Ground-looping on pavement/concrete - RCCanada - Canada Radio Controlled Hobby Forum
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Old 04-29-2012, 04:50 AM   #1
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Ground-looping on pavement/concrete

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Is there anything I can do to avoid having my tail-dragging airplanes ground-loop wildly on take-off and on landing when I am flying on pavement/concrete? - someone said to toe-in my main wheels a bit? - does this work? - any other suggestions? Thank you.
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Old 04-29-2012, 06:54 AM   #2
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Re: Ground-looping on pavement/concrete

Yes, a bit of toe in is important. In some cases without it the plane is almost unmanageable on the ground.

However, due to their nature, even with correct gear geometry taildraggers are rudder aircraft and you have to be diligent.

An identical problem was brought up a bit over a week ago and rather than retype it all here is a direct paste of my reply:

A majority of my planes, including the larger scale are taildraggers and they take a bit of finesse in ground handling and launch. Part of the problem is the main gear is ahead of the CofG and therefore will try to force the plane to ground loop or in the worse case swap ends.

You are correct in that you do not want to honk full throttle right off the bat. You want a bit of up elevator just at the start to keep the tailwheel on the ground for directional control ONLY until enough speed is gained for the rudder to become effective. Once that is achieved neutralize the elevator, (or even a touch of down to lift the tail) The purpose is to keep a flat or slightly nose up attitude until the wing reaches flying speed. You will need to keep active on the elevator and rudder with fine inputs to keep the plane on the correct level and accelerating for takeoff in the direction you want. During this evolution you will slowly advance the throttle. If done correctly the plane should fly itself off.

If you go full throttle too soon the prop torque will try and turn the plane to the left and if you hold up elevator too long, forcing the tail to stay grounded the attitude of the wing means it will likely lift off before you reach minimum control speed. At that stage any aileron input will likely cause a wing to stall and the plane to snap over, usually with no altitude or control to recover. Under any conditions where the plane is nose high and slow you want to avoid aileron input and only use the rudder for directional control.

The other aggravating factor in ground handling is incorrect wheel toe in. You want a slight amount as that helps prevent the tendency to wander as soon as the tail is off the ground.

As I stated, the first thing you need to work on is directional control with the rudder input. This is both with the tail on the ground and with the mains still planted and the tail up.

This takes practice to master and eventually becomes second nature. I believe this is also the reason most trainer planes have a more conventional gear and nosewheel.

Also, don't forget the smaller and lighter planes mean things will happen rather quickly, so before even getting airborne a good practice is to work on ground handling repeatedly, going as far as raising the tail and accelerating, but stopping the process before takeoff. This practice will help in landing as well since the exact reverse is true when decelerating after touchdown.
One thing I neglected to add in that one is to ensure the gear does not deflect too much under loads. How it looks on the build table may not be what is happening during ground ops.
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Old 04-29-2012, 07:11 AM   #3
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Re: Ground-looping on pavement/concrete

Thank you. Is the type of wheel important i.e. hard, soft, treaded, smooth, narrow, wide, sponge, foam, solid, air-filled, flat, round section, etc.,? And I suppose the wider the wheel track the better? And is it better for the wheels to be more - or less - farther ahead of the flying center of gravity of the wing?
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Old 04-29-2012, 07:51 AM   #4
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Re: Ground-looping on pavement/concrete

Originally Posted by happypilot View Post
And is it better for the wheels to be more - or less - farther ahead of the flying center of gravity of the wing?
Maybe I can help you with this one.

The closer the mail gear is to the C of G the worse the problem is. Get the main wheels further forward, it puts more weight on the tail wheel making it more effective and delays lifting of the tail untill there is more airflow over the rudder.

Don't neglect that tail wheel! Larger size helps but more important is that it and the associated linkage is in good shape & free of slop & play. A wobbly wishy - washy tail wheel will cause all of the grief you are talking about!

Make sure that the tail wheel & rudder are pointing the same direction. A rudder poining left & tail wheel pointing right is a sure recipe for a ground loop on landing! Even a few degrees makes a difference.

Hope that helps. Merv

P.S. If all else fails, fly off of the grass beside the runway! Way better!
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Last edited by OLD PRAIRIE RED NECK; 04-29-2012 at 08:16 AM.
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Old 04-29-2012, 08:08 AM   #5
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Re: Ground-looping on pavement/concrete

I would need a bit more info on your airframes to answer some of your questions, but tire choice seems to be more of a personal preference or the option of "Whatever Fits". In my cases one pet peeve is the wheel I.D can frequently be sized now for metric gear. Luckily I have access to numerous diameter drills and work towards getting a slop free and smooth rotation.

The smaller and lighter "Park Flyer" aircraft don't seem to mind sloppy wheels, but as weight and size increase maintaining a good wheel alignment and geometry become almost as important as a flight control system.

I find the same type of tires work on both pavement and grass and they are the "Low Bounce" type of plastic or alloy hub with rounded rubber treads. My own criteria leans towards a tire adequately sized and of a material to deform only moderately under load. This way you ensure the same behavior under most conditions.

For example, if your tire deforms to the point even half of the tire radius is now gone then that same type of deformation has to follow as the tire rotates. Other than a factor of extreme drag the tire is not going to want to go where you aim it or stay consistent. Ever try to drive a car with a low inflated tire, (or a flat)?

Another criterion I use is to look at the surface I will be operating from the majority of the time and work towards that. In our case it is grass, (frequently long and wet) and therefore I need wheels large and wide enough to prevent too much sinking in and the drag that causes. My Skybolt is a bit extreme and looks like I have a set of tundra wheels attached, but it will roll over anything. Those tires are a bit wobbly, but again, with close to 300 flights on that airframe I know its quirks.

On pavement a lot of the drag is removed, but you still need to come up with a set of wheels that fit your requirements. Narrow or too hard a wheel may actually bounce so much they spend little of their time in contact with the ground helping to aim the craft where you want it. They also do little to absorb shocks and transmit them directly into the airframe.

As for the material, since I work predominantly with nitro powered machines I stay with rubber. Foam degrades pretty quickly and also has the annoying tendency to continue to deform under load and remain that way as the tire ages. A wheel with flat contact will not work well if the loads are off centerline, (drive down a road with truck grooving). The wider the wheel the worse this behavior.

In tail draggers the location of the gear can cause more aggravation than it should. If too far forward it will prevent the tail from lifting during takeoff for far too long. If too far aft the plane will have the tendency to tip on its nose at the slightest excuse. The longer the legs the worse it can be since as the tail lifts the position of the tires in relation to the CofG shifts aft.

This brings up another of my pet peeves. Perhaps due to my background I tolerate deflection far less than necessary. What that means is I want the wheel to stay where I bolted it and aimed the way it is supposed to. With single wire spring gear legs this is almost impossible since the wire can and will flex in multiple directions under load. It is designed and fabricated to bend. If too soft there is no way to maintain good wheel alignment. In that case I discard the wire type and go with an alloy or composite gear or double up the wire in a brace fashion, (although of conventional gear type, Ken Willard's Seamaster is the best example I have here of that method). The main gear wire/axle has been supplemented with a smaller diameter trailing wire to prevent any deflection other than vertical.

When working with a very stiff gear of any type some other method to absorb shocks is required and that is easiest with tire type. A material that will absorb loads, not rebound and/or transfer them further into the airframe is the quest. In the final pic I have my Rare Bear retract setup with Robart legs to show in that case the actual leg has a method to shorten and absorb loads. The tire material is still important since with no method to dampen the spings the load can again get transferred up the leg into the gear attach and airframe.

p.s. While I was typing this protracted affair Old Prairie Red Neck pointed to the tail. I neglected that end. My Bad!
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Old 04-29-2012, 08:53 AM   #6
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Re: Ground-looping on pavement/concrete

The further forward, the more chance of groundlooping or switching ends. Think of it like a castor wheel. Put the castor wheel forward of its pivot point and push on it and it will go towards the back. Closer to the CG the better without tilting over on the nose. Better way is to use full up elev. to push the tailwheel down on takeoff and let off on it as it picks up speed and the rudder takes over with the airflow over it. This is why the Fokker DR1 is so notorious at takeoff for switching ends. Sitting on the ground with a very nose high attitude on the ground, the wheels are very far forward of the CG. As the tail comes up on takeoff, it's much more closer to the CG. The little rudder doesn't help much, being blocked by the fuse. but once the tail comes up, it's fine.
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Old 04-29-2012, 09:33 AM   #7
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Re: Ground-looping on pavement/concrete

Here we go again! "TOE OUT IS THE WAY TO GO". When the left wing is high the right wheel is on the ground, when it is toed out it will drag the aircraft to the right pulling the left wing down. If the right wheel is toed in it will drag the aircraft to the left making things worse. Too high of an angle on the ground will also cause ground loops. A slow application of the throttle will also help prevent ground loops. Pretend you are the airplane and your hands are the wheels. Set it up both ways and see what reaction you get. I seen an ultra-light with toed in wheels do a ground loop on landing.

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Old 04-29-2012, 11:12 AM   #8
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Re: Ground-looping on pavement/concrete

It's all in the thumb. If you don't know how fly with the rudder how can you expect to have decent landings and take-offs from pavement. Most pilots over control with the rudder on pavement and get really out of control takeoffs. Go to low rate on the rudder and practice. No amount of toe in or toe out is going to make it easier if you don't know how to use the rudder.
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Old 04-29-2012, 11:47 AM   #9
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Re: Ground-looping on pavement/concrete

Practise!! and tons of expo in your rudder.
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Old 04-29-2012, 12:12 PM   #10
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Re: Ground-looping on pavement/concrete

Originally Posted by JKovats View Post
Practise!! and tons of expo in your rudder.
Amen, and easy on the throttle giving you a chance to correct as you increase to full.
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