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Old Sep 04, 2012, 05:40 PM
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Gear position and ground-loops on a tail-dragger

Fokker D-VII set up with a tail-wheel, vicious ground loop tendency, one problem is the tail wheel gear is too flexible, fixing that, but also see the main gear is a bit farther forwards than spec.

I suspect the further forward the gear the greater the moment arm, and so the greater the ground-loop tendency.

Anyone have any idea just *how* great the sensitivity is?
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Old Sep 04, 2012, 06:49 PM
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Originally Posted by CNY_Dave View Post
Fokker D-VII set up with a tail-wheel, vicious ground loop tendency, one problem is the tail wheel gear is too flexible, fixing that, but also see the main gear is a bit farther forwards than spec.

I suspect the further forward the gear the greater the moment arm, and so the greater the ground-loop tendency.

Anyone have any idea just *how* great the sensitivity is?
Depends on the model, its size and weight and lots of factors, but having the gear too far forward does exactly what you mention......gives a greater moment arm between the CG and the wheels.

With a scale model you are sort of limited to the scale gear location unless you as going sport scale and then you can tweak it as desired.

One old rule of thumb that usually gets it close is to look straight down on the model from above. The axles of the landing gear need to be pretty much directly below the leading edge of the wing, or perhaps a touch further back...but not further forward. Models that have a very large deck angle do cause exceptions to this rule......
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Old Sep 05, 2012, 04:28 AM
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All the above is great advice, but if you can't do anything about the gear position there are a few things you can do with radio setup and flying style that might help...
  • Be pedantic about taking off exactly into wind - any slight crosswind will exascerbate your problems keeping it straight.
  • Reduce your ruddder rates for takeoff, and increase rudder expo;
  • Throttle up slow and smooth, and don't use full power until your wheels are off - with a model like that I'd have thought nice scale takeoffs were possible with 1/2 - 2/3 throttle.
Best of luck!
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Old Sep 05, 2012, 06:29 AM
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Yep, gear too far forward is generally sighted as being a bad thing for ground looping so I'd say dont put it any further forward than the scale location. Narrow set gear is another red flag, as is long landing gear... the DVII probably has all of these things.

You might try toe'ing the wheels out. This is a sensative subject with some swearing by toe-in and some by toe-out. In truth i dont think there is one solution that fits all but for narrow track taildragers I think toe-out works best, maybe worth a try if you can do it easily?
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Old Sep 05, 2012, 08:32 AM
DX5e fatal flaw- PM me!!!!
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Thanks.

The gear is actually forwards of the proper install location, I realized this when I saw the front-most screws poking through the floor just in front of the wood block for the screws. Just another thing on this that wasn't done *quite* right.

So between the better tail wheel and moving the gear back a bit I have hopes the ground handling will be a little improved, anyway.
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Old Sep 05, 2012, 09:19 AM
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Originally Posted by JetPlaneFlyer View Post
Yep, gear too far forward is generally sighted as being a bad thing for ground looping so I'd say dont put it any further forward than the scale location. Narrow set gear is another red flag, as is long landing gear... the DVII probably has all of these things.

You might try toe'ing the wheels out. This is a sensative subject with some swearing by toe-in and some by toe-out. In truth i dont think there is one solution that fits all but for narrow track taildragers I think toe-out works best, maybe worth a try if you can do it easily?
I believe that you'd want to toe the wheels IN to reduce sensitivity. It's why production cars all have front wheel toe IN...When I was auto crossing my Miata I had a tad of toe OUT in the setup and talk about quick responsive handling. Think turn and it was half way through the turn before you could blink...Terrible for trying to drive straight down the freeway, so I had it removed to 0...
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Old Sep 05, 2012, 09:28 AM
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Matty B has some of the answer I had a Stearman that was all wobbly on take off and adding 30% expo cured the problem.Once the plane started going astray, counteracting with too much rudder authority only made it worse The it would go too far the other way and counteracting that with too much rudder just made things even worse. I wouldn't reduce the rudder throw though as it affects ground handing turn circle diameter which could get quite large.

If your TX is capable you could add in some throttel/rudder mix to calm her down. The aboce tip on wheel alignment is good too with just a touch of toe in, toe out will make for a mean handling plane on take off generally.
Other than that,not much you can do with scale gear placement as mentioned above, if you want the look you have to go another route.
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Old Sep 05, 2012, 09:49 AM
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Increase the height of the tail skid/wheel to lower the nose . Wheel axles lined up with wing leading edge . Moderate toe in. Up elevator to keep tail skid on ground during start of take off run. Release elevator when rolling faster to let tail rise and steer with GENTLE nudges of rudder anticipating tail swing and torquing to the left. Take off at 3/4 throttle or less and gain altitude slowly before turning using coordinated ailerons and rudder. I use aileron/rudder mix most of the time. Real WWI bipes had guys running along holdiing onto wing tips to help prevent ground looping. Model will have less ground looping when taking off from grass if grass is short and smooth.
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Old Sep 05, 2012, 11:59 AM
DX5e fatal flaw- PM me!!!!
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Takeoff is actually not much of a problem. It gets the tail up and starts flying fast enough that ground looping isn't an issue, although if I get the tail up too quickly the gyroscopic precession will turn it 90 degrees pretty quickly.

If I can get it to not ground loop during landing rollout I'll be a happy camper.
I went with a bit of toe-in already, but the too-wimpy tail gear is just flopping about, have a better unit in hand and ready to go on.
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Old Sep 05, 2012, 12:27 PM
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As i said toe-in and out is a controversial subject but suffice to say there is some good science that says that toe out should work with a narrow track taildragger (the physics is different from a car!).. All I'm saying is it may be worth a try, you have tried toe in and it's not helped so.. might be worth considering?
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Old Sep 05, 2012, 12:42 PM
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Most of the full scale pilots I know think a little toe out is the way to go.

Discussion of why toe out is a defendable and desireable setup on a taildragger:

http://www.homebuiltairplanes.com/fo...technique.html
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Old Sep 05, 2012, 05:25 PM
DX5e fatal flaw- PM me!!!!
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Toe-in and toe-out may be an impossible conversation to have rationally without also discussing camber.

Also, on a full-scale there may be very important considerations relating to geometry changes as weight is applied, suspension movement during weight shift, and lateral and fore-aft movement of the contact patch (that last could drastically affect the effective caster).

I could see depending on the plane, and on a model, where one or the other could be the better, depending on which effect was greater or more important. It could well be what makes for better handling with no cross-wind works opposite what makes better handling with a cross-wind.

One could analyze the situation as a car being set up with negative caster, and if a car with positive caster is more stable with toe-in, maybe a car with negative caster might be more stable with toe-out.

I'm a nut about getting a good landing, so if I don't get good results with toe-in I will certainly try neutral and toe-out.

It seems this could be a downwind-faster-than-the-wind / will-the-plane-takeoff-from-a-treadmill scale discussion!

Dave
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Old Sep 05, 2012, 06:42 PM
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Try flying from grass, and slowly advancing the power.
My Top Dawg is crazy on the pavement, but a pussycat from grass.
I think the toe-in toe-out thing depends on the model. My Super Sportwin likes a degree of toe in, others like a little toe-out. Time to experiment
Mark
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Old Sep 05, 2012, 06:51 PM
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Quote:
Originally Posted by mrittinger View Post
Try flying from grass, and slowly advancing the power.
My Top Dawg is crazy on the pavement, but a pussycat from grass.
I think the toe-in toe-out thing depends on the model. My Super Sportwin likes a degree of toe in, others like a little toe-out. Time to experiment
Mark
My experience, exactly. It depends on the plane. Experiment to see what works best, if you can.
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Old Sep 05, 2012, 06:57 PM
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Camber is a variable on models with flexible landing gear...Spring steel, bungees like a Cub, etc. It is one thing when lightly loaded at touchdown with the wing still flying somewhat., different at rest and different again if you make a hard arrival and load the gear with more than one G..
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