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General control line discussion => Open Forum => Topic started by: Mike Griffin on March 18, 2012, 04:41:14 PM
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This is a question that I have often wondered about pertaining to wing mounted wire landing gears. Is there any hard and fast rule about how far forward the gear should be for a tail dragger type model? For example.. do you want say..... 1/2 of the wheel showing past the leading edge when you look straight down on the wing? Is there a certain angle from (90 degrees) straight down that the wire should be bent toward the leading edge?
Thanks
Mike
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This is a question that I have often wondered about pertaining to wing mounted wire landing gears. Is there any hard and fast rule about how far forward the gear should be for a tail dragger type model? For example.. do you want say..... 1/2 of the wheel showing past the leading edge when you look straight down on the wing? Is there a certain angle from (90 degrees) straight down that the wire should be bent toward the leading edge?
Thanks
Mike
15º forward of the C.G. seems reasonable to me but it could be slightly more if the ground conditions are poor.
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15º forward of the C.G. seems reasonable to me but it could be slightly more if the ground conditions are poor.
I asked this question several years ago and got just that answer from Ted Fancher. Too far forward and you'll tend to bounce, too far back and you'll tend to nose over. You have to find a happy compromise, and on rough grass that's more toward "don't roll over".
The bounce, by the way, is an aerodynamic one: the farther forward the wheels are, the more the airplane will rotate tail-down when you touch the ground -- the more you rotate, the less speed it takes for the wings to start lifting.
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25 degrees forward from the CG for grass, and I think 15 degrees forward for pavement.
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Nothing special about the angle. Its the distance ahead of the CG that counts. I shoot for .75" ~ 1" from CG for pavement, 1.5" for grass. The angle will vary depending on height of gear and where it comes out of lower wing surface.
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To simplify the landing gear placement and adjusting it, start with a scale. Assuming conventional gear, set the plane on the workbench, raise the tail where the wings are level and slide a scale under the tail wheel. The weight exerted by the tailwheel should be approximately 10% of the weight of the whole plane. The result will be extremely close to where you need to be on pavement. Almost the same for trike gear. 10% of the weight on the nose gear.
Pat Johnston
Scientific Trim Solutions
Skunk Works
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Thank you all for your replies.
Mike
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Mike,
I was always told that the axles should approximately align with the leading edge of the wing. Here is a drawing showing the setup I use on my profile ships. I set up for a maximum 12.5" diameter prop clearance when the model is level, and use 2" diameter (Lectra Lite) wheels. I use a 30 degree exit angle from the wing with 1/8" music wire gear. That provides a little spring, and is conveniently measured with a common 30-60 right triangle.
The other two 3D drawings show the details of my wing mounting method. The gear wire is held in place in the finished wing with a rectangular strip of hard balsa shaped to the curvature of the airfoil on the leading edge bottom of the wing. The gear blocks are all laser cut from 1/8" Lite Ply. They mount between ribs R2 and R3 and these ribs have 1/64" plywood laminates on their front facing surfaces to strengthen them for supporting the gear blocks.
I hope this is useful to you.
Best regards,
L.
"Congratulstions on breaking my record. I always thought the record would stand until it was broken." -Yogi Berra, to Johnny Bench
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Mike,
I was always told that the axles should approximately align with the leading edge of the wing. Best regards,
(Clip)
L.
"Congratulstions on breaking my record. I always thought the record would stand until it was broken." -Yogi Berra, to Johnny Bench
That only works when the geometry of the wheel spacing (tread), wing LE sweep, wing planform, vertical position of the wing, vertical CG of the model, and whatever else puts the axles at about 15 degrees ahead of the model CG. Yes, for "a normally configured stunt ship" (whatever that is), that rule of thumb "generally" works. The 15 degree prameter works, even for full scale aircraft with a conventional gear (non-tricycle LG).
Keith
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Thanks Larry. The CAD drawing showed me what I thought. That helped.
Mike
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I forgot to mention, with the type of removable gear wing mount shown, the spring of the gear involves torsion of the horizontal portion of the gear wire. And the length of this segment affects its "torsion bar" effect - shorter makes the gear stiffer. Allow clearances for the gear wire to be twisted (see my bottom view illustration).
I like this type of gear mounting, it makes finishing easier, and you can easily make multiple sets of gear for different conditions. If you always fly on tarmac, the wheels can (and should) move back for a better landing. On grass, the forward position is usually better.
On the "Shogun" ship I built (bashed SV-11), I "retrofitted" a (foam) wing mounted gear, and it ended up entering the wing aft of my preferred location, with a longer horizontal section I angled the gear strut piece to make it less obvious and it worked out OK, if a bit springier than I would have preferred.
L.
"The walls of medieval cathedrals were supported by flying buttocks." -from Kids Say the Darndest Things
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I am ashamed to say that those wheel pants are not only better than any I've made, but they are better than any I have tried to make.
The cowl interface design looks like something I'd do, though.