riddle me this: my ground looping eindecker...

just built up and ARF eindecker (hobby-lobby).
i was just doing some taxi-test in the street and the thing just wants
to ground-loop no matter what.
i bring up the power slowly, and it loops hard.
i bring it up quickly, and it loops hard.
strangely there is no consistency in the direction, either. it's
equal opportunity.
CG is correct, if it matters.
any ideas?
Reply to
wamanning
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Is this your first tail dragger?
If the LG is way ahead of the CG it'll ground loop. If you're running a skid on pavement it'll want to ground loop. If you're running a tail wheel consider holding up elevator to keep weight on it until the rudder gets effective (don't forget to let off the up elevator before reaching flying speed -- and please don't ask me how I know).
I find that putting some toe-in to the landing gear helps. Others suggest toe-out. Few suggest just leaving it alone.
Reply to
Tim Wescott
Sorry NP but to correct a groundlooping tendency you add about 3° of toe-*in*. I always add toe-in to a tail dragger whether it's needs or not. Back in the late 60's (or early 70s) I converted my KAOS to a tail dragger. On grass it was OK but the first time I took it to a paved strip it wouldn't taxi at any speed without groundlooping. One of the *real* old timers took a couple of monkey grips and eyeballed some toe-in. From then on it taxied like it was on rails.
AAMOF, TNT manufactures all of their landing gear (CF & Al) with 2-3° of toe-in. Toe-out will increase the groundlooping tendency. Don't try to figure it out rationally 'cuz it won't work. ;-) An automotive engineer will explain it clearly if you listen for about 20 minutes . Ed F.
Reply to
Ed Forsythe
| AAMOF, TNT manufactures all of their landing gear (CF & Al) with 2-3° of | toe-in. Toe-out will increase the groundlooping tendency. Don't try to | figure it out rationally 'cuz it won't work. ;-)
Sure it will! It's really quite simple.
Put your hands in front of you, fingers straight, palms parallel to each other.
| | | | | |
(Phear my mad ASCII art skillz!)
This is your landing gear with no to-in or to-out.
Now, add some toe-in --
/ \ | | / \
(The weird bend is just because I can't really show an angle less than 45 degrees with ASCII art very well. Both hands should still be straight, but angled in a little.)
Now picture this going forward, straight ahead (away from you.) You can see that the tires will drag, but just a little, and evenly on both sides, so it won't make it turn either way.
Now picture the plane suddenly turning a bit to the right while the plane itself is still going straight --
/ | ^ / | | direction of travel / | |
This will cause the left wheel to drag more and the right wheel to drag less. This drag will tend to make the plane turn left, counteracting the perturbation that made it turn right in the first place.
If you do toe-out, you can easily see that the drag caused by it not going straight will make it worse rather than better.
| An automotive engineer will explain it clearly if you listen for | about 20 minutes .
You may be right. But it can be explained more simply.
Of course, all of this is needed largely because turning on a tail dragger is an unstable affair. If you're driving your car forward and you turn your wheel to the left and then let go of the wheel, the wheel will tend to turn back to the center. But if you're driving your car backwards and turn and then let go of the wheel, the wheel will tend to turn more and more. Taildraggers, because the steering wheel is in the back, work like a car going backwards.
Tricycle gears, with the steering wheel up front, are more stable.
Regarding cars, this page --
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explains it pretty well. But cars are different too -- where they might want toe-out to make the car more nimble, we just want our tail draggers to go straight as we take off. Nimble is something we want in the air ...
Reply to
Doug McLaren
Hi Doug, I knew that it wasn't necessary for me to explain it because I knew that you would step in and do the job masterfully ;-) You saved me much time - thanks . Ed F.
Reply to
Ed Forsythe
"Doug McLaren" wrote
All this is fine and good IF (I repeat) ** IF ** both wheels stay on the ground. That is not usually what happens.
When the plane veers to the right, it also will have the right wheel lift off the ground. With the left wheel toed in, (turning to the right) it will make the plane veer even sharper to the right, lifting the right wheel off the ground even more, and you have your ground loop, big time.
With the toe out, when the plane gets up on the left wheel, the toed out (to the left) left wheel will make the plane turn back to the left, correcting the errant turn, and getting the plane on both wheels again.
What usually happens when you ground loop? The wing tip on the outside of the turn gets damaged. That means it got up on one wheel, which makes my scenario the correct one.
We will argue this all day long, and never agree, so how about this approach.
Set the wheels dead ahead, 0 degrees and see what happens. My guess is it will be much better than it is now.
I'm curious, asking to the OP. What is the current alignment of the wheels, now? You can measure them by putting a straight edge on the outside of each wheel, and see how it is aligned with the plane's center line. It should be the same distance at the front and back of the sticks.
Reply to
Morgans
But if your gear is tall compared to the width, the opposite will happen: The plane will "lean" onto the left wheel, which is steering to the right, causing more right turn, which causes more lean to the left....
Yes. A taildragger setup is inherently unstable if you ignore aerodynamic forces.
While that is entirely correct, it has nothing to do with the point in question. This effect is caused by suspension geometry which results in the same effect as you get on the wheel on a shopping trolley.
If you want to test a similar situation with a car, drive along and then lock the rear wheels using the hand brake.
Placement of the steering wheels has nothing to do with it. Forklifts have rear steering, and they are perfectly stable.
The core of the problem is that the main gear, which carries almost all of the plane's weight, is located in front of the CG. When a slight turn occurs, the CG want to continue straight ahead, while the wheels steer off to the side. This causes a torque that increases the turn.
But not because the location of the steering wheel. The key is that the main gear is located behind the CG, so the occurring torque will work the other way, dampening out any random error. The trike setup is inherently stable. You will *never* se a trike ground loop.
Reply to
Robert Roland
Getting a taildragger to behave is a matter of delicate balance and compromise. There are many factors that affect the result.
On some planes, toe-in will help a little bit, but it may be just enough. On other planes, toe-out will help a little bit. Experiment and see what works for your model.
There are two things that always help:
1: Move the main gear backwards. If you go too far, the plane will nose-over too easily, of course. Try to find the "sweet spot".
2: Increase the area of the rudder and/or vertical stabilizer. Easiest is to simply replace the rudder with a much bigger one.
On a scale model, neither option may be acceptable. In that case, you can try moving the CG forward. That will affect flying characteristics, of course, so it is another compromise.
Reply to
Robert Roland
Here we go again! The efficacy of toe-in has been proven empirically and scientifically. My only suggestion is that any pilot with a groundlooping problem (not induced by PIO) try toe-in and observe the effect. TallyHo - :-) Ed F.
Reply to
Ed Forsythe
Toe in makes a car tend to oversteer, as the outside wheel loads up it pulls the front into the turn.
It does however stop the car wandering about on a camber. A car with toe in tends to find the flat piece of road!
Reply to
The Natural Philosopher
That is nowt to do with whether the steered wheels are at the front or at the back ..its all to do with the inclination of the kingpins - the castor angle.
Reverse that and a rear wheel steer car will center its steering just the same.
I'll go with your drag theory though..as the tailksid does nothing to stabilise once the tail goes light..
Reply to
The Natural Philosopher
"The Natural Philosopher" wrote
Not that it has anything to do with the discussion (besides people that have a trailer they tow with their airplanes in them) but I have found putting the toe-in on a car that pulls trailers to the maximum toe recommended in the car's repair specifications, tends to make the whole thing a little more stable while pulling the trailer.
Reply to
Morgans
Methinks that statement is backward.
You want the main gear forward on a taildragger.
Reply to
M-M
"Robert Roland"> wrote
OK, I did some more reading on the subject, "again!"
I looked at full sized airplane info, because anything with models is likely to just be opinion, and proves nothing.
* Ladislao Pazmanys "Landing Gear Design For Light Aircraft, Volume I", and he comes to no conclusion either. (way)
*
Darrol Stinton in "The Design of the Aeroplane" (excellent book) clearly states that taildraggers should be set up at 0 deg or some toe out. NEVER toe in.
*A previous comment mentioned that the Pitts factory set their a/c up with toe in. Given than all Pitts a/c have an evil reputation on the ground (and that mine is now excellent with this one change) do you think that this is good thing?
* The Boredom Fighter tracks straight with 1° toe-out, per plans, and does not wander around without rudder input.
*
On the Luscombe buy inspection list: Check there is no "toe-in" on the main gear.
* After a review of "opinion" posts about the whole toe in/ toe out subject, a great majority of high time taildragger pilots that I have grown to respect, almost all say "toe out - no doubt!"
* Another frequent opinion was that it varies from plane to plane, based mainly on landing gear geometry, with aspects of camber having a lot to do with it.
*
Another aspect that makes the toe out case pretty strong is how and when ground loops usually take place. On landing, it is common practice for one wheel to touch down before the other, usually to account for cross winds. If that happens and a swerve occurs at that time, the toe out is going to straighten out the landing, because the "toe in / rolling friction straightens it out" theory is not going to work until both wheels are on the ground.
* It also makes a difference if your landings are on grass or a hard surface. On grass, the toe in may not work as well, as the friction on the slippery grass is not going to help very much.
*
ALL IN ALL ... My take on it. Take the time to adjust from an extreme setting of perhaps 4 degrees toe in, then repeat it with 4 degrees toes out, and stood several landings each way. Then try 0 degrees, and compare your notes. You should be able to get a good opinion on it all, after all of the experimentation!
Reply to
Morgans
"M-M" > wrote
Not necessarily.
Moving it forward will help keep the tail wheel on the ground longer on take offs, and plant it sooner on landing. Good for directional control - more so if you are on hard surface runways. It will also keep you from nosing over as easily.
Moving it forward will also make the CG further behind the gear, which means once the center of mass starts to get out of line with the center line of the plane, that it will come around more violently/ perhaps so much more so that there is nothing you can do about it.
Reply to
Morgans
No. If you place the main gear too far forward, the plane will ground loop. Trust me or try it.
Sure. But only slightly ahead of the CG. Too much forward will cause ground looping. Too much backward will cause nosing over.
If you want proper directional stability, you must place the main gear behind the CG. In that case, however, you no longer have a tail dragger.
Reply to
Robert Roland
Interesting- I see that possibility if you horse it off before flight speed is attained.
But moving it forward for longer takeoffs allows you to gain more speed before breaking ground which prevents groundloops, no?
Reply to
M-M
this is very true. any model rocketeer knows that the CG must be ahead of the center-of-pressure (roughly akin to the main-gear), or your rocket will spin in place once it clears the launch-rod!
w
Reply to
wamanning
"M-M" wrote
For takeoff, perhaps true. It is possible to ground loop while the tail wheel is still down. Having it on the ground just means you have more of an opportunity to prevent the ground loop.
It would mean only full stall (or close to it) landings, or it could be a landing loop!
Reply to
Morgans

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