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Old Apr 27, 2015, 04:31 PM
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square or tapered trailing edges?

hay all

my plane tracks well but im wondering if it can be better. some wag in yaw and hunting in the wind. the plane is a 3dhobbyshop extra 72. i have rebuilt it a bunch of times and have added inch to the elevator and the plane ways 10.5 lbs on 8s. cg = slight pitch coupling on knife edge/ 04% mix rudder to up elevator.

was thinking of tapering the trailing edges of rudder and ailerons finer than the ARF stock. will this help? the other option is to add basa to the trailing edge to continue the lines of stock airfoil.

do pattern guys worry about the trailing edges?

matt
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Old Apr 27, 2015, 05:08 PM
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wag in yaw?... can't say I've ever seen a 72 wag.

if you want to remove the coupling, push the battery forward.

trailing edges actually do have an effect to control, kind of like a mechanical expo of sorts, it wont help the tracking. actually making them worse may help the tracking and centering: add a half inch strip of 1mm plywood to the trailing edge in a T shape. Have done it myself, and pattern guys do it a lot especially on rudder.

Do an image search for F3A planes, they've tried all kinds of crazy things to their planes.


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Old Apr 28, 2015, 02:17 PM
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Yeah, I wish I had the time to experiment with it a little more, but when I was building planes with Godfrey, we used to make sure the trailing edge was no thicker than the two 1/16” balsa sheets plus just enough adhesive to keep them together. Not many planes that track as well in IMAC, but there are other differences. The wings and stabs are in the 16 to 18 percent thickness range as well, which will add a bit more stickiness to the airfoil, and maintain it through a a higher AoA. This is easy to do without adding weight on the sheeted foam wings, but this thickness could add weight to the built-up wings with the material, adhesive, and assembly techniques we typically see from China.

A 50cc kit used 6mm and 3.5mm ply, heavier Monokote, thick aluminum LG (my undercarriage weighed 583g), heavy split 388g cowl, and a thick plastic, heavily framed canopy (327g), and 10mm stringers, and still came in at 17.4 pounds wet (7890g). 35% planes were 22.5 pounds wet, and 40% were 37 pounds. Wag and yaw would self-correct before you had a chance to input a correction if built correctly, yet some guys still think a heavier plane tracks better. I think it's more about design from what I've seen, than about weight. but a 10.5 pound 330 should be no problem.

I just wonder if the thinner wings and stabs we see more of has something to do with it, but all in all, the 3DHS and EF planes track incredibly well and if I had one that didn't, I’d tend to look for a problem somewhere, like a warped control surface, incidence issue, etc.

Another thing to consider is precession and slipstream and how the vertical stab is configured. For a hint about this, look no further than the design files that were declassified after WWII for the P-51 and look closely at what they did.
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Old Apr 28, 2015, 04:43 PM
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i admit i am being picky...... i moved the lighter batteries and esc foreword and it seems better..
so 1/8 inch is the standeed eh? i wont sharpen them when i recover...

what about the p-51? may i have a link?

matt
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Old Apr 28, 2015, 06:11 PM
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Generally with 3D airplanes you won't get them to track arrow straight like a pattern ship will. They can track pretty good but if there is enough wind they'll just wander all over the place especially if there is turbulence. For the most part, they're designed to not fly arrow straight, rather be all over the place, wadded up into a ball
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Old Apr 28, 2015, 07:29 PM
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...well, if ever there was a chance to disagree with Xpress.. it's this one.
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