A question about flaps
 

Hi all, new user here.

Here's a quick question about aileron flaps. My aircraft has nearly full length ailerons and I got to thinking, if you set those alierons to a slightly down position on each wing, would that give the aircaft more lift and better slow speed stability. They would act pretty much the same as the F16's flaperons. (which I'm reliabley told, work quite well!)

If indeed that would work, then in theory, it would be possible to fix your aileron servo into the wing in a way that enables it to move forward and backwards (say on some sort of plastic tracking) and use a 5th servo to adjust its position. If the servo is moved back, the ailerons will drop slightly giving added lift and forward would make the aircraft clean again.

Anyone else done something similar?

Or think it's a ludicrous idea.........


Danny "I still can't prop hang in G3" Cook

RE: A question about flaps
 

Yes that is the way we did it in the days before radios could mix and do all the cool stuff they do now but if your going to add a servo anyway just put one on each aileron and set up the radio for flaperon mixing

RE: A question about flaps
 

Most radios have a "FLAPERON " function... try that .... allows you to have steeper approaches and lower landing speeds

RE: A question about flaps
 

Thanks for your replies guys. :)

Danny "Still can't prop hang in G3" Cook

RE: A question about flaps
 

Be careful with flaperons, when deployed (down, as flaps) they add wash-in to the tips and with this condition the wing is more prone to tip stalls.

RE: A question about flaps
 

I read quite often that when using flaps, you mix in down elevator to compensate.
My experience is that I have to mix in UP elevator or the plane nose dives. It still balloons when I dial in the flaps but if I hold down elevator untill the plane slows down then it flies and descends slowly.
Why is that??

RE: A question about flaps
 

Spoilers will need up elevator, flaps need down elevator.

THe initial attitude of the plane may require a higher mix as the flaps are first deployed. Especially if they are jammed to full all at once. As the plane settles, then a less radicial mix is required to keep the plane in a level or controlled condition.

RE: A question about flaps
 

Yup, this is what I always read.
I have a Polks Lucy 3d with flaperons and, the same as a my 45" scratch built L-19 with flaps, even if I dial in the flaps slowly so that the plane does not balloon, my flaps need UP elevator or the plane rotates downward into a dive.

Here is the drill:
Level flight - Pull throttle back to 1/4
plane begins to nose down,
dial in flaps slowly to hold nose up till level flight / speed stabilizes,
use throttle to cotrol descent rate afterward
Radio is mixing in UP elevator.

Without Up elevator mix OR holding up elevator, when I pull the throttle back, If I dial in flaps the airframe rotates forward in pitch, i.e. dives.

??

RE: A question about flaps
 

Ditto the flaperon mixing. That has another advantage of not being permanent, and there is no weight penalty except that you must use two servos, which you probably were going to use anyway. You may find you don't like flaperons. If you really want flaps, you will probably like separate flaps better, since you avoid the aggravated tip-stall problem that many flaperon configurations can cause.

Many, many modelers speak in terms of flaps increasing lift but this is not technically correct, the way they are used in practice. (Would-be flamers, please note the italicized text).

Increasing camber by extending flaps increases total lift force only if the AOA is not decreased enough to compensate. In practice, the pitch attitude is adjusted and/or speed is reduced when flaps are extended to keep the lift constant. Flaps are used to increase the lift available at a slower airspeed (allowing slower flight), and to increase drag, allowing allowing a steeper descent. They are not typically used to increase total lift over that required for a given flightpath, as that would result in a climb, or a reduction in descent angle.

Put another way, a ten-pound airplane must generate ten pounds-force of lift to maintain level flight (or the same descending flightpath), whether the flaps are extended or retracted. The extension of flaps allows ten pounds-force of lift to be generated at a slightly slower airspeed than with flaps retracted.

RE: A question about flaps
 

This is an excellent observation. The airplane gains altitude as the flaps are initially deployed because lift is momentarily increased. Total lift is increased as the flaps are going down, because speed is still relatively high and the AOA is often not being decreased fast enough. Then, when equilibrium is again reached, the nose of the airplane is usually lower than before flap extension, even if a constant altitude is maintained.

Normally much more down-elevator is required to keep from ballooning AS flaps are being extended, than in equilibrium with flaps extended. Therefore, I would use the amount of mix that gives me the desired glide trim in equilibrium. Making sure speed is already low before flap extension, and being ready with elevator correction minimizes ballooning when flaps are being extended.

RE: A question about flaps
 

> My experience is that I have to mix in UP elevator or the plane nose dives. <

That is my experience also.

RE: A question about flaps
 

Yep. The amount and direction of elevator correction depends on which moment is stronger: the increasing negative pitching moment of the airfoil as the camber is increased, or the positive pitching moment contributed by the tail influenced by the increased downwash as flaps are extended. In other words, it depends on the airplane and how it is flown.

A conventional airplane will almost always have a lower pitch attitude in level flight with the flaps extended.

The elevator may have to be trimmed either up, down, or not at all depending on the airplane, once equilibrium is re-established. Bonanzas require very little pitch trim change after flap extension (well designed), but they do try to climb a little like most planes AS the flaps are being extended. C310s require a large nose-down pitch trim adjustment when and after flaps are extended. It's possible the pitching moment can be strong enough with flaps extended, that the plane dives with flap extension, especially if the airspeed is high. That's why it helps to slow down before extending flaps. Anything's possible, it just depends on how the forces and moments are balanced.

RE: A question about flaps
 

Simply put, since the flaps are behind the CG, and when lowered they tend to push the trailing edge of the wing up / the plane dives. Up elevator counteracts the flaps to fine tune the rate of descent / speed / angle of attack. If you are getting too steep a descent, don't use so much flaps, you might run out of elevator at the wrong time and place.

RE: A question about flaps
 

In most of my warbirds, deploying the flaps (not flaperons) would make the plane balloon and the nose pitch up. "Down" elevator is required to make the plane fly level at low speed.

RE: A question about flaps
 

flaps don't push -or pull - they are just reshaping the wing - changing it's angle of attack

RE: A question about flaps
 

Bingo... for the most part. But altering the camber can change the pitching moment. On some models with barely adequite tail volume this can result in odd elevator needs to balance the new angle of incidence and pitching moments.

Anyway, full span flaperons is a bad idea. Do a search for many past threads about this and the adverse yaw that occurs.

RE: A question about flaps
 

This explanation doesn't use changes in pitching moments, which is really what is going on, and it doesn't account for airplanes that pitch up with flap extension. There are a few, though it is less common.

RE: A question about flaps
 

My Kadet will run out of down elevator, with full-flap and full power. The pitchup is uncontrollable. The only way to stop it is to pull off power, then correct with the elevator.
A modest amount of down, either put in by mixing or by thumb is normal for a flapped plane at low speeds.

RE: A question about flaps
 

That's what I would expect. I didn't write my last post carefully enough (I was referring to net pitch change after equilibrium is restored, which is usually negative). I mean to distinguish between the pitching moment change with flap extension, from the momentary lift change if attitude is not adjusted with elevator as flaps are extended. An airplane will usually climb a bit while flaps are being extended if not corrected. But the pitching moment change depends on several factors and can be nose down even while the airplane is "ballooning" with flap extension.

An airplane can be designed in such a way that if flaps are extended at the proper speed, the pitching moment change is approximately balanced by the change in downwash over the tail, so that little or no pitch trim adjustment is needed with flap extension (the Bonanzas I mentioned earlier, and the Barons I did not).

I cannot think of an airplane I have flown that does not have a lower deck angle maintaining altitude with flaps extended than it does with flaps retracted, though most of them try to climb a bit momentarily when I extend the flaps, and most require nose-down trim after flap extension. Applying full power causes the nose to rise sharply IAW your experience because of the increase in downwash, and is the reason for the elevator trim stall demo in training curricula.

RE: A question about flaps
 

The pitch-change effect with flap extension is more dependent upon the change in airflow over the tail than anything else. High-wing Cessna aircraft have a strong pitch-up tendency with flap extension, even at the proper airspeed. The low-wing Piper singles have a pitch-down tendency with flap extension.

RE: A question about flaps
 

One might surmise from the above observation that the low wing on the Piper influences the tail less with flap extension than on the Cessna. If downwash is the single most important factor (or the only important factor), then why would the Pipers pitch down? A reduction in the downwash force on the tail would seem to simply reduce the amount of pitch up (because the downwash effect is only reduced, not eliminated) unless there is another moment opposing it, like an increase in negative pitching moment, which is well known to occur with an increase in camber. Even if the downwash effect on the tail was totally eliminated by the low wing position, the downward pitching of the Pipers is evidence of an increase in the negative pitching moment. Since this is a fundamental and well known force in aerodynamics, why discount it?

RE: A question about flaps
 

I don't suppose this will come as a surprise to anyone but in case anyone is interested:

This morning I was out flying a C172 around in smooth air and I took a few measurements.

I trimmed for 90 knots in level flight with flaps up and zeroed out the attitude indicator. Then I took my hand off the yoke and quickly extended 10 degrees of flaps. The plane pitched up to 35 degrees, then stalled and recovered. I did this twice with the same results.

Then I did the same thing except I forced the airplane to maintain altitude. When the airspeed stabilized again, I was going 83 knots, and the pitch attitude was 3 degrees nose low. I did not change the power setting. Naturally, if I had increased power (airspeed), I would have had to decrease the pitch attitude further, and conversely if I had reduced power, I would have had to raise the nose to maintain altitude.

Then I extended full flaps (30 degrees). Initially of course I had to lower the nose to around 5 degrees below horizon to keep from climbing, but as the speed decreased to and stabilized at 62 knots, (again without changing the power setting), the pitch attitude was again at the same pitch attitude as at 90 knots with flaps up.

At 62 knots with flaps up, maintaining altitude, the nose is much higher than it is with the flaps extended, and of course requires much less power.

RE: A question about flaps
 

Wait, you had a 172 trimmed for 90Kts. and extended 10* of flaps and it pitched up 35* and stalled? Um, the 172 has a trim change with change in flaps, but I've never experienced anywhere even in the ballpark of 35*, and I've dang sure never had one stall from flap extension, same goes for any of the 8 or so other planes I've flown. My students fail to apply a little forward pressure when they extend flaps all the time, and the plane doesn't even pitch up to the normal climb attitude. I think something was majorly off with your experiment there, but I'm going to try those things out to get my own comparison there.

RE: A question about flaps
 

The amount of pitch up surprised me a little too, as I had never not applied any pitch correction before. I would have guessed on the order of 20 degrees or so. Your mileage may vary, as the CG position may affect how much it pitches up. Remember, I took my hand off the yoke entirely until after the stall recovery. Pitch went off the scale - I had to extrapolate the last 10 degrees. Not that this should make any difference, but it is a CAP 172 with 180 hp. Also, keep in mind that the power and trim were set to maintain altitude at 90 knots - about 2150 RPM - which is more power than normal for 10 degrees flap extension, and a little lower than normal speed. Ususally the RPM is down around 1900 - 1700 before flaps are lowered, at least the way I fly and was taught. The amount of power applied will have a profound effect on the amount of pitch up with flaps. I'm sure you have done trim stall demos.

BTW, I'm a 4600+ hour ATP with CFI/II/MEI and I have flown more than two dozen types and you might want to replicate an experiment before you say something's wrong with it. The amount of pressure it takes to keep the nose down is not great, but in my experiment, NO pitch correction whatsoever did indeed result in a 35 degree pitchup. I did use the rudder to keep heading. If I had not used rudder, the nose would have gone left, the wings would have banked, and the pitchup would not have been quite as great. Oh yes, I was solo, took off with full fuel, with 30 lbs in the back seat, and another 30 lbs in the baggage area.

I am interested to read your results.

<edit: changed "last 5 degrees" to "last 10 degrees". Measurement is unchanged.>

RE: A question about flaps
 

I'll try that on my flights today. I really don't see how that could work considering the normal amount of forward pressure required to keep the nose leve while extending flaps is really light, and lasts but a couple seconds. It takes a good amount of time or a lot of force on the elevator to get the nose up to 35*, not to mention the fact that as soon as the speed goes down, the elevator trim would allow the nose to drop on it's own some to maintain speed. I've got a couple flights coming up today, so I should be able to try that out.