Leadout position vs. Rudder offset

When setting up an older model (kit) with a modern leadout set (closely spaced, aft angle) in lieu of the old wide spaced leadouts, would it be prudent to set the rudder offset 90 deg to the c/l of the leadouts?
ex. -If the leadouts were 3 degs. aft, offset the rudder 3 degs.

They should compliment eachother, right? I don't subscribe to eng. offset and I'm thinking the rudder offset should be a secondary trim aid to leadout pos., depending on fuse type/ moments.

HI Doug. What I have been told and have read and try to do is to set the engine at 1/2 degree off set to the out side just to make sure it isn't set inside. As to the rudder, most just use an airfoil that gives a tendency for the tail to swing to the inside of the circle. Rudder off set usually gives yaw that is detrimental to the line tension we need and actually slows the plane down. The speed of flight is what should keep the lines tight. I personally would never use rudder off set, whether OTS, Classic or modern.
By use of the 1/2 degree engine off set, movable leadouts, and the outside tip weight box, rudder offset makes no sense FWIW

OK, guys, I was just checking to see if anyone acutally read my input. OH sure. Yeah, right.
Note to self: PROOF READ dagnabit!

Ty; What I think you meant was that the rudder offset should swing the tail to the inside of the circle to push the nose out. I agree that normally no rudder offset works most of the time. However, I have seen times when a little rudder offset was needed to make the plane fly right.

UH OH --- TY aka my buddy made a Boo Boo. Be careful Ty Oswald is lurking. LOL Nice catch "cyclone" Ty don't make too many of those. LOL

"Billy G"

Doug,
Rudder off set, if used should be adjustable.
Most often it is not needed and actually presents other trim issues that are very difficult to compesate for. Most noticible is a tendency to be most effective at higher speeds where it is least needed. It then often results in a "wiggle" during square maneuvers that is nearly impossible to eliminate without eliminating the rudder offset.
My best advise is to not use it or at least use no more than is necessary to insure that there is no inset. If you get a chance take a look at nearly any good experts airplane and you will notice no offset on the rudder or if any very very small amount.

Randy C

HI Doug,

That's a dandy question about which we've learned a lot over the years. The best guy to answer this is Brett Buck (who makes his living insuring things that move do exactly what you want them to do) but he's busy driving 2400 miles to play with toy airplanes in Indiana. In his temporary absence I'll give you more than you probably want to know about the subject.

The "modern" assessment of rudder offset (and associated yaw trim devices) follows!

Rudder offset should be avoided to the greatest degree possible. In the first place, with any reasonable powertrain, it simply isn't necessary (you don't need rudder offset to keep a rock on a string on the end of that string when spinning it around, do you?) A two to four pound airplane traveling 60 MPH tethered to the pilot will generate all the pull it needs without any assistance.

Any rudder offset on an otherwise sound airplane will complicate the yaw trim condition. By definition, offset "will" draw the tail into the circle and when doing so will yaw the fuse out and the inboard wing forward from where they really "want" to be if it wasn't for that rudder offset.

In steady state flight, therefore, the inboard wing will be yawed forward causing the leadouts to "bend" a bit as they exit the leadout guide. This will obviously cause friction on the controls which isn't good ... but that isn't the biggest problem.

Any time the pilot attempts to maneuver the ship out of level flight, pulling on the required up or down line will produce a "moment" between the handle and the CG of the airplane trying to "straighten out" the yaw caused by the rudder offset. This will cause an inboard yaw moment for the duration of the control input.

Unwanted yaw is a "bad" thing for stunters. The biggest problem (other than the drag associated with doing so) is the fact that yawing the airplane causes the wings to move forward or aft at the same time. This accelerates the forward moving wing making more lift (lift increases at twice the increase in airspeed) and decelerates the aft moving wing causing less lift. The net result will be that the yaw from control inputs will also result in a roll moment -- one direction going into the control input and the other coming out. Again, not desirable.

So, the best RUDDER offset on a stunt ship is somewhere around zip, zero, nada!

IMPORTANT NOTE: ALL THE ABOVE ASSUMES THE PROPER RELATIONSHIP BETWEEN THE MIDDLE OF THE LEADOUT EXITS AND THE CENTER OF GRAVITY. IF YOU LOCATE THE LEADOUTS ABOUT 2.5 TO 3.0 DEGREES AFT OF THE CG (CALL IT ONE INCH LINEALLY FROM THE BALANCE POINT AT THE TIPS TO THE MIDPOINT OF THE LEADOUTS ON A CLASSIC SHIP AND 1.25 TO 1.5 ON A "MODERN" 60 INCH PAMPAWAGON) YOU WILL BE WITHIN SPITTING DISTANCE OF WHERE THEY'LL END UP WHEN FULLY TRIMMED ... and with ZERO rudder offset.

More stuff you may or may not want to read follows. These comments will be more or less directed precisely at your original question.

Of course, it isn't all that simple. An airplane with an offset rudder can be trimmed to fly without the yaw and resulting roll/yaw couple described above ... but ideally it wouldn't be. Here's how that could come about and, also, why it isn't the best way to do it.

I'm sure you understand about angles of attack and lift and all that happy airplane stuff. You may or may not have thought about the rudder as a potentially lifting surface but just a moments thought will make it clear that whatever effect it has on the airplane "yaw axis" is the result of the Vertical stab/rudder (henceforth we'll just say rudder) producing lift in the horizontal plane.

Also, you surely know that the production of lift requires an angle of attack relative to the air mass through which it flies. Basic aerodynamics. This same logic makes it clear that at some reduced angle relative to that air mass the surface will reach a point where it produces no lift ... because the surface has reached the zero lift angle of attack. Even a highly cambered surface like a highly offset rudder can be pitched negatively to the point that it produces zero lift.

This "zero lift" angle of attack is the secret to using rudder offset without causing the offending yaws discussed above.

If one uses significant rudder offset one "MUST" offset the leadouts far enough aft that the rudder in steady state flight is at the "zero lift angle of attack", unable to cause the ship to yaw relative to its natural angle on the end of the lines.

The way to achieve this state is to increase the leadout rake so that when the CG lines up with the leadout guide in steady state flight (including the rake necessary to account for line drag ... which is where that 2.5 to three degree number came from above) the rudder is at zero lift. When this state is achieved there will be no yaw from control inputs.

The fact that doing so eliminates the yaw/roll couple from control inputs doesnt' make this set-up the equal of the "Best" setup originally described. Here's why.

When the ship is yawed out enough to reach that zero lift rudder condition the inboard wing will be yawed in "all the time" and the outboard wing yawed out "all the time". The inboard wing will be more "efficient" in lift and the outboard wing less so. Much more difficult trimming of roll in maneuvers will result. In addition, the lift generated on the outboard wing will be aft relative to that generated on the inboard wing. The ramifications of that displacement will be almost incalculable given the huge array of aerodynamic forces that derive from flying tethered, especially in windy conditions.

In addition, the airplane will be circulating about the circle in a crab ... i.e. angled out, perhaps dramatically, from its track around the circle. Doing so creates a lot of drag and, as you well know, drag robs horsepower from more important tasks.

On the other hand, when set up "correctly" the fuse (and the prop's thrust) will be aligned almost exactly tangent to the circle. This will still be a tiny crab but that's not all bad because that crab will do all of the things that for so many years were felt to have been accomplished by lots of rudder offset. That is, that tiny bit of crab will help assure the ship will return to the end of the lines should it be displaced by "conditions"!

It may sound like setting the airplane up so as to crab away from the pilot will "improve" line tension. This is one of control lines biggest canards (pardon the pun). While it is true that within reason level flight tension may be modestly increased by doing so. Once you start maneuvering, and get up above level flight the extra drag from that crab will overcome any perceived advantage down low and line tension will suffer (along with all the negative effects on the track of the airplane as you do the tricks).

A mental image which might prove this to you is to imagine the leadouts exiting the tail. AS you hold that bellowing PA .75 powered ship in the center of the circle the tension you feel will be the engine thrust and nothing more -- the airplane will just sit there trying to escape the circle.

Now imagine putting in up elevator and visualize the ship rising off the ground but not moving forward. The tension generated by the powertrain will bleed off dramatically as you go higher. Say your four pound ship was pulling 3.5 pounds of static thrust. Because the airplane has no tethered flight generated centrifugal/centripetal force(take your pick)at some point in time as it rises the line tension will disappear entirely. To the degree the various forms of yaw offset are employed you approach that situation. (do this with a nine or ten inch pitch prop which will produce significantly less static thrust and you might be able to elevate very little if at all)

Finally, if you want a clue of how bad a control line airplane can handle if offsets are taken to extremes, go to the carrier (helicopter) deck and watch slow flight for a while. That should tell you all you need to know about how valuable offsets are for stunt ships. Works fine for the carrier guys' mission, but nobody judges them on how good the airplane looks attempting to hover.

Whew, old motor mouth strikes again. Hope some of that made some sense. If not, wait 'til Brett hits the hotel and he'll say the same thing in four words.

Fly stunt.

Ted Fancher

p.s. FWIW, I do use about a degree and a half of engine offset which I believe has little or no effect on yaw trim ... but Brett will scoff at such a suggestion. For that matter, so does my mentor Bob Emmett. Back when I would beat Brett I could pretend it was because I used the offset and he didn't. Alas, the obvious pertinence fo that logic appears to have disappeared with the onset of my male pattern baldness.

Ted,
I suspect what Brett will say is WHEW! At least that's what I said.
As usual your eloquence in explaining the reasons behind the rudder offset usage is excellent.
Without all of the explanation, however, I think it's pretty much what I said above.
Nice analysis of the problem though.

Randy C

>Ted,
>I suspect what Brett will say is WHEW! At least that's what I
>said.
>As usual your eloquence in explaining the reasons behind the
>rudder offset usage is excellent.
>Without all of the explanation, however, I think it's pretty
>much what I said above.
>Nice analysis of the problem though.
>
>Randy C

Well, yeah, but ...

Ted

>Ted,
>I suspect what Brett will say is WHEW! At least that's what I
>said.
>As usual your eloquence in explaining the reasons behind the
>rudder offset usage is excellent.
>Without all of the explanation, however, I think it's pretty
>much what I said above.
>Nice analysis of the problem though.
>
>Randy C


no body's ever gonna beat you to the last word

>When setting up an older model (kit) with a modern leadout
>set (closely spaced, aft angle) in lieu of the old wide spaced
>leadouts, would it be prudent to set the rudder offset 90 deg
>to the c/l of the leadouts?
>ex. -If the leadouts were 3 degs. aft, offset the rudder 3
>degs.
>
>They should compliment eachother, right? I don't subscribe to
>eng. offset and I'm thinking the rudder offset should be a
>secondary trim aid to leadout pos., depending on fuse type/
>moments.

Others have commented already, but I read into this that you are thinking the leadouts are raked back causing the airplane to yaw out already and you are thus just putting the rudder in line with the path of flight. In fact the leadouts are raked back slightly because the drag on the lines causes them to bow back slightly. With the airplane perfectly tangent to the circle, the lines will bow back and the leadout are set to follow that bow. Thus the rudder, set at zero, is already in line with the direction of flight and no additional offset is (normally) needed. Noted exception would be the use of a Rabe rudder where the rudder "wiggles" with the movement of the elevator--sometimes out and sometimes in to compensate for the gyroscopic precession caused by the rotating prop and nothing else. Again, the purpose is to keep the airplane flying tangent to the circle.

There is one possible reason for having a slight rudder offset, which is that the propwash is helical. As a result, it hits the vertical stab with a slight angle of attack, and would cause a yaw to the left in the case of a prop which rotates clockwise when viewed from behind. As the aircraft accelerates the helix 'stretches', reducing the fin AoA and resulting yaw.

The effect is quite noticeable on powerful full-size aircraft, which (at least in my experience) are often built with a slight fin offset despite line tension not being a consideration! How much it translates to models I don't know, but I think the previous comments are still quite valid - and I've got one particular old-style model with a 20 degree rudder offset which I will be performing surgery on in the near future, to see if it improves the flight characteristics in manoeuvres as Ted describes.

When I built my Humongous, I put in about 1/2 or 1/3 the rudder offset shown on Ted Snow's drawing. I ended up with the leadouts in front of the CG. The airplane flew very well and was a favortie airplane. Flying in a good wind, in level flight, coming into the wind, the wind force on the lines would bow them a good bit and actually bring the nose noticably into the circle. I still had enough tension to keep it flying.

I have build a couple of airplanes which had fixed leadouts because it was structurally difficult to make them adjustable. I made the rudder adjustable instead, and that worked fairly well. The leadouts were not off by much and it only took a degree or two of rudder to make the airplane fly well. But I think adjustable leadouts are the cat's pajamas.

I love this forum! A wealth of info/ experience even if the same problem is approached by different methods.

Len- You get the cookie for the night. Yes, I was imagining the rudder faired with flight path (Ted's zero lift statement) with the plane offset by the leadouts. Most of you clarified the plane should remain tangent to the cirle, lines raked back to compensate for line drag - Got it!

Cowboy- I can see where an adj. rudder would be a good trim aid on "some" airplanes. heck, some models have NO rudder.

Ted- I've remember seeing a couple planes flying level with what looked like a Dutch roll (roll/ couple) going on. Could have been a warped wing I suppose but what you say makes sense. I can see where a crabbed plane could get funky during maneuvers.
Al Rabe's coupled rudder is an interesting solution to gyro prec. but I'm curious what the nominal setting is and is it neccessary with a short nose moment/ light prop (carbon)?

Steve11- I believe what your refering too is the P-factor of the prop. Most C/L planes are over-powered and fly quite level so I not so sure this is a factor. C/L Carrier would be another story.

Thanks all, keep the opinions coming, I'm listening...

(snip)
>Al Rabe's coupled rudder is an interesting solution to gyro
>prec. but I'm curious what the nominal setting is and is it
>neccessary with a short nose moment/ light prop (carbon)?
>
>Steve11- I believe what your refering too is the P-factor of
>the prop. Most C/L planes are over-powered and fly quite level
>so I not so sure this is a factor. C/L Carrier would be
>another story.
>
>Thanks all, keep the opinions coming, I'm listening...

Matt uses large props--usually the largest he can find that the engine will turn with a reliable stunt run. And, yes, this can lead to some precession. ALL (emphasis intended) props will give some precession forces to the plane. The bigger, the heavier, the more they add. And ALL planes will have some P-factor added into the equation. With symetrical airfoils we always fly at some angle of attack--an angle which increases with the intensity of the maneuver.

Now, fortunately, p-factor and precession cause opposite effects on the flight path of the plane. One will always cancel out the other. Precession is most obvious in a violent maneuver as is p-factor. But, again, one is always cancelling out the other. And the winner is....precession! But it is less than what it could be.

And, since it is there, if you can do something to get rid of it, do it. Most (well, a lot) of our airplanes are designed with a lot of aft side area. That barn door side area resists the effects of precession to a degree, smoothing out its effects. But those effects still remain. The Stuka (real life German) has a very narrow fuselage in back. Matt's "sorta look like a" Stukas have a lot of side area built in. Not scale like at all. That is done purposely. Still, he has incorporated the Rabe rudder.

Does it work? That is always the question. First off, Matt builds it so it is full adjustable in both the amount of throw and the amount of assymetry (more out than in) as it moves. Building in an adjustment for the amount of assymetry was his addition to the design and it gives you one more thing to adjust to get things right.

OK, so back to the question, "Does it work?" Matt built his first plane using this "wiggle rudder" with a solid anchor for the rudder so he could disable the feature. He flew the plane first without it and trimmed it to where it was "about" as good as he could get it without it. It flew quite well. Under normal circumstances one would have been happy with the plane and thought nothing more was needed or possible. But he then hooked up the Rabe rudder, and with no adjustment other than the "ballpark" set up, saw and felt an immediate improvement in the flight. As I said, this was without doing any additional adjustments to it.

So, back to your original question of short moment, light prop (such as carbon fiber)... Nose moment won't make any difference. It is the change in direction that makes the difference, not how long the moment is. And Matt uses a carbon fiber prop (albeit rather large).

I once had an aluminum sanding disk with carbon grit that I could put on my hand drill. It was a perfect demonstration of precession. I would have someone hold the drill straight out, turn the drill on, and then try to move the drill up and down. You couldn't do it. It would swing right and left. Precession. The disk was much heavier than our props, although not much larger. The drill, however, was turning at a speed much slower. I have never tried grabbing the airplane with the engine running and swinging it up and down for fear of breaking something, but I have often wondered how much force there really was there. We are tied to a tether at the center of gravity. It doesn't take much force to swing that nose around.

> . . .
>Now, fortunately, p-factor and precession cause opposite
>effects on the flight path of the plane. One will always
>cancel out the other. Precession is most obvious in a violent
>maneuver as is p-factor. But, again, one is always cancelling
>out the other. And the winner is....precession! But it is
>less than what it could be.
>
>And, since it is there, if you can do something to get rid of
>it, do it. Most (well, a lot) of our airplanes are designed
>with a lot of aft side area. That barn door side area resists
>the effects of precession to a degree, smoothing out its
>effects. But those effects still remain. The Stuka (real
>life German) has a very narrow fuselage in back. Matt's
>"sorta look like a" Stukas have a lot of side area
>built in. Not scale like at all. That is done purposely.
>Still, he has incorporated the Rabe rudder.
>
>Does it work? That is always the question. . . .
> . . .

The discussion of whether p-factor can be larger than precession is interesting, but probably over. The reason is, no one has ever demonstrated, to my knowledge, an anti-Rabe wiggly rudder that works in reverse (to add outward yaw on up).

I liked your summary very well, as there are many airplanes that (according to the flyer and other observers) seem to show no benefit -- balanced p-factor and precession forces can exist.

An interesting "mind experiment" would be to imagine a "p-factor max" design. Maybe an unflapped stunter with a very light prop? In an unflapped model, the "effective AOA" of the wing and the AOA of the thrust line are always the same, unlike a flapped stunter. That is, an unflapped stunter experiences greater thrust line AOA changes to create the same amount of lift. If that airplane needed an anti-Rabe rudder, we could call it the "Krauss rudder"?

Larry Fulwider

>When setting up an older model (kit) with a modern leadout
>set (closely spaced, aft angle) in lieu of the old wide spaced
>leadouts, would it be prudent to set the rudder offset 90 deg
>to the c/l of the leadouts?
>ex. -If the leadouts were 3 degs. aft, offset the rudder 3
>degs.
>
>They should compliment eachother, right? I don't subscribe to
>eng. offset and I'm thinking the rudder offset should be a
>secondary trim aid to leadout pos., depending on fuse type/
>moments.

They should compliment each other, an I think your underlying idea is mostly correctly, but I don't think there is any particular relevance to matching the rudder angle to the offset angle. Particularly since the vast majority of the leadout angle is there to compensate for the rearward sag of the lines from drag. For a typical 40-60-sized stunt plane the lines sag rearward somewhere around 2 degrees. So if you set the leadouts at 2 degrees, that's going to want to make the airplane want to fly tangent to the circle.

There are a variety of methods to setting the leadouts and rudder, but this is how I do it:

1. Set the leadouts (center point between the two) to where it is calculated to be equal to the line "sag" using Pete Soule's "LINEII" program. What the program actually does is to calculate the sag angle from the drag and the line tension. I use this as the leadout position.

2. Set the fin/rudder *dead straight ahead*, no offset

3. Fly the airplane and once you get the tweak/tip weight correct, watch for yaw in the corners. If the nose wants to yaw in when you provide control, move the rudder to the left. If the airplane wants to yaw away from you when you provide control, move the rudder to the right. At some point it will neither want to yaw in consistently nor yaw out consistently in the corners - or, it will yaw in at you on outsides and away on insides. This is the position that provides a perfect compliment of the leadouts and rudder.

4. If you don't like the way it flies, adjust the leadouts whichever way you want, then repeat 3 until the rudder is once again "in sync" with the leadouts.

Brett

Ted likes to point out that in tethered flight we fly in an aerodynamically "warped" space. No one has mentioned in this thread as yet that (because of the curved space and some observations of a Greek guy a couple millenniums ago) we have a "free insurance policy" against crooked building. That is, erring toward a tad of right rudder or a tad of right thrust (as insurance against accidentally building in left rudder / left thrust) is actually a second insurance policy.

Simply put, a straight line can only be tangent to a circle at one point. If the fuselage center line is tangent to the circle at the aircraft CG, the engine is pointing slightly out, and the fin / rudder pointing slightly out. Not much, but some.

A typical example with numbers: '57 Nobler flying CCW on 60 foot lines => ~63' radius circle.
At zero degrees yaw, we have
Right thrust: .89 degrees
Right rudder 1.59
Total Difference: 2.48
Yawed out, we have more right thrust and less right rudder. Yawed in, we have -- who cares?
True zero - zero would be the above amount of left thrust and left rudder.

This is not a correction, of course, of anything anyone has already posted. I know (from other threads) that all the experts above are well aware of this fact, but often mistakenly assume others are also aware of it -- that it is just "common knowledge".

Larry Fulwider

>Snip<
>Simply put, a straight line can only be tangent to a circle at
>one point. If the fuselage center line is tangent to the
>circle at the aircraft CG, the engine is pointing slightly
>out, and the fin / rudder pointing slightly out. Not much, but
>some.
>
>A typical example with numbers: '57 Nobler flying CCW on 60
>foot lines => ~63' radius circle.
>At zero degrees yaw, we have
>Right thrust: .89 degrees
>Right rudder 1.59
>Total Difference: 2.48>
>Yawed out, we have more right thrust and less right rudder.
>Yawed in, we have -- who cares?
> True zero - zero would be the above amount of left
>thrust and left rudder.
>
>
>Larry Fulwider

I was aware of this, and the reason I don't like eng. offset, but 1 deg. could mean the sloppiness of your engine mount bolts!
Longer tail moments would give more of an effective arm, etc.

...and what about the P factor of an offset eng. in the verticle axis? -like a Cessna mushing along during takeoff, only turned 90 degs. to the right. A whole `nother "tangent" discussion, lol.

True zero is like a fantasy condition that exists, but could be upset by the slightest wind gust. You'd be relying solely on centr. force.

Larry,

Glad you mentioned that! For those who recall some trigonometry, a good approximation of how to find the "automatic" angles -

Line length is from, say, shoulder to CG. Presume the model flies perpendicular to that imaginary straight line. (The real lines sag aft with air drag, and very slightly below the imaginary line - from their own weight.)

The distance forward of the CG to the rear face of the prop is one "side" of the natural thrust offset angle, presuming the engine was given no structural offset... The other side is the line length. The the Tangent of the natural offset angle is the distance forward, to the prop face, divided by the line length. (Many inexpensive pocket calculators have trig direct and inverse functions, so we can find actual numbers.)

Same way, the Tangent of the natural fin offset angle (presuming it is built exactly along the fuselage centerline) is the distance along the fuselage centerline to the "lift center" of the fin area from the CG, divided by the line length.

This, as I said, is only an approximation, but it's usefully close.

...er, I don't know about flying in "curved space," but we DO fly on a curved surface. Unless things go into the potty, we most always have purt near the same line length through the whole flight, no?

If you can visualize looking straight down on the model from above, in level flight, and you could see the trace of the path of the CG, you'd notice that trace is to the inside of the thrustline, AND of the fin chordline.

Now, imagine you could keep the same relationship, but straightened out the trace of the CG's path. For things to be "like" the real condition, the model's fuselage centerline would have to be curved so the CG is the nearest point to the flier.

I usually add slight thrust offset, since that may be the only recovery source when things go bad. Thrust added to the model by the prop in cruising flight (level, straights) is less than the max thrust the engine could provide at that RPM. The thrust is only enough to match model and line drag, and we use prop, plug, fuel, setting, etc., to keep it at that RPM. In sharp turns, wing drag increases a lot, and prop thrust is all there is to meet the load.

When things go bad, if the model slows drastically, the thrust the prop applies to the model rises accordingly. The small out-thrust vector increases with the change in thrust...

Lou –

To give credit where credit is due, in a discussion of fuel tank taper and tank skew, you were the one who pointed out to me that all the trig we were doing also applied to a lot tethered flight applications – engine thrust line and rudder / fin / fuselage centerline trig calculations, for example.

> (snip)
>...er, I don't know about flying in "curved space," but we DO fly on a curved surface.
>(snip)

It may sound a little pedantic and pretentious to say we fly in an environment more naturally described in polar coordinates and pi flavored radians than an accountant’s Cartesian-degree world.

It sounds a little Trekkier, sexier, and generally “with it” to say we “fly-by-wire” through a curved space-time continuum.

Larry Fulwider

>Lou –
>
>To give credit where credit is due, in a discussion of fuel
>tank taper and tank skew, you were the one who pointed
>out to me that all the trig we were doing also applied to a
>lot tethered flight applications – engine thrust line and
>rudder / fin / fuselage centerline trig calculations, for
>example.
>


Lou wrote a a very nice paper on it, in fact.

One interesting thing is that the offset arising from the curved flight path IS NOT the same as just putting in offset relative to the rest of the airplane, in that it doesn't generate torque by offsetting the thrust line from the CG. The offset from the curved flight path is a "free" improvement. It is however, hardly any effect at all - about 1.5% of the thrust is vectored outboard to increase line tension. The primary effect of conventional offset is not to vector the thrust outboard but to cause a yaw rotational torque - which is generally not what you want.


Brett

'sorry I missed this discussion; I was on my own I-80/90 odyssey of several hundred miles for class and family reunions.

Larry, I'm not quite sure how I (am I the 'Krauss'?) got into the Rabe-rudder discussion, but certainly particularly agree with your discussion of geometrical out-thrust and "right" rudder inherent in a plane flying tangent to the circle, without deliberate engine or rudder off-set.

Brett, I think I'd be correct though in thinking that out-thrust might be desirable to generate a clockwise yaw moment in an upright or inverted engined plane with significant wing asymmetry (longer inboard wing), as compared, say, to a symmetrically winged profile plane with its engine mounted in the traditional right-side laid-over position. The asymmetrical-winged model's center of mass would be inboard of the thrust line, due to needing less right tip weight and having greater left wing mass. This and possibly greater inside wing drag, despite the inside/outside speed differential, would result in counter-clockwise yaw needing compensation. The profile would have greater outboard drag and (needed) weight bias, due to the engine position and symmetry, giving it a built-in clockwise yaw.

I suppose the profile's outer wing would be less efficient, effecting the roll in maneuvers; maybe that would tend to reduce needed tip weight for line tension though? 'just some musings.

SK

Brett,

Thanks! GREAT if subtle point...

Given no structural engine offset, the thrust still aims through the CG (if tip weight and line weight are correct), so no yaw rotation tendency occurs. There is a slight gross outward vector relative to the path, but not relative to the CG.

Thanks again!

>Ted likes to point out that in tethered flight we fly in an
>aerodynamically "warped" space. No one has mentioned
>in this thread as yet that (because of the curved space and
>some observations of a Greek guy a couple millenniums ago) we
>have a "free insurance policy" against crooked
>building. That is, erring toward a tad of right rudder or a
>tad of right thrust (as insurance against accidentally
>building in left rudder / left thrust) is actually a second
>insurance policy.
>
>Simply put, a straight line can only be tangent to a circle at
>one point. If the fuselage center line is tangent to the
>circle at the aircraft CG, the engine is pointing slightly
>out, and the fin / rudder pointing slightly out. Not much, but
>some.
>
>A typical example with numbers: '57 Nobler flying CCW on 60
>foot lines => ~63' radius circle.
>At zero degrees yaw, we have
>Right thrust: .89 degrees
>Right rudder 1.59
>Total Difference: 2.48>
>Yawed out, we have more right thrust and less right rudder.
>Yawed in, we have -- who cares?
> True zero - zero would be the above amount of left
>thrust and left rudder.
>
>This is not a correction, of course, of anything anyone has
>already posted. I know (from other threads) that all the
>experts above are well aware of this fact, but often
>mistakenly assume others are also aware of it -- that it is
>just "common knowledge".
>
>Larry Fulwider

Hmmmm, Larry. I'm sure I've used the word "warp" quite a bit over the years but I don't think I've ever used it in exactly that context

Your post is absolutely right on the money otherwise and, you're probably right, a lot of people might not have given any consideration to the tangency factor that you explain quite well.

As Brett is wont to say from time to time, when it comes time to figure what they're going to do when such and such happens, satellites are easy. Control line stunters are hard!!

Ted

Some years ago I made here some comment on this subject with pictures and so, but I cannot find it right now, but I think I can easy collect all thoughts to following:

1/ Yaw itself is not ill, but changing of yaw during the flight IS ill, as Ted already mentioned, therefore we need to make all forces / moments to be in balance during all maneuvers. We have constant speed (in calm), so all aerodynamic forces are constant and the only changing is line pull (because of arm pulling in maneuvers against the mass of the model, changing centrifugal force, gravity angle and side wind)

2/ If the line pull is changing during the flight, the only trivial solution for zero yaw CHANGES (not the constant yaw) is situation when the CG is in line with handle and leadouts, so the line pull force ha zero arm to make any moment to the airframe.

3/ point 2 means that the CG has zero effect on yaw moments, therefore all yawing aerodynamic moments are in balance. The model airframe moving on circular path HAS some out moment – because of asymmetric air speed on wing sides and also the fuselage makes outer moment - does not matter if rudder is straight or not, simple it is the same situation like chambered airfoil – it tries to point the nose out as Lou wrote. The engine has another contribution in the same direction. The only force which can keep all of that in balance is the lines drag.

4/ Means together and together – CG is in flight in line with handle and LO, line drag is in balance with fuselage, prop and rudder moments = no yaw changes.

So for me the answer for question:

>>> They should compliment eachother, right? I don't subscribe to eng. offset and I'm thinking the rudder offset should be a secondary trim aid to leadout pos., depending on fuse type/ moments.<<<

Is clearly YES.

But here are 2 small points which make little trouble here.

And that is SIDE LIFT of fuselage. It acts like a wing with chambered airfoil and therefore makes some lift in zero AoA. I made another contribution here on this point already too. It looks that the fuselage makes zero lift at yaw around 1.5 deg out. It looks like good position of fuselage in flight. So yes, we need adjustable LO to choose this angle, but it can be easily done at building if we know position of CG for sure – because that adjustable LO is necessary to make LO position relative to CG. But if I know I will certainly fly CG at 22% AoA, then I can do it easily at building and do all trimming only by rudder. I do not like play with LO too much, because it always changes also the handle neutral, so if I need to change position of fuselage little bit (different altitude or different wind sometimes need it), especially on the contest, I do it always only by the rudder.

And second reason is asymmetric flaps effect in rounds and squares. It is possible to trim by LO, but I also do not like it too much and we do it here by small tab on outer flap (trimmed by knife ).

Igor –

I had written this off as a “mature dead thread”, but you got me thinking again.

> (snip)
. . . And that is SIDE LIFT of fuselage. It acts like a wing with chambered airfoil and therefore makes some lift in zero AoA. I made another contribution here on this point already too. It looks that the fuselage makes zero lift at yaw around 1.5 deg out. . . .
> (snip)

Zero AoA where? If the straight fuselage centerline is at 1.5 degrees outward yaw at the CG (relative to the tangent line at that point), the numbers in post #16 become:

Nose AoA: .9 + 1.5 = 2.4 degrees
Fin AoA: -1.6 + 1.5 = -0.1 degrees

My poor joke, poetic license (or whatever we might call it) saying we fly in a “warped space” or “curved space” is less humorous and more true than I was thinking at the time. After thinking on it a bit and looking at the above simple math, how else can a straight line have a different AoA at each end unless we think of a curved environment?

The truth of the “curved space” mathematical model becomes clearer if we mentally try to design a control-line wind tunnel. We would have to build the wind tunnel in such a way that we had a curved laminar flow tunnel with a radius equal to line length. (Changing line length would be a bear though, huh?). Only in that way could we simulate a straight fuselage with a different AoA at nose and tail. Only with a curved laminar tunnel could we measure actual lift and drag differential between inboard and outboard wings.

Even our “perfectly designed” curved wind tunnel is flawed, because the air molecules would still have straight line inertia although “forced” along a curved path. To make the air in wind tunnel both aerodynamically and dynamically correct, we would have to build the tunnel on a mobile platform that we could accelerate at a constant of 90 or so ft/sec² away from the “center of the circle”. Actual acceleration and air speed being a function of lap times in still air – forget even steady wind considerations! That would take a pretty big “flying field” for an experiment of any significant duration.

We tend to think terms like “curved space” and “relativity” as being esoteric Einsteinian terms, which they are not. Einstein mathematically described situations where the more familiar Newtonian relativity and mathematics led to faulty conclusions – in short, non-relativity, in Newtonian terms. Our control line world is easily described in common sense high school trig and plain-Jane high school Newtonian physics. It is not even at college sophomore or airline stewardess complexity levels, two epitomes of sophisticated thought. All’s we gotta do is be a little careful from time to time, and learn to sketch curved vectors instead of straight ones sometimes.

Larry Fulwider

Larry, does not matter how exact are measurements, or having curved tunnel, or whatever. Important is understanding. And for understandig is simple simulation or test on very short lines clear

I do not speak about esoteric Einsteinian curvature. But there is no question that the air is not straight. It is easy to see. I did lot of math myself about our models and I do not think it is "high school simple"

>... I did lot of math myself about our models and I do not
>think it is "high school simple"

Not my high school anyway.

Jim

Igor --

> . . .
> . . . I did lot of math myself about our models and I do not
>think it is "high school simple"
> . . .

I always give you, Serge, Brett, Lou, et al credit for being at a mathematical level beyond many of us, and you all use that talent / knowledge as necessary -- which it often is.

My point was kind of that this specific piece of math is simple enough even I and most everybody else can do it. It doesn't require great math skills or knowledge, as much of your work does.

Larry Fulwider

Igor,

(We've been kicking this stuff around a long time, now, yes?)

VERY important point!: the equivalent of a curved fuselage centerline in straight flight, that we get with a straight fuselage centerline in curved flight, suggests the fuselage side area acts as a cambered airfoil - lifting towards the handle!

It takes some outward attitude to bring that to its "zero lift AoA!"

As you may recall, I began my estimates with what were simplest equations, that checked out consistently in flight at that time. NOT precisely, just consistently. Errors, so long as they were consistent, were useful...

My estimate on line rake was higher than LineII finds, but your comment on the effect of side area of straight fuselage vs curved path is NOT considered in LineII or III or whatever is most current... Speed or racing models do not have side area so large it must be considered... Stunt models do.

Although I have not flown so much lately that I can be very confident of it, I think the EXCESS outward yaw base-point I was using did well for high-speed, unloaded cruise conditions, AND for the various velocity changes in different maneuver-load conditions, and flight above level altitude.

Worked for me, then, and is still comfortably close - for me - when I get a chance to get some practice in...

As ever, I admire and enjoy your contributions to our mutual interests!(edit - spelling.)

>>>We've been kicking this stuff around a long time, now, yes?<<<

... and every time we know more and more

>>>VERY important point!: the equivalent of a curved fuselage centerline in straight flight, that we get with a straight fuselage centerline in curved flight, suggests the fuselage side area acts as a cambered airfoil - lifting towards the handle!<<<

exactly ... and yawing nose out all the time, all the way

>>>NOT considered in LineII or III or whatever is most current... Speed or racing models do not have side area so large it must be considered... Stunt models do.<<<

If that all what I wrote is true (and I do believe it is because I trim my models to fly that way and it works well ) then not only side area is not considered, but also basic condition of that program - tangent position, level flight and zero force in LO is not fulfilled, so I thing that tool is just improperly used on our models (and you know I had it also in my calculator). It is really for tailless speed and especially team racers.

It gives SOME angle which is useful for our models, but I am sure if I take any other "useful" angle I will trim model too. In reality I did it - on my indoor (flying on extremely short lines) which does not have any predecessor, so I was not able to steal "useful" numbers from existing model ... I calculated angle at which flat fuselage (extremely large) makes maximum lift ... and that angle is used on LO. The rest is trimmed only by rudder to keep CG, LO and handle in line. Result is perfect flight at unbelievable offset making lift at speed absolutely not giving centrifugal force to keep the model overhead, but still flying whole pattern with no ill effects

so for me ... the theory is proofed and since time I first time presented that idea, I already know it works

I'm not exactly sure what you are saying and you have my curiosity aroused.. Would be helpful if you could explain the difference between how you come up with your leadout position verses Line-III and where both are located relative to the CG for comparison. You may have already explained this but I missed it or didn't understand what was being said.

Hi Bob, it is simple, I choose some "useful" angle 2 degrees, or 1" or just result of the LINEXX and the I adjust rudder to match CG-LO-Handle to one line in flight.

I use following markers showing LO position and CG position:

[photo not recovered: 29694.jpg]

The CG is marked at half the tank. It is not my model, but it is well visible.

When I have all in line only then I start trimming (for my hand) usual way. I never get out of those markers (at least not visible) and they stay perfectly alighned in level and also overhead and in maneuvers.

Then if I feel it is too much out (too much line tension in level) I make smaller angle an try all again. (or just opposite)

And lately small ill effects I trim all the time (also in different wind an pressure, altitude and so) by rudder, not by LO ... if I feel corners has tendention to open, then I move rudder out. If I feel often overshooting, I put it in. (I hope I remember it well )

Attachment #1, (jpg file)

Bob,

My earliest attempts were based on an article in an Aeromodeller Annual (1971-72?) by Pete Soule'. He analyzed the drag on lines basically for speed and team racing FAI models. It came out with a slightly greater rake angle than the current Line programs - found by analyzing the shape air drag imposes on the lines as an 'accelerated catenary.'

Suspension bridge cable sag follows a basic 'catenary' form. The load from the cable weight and the supported roadway is uniform from pier to pier.

Our flying lines meet main loads of air drag and pull force. The air drag load increases - like any other aerodynamic drag - with the square of the velocity. Velocity depends on radius and rotational rate. So the load, and the resulting curvature, increases as speed accelerates (with the radii) from the handle to the leadouts.

Centri(-petal? -fugal?) force also depends on the square of velocity, so it can be thought (crudely) to be less relevant. The tangent of the line drag curve angle where it meets the wingtip divides one term that includes the square of velocity by another that also includes it.

The later versions of pretty much the same approach are in Soule's current Line programs, much further refined, and including wind exposure, which varies around each lap, and other fine details.

I've mentioned several times over the years, and others mention it in this thread, that - ideally - line pull should be aimed at the CG as the lines pass through the leadouts. If it isn't, but rather 'aims off' it's like pushing on a crank handle away from its axis - it tries to yaw the model until the pull force DOES aim through the CG. If you push on the axis of a crank handle, you apply no turning force, right?

Since our solids or cables are effectively limp cables, capable only of tension loads along their length, to aim the load at the CG means following the rake angle at the tip straight to the CG (from the leadouts on in, it is out of the airstream, no?) We can estimate a starting point for the leadouts if we know where we want the CG to be, and what length and diameter lines we'll use. Laptime on the line length allows us to estimate CF, the basic pull force, but that isn't urgently required in finding the rake angle. (V^2 terms cancel.)

These days we recognize leadout guide position as a very basic trimming tool.