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Author Topic: Leadout Position  (Read 961 times)

Offline Peter in Fairfax, VA

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Leadout Position
« on: January 17, 2019, 08:22:23 PM »
What is some of the thinking on where leadout guides should be positioned?

For combat, I originally flew the Nemesis, a Howard Rush design.  Part of the thinking of that design was that moving the leadouts back would increase line tension, which can be very important when your lines are wrapped several times by your opponents.  The other part of the thinking was that some speed was lost, as "raking" the leadouts toward the rear yawed the plane out, causing some of the engine thrust to be diverted towards the line tension.

In reading the stunter tuning guidelines mentioned yesterday in a thread on flap horn bending, there are cases where moving the leadouts forward may be advised. 

Thoughts?

Offline Dave Hull

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Re: Leadout Position
« Reply #1 on: January 17, 2019, 08:32:14 PM »
Yes.

It turns out that you can precalculate a good starting position if you know the weight of the plane, the target speed, the line length and size. The idea is that the drag of the lines creates a catenary shape, and you want the leadout guides positioned relative to the model's CG so that the you do not excessively yaw the plane.

The link to the line rake calculator is:   http://nclra.org/Programs/index.php

I'd probably set the average of the guide(s) slightly behind this position, and then move it forward in small increments (1/8" -ish) and see if it helps or hurts you. There are certain corners that tell the most about guide position. They should be listed in the trimming tools.

Note that trimming is an iterative process. You get as close as you can on the initial things, like CG location relative to the wing MAC, then get the leadouts aligned to that CG location, etc. (Even the landing gear needs to be located relative to the CG). As you get the response rate you want (if you have to move the CG) then you should count on moving the line guides again, too. And so on. Early in the trim chart you are going to find that you need to get the tipweight adjusted correctly. I suppose my point is that there is an order to these adjustments that allows you to converge on a best answer. If you are way off in one regard, then tweaking the others to compensate will lead you astray....and that's the beauty of the trim guide--we are lucky to have had world champion competitors give us an optimized sequence and help us converge more quickly!

What model are you trimming?  We might be able to give you some more specific info if we knew more about your setup.


Dave Hull

PS--If you are worrying about lines getting wrapped with a competitors in stunt then your circles are too close together, I think. I'll have to check with some of my flying buddies on that one....
« Last Edit: January 17, 2019, 08:52:30 PM by Dave Hull »

Online Dan McEntee

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Re: Leadout Position
« Reply #2 on: January 17, 2019, 08:39:12 PM »
   An  excess amount of yaw outwards can hurt even the best stunt model, and the leadouts being too far aft are frequently found when helping others trim out a new stunter. There is a general rule of thumb on where to put the lead outs behind the balance point as a starting point that I can't remember right now but I'm sure someone like Howard will fill in the blank. Line tension is a different thing in stunt than it was in combat. generally you have bigger engines, bigger props longer nose moments and longer lines and other factors helping with line tension on a stunt model. Most plans show a good starting point, just make sure you can adjust from there both ways. But generally, lines are going to be farther forward than you may think. Now with the advent of electric models, that changes things a little bit, as they have come to discover here as time goes on and progress is made with the power plant. I'm sure those that are devoted to that motive power will chime in also.
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Offline FLOYD CARTER

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Re: Leadout Position
« Reply #3 on: January 18, 2019, 01:04:27 PM »
Line guide position is still "black magic" to me.  Like you say, test flights are the only way to get it set right.

I don't understand why an electric model trim is "different" in regards to leadout position.
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Offline Tim Wescott

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Re: Leadout Position
« Reply #4 on: January 18, 2019, 02:44:25 PM »
I don't understand why an electric model trim is "different" in regards to leadout position.

That comes from Paul and Igor, and I suspect that it's dependent on having Igor's speed-regulating system, which can overcome the loss of speed due to drag.  It would be interesting to know if the situation is the same with a Hubin or KR system.

(Igor does something fancy with rudder adjustments and the yaw stability of his planes, too -- I understand it every time I read it, and then it immediately slips out of my brain once I stop reading.)
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Offline Ken Culbertson

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Re: Leadout Position
« Reply #5 on: January 18, 2019, 03:44:29 PM »
That comes from Paul and Igor, and I suspect that it's dependent on having Igor's speed-regulating system, which can overcome the loss of speed due to drag.  It would be interesting to know if the situation is the same with a Hubin or KR system.

(Igor does something fancy with rudder adjustments and the yaw stability of his planes, too -- I understand it every time I read it, and then it immediately slips out of my brain once I stop reading.)
I noticed it right off when I switched to electric.  For me the most dramatic was from the hourglass forward through the landing, especially the landing.  Better overhead tension and not so tail heavy.  Some of that may be switching to clockwise prop rotation but that came at the same time.  I use the Hubin.  I build light so I may notice the CG shift more than someone who is flying heavy.

Ken

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