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LT-40 Wing Inciddence

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Old 08-01-2010, 11:46 PM
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RICMOD5634
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Default LT-40 Wing Inciddence

LT-40 OS 65LA 12X6 MA Very slightly nose heavy. After Maiden flight we added 1/4 inch to the back of the wing saddle to lower the + inciddence. On second flight it was much better but at the end of the flight we still had added 85 clicks of down sub trim to get it to fly level, hands off at half throttle. If you'd add three clicks of throttle she'd climb like there was no tomorrow. So my ? is,
1. Seeing as how its a triner, leave as is?
2. Try adding another 1/8 to the wing saddle to lower the inciddence more?
3. Or would it be better to try and add some down trust to the engine. Say put a 1/16 thick washer under the top of both sides of the motor mount.
Can I get some thoughts on this please. Thanks
Rich F.
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Old 08-02-2010, 10:40 AM
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Rodney
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Default RE: LT-40 Wing Inciddence

Sounds like you may have a thrust problem, to much up thrust or not enough down thrust. If you CG is where the kit suggested and the thrust line proper, the original set up for wing incidence worked very well.
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Old 08-02-2010, 12:46 PM
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Default RE: LT-40 Wing Inciddence

Every LT-40 I have seen, including mine, always flew with a bunch of down elevator trimmed in......
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Old 08-02-2010, 04:06 PM
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Default RE: LT-40 Wing Inciddence

What exeter acres said. My club has had three of these as club trainers, and all three had a lot of down trim. You can play with the wing incidence and get rid of the need for this, but why bother? It's a terrific trainer in its normal state.
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Old 08-02-2010, 07:51 PM
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Default RE: LT-40 Wing Inciddence

Check this thread:

http://www.rcuniverse.com/forum/m_75...tm.htm#7544125
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Old 08-02-2010, 08:47 PM
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Default RE: LT-40 Wing Inciddence


ORIGINAL: RICMOD5634

LT-40 OS 65LA 12X6 MA Very slightly nose heavy. After Maiden flight we added 1/4 inch to the back of the wing saddle to lower the + inciddence. On second flight it was much better but at the end of the flight we still had added 85 clicks of down sub trim to get it to fly level, hands off at half throttle. If you'd add three clicks of throttle she'd climb like there was no tomorrow. So my ? is,
1. Seeing as how its a triner, leave as is?
2. Try adding another 1/8 to the wing saddle to lower the inciddence more?
3. Or would it be better to try and add some down trust to the engine. Say put a 1/16 thick washer under the top of both sides of the motor mount.
Can I get some thoughts on this please. Thanks
Rich F.
Personally, I would have left the wing alone and added weight to the tail to get it on the specified C.G. and then shimmed out the motor for a bit more down thrust.
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Old 08-02-2010, 09:12 PM
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Default RE: LT-40 Wing Inciddence

I put a 1/4" shim under the back of the wing and my plane flew fine at cruise but I only have a OS 52 Surpass Four Stroke

If you put the throttle forward she would go up.

You need to try the down thrust
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Old 08-02-2010, 10:04 PM
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Default RE: LT-40 Wing Inciddence

If you trim for level at half throttle then throttle to full smoothly it should start a climb at about the same airspeed. If it pitches so steeply that it stalls or slows noticably then yes your incidence is too high or not enough down thrust. Set the plane so that the horizontal tail is level then check for down thrust from that level line. It should be between 3-5 deg's. if it is then reducing the incidence more should help. The trim should keep the plane at a constant airspeed no matter what the throttle setting, and start climbing with more power or descending with less power, but it can take some time to settle in if the throttle is moved too rapidly. Remember throttle controls altitude, pitch (trim) controls airspeed.
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Old 08-02-2010, 11:50 PM
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Default RE: LT-40 Wing Inciddence


ORIGINAL: cfircav8r

If you trim for level at half throttle then throttle to full smoothly it should start a climb at about the same airspeed. If it pitches so steeply that it stalls or slows noticably then yes your incidence is too high or not enough down thrust. Set the plane so that the horizontal tail is level then check for down thrust from that level line. It should be between 3-5 deg's. if it is then reducing the incidence more should help. The trim should keep the plane at a constant airspeed no matter what the throttle setting, and start climbing with more power or descending with less power, but it can take some time to settle in if the throttle is moved too rapidly. Remember throttle controls altitude, pitch (trim) controls airspeed.
I know this is the theory of how it should work but no LT-40 I ever flew worked this way and I've been flying them off and on for 12 years or so.

But using your setup, it would be very nice to have one that did fly the ideal way you describe.

The LT-40 has about +6 degrees of wing incidence in them and I think everything else is Zero?

Somewhere along the line of changing all the incidence angles, the attitude of the fuselage should be observed to have the fuselage level in flight. For with the original positive wing incidence the fuselage would be pointed down for the plane to fly level.

With a OS FP40, the original incidence settings were not bad at all but the bigger engines of today really brings out the problems?

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Old 08-03-2010, 09:56 AM
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Default RE: LT-40 Wing Inciddence

Thanks guys for all the good information. I had left out that this was an ARF and I never checked the thrust angle, but that is what I will do and see where I am starting at. Don't get me wrong on this, I know its a trainer and it does fly great at half throttle. The climb at increased throttle would not be bad, if it wasn't so drastic even with smooth inputs. I'm trying to consider the student at the other end getting shook if he touches the trottle a bit. Will post the outcome of my trials. Might help someone else out.
Thanks Rich
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Old 08-03-2010, 06:19 PM
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Default RE: LT-40 Wing Inciddence

The reason for the design causing the drastic pitch up is it will pull out of a dive quickly if you just let go of the controls. Great for a true newbie, especially if they don't have good help, but not desirable for someone hoping to fly it long term. In other words it is great for initial instruction. The flat bottom airfoil and excessive incidence can be changed somewhat but the due to the overall design it will never fly well as a sport flyer.
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Old 08-03-2010, 06:41 PM
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Default RE: LT-40 Wing Inciddence

AH YES Very good point you make. If you stop and think about it, it makes lots of sence. I guess thats what comes when you get old and fly a lot of different birds. You forget what it was like starting out.
Thanks Rich
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Old 08-03-2010, 06:48 PM
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Default RE: LT-40 Wing Inciddence

I've looked at other's LT-40's for years and unless the guy is a absolute newbie, the plane has a shim under the rear of the wing
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Old 08-10-2010, 10:51 AM
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Default RE: LT-40 Wing Inciddence

Well after checking the thrust angle of the engine, Which by the way has plenty of down thrust. And seeing as how I have lowered the positive incidence of the wing by half of what it was. Have decided to leave well enough alone. It flies great at half throttle, so that is how it will stay. NO sence messing with a good thing, I guess.

Thanks for you thoughts;
Rich
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Old 08-10-2010, 11:09 AM
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Default RE: LT-40 Wing Inciddence

This has more or less been my experience also
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