Thoughts on Control Lines

[Howard Rush]

I've been wondering about line stretch, hinge moments, and the like.  I figured that line elasticity and the bow of lines due to drag might come into play.  Brett mentioned something that got me wondering about line weight, too.  Below is a plot of forces normal to a seven-strand, 70-foot .018" line on an airplane flying at 5.3-second laps turning a 15.6-ft.-radius corner while level.  X axis is distance from the handle.  Y axis is force.  Units are weird, but the same for all three forces.  I'd like to calculate how lines whip around in a maneuver, but I doubt if I'm either smart or industrious enough.

[Tim Wescott]

You'd probably need to do it by FEA.  I always start this thing by trying to prove that I'm manly by solving the differential equations symbolically.  After a while (sooner if someone's throwing money at me to buy a solution) I give up and figure out how to solve it by simulation.

Cutting each line up into 70 one-foot segments and then solving for drag, weight, mass, springyness, etc., will probably do.

If you didn't leave work with a copy of Matlab, check out Octave or Scilab -- learning one of those will keep you off the streets for a while.

[Howard Rush]

I am trying to do finite difference method with Excel.  I do have Octave on another calculating engine and should probably learn to use it. 

[Tim Wescott]

I am trying to do finite difference method with Excel. 

I can't even begin to imagine the amount of screwing around you'd have to be doing with Excel to try to choke this answer out of it.  Excel is a spreadsheet program, it's for bean counters.  Bean counters don't do FEA.

I do have Octave on another calculating engine and should probably learn to use it. 

Yup.

[Howard Rush]

Excel did the trick.  I started with ten chunks along the line, then switched to a thousand.  So far it looks like the bow in the lines for drag doesn't contribute much to the springiness.  I only looked at adding line tension, though, and it's way nonlinear.   Next I'll do differential tension, as with handle inputs and hinge moment.   

[tom hampshire]

     Might it be of interest that Spectra lines weigh about one fifth as much as steel?  There seems to be noticeably less whip/inertia when flying with them.

[Tim Wescott]

So, were you simulating a maneuver in Excel, or just steady flight?

[Howard Rush]

Just the constant control deflection sorta-static case so far.  The effects of differential line tension due to hinge moment look pretty interesting.  Because maneuvers tug on both lines about the same, it's reasonable to separate the "centrifugal force"  on the lines from the other stuff.  Line oscillation is another interesting topic, but I'll save it for later. 

[phil c]

Kinda fuzzy on the colors, but shouldn't the force on the lines from the turn also be a curve based on the rate of turn?
a=w^2*r

It is pretty obvious that doing high rate turns at low speed on long lines cause very large effects from the bow in the lines swinging around.

[Howard Rush]

Omega is constant, r is linear with distance from the handle, so acceleration is linear with distance from the handle.  Yes, when a southbound airplane suddenly becomes northbound, the lines slosh around some.  The first-mode resonant frequency for a combat plane flying on 60-foot .018" lines is right at the frequency of control inputs for square eights.  My guess is that stunt guys trim their airplanes somehow to cancel the line oscillation so the airplane never twitches.   

[phil c]

The turn rate is not particularly constant due to induced drag.  As a rough observation it looks like the rate settles down after about 270deg. of turn.

[Brett Buck]

The turn rate is not particularly constant due to induced drag.  As a rough observation it looks like the rate settles down after about 270deg. of turn.

    The turn rate is not constant due to finite angular acceleration.

    Brett

[phil c]

Omega is constant, r is linear with distance from the handle, so acceleration is linear with distance from the handle.  Yes, when a southbound airplane suddenly becomes northbound, the lines slosh around some.  The first-mode resonant frequency for a combat plane flying on 60-foot .018" lines is right at the frequency of control inputs for square eights.  My guess is that stunt guys trim their airplanes somehow to cancel the line oscillation so the airplane never twitches.   

When you hang a 70 oz. plane on 018 lines at 55 mph the required leadout rake gets cut in half.  There is much less curve in the lines to oscillate around and upset the ariplane.  Also, the massier plane takes more force to upset. 

You can see the curve in the line roll down the lines to the plane when you try to do a square loop with a combat plane.  It usually does arrive at the plane just as you are hitting the second corner when the curve hits the wing tip and pulls it back and down, turning the plane in at you.  Not good.