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Differential Aileron

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Old 06-29-2004, 12:20 PM
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jagnweiner
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Default Differential Aileron

I hope this question makes sense. I am just finishing a GP 60 size Cub. It has a servo on each side of the wing for the ailerons. Radio is a Futaba 6XAS. The instructions say to adjust the servo arms to set the ailerons up for less travel down than up (presumably to reduce adverse yaw).

I assume this means the arm should not be straight up and down when the aileron is in neutral position. My question is this: Should the servo arm be more forward from vertical when the aileron is in the neutral position? Or should it be more to the back? My gut tells me more forward, but I'm not sure why. Or maybe I'm missing it completely.

Also, is there a way I can set this up on my radio? In retrospect, I'm thinking maybe I can, since each servo is on a separate channel.

-Scott
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Old 06-29-2004, 12:34 PM
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Default RE: Differential Aileron

Scott,
Assuming your aileron horn is on the bottom of the wing, your intuition is correct. Mount the servo arm rotated towards the front of the wing. It can also be programmed into computer radios, but some of us who started this hobby in "the old days" still like to set things up mechanically.
Chuck
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Old 06-29-2004, 12:38 PM
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jagnweiner
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Default RE: Differential Aileron

Chuck-

Wow, that was fast! Thanks for the reply. I agree, I'd like to start out getting it close mechanically, then I can fine tune with the computer as necessary.

-Scott
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Old 06-30-2004, 12:07 PM
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Kaos Rulz
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Default RE: Differential Aileron

Scott,

Set it up as close as you can mechanically, then go to the ATV function in the programming mode for each channel and adjust the down side for correct throw. The same procedure will work for the up function should you need to do so.

Good Luck
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Old 06-30-2004, 04:03 PM
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leftnut
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Default RE: Differential Aileron

i've been wanting to ask this question.
can someone please define the term aileron differential for me.
i read about it on some other thread... it's confussing me.
now this thread..

i've always thought (servos return back to center) depending how
the airleron's are... that would effect the pitch of the plane and air foil.
point the plane up to test straight veritcles.

if set too low on an under powered, over size ailerons.
the plane would yaw and loose altitude from drag ?

i always try to set linkages/ horns at 90/90 no matter where it's at.

but... what the heck is aileron diff??[sm=spinnyeyes.gif]
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Old 06-30-2004, 10:25 PM
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Default RE: Differential Aileron

Just means the amount of up deflection and down deflection are "different". You need more up than down. say 1/2" up, 1/4" down. A down deflected aileron creates drag, tends to make the plane yaw in the direction of the down aileron. This is a momentary condition, as the increased lift will soon overcome the drag, it just makes turns sluggish. If you were seated in the plane, you would feel the slip in the seat of your pants.

The easiest way to visualize this is to look at the plane from the front, at an angle slightly below the wing (because the plane flies at a positive angle of attack). If you give full right aileron, you can barely see the up right aileron, but you can see the down left aileron completely, and this is what the "relative wind" also sees. It's like a huge flap hanging down.
Perhaps a better way of describing the action is to say the wing with the down aileron slows down, at a time when it needs to speed up in the turn. As the turn progresses the slow wing tries to play catch up and rushes forward, sometimes resulting in over banking. It just ain't smooth.

Proper turns also require coordinated rudder. The rudder does not create the turn, it merely helps to counteract the adverse yaw in the wrong direction. Piper Cubs in particular are said to be hard to turn without differential and rudder. With today's modern computer radios, you can mix in the rudder for turns, but this is not a proper way to learn correct flying techniques. Years ago, all trainers were 3 channel, and the use of the rudder to "turn" created the wrong concept for the pilot. Even then, the rudder didn't make the turn, it simply created yaw, making the outboard wing move faster, and the relative wind was then able to act on the dihedral, and the turn resulted. Etch in your mind...Rudder does not turn the plane.

It is best to set up the differential at the servo by having the linkage point forward of the servo center. (On a high wing) Then when the servo pulls the rod for down aileron, the path of the servo arm connection point is an arc instead of a "straight" line. Setting the servo arm on the spline about 30 degrees forward of the center line is usually sufficient.

Bob
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Old 07-01-2004, 10:18 AM
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Default RE: Differential Aileron

Was just gonna post about this subject. Wow, lot of info there to soak up. Im working on a PT-40 trainer, which have only one servo for the ailerons. Just gotten hold of an non-computer 4c radio (Hitec Laser 4). It does have ATV on channel 1 and 2 (ail./rud.), and im guessing i should set up differential on my ailerons too. Would love to have some automated rudder-input (similar to auto-rudder in MS Flight Sim i guess ) too, but i guess thats not good learning.

Anyways, i get how you can set like 1/2 up and just 1/4 down on a computerradio and when the ailerons are operated by two seperate servos, but how do you (if you can) do this on a single-servo aileron wing?
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Old 07-01-2004, 10:31 AM
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Default RE: Differential Aileron

ORIGINAL: brsseb
Anyways, i get how you can set like 1/2 up and just 1/4 down on a computerradio and when the ailerons are operated by two seperate servos, but how do you (if you can) do this on a single-servo aileron wing?
In the end of the manual for the PT-40 there's instructions on how to setup the single aileron servo with differential. Using a wheel horn on the servo, just mount each aileron pushrod 30 degrees up from the center line. Have a look in the back of your book where they do the servo mounting, it's pretty clear there.
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Old 07-01-2004, 11:05 AM
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Default RE: Differential Aileron

Note to self: Next time it says "read though the whole manual before starting assembly" in the start of the manual, ill might actually do just that
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Old 07-01-2004, 12:50 PM
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Default RE: Differential Aileron

ORIGINAL: newbieT

ORIGINAL: brsseb
Anyways, i get how you can set like 1/2 up and just 1/4 down on a computerradio and when the ailerons are operated by two seperate servos, but how do you (if you can) do this on a single-servo aileron wing?
In the end of the manual for the PT-40 there's instructions on how to setup the single aileron servo with differential. Using a wheel horn on the servo, just mount each aileron pushrod 30 degrees up from the center line. Have a look in the back of your book where they do the servo mounting, it's pretty clear there.
Is aileron differential beneficial on most/all aircraft? It sounds like it from a previous (bojangle's) post. I have a WM trainer and am curious if it would make a noticable difference. It has a single servo in the bottom of the wing that is attached to torque rods on the strip ailerons.

Thanks,
dano13
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Old 07-01-2004, 01:22 PM
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Default RE: Differential Aileron

[/quote]
Is aileron differential beneficial on most/all aircraft? It sounds like it from a previous (bojangle's) post. I have a WM trainer and am curious if it would make a noticable difference. It has a single servo in the bottom of the wing that is attached to torque rods on the strip ailerons.

Thanks,
dano13
[/quote]

Depends on the airplane design. High wing, Cub-like scale ships and trainers tend to require some rudder to make a coordinated turn. In a Cub, when you roll in aileron, the drag created by the down aileron yaws the plane in the opposite direction to the desired turn. This is called yaw coupling-yaw results from the roll. The rudder is not there to make the plane turn as much as it is there to stop the plane from turning (the wrong way). Diffy ailerons reduce this yaw coupling and reduce the need for rudder input to make a coordinated turn.

Now, if you fly a high-performance low- or mid-wing design (like a pattern ship), it will roll with ailerons and show little if any yaw coupling. So the diffy ailerons aren't needed. Any bits of yaw or roll coupling can be dialed out with a computer mixing radio.

On your trainer, it sure won't hurt to have diffy. It might not be noticeable, but then it might. Try it-it may fly a little nicer.
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Old 07-01-2004, 02:47 PM
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Default RE: Differential Aileron

So, correct me if im wrong, this is a problem on planes with non-symmetric wing profile (where the aileron poiting upwards are not that visible as the one pointing downwards, and you get extra drag on one side), while planes with a more symmetrical wing profile you dont have this problem. Right?
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Old 07-01-2004, 03:47 PM
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Default RE: Differential Aileron

kdheath, thanks for the info I may give it a try next time out.
I am also in the process of building a cub, and the plans/instructions do call for aileron differential. But it uses two wing mounted servos. Should be no problem there. My radio is non-computer so I guess I will have to do it the "old fashioned" way.

Thanks again for the info,

dano13
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Old 07-01-2004, 11:52 PM
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Default RE: Differential Aileron

The topic is about differential (on a Cub), and I indicated that diffy was helpful on a Cub, or most high wing trainers. These planes fly slower, therefore at a higher angle of attack where down aileron creates the most drag.

High performance planes fly at a lesser angle of attack so there is somewhat less yaw coupling. The yaw still exists but not as noticeable. Usually (though not always) pattern planes are flown by those who have learned to use coordinated rudder. Also you would not want diffy if you plan to fly inverted.

The yaw occurs on all wing profiles but it is more pronounced on a flat bottomed trainer. Some high wing trainers have symmetrical airfoils to allow a student to practice some aerobatics including inverted flight. Again, you would probably not want diffy on these planes.

Bottom line...Is diffy absolutely needed? No. Is it helpful? For a beginner, it's a mixed bag. It will help make smoother, more scale like turns, but it should not negate proper use of rudder in turns.

Bob
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Old 07-02-2004, 03:47 AM
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Default RE: Differential Aileron

I trust these words of experience applies to bi-planes too? I am building a 1/5 Waco and never heard of this. Makes since!! Should I???
Donnie
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Old 07-02-2004, 04:02 AM
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Default RE: Differential Aileron

Sorry,,, thinking about it.... The wing that needs to go faster needs to have the least drag,,,, right??
Thats what makes you turn!
Donnie
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Old 07-02-2004, 05:31 AM
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Default RE: Differential Aileron

Ok, thanks for clearing it up. Ill be sure to add some diff. to my trainer. It got abit of ditheral on a flatbottom wingprofile, so it wont do much inverted flight anyway.
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Old 07-02-2004, 05:22 PM
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Default RE: Differential Aileron

thanx Bo !!
i'm still[sm=spinnyeyes.gif] but it ain't from aileron diff no more.
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