Diferential aileron needs

There used to be a formula or diagram on the net as how to make a standard servo throw to one direction more than the other, but using the entire angular swing.

If the servo arm/disk were rotated 45 degrees each way from neutral, there is supposedly a way to make the movement about 85 percent/115 percent at the aileron. This was using either an angular torque rod on the aileron, or some offset position for the pivot on the servo. Want to do this diferential movement the old fashioned way instead of using a special setting on the transmitter.

Someone had a series of diagrams and formulae posted last year. Now where is that?


Wm.

Um, might I suggest you draw up that diagram?

Step 1. Draw an arc that depicts the servo disk or the aileron control arm showing the servo center point. Differential can be created at either or both points.

Step 2. Measure the distance of the servo movement or aileron control arm movement with the normal servo throw and lay that out on a translucent vellum using the same arc used in step 1. Be sure to mark the servo center point on the arc.

Step 3. Next overlay the vellum over the step one drawing.

Step 4. Determine how much differential you want by rotating the velum. The further the vellum is rotated the more differential is created.

Step 5. Measure the distance the push rod will travel by using the vellum end points.

Step 6. The vellum servo center point becomes the point to attach the aileron linkage.

Pretty simple! I have no clue why they invented radios that can do this with programming when it is more fun to do it with linkage.




Or for less fun use the calculations in the chart in the next post!

"Vellum"???
Didn't Leonardo use that?
From not that far back... (Nov. 2005)

Well, Paul. That don't work. Last night, mentioning this to a friend last night, he found an old book by Commander Whitehead of the U.K. on how to set up the differential throws. This has been known for perhaps twenty years now, but I never saw it in print or diagram. Part of your idea is correct, but the remainder is otherwise infeasible.

The situation of placing the pivots on the near side (or interior side) of the servo is correct. However, what you have shown is only applicable to one servo and for one aileron. What would be needed to be located is a beam arm of included angle of about 130-140 degrees. Not 180 as most commonly available today. The angle between the pivots and the center rotation spot, must be less than 180 degrees to obtain ANY amount of differential. It is this interior Vee angle which give the differential to BOTH push rods, not just one. There are so many servos on the market today, that even if there was such a thing as a Vee shaped beam aftermarket arm, it would be obsolete next month. Read original note.

Furthermore, in the caption from the book, it indicates that the connector link (wire) from the servo MUST be at an angle greater than 90 degrees for this principle to work. The push away from neutral and the pull to from neutral is known, nd only way to add more or less differential is to make the movement at an angle. Leaning over the torque arm will emphasize the movement even more. Your diagram is drawn near perpendicular, which makes the fore-aft movement exactly equal to that of the servo displacement.

Wish I would have located that book earlier.


Wm.

Wm,

That’s why I like to draw it out on good old paper and vellum! It also lets you figure out the differential available by changing the angle of the control arm on the ailerons. The differential created by rising or lowering the servo position in the wing can also be analyzed. Years ago Kavan made servo output arm disks without holes that were marked in degrees and you could drill your own holes. Once you figure the differential for one aileron then you just repeat the linkage setup for the other side.

I'll stand by the math.
It's up to you to understand the principle as shown.

The "drafting method" 8178 suggests works well and makes it easy to see what's going on. You can't go wrong with that.

Over the years there have been several formulas and even computer programs to do the same thing. For several years I've used Blaine Beron-Rawdon's "Linkage Design" program. It's an Excel spreadsheet that includes an interactive schematic display of the servo and control surface that you can move over the full range of travel and watch what the linkage is doing. It also computes the hinge loads and the basic servo requirements.

It's done well by me.

Tony

8178194875...

I can remember those disks by Kavan. I think at the time though, most of my needs pretty well dictated a need for 90 degree or 180 degree holes. There was a poop sheet that went along with them to let the builder know how to calculate tne position for drilling the holes.

So who then wants to mark and drill for special holes?

I sort of wish they can ready for currently available servos, but there is such a variety of mountings available now. Square shaft, splined, and just how many splines too.

Wm.

Pretty neat stuff Tony! Here’s Blaine Beron-Rawdon's website http://members.cox.net/evdesign/pages/products.html

Wm.

I think Futaba is still shipping this output disk with their servos. The disk is about 1 5/8� wide. I have some of the Kavan wheels around here somewhere that came with my Kavan Jet Ranger.

Don' need no steenken' vellum....

If you're happy with a 45 degree angle on the arm and the amount of differential that provides then you can just put a + arm on but angled so it's a X shape.

Remember too that when you're working with arcs like this that you are also introducing a non linearity in the amount of throw compared to the stick movement. Not so bad on the one that is pulling in Pauls second pic but on the other one that is pushing the arc travel is reducing quickly as more throw is used. So the differential %'s will be very low for smaller travel amounts and higher for extreme stick positions. But that's OK since the adverse yaw you're compensating for is more noticable with full throw than at small movements.

If the ail. horns are on the bottom of the wing, moving them back on the ail. and leaving the servo horns at 90 degrees will give the diff. too.

Learn to use the rudder, and don't worry about setting up differential on the ailerons.

LOL! We have a winner!!

Thanks for the diagram, and everyones help in discussing aileron differential. I have always use
a circle servo disc for my ailerons in order to get the differential. Will have to consider using arms for getting differential during the next construction.

Rich S.

Doesn't just about every radio sold today come with the option of dialing in aileron differential? Of course that doesn't work where a single servo controls both ailerons. I don't use aileron differential because I don't care to use the rudder, I use it to obtain axial rolls; I doubt the rudder can help me much there.

DKjens

The rudder can indeed help you there. It can be used to stop adverse yaw beginning the roll, and to cause the fuselage to generate lift during the roll. It's common to get off heading slightly opposite the direction of roll - a tiny blip of rudder at the beginning of the roll can help prevent that.

I have never seen an adequate defenition of the term "axial roll". I don't think I like it, as to perform a proper roll requires that the longitudinal axis of the airplane rotate (nose) up and down somewhat during the roll.

I stand to be corrected on this, but the first aircraft with differential throws was the Tiger Moth. The linkage was a horizontal disk with an attachment point at about 60 degrees forward of the mid point. A pushrod extended back to the control horn on the hinge.

I don't know the math, but if it was good enough for Geoffrey de Havilland, it's good enough for me -- so that's how I set up diff throws.

Elevator will take out the up and down motion of the nose during a proper roll and rudder will keep the nose up when the wings are vertical. Also the more rearward the CG the less rotating motion of the nose.

Snitch,
You've got an excellent point. And on that same line, a lot of the WWI fighters had externally rigged flying surfaces. You can see the horns in good photographs. Actually you can see the horns and the control wires as well. They were all "pull-pull" connections for all the surfaces.

On some of the aircraft, the horns were adjustable somewhat for tuning the individual airplane. And there is documentation that some airplanes were tuned for the pilot.

Some pictures include enough subject that you can see that some of the horns were designed such that there would be asymetrical movement of the surface because of the "off center" connection point for the horns. BTW, their connection geometry was exactly the same as our RC geometry. The difference was only that there were no servos, just a pilot's hands and feet.

The Tiggie had extreme differential...something like 1/4" down, 3-3/4" up.
I've used 100% differential... no down at all... on some of my Lifter airplanes, which fly in the tip-stall area all the time.
No down aileron prevents tip stalls due to aileron deflection.
.
"..no servos, just the pilot's hands and feet."

Imagine flying something like an Eindecker, with the full flying tail surfaces, which had to -held- in the trim condition all the time!

Imagine having to hold trim while flying something like a WWII Bf109. I think at least the first few models had no pilot adjusted rudder trim. I know of one memoir that mentions how tiring it was to fly because of the lack of trim.

BTW, there are a number of modern aircraft that use spoilers instead of ailerons. Want to turn left? The spoiler on the left wing comes up and the airplane rolls in that direction and yaws in that direction. Most efficient way to turn left.

The F4 Phantom uses spoilers, and down-aileron only on the other side for rolling.
The SuperBug.. F/A 18E, uses spoilers for roll, with the tailerons deflecting opposite the expected direction for rolling assist.

Ercoupe had 10 degrees down and 30 degrees up.