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Building Tips and technical articles. => Building techniques => Topic started by: kevin king on October 31, 2022, 05:20:23 PM
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I cant figure out what to do now. I read the elevator Tapering instructions and it calls out for a 3/8" rod for 3/8" elevators. But both the stab and elevators included in this Vector 40 kit are a 1/2" thick. So how do i properly taper that? Do i need different rods??
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Side view
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Assuming it remains an 1/8" taper.
Then, a 1/2" rod at the leading edge a 3/8" rod at the trailing edge, turn it over and a 1/4" rod at the trailing edge.
Craig
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But the side view on the plans seems to indicate the elevators are 3/8" thick. Maybe the solution is two 3/8" rods front and back of the 1/2" elevator and sand then down to 3/8". With my luck they will warp.
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My Brodak plans, which I think are the same as yours, says the following.
Elevator from 1/2" balsa, then, use the 3/8" and 1/4" rods etc.!
I guess you sand it down to 3/8 at the LE. Not sure why they didn't just use 3/8"?
Craig
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At the time this was designed, it was common practice to have the elevators slightly thinner than the stabilizer. This is supposed to help eliminate hunting, I think, in place of having slop in the elevator control horn hole. It , in theory, makes a sort of "dead" area of airflow over the elevators. If you start with 1/2" thick wood, by the time you finish you should have a precise 3/8" thick elevator leading edge. If you start with 3/8", You have to be careful you don't inadvertently make the leading edge too thin. That's my take on it.
Type at you later,
Dan McEntee
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Ok, i think I understand now. I guess i was assuming because the flaps use a 1/4" bar on 1/4" flaps I mistakenly assumed the 1/2" elevator should use a half inch bar. Thanks for clearing that up.
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I cant figure out what to do now. I read the elevator Tapering instructions and it calls out for a 3/8" rod for 3/8" elevators. But both the stab and elevators included in this Vector 40 kit are a 1/2" thick. So how do i properly taper that? Do i need different rods??
Kevin, see Al Resinger’s post and mine that follows it in the following thread. In short, you can not make proper tapers where the planform of the flap or elevator tapers using the rod method without a jig.
https://stunthanger.com/smf/building-techniques/sanding-a-taper-in-flaps-stab-ect/msg568720/#msg568720
Kim
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Kevin, see Al Resinger’s post and mine that follows it in the following thread. In short, you can not make proper tapers where the platform of the flap or elevator tapers using the rod method without a jig.
https://stunthanger.com/smf/building-techniques/sanding-a-taper-in-flaps-stab-ect/msg568720/#msg568720
Kim
You would be correct, assuming the flap is completely rigid. Fortunately for this purpose, it isn't, and also fortunately, sanding a thick balsa sheet to a taper releases enough other stresses that it swamps the exceedingly low stress induced by sanding these two "out-of-plane" surfaces on a flat tabletop. So the "two rods" method, with or without a jig, is a perfectly satisfactory way of achieving the goal.
I will elaborate later, however the point Alan makes is that, at least as described, the faces of the flap are not planar figures- the surface has a twist in it. It is "fine" for the first surface because the other side of the sheet holds it rigid against the surface, but when you flip it over it does not want to sit flat because it is not in a plane. He has a method of getting around this, but I think it is unnecessary from a practical standpoint, and this particular method *does not* induce any unexpected twist in most practical cases.
Brett
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You would be correct, assuming the flap is completely rigid. Fortunately for this purpose, it isn't, and also fortunately, sanding a thick balsa sheet to a taper releases enough other stresses that it swamps the exceedingly low stress induced by sanding these two "out-of-plane" surfaces on a flat tabletop. So the "two rods" method, with or without a jig, is a perfectly satisfactory way of achieving the goal.
Brett
Yes, you can manage with 2 rods , but you will have to glue the t.e. rod temporarily to very centreline of t.e. Cutting a well-fitting groove in the c/l helps a lot with that.
If you want to have both rods against the table, you'll need 3 rods;
-1st t.e. rod with ø= (flap total thickness :2) + (final t.e. thickness : 2)
-2st t.e. rod with the ø=final t.e. thickness.
-And the front rod with same height/ø than flap l.e. thickness, of course.
That is naturally more complex and has a bigger risk of introducing an error to the flap, so in my opinion gluing the rod in flap c/l is better.
But even better, more stable and often with a better stiffness/weight ratio, is a built-up flap with 1/20..1/16" skins and some ribs inside.
Using same kind of highly complex calculations as above, it is very easy to sand or hot wire-cut a foam building jig with correct surface twist to produce a straight flap.
I'm just about to build such flaps for my new model, so I can document the process in here. But don't hold your breath, it'll take a while. L
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Yes, you can manage with 2 rods , but you will have to glue the t.e. rod temporarily to very centreline of t.e. Cutting a well-fitting groove in the c/l helps a lot with that.
If you want to have both rods against the table, you'll need 3 rods;
-1st t.e. rod with ø= (flap total thickness :2) + (final t.e. thickness : 2)
-2st t.e. rod with the ø=final t.e. thickness.
-And the front rod with same height/ø than flap l.e. thickness, of course.
Which is exactly as shown on the plans on the first post. This issue (if there is one, which I there contend is not), is when you go to do the second surface, and it won't lie flat on the table because it is not a plane. You have to stress the flap to sand the second surface - but that is not an issue because that stress is negligible compared to the unaccounted stresses that you release by sanding 1/4" of wood away from the surface.
Brett
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Oh sorry, I saw the picture too late.
I think you'll introduce more (permanent) stress by sanding with too high pressure/dull sandpaper. L
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Another way which I use is to glue a basswood strip (3/32 sq.) down the center of the trailing edge, then sand down to it. The strip can be removed or just left on to help prevent dings on the trailing edge .
Dave
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Another way which I use is to glue a basswood strip (3/32 sq.) down the center of the trailing edge, then sand down to it. The strip can be removed or just left on to help prevent dings on the trailing edge .
Dave
I do the sme thinAnother way which I use is to glue a basswood strip (3/32 sq.) down the center of the trailing edge, then sand down to it. The strip can be removed or just left on to help prevent dings on the trailing edge .
Dave
I use a 3mm x .5mm strip to do the same thing. Not a single warp in stab or flaps where I have used the cap.
Ken