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Aileron set-up

Old 03-26-2016, 12:42 AM
  #1  
silent flyer
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Default Aileron set-up

I have a question;
If a model bi-plane is fitted with a 'Y' lead for aileron control and the control movement is supposed to be 10 mm each way.
What should the control movement be if the 'Y' lead is replaced with individual servo leads and differential is introduced?
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Old 03-26-2016, 12:53 AM
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fhhuber
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The same initially.

Then dial the differential as desired.
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Old 03-26-2016, 12:55 AM
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JetPlaneFlyer
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It's not an exact science but if you keep the upgoing aileron at 10mm and reduce the throw of the downgoing one that should be fine as a stating point. You can adjust throws after flying to suit your preference.

Bear in mind that differential makes the ailerons more effective even though the travel of the down aileron is reduced slightly.
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Old 03-26-2016, 02:58 AM
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firemanbill
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I generally seem to find that with a little bit less down they fly better. I tried to avoid a y harness on anything if at all possible.
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Old 03-26-2016, 02:42 PM
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Bald Paul
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Aileron differential can also be built in mechanically by adjusting how your linkage connects to the servo arm.
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Old 03-26-2016, 02:48 PM
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JetPlaneFlyer
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Originally Posted by Bald Paul View Post
Aileron differential can also be built in mechanically by adjusting how your linkage connects to the servo arm.
Very true, and also by offsetting the horn on the surface in front of or behind the hinge line.

That's how it had to be done 'back in the day' before computer transmitters.
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Old 03-26-2016, 03:56 PM
  #7  
silent flyer
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Default Aileron set up

Thanks to fhhuber, JetPlaneFlyer, baldpaul and firemanbill for your inputs.
The model is a foamie Wots Wot and I understand they fly well, but fancied experimenting with flaperons. Then thought go one better having put in the independent servo wires and try differential, but how much.
All been very helpful.
The manufacturers/designer already suggest a rudder mix with 30% up elevator as the rudder is so powerful.

So one or two more mixes to play about with.

Thanks again for the advice.
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Old 03-26-2016, 04:02 PM
  #8  
JetPlaneFlyer
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Originally Posted by silent flyer View Post
The manufacturers/designer already suggest a rudder mix with 30% up elevator as the rudder is so powerful.
Otherwise known as a knife edge mix, as it is used to get the plane to fly straight in knife edge without having to correct with elevator. 30% seems a lot but each plane is different. Most aerobatic planes also benefit from some rudder to aileron mix to trim out any rolling tendency when rudder is applied.
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Old 03-26-2016, 06:47 PM
  #9  
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Originally Posted by JetPlaneFlyer View Post
Otherwise known as a knife edge mix, as it is used to get the plane to fly straight in knife edge without having to correct with elevator. 30% seems a lot but each plane is different. Most aerobatic planes also benefit from some rudder to aileron mix to trim out any rolling tendency when rudder is applied.
I'd not come across this before in instructions, so, because I trust Chris Foss' stuff, will be using it.
As the elevator movement suggested is only10mm each way and the rudder is at 50mm, it is actually very small amount of movement.
Regards the aileron/rudder mix, I can add that in at a later stage if the flaperon mix really isn't required. Only have two optional mixes on DX6i Tx.

Cheers
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Old 03-26-2016, 07:20 PM
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I was forgetting that the Wots Wot is a biplane.. Yes, they often need a lot of elevator mixed with rudder. I've got a Precision Aerobatics Ultimate AMR, it's a roughly similar layout to the Wots wot and it needs a lot of mix.

The thing to do is fly it in knife edge and see what it does. Adjust the mixes until it will fly straight in knife edge by holding only rudder.
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Old 03-26-2016, 08:09 PM
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Originally Posted by JetPlaneFlyer View Post
I was forgetting that the Wots Wot is a biplane.. Yes, they often need a lot of elevator mixed with rudder. I've got a Precision Aerobatics Ultimate AMR, it's a roughly similar layout to the Wots wot and it needs a lot of mix.

The thing to do is fly it in knife edge and see what it does. Adjust the mixes until it will fly straight in knife edge by holding only rudder.
Thanks JPF, all I need is a few days decent weather and a chance to get out there. Not touched the sticks since late October, apart from a little indoor quad and a nano-stick!

Cheers
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Old 04-05-2016, 07:27 PM
  #12  
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Let me add a bit to this discussion. I've taken the time to draw some "cartoons" to show how to accomplish aileron differential. Note that this works great in an upright attitude. But as the plane goes inverted the differential is effectively reversed. This then ADDS to adverse yaw issue, particularly so if you are timing the elevator to keep the nose on the horizon. With that disclaimer lets get into the drawings.

The key to this that the relationship of rotational movement and linear (straight line) is not linear (one to one) when look at in one axis (push rod movement).

Now you might hear that you can get this aileron differential by using two aileron servos (one for each side of the aircraft). This is somewhat true, but is often limited in functionality by the radio's programing. Most low and midrange radio offer aileron differential by limiting the servo's end point (stoping point). That is the servo movement IS linear and offers NO differential until one servo hits its endpoint (usually the downward moving aileron). By this time the nose is already moving in an adverse direction to the intended roll! The aileron differential only happens as the upward aileron servo continues to move as you add more extreme aileron stick control movement. This servo end point way of achieving aileron differential should be avoided! It should be reason enough to purchase a higher end computer radio!

Note differential (or Ackerman Effect) can be used to achieve a balance feel on other controls. For example allowing for more down elevator than up elevator movement can make models with high cambered airfoils feel more balanced flying inverted.

Now there has been some research at NASA that shows wings can be made to exhibit pro-verse yaw. There are still issues with spin and spiral stability, well last I looked into these developments.

All the best,
Konrad

P.S.
Found a nice high level link to this and the NASA Dryden research
http://www.uasvision.com/2014/03/19/...-aerodynamics/
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Old 04-06-2016, 06:47 PM
  #13  
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Default Aileron set up

Thanks Konrad.
Very technical descriptions, but I understand.
I've used off set/mechanical aileron differential before on a couple of models. A Dakota and an SE5A. Both worked well, but this time I wanted to experiment with the differential set in the Tx.
The Wots Wot has still not flown, but I'll let you know when it does.
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Old 04-06-2016, 11:01 PM
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Originally Posted by silent flyer View Post
Thanks Konrad.
Very technical descriptions, but I understand.
I've used off set/mechanical aileron differential before on a couple of models. A Dakota and an SE5A. Both worked well, but this time I wanted to experiment with the differential set in the Tx.
The Wots Wot has still not flown, but I'll let you know when it does.
I like to set up my planes using the mechanics as much as possible. I then use the electrical programing of the radio TX to "fine tune" things.

All the best,
Konrad
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Old 04-07-2016, 11:01 AM
  #15  
solentlife
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The point made about inverted and differential acting differently really comes down to when you are inverted and NOT completing a roll.

In practical terms - which basically most of us allude to !! ... if the model rolls better with differential set-up ... the inverted part of the roll is too short to cause problems. The models momentum through the roll will overcome it and roll completes.
But if the model is flying straight and level inverted - THEN you may see a difference and problems.

But lets be honest ... the sort of model that needs Aileron Differential .... and the Wots Wot is definitely NOT in that class, as all of Chris Foss designs are fine without - the models that need it - are not usually good candidates for steady inverted flight.

Being honest - I cannot see any point of differential on a Wots Wot ... or any other Wot design ...

A modification I can see for any Wot model though is increased control surfaces to push it into 3D work. Chris's designs are excellent aerobatic machines and only need a small push to break into extreme arena.

(I flew two Wot 4's with piped 61's .... and two Skymaster Biplanes - still flying one now .... both with Chris's stamp on them !)

Nigel
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Old 04-07-2016, 12:34 PM
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Some of the top pattern aerobatic flyers recommend some differential, not to address adverse yaw in turns, but to make rolls more axial.

I've tried it for that purpose but at my level I cant honestly say that I can tell the difference!
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Old 04-07-2016, 02:04 PM
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Originally Posted by JetPlaneFlyer View Post
Some of the top pattern aerobatic flyers recommend some differential, not to address adverse yaw in turns, but to make rolls more axial.

I've tried it for that purpose but at my level I cant honestly say that I can tell the difference!
That IS adverse yaw (not maintaining an axial roll, nose points away from the roll). You can really see this in slow rolls (flick or snap rolls not so much).
Now the amount of differential varies a lot depending on airfoil, wing platform and rudder(vertical tail). For example most glider respond well to 20% aileron differential (some as high as 200%). Pattern planes often use much less (3% to 5%) as they have symmetrical airfoil and large vertical fin area. All are trying to keep the nose on heading.

I like to use this chart in setting up my ships.
http://www.wtp.net/DBEST/trimchrt.html

And yes adverse yaw it is very noticeable. Some times, like in a scale Citabria or Aeronca Champ, it can results in loss of directional control, if not addressed.

All the best,
Konrad

Last edited by Konrad; 04-07-2016 at 02:23 PM.
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Old 04-07-2016, 02:21 PM
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Originally Posted by solentlife View Post
The point made about inverted and differential acting differently really comes down to when you are inverted and NOT completing a roll.

In practical terms - which basically most of us allude to !! ... if the model rolls better with differential set-up ... the inverted part of the roll is too short to cause problems. The models momentum through the roll will overcome it and roll completes.
But if the model is flying straight and level inverted - THEN you may see a difference and problems.

But lets be honest ... the sort of model that needs Aileron Differential .... and the Wots Wot is definitely NOT in that class, as all of Chris Foss designs are fine without - the models that need it - are not usually good candidates for steady inverted flight.

Being honest - I cannot see any point of differential on a Wots Wot ... or any other Wot design ...

A modification I can see for any Wot model though is increased control surfaces to push it into 3D work. Chris's designs are excellent aerobatic machines and only need a small push to break into extreme arena.

(I flew two Wot 4's with piped 61's .... and two Skymaster Biplanes - still flying one now .... both with Chris's stamp on them !)

Nigel
The comment about inertia/momentum is a real. Once the nose starts to yaw the wrong way (adverse yaw) the game is largely up (lost). This is why one should set up the mechanics to initiate the differential at the beginning of the control input. NOT like with most entry level computer radio at the end of the control movement, as is done with servo end points. This might be why some folks don't notice much effect in their efforts to trim for rolling.

All the best,

Konrad
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Old 04-07-2016, 02:52 PM
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Originally Posted by Konrad View Post
And yes adverse yaw it is very noticeable. Some times, like in a scale Citabria or Aeronca Champ, it can results in loss of directional control, if not addressed.

All the best,
Konrad
Yes, I agree totally. In planes like Cubs, sailplanes etc differential is very noticeable. It's the pattern/3D type aerobatic ships with symmetrical airfoils and likelihood to be flying inverted or in the vertical orientation where I personally don't notice the difference so much and don't see the same theoretical argument for it either.
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Old 04-07-2016, 03:10 PM
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Originally Posted by JetPlaneFlyer View Post
Yes, I agree totally. In planes like Cubs, sailplanes etc differential is very noticeable. It's the pattern/3D type aerobatic ships with symmetrical airfoils and likelihood to be flying inverted or in the vertical orientation where I personally don't notice the difference so much and don't see the same theoretical argument for it either.
3% to 5% is often difficult to notice. But theoretical/practical arguments are the same. The added lift (from a control, aileron) adds drag. The loss of lift (from a control, aileron) results in less induced drag. As a wing in roll* does NOT have equal lift it therefore has asymmetrical drag which results in yaw. Now differential control throws might not be the most practical solution for all flight attitudes, has been noted before.

*A good test to see if you have other sources of asymmetrical drag is to unload the wing in a vertical climb and perform vertical rolls. For example misaligned control surfaces will show as non axial vertical rolls.

All the best,

Konrad
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Old 04-07-2016, 03:27 PM
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FWIW.. the standard explanation for the cause of adverse way is the extra drag caused by the downgoing aileron / induced drag due to extra lift produced.

Prof Mark Drela (prof of Aeronautics at MIT and world renown sailplane designer) explains that although those things are factors adverse yaw is equally if not more the result of the tilting of the wings lift vector... see attached pdf.

PS.. wings in a sustained roll 'do' actually have equal lift. it's only when the the aileron is initially deflected and the plane is accelerating in it's rate of roll that the lift is uneven.

In Mark's words: http://www.rcgroups.com/forums/showp...0&postcount=31
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Old 04-07-2016, 04:14 PM
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That tilt (draw a chord line from the LE to the TE of the control surface) is what is the true source of the lift change The tilt changes the effective angle of attack between the two wing half, through the change in camber. The Wright brothers did this directly with wing warping, less drag but it results in some structural issue with flutter.

I was hinting to this with the comment "Now differential control throws might not be the most practical solution for all flight attitudes, has been noted before." (What I've been saying is the change in lift result in a change in drag. Not that the ailerons are cause drag like dipping into the slip stream. What Dr. Mark Drela is saying is that as a result of the apparent change in the cord line (tilt) the actual direction of lift changes. He states that this has more effect than the added drag from the change in the coefficient of lift as a result of the change in camber. I don't question this as I'm sure he has the wind tunnel data to support this claim)

Dr. Mark Drela is trying to show that it is not the aileron (or parasitic drag associated with the aileron) but rather the whole chord (tilt) of the wing that should be look at.

Now since the wing in a steady state of lift is at an angle of attack (offset) the amount of tilt needs to be different to maintain proper induced balance. If the wing does not have this offset, angle of attack (zero lift, like in the vertical) then tilt (control deflection) would need to be balanced. But if the wing is generating lift like in a slow roll or one with timed elevator inputs the lift is not equal on both wings and adverse yaw raises its ugly head. (Dr. Mark Drela recommends the use of rudder rather than aileron differential)

Of note he states that to keep from rolling into a thermal you should use opposite rudder rather than ailerons to maintain the most efficient configuration of the wing and overall drag profile of the aircraft. When I started to fly like that I did notice a rise in my flight times.

All the best,
Konrad

Last edited by Konrad; 04-07-2016 at 04:47 PM.
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Old 04-07-2016, 04:31 PM
  #23  
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A bit more on the Adverse effects of aileron differential
http://www.rcgroups.com/forums/showp...99&postcount=7

This is noticeable when you are flying at high coefficients of lift (close to stall). Not so much while burning burning holes in the sky with a WOT or Ugly Stick.

Remember when trimming a model it is all about "balance". And I don't mean just mass!

All the best,
Konrad
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Old 04-07-2016, 06:16 PM
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Originally Posted by Konrad View Post
Dr. Mark Drela is trying to show that it is not the aileron (or parasitic drag associated with the aileron) but rather the whole chord (tilt) of the wing that should be look at.
Actually that's not what he's showing. The AoA of the wings is actually about the same in a steady rate roll because the offset in the relative wind direction (due to the roll components) offsets the aileron deflection to equalise the AoA. It's actually the forward and aft 'tilt' of the lift vector that causes adverse yaw. That's why his diagram shows the forward and aft lift component.

In Mark's words:

It's important to realize that adverse yaw is mainly caused by a fore/aft rotation of lift forces due to roll rate, and not by a drag imbalance[clip]
The roll rate which ramps up in a fraction of a second after the ailerons are applied mostly cancels the aileron-caused lift imbalance. And if the left/right lift is nearly the same, the drag will also be nearly the same as long as the flow stays attached

Last edited by JetPlaneFlyer; 04-07-2016 at 06:50 PM.
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Old 04-07-2016, 07:00 PM
  #25  
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Rill (eddy currents?) you got me on this term. You have a link?
This I don't understand.
"...caused by a fore/aft rotation of lift forces due to roll rate,.." More of less vortices coming off the tips as a result of the lift changes? And that this changes the effective lift vector?

I had thought that his description was showing that the lift vector (center of pressure) moved as a result of the corresponding tilt in the cord line (camber change).


Yes, to the if.
"And if the left/right lift is nearly the same, the drag will also be nearly the same as long as the flow stays attached"

I don't see the lift being the same as camber is different between both wings during the roll. As I read Dr. Drela's post he is not discounting drag. Just saying that the predominate sources of the adverse yaw is from the change in the lift vectors. So adding drag in the form of aileron differential does not efficiently address the yaw component. He likes to use the rudder (rudder mix).

All the best,
Konrad
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