Aerodynamic forces

[Matt Piatkowski]

I wonder how high are the aerodynamic forces that develop on flaps and elevator of large plane during hard corners.
Large plane: 700 sq.in. total wings area with flaps, 59+ in. span, weight: 60 oz. RTF.

Are we talking 5 lbf. on flaps and 5 lbf. on elevator or perhaps around 30 lbf.?

[Howard Rush]

I wonder that, too.  It would be good to know, because if we knew, we could determine what to do (how big to make bellcranks, for example) to overcome the effect of line springiness, which would make it a lot easier to fly an airplane accurately.  The other parts of the calculation are pretty easy to measure or calculate; aerodynamic forces on the control surfaces are more difficult.

The pertinent parameter in hinge moment, the torque required to deflect the control surface.  It is proportional to air density, airspeed squared, control surface span, control surface chord squared (as Ted found out with his Imitation experiments), and the angle the control surface is deflected from the angle to which it would float if the pushrod isn't hooked up.  For an airplane maneuvering at a positive angle of attack relative to the airstream, this float angle would be up for both the flaps and elevator.  Because flaps are bigger than elevators, and because they are deflected farther from their float angle than elevators, I would reckon that: 1) by far most of the aerodynamic forces (hence hinge moment) on a maneuvering stunt plane are on the flaps, and 2) hinge moment is way more of a problem for planes with flaps than for planes with only elevators.

I can think of four ways to calculate or measure hinge moment:
1. Do the textbook calculation using hinge moment coefficients.  You can get some rule-of-thumb values for these coefficients, but they only come close for the particular wing and flap shape for which they were measured.
2. Do a wind tunnel test.  This could be as easy as mounting a stunt plane on an automobile and measuring the hinge moments.
3. Do a CFD computation, find the pressure distribution over the flaps, and add it up.  There are free programs such as XFoil and Profili that do airfoil calculations, but I don't know if you could trust the data they'd give for pressure distribution over the flaps, where the flow is probably separated.  If you know somebody with access to a fancier CFD progam, you could ask him or her to run your stunt plane airfoil.  I know such people, but they prefer to do this work for paying customers.   
4. Knowing the control system geometry of your airplane, put some strain gauges on your handle and record data for a stunt pattern.  I think this would be the best way.   I have some cheap luggage-scale strain gauge doodads I intend to do this with, but I haven't gotten around to it.

[Howard Rush]

My long-winded piece above didn't answer your question.  Method #1 above would probably come within the factor of 6 that you mention.  I am personally too lazy to do that calculation now, so I'm not much help.

[Ted Fancher]

If we had the ability to measure the combined pull on both lines wouldn't any increase from control inputs be due largely to those forces?  Of course the slowing of the ship during maneuvering would simultaneously reduce the pull somewhat and the recovery capability of the power system could add or subtract as well based on its efficacy for our needs, so...never mind.

Ted

[Howard Rush]

One would subtract the tension in one line from that of the other. 

[Peter Germann]

There is a related piece of software on the net being called "rudermoment3.exe" . It is available for download on  http://www.mfsv-lingenfeld.de
regards, Peter G.

[Brett Buck]

One would subtract the tension in one line from that of the other. 

  In a static sense, yes, and that's probably enough for an 'order of magnitude' feel for the situation. The dynamics are far more interesting to me.

    Brett

[Tim Wescott]

4. Knowing the control system geometry of your airplane, put some strain gauges on your handle and record data for a stunt pattern.  I think this would be the best way.   I have some cheap luggage-scale strain gauge doodads I intend to do this with, but I haven't gotten around to it.

Next build, put the strain gauges on the flap and elevator rods, convert to a pulsewidth modulated stream, and record with a TUT.

Assuming that the TUT guy ever gets his microSD card software working -- it's sitting there, waiting for some unpaid hours spent debugging, hacking, and otherwise messin' around.

[Howard Rush]

There is a related piece of software on the net being called "rudermoment3.exe" . It is available for download on  http://www.mfsv-lingenfeld.de

I assume c_a is lift coefficient, so pick c_a  = 2 and flap deflection = 20 degrees to get hinge moment (Rudermoment) for a square corner.  Enter airplane dimensions in millimeters.

That looks like the textbook method, but some things are a little peculiar.  I couldn't find any documentation for this program, but I see it gives an answer for a c_a of 10,000, so it's at best an oversimplification. 

[Howard Rush]

Next build, put the strain gauges on the flap and elevator rods, convert to a pulsewidth modulated stream, and record with a TUT.

Assuming that the TUT guy ever gets his microSD card software working -- it's sitting there, waiting for some unpaid hours spent debugging, hacking, and otherwise messin' around.

I'd put it on the handle to save airplane weight and have it usable on multiple airplanes. 

Keep going on that dataTUT.

[Howard Rush]

  In a static sense, yes, and that's probably enough for an 'order of magnitude' feel for the situation. The dynamics are far more interesting to me.

I'd record the data with my Labjack and a Raspberry Pi or laptop.  What dynamics would be missing? 

[Ted Fancher]

One would subtract the tension in one line from that of the other. 

Hmmmm.  Of Course!  I knew that...sorta.  I can't be expected to do everything, for Pete's sake!  No. Not you, Peterson .

Ted