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Pitts stab incidence?

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Old 10-05-2006, 08:42 AM
  #1  
zwibelturm
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Default Pitts stab incidence?

After needing lots of up trim on the first flights I re-checked the thrust and incidences on the Goldberg Pitts Monster 12 ARF and got zero up/dwn thrust, zero on the bottom wing, +1 on the top wing, and +2 degrees on the horiz. stabilizer. Goldberg folks say to expect some down thrust, zero on both wings and +1.25 on the stab. Anyone else have to deal with this? What worked?
John
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Old 10-05-2006, 02:19 PM
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scratchonly
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Default RE: Pitts stab incidence?

My pitts S2A has 0 on the stab and top wing and 2 degrees pos on the bottom wing, engine at 0. Can you add 1 degree of pos to the top wing? Make any change small and be carefull.
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Old 10-05-2006, 03:37 PM
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Rodney
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Default RE: Pitts stab incidence?

On all my bipes 2 Phaeton 90's, 2 Phaeton 40's, 2 1/4 scale Moths, 7 Aeromasters, 2 large Lazy Aces, one small Lazy Ace and several others, all flew best when the upper wing was 1.5 degrees less incidence than the lower wing and all liked about 3 to 5 degrees down thrust and 3 degrees right thrust. The worst flyer was an Aeromaster with about 2 degrees positive in the upper wing. When that was changed to negative 1.5 degrees it turned into a nice flying bipe. I have flown two bipes, other owners, with a zero, zero, zero setting and they also seemed to perform well. First do as the designer says then change it if you feel it doesn't fly as you like.
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Old 10-06-2006, 03:03 PM
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Default RE: Pitts stab incidence?

I agree, except he is having a problem with needing a lot of up trim.
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Old 10-06-2006, 03:05 PM
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Default RE: Pitts stab incidence?

Are you sure you are not nose heavy?
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Old 10-06-2006, 06:43 PM
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Default RE: Pitts stab incidence?

Stab 0
Bottom wing 0
Top wing down 1
Engine down 2
Engine right 2 or 3

The reason why you need a lot of up trim on the elevator is because of the positive incidence on the stab. The stab it going to fly flat and level in the air. The leading edge is pointing up, so thats going to make the tail climb up until that stab is flying flat and level in the air. Whats that do? It brings the tail up and points the nose down. So, you need to carry a bunch of up trim to compensate.

Set that stab at ZERO.

Set the bottom wing at ZERO and set the top wing at 1 DOWN.

Work the engine from there. Your going to want 2 or 3 degrees right thrust. Dial that in and get it centered in the cowl. Then use washers to adjust the up and down thrust.
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Old 10-07-2006, 12:36 PM
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Default RE: Pitts stab incidence?

I agree, but your asking him to tear his plane apart.
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Old 10-07-2006, 09:33 PM
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Default RE: Pitts stab incidence?

This might be hard to swallow, but I'd have checked the incidence BEFORE I mixed the epoxy.

The only choice he has now is tear it apart, or live with it. You can't fix it by whacking out the other incidence on the wings and engine. Everything revolves around the stab. because it's the only part thats not adjustable once you install it. So, you set your stab at zero to the longerons under the cockpit -- or the firewall if you can't set it to the longerons -- and then you work everything from there.

On this bipe, I would have set the bottom wing in the saddle and then made sure the stab was zero to that bottom wing. Then mount the top wing and make sure it is down 1. Once you have all that dry fitted--you can glue the stab in. Now all you gotta do is shim the engine.

But of course this is all after the fact. Better luck next time. []
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Old 10-08-2006, 02:43 PM
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Default RE: Pitts stab incidence?

Stab "incidence" is not relative to the fuse; It is the relationship to the wings and engine thrust.
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Old 10-08-2006, 06:14 PM
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Rcpilot
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Default RE: Pitts stab incidence?

It's not?

Is that why everytime I build a kit the instructions tell you to set the stab incidence in relation to the fuselage longerons?

On most every plane I've built, the fuse longerons are parallel to the engine datam line. Thats your zero line, or thrust line of the plane, if you will.

You know what I mean. Unless you want to just argue symantecs. I don't. Anyone thats built more than 5 kits should know exactly what I'm talking about.

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Old 10-08-2006, 06:19 PM
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Default RE: Pitts stab incidence?


ORIGINAL: scratchonly

Stab "incidence" is not relative to the fuse; It is the relationship to the wings and engine thrust.
Okay, so we can set our stab and wings anywhere we want? Just as long as they are all lined up with each other and read 0 degrees relative to each other?

Like this?

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Old 10-09-2006, 09:43 AM
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Default RE: Pitts stab incidence?

Yes, Rcpilet, you could set it up like your illustration and it would most probably fly quite well, just have a lot more drag. Scratchonly is correct, the reference or what is commonly called the "water line" is an arbitrary line and is most often determined by "how it looks".
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Old 10-09-2006, 06:51 PM
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Default RE: Pitts stab incidence?

Well, then I guess we have to agree to diasgree.
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