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Pull-pull ailerons for biplane

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Old 11-10-2006, 05:44 PM
  #1
fledermaus
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Default Pull-pull ailerons for biplane

The project: a 1/6 scale Sopwith Camel from plans.

The problems:
a) plans show no detail of aileron set-up
b) I want a scale appearance, which means external linkages must be pull-pull.
c) wings are just 1 inch thick, pretty tight for a wing-mounted servo

My solution:
The lower wing centre section is a solid 1/4 ply square which not only anchors the wings, but also the undercarriage. This will be a good place to anchor two standard servos, lying sideways.
Each servo push/pulls a Sullivan rod out to a bellcrank. The pushrod is anchored through every wing rib, so there should be very little flex or slop here.
The bellcrank (size to be determined) is fastened to a pull-pull cable, which pulls the aileron DOWN.
The lower aileron is linked to the upper aileron by a cable as well (scale fashion) so the upper aileron goes DOWN too.
The upper aileron is connected to a bellcrank by a pull-pull cable similar to the lower wing.
The bellcrank on the right side is linked across the 1-piece upper wing to the left bellcrank by another Sullivan rod. This will pull in either direction. A cable could be used but I am concerned about tensioning here, where servicing would be awkward.
Set-up on the left side is similar to that described, but linked to the other servo.

A sketch is shown.

My questions:
a) with two servos pulling on a pull-pull loop, I expect some problems balancing the system. Will this be a big problem? How should it be done?
b) is there another solution anyone can suggest?
c) do I need two servos to drive four ailerons, or can I use one higher powered servo to run all four ailerons?

All comments and advice welcomed!
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Old 11-10-2006, 07:11 PM
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Default RE: Pull-pull ailerons for biplane

What you're doing should be possible but it will be tricky. For one thing, the Camel has the aileron hinges flush with the bottom surface so it will be difficult to avoid some kind of differential between the lower aileron horns and the upper ones. Of course differential is good on a biplane if it is arranged correctly.
I would think that one (more powerful) servo would be better than two standard ones, in terms of equalizing the movements to each side. How does this wing break down, if you have to, for transport or storage? On my VK Camel (also 1/6th scale), the lower wings are separate which is why I elected to use two thin servos (HS81) in each wing, to avoid screwing around with the linkages every time I took the wings off. Of course this plane is just small enough to fit in my vehicle without breaking it down but still...
Here is a link to my Camel build on RCSCALEBUILDER where I was dealing with similar issues. As I said above, I elected to go for separate wing mounted servos but, in this early thread, I was considering a center mounted one, getting differential from the center servo wheel configuration. I was even thinking of using the cable idea to power the upper ailerons and would have had to use another dual drilled wheel in the middle of the upper wing, to recompensate for the differentials.
http://www.rcscalebuilder.com/forum/...?TID=4723&PN=1
Of course the real plane used pulleys and cables throughout.
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Old 11-10-2006, 11:26 PM
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Default RE: Pull-pull ailerons for biplane

I've done a pull pull system on the ailerons of a Sperry Messenger bipe. Pics show some detail, i made my own pulleys from 1/32" ply outer discs with a smaller inner disc of 1/8" ply glued together. There's 6 of them all together if i remember right, they were putt in between the lower and upper spars just using a music wire pin that was glued into the spars. Proctor rigging cable was used for the runs with just two clevices at the bellcrank for the servo to push and pull on. It works good and havn't had any problems with it. When rigging the cable, keep the ailerons held rigid, and don't try and get the cables too tight. Try to make it so the cables can't come off the pulleys. I also made my own control horns from aluminum and the top wing ailerons were actuated using a airfoil shaped aluminum rod connected from the trailing edge of bottom aileron to the trailing edge of the top aileron.
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Old 11-11-2006, 01:46 PM
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Default RE: Pull-pull ailerons for biplane

A rotary driver system with dummy pull-pull cables would be a lot simpler.

For servos check out the BlueBird BMS 380MG (50 oz torque @ 4.8 volts) or the BMS 380 Max (57 oz torque @4.8 volts ). These servos are reliable, light weight (about 1/2 oz ) and slightly smaller than a HiTec 85.

Rotary drivers

http://www.irfmachineworks.com/rds/
http://www2.towerhobbies.com/cgi-bin...?&I=LXNN07&P=7


Servos

http://www.bphobbies.com/view.asp?id=V443293

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Old 11-11-2006, 08:37 PM
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Default RE: Pull-pull ailerons for biplane

Thanks for the feedback, guys.

I want to stick with a pull-pull system, but I can see that it should be possible to make a complete pull-pull system by making the pulleys as described by canadagoose. I thought of using pushrods for the invisible parts because I didn't think of a way of setting up pulleys that minimized chances of the cables hopping off. The plywood pulleys are a great idea and I'm ashamed I didn't think of it!

I gather that Campy is figuring I will need about 50-60 oz of torque to run all four ailerons. I really had no idea how powerful a servo would be required, so thanks for the info. I will go with one large servo of the type suggested, I think.

Allanflowers was asking how the wing was removed in my plans. The plan set I have is not quite as authentic as the VK Camel, and uses a one-piece wing that bolts to the bottom of the fuselage. The ply centre of the lower wing is the fuselage bottom plate. The plans are quite good as to outline and come close to authentic construction but deviate in a few places (as in the wing mounting). I will be "fixing" a few of the deviations but as this is my first venture into building from scratch I don't want to innovate too much. (Actually, I am quite experienced at building things from scratch, just not RC airplanes yet. I don't have any problem building and this project is quite a bit more straightforward than some I've done.)

I have not really considered setting up aileron differential. I'll be quite happy as long as all four ailerons work nicely in concert. Flying coordinated turns is no big deal - its how I fly all the time. I figure if the full-scale guys can do it, so can I.

I don't intend to disassemble this Camel for transport. The 56 inch wingspan will fit in my van quite nicely so I can carry the plane fully set-up. I am planning to use working rigging, which I don't want to be taking on and off.
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Old 11-12-2006, 03:33 PM
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Default RE: Pull-pull ailerons for biplane

You can use tubing bent to about 90 degrees also. It works well because the cables can not jump out of them. 1/8" I D Aluminum filled with sand first, then bent will work.Remove the sharo edges at each tube opening. Use as large a bend as comfortable. Friction is not a problem.

Make a quicky setup to have faith in it, buried in a wing.
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Old 11-12-2006, 10:36 PM
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Default RE: Pull-pull ailerons for biplane

The bent tube thing might be okay for one bend but, with a single servo and FOUR bends in the system, the friction buildup would simply be too much. Bell cranks or pulleys would be much better.
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Old 11-13-2006, 12:52 AM
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Default RE: Pull-pull ailerons for biplane

I ordered some servos for this project today - the Bluebird BMS 380 MG for the ailerons and another for the elevator (which will also be pull-pull). I got a BMS 380 for the rudder (p-p) and a BMS 371 for the throttle. Haven't used micro servos before, having only built sport planes up to now. These little guys will definitely make the rather cramped interior of the Camel a bit easier to manage.

No problem making pulleys - but what about the pivot for each one? That will be a friction point to worry about as well. I suppose I could use a brass bushing with a screw or bolt going through for the axle. Ideas?
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Old 11-13-2006, 01:28 AM
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Default RE: Pull-pull ailerons for biplane

Quote:
No problem making pulleys - but what about the pivot for each one? That will be a friction point to worry about as well. I suppose I could use a brass bushing with a screw or bolt going through for the axle. Ideas?
I used a piece of brass tube glued in the center of each pulley with a short music pin through it that acted as the pivot for the pulley. The pin was epoxied into the top and bottom spars. The diameter of the pins in my plane are only 1/16", too small a diameter for friction to amount to anything.
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Old 11-13-2006, 11:02 PM
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Default RE: Pull-pull ailerons for biplane

That's doable! Thanx.[8D]
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