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Incidence Balance

Old 03-26-2003, 09:09 PM
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stevehord
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Default Incidence Balance

I have measured my wing and stab incidence and found that since the last tail reattachment on my PT40 (results of wrecks) the stab is approximately 3/8th of an inch higher than it should be. Before I measured the incidence I tried shimming the trailing edge portion of the wing about 1/16th but found it made the “ballooning” affect worse. Can I simply shim the leading edge portion of the wing to achieve the incidence balance or do I need to lower the trailing edge portion to achieve this? Or as I suspect, am I simply out in left field on this and need to do something else to keep my PT40 from ballooning every time I remove my hand from the elevator stick on my radio.

Thanks in advance,
Steve Hord
Old 03-27-2003, 12:53 AM
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FlyingPilgrim
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Default Incidence Balance

If your incidence is what it should be (I think it's zero degrees on the PT-40, check your plans), it should not matter if you raise one end or lower the other as long as the incidence is correct.

You mention the plane was wrecked. Have you re-checked the center of gravity on it since your repair? It is likely you added some weight to the tail in doing your repair, and it sounds like that may be the cause of the plane balooning.
Old 03-27-2003, 01:29 AM
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stevehord
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Default Incidence Balance

Thanks for the information FlyingPilgrim. Yes, I have made sure the CG is still within the allowed limits. It has only changed less than an 1/8th of an inch from the plans specified mark (4-7/64" back from the leading edge) the manual says it can shift as much as 1/4" forward or back.

I'm still not sure what would be the best method of returning the incidence to normal. If I sand, plane, or cut out from the trailing edge area of the wing saddle I'm not sure what that will do to the integrity of the fuselage, whereas if I add a shim to the leading edge area of the wing saddle to compensate for the raised forward section of the stab I fear that it may have adverse effects. I would really not want to crack the tail and have to reattach it again as I've already done so twice and I don't think it would be good for the fuselage or the stab and push rods where they connect to the elevator and rudder. Am I wrong in my thinking that if I match the incidence of the wing with the current incidence of the stab that this will solve the problem of the plane constantly rising?

Thanks again,
Steve Hord
Old 03-27-2003, 02:38 AM
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mscic-RCU
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Default Incidence Balance

I think the ultimate solution to your problem is to cut off the tail and reattach it with the correct incidence. All other solutions are just band-aids. I know that is not what you want to here, but it is probably the most safe and best solution.
Old 03-27-2003, 04:18 AM
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stevehord
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Default Incidence Balance

Thanks MSCIC. I know, I know you are right, but...

I'm going to attempt to change the wings incidence to match the tail first and see if I can find a match. From all I've heard, most newbies have 3 wrecks following their solos. I'm on my second so whenever the third happens I will tackle the repositioning of the tail.

Thanks again,
Steve Hord
Old 03-28-2003, 01:17 AM
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Stick Jammer
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Default Incidence Balance

Positive or negative incidence of a wing OR stab is measured in relation to the thrust line of the aircraft. If the stab incidence is off due to inaccurate repairs and you change the wing incidence to match it, now they are both off. If you want the plane to fly as it originally did, you need to cut the tail loose and get the stab incidence back where it was.
Old 03-28-2003, 01:25 AM
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stevehord
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Default Thanks, Stick Jammer

That is what I have been thinking about for the past couple of days. It makes complete sense to fix the tail rather than change the wing incidence. Now I'll have to figure out the best, way to accomplish cutting the tail section off so that I can reattach it with the correct incidence.

Thanks again,
Steve Hord
Old 03-28-2003, 02:51 AM
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Default Incidence Balance

If you only change the wing to match the tail, you are changing the thrust line in relation to the flying surfaces (like stated by someone else above). It would be the same effect as having an incorrect thrust line of your engine (right thrust and up or down thrust). Your prop will be pulling one way, and the wing and tail in another slightly different direction, causing.... balooning. Use a razor saw to cut off your tailfeathers and glue 'em on straight.
Good luck.

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