Extreme Speed Prop Planes Discuss the need for speed with fast prop planes (Screamin Demon, Diamond Dust, Shrikes or any REAL sound breakin'''' plane)

Viper 500 Tail incidence?

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Old 07-11-2004, 11:02 PM
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RCAddiction
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Default Viper 500 Tail incidence?

Currently assembling a Viper 500 to build up my reflexes a little bit since I've been mostly flying funfly and 3D stuff of late. Fastest current plane is a 40FP powered Dogfighter SPAD. The Viper will be powered by my Magnum 52-XLS, Hitec 225's, 720mah NiMH battery, etc. I'm not going to compete, just wish to get more comfortable flying quickly. Since I fly off grass, and ULTIMATE speed is not super necessary for me, I installed 1" taller (and much wider and draggier) gear from a SIG Somethin' Extra.

QUESTION
I read some threads about the incidence of the V-tail being off on the Viper. People strive for 0-0-0 to get this plane to fly really well. On mine, there is 2-1/2 degrees positive incidence of the tail, relative to the wing. I will need to shim the rear of the v-tail assembly up by approx 0.110" (7/64" ) to get it to zero relative to the wing. Is this amount fairly common?
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Old 07-12-2004, 01:21 AM
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Default RE: Viper 500 Tail incidence?

I've been out of Quickies for a while. Does the Viper have a semi-symetrical wing?
If it does, I'm sure the positive incidence in the tail really means that the wing is set 2-1/2 deg. neg in relation to the thrust line or the flight path due the negative angle of zero lift for the airfoil. Like I said, I'm not familiar with the Viper, but I have built many Q500's.
Before you alter anything, check the tail in relation to the thrustline, If it's is at zero, then check the wing to see if it is at a negative angle to the thrustline. If so, then the set up is probably correct. The reason designers do this is to insure stabilty at different flight speeds, so that the trim doesn't need to be constantly adjusted with speed increases. Semi-symetrical wings have greater lift at zero angles of attack than that of symetrical wings, so the wing gets set slightly neg. in relation to the zero angle of attack; usually the thrust line, to insure minimal lift and drag at a given angle of attack.

Just some food for thought,

Dan
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Old 07-12-2004, 02:01 AM
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Default RE: Viper 500 Tail incidence?

I do not have a Viper but have had Predators and am currently actively racing with a variety of 424 and 428 Quickees and I can assure you that you want 0-0-0. Both the Pred and the Viper have shown up with widely varying incidence. Use a Robart on the wing and check the tail with a small bubble down in the 'V' before any fairing plate is put in the 'V'

Your electronic choice is a good one. The high gear is OK but remember Don,t taxi this airplane or do throttle up T/O's. Have a helper do a release after full power is stabilized and always land dead stick. If not you will be floating into the next county.

John
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Old 07-12-2004, 07:55 AM
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Default RE: Viper 500 Tail incidence?

Thank you both for your quick replies.

John, I was using a Robart on the wing and another on tail surfaces to measure as directly as possible on the installed V-tail. Thanks for the tip on a bubble vial in the bottom of the V to check as well.

Dan, I've read that on the Predator and Viper, specifically, just like John said, there have been some problems with the tail incidence, as built, and that they don't fly well until set at zero-zero. You may well be right about the design. However, without a plan to read off of, it's hard to know the designer's intended thrust line. I can check the tail and wing vs. the firewall, but of course it's hard to know if the firewall is perfect either. Gotta use 'something' as the reference point, eh?

TAKE-OFFS
I can understand taxi-ing, since there is no good way to steer it, but why is it bad to do a power up take-off with these? Is it because the ruddervators won't be effective to counter the engine torque on takeoff or....?
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Old 07-12-2004, 09:46 AM
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Default RE: Viper 500 Tail incidence?

Lee,

Use the Robart meter to set the wing to zero deg then adjust the firewall and vtail to match the wing 0-0-0.
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Old 07-12-2004, 06:04 PM
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Default RE: Viper 500 Tail incidence?

Make sure you are not measuring the V-tail incidence at the fuse side. Due to the geometry of the V, and the tapering of the fuse width, it will look like there is a lot of positive incidence. With this airplane upright and level on your workbench, lay a piece of scrap 1/4 balsa between the V halves (make sure the sides of the balsa are perfectly parallel), and level it with a small bubble level relative to the wing. At this time you can also make sure the firewall is 90 degrees to your bench. All three should be at 0.
Gary Schmidt
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