Balancing at 25% Chord

several posts over the months have mentioned the benefit of being able to balance a plane back at 25% chord because it lines up with the zero moment axis of a symmetrical airfoil.

I can't figure out how this is supposed to work. Does anyone have a force diagram that can explain the reasoning?

As best I can figure, the wing and the tail are solidly connected by the fuselage. When you move the controls the wing tries to pitch down from the flaps, but the tail, with a longer moment arm, pitches the nose up. I know there are some downwash effects that hit the stab, but I can't see what 25% of the chord has to do with anything. Also, once you drop the flap the wing is no longer symmetrical and will try to pitch like crazy without the tail to stabilize it.

Wow! I know I'm stepping into it on this one, but let me try some common sense observations.

>several posts over the months have mentioned the benefit of
>being able to balance a plane back at 25% chord because it
>lines up with the zero moment axis of a symmetrical airfoil.
>

I think this all depends on where the point of maximum thickness occurs. It may not be at 25% of the chord. Simply considered, the force of lift is probably maximum at the maximum thickness - simply because that's where the airfoil shape transitions. Balancing the loads at that point optimize the force structure.

>I can't figure out how this is supposed to work. Does
>anyone have a force diagram that can explain the reasoning?
>
>As best I can figure, the wing and the tail are solidly
>connected by the fuselage. When you move the controls the
>wing tries to pitch down from the flaps, but the tail, with
>a longer moment arm, pitches the nose up. I know there are
>some downwash effects that hit the stab, but I can't see
>what 25% of the chord has to do with anything. Also, once
>you drop the flap the wing is no longer symmetrical and will
>try to pitch like crazy without the tail to stabilize it.

I'm not sure I agree with some of your assumptions. I don't think the wing attempts to pitch down when the flaps deflect down, assuming you're talking UP control input. There is an optimum - that may be exceeded - where increased drag from too much flap deflection exceeds the increased lift. Generally, though, the flap downward deflection increases the airfoil effect and causes the wing to lift up. In the same manner, an UP elevator deflection, in combination with the stab, forms an airfoil that produces lift in the opposite direction, causing the tail to pitch down. A flying tail or stabilator causes the same thing, but it's just the angle of attack involved.

The point of rotation is a complicated balance formed from the difference of lift between the tail and the wing. Maintaining the center of gravity on the point of maximum lift on the wing allows the tail to achieve maximum leverage. It also minimizes wasted energy used in correcting either the unstable, or overly stable result of the maximum lift point being different than the static leverage point.

Flaps were applied to stunters because it enhanced the rotating effect, while smoothing out the tail leverage. Too much flap, and the plane balloons in the maneuvers, too much tail and the corners and turns are too hard, and difficult to smooth out.

I'm sure Brett has his darts ready. I make no claims except observation and studying years of descriptive accounts.

Tom B

>I'm not sure I agree with some of your assumptions. I don't
>think the wing attempts to pitch down when the flaps deflect
>down, assuming you're talking UP control input. There is an
>optimum - that may be exceeded - where increased drag from
>too much flap deflection exceeds the increased lift.
>Generally, though, the flap downward deflection increases
>the airfoil effect and causes the wing to lift up. In the
>same manner, an UP elevator deflection, in combination with
>the stab, forms an airfoil that produces lift
>in the opposite direction, causing the tail to pitch down.
>A flying tail or stabilator causes the same thing, but it's
>just the angle of attack involved.
>
Tom:

consider a combat wing with the elevator along the trailing edge, deflection of the elevator causes the wing to rotate in the direction of the elevator deflection. "Up" elevator causes an inside turn and "down" elevator causes an outside turn. This is the same as the wing pitching down with down flaps. On full size aircraft lowering the flaps for landing causes the nose to drop. If I understand the reason correctly is is because the deflected control surface causes the center of pressure to move aft resulting in a pitching force opposite the direction of deflection.

LAST EDITED ON Nov-06-04 AT 09:16 AM (CDT)
 
I understand that a tail-less combat wing turns (never liked those designs), but I thought this was an anomaly caused by the zero moment arm of the tail. In a stunter, the leverage of the tail moment arm dampens out this initial tendency (also known as stability), allowing the wing to use the deflection as additional lift. I did allow for the fact that if the deflection is great enough, the reverse will be true simply because the drag overcomes all the other forces.

It seems to me that if this "pitching down" were really true in the theoretical sense, ailerons would work in the reverse. I think it goes back to Gary's thread on aerodynamic forces. The primary component is lift, and whether the air is directed in such a way that differences in speed from the top to the bottom of a surface creates lift. Eventually, if deflection is great enough, then drag becomes the primary component (due to the air flow breaking down in to vortices), and deflection creates a moment arm force to overcome the additional lift. With ailerons, this would be noticed as yaw in the opposite direction.

I continue to doubt the usefulness of full-size aircraft experience. Until they use down-thrust and make the Reynolds Number constant, I'm not sure the analogy applies. Does an RC model's nose pitch down with the application of flaps? Does yaw occur with ailerons unless the model gets so big that coordinated rudder turns are necessary? I haven't seen that happen, but maybe they're too quick with the sticks to let me notice.

Good point. If you are doing some calculations, the quarter mean aerodynamic chord is a useful reference, but if you are trimming an actual rigid airplane with a tail, I wouldn't think there's anything special about it. It may be where modern stunt planes tend to balance, but that could be just a coincidence.

The 25% rearmost c.g. idea may relate to stability near the undeflected setting. First - IF I remember correctly - the neutral point for a symmetrical section is closer to 24%-chord. So 25% would be marginally behind that. While I doubt that most measurements cited are accurate enough to differentiate the two (+/- 1.0%?), such a rearward bias may be possible due to negative pitching moment from slight flap and elevator deflection used to achieve lift for straight and level flight. Still, when the controls pass through neutral, you don't want much of an area for the plane to choose its own divergence rate.

I think that the negative pitching moment due to flap deflection takes the place of lost static margin brought about by moving the c.g. aft. Then a 25% aft "limit" may result from the need for stability about neutral, so that the plane grooves in level flight and doesn't "hunt". Reasonable?

SK

LAST EDITED ON Nov-06-04 AT 01:57 PM (CDT)
 
Phil,

The 25% MEAN chord point (not nasty: 'average')-25%MAC- took over from discussion of Center of Pressure back in the NACA era. To use the 25% (or 24+% as SK points out) point, lifting airfoil data needed another term and value to account for the 'nose down' effect you mention. (With flaps, this is always AWAY FROM the principal direction the wing is lifting, so DOWN is a fitting term.)

The new term is the Moment Coefficient about that 1/4 MEAN chord . Dealing with airfoils as simply shapes in a flow of air, planform, sweep, taper, whatever... don't enter the picture.

Almost all lifting sections have a negative value of Cm(MAC)(current practitioners, please correct this term if it is out of date?). That is, a tendency to rotate the wing to lower angle of attack and reduce the lift generated. You may have seen flying wings use 'reflexed' airfoils -- in effect, a twist near the TE like UP-elevator on those plank combat models. That is a way to reduce Cm(MAC).

A symmetrical section, supposedly, has no Cm(MAC) value about the 1/4 chord. When, however, we deflect the flaps, the airfoil isn't symmetrical any more. It DOES develop a negative Cm(MAC), tending to reduce the Angle of Attack and thereby reduce the lift.

The recent trend toward VERY large tail surfaces brings two things, at least. The fixed stabilizer is proportional to the moving elevator, and either one is much larger than would be needed for stability and control if we didn't do the high-g maneuvers we do.

1) The stab helps more restore stable straight flight -- level or figure sides -- in proportion to its larger size. The elevators, likewise, produce greater pitch producing moments.

2)This power also allows a very "tailheavy" CG position, compared to where CG on earlier designs started to make things go squirelly.

Think of the traditional CG as a shovel situation, with the model's weight at CG as the load, Wing Lift as the lifting hand, and the tail as the other, guiding hand. If we let go with the guiding hand, the torque from the load will twist the lifting hand and the load will rotate down.

If we could center the load under the lifting hand, that torque wouldn't happen. The guiding hand need only 'point' the other hand, the shovel and the load. With the very large area tails, skittish aft-CG tendencies are suppressed, and we CAN control things.

Without a moment arm radius from the CG to the MAC quarter chord, the wing doesn't have that load, added to the accelerated weight, to overcome in a high-g maneuver path. The change from symmetrical to "in effect lifting" airfoil with deflected flaps is there anyway, whether CG is at 18% MAC or 24%, but the nearer to the 25% MAC position, the less the other load is.

>>>The 25% MEAN chord point (not nasty: 'average')-25%MAC- took over from discussion of Center of Pressure back in the NACA era. To use the 25% (or 24+% as SK points out) point, lifting airfoil data needed another term and value to account for the 'nose down' effect you mention. (With flaps, this is always AWAY FROM the principal direction the wing is lifting, so DOWN is a fitting term.)<<<

That quarter chord AC definition is synthetic and has no real background. I learned that value 25% was used because that is the closest easy to remember point where are changes of pitching moment with changes of AoA smallest. ... so tose 25% looks like good number

>>>Almost all lifting sections have a negative value of Cm(MAC)(current practitioners, please correct this term if it is out of date?)<<<
You wrote that counter example - autostable airfoils for flying wings - for example Eppler 182. And if you have flapped airfoil with flap at negative deflection you can come to positive pitching moment and the airfoil can be still lifting.

>>>A symmetrical section, supposedly, has no Cm(MAC) value about the 1/4 chord.<<<
This is not 100% true. It is true only at zero lift. But the moment changes when airfoil goes out of its linear lift/alpha segment because of partial separation on upper surface. Also if there is not enough pressure at LE like it is on our thick airfoils - in this case moment grows directly with AoA as blunt LE does not ballance enought pressure on aft lower side of airfoil. And also CP goes even to 50% on complete separtion on upper side (90 deg AoA) on any airfoil.

>>>This power also allows a very "tailheavy" CG position, compared to where CG on earlier designs started to make things go squirelly.<<<
Yes, and thus help little bit with Netzeband wall. So if you are going to make big heavy flapped model (that makes that unwanted feedback) then the good way is to make large tail, aft CG not needing overcome tose forces by handle of even bring some "power steering" from aft CG

To make things a lot simpler, The Center of Lift is of a wing is at about the 25% point on the wing. This can vary a bit by the airfoil design, but is not a point of issue here. The CG will be forward of this point somewhere. As the CG moves closer to the C/P the plane will become more responsive and less stable because it requires less leverage (the elevator to move the lever arm of the CG/CP distance. I prefer them this way, others don't. When flaps, or the elevator on a plank wing, is activated, there is lift caused by the new camber of the deflected control. This camber at the hinge point of the control surface will cause the center of pressure to move back, forcing the CG to CP ratio to change and the nose will move in the opposite direction of the lift force, ie. if the flap or elevator or flap is moved down, the CP will move back and the CG will pull the nose down as the lift that has been holding the nose level moves back. If the flap or elevator is moved up, the lift has increased on the lower part of the wing aft of the original location and the trailing edge drops, and the total lift force has shifted to the bottom of the wing. So, if the flaps are lowered the nose will pitch down. The elevator at the end of a fuselage works on a longer moment arm, so it has to be big enough and have enough travel and lift developed to overcome the downward pitching moment of the flaps. If you lose the elevator and the ship is rigged truly 0/0 the nose will pitch down when up "UP" control is given.
Now, to the question about flaps making the airplane turn tighter. That depends on the airfoil, wing loading, tail moment (leverage) and other design factors. Generally, a heavy airplane with a high wing loading will perform better with flaps. A long tailed airplane will perform better with flaps than a short airplane. Flaps will not make an airplane turn more quickly. Light airpalnes with light wing loadings do that, and flaps will be a detriment. You have to have more control movement for a flapped airplane to work, so just surface movement alone will take more time. A good combat plane will outturn any stunt ship any day because of the lightness. Pure physics here. ANd they do it with a max of 15 degrees elevator travel.
I could delve deeper into this, but I gotta go, the wife is yelling.
Bare

Comparing stunt planes with combat jobs is like apples and oranges. I still remember when a person by the name of Howard Rush won open combat with a "Nemesis". As I was paying for a set of plans, someone asked where it balanced. He gave them the plane and had them check for balance point. It was setting on their fingers with the nose pointing to the ground. CG was somewhere up on the motor mount. DOC

Modern F2D models balance much further back then a Nemesis would have.
Depending on the fliers preference they are balanced very close to the 1/4 chord. I like them tail heavy enough that adding one penny to the tail can make the difference between flyable and unstable.
Pat MacKenzie

this doesn't have anything to do with the original question, but you can make almost any plane with a stabilator fly with a wide range of CG position as long as the leadout position, control travel, handle travel, and line spacing are adjusted to match. With a forward CG the controls take more force to move and the plane tends to be more "pointable". As you move the CG back it takes less and less force to move the controls so the pilot has to be a lot more aware of what the plane is doing because he has less feedback through the handle.

I suspect this may be some part of the 25% chord balance point thing. With big flaps and a big tail a rearward CG reduces the control forces to manageable levels, but the plane is still stable and pointable due to the large tail.

Why is comparing stunters and combat ships apples and oranges? The same aerodynamics still apply. Physics is physics. I can fly a combat ship through the stunt pattern with no difficulty. Judges don't like it because they can't see it. Judges like those 5.234 sec laps, but that shows how the judges run the contests. A guy should be able to fly at any speed he likes without being downgraded by the judges. But, I digress. As far a CGs go, I would balance a Nemesis or any other combat ship and stunters, too, nearer that 25% chord mark, because that's the way I like a plane to handle, light and quick. I don't like heavy controls and slow responding models. CG location is a personal thing. Some people like a Flite Streak to be balanced near the leading edge, I like them balanced on the spar. Moving the CG forward requires more control input to get the plane to move, slowing down the controls. Flaps add even more to the slowing down of the controls. This is why stunt flyers like to have flaps and forward CGs. It slows down the controls making the model feel more "stable" and pointable. It certainly does nothing to improve the turning radius of the plane. Turn radii are a function of wing loading, airfoil, control input, and speed. Stability is is generally based on the trueness of the structure. If you get the CG too far aft, the model will become unstable, but most people equate instability with responsiveness. Just because a plane has quick controls and is somewhat more difficult to point doesn't mean it's ustable. Just means the pilot isn't good enough to fly it properly. 1/2A combat planes are incredibly quick and tight turning. But combat flyers with any skill at all can easily control them. They make flying large combat ships a piece of cake and stunt ships are just trucks.
Bare

Having flown a number of 1/2A stunt ships; a 35 size airplane seems slow in contrast. My reflexes aren't good enough to watch a modern 1/2A combat airplane, much less fly one. I have a slowed down old fast combat airplane that runs only 100 MPH, the tank is 2 oz, and I am soaked with sweat after one flight with it. So if I am judging, I hope you will not be flying 3 second laps.

Try flying stunt with a good stunt plane. Stunt planes are particularly good at flying stunt.

>Try flying stunt with a good stunt plane. Stunt planes are
>particularly good at flying stunt.

I kind of agree with Barry, but you're still my Combat Hero. Your humor is a little subtle, though. I don't think anybody caught the Firesign Theater reference a while back.

>If you get the CG too far
>aft, the model will become unstable, but most people equate
>instability with responsiveness. Just because a plane has
>quick controls and is somewhat more difficult to point
>doesn't mean it's ustable. Just means the pilot isn't good
>enough to fly it properly. 1/2A combat planes are incredibly
>quick and tight turning. But combat flyers with any skill at
>all can easily control them. They make flying large combat
>ships a piece of cake and stunt ships are just trucks.
>Bare
>

Barry, I absolutely LOVE your preservation work. An order will be coming your way when I get some more disposable funds. Meanwhile, I think I would paraphrase part of what you say with, "Just because a plane has quick controls doesn't mean it has a smaller turning radius."

Maybe I have corner obsessiveness. I think a stunt plane will let you dial in the turn, and stop at any point along the way to change directions. A typical combat job will always want to complete the turn, and it's extremely difficult to keep it from doing so. That's why I always saw so many combat fliers finish a reverse wingover with an outside loop at the corner turn. The combat plane will do the turn and stop, but it takes real guts to stop it and level out inverted. With a stunt plane, the guts are needed to WAIT until the corner.

Another way of looking at it is that the CG affects the pilot-control relationship, with response and feedback. It does not affect the aerodynamic limits of the plane.

>Comparing stunt planes with combat jobs is like apples and
>oranges. I still remember when a person by the name of
>Howard Rush won open combat with a "Nemesis". As I was
>paying for a set of plans, someone asked where it balanced.
>He gave them the plane and had them check for balance point.
> It was setting on their fingers with the nose pointing to
>the ground. CG was somewhere up on the motor mount. DOC

I agree about the Nemesis CG. It always came out nose-heavy, even with a light Fox up front. Compared to a lot of combat planes, it was solid as a rock, and flew exactly where it was pointed.

That's probably Howard's personal style. I remember he went to great lengths to recommend trimming the throws to limit the turn just before any stalling. That probably feeds into the overall strategy of having a very stable plane that you can fly "eyes off."

Having a plane with a more unstable CG and greater throws results in a different combat strategy - fast directional changes and the ability to turn really tight and put on the brakes in an emergency.

>>>If you are doing some calculations, the quarter mean aerodynamic chord is a useful reference, but if you are trimming an actual rigid airplane with a tail, I wouldn't think there's anything special about it. It may be where modern stunt planes tend to balance, but that could be just a coincidence. <<<

Yes, I think that is the shortest mean full answer

That Ted's rule CG at size of tail has no magic theoretical background, it is just so on our typical flapped models and it makes that nice coincidence.

However you have 4 pitching moments here:

1/ CG from AC makes negative pitching moment from centrifugal force (or gravity in straight flight) on arm to AC at 25% of MAC. Properly written - it is lift on arm from CG to AC what makes that moment. And that is also reason why center of pressure or center of lift moves up and down with amount of lift and why it is not really useful as reference point for calculation.

2/ Airfoil pitching moment concentrated on LE of MAC and its force is also on arm to AC - it is close to zero in level flight, but negative in corners because of flaps and it also dramatically changes before and at stall.

3/ Lift of tail on arm from AC of tail to AC of wing

4/ Pitching moment of tail airfoil - as you wrote fuselage is rigid so it is the same in AC of wing

So you see now you have all concentrated in one point - AC and sum must be zero (less acceleration).

And now comes the trick. If you want statically stable vehicle, then if angle of attack grows, then net effect of all changes in those 4 values must be negative. Means if you make smaller tail and that makes less lift, then you must move CG little bit front to replace that deficit ... or just make longer tail, or higher pitching moment from flaps or smaller from elevator.

And really does not matter (from static stability point of view) where the absolute position of CG is. It could be at 60% of MAC and it could be still stable enough. It depends on those other 3 factors – canard is good example. Stability is about that CHANGE as reaction to AoA.

But we have much easier way to examine stability. We have that mentioned static margin – means how far is CG from neutral point of model. Having CG front of AC looks like bullet proof way .

So our model will fly also if CG will be aft of 25%, however there IS one reason why not to go further. If model flies in level and flaps are straight, the pitching moment of airfoil is ~0 and CP is at AC. If we go with CG to say 30%, then the tail is already lifting – it must overcome positive moment from CG aft of AC. But if you deflect flaps and elevator, then negative pitching moment from flaps forces tail to go negative lift mode. That transition can make problems and can be reason of hunting or imprecise transitions in figures. But such model definitely CAN fly and fly well.

Anything that flys, wants to rotate around it's CG when it turns.

The area in front of the CG digs into the turn and makes it turn quicker.

The area behind the CG drags behind and make the plane more stable.

If you have too much area forward of the CG, it will want to turn all the time ( unstable ). Too much aft and it will take too much effort to make it turn (slugish).

I flew real planes in the Air Force. As I remember, the allowable CG range was 22% to 35%, just like out models. We had a TRIM METER in the cockpit, and I could jockey fuel to move it. We went faster and burned less fuel when we pushed the aft limit.

It works the same in NASCAR and pickup trucks.

>>>Anything that flys, wants to rotate around it's CG when it turns.<<<

That could be true at a flying rock, but once it makes a lift, it is not true anymore. ... for example model in loop has center of rotation far from CG

>>>>If you are doing some calculations, the quarter mean aerodynamic chord is a useful reference, but if you are trimming an actual rigid airplane with a tail, I wouldn't think there's anything special about it. It may be where modern stunt planes tend to balance, but that could be just a coincidence. <<<
>
>Yes, I think that is the shortest mean full answer
>
>That Ted's rule CG at size of tail has no magic theoretical
>background, it is just so on our typical flapped models and
>it makes that nice coincidence.

That is what I figured from the hand-waving explanations floating about. Since all the planes are very similar in design and purpose they ended up with the CG at a similar spot. That's opposed to designing to put the CG at a particular point because there are aerodynamic reasons driving it. I always liked Wild Bill's dictum- balance the plane ahead of the MAC and you are virtually guaranteed to have a stable design. Given the physics of pitching moments, tail sizes, etc. it is really difficult to design an unstable plane following his rule. I don't think the CG at 25% "rule" has that kind of ammunition behind it.

>So our model will fly also if CG will be aft of 25%, however
>there IS one reason why not to go further. If model flies in
>level and flaps are straight, the pitching moment of airfoil
>is ~0 and CP is at AC. If we go with CG to say 30%, then the
>tail is already lifting – it must overcome positive moment
>from CG aft of AC. But if you deflect flaps and elevator,
>then negative pitching moment from flaps forces tail to go
>negative lift mode. That transition can make problems and
>can be reason of hunting or imprecise transitions in
>figures. But such model definitely CAN fly and fly well.

I'm lazy Igor, what is a typical figure for the AC position on a typical stunter like an Impact?

LAST EDITED ON Nov-08-04 AT 01:00 PM (CDT)
 
As far as I know all of them comes between 18 and 24%. The only exception can be my model from WC at Sebnitz with CG at 15%, but it was with logarithmic unit and can not be used as typical example ... anyway, it well shows relations - less flap asks for more CG effect.