Aileron differential

[trebor]

I have never needed any differential movement on ailerons before but is it possible to set 16mm up and 9mm down using a y-lead on one channel or will I have to use two separate channels to do this ?

[Essjay]

It should be possible by moving the servo arms off center to get the differential, but easier if you can separate the servos and do it via the transmitter.

[Denis Watkins]

A Y lead suggests one servo Trebor, as suggested, you select a circular servo disc and set your linkages beyond the centre line.

I find it easiest to fit ball joints, and turn each bit by bit to make the final adjustment

Be aware the greater differential is made where the connections to the servo disc are very close together, beyond centre

[trebor]

Thanks, I hoped there was a magic way to do it on the tx. Wishfull thinking, I plumped for freeing up a spare channel so I could adjust it in the field. Now all I have to do is figure out how to program the Frsky tx I've done this on Spektrum before but I feel a headache coming on.

[Denis Watkins]

As you say, it is simple to do with 2 channels on the Tx through 2 servos.

Servo travel is selected on the Spektrum, and most other makes

[Frank Skilbeck]

You can do it by adjusting the servo travel, but often a differential setting is best, that way you can have different amounts of differential for different flight phases and also suppress aileron differential when both ailerons are raised, for spoilerons or part of crow/butterfly braking, if not you end up with very weak aileron response when the spoilerons are deployed. Pretty sure that's how my Taranis glider buddies do it.

[Keith Sharples]

Not trying to steal the thread but I had a similar problem last week. I have altered a 6ft wing span glider from a 2 channel to a 4 channel by adding ailerons and lecy motor. On the maiden when inputting ailerons it was not happy turning and would appear to stall on the one wing (the side that would have down aileron) and turn the other way and dive. It is a clark Y profile wing. So I'm thinking (and back to Trebors thread) to set it to only give up aileron to turn, it was easy to do using the 2 servos on a Spektrum DX8. Is my thinking worng? Can anyone give any light on the aero dynamics of ailerons on Clark Y profiles. Also answer Trebors question and as to why he needs differential movement.

Not flown since adjusting aileron movement as there was another problem the propeller was windmilling so I need to brake it using a program card on the ESC, getting one soon.

[Simon Chaddock]

Trebor

I set most of my planes with mechanical differential almost as a matter of course. With an under wing linkage by setting the servo arm forward to give a 'mechanical' differential (more up than down).

On top tape hinged ailerons a true mechanical limitation is desirable particularly if the V slot limits the down aileron travel.

This the aileron linkage on my 4 ch " y lead aileron" Depron Super Cub.

Simple and once set up is 'fit and forget'.

[Denis Watkins]

Some explanation of differential can be seen on many aircraft including jets that do not even have a down aileron at all.

These aircraft have ailerons above the wing, and one aileron at a time lifts into the airstream to initiate the turn.

Many years ago, before computer radio, the kit built Flair Harvard/AT6 had a single servo built into the wing with instructions to set the servo off set for differential before sheeting and covering., more up than down aileron

 

Edited By Denis Watkins on 22/07/2017 21:37:34

[MattyB]

Posted by trebor on 22/07/2017 19:52:26:

Thanks, I hoped there was a magic way to do it on the tx. Wishfull thinking, I plumped for freeing up a spare channel so I could adjust it in the field. Now all I have to do is figure out how to program the Frsky tx I've done this on Spektrum before but I feel a headache coming on.

Diff in OpenTX is easy to set as a variable in the mixer line, but there is a subtlety to be understood around the order it is applied in relation to the trim offset. Luckily Mike Shellim has authored an excellent guide to help you out...

Edited By MattyB on 22/07/2017 22:08:23

[trebor]

Just had a look at the moment it's as clear as mud to me. I've still got to finish the wiring off before I have a go. I can't set it on companion as my computer died so I'm on the other half's iPad. What I need is a dumbos guide

[Martin Harris - Moderator]

Keith,

With a high aspect ratio wing (as per gliders) the effects of adverse yaw are very apparent. The downgoing aileron alters the effective angle of attack which produces more lift and the upgoing one reduces it. One of the affects of producing lift is creating induced drag. This means that the upgoing wing's drag and downgoing one's reduced drag act to yaw the glider in the opposite direction to that expected and also pulls the nose up. You've seen this happening yourself!

Aileron differential reduces these effects by limiting the drag differential. In full sized gliders, a hefty application of rudder is needed to stop the nose swinging the wrong way and to keep the turn balanced and efficient.

The other thing which will complicate things when converting to ailerons is that the model will have more dihedral than required as in order to turn efficiently with rudder only, much more is required. Guess what happens when the nose yaws with that adverse yaw? The wings try to roll against the aileron input!

Short term answer is to use rudder in co-ordination with aileron, preferably in conjunction with providing aileron differential and reducing the dihedral. You might like to mix some rudder to aileron in your transmitter to reduce the workload.

[Keith Sharples]

Posted by Martin Harris on 22/07/2017 22:45:50:

Keith,

With a high aspect ratio wing (as per gliders) the effects of adverse yaw are very apparent. The downgoing aileron alters the effective angle of attack which produces more lift and the upgoing one reduces it. One of the affects of producing lift is creating induced drag. This means that the upgoing wing's drag and downgoing one's reduced drag act to yaw the glider in the opposite direction to that expected and also pulls the nose up. You've seen this happening yourself!

Aileron differential reduces these effects by limiting the drag differential. In full sized gliders, a hefty application of rudder is needed to stop the nose swinging the wrong way and to keep the turn balanced and efficient.

The other thing which will complicate things when converting to ailerons is that the model will have more dihedral than required as in order to turn efficiently with rudder only, much more is required. Guess what happens when the nose yaws with that adverse yaw? The wings try to roll against the aileron input!

Short term answer is to use rudder in co-ordination with aileron, preferably in conjunction with providing aileron differential and reducing the dihedral. You might like to mix some rudder to aileron in your transmitter to reduce the workload.

Thanks Martin, great explanation, looks like I was kind of going down the right path. Yes I will look at adding rudder either seperate or as a mix, more than likely mix. I have reduced the dihedral from 3 ins at each wing tip to 1 inch at each. I have to sort the windmilling prop/motor and try it again. Again Thanks

[Broken Prop]

I recently built a gull winged vintage model from a very old plan which exhibited considerable adverse yaw even with 50% aileron differential. (The wing had a constant chord, fat section, with rounded tips so definitely not high aspect ratio).

It required large amounts of rudder in order to turn at all, however too much rudder caused the lowered wing to tuck under without warning. It was not a pleasant model to fly!

Unfortunately I wrote it off due to other reasons before I was able to sort the settings out properly. Perhaps it was a blessing in disguise.

The moral of this? Be careful of putting too much rudder in when sorting out the settings....

Edited By Broken Prop on 23/07/2017 07:47:46

[Frank Skilbeck]

A good starting point for differential is for the down movement to be 50% of the up and then adjust from there. You should be able to put the differential mix % on another control and then fine tune it in flight.

You might think you don't need differential, but it can make a tricky model handle much easier.

[trebor]

Good morning, well the plane is an old Flair Astro hog with a built up wing. It's so old it's got hook n eyes for hinges and a repaired wing that got stood on by a horse. I flew it last year on Futaba as the chap gave me the tx to go with it , on going repairs, leaky fuel tank, re wire, engine bearings, push pull wires, the list goes on and on.

Flying it this year so far it's like a narrow boat, turns don't happen until it suddenly goes too far and then I have to correct it the opposite way very quickly or it could turn out nasty. I had equal up and down ailerons so I'm hoping differential will cure it. Getting used to Spektrum then Futaba I've decided to get rid and go Frsky and I'm slowly replacing all the Rx's I have.

[trebor]

Well I managed to program the differential in, am I right in thinking if it's set in the mixing section and I add medium rates and high rates in the imputs section the differential will automatically work on the new imputs ?