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Gear position and ground-loops on a tail-dragger

Old 09-05-2012, 12:41 AM
  #1  
CNY_Dave
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Default 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 09-05-2012, 02:02 AM
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hayofstacks
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On my mini ultra stick, I had a problem with the stock foam wheels and the tail skid was broken. I put a .40 size tail wheel on it to make it balance better, but still struggled to keep it straight. The tail would lift well before I had control with rudder, even with elevator into it. Adding a 1" large rubber wheel and some 2.5" "sport" foam wheels made it bareable to taxi while it was on the ground, and stopped the tail from trying to come around on 3 point landings.

The further your main ger is from the tail wheel should slightly dull the quickness, but then it takes more speed to get the tail up, and can possibly cause it to catapult upwards into a stall if your not careful.

I do hate to tell you this, but ww1 planes weren't known for their ground handling.
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Old 09-05-2012, 03:35 PM
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Yeah, it'll never handle great, but I figure it should be able to taxi faster than a crawl without swapping ends.
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Old 09-05-2012, 10:29 PM
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gyrocptr
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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.....
Article from EAA may be of interest. Starting at page 3.
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Old 09-06-2012, 12:30 AM
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Ah, I will have to read that. Thanks.
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Old 03-08-2013, 05:29 PM
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7car7
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Originally Posted by gyrocptr View Post
Article from EAA may be of interest. Starting at page 3.
Great article.
I've tried to put toe out on the gear I've built, as I've luckily had an un-official, un-educated opinion that planes needed toe out. However, on planes that were bought, I've honestly not checked them as well as I should. I have a Nieuport 17 that is really bad on takeoff and landing. It has a straight axle. Sounds like that needs to change!
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Old 03-08-2013, 05:52 PM
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toe out is worth a try but narrow gear set well forward will always tend to be tricky. Moving the main-gear back should help but at the expense of increased nose-over tendency if you go too far.
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Old 03-08-2013, 06:20 PM
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CNY_Dave
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Toe out worked very well on my Fokker.

The dynamics are sufficiently complicated that in, neutral, and out should be tried, any might work best.
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Old 07-05-2013, 12:02 AM
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solentlife
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Watch this for ground tricks ... this baby NEVER got straight in every flight I had ... only time I had any near success was the day she took 8th place scale in Southern UK ...

[media]http://www.youtube.com/watch?v=G0CBjJjzYu0[/media]

OK ... it's last minute or so of the video ... Sorry about quality - but it was shot on 8mm Cine film and then transferred to video tape ... then digitised ! It's from 1980's ..... Yes that's me with the Nieuport 28 ...

Nigel
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Old 07-05-2013, 06:04 AM
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maxflyer
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I think part of the problem with models may be due to the fact there is just not much weight on the tailwheel. It will skip off the ground with the slightest force and lose contact, motor torque, a bit of wind, small bumps. At that moment only the rudder will have control (or not, if there isn't enough air blowing past it). I am assuming you are using plenty of up elevator to apply downforce on the tail as you begin your run. If your tailwheel is rigidly tied to the rudder it will only take a tiny stick movement to start the tail swinging around, especially on a short-coupled design, and you probably won't be able to stay ahead of it. Even worse, these tailwheels that are tied to the rudder have very little trail, or caster, so the slightest twitch starts them into a turn. The same issues apply to real airplanes, especially very lightweight ones.
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Old 07-05-2013, 03:25 PM
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solentlife
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Which is why I use the 'rubber band' system where tail wheel is connected by band to rudder.... means wheel resists initial turn and rudder can turn more initially. You put stick back to neutral before wheel has reached command position.... bit like having expo just on the wheel but rudder still as needed.

The Newport above dudn't gave anything like that because she was scale comp machine. Boy was she a pain. I swapped it in the end for a P51D !

Nigel
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Old 07-05-2013, 04:07 PM
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Here is an example. The first image is the tailwheel currently installed on my open-cockpit 370 lb. homebuilt. The second image shows the original that it replaced. The original had poor geometry (tailwheel post tilted too far forward) and was controlled by a rigid steering rod. The slightest pressure on a rudder pedal could cause the tail to depart sideways, inviting a ground loop. Ground handling was super-twitchy. As you can see, the replacement uses springs instead of a rod, to give the same effect Nigel is describing, and the tailwheel itself has much more trail and caster. This replacement changed an aircraft that handled on the ground like a Pitts biplane, to a real (board won't accept this word)-cat. Before the changeover, a good crosswind could also cause the lightly-loaded tailwheel to break loose from the pavement and skid to the side. What finally pushed me to make the change was when that Aluminum steering rod broke in half on a landing, sending me off the runway at about 25 mph, through very rough terrain, which wiped out my main gear and necessitated a four month repair. Our models are not immune from this. If anything, they are worse because of their scale and lack of weight.
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Old 07-12-2013, 05:17 PM
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Love the write-ups here guys. I've thought of a lot of things, and improved a lot of things, and have shared a lot of things on these boards, but it's never really occurred to me to make the tail wheel spring (or rubber band, or whatever) loaded.

I WILL be doing some changes to a couple of my planes after seeing this info.

I have a P-47 that has always been too twitchy with it's steerable tail wheel. That'll now change.
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Old 07-12-2013, 06:06 PM
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My bipe has a tailwheel where a spring goes into the pivot mechanism, the other long straight end I have just following the curve of the tail, then bending 90 and entering a hole drilled in the balsa.

That way I have not just the spring section but also the part that's just a thin wire.



Again, I need to say the best piece of advice I found was at speed to treat the tailwheel not as a direction-control device, but as a slew-rate control device, where leaving the control 'neutral' meant to maintain the current slew rate.

If the tail swings one way, hit rudder to get it swinging the other way, then just before the tail is 'straight' stab the rudder (tailwheel) the other way.
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Old 07-12-2013, 06:52 PM
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Also check forward rake angle. I have to adjust on all my tail draggers. So toe out but also rake. I'll throw out another thought. I rolled my T-6 Texan (ground loop) on her back a few weeks back. How? Lack of proper air speed during take off roll. In other words, rotating too early.
Two cents

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Old 07-24-2013, 02:14 AM
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on some planes, unused a rubber tail wheel. think of it like this....a hot wheels with rubber rear tires...the car will never spinout. I tried a similar principal on my neiuport but wrapped electrical tape around the skid to create more friction. it was a little better in tracking the landings.
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Old 07-24-2013, 03:47 PM
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Like a sea-anchor... I haven't flown a dragger yet, probably would have left my bipe a dragger but at the time I was flying off pavement.
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Old 11-15-2013, 11:09 AM
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Forgot to add this to my previous post ...



Nigel
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Old 11-15-2013, 08:57 PM
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Originally Posted by gyrocptr View Post
Article from EAA may be of interest. Starting at page 3.
Great article, thanks for posting....
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Old 11-16-2013, 01:06 PM
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your plane will track better it you bend the front wheels turned in a little. also bent a little forward will prevent nosing over on take off & landing.
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Old 11-16-2013, 04:02 PM
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Have the wheel's top-out/bottom-in just a bit with the plane sitting in a level attitude. this gives effectively more toe-in as the tail is further down.

Apply power SLOWLY and carefully with taildraggers, especially if they have a skid or a castering tailwheel instead of a steerable tailwheel.

A skid will be easier to control on short grass than on smooth pavement.
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Old 11-16-2013, 04:13 PM
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solentlife
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Originally Posted by fhhuber View Post
Have the wheel's top-out/bottom-in just a bit with the plane sitting in a level attitude. this gives effectively more toe-in as the tail is further down.

Apply power SLOWLY and carefully with taildraggers, especially if they have a skid or a castering tailwheel instead of a steerable tailwheel.

A skid will be easier to control on short grass than on smooth pavement.
I've usually found that 'a quick blip of throttle' to straighten a wayward tail is good ...

Here is my Skymaster when the rubber band was broken on the tailwheel link .. so was free-castoring ...

[media]http://www.youtube.com/watch?v=HwPxHPpxpwg[/media]

Ignore the last bit of movie ... she had terrible trouble with such power plant ..

Nigel
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Old 11-16-2013, 06:05 PM
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Originally Posted by solentlife View Post
I've usually found that 'a quick blip of throttle' to straighten a wayward tail is good ...

Here is my Skymaster when the rubber band was broken on the tailwheel link .. so was free-castoring ...

[media]http://www.youtube.com/watch?v=HwPxHPpxpwg[/media]

Ignore the last bit of movie ... she had terrible trouble with such power plant ..

Nigel
But... you have experience dealing with difficult to handle tail-draggers.

A blip of the throttle at the wrong timing and/or without proper coordination of rudder can make things worse very fast.

Initial methods should be the ones a relative novice can use successfully.

With the basic stick skills practiced, then add more complex methods such as differential thrust for a DC-3 with a castering tail-wheel on very smooth concrete. This example can get very sensitive at low speeds when the rudder is ineffective and the props are not able to blow significant air over the rudder.
Having the basic setup correct with wheel's angled properly is very important here.
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Old 10-29-2015, 05:34 AM
  #24  
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to start with, your wheel axle should be slightly forward of the C/G
I put mine about 1/4 inch forward of the lower wing leading edge at level.

When you throttle up. do it GENTLY, until almost lift off. This will help you maintain control. The D V11 should not be hard to control on the ground.
I have Two Pups 1/4 and 1/3...a D V111 1/4....a DR1 33% three Tiger-moths one 1/5 scale, one 1/4 scale one 1/3 scale. NONE of these are any problem on the ground....if it tends to nose over on landing, add tail weight a little at a time until it acts properly on the ground. or until it flies with no down trim. this will smarten it up nicely big cubs need toe in, AND reverse expo on the rudder.
since you need instant reaction to a small rudder input, and less sensitivity as you get outside the little middle section of movement. Cubs can be very tricky to take off, or if properly balanced and set up ie. toe in, they are a cat.
The D V11 should be an easy fix.
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