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Leadout Wires

Old 11-20-2008, 11:42 PM
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Default Leadout Wires

I'm looking for some advice. I'm building a balsa wing to replace a Styrofoam wing on an areobatic plane. The original bellcrank will be used. The bellcrank has 3 1/16" leadout wire to leadout wire holes. The original symetrical wing has a plastic guide attached to the wingtip which is 1" wide with six wire guide holes spaced 3/16 " apart. The exit guide is located at the thickest part of the airfoil. I know from reading web articles about this aircraft that the pilots have had greater and lesser success utilizing different exit wire guide holes. It's not clear to me why. It is also not clear to me why I don't just match my wingtip exit holes to the bellcrank holes. That is, two parallel lead out wires. I plan on running the leadout wires through aluminum tubing tp prevent wear and tear on the balsa ribs and to make leadout wire replacement easier. I'm a novice and probably would not appreicate different wire spacing anyway. Any suggestions?

Dumb as a rock in Michigan
Bill
Old 11-21-2008, 01:33 AM
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Default RE: Leadout Wires

The position of the wires at the wing tip leadout determines how the plane flies, either nose in or nose out (yaw), and is more or less a straight line between the centres of the two wires at the wingtip through the CG of the model. It doesn't matter where the bellcrank is located. It's desireable to have a slight sweep back of around 1 1/2 degrees which means the centre between the leadouts should be slightly behind the CG. This is easy to see by just hanging the model evenly by both the leadouts and looking for a slight nose down attitude of the fuselage. That's what's showing in the first photo. Looking at that photo it's easy to imagine that if the leadouts were further forward on the wingtip then the nose wouldn't be pointing down as much.

But then there's the next trick. In the photo the model is suspended evenly by both the up and down wires the same as in level flight. However, when you're doing a loop or any other manoeuvre then there's a different force on each wire. Say you give full up, then there's more strain on the up wire but an equivalent amount less on the down wire. That shiftes the average position that determines what angle the model hangs to the CG so the yaw attitude in flight will change. To make that change as small as possible the two leadout guides should be as close together as practicable. With your leadout guide having 6 holes you'd have to choose the two closest holes that also gives the correct yaw angle. The trouble is you can only find that when the model is completely finished and ready to fly by checking exactly where it balances then fixing the leadout guide to give that roughly 1 1/2 degrees sweep back.

The better way is by using an adjustable lead out guide like in the other two photos where you can simply slide the guide in a groove by loosening the clamping screw. The one in the photos is just a roughy I made up for that basic stunt trainer I made for my son and the two lines should really be a bit closer together but it's good enough for the purpose.
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Old 11-21-2008, 09:14 AM
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Default RE: Leadout Wires

Where are you in Michigan? We have a group that flies here in Flint. There's also a group that flies in Lansing as well as the Detroit area.
What plane are you rebuilding? Sounds like either a combat style plane with a foam wing or a 1/2A.
In any case you don't need to make it easier to replace the leadouts if you are using good leadouts in the first place. Solids are good but I use wire on all my planes.

Hope to see you around this summer.

Jay
Old 11-21-2008, 04:04 PM
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Default RE: Leadout Wires

Interesting pictures and ways to adjust the lead-outs. But here is a question for your consideration. I have an old Lil Thunderbird kit (Veco/Dumas if I remember correctly). Instead of the lead-out wires being parallel or like a double-barreled shotgun with the barrels alongside each other, the markings on the wing ribs and on the plans call for an over and under arrangement (like an over/under shotgun with one barrel being above the other). One lead-out (up-elevator) comes out above the wing tip and the other (down-elevator) below the wing tip. Any comments?

Dave[8D]
Old 11-21-2008, 06:50 PM
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Default RE: Leadout Wires


ORIGINAL: olenorski

Interesting pictures and ways to adjust the lead-outs. But here is a question for your consideration. I have an old Lil Thunderbird kit (Veco/Dumas if I remember correctly). Instead of the lead-out wires being parallel or like a double-barreled shotgun with the barrels alongside each other, the markings on the wing ribs and on the plans call for an over and under arrangement (like an over/under shotgun with one barrel being above the other). One lead-out (up-elevator) comes out above the wing tip and the other (down-elevator) below the wing tip. Any comments?

Dave[8D]

It's often a real good idea to have the two leadouts on the same line with the centerline of the airplane. The idea is that there is less drag from one leadout following directly in the wake of the other, compared to two of them cutting their own path through the air. Once you've got a couple of loops in the lines, it doesn't matter so much (if it did in the first place).

Some guys used to theorize that placing each leadout so it'd "pull back" and help fight the natural yaw was the way to go. So they placed the up line aft the down line. Nose up pitch would give p-effect that would yaw in, so that line being behind the perfect spot in the tip should fight that yaw. Same idea with the up/down leadout relationship for roll couple.

Considering the real effects, it was a lot of fun to discuss.
Old 11-21-2008, 08:19 PM
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Default RE: Leadout Wires

Bill,

What Downunder posted is an excellent start. It can be much more involved, very involved... which isn't needed unless you are going for broke, and want the most absolutely pure perfection... A few of us do, you know...

The other factors are gyroscopic precession and line "rake" angle...

As Downunder said, the center of pull force shifts between leadout positions as we input control. The air resistance of moving the elevator and/or flaps into the breeze changes the load to more on the "working' line and to less on the other one. Traditional layout has the pushrod outboard of the bellcrank pivot, and control horn under the elevator. In this case the forward leadout is the DOWN line. Think about that...

GYRO: You've played with a toy gyro, I'd guess. When you get one spinning, and try to tilt the axis, the gyroscope pushes in another direction - 90° "later" in the flywheel's direction of rotation... The prop also acts as a gyroscope. Most of our engines turn counter-clockwise, seen from the front. When we fly and pull in some UP, in efect, we are trying to tilt the prop back at the top. The gyro responds by pushing the model's nose to its right. (Same-same for DOWN controls moves, but the other way.)

If we flip the bellcrank over, so the pushrod comes off the inboard side of the bellcrank pivot, we've made the forward line the UP line. It tends to move the effective pull force forward (left, or nose inboard) while the gyro effect tries to push the nose to the right. Same in reverse for DOWN control. Theoretically, we might be able to "design" things so they exactly cancel out. Lotsa luck..

RAKE: We can see the flying lines sag "aft" from the straight line to the model as we fly. There is a program - Line III - available to estimate what that curve means in terms of the angle with which the lines meet the wingtip. Our lines are cables. They can only carry tension (pull) along their lengths. So, pull force aims at the angle with which the lines reach the wingtip.

The CG will align with that angle, from the point where the lines meet the wing. Other things, like rudder offset, don't have much effect.

That makes it important to put the leadouts in the right place. We want the model to fly as tangent to the flight circle as possible. That's the cleanest in terms of drag, and it prevents crabbing (very slightly) that can mess up tight maneuvering.

As I said, it can get complicated. Doesn't need to. What has worked since the early 1940's still works. These are just thoughts you might follow up if they interest you, over time. Hobbies like this can be VERY satisfying - they are open ended into all kinds of detail, or as simple as an arf with the engine running in the box when UPS shows up...
Old 11-21-2008, 08:25 PM
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Default RE: Leadout Wires

...to follow up...

The over and under leadout setup mentioned had no ability to counter the gyroscopic precession yaw tendencies.

Bob Palmer, who designed two Thunderbirds and probably also the Little T-Bird, tried the over and under leadouts for the Thunderbird II, but went back to fore and aft layout in hiw later designs, ASIR. The "trailing in the front line's wake" idea seems to have more benefit... Also, if the model for any reason starts to twitch in roll, that gives a slight up or down control input all by itself...

(I'd been timed out in attempting to put all this in one post, so I'm running hard, here, gasping and fighting cricks in my legs...[:-]

So, enjoy! Worry about real things, and enjoy the tactile joy of having your model actively fighting your personal arm muscles!
Old 11-21-2008, 08:50 PM
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Default RE: Leadout Wires

Downunder has illustrated the basic 'slider" type adjustable leadout guide. I make my slider out of 1/8 ply with a blind nut for the holding bolt and use the golden-rod type yellow plastic tubes, about 3/8 long, for the leadouts to go through. I glue them in with thick CA. Get the leadouts as close together as you are comfortable with. Separation will generate yaw when you give control one way or the other. On a 1/2A size airplane, I like both leadouts to come out one hole.

Where adjustable leadouts help you is when you have good line tension down low, but poor line tension above 45 degrees. This can be corrected by moving the leadouts forward. Move them about 1/8 in at a time until the airplane pulls to suit you. If you get them too far forward, the airplane will yaw in when you hit a hard corner.
Old 11-22-2008, 06:29 AM
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Default RE: Leadout Wires

Where adjustable leadouts help you is in trimming the airplane. Where they wind up can help the airplane fly better, but where that is is up to you. Just having adjustables won't do squat by itself.

If you're one who adjusts your CG in your trimming flights, or adjusts your surface throws to get better turns, then you'll benefit from an adjustable setup. And you'll need to keep it adjustable until you've got the sucker sorted out.
Old 11-22-2008, 06:35 AM
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Default RE: Leadout Wires

Having the two leadouts as close together as possible can be a good idea. But keep in mind that your line connectors are going to hang up on each other given the chance. And if the leadouts are the same length, they'll have the chance from the git go. And the closer the leadouts are to each other the more chance they'll have.

So if you're new to this, it's a good idea to have the leadouts close to each other, but if you do, then make your leadouts sufficiently different lengths to solve the connector fouling problem. Have you ever noticed how long the adjustment is on most guy's handles is? That's to make up for the different length leadouts on their airplanes. There won't be connector fouling at your handle, but there will be at the airplane. So the handles are made to deal with what happens at the airplane.

Lots of little details matter.
Old 11-22-2008, 02:36 PM
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Default RE: Leadout Wires

I think you would be better off having the leadouts far enough apart to not hang up, than having them different lengths. That would be a big headache I believe. I like to keep all of my stuff the same length if possible. I mostly only fly combat aircraft though and we hook our lines directly to the bellcrank. No leadouts required.
Also, I generally setup my leadout location so the fuselage is parralell to the ground. This is sufficient for proper line tension (which is the main reason in determining the location). If the plane is yawed extremely away from the circle this can create unnecesary drag and can actually create too much pull. Depending on the aircraft, you can get your arm tired. I prefer mine to be a little lighter. This is personal preference though.
Old 11-22-2008, 02:38 PM
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Default RE: Leadout Wires

I am interested to see this aircraft. Can you post some pictures? We can get really scientific and extravogant in our explanations when this could just end up being a sheet balsa 1\2 A!
Old 11-22-2008, 07:04 PM
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Default RE: Leadout Wires

ORIGINAL: vertical grimmace
We can get really scientific and extravogant in our explanations when this could just end up being a sheet balsa 1\2 A!
Actually the model in my photos is just a sheet wing thingy. Except it's got a Magnum .53 in it..........................
Old 11-22-2008, 07:24 PM
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Yes, and that looks like a very nice plane.
Old 11-22-2008, 09:26 PM
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Default RE: Leadout Wires


ORIGINAL: Ram Jet

I'm looking for some advice. I'm building a balsa wing to replace a Styrofoam wing on an areobatic plane. The original bellcrank will be used. The bellcrank has 3 1/16" leadout wire to leadout wire holes. The original symetrical wing has a plastic guide attached to the wingtip which is 1" wide with six wire guide holes spaced 3/16 " apart. The exit guide is located at the thickest part of the airfoil. I know from reading web articles about this aircraft that the pilots have had greater and lesser success utilizing different exit wire guide holes. It's not clear to me why. It is also not clear to me why I don't just match my wingtip exit holes to the bellcrank holes. That is, two parallel lead out wires. I plan on running the leadout wires through aluminum tubing tp prevent wear and tear on the balsa ribs and to make leadout wire replacement easier. I'm a novice and probably would not appreicate different wire spacing anyway. Any suggestions?

Dumb as a rock in Michigan
Bill


My sincere thanks to everyone:

downunder, CLpilot, olenorski, de rock, Lou Crane, Jim Thomerson, verticlal gimmace

You all put a significant amount time and effort into responding to my question. This should be collectively published somewhere. Thanks to my HP printer I have a copy of all for my files. My HP printer also enabled me to take a copy of a wing I took off the Net and scale it to my application (see attachment). I was asked about my project. I'm turning a Cox Hyper Viper into a Super Hyper Viper. I have become disillusioned with the ol' Cox .049 engine and I didn't like the Styrofoam wing. I'm "stuffing" an Enya .09 into the orginal airframe, building a proper balsa/basswood wing of 20% increased area, adding wheel pants, new aluminum landing gear legs, installing a very long sexy spun aluminum muffler (tuned pipe?), and a Brodak polished aluminum spinner. I'll probably have to add about 10 lbs. of tail weight for balance. Anyway you guys came to my rescue as I'm at a crossroads in my wing construction and since I'm locating the leadout wires and bellcrank inside the wing there won't be much opportunity to change things later. I remember my dad's Flying Clown as having the leadout wires exiting the wingtip with the same spacing as the bellcrank. I guess if this information was available in 1950 when he built it he would have done things differently. Cox's installation is in absolute harmony with your recommendations. C/L pilot, I'm in Ft. Gratiot. Flint and Detriot is a bit of a haul but who knows.

I've learned:
* Leadouts in line with the center of gravity and as close together as possible.
* Adjustable leadout guide is highly desireable and should end up angled 1 1/2 degrees back from a balanced yaw position.
* I think I will avoid an over/under leadout installation.
* Up and down controls as they relate to the gyroscopic effect of the prop - I never would have considered THAT!
* I can see where the leadout teminations could hang up if the leadouts are the same length. Since I bought a Sullivan Inst-A-Just control line handle I think I will stagger the leadout temination - wouldn't have thought of that either.
* I like the idea of the pushrod hole being inboard of the bellcrank pivot. Brodak says a bellcrank of this configuation leads to "smoother take offs, flying, and landings". Cox set their's up this way.
* Combat aircraft with no leadout guides? My mind boggles. Combat aircraft must be a-bit scarey to fly. Heck I'm challanged enough as a novice.

I'll try and get some photos to you guys. I'm thinking about buying a digital camera or maybe I could borrow one. Hey verticle grimmace, I've got plans for a Stunt Chimp which should be right up the sheet balsa 1/2 A alley. I tried to upload the plans but the software won't let me. If anyone wants them I'd be glad to mail you a copy. I could have built 20 of them in the time I've already spent on the Hyper Viper. A great plane to introduce your kids or grandkids to modeling.

Thanks again,
Bill

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Old 11-23-2008, 07:08 PM
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Default RE: Leadout Wires

The Combat aircraft have a leadout guide at the tip. But the lines themselves connect to the bell crank.
Old 11-24-2008, 02:49 PM
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Default RE: Leadout Wires


When the airplane is flying, the resistance of the air against the lines causes the lines to form a catinary curve (Sag(approximate)) = Drag x length x length /8 x tension), ignoring the weight of the lines. I believe that Mr. Rush determined that the bottom/belly of the curve was about 2/3 the length of the lines (on 60ft lines it would be at 40ft, 20ft from the airplane). If the leadouts are parallel and straight out from the bellcrank, that would cause the airplane to be pointed into the circle somewhat. In order for the airplane to be either pointed straigtht ahead, or a little out, the leadouts have to be raked toward the trailing edge of the wing. Adjustable leadouts allow rake to be varied to where the plane is flying tangent to the circle or a little out, which also adjusts how hard the airplane pulls. That is why your Sneeker gets light on the upwind side of the circle, not enough rake in the leadouts.

For electrical transmission lines, Sag= (Weight of the lines)x(length)x(length)/(8)x(tension). Good enough for spans up to 1000ft.

A rather poor explanation, but the best I could do, mis-spelt words and all.

Hothandle[sm=lol.gif][sm=lol.gif]


Old 11-26-2008, 02:53 PM
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Default RE: Leadout Wires

H/Handle, Great start on the idea of the catenary curve...

Suspension bridges and telephone or power lines don't usually face a uniform increase in wind speed from one pole to the next. That, however, is the standard case for our flying lines. Speed is distance divided by time, right? Miles per hour... feet per second... We don't have a simple catenary curve, we have an accelerated catenary. That fits with Howard's comment that the "bottom of the belly" (sounds anatomical!) of the lines - er, try it this way - the point where the lines are furthest back from the line-of-sight from handle to model is about 2/3 out from the handle (40/60 is 2/3, right?).

We also have to deal with one other problem: Speed changes uniflormly from the handle to the model, but drag changes with the square of the speed.

Lift and drag - aerodynamically -calculate from the square of velocity. If the model is flying, say, 60 miles per hour, the point halfway out the lines to the model, obviously, is "flying" (moving, anyway) 30 miles per hour. Full distance to the model is twice that half distance. Well, duhhh...

If the lines could lift, the lift at the model would be four times its value at the halfway point on the lines. Same way with the air drag on the lines. The model is twice as far as at the halfway point, so is going twice as fast. Twice squared is four times as much...

So, the angle the pull force aims when it gets to the leadouts is affected, too. Their curve - if you could look down on the lines from above during a flight - tightens as you get nearer the leadouts. The natural state is that the CG will align to that pull force direction, as aimed through the leadout guides. IMHO, the leadouts should be lined up so they practically float free inside the guides in level or straight flight. Any change in direction, whether we add handle input, or the model rolls or yaws for whatever reason, moves the leadouts some, at least. That aims the pull force away from the CG, which tries to get back in alignment.

Some studies I've played with suggest that this is a very powerful stabilizing factor for our models. That's why I'd like it aimed RIGHT for simple flight. That's the center of the different moves we can make in maneuvers and altitudes. With any luck at all, we'd have about the same range of deviation from simple CG-aligned flight for outside and inside turns. I like that.


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