Enlarging a micro jet CG? - Page 2 - RC Groups
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Jan 24, 2013, 03:06 PM
B for Bruce
BMatthews's Avatar
You say it misbehaves during the glide. But how was it during power on when the CG was back? And something more descriptive than "misbehaves" would be nice to see. It's not possible to really understand what could be the possible issue from that one word.

One telling point is to try some dive testing.

That is where you trim for a moderate speed hands off level flight. Then push into a 45 dive and release the stick. If the model pulls up sharply and quickly then the CG is well forward of where it could be. If it continues in the dive for a long ways before slowly coming back to and past level then it's very near the neutral point. If it tries to tuck then it's behind the neutral point.

For a small model of this sort which is flown in a "jet like" manner I prefer that the recovery from such a dive test be a fairly slow "near neutral" one which has some but not a strong pitch stability.

This dive test result goes hand in hand with the inverted flight test. If you need a LOT of forward stick to hold level in inverted then it's the sort of deal where it would do a snappy recovery during the dive test.

Kevin raises a good point about small servos and limiting the surface travel by controlling the throw volume from the Tx instead of using mechanical options in the model.

You may also want to check the control system for any "deadband". Namely when you reduce the servo throw by using the dual rates or control volume can you see some degree of poor centering of the elevator when coming slowly back to neutral from up vs down? Clipping a clothes pin to the elevator to extend it will make it easier to see such deadband in the system. But in that case have the model perched in a "knife edge" attitude during the test so that the clothespin doesn't load the surface down and eliminate the slop you are checking for.

Such deadband can come from poor servos or all the sources of slop in the system adding up. If the pushrod wire isn't a snug fit in the servo arm and control horn you can easily get such an issue. Or if the wire pushrod is binding and flexing in the fuselage you can also get this sort of thing. And when the CG is back where it logically should be the elevator is more sensitive. And any deadband in the neutral centering will most certainly show up.

I had this happen on a T tail glider. I had used the cable in sleeve to form an "S" in the system to get the elevator to work with a regular control horn. The sleeve "S"ed from the fusealge up and through the fin to exit below the elevator still level. The drag and slop of the cable in the sleeve was more than enough to produce a bad dead band in the elevator that had me constantly hunting for neutral until I got around it by always adding a quick little dab of up then release after using some down elevator. I didn't keep that one for long before building a new fuselage to correct the issue.
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Jan 25, 2013, 04:30 AM
Registered User
stegla's Avatar
Misbehaving.........The signs that I attributed to tail heaviness is when a tiny amount of up elevator input would make the model zoom up and a tiny bit of down would make the model zoom down, making the model unflyable.

This is going to be interesting to see if the CG can be moved back!

What I shall do is make longer elevator control horns to reduce the throw mechanically and then test fly the inverted and dive tests.

I'll post some video of the elevator with a clothes peg attached, and fly as soon as the weather allows.

Again, many thanks

Jan 25, 2013, 11:35 PM
B for Bruce
BMatthews's Avatar
Ah, thanks for the clarification.

And now I can firmly say that Kevin hit the nail on the head a few posts back when he mentioned that the elevators become more sensitive as the CG moves back.

I'll also refer you back to my description of dive testing to find a CG location that produces a manigable form of pitch stability based on what you want the model to fly like.

The key is to get the model to fly correctly and then to tailor the elevator throw to provide a harmonious elevator response by altering the mechanical linkage as suggested by Kevin.

I can also say that smaller models set up for a wide range from low to high speed tend to be touchy at the high speed end of the spectrum. This is also apparent with bigger models. But the difference does not seem to be as great as I've seen on smaller models that can glide in like a butterfly yet still zip around like a bumble bee on steroids when at full throttle and coming out of a dive.

The longer elevator horns and full travel will increase the control surface resolution and reduce the effect of any deadband travel in the system. I think you'll be pleased with the results.
Mar 04, 2013, 10:03 AM
Registered User
stegla's Avatar
The biting easterlies have finally abated enough to go and fly.

The cg was moved back from the usual 10%MAC to 15%MAC and the flying was good, indeed a lot better.

To move the cg back to 20%MAC I had to drop from 3s to 2s. The flying was not so good as I sorely missed the power and the higher wingloading of 3s. Nevertheless the model was very controllable with the pitch response quite lively.

So..............indeed the elevator is very powerful and only needs a tiny throw (an anomily with the smallness of the model?) that I mistook this for tail-heaviness.

After some more testing on 2s and cg at 20%MAC. I will probably go back to 3s and 15%MAC.


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