Aileron to flap alignment

[Anticept]

What is the purpose of the flaperons? We know they reduce roll effectiveness at higher flap settings, so there must be an advantage to make that a worthwhile trade off. Do they increase the effectiveness of the flaps by giving some additional "virtual" flap area?

 

Full length flaps increase lift a lot more than just inboard flaps. However, they are heavy on the controls, and decrease roll effectiveness. Having secondary flaps, or flaperons in this case, means we meet in the middle in terms of inboard flaps vs full length flaperons. The big boys have so much wing surface area, that they can use secondary flap systems which are actually multiple flap segments!

[Doug G.]

Ok, I am still trying to figure out this flaperon idea. I realize that the ailerons move (" droop") with the movement of the flaps, but, as I understand flaperons, the flaps should move in conjunction with the aileron movement. Am I wrong in this understanding, or have I simply not seen my flaps moving from the place they are set?

If the flaps do not move with the aileron movement, isn't the differential description a better one?

[Tom Baker]

The flaps are just flaps. The ailerons move with the flaps, so they are flaperons. The reason they do this is to get a slower stall speed. The reason some wanted this removed is with flaps down and a correction for a crosswind you a very different angle of attack between the two wing tips. As you near stall one wing will quit flying quite a bit before the other dropping the wing and creating a big problem if you are not right at ground level.

[sandpiper]

This system is nothing new and is not unique to the CT. I don't know when it was first used but it probably predates WW II. My first experience with it was with DHC-2 Beavers which were manufactured in the 1950's. Probably the best know to GA is the Robertson STOL kit that was (is) available for many Cessnas and Pipers , probably others. Robertson conversions were pricey but effective. Like the CT, they did cause some cross wind control problems for some pilots, especially on the C-180/185, not so much for the C-206.

[Ed Cesnalis]

In the light sport world flaperons are sometimes used for simplicity. My Challenger had full span flaperons which saves weight and complexity.

 

You also see "junker" flaperons on light designs.

 

 

[Znurtdog]

I wonder if having no aileron interconnect could eliminate the mixer complexity and possibly make for better control feel ie lighter.Might lose some weight too.

[Ed Cesnalis]

I wonder if having no aileron interconnect could eliminate the mixer complexity and possibly make for better control feel ie lighter.Might lose some weight too.

 

Its not like the flaps and flaperons resist when you move the stick, more like the flaps and flaperons resist when the flap motor is actuated.

[FredG]

Tom B., you wrote: "The flaps are just flaps. The ailerons move with the flaps, so they are flaperons. The reason they do this is to get a slower stall speed. The reason some wanted this removed is with flaps down and a correction for a crosswind you a very different angle of attack between the two wing tips. As you near stall one wing will quit flying quite a bit before the other dropping the wing and creating a big problem if you are not right at ground level."

 

I am having trouble understanding this. GIven that 1) the ailerons (flaperons) move in the same direction and in the same amount for each added increment of flaps (ie. the left and right sides are affected equally at any given flap setting) and 2) the total aileron range of travel from full up to full down is not affected by flap setting (ie, the full range of aileron travel is available at all flap settings), I am unable to understand how this mechanism creates greater differences in angle of attack (ie cross control) than an equivalent airplane with ailerons that have no linkage to the flaps (ie, true ailerons rather than flaperons). Can you help me with this?

 

Thanks,

Fred

[Tom Baker]

Fred, go out to your airplane and lower the flaps all the way. Now push the stick to the Left until the aileron is lined up with the wing tip. Now look how far down the Right aileron is. The cord line is a line from the leading edge of the wing to the trailing edge, and this line changes with application of flaps or aileron. Now think about the relative wind being parallel to the ground. The left wing has 0° angle of attack, and the Right wing has maybe 4°-5° angle of attack because of the aileron being down. A couple different things happen on landing because of this. One because of induced drag the airplane wants to turn away from the crosswind and this force increases as you increase angle of attack. The other is the down wind wing has a higher angle of attack and as you approach the stall it will drop sooner and away from the crosswind. The first one is likely the biggest factor because when the wing stalls you also have a bunch of asymmetrical drag.

In an airplane with out flaperons you will get the difference in angle of attack, but the ailerons often have differential deflection. The one that goes up travels farther to create drag to help the airplane turn. Because of this you don't have the problem with asymmetrical drag.

[Ed Cesnalis]

For my CTSW the fix I would like would be a reversal of the drooping when going from 30 to 40 degrees.

[Tom Baker]

There are some who would like to do away from the flaps drooping altogether. I guess for the guys over in Europe on the 300 meter strips need that little extra the drooping the flap gives them.

[sandpiper]

For my CTSW the fix I would like would be a reversal of the drooping when going from 30 to 40 degrees.

 

As I recall, the Robertson conversion on the C-185, does similar. C-185 has 4 flap positions (other than zero) at 10, 20, 30 and 40 degrees. With the Robertson, 10 and 20 degree positions include the ailerons drooping but when you go to 30 degrees, the Robertson preferred T/O position, the ailerons retract a little bit. I don't remember what happens with 40 degrees.

 

Because of what Tom describes, more than one C-180/185 got rolled up in a ball.

[FredG]

Hi Tom, thanks for your reply.

 

I understand why cross-control (a sideslip in the case of landing with a crosswind) results in a difference in the angle of attack of the wings. By this I mean (and I think you are saying) that downwind wing with the aileron deflected downward has a greater angle of attack than the upwind wing with the aileron deflected upward. Hence, the downwind wing will reach the critical angle of attack first, and lose lift first (and drop first). I also understand that any use of ailerons results in adverse yaw - the wing with aileron deflected down has more lift and, hence, more induced drag than the wing with the aileron deflected up (reduced somewhat in the CT because of differential flap movement). These aerodynamic phenomena occur in planes with ordinary ailerons and ordinary flaps, however.

 

What I don't understand is how having symmetric downward deflection of the ailerons when the flaps are deployed (making them flaperons) affects these aerodynamic phenomena. If I read your first post correctly, I thought you were saying that some pilots want to eliminate the flaperon feature and have only ailerons that are unlined to flap deployment in order to limit differences in the angle of attack between the wings that occur during a sideslip on landing in a crosswind (or that, somehow, the flaperon arrangement makes this problem worse).

 

Given the symmetry of flaperon displacement with flap use, I don't see how the flaperon arrangement changes (worsens) the tendency for the downwind wing to stall first during a sideslip, in comparison to what would happen with ailerons that are not linked to flap movement. Basically, what I am saying is that eliminating the flaperon linkage to flaps (making them into ailerons) won't affect the differential angle of attack issue that occurs during a crosswind sideslip landing.

 

Thanks again for discussing this.

fg

[Ed Cesnalis]

I think there are many factors at play that may or may not add up to a wing tip stall. When we deploy flaps and flaperons we lose stability as the center of pressure moves towards the wing's root. Stalled or not if the lift develops a short moment arm that wing can drop.

 

AOA isn't the whole story, flaperons can add wash-in and with crossed controls the effect can become asymmetrical. With both flaperons drooped but one down and the other up the wing tip vorticies probably become asymmetrical as well.

 

Throw in rigging differences and other changes from being in service and the number of variables to consider grows even more.

 

I won't land with 40 degrees if it requires a lot of crosswind correction or if I do its with the throttle partially open. That margin for error has served me well for 7 years but it is no guarantee that it will continue to be adequate. Here's a good reason to practice stalls, to see where you are prone to drop a wing and then avoid that configuration when landing. If your CT develops a wing drop due to time in service or maintenance you can learn it altitude instead of the expensive way.

[Anticept]

Hi Tom, thanks for your reply.

 

I understand why cross-control (a sideslip in the case of landing with a crosswind) results in a difference in the angle of attack of the wings. By this I mean (and I think you are saying) that downwind wing with the aileron deflected downward has a greater angle of attack than the upwind wing with the aileron deflected upward. Hence, the downwind wing will reach the critical angle of attack first, and lose lift first (and drop first). I also understand that any use of ailerons results in adverse yaw - the wing with aileron deflected down has more lift and, hence, more induced drag than the wing with the aileron deflected up (reduced somewhat in the CT because of differential flap movement). These aerodynamic phenomena occur in planes with ordinary ailerons and ordinary flaps, however.

 

What I don't understand is how having symmetric downward deflection of the ailerons when the flaps are deployed (making them flaperons) affects these aerodynamic phenomena. If I read your first post correctly, I thought you were saying that some pilots want to eliminate the flaperon feature and have only ailerons that are unlined to flap deployment in order to limit differences in the angle of attack between the wings that occur during a sideslip on landing in a crosswind (or that, somehow, the flaperon arrangement makes this problem worse).

 

Given the symmetry of flaperon displacement with flap use, I don't see how the flaperon arrangement changes (worsens) the tendency for the downwind wing to stall first during a sideslip, in comparison to what would happen with ailerons that are not linked to flap movement. Basically, what I am saying is that eliminating the flaperon linkage to flaps (making them into ailerons) won't affect the differential angle of attack issue that occurs during a crosswind sideslip landing.

 

Thanks again for discussing this.

fg

 

Hi FredG!

 

Ailerons don't change the angle of attack of a wing. Contrary to popular belief, neither do flaps. In fact, they don't change the wing chord either! They change the camber, which leads you to a change AoA if you want to hold the same descent rate and airspeed. Yes I know the FAA teaches that these things do change AoA, but they've been wrong for decades, this isn't taught this way in aeronautical engineering.

 

Also, if you are forward-slipping and developing a skid (you can't hold a heading if you are skidding) then the lower wing stalls first, because that wing slows down, while the raised wing speeds up. However, in a properly executed slip, the raised wing will stall first. Proper slipping actually doesn't result in spins either. Usually a spin turns into a skid just before stall, and that is why the spin develops.

 

CharlieTango:

 

Another factor is that the fuselage in a severe slip, will decrease the effectiveness of the flap/flaperon/wing that is hiding behind the fuselage from the relative wind. That wing will see decreased lift, and the two wings will have air movement over them that is not head on, so this will greatly affect the vorticies. However, vorticies affect drag, not lift. Vorticies are the product of lift.

[FredG]

Anticept, thanks. I am familiar with the stall characteristics during slipping and skidding flight.

 

I was trying to understand what the thought process was for some pilots wanting to disable the flapperon feature for the reasons that Tom articulated.

 

Regarding AOA and flaps, I would like to understand this more. Clearly, alerons don't change AOA the way that pitch changes do. But, there does seem to be an AOA-like effect of a downward aileron (lift increases, drag increases, and the wing is closer to critical AOA and stalls before the wing with ailerons deflected upward). So, technically, while ailerons may not change the AOA, they affect flight in such a similar way as to an AOA increase that, for this discussion, we can think of it that way.

 

Thoughts?

 

fg

[Ed Cesnalis]

Anticept,

 

I agree, I said lots of factors are at play, I was including the direction of the relative wind even if I didn't list it.

 

When a wing drops lift has become asymmetrical and the product of asymmetrical lift will be asymmetrical vorticies. I said the vorticies would probably become asymmetrical I didn't say it wasn't the result of lift changing. I think that's my whole point, when that wing drops the one thing we know for sure is lift was effected.

 

Its hard to answer Fred's question for the reason that Tom stated. I too would prefer a change, the droop at 40 degrees produces quirky stall behavior. I would say it this way, flaps and flaperons reduce stability and in my CTSW that reduction in stability is a real issue at 40 degrees.

[Anticept]

Anticept, thanks. I am familiar with the stall characteristics during slipping and skidding flight.

 

I was trying to understand what the thought process was for some pilots wanting to disable the flapperon feature for the reasons that Tom articulated.

 

Regarding AOA and flaps, I would like to understand this more. Clearly, alerons don't change AOA the way that pitch changes do. But, there does seem to be an AOA-like effect of a downward aileron (lift increases, drag increases, and the wing is closer to critical AOA and stalls before the wing with ailerons deflected upward). So, technically, while ailerons may not change the AOA, they affect flight in such a similar way as to an AOA increase that, for this discussion, we can think of it that way.

 

Thoughts?

 

fg

 

There are two ways to change AoA: rotate the airfoil, or change the relative wind direction. Flaps and ailerons have no direct affect on AoA, and nor do they have any affect on a wing chord (in aeronautical engineering, we don't redefine wing chord by changing configurations, we only base it on cruise flight). The old twisting-wing style aircraft in the Wright Brothers days are wings that changed AoA to turn. With modern control surfaces (except spoilers and flaps), we just play on Newton's Third Law (equal and opposite reaction), with a tiny bit of help from Bernoulli. Flaps and spoilers however, play heavily with Bernoulli, and a little bit with Newton's Third Law.

 

Anyways, you can trigger a stall at very low speeds with ailerons because you are changing the wing camber, which can lead to increased or decreased lift, which in turn changes the relative wind, leading to a stall if you are already near the critical airspeed. You would need to make significant changes to aileron deflection though, because they don't have much affect for normal flight operations.

[FredG]

I get your point, you change AOA by rotating the airfoil (pitch change) or by a change in the relative wind.

 

This raises the next question, when the aileron deflects downward (because the stick is pushed to the opposite side), why does the airplane bank? Does the wing with the downward aileron deflection have more lift (I think yes)? Does it have more drag (I think yes)? And is it closer to its critical angle of attack angle for that configuration (I think yes)? If it is not closer to its critical angle of attack for that configuration, why does the wing with the downward deflected aileron stall first while in a slip?

 

Thanks,

Fred

[Ed Cesnalis]

Fred,

 

The wing with the downward aileron is prone to drop but you are asking why it stalls 1st, maybe it isn't stalling. With the change in lift and drag, and with the fuselage blocking wind and the relative wind not on the nose perhaps all that is happening is the center of pressure is moving inboard on that wing. It can drop due to the short moment arm.

 

After all you just agreed that the wing needs to rotate or the relative wind has to change in order to change the AoA. The wing's rotation and the relative wind with the exception of disturbance from the fuselage change in unison.

[Tom Baker]

Fred,

what's going on could be simply be due to the asymmetrical drag. The normal aileron set up is designed so that when on goes down the other goes up farther to keep drag equal. More surface exposed on the up aileron to counter the induced drag for the down aileron. We agree that whem you apply flaps the ailerons droop increasing induced drag on both ailerons. Now when you apply aileron you are increasing induced drag more for the one that is going down, but you are decreasing drag from the flaps deployed position on the aileron that moves up. With flaps deployed the up aileron never produces enough drag to compensate for the increase in induced drag on the aileron that goes down.

This increase in drag along with the low aileron wing stalling first could be what causes the problem.

[FastEddieB]

Does the wing with the downward aileron deflection have more lift (I think yes)?

 

Yes. That combined with the opposite wing having less is what banks the plane.

 

Does it have more drag (I think yes)?

 

Yes. Hence adverse yaw and the need for rudder. Whenever you increase lift you increase induced drag.

 

<<And is it closer to its critical angle of attack angle for that configuration (I think yes)?>>

 

Not sure there. The more cambered overall profile with the aileron down should make it a more efficient low-speed airfoil.

 

<<If it is not closer to its critical angle of attack for that configuration, why does the wing with the downward deflected aileron stall first while in a slip?>>

 

Let me get back to you on that, though I assume fuselage blanking is involved.

 

Also good to remember is that in a medium turn, ailerons should be neutral - that help defines a medium turn. In a steep turn you will typically be holding aileron against the turn to prevent over banking.

 

And also good to remember is that the "bible" for this sort of thing is available online - for free!

 

http://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/00-80T-80.pdf

[WmInce]

 

And also good to remember is that the "bible" for this sort of thing is available online - for free!

 

http://www.faa.gov/r...a/00-80T-80.pdf

 

In paperback, at Amazon - $10.88 By H.H. Hurt

[Ed Cesnalis]

With no reaction to my suggestion that maybe the wing dropping isn't from stalling I'll ask this. What makes you guys think that this quirky behavior that leads to a wing dropping is a stall?

[Anticept]

With no reaction to my suggestion that maybe the wing dropping isn't from stalling I'll ask this. What makes you guys think that this quirky behavior that leads to a wing dropping is a stall?

 

CharlieTango makes a valid point. We really should define a stall. There are varying degrees of a stall, and a stall is defined to first begin when the lift coefficient has peaked, and continuing to increase AoA is now stalling. You can still fly while your wings are stalled. It's when lift becomes less than weight, that we have the sudden drop that pilots are familiar with, since a stall becomes a positive feedback loop. What that means is, the wing starts to drop slowly because it can't support the weight with lift, steepening the stall, which means the wing drop accelerates quickly, until you lose control.

 

why does the airplane bank?

 

&

 

If it is not closer to its critical angle of attack for that configuration, why does the wing with the downward deflected aileron stall first while in a slip?

 

I couldn't post another quote block, so i'll start with airplane banking.

 

The major contribution to a bank in flight will take a moment to explain. First, when you are flight straight and level, all forces are equal. When you bank, you are decreasing your vertical lift for horizontal lift. Since you have lost some of your vertical lift, you need to pull back on the stick to maintain altitude. But when you do, you are apply that back pressure, you are also applying some additional horizontal "lift." What then occurs is you are creating a net inward force, towards the center of the turn. This is called centripetal acceleration. Pulling back on the stick also causes your aircraft to rotate around it's lateral axis, which means in a coordinated turn, the top of the plane is always centered with the center of the radius of the turn.

 

Think of it this way: When you are in level flight, and you pull back, you will do a loop (assume airspeed remains constant). When you bank and try to hold altitude, you have shifted the loop's peak partially into the horizontal axis. Now, if we consider gravity, which is trying to pull you to the ground, this further shifts your loop towards the horizontal plane. When you hold altitude constant, what you have found is the balance between the two competing vertical forces of lift and gravity, but your sideways momentum does not have a counter-balancing force, therefore your loop is now sideways. Does this help?

 

 

Now for this aileron effect you speak of:

 

The higher wing, not the lower wing, stalls first in a forward slip. The higher wing is gimped because it's being shrouded by the fuselage of the airplane, and you have more spillover at the tip, contributing less to lift.

 

Let me draw another scenario. Assume we are slipping to the right, in a forward slip. This means we are slipping to a landing, with the left wing up, and the right wing down. What we have done, is the spillover on the right wing is less, and it has full exposure to the relative wind. The left wing however, is partially shrouded by the fuselage, and has more wing spillover. If we maintain this slip properly and stall, then you will not spin, you will only drop (this is because the two wings are still generating equal lift), or the higher wing will stall, forcing a roll out.

 

However, when you skid, the lower wing stalls first, because in a skid, it means the tail is trying to move towards the relative wind. If we are right-forward-slipping (right wing down, left wing up) and begin to skid, it means the nose is going to bear right, and the tail will bear left. Your right wing, the lower wing, is going to stall first, because it is not travelling as far as the left wing, and therefore is moving slower. Since gravity is already trying to pull the nose down and right, this affect will turn into a positive feedback loop in a stall, and possibly pull you into a spin.

 

I will repeat what I've previously said: ailerons will have nearly zero direct affect on the wing's stalling characteristics. If you try to bank close to a stall, and one wing drops from a stall, it's because you were uncoordinated. If both stall evenly, it's because the load factor became too great, causing the wings critical AoA to be passed.