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Down thrust

Old 04-26-2007, 06:32 AM
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Default Down thrust

Hello Don!

How is the pattern season going?

I have a question about down thrust, that is puzzling me these days. I'm trying to setup my plane for a really straight vertical up line.But, when I pull up and advance throttle, the plane slowly pulls to the canopy. Given that behaviour, I added a good deal of down thrust, but the problem is still there.

I've trimmed my elevator for hands-off level flight, at about half throttle (my cruise speed, more throttle during vertical manouvers). CG is quite forward, but it is not exaggerated: inverted flight requires a good push on the stick.

An interesting thing I noticed after many pulls is that the unwanted pull to the canopy occurs a little while after the pull to the vertical,BEFORE the airplane starts to slow down: this makes me think that it is more a trim problem, than a thrust one. Perhaps I have too much up trim?

What do you think about that Don?

A huge thanks, as usual!!!
Old 04-28-2007, 09:58 PM
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Default RE: Down thrust

I had this exact problem and it ended up being an elevator pushrod that was oscillating. During normal flight the plane would trim, at full throttle during a verical up line the plane would act like it would do a large loop on its own. Likewise on a vertical downline it would pull out. Are you useing a single elevator pushod from center of fuselage?

Don
Old 04-29-2007, 07:19 AM
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Default RE: Down thrust

Thanks Don for the reply.

I have two Futaba mini digital servos on the stabs, with a 3mm threaded rod and ball links on both ends.

I've just checked the fit and it is still good...

The problem could not be due to some excessive down thrust? I've noticed that adding down thrust requires further up trim also, perhaps this can be the culprit?
Old 04-29-2007, 10:26 PM
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Default RE: Down thrust

What you want to do, on a fairly calm evening is trim the plane very carefully. Set your trim rate to 1 percent. Fly hands off at a little over half throttle. It should go from one side of the field to the other at the same altitiude.

Then do you vertical up lines and let go of the stick. Do this with pull ups flying from left to right, and right to left. This is to cancel out any wind or other factor. If it pulls to the canopy both times, continue to add down thrust til it goes up straight as an arrow.

One other thing you may double check is your wing and stab incidence. If they are off significantly, this could impact your pitch at different airspeeds and attitudes. For example, if you have negative wing incidence and negative stab incidence, it gives a perspective from the engine's point of view that it has up thrust. If the incidences are both ok then do the thrust adjustments as above. Don't be bashful as if you put in what it takes it will be very effective. It normally takes more thrust changes than it seems it will need. Let the vertical uplines do the talking.

All the best,

Don
Old 04-30-2007, 08:32 AM
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Default RE: Down thrust

Don,
you're GREAT!!!

This suggestion is worth like gold. In fact yesterday I did just that, and guess what was the problem? Actually the plane was not well trimmed to keep a constant altitude. I've realized how difficult is to say "ok, now it flies straight." If we add some wind gusts, the target becomes nearly impossible!

I retrimmed the elevator flying a few meters above the field, now I'm quite sure it is correct: in fact the uplines were good too! It is amazing how a few "cliks" of trim can affect the uplines...

Thanks again Don!!!!!!!!!!!!!!!!!!!! Now I'm very happy and eager to fly again

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