riddle me this: my ground looping eindecker...

wamanning@email.com wrote:

Try some toe out.

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 <VBG>.
Ed F.

| 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 <VBG>.

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 --

   http://www.ozebiz.com.au/racetech/theory/align.html

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 ...

--
Doug McLaren, dougmc@frenzied.us
Nothing lasts forever.  Where do I find nothing?

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
<BG>.
Ed F.

"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.
--
Jim in NC
--
Jim in NC

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.

On 10 Aug 2007 20:46:02 GMT, "Doug McLaren"

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.
--
RoRo

       Hold on a minute. You must never have driven an old forklift
with the steering a bit worn, and probably never any forklift at
higher speed. They are nasty beasts. The CG is behind the non-steering
wheels and they don't want to go straight for anything. I nearly
rolled forklifts a few times. The forklifts you see cruising on the
streets have extremely rigid, hydraulically-boosted non-feedback
steering, and some are articulated for highway travel to control the
nastiness.
       People who want to see taildragger instability should fool with
a grocery cart: push it forward and see it roll straight, then push it
backward and watch it swap ends. Don't knock over the mayonnaise
display:-)




       Yes you will. Seldom, but possible. A too-fast touchdown,
especially with flaps applied, will cause such a low deck angle that
the nosewheel alone will be on the ground; now we have a really bad
taildragger configuration with the CG a LONG way aft of the only wheel
on the ground. It's called wheelbarrowing and it's a result of very
poor flying skills. It has broken many perfectly good full-scale
airplanes and hurt a few people. Many pilots forget that angle of
attack is related to airspeed, and that AOA decreases with airspeed
and increases as airspeed falls, in a 1-G condition. The too-fast
touchdown can only be done if the AOA, and therefore the pitch angle
in this situation, is very low. Dropping flaps changes the angle of
incidence of the wing and makes it worse. The same scenario can result
in porpoising, another airplane-buster.
         Back to the toe-in/toe-out debate: The toe-out works on full-
scale airplanes, while many modelers find that toe-in works for them.
Many model landing gears are simple wire affairs or thin leaf springs
that have little torsional resistance, and the wheels' rolling drag
tends to torque the wheel's axle to a toe-out condition. In applying
toe-in, I think they are unconsciously correcting for gear leg twist;
toe-out for them would just give them further toeing-out when rolling
that would just make the airplane nasty.
          The full-scale gear is pretty rigid and the wheels are so
big that surface drag is much smaller, in proportion, than that
experienced on models. They don't twist much at all. The manufacturers
of taildraggers usually call for zero (straight-ahead) to maybe a
degree of toe-out. I have been flying real taildraggers for a long
time and have encountered some with toe-in; they're no fun at all. One
had one wheel straight and the other toed-in; if I didn't touch down
in a slight crab so that the airplane's rolling track bisected the
wheels' tracks, it would jump to one side away from the toed-in wheel,
and do it quite violently. I took the grass whenever I could.

         Dan

Doug McLaren wrote:

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..

Ed Forsythe wrote:

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!

"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.
--
Jim in NC

wamanning@email.com wrote:

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.

--

Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

Do you need to implement control loops in software?
"Applied Control Theory for Embedded Systems" gives you just what it says.
See details at http://www.wescottdesign.com/actfes/actfes.html

On Fri, 10 Aug 2007 17:51:20 -0000, wamanning@email.com wrote:

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.
--
RoRo

Methinks that statement is backward.

You want the main gear forward on a taildragger.

--
m-m
http://www.mhmyers.com/rc.tn.html <--Lots of RC photos

"M-M" > wrote

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.
--
Jim in NC

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?

--
m-m
http://www.mhmyers.com/rc.tn.html <--Lots of RC photos

"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
--
Jim in NC

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.
--
RoRo

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