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Low Speed Quirks

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Low Speed Quirks

Old 06-25-2004, 02:25 PM
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JeffH
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Default Low Speed Quirks

I am flying the GP GeeBee arf, and other than trimming for landing, it flies great. As the plane slows down, it becomes pitch unstable. I relate it to walking on a ball. You breathe on the elevator and is shoots up or down. Obvisouly, the answer is land faster, the problem is that it will not get on the ground at any faster speed. I am not sure if this is a design problem, incidence, thrust, etc. In the air, it does not exhibit any odd traits. You can even pull a full elevator loop without it snapping. It will slow to nothing and not drop a wing, but the elevator is hyper sensitive, but ONLY at low speeds. What should I do?
Old 06-25-2004, 09:49 PM
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WS
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Default RE: Low Speed Quirks

Sounds like an airfoil stability problem. That is, the center of pressure moves forward as the angle of attack increases. One solution is to move the CG forward and possibly increase your elevator travel to compensate.
Old 06-25-2004, 11:43 PM
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tommy321
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Default RE: Low Speed Quirks

I agree... moving the CG forward a little bit will reduce the elevator effectiveness slightly which might help the slow speed characteristics. You probably won't even notice the reduced effectiveness at the high speed ranges. You might try using dual rates to tune down the elevator for landing. Or perhaps reduce the elevator throw by adjusting the clevises (this way you won't run into servo resolution problems).

You're probably running into the same problems that the real GeeBee's had. They were built for speed, and didn't handle well at low speeds. They were so much faster than anything else of the time that very pilots could handle them well. Your GeeBee comment made me do a quick google searches, and I found a couple really interesting historical sites about them.

http://www.donhollway.com/writing/do...doolittle.html

and

http://www.geebee.com/

Cheers,

Tom
Old 06-27-2004, 11:15 PM
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acropilot_ty
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Default RE: Low Speed Quirks

I have had this type of thing happen with a model that I put leading edge slats on, while pulling up to high angles of attack the aerodynamic center would move foreward of the center of gravity and the thing woud pitch to vertical and tail slide. With that airplane I had to start a flare with up elevator and then control it by backing off, or even pushing. There are two fixes, move the cg foreward, or learn to do good wheel landings so that you can touch down at a higher speed on the main wheels. To wheel land fly the plane as low as you can over the runway with a bit of speed, then land as soft as you can (you still have to flare, just not all the way to 3-point) When the mains touch return the elevator to neutral if you have been pulling back, or possibly a small amount of foreward stick. Timing is important, if you don't release the back stick soon enough it will bounce, if you push before it touches it will hit hard and bounce even worse. I had a Goldberg chipmunk that was more fun to do wheel landing touch and goes then it was to do acro...

Ty
Old 06-28-2004, 07:03 AM
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JeffH
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Default RE: Low Speed Quirks

To do a conventional 2 point type landing at a speed that the elevator remains 'normal' , it rolls for several hundred feet of grass. Every roll in the dirt, clump of grass, etc send the plane back into the air. I could truly care less about trying to 3 point it, I am trying to wheel land it when the elevator goes funky. I am afraid to go forward with the CG anymore since it is already further forward than called for which puts it about 18%ish. I am going to try raising the ailerons just a hair in the landing configuration to help kill some lift and cause slighty great vertical speed versus horzontal speed.
Old 06-28-2004, 10:00 AM
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ksechler
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Default RE: Low Speed Quirks

Tommy-
Interesting reading. That thing sounded like a REAL pig to fly. It's amazing that the thing is held in such high regard by modelers. It seems to me that it was an engineering disaster. The only thing that put it in the history books was Jimmy Doolittle's skillful piloting.
Old 06-28-2004, 10:11 AM
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JeffH
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Default RE: Low Speed Quirks

I just read that on another thread that the model is designed with positive two degrees in the wing, negative two in the the stab with four degrees of downthrust in the motor. Is it me, or does these numbers seem a bit odd? The wing set positive will cause the plane to rise, the stab at negative will push the tail down, thus causing the plane to climb. These climbiing forces are evidently being countered by the downthrust. This seems to me as the throttle is reduced, the downthrust quits pulling the plane back level and all sorts of bad stuff will start happening. I was acutally thinking of removing some downthrust, but that is probably going to cause all sorts of pitch changes with speed. Anyone wanna buy a GeeBee? The design is fine, but the incidences should be checked before assembly to insure they are 'logical'. The manual says that everything was highly tested and not to change anything, but I think they are wrong. It seems as if they were chasing problems, and ended up with the cobbled together mess of funky incidences.
Old 06-29-2004, 11:10 PM
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acropilot_ty
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Default RE: Low Speed Quirks

That must be it,

I hate downthrust, pilots should learn to use trim!!

my 2 cents,

Ty
Old 06-30-2004, 12:07 PM
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Default RE: Low Speed Quirks

ORIGINAL: JeffH
I just read that on another thread that the model is designed with positive two degrees in the wing, negative two in the the stab with four degrees of downthrust in the motor. Is it me, or does these numbers seem a bit odd? ... This seems to me as the throttle is reduced, the downthrust quits pulling the plane back level and all sorts of bad stuff will start happening.
This is not as odd as you might think. Models are set up this way for stability. It started with free flight models with no real-time input from the "pilot." Then the early RC models were little more than free flights with some corrections given by the pilot. The models need to be very stable, as a result.

RC trainers were always (until recently, aparently) set up this way. As throttle is reduced, the model will slow down and self-right. This (excessive) pitch stability is undesireable in a lot of RC situations, which is why you now see more 0-0-0 setups.

have fun,
-David
Old 06-30-2004, 01:10 PM
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JeffH
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Default RE: Low Speed Quirks

THe problem is that it is UNstable in pitch. longitudinal control becomes iffy at best as the plane is slowed.
Old 06-30-2004, 01:14 PM
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davidfee
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Default RE: Low Speed Quirks

Did you try moving the CG forward, as was suggested?

-David
Old 06-30-2004, 01:21 PM
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JeffH
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Default RE: Low Speed Quirks

It is already forward of what is called for. It is about 18% now. If i remember, the book calls for around 20-22%. They tell you that moving for or aft could cause problems, but hell where they have it causes problems.
Old 06-30-2004, 02:21 PM
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davidfee
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Default RE: Low Speed Quirks

Do you have some strange dual rate thing going on? Or maybe a flexible pushrod? Normally a pitch-unstable plane gets worse as you speed up, not as you slow down. Something sounds fishy.

-David
Old 06-30-2004, 02:26 PM
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JeffH
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Default RE: Low Speed Quirks

the fishy part is that it is a GeeBee. I have 70 oz servos, one per elevator and graphite arrow shaft rods with 4-40 steel on each end.
I have been building and flying for 20 years now, and I have never seen anything like this. At speed the plane is rock steady. It may be a case of simply finding a new technique. I am going to try adding a touch of spoileron to kill some lift and bring it down steeper.
Old 06-30-2004, 04:07 PM
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davidfee
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Default RE: Low Speed Quirks

I just looked the plane up. I think the problem may be an undersized (for the model) horizontal stabilizer. Build a new stab, maybe 10-20% bigger, and I bet most of these problems will go away.

-David

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