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Largeprime

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StRaNgEdAyS said:
twisting moment along its length (it IS over 2M (6 1/2") long) would be a significant flutter risk at high speed.

how does the design above reduce this?
It would seem it has the same length.

The stiffness should be a matter of material selection
 

StRaNgEdAyS

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hmmm perhaps I should have been a little more specific there.
The length of the tube if it were to run from the aileron to inside the fuselage would be well in excess of 2M long. As it is drawn now, it is only about 1M long.
Because of the aileron ribs and the skin, any twisting moment in the pivot/spar tube supressed, however if I was to extend the tube further to inside the fuselage, there would be more than the ailerons' length of essentially unbraced tube section leading up to it.
Using the pic as a reference, the aileron and the flap follow a constant 30% chord. The linkage set up I have drawn (you can just see it against the rear spar in the centre of the wing) eliminates the possibility of any twisting moment leading up to the control surface.
 

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Norman

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You can reduce any twisting moment even further by moving that bell crank out to the aerodynamic center of the aileron. Yeah Yeah, I know, you don't want a bump in the wing.
 

Largeprime

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In your latest pic could not the "tube" be braced by the ribs it penetrates?

Are you saying that one 2m tube is less stiff than 2 1M tubes?

Are you saying from the inboard edge of the aleron to the fuse is 2M or 1M?

The distance from the inboard edge of the aleron to the fuse will have to be covered in all cases, Right?

How does traveling that distance to a join end up stiffer than traveling that distance to the aleron itself as a sloid peace?

If you are saying the inboad distance is 1M and the aleron is 1M so there for it is a 2M vs 1 M thing then I understand it differently.

The aleron itseld NEEDS to be stiff over its length. This effectivly eliminates that 1M of the tube from a stiffness calculation. (running the tube straight through only avoids a joint. It might have a structual use) All thats left is the distance from the fuse to the aleron, which has to be covered anyway.

I think there is an advantage in doing it without joints.
 

StRaNgEdAyS

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The ribs the tube would penetrate on it's way to the fuselage belong to the flap. I'm not however saying that the suggestion is not practical, and it may well bear further investigation.
The distance from the inboard edge to inside the fuselage is about 1.6M (63") The aileron itself is just over 1.2M (50") long and the flap beside it is just over 1.3M long (51"). There is a 60mm (2 1/3") gap between the flap and the aileron. Both surfaces run at 30% constant chord length from the TE. To run a tube from the inboard edge, through the flap tube, would provide a significant length of, for all intents and purposes, "unbraced" tube section, because it will need to rotate to be functional. Yes, you could support it along it's length with bearings, but that will not preclude any twisting moment from occuring.
What I was saying was that I was concerned about the twisting forces that high speed and/or high G maneuvering would transmit along a tube of that length. Bear in mind that I am looking at anticipated speeds in excess of 300kts.
 

wsimpso1

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Strangedays,

By the time that your metal skinned control surfaces are sturdy enough for handling and the like, you will only need a thicker panel of aluminum to use as a spar along the the hinge line. The skin (backed up by ribs) carries torsion really well, but you do tend to need the spar to support bending loads between hinges.

So the spar is usually just a thicker panel of aluminum at the hinge line that runs from top to bottom surface, and a brake is used to bend a flange on the top and bottom ends. Attach the skins, with the hinges, bellcrank, ribs and any nose pieces attached to the spar. If you are hoping to use a rod or tube as a spar, it will be very flexible in bending, and thus not much of a spar. You need depth in the control surface spar to keep it light, so make it full depth there.

Work out the surface loadings required, then the shear and bending in the control surface, and you may see a need ofr all of this bending stiffness...

Look at the rudder or aileron or elevator on any certified metal aircraft, except maybe a Grumman (they are bonded, so it is difficult to see where the joints are) and you will see where the spar is, how the skin is attached (sometimes one piece, sometimes two), how few ribs there are, etc.

Billski
 

Largeprime

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Flap, Right

ok I understand about the flap now.

What I dont understand is no matter how you do it you have to have a tube go at least 1.6M to the aleron right?

No matter where it goes it has the same tourque problem right?

So even if the tube through the flap is a non starter, the problem is not torque, as no matter where it goes it has to beat the torque requirments, right?

Or did I miss something about how less torque will hit the part if you run it further up in the wing?
 

orion

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There are two aspects to this discussion and if I read the comments correctly, the two issues probably need to be clarified. As I read the above, the discussion can be divded up into the subjects of control linkage and the aileron itself. First, the control linkage as it sits and as a couple of us recommended several posts ago, is all done trough push-pull members where all the tubes and/or rods, are in axial loading. In this way one achieves the maximum level of control system stiffness.

If you tried to activate the aileron thorugh a torsion tube, as it was first proposed, then there would be a significant issue of stiffness to deal with since a torsional tube, extending from the fuselage to the aileron, may have sufficient radial flexibility (low torsional spring constant) that could increase the risk of control surface flutter.

By arranging all the control linkages so that they push and pull, the potential flexibility of the control system is dramatically reduced. However this is why it's important to pay attention to the details since things like the ball ends discussed earlier, would definitely reduce the stiffness and again potentially create problems.

The second issue deals with the torsional flexibility of the aileron itself. Here we actually deal with two issues, those being the torsional rigidity of the aileron structure and the mass coupling between the actuating point and the mass center of the surface. The latter also depends on the location of the counterbalance mass.

Basically, the shorter the distance between the surface's mass center and the actuating point, the higher the vibrational frequency of the aileron's structural system. By locating the actuating horn further out from the aileron root (closer to the aileron's mass center), you reduce the chance of a coupling motion resulting from the aileron's inherent torsional flexibility. Ideally, the hook-up should be somewhere about one third the distance from the surface root to the tip, maybe a bit more since the aileron is not an actual triangle. (On a rectangular surface this location would be half way out.)

How critical all this is though is a function of the design of the aileron, and the material gauges you select for within the assembly. At this point my guess is that if you use a bit of common sense in the layout of your aileron and the selection of your gauges, the mass coupling issue may not be all that big a deal. However it still may be worth investigating - up to you.
 
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Largeprime

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Of course
You all were on to push pull and I was playing catchup with twisting tubes

Thanks for your patience.
 

wsimpso1

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Hey strangedays,

The aileron is hung from the ends only? Am I reading this correctly? Why run a rod (or tube) all of the way through? It is just extra weight and I bet that you could find a nicer way to carry the load at the ends. And a nice full depth spar made from bent sheet metal will both be lighter and stiffer than the rod (tube).

Usually you pick the number of control hinges based upon what is lightest and/or is stiff enough and/or is strong enough:

Light enough is easy, pick two and three hinges, design and see which gives the lightest total system. If three is favored over two, investigate four, and so on;

Strong enough is easy too. Compute max conditions, compute bending and shear and torsion, apply factor of saftey, and beef to not fail;

Now stiffness - the control surface when under its maximum loading can not touch the adjacent parts, must be resist unloading by twisting, can not flutter, etc. This one can be tough when the Machmeter begins to register.

In any event, in little airplanes we have a tendency to use either two or three hinge points. Now two at the ends means bending deflection at the middle could be significant, leading to the need for a stiffer spar... Flutter resistance would take you the same place. Twist stiffness might require upgauging the skin and ribs.

Or you could investigate going to three hinges, which is way stiffer in bending. Given that your span is pretty small, I suspect that two hinges is lighter and OK , but I would actually bury them inboard from their tips a little to reduce bending and deflection. That means cutting a little slot in the nose of the aileron at each hinge point and fabricating the hinge reinforcement, but you probably can not get away with just using an unreinforced rib to hang the pivot anyway... A lot of really great airplanes have slots in the leading edges of their control surfaces.

Am I making any sense with this?
 

StRaNgEdAyS

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Hey there Billski.
Yes, they are hung from the ends only for now.:ponder:
These drawings are representative of my original ideas for hanging them, I hadn't decided whether a central pivot point will be needed or not yet.
The main reason the tube (rod) ended up being there was because when I started drawing them up, I started from the pivotal axis, made a SWAG at what would be an appropriate diameter for spherical ball supports and came up with a tube of10mm OD with a wall thickness of 2mm. (I drew a rod by mistake and have not yet gotten around to changing it) I continued this right through so I could align the control horn and the positions of the end sockets on it, and it has pretty well stayed there since.
I'll certainly put some thought into some changes to improve it.
Anything that goes towards minimisation of control flutter is good advice, and worth considering.
This one can be tough when the Machmeter begins to register
Just an aside thought here, but wouldn't it just be too cool to have a machmeter on the instrument panel???:gig:

Reinforcement of the mounting points was always on the agenda, I just hadn't got around to drawing it in when I posted this...:eek: (I know it's terribly slack of me! :gig: ), so here is the means by which I have attempted to spread the load. These rib sections are attached to the rear spar and are 60mm apartso they should be fairly stiff. That spherical socket is olny 50mm long x 32mm high.
 

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pylon500

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WOW, You have to be quick to keep up with this thread!! :eek:
OK, I think what's happening here is a little too much aeromodelling influence. :rolleyes:
As
wsimpso1 said, you don't really need to run a rod (or tube) through the aileron or flap to create stiffness like you would with an R/C model.
If the control surface is fully skinned, it becomes a triangular sectioned tube and quite strong torsionally, its weakness being the flat surfaces, so just add a few ribs and there you have it. :gig:
You may have seen some FABRIC covered control surfaces with a tube running through them, this is so each rib can support the trailing edge.
If you are worried about flex in the wing or aileron, you can use self aligning bearings at the hinges, it's better to design around having flex in the system than trying to reinforce against it!, that just adds weight and eventually something cracks anyway. :mad:
As for number of hinges, have a look at a 767 outboard flap, it's about 20 feet long, made of composite and hinged at two places!!
This thing 'flaps' all over the place, but the Boeing boys seem to be happy with it!
On the hinge attach question,
wsimpso1 says use a thicker sheet to make the aileron spar compared to the skins, this helps to spread the load from the immediate attach point to the rest of the structure.
The usual route is to fit doublers behind the spar at the attach points to absorb any concentrated loads.
Just as a side note, the use of torque tubes for aileron control is typical in the early BEDE aircraft.
I once flew a BD-4 and found that on the ground the controls were free and smooth, but once airbourne the ailerons became fairly heavy due to control friction as the wing flexed. :(
I'm not sure that I would use the same system in my BD-5 kit, just too flexable. :whistle:
Arthur.

ps, do you really need that 60 m/m fixed piece between the flap and aileron?
 

StRaNgEdAyS

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Hey there.
Yes I have been pretty busy.
The reasoning I had behind hinging at the ends was to have as little outside the lines of the skin as possible. I used the spherical ball type bearings to allow for twist and flex, as they self-align.
I left the rod in there to ensure that the twisting forces from the control horn is transmitted to every rib in the aileron structure, as due to the limited space I had to mount the internal control horn pretty close to the root of the control surface. I have backed this up with a (preliminary) spar running the full depth and length of it.
That 60mm section in the middle allows for the mounting of the pivot points for the aileron and the flap to sit inside them and stay out of sight.
I still have some workling out to do on this, so while I think on it for while, I am adding to the (so far) permanant portions of the design.
 

Dust

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I think one of the criteria for the flapping around like crazy controll surfaces is to make sure that it will not bind when it flaps around.

On our elevator we need(from memory) 1/8 to 3/16 required gap between fuselodge and elevator, we opted for a heavy 3/16.

Gaps are important

enjoy the build

dust
 

Largeprime

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Feb 18, 2004
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This might be of use
On the cozy (and rutan ez's) the alerons (and rudders) use a piano hinge mounted on the top of the aleron and wing.

Seems alot lighter and simpler than the rod and bearing setup.
 

Largeprime

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On mike arnolds video he has a bell crank thing like you pictured above.

One thig he had a cable for pull only, but i think he had friese alerons
 
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