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Old Nov 17, 2012, 04:31 PM
M0unt@in M0del$ minion
turbojoe's Avatar
United States, AZ, Mesa
Joined Jul 2002
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Help!
Incidence with a "V" tail?

A couple of us are converting Herr V-35 Bonanza rubber power free flight kits to electric R/C and adding a few mods along the way. I've built a ton of planes from kits and plans but this is my very first V tail. My build thread is HERE.

It had been brought up by glewis (a previous Bonanza builder) that the flat bottom wing while fine for a free flight rubber power design pretty much sucked for his R/C conversion and suggested going with a Clark Y design. rtibbetts kindly obliged and plotted out ribs for us in Profili. He did an awesome job! My question now is a matter of incidence. What's the best method to measure it with a V tail? I do have a Great Planes laser incidence meter but there is no way I can hang it on this tiny tail. Wing yes. Tail, no way Jose. One good thing (I hope) is that with the tail in place the joining point of the "V" lines up with the fuselage longeron that just happens to be the datum point on the plans. Will that V joining point actually be the true aerodynamic "0" incidence? Or does the V design require a completely different approach?

I'm at the point where I can still easily sand the fuse to change the tail incidence and I have yet to cut the wing fuse saddle to accommodate the new rib profile. Having zero V tail experience I'd like to get it right before finalizing the needed modifications to the fuse. Thoughts on whether incidence would be best set at the tail or wing on this design will be welcomed! I think this is going to be a fun little plane if I get everything set just right.

Thanks for any help guys. Please feel free to make suggestions in my build thread too as based on the amount of thread views it seems a lot of people are interested in converting this particular kit. I'd like to see them have success as well.

Joe
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Old Nov 17, 2012, 04:58 PM
M0unt@in M0del$ minion
turbojoe's Avatar
United States, AZ, Mesa
Joined Jul 2002
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OOPS! Just took a quick look at the plans again. That fuselage datum line actually slopes 3/32" negative to the front of the wing saddle as measured from the tail.

Joe
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Old Nov 17, 2012, 05:11 PM
"...certainty is absurd."
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Joined Jan 2007
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Since it looks like a flat stab in your build photos, the bottom of the stab or the "v joining point" will be the zero incidence angle of the stab.

You do realize the Clark Y is also a flat bottomed airfoil? It is a good airfoil even a pretty low Re, but your scale will be pushing past the limits. Thinner airfoils work better at that low of Re.The zero lift line of the Clark Y is about 4 degrees above the chord line, so if you set the chord line to zero incidence, you actually have about 4 degrees of positive incidence in the wing.

It is not the flat bottom or thin airfoil that makes an airplane have pitch/speed coupling. It is a forward CG, used to trim out an airplane that has too much incidence in the wing. If you move the CG to 10% static margin, and reduce the wing incidence so you don't have to move the CG forward to trim it, the pitch trim will stay much more neutral with speed changes (assuming the thrust line is correct).

Kevin
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Old Nov 17, 2012, 08:36 PM
M0unt@in M0del$ minion
turbojoe's Avatar
United States, AZ, Mesa
Joined Jul 2002
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OK, to simplify things I sanded the tail mount area slightly so that the joined edge of the V lines up with the longeron and that can now be used for the datum point. The bottom edge of the blue tape runs along the longeron and was used simply because it shows better in a photo. As the template is taped to the fuse side it is 49mm from the fuse datum to the rib datum. As I understand this would be 0 incidence right? If I measured to the flat area of the rib bottom it would be 4 degrees positive incidence that you described as inherent in the Clark Y design?

Joe
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Old Nov 17, 2012, 10:06 PM
"...certainty is absurd."
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Joined Jan 2007
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Joe,

I was referring to the chord line of the Clark Y - the line thorough the foremost and rearmost part of the airfoil. There is about 6 degrees between the flat bottom of a Clark Y and it's zero lift line. It is actually closer to 3.5 degrees than my original 4, between the chord line you show and the Clark Y's zero lift line.

The angle between a cambered airfoil's chord line is roughly proportional to the % camber - A Clark Y has 3.8% camber, and about 3.5 to 4 degrees between it's chord line and it's zero lift line.

On a symmetrical airfoil, the zero lift line and the chord line are the same thing. Using the chord line or the flat bottom to align a cambered airfoil will result in a lot of incidence in the wing. The only way to trim it then is to move the CG forward. This results in a large amount of pitch stability, which makes it have a large speed/pitch sensitivity.

I've attached a drawing of the Clark Y angles. Usually the wing is drawn at it's cruise angle of attack, maybe -1 degrees for the chord line of the Clark Y, giving 3 degrees above it's zero lift line. This would be a fairly slow cruise. Then the fuse is put at it's least drag angle over that, usually zero angle.

Kevin
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