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General control line discussion => Open Forum => Topic started by: Leester on November 05, 2010, 04:22:10 AM
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I have a Larry Richards kit of the P-63 King Cobra, (the old Midwest version) the plans show to start tapering the profile fuselage 9" from the end to a 1/8" thickness where the rudder attaches ??? Is this to thin ??? or should I make the rudder 3/16" or 1/4" so there won't be as much flex in the tail ??? Crist Rigotti cut some foam cores for it so I'll lose the baseball bat leading edge and trailing edge. I hope to have time to work on it this building season. Any other tips or suggestions would help. TIA
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If you are worried about the fuselage being too flexable, you might tried some hardwood strips let in to the balsa fuselage. Then after sanding the taper in. Use silk span with grain running the length of the fuse. Or use epoxy with carbon veil. I had one of the original kits and never gave it a thought when the tails surfaces were flexing while sitting on the ground waiting to take off. Even with thin airfoil and flexing fuselage it flew great. H^^
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This gets into a kinda esoteric subject called aeroelasticity. I think I got a C in that class. Generally, we try to make everything on a stunter stiff. Because aerodynamic loads vary with the square of airspeed and structural forces are independent of airspeed, the shape of a wimpy airplane will vary with speed. On a wimpy-fuselage profile, for example, the tilt of the stab can vary with speed. This can make an airplane's performance unpredictable or downright squirrely. If you put in a lot of effort so you really know what you're doing, you can make things intentionally elastic and get some benefit from it. This is beyond my level. The Beringers do it.
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Since it is a profile model, I would start the taper right at the elevator hinge line. If this also is where the rudder hinge line is, I would simply round off the back end of the fuselage so almost all of the area under the stabilizer is 1/2 inch width.
Don
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What Don said, or, I usually start the taper an inch or so in front of the stab L/E and then only bring it down to 1/4" or 3/16" depending on the rest of the tail design.
IMO, this is one of those things that doesn't need to be overthought a whole lot. Forget the plans and go with your gut. It's definitely better to leave a little extra "meat" than take too much. Keep it strong in this area of the build more than worrying about weight. Mainly just take into consideration the hardness, density, stiffness etc. of the wood itself.
Or you could do this, sand in the taper afterward, and then sheet it...
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Thanks, I guess I should have checked the actual parts first as Larry has made the fin and rudder 3/16" instead of the 1/8" the plans show. He also made the stab. and elevator 3/16". I do think I'll start the taper closer to the end of the fuselage than the plans indicate as Don suggests. Nice job going there Wayne, that Zero will be another winner I'm sure.
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Thanks Lee, I've actually got one of Larry's P-63's on my shelf too. Another one to get at in my "spare" time. I've always thought I'd like to do it as "Tipsy Miss", John Sandberg's old Reno racer...
http://www.flickr.com/photos/lightning72/3876538925/
I'd really like to see some build pics as you progress on yours. y1
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Thats a cool picture !!!
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After seeing various flexi-fliers shake their tail feathers I no longer taper much. I've been rounding off the top and bottom. Clunky. Primitivo. A Flint Stone club. IT'S A PROFILE. I know, aesthetics intrude. PROFILES ARE SO SIMPLE, there's a devilish impulse to get complicated.
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So true. H^^
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I have a Larry Richards kit of the P-63 King Cobra, (the old Midwest version) the plans show to start tapering the profile fuselage 9" from the end to a 1/8" thickness where the rudder attaches ??? Is this to thin ??? or should I make the rudder 3/16" or 1/4" so there won't be as much flex in the tail ??? Crist Rigotti cut some foam cores for it so I'll lose the baseball bat leading edge and trailing edge. I hope to have time to work on it this building season. Any other tips or suggestions would help. TIA
Yes Doc,
That would be too thin. Mine had the fuse full width all the way. I think Whitely told me to do it that way. It was a good model, except the tail is too small. I'd scale it up a bit. The kit leading edge shape is very pointy, too sharp for my taste. Mine was a kit with a Fox 35 and was 5th at the first VSC. I probably flew it 500 flights and it was an OK model for Classic. There are a lot better designs out there!
I redrew the side profile to look like Sandberg's Tipsy Miss. I always had a love for the racing King Cobra's and John's GM Orange one looked the best.
Chris...
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Chris As I said I'm going with a foam wing and will be sure to make the leading edge more blunt than shown. Speaking of foam wings I got out my wing skins video from Windy which I converted to dvd and arn't you the young !! assistant to Scott Smith ?? That must have been a few years ago. The Tipsy Miss does look very good, I'd like to see the Orange one.
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That picture I linked is the orange one Lee. y1
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I knew that ..ya LL~ LL~ LL~
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I'd keep the fuselage full width, preferably 3/4 in., and taper it down starting just behind the leading edge of the stab. If you are going to go to the trouble of building it up, do it the easy way and use 1/16 in. sides and a foam core. Saves tons of weight over a balsa fuselage.
Going from 1/2 in to 3/4 in will make the fuselage at least 2.25 times as stiff. But you will have to do other things to keep the weight down. All balsa will be too heavy.