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May 08, 2017, 03:40 PM
Flying R/C since 1964
kallend's Avatar
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Trike landing gear directional stability


I have a scratch built model (delta wing) with tricycle LG that is quite directionally unstable on the ground. It's very hard to keep straight on take-off and has a tendency to ground loop on landing. The wheels turn freely, no binding, and all the nosegear steering linkages are slop free. The mains are about 1 inch behind the CG. There is no toe-in or toe-out on the mains.

Does anyone have any insights on landing gear design that might help? I expect directionality problems with conventional gear but this is the first time I've had problems with trike gear.
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May 08, 2017, 04:57 PM
Registered User
mains need both toe-in and camber .... so the unstable reverse doesn't happen under load
May 08, 2017, 05:05 PM
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rear view

May 08, 2017, 05:17 PM
Registered User
Restrict the movement of the front wheel to +/- 10 degrees ... or even less ... too great a movement is a common cause of this squirreling.

Also, it might be that you are holding the model down too long. If the squirreling is predominantly on departure, what sometimes happens is that the wing provides sufficient lift to unload the main gear, whilst the nosewheel remains firmly stuck to the ground. This "wheel-barrowing" is very unstable, because the CG is now a long way behind the point of ground contact.
May 08, 2017, 06:20 PM
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exf3bguy's Avatar
Make sure the nosegear linkage has no slop or bind and then put some friction on the mains. A short spring and washer between the wheel and wheel collar or a peice of fuel tube and washer works well. If the instability continues then it could boil down to a case of over controlling. How much rudder throw do you have and are you running any expo? Is there any right thrust built into the firewall or engine mount?
May 08, 2017, 06:52 PM
Registered User
Also, the further ahead of the CG the front wheel is, the more it will tend to ground loop. I think most trike gear planes have a slight "nose down" attitude when sitting on the runway, both to help when braking and to make sure the nose wheel unloads as soon as the plane starts to lift on the wing.
May 08, 2017, 07:03 PM
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exf3bguy's Avatar
Maybe a whole different thread but if the model is sittin nose down that means the wing is sitting at a negative incidence and will never " lift " until that incidence is changed to positive. The more speed the airplane builds while on the ground, the more downforce will be applied. The best stance for a trike geared airplane would be what has the wing sitting at zero incidence. Personally I have never had any airplane with a ground loop issue.
May 08, 2017, 07:17 PM
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Depends on the wing section and rigging angle, obviously.
May 08, 2017, 07:38 PM
AndyKunz's Avatar
Quote:
Originally Posted by xlcrlee
mains need both toe-in and camber .... so the unstable reverse doesn't happen under load
+1.

It's also very easy to "break" a good setup with one rough landing.

Andy
May 09, 2017, 05:16 AM
Registered User
Andy, you are exactly right!

I posted this pic of the tiny thin wire all-in-one trike gear to make it easy for readers to visualize the effect of the various ground, thrust, weight and lift forces on the LG: they bend backwards plus up and" out" from ground-drag and bouncing forces and to axial and pitch-wise wing-tipping, etc. The setup shown is extremely stable and WORKS! (somewhat exaggerated due to very light weight and size)



With larger planes I ALWAYS check and usually have to re-bend the gear back after each landing, even if it seemed smooth
May 09, 2017, 07:01 AM
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exf3bguy's Avatar
Sorry, no magic toe in or out or stance to be found here. It mostly boils down to how well the airplane is flight trimmed and of course PRACTICE. You need to fly the airplane on and off the runway. 40# Extra 330, wheels set to zero but airplane is very well trimmed and FLIES off the runway quite easily.

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May 09, 2017, 07:29 AM
Flying R/C since 1964
kallend's Avatar
Thread OP
Quote:
Originally Posted by exf3bguy
Sorry, no magic toe in or out or stance to be found here. It mostly boils down to how well the airplane is flight trimmed and of course PRACTICE. You need to fly the airplane on and off the runway. 40# Extra 330, wheels set to zero but airplane is very well trimmed and FLIES off the runway quite easily.

https://www.youtube.com/watch?v=GQTLS6l28fo
I've been flying R/C for over 50 years. I DO know how to trim and fly an R/C plane.

There is clearly a geometry issue with this particular plane and I'm trying to figure it out.
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May 09, 2017, 07:58 AM
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exf3bguy's Avatar
Quote:
Originally Posted by kallend
I've been flying R/C for over 50 years. I DO know how to trim and fly an R/C plane.

There is clearly a geometry issue with this particular plane and I'm trying to figure it out.
Great, then you won't mind giving some specifics on your setup that may lead to a better assessment. What are your thrust angles? Incidences? Rudder throw degrees? Amount of expo being used? Tail moment? Size and weight of airplane? Power plant and prop? Description of your throttle curve?
May 09, 2017, 08:47 AM
Registered User
I notice that the experimental full-size Delta types often seem to have long nose-wheels and sit at a positive angle on the ground.

Is it possible that the relatively -weak elevator authority (short moment) is creating a condition where you have to accelerate to a high speed before the aeroplane will rotate?

Coupled with main gear that is well aft of the CG, this would lead to that "wheelbarrowing" condition which I mentioned earlier ... where the wing is unloading the main wheels, whilst the wing's pitching moment is loading the nosewheel.
May 09, 2017, 08:47 AM
Registered User
Quote:
Originally Posted by exf3bguy
Sorry, no magic toe in or out or stance to be found here.
And no tricycle gear either.


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