why have aileron differential?

Steph280 · Sep 24, 2001, 01:25 PM

Sorry if this question sounds ignorant, but what is the purpose of aileron differential? I've never had a plane that required it, until I started to fly electric gliders.

I am wondering if it has anything to do with the problem I had this weekend with my Jerry (see post "Aileron Problems").

TIA for any pointers.

Stephen

Bill Glover · Sep 24, 2001, 01:49 PM

This gets raised regularly, if you search on 'differential' I'm sure you'll find lots of hits.

Basically, differential aileron is required to offset adverse yaw. Adverse yaw is where the downgoing aileron creates more drag than the upgoing one (this effect can be caused by lots of factors like wing section, airspeed, angle of attack, etc.). So for example the plane rolls right, but yaws left (as if you'd input left rudder at the same time as right aileron). If you have dihedral, this left yaw is converted to left roll - opposing the right aileron you've input. This will reduce the roll rate ... in extreme cases you can get a model that rolls in the opposite direction to your control input (not nice). Typically this is at low speed/high angle of attack i.e. right after launch! Note that this only happens if you have dihedral, otherwise the roll rate won't be affected (though the model will look a bit 'odd' in turns as it yaws in the opposite direction).

Differential movement makes sure that the upgoing aileron causes as much drag as the downgoing one (or more). The amount that's needed varies depending on the aerodynamics of the individual model.

DICKEYBIRD · Sep 24, 2001, 01:52 PM

Aileron differential throw (in most cases, less down aileron movement than up) is used to counteract the yawing effect of the induced drag of the wing with the aileron deflected downwards in a turn.
ie: left roll command/right wing aileron goes down, right wing induced drag pulls right wing back, aircraft yaws right requiring input of left rudder to keep fuse flying tangent to the desired turning circle.

If you reduce the "down" aileron throw by a goodly amount, the induced drag is reduced as well, allowing smoother turns....until you roll inverted!

My $00.02

Bill Glover · Sep 24, 2001, 02:06 PM

Quote:

Originally posted by DICKEYBIRD
....until you roll inverted!

Aileron differential still works correctly when inverted, remember you're effectively flying a model with anhedral (which responds in the opposite direction to yaw) when upside down.

Steph280 · Sep 24, 2001, 02:46 PM

Thanks for all the responses. Yes I should have done a search before posting the question. Sorry

this makes the problem I had on my jerry interesting. When I apply right aileron, it begin slowly, and then all of sudden banked so hard that it almost did a roll, as though if the wing lost lift all of sudden. Correcting it by applying left aileron did the samething but to the left.

Since I have "down = 1/2up" differential, does that mean I have too much drag on the right aileron when I apply right command, which causes the right wing to loose so much lift that it just falls out of the sky? The wing does not have any dihedral. I didnt notice any yaw when this happened, but then again the plane was too far away to see if there were any yaw.

Your inputs are greatly appreciated.

Bill Glover · Sep 25, 2001, 05:15 AM

Assuming that there's no problem with the servos or linkages (if you have sloppy or 'sticky' control runs you can get the symptoms you describe) I would definitely try reducing the aileron differential. It's unusual to need that much on a design that has no dihedral.

Bernie Wolfard · Sep 25, 2001, 06:54 PM

Aileron differential should be more like 1/2 down = 1 up (aileron direction, not wing movement direction), not the other way around. The reason for differential is that lift and drag are related, more lift = more drag, less lift = less drag. The aileron that lifts the wing (down aileron) also increases that wings drag, the aileron that lowers the opposite wing decreases that wings drag. The result is yaw opposite the desired turn direction. If you are set up with 1 down = ½ up (direction of aileron movement, not wing movement) then the adverse yaw will probably overpower any turning moment of the roll response and also cause violent tip stalls.

1/2 down = 1 up (direction of aileron movement, not wing movement) is not a bad place to start. All planes should have some differential but it is more important for gliders because coordinated turns are essential to efficient flight and the rudder needed to counteract adverse yaw is just more drag.

Steph280 · Sep 25, 2001, 07:01 PM

Sorry I didn't word it correctly.

When I said "down = 1/2 up", I meant the down throw is half of the up throw. For example, if I have 40 degrees up, then my down will be 20 degrees.

Besides, the ailerons on the Jerry is hinged that the down can only get 20 or so degrees.

I will try to reduce the overall throw on my next attempt. probably use 60% dual rate or something.

Thanks.

Sep 24, 2001, 01:25 PM	#1
Steph280 1.21 Gigawatts! Join Date: Oct 2000 Location: Irvine, CA, USA Posts: 5,440	why have aileron differential? Sorry if this question sounds ignorant, but what is the purpose of aileron differential? I've never had a plane that required it, until I started to fly electric gliders. I am wondering if it has anything to do with the problem I had this weekend with my Jerry (see post "Aileron Problems"). TIA for any pointers. Stephen

Sep 24, 2001, 01:52 PM	#3
DICKEYBIRD Registered User Join Date: Jun 2000 Location: COLLIERVILLE, TN USA Posts: 617	Differential Aileron differential throw (in most cases, less down aileron movement than up) is used to counteract the yawing effect of the induced drag of the wing with the aileron deflected downwards in a turn. ie: left roll command/right wing aileron goes down, right wing induced drag pulls right wing back, aircraft yaws right requiring input of left rudder to keep fuse flying tangent to the desired turning circle. If you reduce the "down" aileron throw by a goodly amount, the induced drag is reduced as well, allowing smoother turns....until you roll inverted! My $00.02

Sep 25, 2001, 06:54 PM	#7
Bernie Wolfard Castle's Chaos Corner Join Date: Jun 2001 Location: Olathe, KS (next to the Castle) Posts: 1,383	Backward differential? Aileron differential should be more like 1/2 down = 1 up (aileron direction, not wing movement direction), not the other way around. The reason for differential is that lift and drag are related, more lift = more drag, less lift = less drag. The aileron that lifts the wing (down aileron) also increases that wings drag, the aileron that lowers the opposite wing decreases that wings drag. The result is yaw opposite the desired turn direction. If you are set up with 1 down = ½ up (direction of aileron movement, not wing movement) then the adverse yaw will probably overpower any turning moment of the roll response and also cause violent tip stalls. 1/2 down = 1 up (direction of aileron movement, not wing movement) is not a bad place to start. All planes should have some differential but it is more important for gliders because coordinated turns are essential to efficient flight and the rudder needed to counteract adverse yaw is just more drag.

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Sep 24, 2001, 01:49 PM	#2
Bill Glover Registered User Join Date: Nov 2000 Location: Bracknell, Berks, UK Posts: 8,291	This gets raised regularly, if you search on 'differential' I'm sure you'll find lots of hits. Basically, differential aileron is required to offset adverse yaw. Adverse yaw is where the downgoing aileron creates more drag than the upgoing one (this effect can be caused by lots of factors like wing section, airspeed, angle of attack, etc.). So for example the plane rolls right, but yaws left (as if you'd input left rudder at the same time as right aileron). If you have dihedral, this left yaw is converted to left roll - opposing the right aileron you've input. This will reduce the roll rate ... in extreme cases you can get a model that rolls in the opposite direction to your control input (not nice). Typically this is at low speed/high angle of attack i.e. right after launch! Note that this only happens if you have dihedral, otherwise the roll rate won't be affected (though the model will look a bit 'odd' in turns as it yaws in the opposite direction). Differential movement makes sure that the upgoing aileron causes as much drag as the downgoing one (or more). The amount that's needed varies depending on the aerodynamics of the individual model.

Sep 24, 2001, 02:46 PM	#5
Steph280 1.21 Gigawatts! Join Date: Oct 2000 Location: Irvine, CA, USA Posts: 5,440	Thanks for all the responses. Yes I should have done a search before posting the question. Sorry this makes the problem I had on my jerry interesting. When I apply right aileron, it begin slowly, and then all of sudden banked so hard that it almost did a roll, as though if the wing lost lift all of sudden. Correcting it by applying left aileron did the samething but to the left. Since I have "down = 1/2up" differential, does that mean I have too much drag on the right aileron when I apply right command, which causes the right wing to loose so much lift that it just falls out of the sky? The wing does not have any dihedral. I didnt notice any yaw when this happened, but then again the plane was too far away to see if there were any yaw. Your inputs are greatly appreciated.

Sep 25, 2001, 05:15 AM	#6
Bill Glover Registered User Join Date: Nov 2000 Location: Bracknell, Berks, UK Posts: 8,291	Assuming that there's no problem with the servos or linkages (if you have sloppy or 'sticky' control runs you can get the symptoms you describe) I would definitely try reducing the aileron differential. It's unusual to need that much on a design that has no dihedral.

Sep 25, 2001, 07:01 PM	#8
Steph280 1.21 Gigawatts! Join Date: Oct 2000 Location: Irvine, CA, USA Posts: 5,440	Sorry I didn't word it correctly. When I said "down = 1/2 up", I meant the down throw is half of the up throw. For example, if I have 40 degrees up, then my down will be 20 degrees. Besides, the ailerons on the Jerry is hinged that the down can only get 20 or so degrees. I will try to reduce the overall throw on my next attempt. probably use 60% dual rate or something. Thanks.