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StRaNgEdAyS

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I'm probably one of the fussiest people I know when it comes to some things.
One of those things is the way I want my plane to look. I know the usual method of linking the control surfaces on smaller craft, be they either cable or rod driven, is by draggy looking horns, that poke out from the nice clean lines of the wing.:ermm:
I thought about this problem for a while, I just know I hate the idea of a nice clean sleek looking plane like the one I am designing having pokey protruding control horns. :wail:
So I sat and I tinkered, and I thunk a bit more, and I came up with this, It's designed to sit inside the trailing edge, attached to the pivot axis of the control surface. :D
It will actually be housed completely inside a box to prevent fouling of the gears and to hold them securely, and the control rod will attach directly to the longer end. As drawn it gives a total of 120 degrees of movement, which is more than enough for most surfaces.
The unit itself is small, standing 2.5" x 0.5" and being about 6" long. Made from stainless steel components, housed in an alloy box, it will undoubtably weigh a little more than a standard hookup, but for this small improvement in the aesthetics, I'm willing to take the extra.
What do you all think? is this a do-able idea, and if so, has someone already thought of it, saving me the trouble of making them?
 

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StRaNgEdAyS

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And here it is attached to the alieron.
I'm working on other ways to put it in there, here it is directly mounted along the pivot axis, I'm also considering a flexible coupling (or uni joint)and mounting the actuator flat on the centre rib TE, as opposed to thru it as pictured.
 

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Dust

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I don't get the last picture. If you just don't want a , i believe it's called a bell horn exposed, you could use the cozy system.

I know, I know, cozy, cozy, cozy, it is just what i know.

a round whole is at the base of the wing root and an oval whole is at the inside of the aileron pocket of the wing and the bellhorn is in the wing root connected to a tube that is imbeded into the aileron.

No moving parts showing and simple

enjoy the build

dust
 

wsimpso1

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Strangedays,

First off, the reason that some birds use external bellcranks is because the designers are trying to keep the forces in the pushrods or cables down, and that means longer levers. Keeping control forces down means lighter pushrods or cables, lighter bearings, at the components, and generally less control drag... In a relatively slow airplane, it is a good tradeoff.

In relatively clean, fast aircraft (Lancair, Galsair, RV, Midget Mustang, etc) they apply the pushrod within the other structures. Shorter lever arms, higher forces on that pushrod and the aileron bearings. Usually the long rods can have longer throws and lower weight, but then with a bellcrank with different length arms you increase the force for the pushrod out at the surface at a bellcrank. For ailerons, the bellcrank is necessary anyway to get the corner turned.

Many of these birds have the aileron pivot either on the top surface (Lancair, all Rutan Long EZ derivatives, etc) or on/below the bottom in conjunction with a slotted flap type aileron (RV series, Cirrus). Either way, that allows an internal lever nearly as thick as the aileron. Many of the Glasairs, even have the rudder on a hinge on one side, with a pushrod actuating it.

I am a fan of the slotted flap style aileron, even though it leaves the pivots slightly below the wing bottom. It improves aileron response at low airspeeds, so I will have fabricated a pair of bump molds to make fairings...

Now your design just uses the same high forces, eliminates the option of designing in differential aileron (to counter adverse yaw), is more complicated, is likely to have higher control drag, and you will have to build it with zero backlash (difficult to do) to avoid having a dead band...

I know that RV's are popular in Oz (a friend of ours has rides in several during her visits) (in her own words, she is "a brazen hussy- when she sees something that she wants to do, she asks nicey" she is also a very charismatic 83 year old lady , self described as an Unconcerned Flying Octogenrian). Go look at them, and everything else too. There is lots of good info out there for the looking as people are building their birds.

Billski
 

orion

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Very clever and to answer your question, no, I haven't seen this type of actuation in an aircraft application.

One word of caution though: In designing your mechanical elements, you will have to be extremely careful, especially at the speeds you are anticipating, to minimize any level of free play. The contact within the rack and pinion will have to have zero backlash, as will the rest of your control systems.

In a more conventional setup the stability of the components is well understood and anticipated since all contacts can be controlled through the use of thicker gauges where necessary, as well as bearings, bushings, or the like.

Personally, I do not see a problem with what you are trying to do, especially if you take care with the design of the gear teeth, and that of the positioning and alignment mechanisms.

The only drawback is that this will work fine with a conventional control surface but will not have application to something like a Frise type aileron.

Also, pay particular attention to the control surface forces and the moments required to deflect the surfaces at Vne. Make sure you take into account the bit of vibratory motion that might happen if something loosens up (substantial design safety factors). That will design the size of your gear teeth. You certainly would not want to have the rack fail and end up with a freely fluttering surface.

This may require the use of something like 17-4 stainless (~100ksi yield) as opposed to something more common like the 304 alloy (~35ksi yield).
 

Marilyn

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Well, I've only fiddled with designing

But no I've not seen this sort of actuator. I will note that 120 degrees is about 95 more than you need, since most aileron designs I've looked at have limits at 15 down and 10 up.
That said, it seems to me that if you are aiming to eliminate the exposed control horns, then the logical way to do so would be to bury the horn inside the aileron itself.
Kinda like this. (sorry about the crappy drawing, but it was the best I could throw together with Visio - I'm supposed to be at work)
 

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orion

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The buried control horn (or other type of rod attachment) is the most common where drag and/or aesthetics matter. In those applications though it is important to pay particular attention to the forces required to deflect the surface as this will design everything from the surrounding structure to the control horns and rods themselves.

One note though on the last post, ideally the movement of the aileron in the up direction (trailing edge up) should be greater than the movement in the down direction. The ratio of up to down should be about 2:1 to 3:1 so, this means that the upward deflection should be about 25 deg. or so, while the downward movement will be only about 10 deg. This differential movement is set up so both surfaces create an identical amount of drag when deflected, thus minimizing adverse yaw.

Set up just right, the deflected surface on the inside of the turn will actually create more drag than the aileron on the outside of the turn, thus yawing the aircraft nose into the turn rather than to the outside. This results in a coordinated turn with little or no rudder input.
 

Marilyn

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Doh!

Originally posted by orion
One note though on the last post, ideally the movement of the aileron in the up direction (trailing edge up) should be greater than the movement in the down direction.


Slaps her forehead! I knew that, so why didn't I notice I had typed it wrong?

Silly fingers.

And you may be right about the 25 degrees up to 10 down, I know that 15 degrees is in the quation someplce - it's what I get for spouting off without verifying against my plans. But, if I may offer the excuse, my plans are in North Carolina and I'm in California on business.
 

StRaNgEdAyS

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I'm trying to het a handle on how differential alierons hook up.
No matter how I look at it, when a control rod moves to put say 25 degrees up on the left alieron, since the controls are linked, the horn on the right elevator moves the same distance in the down direction, which is...25 degrees! :suprised: :ponder: :mad:
I never built a model with differential alierons, so I never had to work out how it goes together. :( Guess There's no time like the present to work it out eh? :D
The closest I've come so far is placing the bellcrank so that the arc travels from 90 degrees to the alieron pushrod downwards, so that on the up stroke, the arc travels to the outside tangent to give maximum linear travel, and on the down stroke, it is travelling towards the bottom of the arc, giving less linear travel.
I don't know it I am making sense here, but I'll draw something up to help explain it.
All I really want to know is, "Am I on the right track?"
 
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Norman

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yep, different angled bell cranks like described on this page:
http://www.mindspring.com/~cramskill/diffthro.htm

Have you looked at frise ailerons? If not here's a very brief description:
http://www.faatest.com/books/FLT/Chapter4/Differential.htm


NACA LARK is a good source of design info if you can stand to sift through the results of the lousy search engine.

Here's a NACA report that looks relevant to aileron linkages:
http://naca.larc.nasa.gov/reports/1938/naca-tn-653/naca-tn-653.pdf

BTW the low AR and high speed are in your favor here. Since adverse yaw is caused by induced drag and to go fast you want the Cl to be as low as possible the induced drag isn't a big issue at cruse speed but it is important at landing. The low AR means that the unbalanced drag is acting on a short arm thus the yaw moment due to aileron drag isn't as big as for a more conventional plane
 

StRaNgEdAyS

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Well, I've got some work done, I have decided that due to my low AR and relatively high cruise speed, I'll not worry too much about friese alierons, but I will be using a differential system.
To this effect, I have begun to draw in the control linkages, using an internal horn.
The total linear movement is 19.5mm, with 13mm forward to give 25 degrees up alieron, to 6.5mm back for 15 degrees down.
Now all I have to do is work out an appropriate bellcrank, which shouldn't be too hard.
 

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wsimpso1

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Strangedays,

You might want to do some book work on control flutter...

For my bird, I did investigate using torque tubes, then looked over push/pull tubes and concluded that the fairly standard push/pull tubes and bellcranks had a much higher resonant frequency when built to stand FAA Part 23 stick forces. That is why cables and pushrods are used most of the time. Of course in our conventional airplanes, the ailerons are out there a ways, while your bird may have such a short reach from the cockpit to the ailerons that the torque tube is better - I am using torque tubes for the flap actuation...

Yeah, the push/pull tube system does have to be covered for buckling (Euler's method), which enlarges the tube diameters some, but you can help that out by choosing the gearing to keep the long pushrods at relatively low forces and then bring the forces back to where you need them with the bellcrank (longer arms on the longer tubes) that converts spanwise motion to fore-aft motion, where the tube will be short.

You get your differential aileron action from the position of the control horn relative to the aileron pivot and the angle between the spanwise and fore-aft arms on the bellcrank. You can dial in the amount of differential aileron by changing the bellcrank angles, which is easy to do by fabricating new ones, and then swapping them between flights. Differential ailerons with your schem may be a bit fussy to change ratios on... Just something else to think on.
 

StRaNgEdAyS

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Cool, well I finally got something that seems to work OK.
I'll do some reading up on control flutter.
I have changed it around a bit, I moved the horn out along the alieron a little so the amount of throw came down to 17.5mm with the same 2:1 up:down ratio and the same 40 degree deflection range, 25 up and 15 down. Then I gave the bellcrank 55mm input travel in each direction, and the end result looked something like this.
(those ball joints have 10mm balls on them, I might reduce those to 8mm ones, as the do look a little large) :ermm:
 

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wsimpso1

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Strangedays,

OK, I see what you are doing now. Bellcrank forward, control horn on the aileron, spherical joints at both ends, short push rod between them, long pushrod going inboard. Pretty standard stuff. You might still want to read up on flutter still. Your bird will be capable of a pretty significant chunk of Mach...

There are whole selections of real aircraft quality spherical joints, tubes, male and female rod ends, etc, available from Wicks, Aircraft Spruce, and other homebuilt suppliers. They could save you soe troble on sizing your parts...

Billski
 

orion

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Be cautious in using those ball linkages - personally I would advise against them for the same reason I would advise against using any torque tubes - flutter due to insufficient control stiffness. The balls react the load above the bellcrank arm plane and thus cause a torque on the arm they're mounted to, which will result in a bit of twisting motion.

When the whole thing is put together, the summed up motion of all the arms twisting can be more than enough to initiate flutter. A control system that is expected to operate at the speeds you are anticipating (we'll see about that when I do your analysis) needs to be as stiff as possible. This means all actuation should be axial and all bellcrank connections should be in-plane, or centered on, the bellcrank arm plane.

I would suggest doubling up the bellcrank arms and sandwiching between them a rod-end fitting on the end of each push rod.

Also, make sure to allow for the counter-balances you will need since all movable surfaces (except flaps) need to be at least statically mass balanced about the hinge line.
 

StRaNgEdAyS

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Be cautious in using those ball linkages
Yes I did some quick checking and I really cannot afford even the smallest amount of play.
I remember from using these ball linkages on automotive applications, and more so motorcycle gear shift linkages, that they do twist under load and provive a certain amount of play. Even using aircraft quality joints, I could still easily end up with more free play than I need.
 

StRaNgEdAyS

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OK Here it is, not bad for a day's on and off work. :D
I have my three kids here with me at the moment, so I'm not able to put in the same time as I would normally when I sit down to draw. :ermm:
I raised the output arm 35mm because there was a huge (well thats how it looked :para: ) "bend" in the linkage to the aileron. now the connecting rod runs pretty much a straight line. :ponder:
 

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