Why more lift needed outboard?

LAST EDITED ON Oct-23-03 AT 05:34 PM (CDT)
 
I am not arguing against the fact that we have found that we need bigger flaps on the outboard wing panel, but I don't understand the "why" of it.

The outboard wing is flying faster, so it generates more lift at any given angle of attack and flap deflection (lift goes up with the square of the velocity). At the same time, it is flying a larger maneuver than the inboard panel (it is on the outside of the cone generated by the fixed point of the handle). Both of those are factors that would indicate a need for less lift capability required by the outboard panel than the inboard.

Compared to actual experience, this does not compute!

Anyone have a clear explanation of what factor(s) I am missing?

Yes, you are forgetting that our most useful adjustable or aerodynamic trimming device is tip weight. A wider outboard flap allows us to carry a bit more of it. Also it geometrically reduces, balances or reverses the difference in flap area in airplanes with asymmetrical length wings.

LAST EDITED ON Oct-23-03 AT 03:44 PM (CDT)
 
I always presumed that the tip weight was just to balance the weight of the lines. If so, then we don't need the extra lift as the plane is in balance.

If, however, we are adding extra tipweight and then compensating with extra flap (assuming a symmetrical plane), aren't we just adding drag in the one flight condition where we can least afford it? In addition, wouldn't the plane tend to fly outboard tip low in level flight as a result?

One possibility that has just occured to me, is that at a high angle of attack, the lines are effectively generating lift in excess of their weight, and pushing the inboard wingtip toward the inside of the turn. That would require extra lift on the outboard that a reduction in tipweight could not properly compensate.

So as I see it, tipweight compensates for line weight in level flight, leadout rake removes the drag component so the plane flies straight tangent to the circle. In a corner, this precise balance is upset by a drastic change in angle of attack, and the line drag vector now has a component in the lift direction, plus a slight reduction in the yaw component. The plane yaws outward slightly and the inboard wing gets a boost toward the inside of the turn.

Extra flap outboard would compensate correctly for the lift, but would tend to aggravate the yaw problem.

You and I were writing at the same time, Larry. You said it in a lot fewer words than I did.

MOST interesting discussion. It reminds me of reading a technical piece on the (full-size) P-51 Mustang racer "Beguine"- a great deal of the redesign work was the brain child of Walter Beech to test some of his theories. Worked pretty well- they mangaged to clock over 500 MPH in level flight at altitude- not bad for the era. They got some slight "boost" effect from the wing tip radiator pod exhausts and there was a bit about the "vortex" airflow effect of having one wing 18" shorter than the other one.
This last feature has been speculated on as the possible cause for the crash at the '49 Thompson: at low level, Bill Odom thought he had cut (or was about to) a pylon and turned into the short wing and over he went. A shame, I always have loved that airplane, I still think it was one of the prettiest P-51s ever built. Probably wouldn't make a decent stunter though I guess...

LAST EDITED ON Oct-23-03 AT 10:57 PM (CDT)
 
Jake,

This subject of the Beguine having one wing longer than the other is based upon an theory that Walter Beech had but was not applied to the Beguine. reference Air Classics volume 38, number 6, page 16. As for making Beguine into a semi-scale stunter or a Cavalier with its tip tanks, the objects on the tips of very thick stunt wings barely extend above and below the wing surface when built to scale. I tried. I carved and hollowed tip tanks for my first Mustang in 1968 and was very disappointed in their appearance.

>Jake,
>
>This subject of the Beguine having one wing longer than the
>other is based upon an theory that Walter Beech had but was
>not applied to the Beguine. reference Air Classics volume
>38, number 6, page 16. As for making Beguine into a
>semi-scale stunter or a Cavalier with its tip tanks, the
>objects on the tips of very thick stunt wings barely extend
>above and below the wing surface when built to scale. I
>tried. I carved and hollowed tip tanks for my first Mustang
>in 1968 and was very disappointed in their appearance.
>
>Al

Al,

Well you have a LOT more '51 time under your belt (both big and model) than I have (or am ever likely to), but from what I have read on websites on the 'net, the wing mod WAS done, just not when the ac was first modified. I have seen info that states it was done either just before or following the belly landing during the ferry flight.
The roll control problem is probably what killed Odom- not enough aileron area. My guess is he tried the turn and got inverted- then couldn't roll out upright and panicked and made the classic human mistake of pulling the stick back trying to "go up". This would have led to exactly what happened: diving into the house inverted at a 45 degree angle.
He was too low and going too fast- simply not enough "time" to sort it all out...

Jake

You of course are correct that you cannot always translate what exists in "full-size" to model scale and make it look "right", or work either...

>I am not arguing against the fact that we have found that we
>need bigger flaps on the outboard wing panel, but I don't
>understand the "why" of it.
>
>The outboard wing is flying faster, so it generates more
>lift at any given angle of attack and flap deflection (lift
>goes up with the square of the velocity). At the same time,
>it is flying a larger maneuver than the inboard panel (it is
>on the outside of the cone generated by the fixed point of
>the handle). Both of those are factors that would indicate
>a need for less lift capability required by the outboard
>panel than the inboard.
>
>Compared to actualy experience, this does not compute!
>
>Anyone have a clear explanation of what factor(s) I am
>missing?
>
>

Al mentions that it "allows" us to carry more tip weight, which is true. I would prefer to say it allows us to carry the correct weight. With assymetry, because there is more lift on the inboard wing, even more outboard flap is needed than with equal panels (or less weight is allowed in the outboard tip to keep it from wiggling). But, even without assymetry, and all of the above factored in, a larger outboard flap is still called for.

Matt's latest is a good example. Equal panel wings, extremely sharp corners, outside tip dropping even with larger outboard flap. More was needed. OK, enough prelude. Here is my theory: Line rake, pure and simple.

We have learned that we need to locate the leadout position behind the center of gravity to compensate for line rake. And we have also learned to put adjustable leadouts in our airplanes so that we can set this position to optimum. Basically we adjust our leadout position to line up with the amount of bow in our lines--the more bow, the more we have to move our leadouts back. And the amount of bow is dependent on several factors--line size, line length, airplane speed, amount of "pull" on the lines. OK, we adjust it for when we are flying level. But what about when we are flying maneuvers? What about up and down?

"Up and down?", you ask. Yes, "up and down". The lines are always lagging behind the airplane while in flight, and in level flight we are able to compensate for this with a proper placement of the leadout guide. We move it back until the plane is flying with little or no yaw caused by the lines. However, what happens when we turn? Here is where the problem enters in

Remember, the lines are always lagging behind the plane in flight. When we are flying level, the lines are lagging behind wanting to pull the inboard wing back. But when we suddenly turn "up", the plane pivots rather quickly, but skids to a degree before it moves out again in the new direction in which it is pointed. This is why we never turn the prescribed 5 foot corner, even though it "looks" like it sometimes. The plane pivots, but the flight path hasn't followed the pivot of the plane.

So, let's say the plane is performing an inside square loop. In the downward leg of the loop the lines will be lagging behind, or bowed "up". This upward drag or pull is no problem, since this line rake or bow is compensated for by the leadout guide position. Now, when we turn the corner quickly, the plane rotates to level, but the lines are lagging behind and are still pulling "up". The result is the inboard wing is held back in the turn or pulled "up" and this gives the impression of the outboard tip dropping.

Now, since we cannot compensate for this by moving the leadout guide up (it would really mess things up then in an outside square), what is needed is more outboard flap (or flap movement) to add lift to that wing and compensate for the pull of the lines. In level flight the flaps aren't deflected, so the extra flap area is meaningless. But in the turn, when we deflect the flaps, the extra area gives us added lift to the outboard wing just at the time that the lines are pulling back (or "up") on the inboard wing.

In a round loop the lines follow much more closely to the path of the flight of the plane, so less compensation is needed. And in a round loop the flaps are deflected less so less compensation is given.

It is not perfect, and I wish there were a simple way to adjust for the amount of extra outboard flap we could give to a design. As you move the cg back or add more elevator deflection to increase the rate of turn, you could then add additional area to the flap to compensate for the "wiggle". Ah, maybe that is the next thing to work on. Or, maybe adjustable flap rates are the perfect answer. Change the ratio of outboard flap movement to inboard flap movement as just another trim tool at our disposal.

A couple of things, maybe they were mentioned and I missed it. I have heard it said that our ships "crab" in flight on the tether, and consequently the outboard wing is "shadowed" by the fuselage. I don't know whether I buy that idea or not, but I can't discount it.

However, notice that the outboard wing tip actually has to move vertically a greater distance, our ship wing axis being normal to the flight sphere. So we make the outboard FLAP larger..

The outboard wing itself is always smaller or equal to the area of the inboard wing on CL stunters, while the outboard flaps are equal to or greater than the inboard flaps in area. There's a reason champs like Ted Fancher add those little "warts" on the tip of the outboard flap on some (e.g. classic) ships - they obviously fly better with increased area.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Never play cards with a man named Doc and never eat at a place called Mom's." -John O'Hara

Larry,
I think you are refering to "spanwise flow" on the outboard wing. My understanding is that it is a factor and is one of the reasons a flap chord ratio increase is more effective at the tip.

Is it possible that there is another factor to consider. One of Newton's laws of motion is an object in motion will tend to continue in the same direction unless acted upon by another force. Applied to the tip weight the tip weight will continue to go upward on the upward leg of the square loop. As you turn to the horizontal plane the tip weight inertia will continue upward hence the outboard wing will rise unless compensated for by additional up flap "lift" often accomplished with a trim tab. Additional tip weight can be tolerated in such compensating situations up to the point where the outboard wing drops. This situation is probably the most abused trim mistake in many models (carrying too much tip weight).

Bob Smiley
A Lancair 360 builder/pilot trying to be a modeler

Bob:
I think you are right about the inertia of the wingtip weight and its effect. Don't forget that the kinetic energy increases with the square of the velocity. As a simplistic example if we were flying a plane with a 6 ft wingspan on 60 ft lines (eyelet to eyelet) the outboard tip would be traveling 110% of the inboard which would mean that the kinetic energy would be 121% greater.

Bob,

We call this "hinging".

Ron B.
F4Fguy

Most of the replies to this question are only addressing the level flight trim,which,it's true ,only needs to balance lines and asymetric lift due to speed differences in/out-board.In maneuvering however,G forces due to inertia are much greater on the outboard wing.At 5-10 G that one ounce tip weight is 5-10 oz.This is clearly demonstrated by the "hinging" which occurs with too much tip weight.
The roughly 1/2 of the lines statically balanced by the tip weight are not going nearly as fast (roughly 3/4 speed)and don't generate nearly as much inertia force.This in turn poses a need for a high lift device which acts on the outboard wing only in maneuvering.
This has been covered time and again by such as Al Rabe,Bill Netzband,and others.
"Those who do not know history are condemned to repeat it."

LAST EDITED ON Oct-23-03 AT 10:36 PM (CDT)
 
This thread has prompted a lot of very technical speculation but little scientific data. All that is really known is this configuration has been successful since 1974. I can make very technical guesses too, but my speculation wouldn't add a bit of useful, practical information to my thirty year old observation that tip weight and reverse asymmetry seems to work best. We can speculate about line drag, spanwise flow, CG location, pitching, rolling and yaw moments, inertia, kinetic energy, longitudinal dihedral, gyroscopic precession, progressive propeller pitches, "P" effect, spiraling airflow, torque, the dynamics of tethered flight and a host or other possible effects which may or may not have a measurable effect on the way a stunt ship flies, but would this add anything applicable to improve the performance of our stunt ships? This topic has a very low signal to noise ratio.

>...possible effects which may or may not have a measurable effect on the way a stunt ship
>flies, but would this add anything applicable to improve the performance of our stunt ships?
>This topic has a very low signal to noise ratio.

Sometimes the primary purpose is understanding, rather than creating a better product. All of this has been addressed before - but not exactly in this same way. I guess some, like me, are easily entertained.

SK

>This thread has prompted a lot of very technical speculation
>but little scientific data. All that is really known is this
> configuration has been successful since 1974. I can make
>very technical guesses too, but my speculation wouldn't add
>a bit of useful, practical information to my thirty year old
>observation that tip weight and reverse asymmetry seems to
>work best. We can speculate about line drag, spanwise flow,
>CG location, pitching, rolling and yaw moments, inertia,
>kinetic energy, longitudinal dihedral, gyroscopic
>precession, progressive propeller pitches, "P" effect,
>spiraling airflow, torque, the dynamics of tethered flight
>and a host or other possible effects which may or may not
>have a measurable effect on the way a stunt ship flies, but
>would this add anything applicable to improve the
>performance of our stunt ships? This topic has a very low
>signal to noise ratio.
>

Knowing *how* things work, as well as *if* they work, is very important, to me, anyway.

Bret

>This thread has prompted a lot of very technical speculation
>but little scientific data. All that is really known is this
> configuration has been successful since 1974. I can make
>very technical guesses too, but my speculation wouldn't add
>a bit of useful, practical information to my thirty year old
>observation that tip weight and reverse asymmetry seems to
>work best. We can speculate about line drag, spanwise flow,
>CG location, pitching, rolling and yaw moments, inertia,
>kinetic energy, longitudinal dihedral, gyroscopic
>precession, progressive propeller pitches, "P" effect,
>spiraling airflow, torque, the dynamics of tethered flight
>and a host or other possible effects which may or may not
>have a measurable effect on the way a stunt ship flies, but
>would this add anything applicable to improve the
>performance of our stunt ships? This topic has a very low
>signal to noise ratio.
>
>Al

Dang, Al! You bring up even MORE possibilities!

I fear if we had good analytical data in this discussion, it might be a REAL turn off. Although we have several techie cones regularly posting here, as Wild Bill demonstrated in SN, the MATH part tends to be less interesting to most stunt grunts. We tend to want to know WHAT works, and WHY is of less conseequence.

I'm enjoying the discussion here, all the same. That's the beauty of Leonard's forum, a diverse set of subjects.

[photo not recovered: 3e69bb3870f12ad5.jpg]

"Research is an organized method for keeping you reasonably dissatisfied with what you have." -Charles Kettering

That's a good one about the low signal to noise ratio, Al. Kind of reminds me of old stories about the AN Ranges. But what is longitudinal dihedral?
Chris...

Any positive incidence in the stab/ele system can be considered "longitudinal Dihedral" I guess Anhedral actually. Dihedral would be negative stab incidence.

I think this is it.

Curt (Integrating to improve the S/N)

Thanks Curt,
Never thought of it that way.
Chris...

This hairbrained idea that entered my mind today. Not thought out at all but intrigues me to the point where it might be interesting what you guys say about it and if it may be worth experimenting with or have you already crashed a test bed with this "hairbrained" idea

GENESIS

Elevators on general aviation aircraft have counterweights to balance the elevators to eliminate flutter. Aerobatic aircraft sometimes have counterweights and also flat flying surfaces on arms below and forward of the aileron hinge line to reduce moment forces at the control stick.

IDEA
Make a tee bar and affix a flat surface on the top of the tee ie a weather vane. Attach a swivel mount with a hole and set screw on the end of say a Twister wing at the CG point. The flat surface is a trim or "weathervane going back just behind and parallel to the flap. Affix two wheel collars on the middle of the shaft attach weights between the collars and attach the shaft end into the swivel mount on the wing. You thus have a weather vane swiveled at the end of the wing and the flat plate parallel and behind and outboard of the flap with weights between the two. It may be possible to attach a wire stop wire on the wingtip to limit up and down movement of this vane. This could also be adjustable.

THEORY OF OPERATION

The moment arm of the weather vane "tab" and the weight location and weight are totally adjustable. Through testing, an optimum setting might be achieved for your desired style and turn rate.

At rest the weight and tab are down. Beginning of flight there would be uplift from the tab and a down force with the tip weight. Balanced weight, moment arms and vane size would be important here.

In level flight the vane and weight would trail in parallel with the relative wind.

An inside loop would throw the weight to the outside of the turn or downwards thus forcing increased lift to the inside of the circle. Similar action would occur on outside maneuvers. The rotation would be in the same direction of the flaps but respond to directional g force changes as they occur, not control inputs.

If hinging is still present, slide the weights toward the weather vane, If hinging is to the inside of the circle, move the weight to the swivel.

The model would have wingtip weight installed in the traditional manner as well as on this device thus you recieve the benefits of centrifugal forces for line tension from both weights but also receive counteracting lift forces from the vane to ameilorate the outward centrifugal forces caused by the attitude change of the model. The model can support more wingtip weight with this device without the corresponding drop of the outboard wing during flight and maneuvers.

More complex? Definitely but it dynamically responds to laws of physics and aerodynamics in a direct relational manner.

Would it eliminate hinging at any turn rate? I don't have an answer. It would appear so.

Can you increase tip weight without corresponding hinging problems. It appears that it may be possible with varying vane sizes.

What do you think of this hairbrained idea? Any other ideas you might add to the forum?

Bob Smiley
A Lancair 360 builder/flyer trying to be a modeler.

LAST EDITED ON Oct-24-03 AT 10:04 AM (CDT)
 
Good point about the blanketing; it is a valid consideration. It's interesting that no one seems to have remarked the difference between side mounted profiles and full-fuselaged CL models. The profile's cylinder/tank complex disrupts the air flow over the outboard wing and adds drag asymmetrically. These ships, if any, would be candidates for spanwise symmetrical wings with larger outboart flaps, even though I think that the added outboard drag from such flaps is already a factor in their acceptance. If I recall correctly, Martin Hepperle's computation (integrating spanwise) shows that the center of lift for our models is typically .5" - .8" outboard of the wing center line on a symmetrically spanned plane. This is near the lateral c.g. position of my profile with tip weight, leaving a net clockwise (outward) yaw moment from thrust, if not drag, without the enlarged flap. Assuming appropriately raked leadouts, the resultant of increased drag of model parts inboard of the c.g., opposed by the higher V^2 drag of faster moving outboard parts is hard to evaluate, but I suspect that it's counter-clockwise (inboard). Still, the "blanketing" from engine and tank and outboard c.g. make added lift and drag from an enlarged outboard flap seem desirable on profiles.

SK

Edit: Just a note that I realize, of course, that this primarily addresses static considerations, which as evident elsewhere, are only part of the story. Still, they should be considered.

LAST EDITED ON Oct-24-03 AT 12:05 PM (CDT)
 
Serge,

Since you bring up the subject of "blanketing" the inboard wing with disrupted airflow, I thought I'd point out that the 1978 Mustang article "Evolution" details my concern with possible blanketing of the inboard wing and tail surfaces with disrupted airflow. This is why I installed the engine "canted" to get the required muffler inside the airplane. It was an aerodynamic consideration and a calculated risk that the canted engine would still run OK. It did. I was trying to design an ultimate competition stunt ship with a reasonably realistic appearance as long as appearance wouldn't affect the airplane's ability to fly competitively. I still think the airplane wasn't fully understood or appreciated for its innovative aerodynamics. In many ways, it was the first modern stunt ship. "Evolution.." was a good article. It was reprinted by Stunt News and in several foreign languages. I still have the British and Japanese copies.

With modern rear exhaust engines, the revised "Snaggletooth" no longer needed a canted engine to get the muffler inside, so I omitted that feature. It simplified the structure and virtually guarantees a decent engine run. The canted engine required a built in tank to stiffen the fuselage structure which sometimes caused a bit of difference in inside/outside lap times. Now the tank can be removable and adjustable without adding excessive weight.

>I am not arguing against the fact that we have found that we
>need bigger flaps on the outboard wing panel, but I don't
>understand the "why" of it.

>Compared to actual experience, this does not compute!
>
>Anyone have a clear explanation of what factor(s) I am
>missing?

The lines aren't a dead mass, but a compliant system. What you need to balance the line weight in steady state ends up being way too much in the short term. So you create a lateral CP shift that only works at large deflection (for a short period) to "hold up" the excess, and has little effect in steady state.

Larry, the proper question is WHEN you need more lift. Brett wrote it well, lines does not make consistent load on inner tip, so we need more lift in corner on outer wing – just because inside wing is temporarily unloaded and outer tip weight is in that moment excessive. Enlarged flap has minimal effect if flap is straight (difference to tip chord) – means in level flight; but it is very effective at deflection (difference to flap chord). So it effectively handles that excessive weight.

But it is not all, we have very asymmetric models. We fly in circle so we have different speeds inside and outside (moves AC out), we fly nose out – means we have side flow (moves AC in), we have misaligned AC of wing and AC of elevator (makes moment to roll). Most of that can be trimmed by proper tip weight and rudder offset. But typically, if you use symmetric wing, you will end up with lot of rudder. It is because you must move AC of elevator LEFT of the AC of wing to balance that unbalanced tip weight in corner (tail has negative lift in corner and thus it rolls inward – against that excessive lift). But it has ill effect to line tension overhead, so you will keep with rudder down and the result is either out roll in corners or in roll in round figures (depends on tip weight). In this case - if you open eyes while trimming you will clearly see that you need more tip weight for proper round figures or less tip weight for corners. The only solution is more rudder out or extra tab on outer flap. That is all, trivial and easy to trim if you can recognize it. A helper out of the circle is very helpful – he can see it well in intersection of round eight overhead.

I do it simple – I make outer flap 4mm longer in chord – that is about proper, and I trim minor misalignment by rudder. It always end up straight or minimally out. Once I did it too long and rudder had to be “in” but that is not good in wind, and I rather cut it out little bit.

>(snip)I do it simple – I make outer flap 4mm longer in chord –
>that is about proper, and I trim minor misalignment by
>rudder. It always end up straight or minimally out. Once I
>did it too long and rudder had to be “in” but that is not
>good in wind, and I rather cut it out little bit.
>
>igor

Question for you, Igor, your 4 mm is about, oh, 1/6 of an inch (0.1574798). Do you make the outboard flap a constant 4mm wider from root to tip, or taper it so the root is the same and the tip is wider than the inboard? We have been doing the latter, both for looks and the fact that the outer part is more effective anyway. Just curious.

I do not think it is important too much, the only signifficant value is difference of chord between inboard tip and outboard tip. Also panel length (span) does not change it too much. However I do it equivalent at root (70mm) and 30/34mm at tip. Wing asymetry is 10mm more on inboard pannel

It is on wing with lot of sweep back - over 3.5 deg and aspect ratio 5.3

All this talk about wing/flap asymmetry and tip weight reminds me of discussions we've had within Combat circles about the same issues. Phil's been the one who's written the most about them in his magazine articles.

1) Larger inboard wing/flaps - this moves the center of lift inboard of the center of gravity. This is what causes the rolling in high-lift situations. Note that this also moves the center of DRAG inboard of the center of THRUST, reducing line tension. Too much of this is bad, most modern Combat planes use very little, if any. However, the more you use, the less tip weight you need. On a Combat plane, the tipweight is a larger percentage of total weight, so reducing it is advantageous.

2) Tip weight. Some is always needed because the center of gravity should be ouboard of the center of THRUST. This is to keep the airplane from yawing in at you upon a hand launch (and Combat hand launches are rocket-like, so this relationship is important). You also need the center of gravity either on or slightly outboard of the center of lift. In flight, you can usually get by with none in a Combat plane, but takeoffs may then be dicey.

3) Right thrust - this is to move the thrust line inboard of the center of drag. It also moves it inside of the center of gravity. On a Combat plane, might be undesirable since it means your engine is going to hit at an angle in a crash, increasing the chances of shearing off the mounting lugs.

I've experimented, in 1/2-A Combat, where line tension is of most concern, using a long OUTBOARD wing, and a larger than usual amount of tipweight. Worked great.

Phil's solution was to use a thicker center block, mount the boom and tail on the plane's centerline, but mount the engine bearers inboard of the centerline. You therefore move the center of thrust inboard, while keeping the center of lift and center of gravity in the same place, without the need for excessive tipweight or right thrust.

In the past, I used an asymmetrical tail - the outboard being longer. The theory was that the added drag (particularly during turns) kept the airplane from yawing in. I ended the practice after a spate of bent tail booms (I was using aluminum back then). Funny thing was the planes flew great with the bent tail booms, better than with straight ones, but another crash or two would mean a broken boom.