Flight Troubles?

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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blue cobra
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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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sr243
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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

-leetx
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blue cobra
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Re: Flight Troubles?

Postby blue cobra » March 9th, 2010, 1:53 pm

<SNIP>No my wings r not curve and don't plan on having them curved.
I thought I might have distracted your attention away from this, and I think (but I'm not positive) that it's a problem.
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jander14indoor
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Re: Flight Troubles?

Postby jander14indoor » March 9th, 2010, 7:05 pm

If I'm reading sr243 right you are talking curve front to back of the wing?

Typically that shape is called the airfoil and the amount of curve the camber. If your plane is close to 7.0 gm, and your wing largish, say over 10 cm, it will fly with enough angle of attack. Not as well as a curved airfoil, but better than you might expect. BUT, you'll need lots more angle of attack than we've spoken of here for curved aerofoils. I'd say try 1/4 to 3/8 inch higher leading edge than trailing edge for a start. And probably more negative incidence in the tail. Leading edge of tail say 1/8 to 1/4 lower than the trailing edge depending on tail chord.

Now, if you are talking side-to-side curve, that's called dihedral. A flat dihedral won't affect lift, but it will certainly affect stability. Once your plane starts rolling to either side, it probably won't stop. But that doesn't sound like the problem you are describing.

BTW, wings with flat span and tip plates do have effective dihedral, even if we call the wing flat. Try putting the tip plates on bottom some time and watch how important those plates are to stability with a flat wing. I made that mistake once not thinking and after the plane flipped completely over it didn't fly half bad. Inspite of the aerofoil being upside down!

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sr243
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Re: Flight Troubles?

Postby sr243 » March 11th, 2010, 4:40 pm

thanks for all ur help, though my plane doesn't work, BUT my wright stuff team put together a good plane and it glides for 10 seconds if u run with it, but the problem is my rubber band is too short and doesn't spin more than 400 times. The total time of the rubber band is only about 30 seconds. The rubber band is just over 1.4 g so it is just under the limit. How long should the rubber band be? right now it is about 26 inches if i were to dangle it.

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Re: Flight Troubles?

Postby krisharmaan » March 11th, 2010, 4:44 pm

I need major help soon. Competition is on Tuesday and I can't seem to get my plane to fly very well. The best time iv'e had so far is 1:20. I am using the Freedom Flight Models Kit and I built it exactly to specifications. The CG is about 1.5 cm behind the wing post and it weighs 7.1 g. I can't seem to find the perfect level of flight to not have any stalls and when i wind it to 1200, it goes up about two feet and then after 1/4 of the circle, it dives down and crashes at half of the circle. Any ideas on how to increase my flight time?
2010 Regionals:

Elevated Bridge- 1st
Bio-Blitz- 3rd

2010 States:

Battery buggy- 1st
Wright Stuff- 4th
Elevated Bridge- 5th

Overall, 2nd place in states. Nationals maybe depending on invitation.

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Re: Flight Troubles?

Postby jcollier » March 11th, 2010, 6:22 pm

How does it fly when you wind it less, say 900-1050 winds?


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