Control Lines For all you fly-by-wire fanatics!

Adjustiable lead-out Guides

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Old 06-10-2017, 11:59 AM
  #1
straitnickel
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Default Adjustiable lead-out Guides

I have a P-40B from Brodak, this is my first on with adjustable lead-out guides, so my question is does this eliminate the need for engine and rudder off-sets?
thanks in advance for all replies.
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Old 06-10-2017, 03:58 PM
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Lou Crane
 
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Straitnickel,
CL models can be thought of a having all their weight concentrated at a single point - the center of Mass, usually called the CG (Center of Gravity.).

Balance points can be found by hanging or supporting the model where it lines up straight below (or above) a point in three "axes." Relax, this should stay simple - because it is.

We all know "where the CG" (center of gravity) should be; there's usually a symbol that looks like a BMW logo, or wording to "balance the model x inches back of the leading edge," or some such. That only considers where the "Mass" should balance when we look at the side view, and it is vitally important for controllability AND flyability. The line pull will always tend, or try, to line up to the CG. But consider...

If the model is a high-wing type (can you say Piper Cub?) and your leadouts come out through the wingtip, when you hang the model by the leadouts it will bank. The vertical CG is not in line with where the leadouts enter the model, so it will not hang vertical, but try to tilt to where the mass IS directly under the leadouts. If you fly a model with the vertical CG way off, it WILL bank to try to line up the line pull with the leadouts. For a high wing model, that is a bank IN, towards you at the center of the flight circle. That will try to turn it in towards the center, reducing the line pull you need to control flight! NOT good!

The third "axis" is yaw - nose left or right of where you want it to be. ...Or just where you want it to be so the model flies as nearly as possible with the long axis of the body "tangent" (remember high school geometry?) to the flight circle. Adjustable leadouts let you "dial in" their location so that the line pull doesn't cause a nose-in yaw - reducing pull too much. ...OR cause too much nose-out yaw, which causes drag (dragging the model around kinda sideways) and messes up stability as well as maneuvering. The 'ground test' for good leadout location is to assure that the model DOES hang slightly (SLIGHTLY!) nose-out when hung by the leadouts. That is, the fuselage centerline should hang 2 to 3 or 4 nose down when hung that way.
...the reason is simple, too. The pull tries to aim at the center of mass - CG - , from where it reaches the leadout guides. The lines sag behind a straight line from the handle, because they have drag from flying through the air. Lines - thin wires - can't push sideways, but they can AIM the pull on them along their length. We want it to go through the front-to-back CG location, right? If it aims too far forward, it will try to pull the nose into the center - not good, right? If it aims too far back, we're back to the condition of dragging the model around part sideways to the direction of flight.

These thoughts are important because line pull is the strongest force applied to the model in smooth flight (sharp maneuvers bring their own problems, but if you are "centered" between the extreme cases with those, at least the effects will increase similarly for "inside" and "outside" turns as you go further from smooth level flight.) Thrust? remember, only enough thrust is applied to the model to overcome the fairly small drag; in level flight, that's not a lot. Take-off acceleration and meeting maneuver drags are temporary loads, which drop back to the low drag smooth level flight condition.

So, summing up: adjustable leadouts can help you tweak your model to a better flying condition, which is always welcome. They are a slight bother to build in, but well worth the weight and bother to be sure it won't destroy your enjoyment, or worse: your model.
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Old 06-12-2017, 12:32 AM
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I neglected mention of tipweight. Sorry...

Think of a model as reduced to its mass, at a single point, where it would balance front-to-back, top-to-bottom and left-to-right , and forces with points where they are applied and their directions, or "lines of action."

Thrust - applied at the crankshaft center, about at the back face of the prop. Its direction is along the shaft centerline. Simple enough. right?

Lift? reduce its total as acting at a single point somewhere between the inboard wingtip and the outboard. Because the outboard wingtip flies at a larger radius than the inboard, it travels further In the same one-lap time. Airspeed increases from inboard to outboard wingtip. Lift varies with airspeed - and other things. So, the point where lift is equal to both sides will NOT be at the center of the span structure. (The actual place can be estimated, but let's skip that for now.)

The center of lift should be at, or very close to, the spanwise center of mass. If not, lift will tend to roll the model accordingly.

The solid pieces that make up the actual model have a spanwise CG, too. BUT, there's also the weight of the lines, on the inboard side, hanging on the model at the leadout guides. We use tipweight, in part, just to balance the model spanwise, so this weight doesn't try to roll the model in, to the center, which is still a bad idea.

Consider the weight of the lines as supported equally at two points: the handle and the leadout guides. A decent start to adding the correct outboard tip weight is to balance half the lines' weight. How?

A quick and simple method:- Put the reel with the lines you'll fly the model with, halfway out from the fuselage on the inboard wing. Put an empty reel of the same type the same distance out on the outboard wing and add tip weight until the model balances on the fuselage centerline. Easy to do with the model on its back on a towel or some such. It's the fat kid/skinny kid see-saw situation;- full weight at half the distance on the inboard panel is the same as half the weight at the full distance, and balancing that should be very close to balancing the model - in flight - on the centerline of the fuselage, where lift, drag, thrust and weight force lines of action should all pass through , or very near, the mass center.
Aerodynamic stability forces - not the varying lift needed for maneuvers and its drag - act the same all the time, and can adjust some force imbalances, but maneuver forces come and go at different points in a flight. They're dynamic - due to forces that only act while in flight. Yet, making the actual hardware balance while NOT in flight - static balance - can help, but there will always be some odd effect that only appears in flight. That's why adjustable leadouts and tipweight are so useful.

There's more, but these should get you started safely. Our CL models are NOT simpler than other flying contraptions. We have another whole set of forces that "free-flying" aircraft don't have: - the tether, our flying lines. We have the same lift/weight, thrust/drag, left/right conditions, yes, but we also have the tethered path, with its forces and loads..
And, we are close enough to our model to see it fly as we interact with it in direct, tactile contact with the air it flies through. GRAND!

Last edited by Lou Crane; 06-12-2017 at 12:44 AM.
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Old 06-18-2017, 05:33 PM
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Thanks for your answer Lou Crain,
ive decided to build as always and use the adjustable lead out as a means to find tune the plane.
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Old 06-24-2017, 06:49 PM
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Brodak finally got back to me and they said; no engine or rudder offset is needed with the adjustable lead-out's utilized.
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