aileron differential

[albyhouse]

I'm in the process of building a Great Planes Electricub and am having trouble with the aileron differential. The wheel that comes with the Hitech 225 Mighty Mini servo is to small to get the recommended control throws, 3/8 up and 1/4 down. No matter how I set it up I always get more down movement than up. For example now I get 5/16 up and 3/8 down, will this be ok? Or else I need to find a bigger servo wheel that fits Hitec servos and I am having trouble finding one. I could swap one of my GWS servos to the wing but they are bigger than the Hitec and I don't think it would fit considering I had to enlarge the servo mounting hole just to fit the Hitec servo. Thanks,

Mike

[Mark Wood]

The guys in Modeling Science can probably help here.

mw

[gouda]

can you post a photo of your setup? That would help quite a bit in figuring out what's up. What angle are your ailerion horns in relation to your ailerions?

[steve lewin]

It certainly sounds like you have the linkage set up wrong somewhere. Is this a torque rod/single servo set up ? Is the servo above or below the wing ?

You don't need a bigger "wheel" because you can always get more total throw by making the aileron horns a bit shorter. But getting up and down differential right means getting all the angles correct.

Steve

[Ollie]

You must have more up travel than down travel on the ailerons to combat adverse yaw. If you have more down than up the turn response will suffer because that kind of differential increases adverse yaw. What kind of transmitter are you using? Are you driving the ailerons with one servo or two? If you are using two servos, are the linkages on top of the wing or on the bottom?

[albyhouse]

It's just a one servo setup on the bottom of the wing.

[Ollie]

Instead of a straight bar for the servo output, use a wheel. Attach the pushrods to the wheel about 30 degrees ahead of the present location. This will give more up travel than down travel.

[albyhouse]

I hear what you are saying but the problem is the wheel that came with the servo is to small so when the servo rotates the linkages bind up against the servo. I need a larger servo wheel to fit a Hitec mighty micro but cant seem to find one on the web. I got gobs of Futaba ones, but of course they don't fit the spline on a Hitec servo. I have spent more time messing with this then building the whole plane! I love this hobby:D By the way, thanks for all the suggestions. I'll figure it out.

Mike

[Ollie]

There won't be any binding if you use a Z-bend instead of a clevice at the servo wheel. You only need length adjustment on the control horn end of the linkage so you don't really need a clevice. If you don't like the simplicity of a Z-bend, you can use a ball joint coupling on the servo wheel. No binding with a ball joint either. The ball fitting has to be fixed firmly to the servo wheel to prevent rocking. The firmer the mount to the wheel the better..

[Al-Parkflyer]

Another option:
Shorten your pushrods a bit and twist the aileron lingkages so the aileron linkage arms are at more than 90 degrees relative to each other. This will make aileron up travel more pronounced.
This is valid only if the picture above shows the bottom of the wing.

[max z]

Another option: Shorten your pushrods a bit and twist the aileron lingkages so the aileron linkage arms are at more than 90 degrees relative to each other. This will make aileron up travel more pronounced.
This is valid only if the picture above shows the bottom of the wing.

Uh....no. That works the wrong way. You could bend the arms at less than 90 degrees, slanting them backwards. But if I see correctly you are using screw-on lugs, with the holes forward of the arm. This already gives you a slant forward, i.e. the wrong way (which accounts for your current "reversed" differential). You would have to bend them a long way back to get the desired result.
What you could do at the servo is fit a wheel in the way Ollie suggested, and use the the cylinder-with hole-and-setscrew type (sorry, forgot the brand name, maybe Dubro E_Z connector?). Forget the clevises, use a z-bend at the aileron arms, and use the setscrew for adjustment.

Max.

[Helge Sondresen]

Mike,

Bend the actuating arms forward....relative to the ailerons(the arms that are with lugs)
Shorten the rod to the servo.Keep the equipment you have.

When pushing aft the arm will now describe a larger movement then the one going forward.Differential should be ok.
:)
Helge

[max z]

When pushing aft the arm will now describe a larger movement then the one going forward.

Sorry Helge, not true. Look at the little sketch I made. The red, grey and green straight lines represent the link, and are all the same length. For equal rotation angles of the servo wheel (angles a and b), the down movement of the aileron arm (angle A, green) is greater than the up movement (angle B, red), and thus the aileron is going more down than up, which is what you do NOT want for aileron differential.

Max

[Helge Sondresen]

Max..
There is no wheel on servo in my suggestion.

So I say again bend it forward..

See pic.
And if you don't believe it make yourself a mockup and see..
If bend forward in this exampel the resultant arm (distance from aileron turning senter)will be shorther for the arm going forward giving less movement downward.
I am sitting with a wing in may lap so i can see what is happening.

The example you have incorporate a wheel on servo with differantial and that's another matter.
:)helge

[Helge Sondresen]

picture 2

Remember the servo is on the underside of wing.

[max z]

No differential on the servo wheel, Helge. Angle a=angle b.
I did mess up my picture, so I've updated it now. Let others decide who is right, I am still convinced I am :) .

Max.

[Helge Sondresen]

Max,

Well I am 100% sure that what I said is right as well..

Why don't you draw it with a straight arm on servo and see what deflection you get??
Helge

[mockingBirds]

I've seen a different differential input scheme: take a medium size cross servo horn, cut off two adjacent arms, leaving two arms at 90 degrees orientation to each other. attach this to the servo so each arm is 45 degrees off centerline of the aircraft. The control horn points forward or backward, depending on if the servo is on top, or on bottom of aircraft. This setup should give the desired differential throws.

John

[max z]

Why don't you draw it with a straight arm on servo and see what deflection you get??

I am not sure I understand you, Helge. The end of a straight servo arm still makes a circular motion, same as a wheel.

I am attaching a sketch of the setup that Ollie proposed in post #4, maybe that will clear it up. Remember that the aileron arm I show in the sketch is an (imaginary) line from the connect point of the pushrod to the hinge line of the aileron. Like I said Steve's case this is the hole in the screw-on lug, not the end of the metal arm.

Max.

[Ollie]

Max and Helge,

You are both right in your own frames of reference but only Max's frame of refernce applies to the original case. The confusion arises because the original case has the servo mounted on the bottom of a top mounted wing and Helge's picture looks similar but I think it is for a servo mounted on the top of a low mounted wing.

For a servo mounted on the bottom of the wing the aileron horns should slant back and for a servo mounted on the top of a wing the aileron horns should slant forward in order to produce aileron differential.

[Helge Sondresen]

Thanks Ollie and Max,
I see it now... I was thinking in reversed order.
Don't bend arms forward!!!
I was wrong..:)
To day things were much clearer after Ollies explanation..
Shame on Me..
I owe you guys a beer..:)

[max z]

I owe you guys a beer..

Now that is the bit I like about being right..... ;)

No worries, Helge. Keep up the good work of building such nice models.

Max.

[albyhouse]

I got it:D I used z bends at the servo and mounted the so there are on top of the servo arm so they do not bind against the servo when there at the top of there movement. I did tweak alittle bit on the torque rods also. Thank you all for your help,

Mike