bellcrank location

Where is the best location of the u/c bellcrank in the wing.

Practicaly speaking, it dosen't matter much. Glue the bellcrank mount to the spars and don't worry about it.
Al

Like Al says, it really doesn't matter where the 'crank is mounted as far as flying goes. However, you might want to consider where it might be the most convenient for assembly & ease of connecting with the flap horn or elevator. For example, you might want to mount the bellcrank slightly to the left (or right) of center for a profile fuse setup for easier alignment. For a full fuse ship without flaps you might want the pushrod "sorta" centered. Btw, I prefer to mount mine mounted so the front line is up, my stunters seem to handle better that way.

FWIW

LAST EDITED ON May-12-02 AT 02:48 PM (CST)

LAST EDITED ON Apr-29-02 AT 11:31 PM (CST)

Guess who was the first to not only invert the bellcrank to make the front leadout the up line and offer explanations why this configuration had possible advantages? Bearcat article in American Aircraft Modeler, March 1970. (The same article also introduced the leadout slider).
Al

LAST EDITED ON May-12-02 AT 02:48 PM (CST)

Fubar
Only Dangerous when Bored!

LAST EDITED ON Apr-30-02 AT 07:24 AM (CST)

Yea, Fubar,
It was no big deal. I think it is fairly obvious that stunt ships tend to yaw outward on inside corners and inward on outside corners from propeller precession. Pulling on a leadout with a flying wire tends to cause that leadout to swing slightly into alignment with the airplane's C.G. I just thought it made sense to use leadout location to offset some on the gyroscopic precession. I said at the time that even if having a front "up" leadout didn't help, it for sure wouldn't hurt and was worth trying. While I was considering leadouts, it also seemed logical to make them more easily adjustable.
I also fly inverted with my hand palm up. When I was learning the pattern, it was easier for me to think up/down when flying upright and "leading" and "lagging" when inverted. This technique also had the advantage of applying a "get away from the ground" control application when I would get into trouble flying inverted and pulled the handle into my stomach which applied "down".
Al

A little knowledge may not be a dangerous thing. I was out of the hobby in the early 70's when Al wrote the articles mentioned, and was not aware of them when I started flying stunt in 1977.

My first airplane was a Magician, stock, except for the controls being on the inside side of the fuselage. The leadout spacing and location were as per the kit plan (front line down). I had the problem of getting really light on outside corners. After I drug the rudder off several times on the bottom of outside squares, something had to be done.

I didn't know much, but I had read of a "Rabe rudder" which moved out when control was applied. Anyway, I hinged the rudder and made some kind of connection between the rudder and the elevator, such that the rudder went out about 1/4 when the elevator was at full down. It worked, and I came in second in Beginner (full pattern in those days) at my first stunt contest in 1978.

Good stuff!

I knew for sure it wasn't me! It's a good thing guys like me have guys like you to figure all this stuff out
Thanks Al!

LAST EDITED ON May-12-02 AT 02:49 PM (CST)

Guilty as charged.

>Where is the best location of
>the u/c bellcrank in the
>wing.

From the dynamical point of view, I'd want to mount my bellcrank at the CG (center of gravity) of the fully finished plane, less fuel. The CG location on most stunt planes varies somewhere between 15% and 25% of the MAC (Mean Aerodynamic Chord).
The reason why I would want my bellcrank at CG is because otherwise you would have created an undesirable moment (Force times Arm) that may lead to yaw on sharp corners. Say, your bellcrank axle is 1" behind the airplane's CG. During level flight, the centrifugal force, which is (see other threads) a major contributor to the line tension, is acting upon the airplane at its CG point. The airplane, howevere, is attached at the ballcrank axle, which is, as we said, 1" behind the CG location. You have two forces (centrifugal - outward, and line tension - inward) acting on a 1" lever. This results in a moment, which turns the airplane slightly nose outward. You'd think it's good -pointing engine outboard helps keep lines tight. However, when you yank on the controls to go for that sharp, Nats-winning corner, the flaps and elevators act as brakes, and slow the airplane down. Centrifugal force goes down, and line tension decreases. Then engine kicks in, airplane accelerates again, and the yaw moment comes back full force. Due to the mass of the airplane and inertia associated with every yaw, this results in an oscillation (I've actually seen it - mostly on square 8's, when you have a lot of cornering).

From the structural point of view, however, it is more sound (and practical) to attach the bellcrank axle to the spar - which is usually the main span-wise load-bearing element of a wing.
Luckily, the spars on most designs are located close to 23%-25% of MAC, which is also a quite acceptable CG location.

So there you go - if you're still awake after reading all of the above, figure out your spar location, attach bellcrank to it, and keep that location in mind when trimming your plane.

> The reason why
>I would want my bellcrank
>at CG is because otherwise
>you would have created an
>undesirable moment (Force times Arm)
>that may lead to yaw
>on sharp corners.

Mike, aside from the fact that you would cause your leadouts to have to flex around a corner if the belcrank is located too far forward or back, the location of the belcrank makes no difference at all. It is the location of the leadout guide in the wing tip that is the determining factor. That is the point from which the airplane "swings" on the lines, and the rest of it, except for drag on the leadouts, is practically immaterial. For logic's sake, placing the belcrank on the CG means the leadouts can come pretty much straight out the wing, but otherwise, the location of the belcrank makes no difference at all.

LAST EDITED ON May-12-02 AT 02:49 PM (CST)

Fubar,
You miss a very important point. For all practical purposes, the airplane dosen't rotate about the bellcrank pivot in yaw. It rotates in yaw around the C.G. regardless of where the bellcrank pivot happens to be. This is exactly what Leonard said and is sufficiently correct in the strict aerodynamic sense. Is this really worth ultra techinical debate? Mount the bellcrank on the spar. If there is a technically superior location, you will never know it from the way the airplane flies.
Al

LAST EDITED ON May-12-02 AT 02:50 PM (CST)

There was an article in MAN, I think, many years ago, saying that bellcrank location makes no difference. There was a similar article in Aeromodeller in the last couple of years. Both articles were based on flight tests. Personally, I like the bellcrank pivot near the CG and tied into the spar if possible.

Many years ago, "Wild Bill' Netzebaum settled this issue for all time. The article was published years ago in MAN I believe.

He built a plane in which the bellcrank was mounted near the tail. It flew fine and proved that it's the leadout position that really matters.

> please explain the effects of my experiment and how
> that does not apply to the dynamics.

Reading your note carefully, I see that you didn't actually
build and observe anything. Instead, you've planned an
experiment and you've got a hypothesis.

You've also got some rationalizations about the imagined
results, but you haven't carried out the experiment.
Try it! (Netzeband did.) Then look at explaining the results.

Fubar,

You left out some very important parts of the equation in your experiment. Yes if you were actually using only centrifical force to keep the your model at the end of the lines your experiment would probably be true and correct. This would mean that the force that keeps the plane out there is applied from the elines and the center of the circle. This is where you forgot two parts. THE MOTOR AND THE RUDDER!

These two parts of the equation fit in there nicely. The motor pulls the plane along creating a force that pulls the plane in a straight line or slightly away from the pilot and the rudder helps steer it away from the pilot also. This will cause the fuse to fly in a straight line tangent to the anchor point. The motor and the rudder keep the model filying in a straight or slightly canted outward attitude. The lines keep it attached and flying in the desired circular path.

This is why your experiment does not apply. You were basing on sheer sling method and centrifical force. If this were the case then yes you would be correct and those lines would straighten out every time. Like a whip glider would probably prove your experiment correct. The centrificcal force would move the tail out or the nose out or vice versa. We do not fly on centrifical force or it does not seem to me to be that way. The plane wants to go one way but we trap it and wont let it go. It creates it own outward force. the motor and the rudeer fly the plane straight and keep it tangent to the position of the LO.

However I would suggest to all who read this mount your bellcrank on the spar or the highpoint or close to cg. Mounting it elsewhere will cause the LO to see a large amount of stress duing pull tests. This is where the lines will want to straighten out during the pull and can cuase serious tip damage if you have an uneeded amount of rake in you internal lines.

>The motor and
>the rudder keep the model
>filying in a straight or
>slightly canted outward attitude.
>The lines keep it attached
>and flying in the desired
>circular path.
>
>This is why your experiment does
>not apply. You were
>basing on sheer sling method
>and centrifical force. If
>this were the case then
>yes you would be correct
>and those lines would straighten
>out every time.

>DMoon

I don't believe this is quite accurate, either, Doug. Although the aerodynamics will definitely affect how the plane "hangs" on the lines, If it were purely swing around, the plane would line up with a line from the leadout position to the CG. It would still have nothing to do with the belcrank location. As it is, the plane is flying as well, but all this aside, we set the leadout position just slightly behind the CG simply to account for line rake that comes about due to drag.

It is a complex situation, but the bottom line is, the belcrank location has nothing to do with how the plane flies.

LAST EDITED ON May-02-02 AT 00:12 AM (CST)

LAST EDITED ON May-02-02 AT 00:09 AM (CST)

> For all practical purposes,
>the airplane dosen't rotate about
>the bellcrank pivot in yaw.
>It rotates in yaw around
>the C.G. regardless of where
>the bellcrank pivot happens to
>be. This is exactly
>what Leonard said and is
>sufficiently correct in the strict
>aerodynamic sense.

What Al and Leonard have said is correct. John Miller and Jim Thomerson have made references to previous magazine articles on this subject.

Bill Netzeband wrote in his Control Line Capers column, American Modeler, October 1962:

"Misconception Blasted. Pictures illustrate that the bellcrank has NO effect on locating the CG. Rather, the bellcrank should be ON the CG to prevent binding (Pic #2). CG will always fall in line with the center of line lead-out spacing (1, 2, & 3) during flight. Locate your leadouts relative to the CG to produce desired yaw angle for proper tug. ... Next time a guy tells you to locate your CG forward of the BC or in front of the lead out, shun him. He don't know the facts. The CG comes first, all else is related to it."

There are three pictures in this column that show his test rig that represents a wing tip holding the leadout position relative to a fuselage where the bellcrank can be located in different locations. This test rig was suspended by the leadouts. Regardless where the bellcrank was located, the CG of the structure lined up with the leadout position at the tip.

There is another article in American Modeler, JulyAugust 1966, by Walter Williamson. Photographs show an all wood model that appears to have about a 40 to 50 inch span with a 35 size motor. The fuselage looks like a horizontal flat plate based on a crutch assembly to hold the upright engine. The bellcrank was externally mounted on the bottom of the fuselage and could be located in various positions along the length of the fuselage. Williamson ran flight tests with the bellcrank mounted in nine different locations from six inches in front of the CG to ten inches behind the CG. His conclusion was that "It makes no difference where the bellcrank is located. What matters is where the leadouts exit through the guide plate."

In the first installment of Bill Netzeband's series of articles titled Control-Line Aerodynamics Made Painless, American Modeler, July/August 1966, he talks in great detail about leadout position, line rake and bellcrank positions. This installment provides the nomographs to determine the line rake relative to the CG of any control line model based on line diameter and length, model weight and model speed. Bill makes an interesting comment that "If it were possible to mount the bellcrank pivot exactly on the cg and the load of each line didn't change (it does) we could fly without a line guide." Bill also writes of an experiment similar to what he showed in his October 1962 column regarding bellcrank position and representative of the actual dynamics of what goes on when we hook lines onto a model. Bill's results are significantly different than what Fubar described above in a different procedure. I am not sure if I completely understand the procedure that Fubar described, but it reads like whatever he is pulling on or whatever else he is doing, it does not represent the forces actually experienced by our control line models. The following is from Netzeband's 1966 article:

"For years we were fooled into believing that the position of the bellcrank in the airplane controlled its attitude. Not so. The bellcrank can be almost anywhere in the airplane.

"A simple cardboard outline of an airplane will show this relation. Place an eye (U-shaped bent pin) in one wing tip and tie a pin on the end of a thread. Run the thread through the eye and stick a pin anywhere in the cardboard. Observe how the thing hangs and then move the pin (string end) to some other location. Right! Anywhere you stick that pin, the airplane hangs the same. Now move the eye (line-guide). The airplane assumes some new position. The CG of the airplane will line up with the line-guide. (Note: It would be nice to mount the bellcrank on a line between the CG and the line-guide, because the control lines could lead straight through the line-guide, therefore operating with the least stiffness.)"

Given all of the above, I do not intend to get into any long discussion about what others might think, either correctly or incorrectly. I think it all boils down to what Al Rabe just said. Mount the bellcrank at the spar. It is desirable to put the bellcrank at or near the CG. This will reduce the rubbing by the leadouts at the tip guides.

Another subject but related: It is really not necessary to use the wing spars to anchor the bellcrank. I much prefer to run the bellcrank post through the wing and tie it to the fuselage doublers. So basically, none of the loads imposed on the bellcrank are carried to the wing structure. The fuselage has to carry this load anyway so all of the structure that would otherwise be needed to hold the bellcrank inside the wing can be eliminated. The pull test is simply accomplished by applying the force at the point where the bellcrank structurally ties in to the fuselage. The wing structure does not have to carry any of this load.

There is an issue which hasn't been mentioned here. It does not matter where the bellcrank is mounted with regard to suspension of the ship on the lines. However, the geometry of the system of bellcrank, flap horn, and control rod IS affected.

If you plot the rotation of the flap horn versus the bellcrank, you can see the effects of control rod length (as well as drive radii on both the bellcrank and control horn). Linearity is improved by a longer control rod and shorter drive radii. In addition, symmetry can by improved by tilting the bellcrank at approximately 7 degrees, and adjusting the neutral bias angle of the control horn.

Around neutral, a very small bias in the length of the control rod has dramatic effects on this little transfer function.

(I know, I know. None of this amounts to a hill of beans, and by extension is not worth doing. But some of us can't help ourselves. )

Keith,
The new design of the BBQB bellcrank mount is lighter, stronger, provides better support to the pivot, and has a projection above the wing which becomes part of the fuselage structure, not only for flight loads, but to provide a hard spot for pull tests which provides a load path to the bellcrank.
Al

Gentlemen,
I am now absolutely convinced of the correctness of your statements. I have to admit - my theory of yaw moments due to bellcrank axis location had no grounds. I just ran a little vector force diagram on a piece of paper (for myself), and had a chance to see why physics textbooks get published every year without many changes: Physics Rocks! (and it works. Always.)
I am also glad to see that every BS (Bachelor of Science) gets thoroughly filtered for correctness on this forum.

It would be interesting to note that my vector diagram revealed one relationship: the further the bellcrank is from the CG, the more side force is applied by the leadouts to the guide, which increases probability of the bind in the controls (leadouts).
Combined with Larry's tilted bellcrank/control linearity consideration, and through-the-wing bellcrank axle mounts, this makes for another great chapter in the unwritten "Complete Stunt Reference Handbook".

FUBAR,

If you care to see the visual proof and read the articles by Netzeband and Williamson, I will be so glad as to copy them and send them by USPS to you. The bottom line of their experiments was that only the leadout position in relation to the CG, both vertical and horizontal mattered.

If the lines make an angle because of whatever reason, there seems the possibility of wear at that point. Would it be adviseable to use some type of bushing at the guides - especially if the guides were the adjustable type?

>If the lines make an angle
>because of whatever reason, there
>seems the possibility of wear
>at that point. Would it
>be adviseable to use some
>type of bushing at the
>guides - especially if the
>guides were the adjustable type?
>

There is always some wear potential. If there were no pressures somewhere, there would be no need for a guide. The nylon sliders seem to work fine without a bushing when using solid leadouts. I am never sure what the effect of flexibles is. Matt makes his own adjustable guide out of plywood and uses brass bushings in these. I guess, when in doubt, it is best to put a bushing in.

Mike,

You are right. It is everything about moments. If you have firm body, the applied moment is valid everywhere. So does not matter if it is CG, or AC or bellcrank pivot the moment is conducted to whole body and thus you can count is everywhere. You wrote that if the bellcrank is 1” aft of CG the CG makes a moment out of the circle thanks centrifugal force. It is true. BUT you must consider also another force – line pull that is thanx law about action and reaction equivalent to that mentioned centrifugal force. That force makes another moment coming from that force and arm made by warped lines. Just extend lines going to the wing tip from the pilot and make a parallelogram to them via bellcrank pivot. The “height” of the parallelogram (or orthogonal distance between pivot and lines) makes the arm for line tension. The coming moment is opposite to the moment from CG, centrifugal force and pivot to CG arm.

Now the question is when are those moments in balance. Forces are equivalent so also arms must be equivalent. You can easily find that balance happens if extended lines runs via CG position.

If you now move pivot to any other place in the body you will see that really does not matter where the pivot is, lines must go thru CG. I think it answers what was written many times already: “does not matter where the pivot is” and that only important is leadout. The only what is changing with pivot position is warp of lines ant thus its friction and wear.

Thank all of you for your very helpfull information. It is thoroughly appreciated.

Boy, it sure took alot of words to say "it doesn't matter but put it at the cg"

LAST EDITED ON May-12-02 AT 02:51 PM (CST)

A civil engineer? Just checking.

>Finally someone drew it out on
>paper. You make a dot
>on paper representing the bellcrank
>pivot. Place a dot that
>represents the handle now place
>a third dot the represents
>the leadout guide.
>

>
>Hey I'm an engineer and I
>just have to nit pick
>stuff.
>
>
There are some other engineers that explain it differently. You need to put another dot on your paper. That new dot represents the Center of Gravity (CG). That is the dot that will line up with the lines and the leadout guides, no matter where the bellcrank is. The only thing that would change this is if the leadouts are inflexible or nearly inflexible rods that extend most of the way to the handle. (Intersting, Netzeband talks about this as well in his 1966 article.)

Earlier, I was giving you some benefit of the doubt that I had ragarding your earlier post about an experiment that you said you had ran. I wonder now if you really did perform that experiment or any other real test. If your experiment truly represented how a control line model is suspended in flight at the end of control lines, you would not come to the conclusion that you seem to be grasping for.

There is photographic proof from 1962 that shows the CG lines up with the leadout position, regardless of the bellcrank position. These photographs are of a fixture designed by an engineer to examine this matter 40 years ago. I doubt if the laws of physics have changed since that time.

As a few have already commented, we have used a lot of words to end up saying that it is still best to put the bellcrank as close as practicable to the CG. But the reason for doing so is not to avoid some force that will try to align the bellcrank with the leadout guide as you suggest. The reason is to minimize the friction of the leadouts at the assumed optimally positioned leadoout guides.

LAST EDITED ON May-12-02 AT 02:51 PM (CST)

>A tight rope is also flexible,
>but tension it enough on
>the Z axis and it
>acts like rigid object on
>the X and Y axis.
>Even easier take your wedding
>ring and hang it on
>a string, it droops because
>of the weight. Now pull
>the string taught, what happens?
>The ring moves! It has
>to! What terminates the centrifical
>force generated by the flying
>airplane? The handle and the
>bellcrank pivot. The lines are
>the string think of the
>leadout guides as the ring.
>(Why else is the reason
>for the pull tests)
>CG is a value not a
>physical anchor. Get the picture?

Your example is not representative of a tethered airplane in flight with lines attached through a wing tip. No matter how much centrifical force is being generated, the force vector goes through the center of gravity. And that force vector will line up with the tip leadout guides, regardless where the lines terminate after they pass through the leadout guides.

I challenge you to do the simple demonstration described by Bill Netzeband in his 1966 article referenced in the #18 post of this thread. You could make the airplane he talks about out of cardboard or from a bar of lead (which would represent more G's or a greater centrifical force which results from a heavier airplane or a faster airplane flying in a circular path or a combination of both). Suspend the thing from the wire or string that passes through the wing tip guide. No matter where the bellcrank is positioned, the cg of the unit will always line up with the tip line guide.

Your test subject does not have to be an airplane. You can glue two sticks together to make a "T". Put an eyelet at the end of the one stick that represents the inboard wing of a model airplane. Run a cable or string trough the eyelet and anchor it anywhere on the cross piece that represents the fuselage. Suspend this by the cable running through the eyelet. Notice that the attitude of the assembly when suspended does not change, no matter where the cable is attached on the cross piece (fuselage). Now put a one-pound lead weight or five pound anything or whatever (It could be 100 pounds) and attach it to the "T" assembly at the intersection of the two sticks. (This represents either a heavier airplane or a faster airplane or both, it is more G's or more centrifical force which puts more tension on the line.) Again move the anchor point anywhere on the cross piece (the fuselage). Behold! Nothing changes. The CG will always line up with the tip line guide.

Please try this and tell us what happens. Maybe it will work different in Florida.

>Ever notice a plane that yaws
>to the out side up
>wind in a wingover? and
>then the nose snap into
>line when it pulls out
>on the down wind pullout
>when the tension comes back
>in spades?

No.

>Well what forces do you think
>does this?

I would like to be enlightened.

>
>Only Dangerous when Bored!

Hey Fubar, you wrote a great piece about first experiences with model airplanes. What you have written here is not dangerous, but it is wrong. Does that mean that you are starting to get bored?

FUBAR,
What you say about the wedding ring is true (you single guys will have to trust us), but the force to straighten the string far exceeds the weight of the ring.
What you say WILL probably happen to the plane if you far exceed the centrifugal force and design parameters of the model. Bill Netzeband's article explains what happens to a plane under normal flight conditions. Yes, the controls would probably eventually straighten out, but long after the model has been ripped apart.
I think this discussion varies from practical application to theoretical extremes. But then, I'm certainly not an expert.
And when you get right down to it, a computer does not multiply, it only adds and shifts
No disrespect intended to anyone.

Looks like a miracle here, I hope a picture will really help.

First picture shows regular balanced condition. The centrifugal force on CG is Fcf. It acts on body by moment of Fcf x Armcf and tries to yaw out because we are fixed in bellcrank. There is also pull force in lines, as stated by Fubar, and it tries to yaw in, because lines are warped – it makes the moment Fpl x Armpl. Both forces are equivalent and arms Armpl and Armcf are also equivalent – and it happens exactly if CG is in line of lines – and this fact is invariant on position of pivot and also invariant on value of centrifugal=pull force and also what kind of engineer you are J.

Second picture shows why the balance happens exactly if CG lined up with lines – assume that it is not. If model is yawed in, the forces are still equivalent Fpl=Fcf but arm for pull is smaller than arm for centrifugal force Armpl<Armcf. It means moment out of the circle is higher than toward the circle – thus – the model must yaw out until moments are equivalent like shown on first picture.

Helps?

igor

LAST EDITED ON May-12-02 AT 02:52 PM (CST)

>Yes exactly, more than once I
>said that the forces are
>not sufficent to make this
>a real concern. But they
>are there. That was all
>I was trying to say.
>
>
>Only Dangerous when Bored!

No, they aren't there. That's what we have been trying to tell you all along. George had it wrong, too. What we have been trying to say is that as a force, only the location of the leadout guide and the CG make a difference, not the location of the belcrank. The location of the belcrank can be in your pocket if you can figure out a way to do it and the plane will still fly in the same attitude. It makes no difference!

Geez, the light finally came on, albeit a little dim at first. When I read Wild Bill's atricle I accepted it and thought I understood it, but I guess I only understood part of it.
This has been a very enlightening thread.

Hi Keith,

You've managed to explain it very clearly and I cannot agree more.

It is now easy to understand that also vertical location of the bellcrank is of no importance. So you can run your leadout cables IN the wing in case of dihedral. Right Al?

This is a subject that pops up every year or so and the discussion is always great. I love it.

Beware. Many topfliers in the world are non-believers!

Paul

Ho Hum,

Theory versus practical adaptation. Gosh, do we have a new and better working wheel now? I think the moral of this lesson is to research things before stating things are different than what has been experimented with and proven valid for a particular given condition. Of course, something proven valid for a given condition may not be valid for ALL possible conditions. I think this is the fundamental cause of the difference in opinions here. Anyway here's what I do. I mount my bellcrank as close as I can practically get it to the CG, but more importantly I tilt it slightly, about 7 degrees as Larry said, to help overcome control throw geometry inconsistencies. I think overcoming control geomerty problems is somewhat more important that getting the bellcrank exactly on the CG.

Aduacter et Strenue

Jim-

With all due respect, this is not really a matter of difference of opinion and application - unless one wishes to repeal Newton's Third Law and other long-standing principles of physics. The wheel is not being reinvented.

Centripetal force directed inward along the lines to hold the plane in circular flight acts through the center of mass (or c.g.) of the plane. The equal but opposite reaction force of the plane against the lines, termed "centrifugal", acts outward along the same line. The leadout guides (being at the leadouts) must lie along that same line. It's easy to get lost in the inner engineering complexities and not be able to "see the forest for the trees", but that's the whole story: the center of mass will line up with the flight wires, regardless of bellcrank placement.

Were it not for the leadout guides (or any other intervening structure) to hold the yaw attitude, then the bellcrank would find itself right in line with the flight lines and center of mass.

Newton's Third Law: "For every action (force), there is an equal and opposite reaction." What could be more practical?

SK

P.S. After writing this, I did the experiment: 'CA'd two sticks together (wing/fuselage)with eyelets on one (leadout guides at different 'semispans') and pins at thr center and both ends of the other (balanced bellcrank positions). A penny at the intersection served as a dominant mass. A fine thread passing through either eyelet and attached to any of the three pins served as the control lines. The attachment ('bellcrank')position made no noticeable difference in the orientation (yaw) of the assemblage when subjected to gravity ('g-force'), even with extreme relative placements.

So, despite my own problems in analyzing the influence of the leadout guides, I think the experimental results are convincing and show no sign of subtle contrary effects.

Serge,

I don't think that the laws of physics can be repealed by any one of us at least. Nor was I trying to say that! I see the new wheel gag I put in my post got ya! What I was actually alluding to was that a model plane at our flight speeds with a bellcrank and leadouts and leadout guides is a little bit different than a mass (so called rock) of weight being slung around on a string until the centrifigal and cintripeal forces creat enough force to cause a straight path along the string to the center of mass of the object. Oh well, i've read all the articles mentioned and I mount my bellcrank how I think it best be done. Doesn't everyone do that?

Aduacter et Strenue

Jim-

Yeah, I'm easy. Fortunately (?), with line rake and so many other inertial, aerodynamic, thrust, torque, precessional,... etc., etc.,... effects, it's also a much more interesting and challenging hobby than rock slinging! 'keeps me entertained.

SK

P.S. Bellcrank orientation and designguration (c.f. Larry's post) - now THERE'S an interesting subject too.

'CA'd two sticks
>together (wing/fuselage)with eyelets

Serge, that's where all your eyelets go - on physics experiments! What are you gonna use for your flying lines?

>
>Ever notice a plane that yaws
>to the out side up
>wind in a wingover? and
>then the nose snap into
>line when it pulls out
>on the down wind pullout
>when the tension comes back
>in spades?
>
>Well what forces do you think
>does this?

I think they call this "weathervaning". It happens if you have a lot of wind and a lot of side area behind the center of gravity. The back end of the plane will blow in upwind and blow out down wind. It has nothing to do with the topic at hand.

The biggest point that I believe you have missed is that it is the center of gravity, not the center of the point of attachment at the belcrank, that is going to "straighten the lines."

>Oh by the way it Telecom
>/ Computer engineering. Where we
>deal in absolutes zeros and
>ones, where it either works
>or it is FUBAR.
>

I hope that was a Freudian slip.

>A tight rope is also flexible,
>but tension it enough on
>the Z axis and it
>acts like rigid object on
>the X and Y axis.
>Even easier take your wedding
>ring and hang it on
>a string, it droops because
>of the weight. Now pull
>the string taught, what happens?
>The ring moves! It has
>to! What terminates the centrifical
>force generated by the flying
>airplane? The handle and the
>bellcrank pivot. The lines are
>the string think of the
>leadout guides as the ring.
>(Why else is the reason
>for the pull tests)
>CG is a value not a
>physical anchor. Get the picture?
>
>
>*Again the amount of tension placed
>on our lines would not
>create the forces that would
>make any significant difference. But
>these forces are at work
>and would increase proportionally with
>the G's.
>
>Pull your string tight enough and
>rather than your wedding ring
>you could support a car
>tire on a straight line
>of string.
>
>Ever notice a plane that yaws
>to the out side up
>wind in a wingover? and
>then the nose snap into
>line when it pulls out
>on the down wind pullout
>when the tension comes back
>in spades?
>
>Well what forces do you think
>does this?
>
>Oh by the way it Telecom
>/ Computer engineering. Where we
>deal in absolutes zeros and
>ones, where it either works
>or it is FUBAR.
>
>
>Only Dangerous when Bored!

FUBAR,

I went back through my old issues of American Modeler and found the articles by Bill Netzeband and Walter Williamson concerning the issue of leadouts and bellcrank position and which really mattered. The magazine issue is July/August 1966 on pages 22 and 26. Walt Williamson's practical test of the importance of leadouts versus bellcrank position is classic empirical engineering. I hold a degree in Mechanical Engineering by the way. Walt built a kluge testbed that allowed him to move the leadouts from several inches behind to in front of the CG and move the bellcrank from 6 inches in front to 10 inches behind the CG over 9 positions. The bottom line is that only the leadout position matters at all. Walt's final admonition to configuring a stunt ship is as follows: "The way I check a plane is hold it up by both lead-outs. The way it hangs is the way it should fly, if I have no offset in the motor or rudder." And this is exactly how I set my leadouts prior to the first flight of a new ship. Bellcrank position matters not at all other than to make sure it is solidly installed, not binding and the controls work smoothly and linearly. Physics is physics.

Again, I offer you these articles by USPS if you are at all interested in how to determine where to set the leadouts.

Here is hoping you have tight lines.

Tom McClain

I have no degrees of any kind that have anything to do with any kind of engineering. I fully understand that BC position is not a factor in flight characteristics. The LO do the work.

Ok, this is all very interesting, or not. There has been a lot of very learned explanations about leadouts and C.G.s. I am not an engineer. I was a fair flight instructor and taught a lot of aerodynamic ground school. Now that we have an amassing of engineering talent, I'd like to know from the professionals, was I correct in my Bearcat article in 1970 which stated that turning the bellcrank over to make the forward leadout, the "up" leadout, should somewhat offset the effects of gyroscopic precession, and failing that, would be harmless?
Also, I have been flying airplanes with dihedral. The bellcranks are mounted on the wing center section vertical centerline, and leadouts centered on the wing tip vertical centerline. This causes up to 1 1/2" of misalignment between the bellcrank and the vertical C.G. My Mustang, " Snaggletooth" has well over 2200 flights on it with just such misalignment and shows no obvious wear on the brass leadout guides or flexable Sullivan C-D leadouts. If they haven't worn significantly, then what are we arguing about? It has been said too often already, the bellcrank location dosen't matter much, so why are there so many serious posts about such an insignificant subject?
Is it possible that all of the builders who followed my suggestion of making the front leadout "up", deluding themselves by thinking their airplanes fly better that way?
Al

Glue the bellcrank mount to the spar and don't worry about it.
Al

>Glue the bellcrank mount to the
>spar and don't worry about
>it.
>Al

AL,

You were correct when you inverted the bellcrank. Ii wouldn't have made a difference if one could have both leadouts exit the wing in the same hole, but that is an impossibility or darn right difficult. Therefore, because there is a linear distance of some measure, and stresses on the flying lines do change as one moves the flight controls, inverting the bellcrank and having the up line forward did help by reducing the tendency of the ship to yaw inward. All of this discussion has been for the purpose of educating all serious stunt aficionados that leadout position and its relation to the CG is the primary worry . You understand that perfectly. The wear question is easy. There is not enough continuous movement to wear out the leadout guides and the leadouts are not abrasive, but low friction. Now, if one wanted to use abrasive jeweler's cutting wire for leadouts, I might have to eat my words.

Press on.

LAST EDITED ON May-12-02 AT 02:53 PM (CST)

>Now was'nt that fun?

Fubar,

Does this mean that you finally understand that the location of the bellcrank will have absolutely no effect on the yaw characteristics of a control line model airplane? Did you really test this or are you still drawing dots on a piece of paper or sliding a ring or a tire on a string? I was also hoping that I could be enlightened by your explanation of "wing-over yaw" and "wing-over nose snaps".

Gee, befor SSW, i cudn't even spell enguneer and now i are one and now i git to nit-pic and i guess i like to beat a dead horse.

LAST EDITED ON May-12-02 AT 02:53 PM (CST)

>So after this discussion, I get
>a feeling that more people
>actually understand the dynamic that
>they did before just taking
>the word of others. But
>darn it sooner or later
>someone is going to admit
>the airplane hangs on the
>bellcrank pivot Not the CG.

So either you still don't understand the principle, or you do, but you're attempting to make some other point?

A quiz question: If you build two identical models, CG in the same place, and leadout guide in the same place, one with the bellcrank pivot at the wing high point, and the other with the bellcrank pivot 2" behind the high point, and hang them from the leadouts, will the fuse be

(a) at the same angle relative to the ground, or

(b) will one be tilted relative to the other?

and part 2, will they fly

(a)identically, or

(b) will one fly yawed differently from the other.

LAST EDITED ON May-03-02 AT 01:03 AM (CST)

Muhahahahaha,

I was wondering how long it would take before someone noticed that my point shifted form yaw to the geometry of leadout and bellcrank position. But the thread developed a momentum of a freight train and once set on it's course it has become juggernaught. Like a supertanker very slow to change directions. Some where around post 40 or so, I had dropped any mention of yaw and focused on the forces acting on the leadouts when tension is applied. Hence the ring on a string example.

I had come to realize that my model was too simple, reducing the problem to a single pivot, that of the bellcrank alone, when there are actually 2 the second being that of the airplane airframe itself (hence the CG pivot point) line stresses are applied the the bellcrank pivot point and the airframe rotates around the Logical GC pivot point.

The wing tip lead out guides limit how much the airplane will Yaw around the CG pivot point, while physical tendency as result of gentripedal forces the angilar moment of the leadouts in relation to the bellcrank result in increased friction as the leadout guides are the limiting yaw and also preventing a straight line shot for the leadouts in a radical installation.

Only one person I recall tried to present the point that I was not considering the entire problem. That individual said that my pad dawing should hve one more dot and that is the CG location. Yet he fell short and did not explain the entire hypothisis.

Mainly people just cited articles and and tests done by others of imperical studies done by others. Well to tell you the truth, I never could stand the answer " WELL THAT THE WAY IT IS" and that that. Hated when my teachers did it, still hate it.

Me I never was content with knowing, I am most happy understanding. I at least now I've got the impression that more people now understand rather than just know, and thats a good thing. As I said befor it makes you think rather than just absorb.

The answers:

A. The same angle to the ground but with a up elevator deflection when hung with the rear bellcrank mounting position.

B. It certainly would not fly the same as you will be fighting varying lead out friction for the entire flight.

>But darn it sooner or later
>someone is going to admit
>the airplane hangs on the
>bellcrank pivot Not the CG.

Make a fixture that has been describe previously in this thread. It takes all of 10 minutes to do. You will be surprised, and then you will wonder how anyone cannot understand that a control line airplane aligns its CG with the leadout guides and that the bellcrank position has absolutely nothing to do with it. (Given this "hint" you might be able to answer Bret's quiz.)

>Some comments make it sound as
>if the model hangs on
>the CG not the bellcrank.

That is what some people have been trying to get across here. It has been proven. Why don't you check it out physically, not with dots on a paper nor with a string holding a ring. However, a properly drawn force diagram will illustrate what is going on here. Some engineers can do that kind of thing.

>If thats the case why
>do we not pull test
>the CG?

For whatever the relevance is of this question, the pull test is to assure the integrity of the control system so that the airplane will stay attached to the lines and for the lines to stay attached to the handle.

Are you just having a good time with us and really have your tongue firmly planted in you cheek when you submitted this last post? If you are, it is very well disguised. It is certainly not contributing to any understanding of the dynamics of control line models. I fail to see any humor in what you are doing. Credibility is still a good word.

I will repeat a quote from an earlier post in this thread. Bill Netzeband wrote in his October 1962 American Modeler column:

"....Next time a guy tells you to locate your CG forward of the BC or in front of the lead out, shun him. He don't know the facts. The CG comes first, all else is related to it."

LAST EDITED ON May-12-02 AT 02:54 PM (CST)

LAST EDITED ON May-03-02 AT 08:01 AM (CST)

LAST EDITED ON May-03-02 AT 08:00 AM (CST)

>>Make a fixture that has been
>>describe previously in this thread.
>> It takes all of
>>10 minutes to do.
>>You will be surprised, and
>>then you will wonder how
>>anyone cannot understand that a
>>control line airplane aligns its
>>CG with the leadout guides
>>and that the bellcrank position
>>has absolutely nothing to do
>>with it. (Given this
>>"hint" you might be able
>>to answer Bret's quiz.)
>
>
>What does this have to do
>with the airplane physically hanging
>and secured by the bellcrank
>pivot?

FUBAR,

I hope you are having fun. But if you are serious, I will try to respond. The test fixture described represents the configuration of a control line model airplane. It has a tip lead out guide. It has provisions to mount the bellcrank at any location relative to the wing tip and the CG. So it has everything to do with examining and demonstrating what happens when the bellcrank is located at different locations. I did this. It only takes a few minutes to set up. The results are surprising. As you suggest, it was more than just taking some written word that this is the way it works. In the same token, the physical examination I made shows that the written word from another quarter (yours) is in error.

I apologize if I did not explain very well my comment to you earlier that I thought you needed to add a CG dot to the diagram you were describing. I was having some difficulty in understanding what it was you were talking about as well as the test or demonstration that you had previously described. Since yu had been ignoring it, I thought the suggestion that the Center of Gravity should be a factor in any consideration of this topic would be useful to you.

There are two forces involved here besides the tension through the lines. There is a resultant force, mostly along the longitudinal direction, at the tip line guide. There is another resultant force at the bellcrank attach point. That resultant force at the bellcrank mount totally and equally offsets the resultant force at the tip line guide as well as the force to yaw the airplane about its vertical axis. The CG will still be in line with the tip leadout guide with no induced yaw. Indeed, as the bellcrank is mounted further from the CG and/or as the weight/mass/centrifigal force of the model becomes greater, the longitudinal force at the tip line guide becomes greater and results in increased friction of the lines at this guide. But there is no change in yaw. (It is still advisable to place the bellcrank at or as near as practicable to the longitudinal center of gravity.)

>>>Some comments make it sound as
>>>if the model hangs on
>>>the CG not the bellcrank.
>>
>>That is what some people have
>>been trying to get across
>>here. It has been
>>proven. Why don't you
>>check it out physically, not
>>with dots on a paper
>>nor with a string holding
>>a ring. However, a
>>properly drawn force diagram will
>>illustrate what is going on
>>here. Some engineers can
>>do that kind of thing.
>
>
>Back to this again You are
>saying that the bellcrank is
>not keeping the airplane at
>the end of the lines?
>Are you aledging that it
>is the CG that secures
>the airplane to the leadouts
>and to the lines?

Where did I say that the bellcrank is not keeping the airplane at the end of the lines? What has been explained before is that the CG will line up with the tip line guide regardless of where the bellcrank is located.

>>>If thats the case why
>>>do we not pull test
>>>the CG?
>>
>>For whatever the relevance is of
>>this question, the pull test
>>is to assure the integrity
>>of the control system so
>>that the airplane will stay
>>attached to the lines and
>>for the lines to stay
>>attached to the handle.
>
>Hmm. So the pull tests are
>not for also making sure
>the Bellcrank stays within the
>airplane? And is only a
>test for line failure?

My statement said that the pull test was to assure the integrity of the control system so that the airplane would stay attached to the lines. I think it is understood that the bellcrank is part of the control system, so that my statment includes your concern that the pull test is "also making sure the Bellcrank stays within the airplane." Where did I state or even imply that the pull test "is only a test for line failure"?

>
>>Are you just having a good
>>time with us and really
>>have your tongue firmly planted
>>in your cheek when you
>>submitted this last post?
>>If you are, it is
>>very well disguised. It
>>is certainly not contributing to
>>any understanding of the dynamics
>>of control line models.
>
>Well if that is the case
>then why are you spending
>so much time trying to
>make a point? If it
>is not to contribute to
>the understanding of the Dynamics
>in CL airplanes. Do you
>think so little of your
>contributions. Is it a sin
>to think and to air
>all angles of a hypothisis?

It is fine to search for truth and understanding. You have obviously thought about this a great deal and I believe you are serious about the convictions you have. There is evidence contrary to the statements you have made. This evidence can be easily tested. I have not just depended on what was written. I tested it and have tried to explain to you what actually happens. It is not a matter of theory. It is a physical fact.
>
>
>I fail to see any
>>humor in what you are
>>doing. Credibility is still
>>a good word.
>>
>>I will repeat a quote from
>>an earlier post in this
>>thread. Bill Netzeband wrote
>>in his October 1962 American
>>Modeler column:
>>
>>"....Next time a guy tells you
>>to locate your CG forward
>>of the BC or in
>>front of the lead out,
>>shun him. He don't
>>know the facts. The
>>CG comes first, all else
>>is related to it."
>
>Well that is a arrogant attitude,
>Shun him, well that statement
>really imparts a understanding of
>principals. And reading what some
>one states as fact is
>absorbing information. And that is
>a far cry from understanding.
>Knowing the sun rises in
>the east in the morning
>is a far cry fron
>understanding how and why it
>does. For some people knowing
>is enough, but as for
>me, it never was.

I quoted a statement regarding this topic that was written 40 years ago. It seemed appropriate.

I have read several accounts regarding the location of the bellcrank. I have set up a fixture to demonstrate to my own satisfaction what has been previously documented. In your quest for understanding, truth and enlightenment, I find it interesting that you have not responded, at least in this open forum, the comments in post #21 of this thread about the testor experiment you said that you had conducted.
>
>NOw I ask again was'nt that
>fun? I enjoy a good
>excersize of the braincells. And
>if nothing else this thread
>had people excersizing some of
>em. And not just regurgitating
>previously written articles.

I find there is nothing wrong in using references to explain any concept as long as there is some credibility in those references. In the case of Bill Netzeband, I have no reason to question the technical accuracy of his writings. Indeed, it was fun to replicate his test.

Keith

>
>Only Dangerous when Bored!

This fixture thing has been suggested and ignored. I am going to try to offer two suggestions here.

First, make a cardboard cut out of a top view of an airplane. Put a thumb tack through the left (or right) wing tip simulating the normal leadout guide. Now add some weight at the normal center of gravity location. See how the airplane hangs. Move the weight back towards the rear of the plane. Watch what happens. Now push the thumb tack to the rear of the wing tip and watch what happens again. It will always line up in a straight line from the wing tip to the center of gravity. There are no leadouts, no belcrank involved here.

Second. Imagine a real plane with the belcrank located in the back of the inboard wing tip. (This is going to involve some interesting geometry for the pushrod, but we shall ignore that for the moment.) Now place the leadout guide in the normal position but inboard from the tip just slightly. Yes, the airplane will fly just fine with the leadout guide inboard a bit. It doesn't have to be all the way to the tip. Now put a pully in the wing for each leadout to travel around to hook back up with the belcrank. We are trying to reduce or eliminate friction here (which seems to hang you up as well). It would help if someone could draw this up in a diagram for me. I don't have the capability to do this other than crudely. Now, the questions:

How will that airplane fly? If it is in proper trim and the CG is properly located along with the leadout location, it will fly just fine.

Now, let's change things and make this really fun. Let's put two pullys on top of that wing in place of the leadout guide so that the leadouts exit the pullys at the same point as they would have previously through the leadout guide itself. The belcrank is still located outboard and to the rear of the pully exit for the leadouts.

Fly the airplane again. And assuming we don't lose the leadouts from the pullys, how will it fly? Just fine. Will there be any tendency for the leadouts to straighten themselves out and for the plane to suddenly reverse itself and come in so that the mass of the plane is now on the inside and the belcrank is on the outside? (This is your theory, remember, that the tendency would be for there to be a straight line to the belcrank.) Hopefully, you don't say, "yes" on this one.

And, if we did a pull test, what would we be testing?
Answer: The handle, the lines, the pullys (because there is a force around these) and the belcrank itself. Even though it would be outside in the tip, it would still be an anchor point. But that has nothing to do with the dynamics of flight.

We don't pull test the CG. We pull test the flight control system (handle to belcrank, and everything in between--even if the lines make a sharp bend at the lead out guide.) But the key forces in flight are being applied in a straight line from the leadout guide to the CG, with no concern as to where the belcrank is located.

You can lead the horse to water, but..

REALLY reaching, the only effects I can see for moving the bellcrank:

a) geometry of the control system is changed, and

b) the location of the CG might change some miniscule amount due to the change in position of the mass of the control system.

c) leadout geometry (length of wires) might be affected in some small manner.

d) if the leadout path is displaced, the leadouts may experience more friction on their guides.

None of these effects counter the argument about the force acting through the center of mass and passing through the leadout position at the inboard wing tip.

All of my airplanes since 1967 have been built with 2 degrees of engine offset including the four stroke Bearcat which flies marvelously. Excepting the Mustunt series, they all have had swept forward hinge lines and dihedral. Most have required some rudder offset to perform to my taste (fairly heavy line tension). They have all flown well. I have made my own peace with yaw, hinging, stalling, buffeting, line tension, C.G. location and engine problems. The answers to all things stunt are not necessairly to be found in net forums. There is enough B.S. here to ruin anybody's shoes. It is easy to build a stunt ship and to get it to fly well, but not if one were to believe many of the "rules" suggested by the "experts" here. Unfortunately, there are people replying to this thread speaking with great authority whose knowledge of aerodynamics was picked up word of mouth from other "experts" and such personal experience as they may have had with control line airplanes. Most emphatically, I am not refering to Leonard, Larry, Keith, Howard, Paul or Brett all of whom I very much respect for their knowledge and experience. There are others who also speak with righteous authority whose names I don't recognize.
Guys, we need to keep it simple to be useful to fliers who need help. The only answer necessary to adequately answer the posted question was in the first reply.
Put the bellcrank on the spar and don't worry about it.
Al
Yes, of course I meant "esoteric".
Al

The
>answers to all things stunt
>are not necessairly to be
>found in net forums.

True. One can get all the answers by trial-and-error, but it may take a while, so it's quicker to ask. It is everyone's responsibility on this forum to filter out the information they want.

>It is easy to build
>a stunt ship and to
>get it to fly well,
>but not if one were
>to believe many of the
>"rules" suggested by the "experts"
>here.
> Most emphatically, I am
>not refering to Leonard, Larry,
>Keith, Howard, Paul or Brett
>all of whom I very
>much respect for their knowledge
>and experience. There are
>others who also speak with
>righteous authority whose names I
>don't recognize.

When joining this forum, I didn't realize that the privilege of posting here was reserved for the persons listed above (you forgot Ted, he posts here often). "Forum" generally means "a medium of open discussion or voicing of ideas", not a "consulting bureau".
But even if we assume that only "experts" can post here, what "expert" will decide which of the "experts" is correct? I've seen Howard Rush disagreeing with you, Al, on another thread. Does that mean you're banned, too?
I would be interested to hear Leonard's opinion on this as the forum moderator.
Until then, I probably should get back to my shop, to work on making my name a little more recognizable.

>The
>>answers to all things stunt
>>are not necessairly to be
>>found in net forums.
>
>True. One can get all the
>answers by trial-and-error, but it
>may take a while, so
>it's quicker to ask. It
>is everyone's responsibility on this
>forum to filter out the
>information they want.
>
>>It is easy to build
>>a stunt ship and to
>>get it to fly well,
>>but not if one were
>>to believe many of the
>>"rules" suggested by the "experts"
>>here.
>> Most emphatically, I am
>>not refering to Leonard, Larry,
>>Keith, Howard, Paul or Brett
>>all of whom I very
>>much respect for their knowledge
>>and experience. There are
>>others who also speak with
>>righteous authority whose names I
>>don't recognize.
>
>When joining this forum, I didn't
>realize that the privilege of
>posting here was reserved for
>the persons listed above (you
>forgot Ted, he posts here
>often). "Forum" generally means "a
>medium of open discussion or
>voicing of ideas", not a
>"consulting bureau".
>But even if we assume that
>only "experts" can post here,
>what "expert" will decide which
>of the "experts" is correct?
>I've seen Howard Rush disagreeing
>with you, Al, on another
>thread. Does that mean you're
>banned, too?
>I would be interested to hear
>Leonard's opinion on this as
>the forum moderator.
>Until then, I probably should get
>back to my shop, to
>work on making my name
>a little more recognizable.
>

Oops, Let's back off again guys. This has been a fascinating thread and, even with the strong differences of opinion evident between the "adversaries", has been refreshingly free of acrimony.

I clearly throw my hat in the ring with the "CG" tribe but have found the discussion itself to be agreeable and was looking forward to the final answer to enlighten every one in the end.

Mike, I don't think for a minute that Al was trying to limit input to the forum to self (or *star rating* nominated) proclaimed experts. I think he was trying to make the point that many of those contributing have an excellent record of success in actually making control line stunters perform (and most still have some question in their minds about exactly how these creatures ultimately work).

There is merit in recognizing such success as validating of a point of view on subjects of contention and, if carefully worded, pointing such out can be a legitimate means of compounding evidence to support a given side of a debate. These guys have proven their ability to utilize the knowledge base to their advantage in direct competition with both others and their results lend credence to their ability to separate the wheat from the chaff (not that we all will always agree...right, Al?)

On the other hand, I respect FUBAR for approaching the subject matter from his trained perspective and coming to (I feel, unwarranted) conclusions and pursuing the debate in search of a conclusion he feels is well founded. I was silently following the give and take seeing how long it would take (and what would be the "light bulb illuminating" argument) that would make FUBAR eventually say "Aha" or "voila" or "no s*&%".

Ultimately, I think the well founded position of the "CG Tribe" will prevail and perhaps we can move on to a subject that still stumps most of the guys whose wisdom of the event I admire. I'd like a physical description (and a proof of the axiom in words even I could understand) as to why the same airplane with a different powerplant can fly equal lap times and be so obviously superior in performance and line tension.

The physical theorists amongst us will likely say that only centrifugal/centripital force--i.e. mass, velocity, radius--have anything to do with it. My pragmatic experience (and that of most with whom I've discussed the issue) is that there is more involved and that a competent pilot can tell within a lap or so of takeoff whether one powertrain's delivery will provide the performance desired even when lap times are identical with that of a different powertrain which seems whimpy.

I loved the theory (I think from Bob Z.) regarding the different power impulses from four strokes. I've no idea whether there is ultimate merit to the theory but the innovative thinking was fabulous. I suspect the answer to the line tension/aircraft performance question lies somewhere in the jumble of torque, horsepower, BMEP, etc. all of which apply themselves to the propulsive force of the prop.

Anyone want to try that one?

Ted,

I see you have been flying your KA-10. How goes it with the ship? Might I see it in Muncie? Also, I think the original theory of the pulses and lack thereof on every other stroke on a four stroke helping cornering ability was initially put forth by Mr. Bradley Walker, otherwise known as the Big Green Lizard "Godzilla", at least within this forum anyway. Even if you don't get to Muncie with your KA-10 it will be nice seeing yourself and the western crew again.

>Ted,
>
>I see you have been flying
>your KA-10. How goes
>it with the ship?
>Might I see it in
>Muncie? Also, I think
>the original theory of the
>pulses and lack thereof on
>every other stroke on a
>four stroke helping cornering ability
>was initially put forth by
>Mr. Bradley Walker, otherwise known
>as the Big Green Lizard
>"Godzilla", at least within this
>forum anyway. Even if
>you don't get to Muncie
>with your KA-10 it will
>be nice seeing yourself and
>the western crew again.
>
>Jim Pollock

I, Jim, don't I just wish I was flying the KA-10. There are some finish issues (both paint and just plain "finishing" the engine installation) that have put it on the back burner. It isn't in the air yet and may just get the paper stripped off the wing and recoverd before it does get airborne.

I made a bad mistake when building the wing using Tom Dixon's kept foam system. On by own (Tom DOES NOT suggest doing so) I cut out the foam from between the pseudo cap-strips in an attempt to save 2.3867 grams of weight. BIG mistake. Doing so does not leave enough substance to the remaining "rib" structure and they will distort when the covering begins to shrink. Todd Lee's Mustang (I learned too late) was a victim of just this sort of mistake and the foam beneath the cap strips eventually pretty much disintegrated leaving his infamous (and apparently quite functional) flexi-flyer airfoil which consists of only the capstrips and the covering in the open bay areas.

As a result I'm staring at this distorted wing every time I go in the shop and haven't had the gumption to either finish it with my eyes blindered to the eventual outcome or to rip off the covering and remove the self-destructing structure and replace them with real ribs. As a result I'm looking at yet another year of pond scum.

Ted

p.s. looking forward to seeing you at Muncie as well.

Ted,

Sorry to hear about your woes with the KA-10. Good luck on any rib support re-design of the wing.

Ted,
Know exactly what you mean about engines. When I was actively competing in the pre PA days, I would overhaul an engine (fit new rings and bench break in) and begin to fly it. I had many experiences where the engine would give fantastic performance and just pull your arm off, only to have the great characteristics fade to ordinary during the first fifty flights. I think I was losing my "seal" somehow. I tried endlessly to prolong those great flights with experimants in cowling configurations for improved cooling and ground cooling between flights by squirting alcohol on the cylinder after each landing. Nothing ever consistantly gave me those great engine runs.
Times change. So do engine technologies and metallurgy. Since I returned to stunt, after a twenty year absence, I have had nothing but great runs (excepting tank problems). The PA's always give me the engine runs I had so desperately wanted years ago. And now we have the four strokes. Wow! I never even hoped runs could be so strong. With every flight I'm waiting for the other shoe to drop. Will this one ultimately fade like those engines of long ago? Time will tell, but for now, I'm having a ball.
Al
On second thought, maybe times havent caught up with smaller engines with less sophicated technology (affordable 30's and 40's). I hate to tell a new stunt flyer that his out of the box engine probably isn't going to run like a competition engine until it has had the attention of a gifted engine man. Thank goodness, I haven't had to fight that battle again.
Al

Len and Ted,
thank you for the words of wisdom. My previous post was not an attempt to turn the discussion personal, but to make it more civilized, and also to protect the less *famous* of us from further arguments of that kind.

Both of you are oh-so-right about how much weight a person's "stunt circle rating" adds to his statements in a discussion about any stunt-related subject. Most of us, the less experienced /knowledgeable / successful stunt folk are hanging out at this forum primarily because of YOU GUYS! And yes of course, that includes Al. I have tons of respect for everyone of you, and I can state that over and over again. Without experts like you, this forum would be a mere amateurs' babble on the subject of wire-tethered toy airplanes.

My point is, however, that there is no need for an expert, let alone a PAMPA Hall-of-Famer, to emphasize or imply his rating for the sake of an argument. One too many times in my life, when I asked "why", I've been told "'cause I said so". One's expertise or superiority clearly helps in an argument, but cannot be used as the argument itself, especially with a purpose of ending the discussion. I try to use the laws of physics when reasoning on the subjects of airplane dynamics, but not the fact that I have a Master's degree in engineering or 20 years of modeling experience. I may be mistaken about certain things, but I have courage and courtesy to admit so, when given reasonable explanations (see this thread, above). Those explanations can be based either on theory and calculations, or a successful practice (experiment).
If someone insists on their point of view, but is clearly mistaken, it is up to you to continue trying to convince him, or withdraw yourself from the conversation altogether. But I will never, ever accept a "shut up, stupid!"-kind of statement. It is that kind of behavior that makes a difference between an expert and a champion. A champion is a leader AND a gentleman!

I promise that this is my last post of this nature. Sorry if I've been blunt at times. Now let us resume our fascinating exchange of ideas.

I don't think Al meant anything that way, Mike. We welcome all comers to this forum. I do qualify, however, that you can't always believe everything that is posted. Sometimes you can get the wrong answers even from the "right" guys.

And sometimes there is more than one answer that can work. ("What is the best stooge?" "I don't know. Try this or this." "What's the best handle?" "What engine should I use on a ...." you get the idea.)

But there are some questions that have only one answer. Don't use a fuel with a low oil content on a Fox 35. Don't worry about the precise location of the belcrank on your super stunter unless you are worried about geometry, It will not affect attitude of how the airplane will want to fly.

On another note, Matt tried a moveable rudder on his latest ship. He made it so he could put it in a fixed position and not touch it, or so he could set it up Rabe style. He was going to wait until after the first contest in June to hook it up as a Rabe rudder, but the wait got the best of him. He was losing tension up on top of the hour glass. It was flyable but not pulling like he wanted. So he hooked up the Rabe rudder and found an immediate improvement. It is undoubtedly not trimmed perfectly yet (maybe after another thousand flights) but it was enough to convince him to retrofit this feature on his previous pipe ship when he finishes rebuilding it. Making it so it can go both ways is a definite win, no lose situation.

Again, he had a test bed where he could try it one way and then the other to see what the net gain (or loss) would be. In this case, with his present set up, it definitely helped.

But, again, it is another function of trim. One is never done with the trimming process until the airplane flies its last flight.

Len,

My piped Intrepid tends to lose a little tension at the top of the hourglass and vertical eights in much wind if my lap times are kept where the plane flys best. I have been using some ballet like body motions to help keep tension in those conditions. I Reach high on the second corner of the hourglass and them pull down some to keep the line tension. I duplicate that in the vertical eights when the wind is such that it causes tension problems at the top. I guess it takes a little getting used to doing such, but I have seen others do the same in similar circumstances.

Matt has learned to do the same thing for a while. We saw Dan Banjock (where is he by the way? Come on in Dan and speak to us.) fly at the Nats a couple of years ago and actually fell to the ground trying (and succeeding) to keep his plain at the end of the lines in the overheads.

Although some even prefer to fly as light as possible (while still maintaining adequate tension) we like a little (well, a lot) of pull on the lines just to know that it is up there.

So far we have confused the issue without many facts.

The important fact is that the centripital force works about the center of mass of the object (rock, car, spacecraft, human, or in our case a model airplane). As Igor pointed out, this force is outboard, and is at the CM, and this is a basic physical law!

Not simple statics take over (this is usually a required corse for any engineering student). If we freebody the airplane in standard flight, the centripital force is balanced by the line tension. (In this example, let's ignore the drag on the lines as it makes no difference) Now, let's extract the control system and freebody it. With the bellcrank on the CM, it is simple as the centripital force being balanced by the line tension. Now, let's move the bellcrank aft of the CM by 5 inches. Also assume the span from the CM to the leadout guide is 20 inches and there is 10 pounds of line tension.

Thus, at the guide, the leadouts take a turn of Atan 5/20 = 14.036 degrees. Therefore, the forward acting force (as applied to the lines) at the guide is 10 * Tan 14.036 = 2.50 pounds. Sounds substantial doesn't it! Now at the bellcrank, assume the leadout force goes through the pivot (it really goes into each attach point and then to the pivot, but the summation is the same). The line tension on the pivot is now 10 / cos 14.036 = 10.308 pounds. Now, since these forces do not act on, or in line with, the CM, we must account for their moment at the CM.

The guide's moment is, -2.5 * 20 = -50 in-lbs (+ is clockwise looking down) (also, the sign on the force is negative as this is the force applied to the airplane, as opposed to the force on the lines) The monent from the bellcrank pivot is, 10.308 * cos 14.036 * 5 = 50.0 in lbs.

Adding the two together, -50 + 50 = 0 in lbs. This states that the system is in balance as is, with no additional yaw, with the bellcrank 5 inches aft of the CM.

As has been stated earlier, the problem with moving the bellcrank pivot 5 inches aft is the considerable force on the guide and it's associated wear and drag on the control system (and yes Larry, the funny control system geometry you get). It is for this reason (the drag) that I put the bellcrank on the CM and let the lines sweep back due to (air) drag into a position of minimum drag on the control system.

Hope this helps convince some people.

Glad I read this thread. After much thought and discussion, I think I'll put my Bellcrank, where the plans say!

Might try that inverted thing though on the next ship.

Go fly guys, its gonna be a beauty-full day.

From what I've seen lately, your approach to trimming airplanes may be getting quite popular.

>Second. Imagine a real plane
>with the belcrank located in
>the back of the inboard
>wing tip. (This is
>going to involve some interesting
>geometry for the pushrod, but
>we shall ignore that for
>the moment.) Now place
>the leadout guide in the
>normal position but inboard from
>the tip just slightly.
>Yes, the airplane will fly
>just fine with the leadout
>guide inboard a bit.
>It doesn't have to be
>all the way to the
>tip

Sorry to be sort of off topic here, but the plane you are suggesting was done. It wasn't a stunter, but a speed (dyna jet) plane. It had the bellcrank at the inboard tip with the pushrod going to the inboard tip of the elevator. It also had no leadout guides. The plane hung from the lines the same as a normal plane (leadouts, centre or othewise located belcrank line guides, etc.) Withstood the pull test just fine (higher than I've seen a P.A. plane get put through). In the air it was almost perfect, hunted a bit "maybe" as I couldn't see it, but the pilot said it was. This may not on topic but I figure that a plane doing over 150 needs to be as precise as a stunter because any little input gets magnified fast and if the oscillation (up and down flying) gets set up, it's only a matter of time till splat. They bust up real good when that happens.
Anyways this thing convinced me that it doesn't matter where the B.C. is located as long as the plane is in trim.
Ron
p.s. The builder adjusted the wing incidence and the hunting problem went away, he said he could fly it all day the way it was set up. The builder also sport flies combat so he does more than straight and level.