Flight Troubles?

calgoddard
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Re: Flight Troubles?

Postby calgoddard » January 22nd, 2010, 10:01 am

See my last post.

The largest Soviet ground effect vehicle was called the Ekranoplan.

Here is a link to a Youtube video showing the same in flight, and giving a history of its development:

http://www.youtube.com/watch?v=rUTWWsh6iGA

jander14indoor
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Re: Flight Troubles?

Postby jander14indoor » January 22nd, 2010, 10:04 am

calgoddard got it exactly right about why it flys nice close to the ground. Ground effect improves the lift of any wing.

As to not climbing, the usual suspects are too heavy, motor not wound HARD enough, too high prop pitch (or too skinny motor), not enough elevation from the rudder.

Oh, and for nice cruise in mid flight, you want the plane cg and elevator trimmed VERY close to a stall, just short of it. In fact as you descend on low power you may see some stalling. If so, you are close, trim for just a hair less elevator at that point. That should give you a good climb at the beginning with a FULLY wound motor. And yes I know about backing off to avoid ramming into the ceiling. You have to get TO the ceiling first.

IF everything else is right, (minimum weight, max wing, trim seems right, motors wound to near breaking, right prop motor combo) and your plane just won't climb, try adding a tweak of wash-in to the inside (usually left) wing. Washin means you raise the leading edge SLIGHTLY wrt to the trailing edge at the tip vs at the root of the wing. Said another way, the angle of attack for the inside wing tip is higher than the angle of attack for the root or outside wing tip. In still other words, if the wing is level to the motor stick at the root rib (center of wing), the leading edge should be say 1/16 to 1/8 inch higher than the trailing edge at the tip. Go slow here. You are trying to make the plane fly level in the circle, maybe SLIGHTLY inside wing low. Note, the outer wing should remain flat, same angle of attack at root and tip. I'm not really sure why this works, but it does, repeatably.

Jeff Anderson
Livonia, MI

sr243
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Re: Flight Troubles?

Postby sr243 » March 4th, 2010, 5:16 am

I am not doing this for competition but needs to help my team not fail at this event. So far i have built a plane that glides decently with the propeller but the plane doesn't fly. I am not sure exactly why but i think it might have to do with the rubber band. The rubber band was from a kit and worked kinda well except after 1000 rotations, it snaps. When i use half the length, i can only get 500 rotations and it dies in 15 seconds, but my plane fly upwards for a while. When I use the full length (about 70cm) it doesn't fly and fall down after 10 seconds. Otherwise i suspect my wings aren't getting enough lift since I attached them on to the motorstick directly. As for weight, the plane is about 9g. Regionals are next week so I am trying to get a plane that goes for a minute by then.

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Re: Flight Troubles?

Postby jcollier » March 4th, 2010, 7:27 am

One week? OK, from what you are describing, it sounds like your plane needs a lot of power to fly at all. Your center of gravity may be too far forward (probably is), or the wing's angle of attack is not giving enough lift. With only one week to go, the 9 gram thing (compared to the minimum 7) is something that will have to wait. You can try putting a little bit of clay at the very far end of the tail boom, right at the stabilizer to help your CG. That is something you can try without changing your plane. I saw some 9-10 gram planes fly for close to a minute at states last year, so it can be done. A better way would be to cut the wing off and use paper tubes and wing posts to allow you to adjust the wing. Little changes make a big difference. Last night, my son got an added 20 sec. with a 1mm wing post change. That may be too risky for you with so little time to regionals, so you have to decide what you are willing to risk.

sr243
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Re: Flight Troubles?

Postby sr243 » March 6th, 2010, 10:57 am

Thanks for the advice, and I have redesigned the plane and now it should be around 7 g but my question is where should the center of balance be for the plane? Right now the center of balance for my new plane is about 20 cm from the front and 28 cm from the back. Should I add weight to the back? Also is it okay to test outside because I don't have a gym I can access for testing? BTW I live in Wisconsin where thre is snow on the ground but not much and no wind.

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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.
#ACESWILD

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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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jander14indoor
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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

Jeff Anderson
Livonia, MI


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