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Ground loops - 1/1/2004 9:04 PM   
rc flylow



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From: Friendswood, TX, USA
Status: offline 		I have a scratch-built bipe (builder designed as well) that is a significant problem in the take off. Once airborne it flys relatively well trimmed out and it lands OK. I have a built-in left thrust angle on the Magnum 61 four-stroke derived from washers under the right side of the motor mount (don't have a way to measure the angle precisely, but it looks like about 2 degrees.)

I know to keep the tail down with elevator on take-off until I get airflow across the fin and I know to keep some right rudder when the tail lifts until the plane gets some speed (the tail-dragger norm.) The problem is that I think the plane needs too much right rudder. I'm wrestling this thing down the runway (asphalt) and dipping wingtips all the way into the air. I've had a little better luck on grass. I've made sure both main wheels spin freely.

I'm flying it with a 12x6 prop. Would more or less left thrust be an answer to counteract torque or would I have better luck centering the engine thrust? Is left thrust the way to counter the problem? (My logic tells me if the airplane is wanting to go left on power that right thrust on the engine is needed.) I don't want to screw up the flight characteristics trying to compensate for hard take-off characteristics. (This bipe is more difficult to take off than a Cub - if you know what I mean.)

I'm up to experimenting but I'd like to do things in an aerodynamically sound manner, not just moving things around willy nilly.

Thanks for your help!

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RE: Ground loops - 1/1/2004 10:23 PM   
RichLockyer



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quote:

ORIGINAL: rc flylow
Is left thrust the way to counter the problem? (My logic tells me if the airplane is wanting to go left on power that right thrust on the engine is needed.)

Your logic is correct, your original design is flawed.
Model aircraft with tractor (front mounted) engines require RIGHT thrust (generally, about 2 degrees), and, depending on the design, possibly down-thrust.

Of course, to accomplish this, you're going to have to move your mount to the left to be able to re-center the prop

The need for down-thrust can be determined during flight. Fly at half throttle, straight and level, trim elevator for "hands off".
Go to full throttle. If it begins to climb, you need downthrust. If it begins to drop, you have too much downthrust.

Edit: The only time you would want left-thrust would be the left engine on a twin.

< Message edited by RichLockyer -- 1/1/2004 2:26:37 PM >

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RE: Ground loops - 1/2/2004 5:26 AM   
acropilot_ty



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Status: offline 		If the right thrust doesn't do it there are a few geometry tricks for tailwheels as well. 1, the closer the main gear is to the Center of Gravity the better it handles, if the main gear is too far foreward of the CG it will be a handfull. 2, the wider the landing gear the better... this has more to do with tipping and dragging wingtips then directional control. 3, the lower the deck angle in 3 point attitude the easier it is to handle... this is limited by prop clearance and the shape of the fuselage, but a high deck angle means lots of P-factor on takeoff, In fact this is where the right thrust comes in. 4, the wheels should be toed in slightly.... a little toe in makes a big difference on ground handling. Even with all of these things done it is still a tailwheel, I have about 1500 hours of tailwheel time full size (most of it teaching tailwheel) and all of my models have been tailwheels so I know a thing or two about them. Tailwheel airplanes are unstable on the ground. This means that no matter how well the geometry of the model is setup the pilot will have to do a rudder dance on the ground. The key here is quick, short duration, rudder inputs... if you see the model swirve left and you apply right rudder until you see the model begin to turn right you have held the rudder too long.. now you'll just have to recover from the swirve that you have created to the right... this usually goes on back and forth down the runway until you get flying speed and it leaves the ground. The correct way to handle a tailwheel airplane on the ground is to respond to a swirve with a PULSE of opposite rudder. For example, if the model begins to swirve left respond with a stab of right rudder and then go back to neutral... there are three possible outcomes... 1, you didn't hold it long enough and the model continues left, then simply continue with more stabs of rudder going back to neutral each time until you stop the swirve. 2, the swirve is stopped and the model is then tracking straight again, which is unlikely. or 3, you held the rudder too long and you have a swirve in the opposite direction now... this is the most common. The best way to learn this is to stand directly behind the model instead of beside the runway like most fields.... and just use about 1/4 throttle to do taxi runs down the runway... force yourself to get the hang of the handling on the ground before you go the next step and takeoff.

Ty
The combo of good setup and good practice should take care of your problem.

Ty

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RE: Ground loops - 1/3/2004 2:21 AM   
rc flylow



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From: Friendswood, TX, USA
Status: offline 		Thanks guys! I'll see what I can do about the thrust angle and learn to "pulse" the rudder. You are right on explaining the way I was "driving" the plane down the runway. Too much over control causing the plane to whip back and forth. I'll get the hang of it! Really good help from both of you!

I figure if I can learn to fly this contraption I'll have an easier time flying a straight model. I love the challenge of this hobby!

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RE: Ground loops - 1/5/2004 4:22 PM   
ksechler



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Status: offline 		Try toeing the gear in too.

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RE: Ground loops - 1/6/2004 10:57 PM   
seanpatsdad


 

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From: Martinsville, IN, USA
Status: offline 		My problem is that my plane wants to nose over when starting out.i was told use a bigger wheel.I am running 2.75 inchers now.The plane is a Goldberg Falcon III with a OS 40.Any ideas.Thanks in advance.
John

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RE: Ground loops - 1/6/2004 11:14 PM   
Montague



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Status: offline 		For nosing over, check to make sure the wheels spin freely. If the gear is wire, see if you can flex it forward to move the gear a bit more in front of the CG. Check your CG to see if it's not too far forward. Make sure you aren't starting in a pothole, or overly dense or tall grass if you fly from grass.

Beyond that, when you taxi, hold up elevator. When you start the take off run, start holding up elevator, then let off the elevator as the plane gets rolling and the nose-over danger goes away so you an avoid jumping off the ground too soon.

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RE: Ground loops - 1/7/2004 12:25 AM   
Tattoo



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quote:

a little toe in makes a big difference


Don't over look this one...it does indeed make a big difference...in fact a HUGE difference on a model. Especially if the opposite is currently true and there might be even an ever so slightly toe out in the gear. You are basically screwed before even throtteling up.

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RE: Ground loops - 1/7/2004 8:04 PM   
CafeenMan



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Status: offline 		I only read the original post and parts of the rest. I'm not going to attempt an aerodynamics answer, but a practical one that may save your wingtips. My Herr Pitts Special is a ground loop nightmare. About 1 in 4 take-offs is successful and the other three result in it scattering it's struts across the field.

What a friend discovered is that if someone holds the the fin while the engine is moved to full throttle and then releases the plane, the aircraft will immediately start tracking straight. Not exactly what you'd want to do for an audience, but the ratio of ground loops has gone down considerably.

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RE: Ground loops - 1/12/2004 12:15 AM   
Ick



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From: Sterling, CO, USA
Status: offline 		I had a pieper cub I could hardly take off. Anthing from wing tips and props hitting the ground during looping. I toed the tires in, and never had a problem again. Made 100% difference without changing anything else.

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RE: Ground loops - 1/12/2004 3:42 AM   
Flypaper 2



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Status: offline 		Same problem with the Herr Pitts here too. Also found out ,one had best be close to the field when the engine quits. Fun plane to fly though. Have an OS 12 in it.

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RE: Ground loops - 1/12/2004 3:50 AM   
CafeenMan



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Status: offline 		Flypaper - I was in the service from '92 - 2003. That was the first R/C kit I built after joining. I built mine in 2001 so I had forgotten a lot. Mine came out way nose-heavy. If I'd thought about it at the time, I could have saved a lot of weight by putting the servos in the rear instead of dead weight. It is fun to fly, but mine is over-weight and is pretty fast with an O.S. .15 CV-A in it. I have to keep a spotter handy because I tend to let it get away and lose orientation easily. I'd build another one though and use lessons learned to keep it light and make it better overall. I just wish the cowl was better.

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RE: Ground loops - 4/30/2005 2:25 PM   
SkyChaser



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From: Riverdale, NY, USA
Status: offline 		I know this thread is over 1 year old but I like to acknowledge when RCU users solve a problem. The diagram in Tattoo's post #8 is terrific and the toe-in discussion completely solved the ground loop issues with my Seagull Decathlon. Thanks!

Now if only I could stop landing so hard and retain the proper gear alignment...

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