Flight Troubles?

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eta150
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Re: Flight Troubles?

Postby eta150 » March 6th, 2010, 12:25 pm

It actually depends more on the wing. With a large wing (12+cm) you'll want the cg to be just in front of the rear post of the wing. With a medium wing (~10 cm) you'll want it right around the rear post. Any smaller of a wing and the CG should be rear of the rear post.
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Re: Flight Troubles?

Postby jcollier » March 6th, 2010, 1:31 pm

I would not try flying any of these planes outside, unless you...

REALLY enjoy repairing damaged planes or
REALLY enjoy building new planes in a short time.

They have enough trouble flying with ventilation on.

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Re: Flight Troubles?

Postby sr243 » March 6th, 2010, 4:21 pm

thanks for the answers but no matter how hard i try, my plane wouldn't go up. I have been rotating it about 500+ times because i am trying to do it in my basement but all it does is fly slowly down. This is with a propeller that is 19cm. Then i tried with a 13cm propeller and it goes up for 2 seconds before the plane falls backwards and dives back and gets destroyed. So what problem do I potentially have. My CG is about 6 cm behind the back of the wing. Sorry if I am asking a bunch of questions.

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eta150
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Re: Flight Troubles?

Postby eta150 » March 6th, 2010, 4:55 pm

No matter what your wing size is, a cg that's 6 cm behind the wing is a serious problem. However, your flight characteristics don't make sense for a cg that's that far back. If it really does have that cg, then it should hardly be going at all, but rather, and should be stalling. You should try to move the wing back or weight the front of the plane. Moving the wing back is probably preferable, for then you don't incur a weight penalty. Depending on your wing size, follow the guidelines I put on my previous response to your question.
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Re: Flight Troubles?

Postby jander14indoor » March 6th, 2010, 6:53 pm

And 500 turns of a reasonable size motor(say 3/32 inch) on a properly trimmed 7 gm plane won't do much more than cruise around level or slow descent. You need lots more turns than that to fly.

As to cg, if you are really 6 cm behind the wing I'd expect stalling too. How do you measure balance? It should be with the rubber and prop installed.

As to flying outside, like someone else said, plan on repairing or replacing your planes often. What seem like VERY soft breezes will tear these things up. If its REALLY dead still you can get away with it, but I mean DEAD still. Don't trust your own feel. I'd tie a long, light strand of yarn to a stick and hold it high above the ground. If its moving at all, its to much.

Good luck

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sr243
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Re: Flight Troubles?

Postby sr243 » March 7th, 2010, 6:54 am

The only reason why it doesn't stall was the propeller i used was hitting the plane so it made it weird, but once i switch propeller, it started to stall. Thanks for all the help, I think i will have to just manual add some weight to the front so the cg is right. I guess i have to wait till i go to school for testing.

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Re: Flight Troubles?

Postby sr243 » March 8th, 2010, 5:29 pm

Sorry about the double post cuz I thought I got it to work but it won't climb. Right now it just go in a horizontal line downwards and hit thE ground after 10 seconds. I winded it about 1250 times and the cg is just behind the rear wing about 2cm. So how I get more lift? No my wings r not curve and don't plan on having them curved.

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Re: Flight Troubles?

Postby blue cobra » March 8th, 2010, 5:42 pm

Why don't your wings have any camber (curve) and why don't you want them to? That's kind of an important part...

Otherwise, are you measuring CG with prop and rubber in place? If not your CG could still be too far back/your wing's Angle of Attack (AoA) could be too low (leading edge not high enough above trailing edge). Adjust accordingly. And for this event always keep in mind two quotes from Douglas Adams, "Flying is learning how to throw yourself at the ground and miss." and, "It is a mistake to think you can solve any major problems just with potatoes."

But I think a flat wing has a lot to do with it...
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Re: Flight Troubles?

Postby sr243 » March 8th, 2010, 5:46 pm

yea my CG is with propeller and rubber band included. I really don't want to build a wing and it is going to take too long since regionals are within a week and i don't have time for this event (not even mine). My last resort is to wind the rubber band up with half the length so it will last like 15 seconds but it will stay up in the air. I think some people had planes without curved wings that worked.

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Re: Flight Troubles?

Postby leetx » March 8th, 2010, 6:07 pm

yea my CG is with propeller and rubber band included. I really don't want to build a wing and it is going to take too long since regionals are within a week and i don't have time for this event (not even mine). My last resort is to wind the rubber band up with half the length so it will last like 15 seconds but it will stay up in the air. I think some people had planes without curved wings that worked.
It sounds like either a) your wing doesn't have enough positive angle of incidence or b) your horizontal stabilizer has too much positive angle of incidence -- or both. The angles of incidence here is relative to the propeller thrust line, which depending on how you built your plane, may or may not be in line with your motorstick. As a starting point, you want the wing to have approximately 1 degree of positive incidence and the horizontal stabilizer to have 1 degree of negative incidence.

I also noticed in a previous post that you said your rubber motor was 70 cm long. I assume you mean that the total rubber used is 70 cm, so that the rubber motor, when tied in a loop, has a length of 35 cm. I also noticed that you mentioned shortening your propeller to 13 cm. Be sure that:
- your motor weighs less than 1.5 g
- your propeller is 20 cm

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