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trainer nose pitching up with gusts

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Old 04-04-2017, 10:03 AM
  #1  
pollemix
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Default trainer nose pitching up with gusts

Hello all
I am trying to set up a homebrew trainer. I noticed that the nose tends to pitch up at the very slight gust. CG seems to be right although I canīt figure out whatīs causing such behaviour. Could it be some angle of incidence?
Thanks for your comments,
-apg
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Old 04-04-2017, 01:27 PM
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To some extent this is normal, but a forward CG make it worse. How did you test to see if yours is right?

There are many methods. The one I use is to trim for level flight at moderate throttle, and then at the same speed put the plane in a dive, then release the stick. If it keeps right on going down, it is neutral in pitch stability. Most pilots prefer a little stability, which would show up as a modest tendency to gradually pull out of the dive on its own. If it pulls out pretty quickly, it is nose heavy.

Jim
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Old 04-04-2017, 07:07 PM
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One more point to add is that a flat bottom airfoil will amplify this tendency. That's because as the nose rises in response to the gust, the center of pressure moves forward. Of course, that is the kind of airfoil most trainers have. A symmetrical airfoil does not do this, so it's reaction to gusts is milder because the center of pressure doesn't shift as much.

The bottom line is still the same. Gusts coming toward the airplane will cause the nose to rise. The pilot must counter it. If the plane is nose heavy, the tendency will be worse.
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Old 04-05-2017, 08:15 AM
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Also you could do a little test by adding a slight shim under the back edge of the wing.. It might fly a bit better, maybe an 1/8th of an inch or so. Its probably a bit tail heavy as mentioned.. However all of us that have built any flying object HOMEBREW... know that sometimes it takes some tweaking.. Be glad you still have something to tweak ! I have had a few experiments go wrong on the first flight ! hahahahahaha
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Old 04-05-2017, 10:31 AM
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I expect it also tries to climb strongly when you add throttle too. At least it would try to do so unless you are using a serious amount of downthrust.

The "cure" is to move the CG back and reduce the amount of wing to stab angle often called "decalage". You want to flatten out the wing to stab angle by some amount. This changes the pitch trim too. So that's why you do this change in concert with a shift of the CG. The two are locked together in this relationship. And when done together in the right amounts the stable medium power level flight trim will be the same.

Clearly as it is a trainer you want to have SOME pitch stability to aid in self correcting. But as you're finding out too much of a good thing is still too much. Ease it back using the tests and results posted by Buzzard Bait above to find the happy middle ground.

You'll likely find that you can remove some of the downthrust you likely have as well. Or, if you shim the wing to reduce the wing to stab decalage angle, you'll find that the wing to thrust line angle also reduces. So that would be the easy and effective way to adjust the trim on your model.

Just be aware that when you shim up the wing's trailing edge that you MUST also move the CG. If you don't then you'll just add back in the shim in the form of up trim at the elevator. Reducing the decalage to reduce this pitching sensitivity to gusts has to be done in concert with a shift of the CG to the rear or it's all for nothing and no change will result.
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Old 04-13-2017, 12:56 PM
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It depends how well your plane is balanced, If your tail heavy, you can still fly,but your nose will be very unstable to any input. Also Maybe,when you mounted your motor, you where not centered on your Datum line on the firewall. Also, your plans might have called for a few degrees of Pan up,down,right or left. Hope this helped a bit
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Old 04-17-2017, 10:47 AM
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Originally Posted by F4 Phantom blue angles View Post
It depends how well your plane is balanced, If your tail heavy, you can still fly,but your nose will be very unstable to any input. Also Maybe,when you mounted your motor, you where not centered on your Datum line on the firewall. Also, your plans might have called for a few degrees of Pan up,down,right or left. Hope this helped a bit
This is true. There is no doubt that the models become more sensitive to the elevator as the balance point is shifted back within the allowed range. But that's easily dealt with by altering the control linkage to reduce the total elevator throw. The model will respond similarly to before if this is done right. But the sensitivity to speed changes from throttle or gusts is reduced by a lot.
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Old 04-17-2017, 11:34 AM
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It is like the other replies ,Your either tail heavy,or you might have to put a few degrees of down thrust on your motor mounts. You never want to fly tail heavy. When you build,remember that for every 1oz.in tail,you have to add about 3oz. in the nose. If I remember right. Please correct me if I'm wrong. One more thing, If your tail heavy, you will probably have to land under power,or allitle hot.
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Old 04-17-2017, 12:22 PM
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I guess it depends on your idea of "tail heavy". Some of that also depends on the type and role of the model.

For most sport flyers we tend to like the CG back close to but not quite at the point of neutral stability. And some even like it pretty much spot on neutral. All this would be too much for beginners or folks wanting a more relaxed flying model.

For myself I don't consider "neutral handling" as "tail heavy" but it certainly doesn't qualify as "friendly to fly" for anyone that isn't ready for more or less constant little nudges of elevator. It's much like driving a car down the highway and the need for regular steering nudges.

And certainly as I get my models that I want set up that way back closer to neutral I do reduce the amount of throw to avoid them becoming too sensitive.

Speed plays a part too. Oddly enough the 3D Flat Foam Flippy Flier which I deliberately set up with a moderately unstable CG behind the NP proved to be surprisingly easy to fly due to the slow speeds that slowed down the response in all axes. I followed the trend with that style of model to set it up as unstable because that aids the stability in hover. Go figure.... It did need constant attention when flying "normally" but because it flies so slow it wasn't anywhere near as bad as I thought it would be. No worse than a car on the highway in some cross wind wanting to wander here and there.

For landing the neutral handling model is very normal. The FFFF didn't need power or anything either. Just the "flat" elevator trim it was rigged for. Or by that time perhaps I had a nudge of down trim in it to fly level at moderate power when hands off. I don't recall at the moment. I do know that it did not try to nose up or down at all with power changes.

Last edited by BMatthews; 04-17-2017 at 12:25 PM.
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Old 04-19-2017, 11:42 AM
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One thing about flying tail heavy,you can do crazy things with it in the air,However,These are things that a full scale can"t do.Im just saying,If your having fun,thats all that matters! I On the other hand like to fly as realistic to a full scale as possible. Enjoy!
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Old 04-19-2017, 11:50 AM
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Once you have mastered flying your plane tail heavy,(1-3) oz.approx.You can do things with it that are crazy,However,These are things that a full scale plane can 't do. I like to fly as close to scale as I can. This hobby is about having fun,Thats all that matters!
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