[rebel-builders] Fw: Instability

[Garry Wright]

At one point I had a fairly serious instability on pavement because the
drag strut on one side was longer than on the other. Murphy changed
lengths on these in the early days and my gear ended up with one of the
new and one of the old. Until I figured that out it was a bit of a bear
to land. 7/8" difference made a big difference in handling. Others might
suggest you have some toe out or other differences in the gear.

Garry

Mike Davis wrote:

----- Original Message -----
From: "Rebel" <rebel@dcsol.com>
To: <mike.davis@dcsol.com>
Sent: Monday, January 18, 2010 6:15 AM
Subject: Instability

Hi
I own a Rebel series 615-R, equipped with 2.2 liter Subaru engine, with
serious stability problems while on ground.
-During taxing, tends to move to the sides.
-While on take-off run, at full engine, the instability diminishes and its
possible to take-off without problems.
-But, if during the take-off run the power is cut, the plane becomes
extremely unstable and it becomes impossible to keep it going staight on
the
runway. Generally it ends up off the runway looking to where we were
coming
from.
-To sump it up, if the plane is at speed but without traction it becomes
completely unstable while at ground.
-Ive got experience with planes of conventional layout, but its the firs
time
Ive met such an unstable one. The rear wheel is in perfect conditions and
has
been recently lubricated.

Is there a special aligning for the wheels, some specific tire pressure or
some
other element I should check or modify ??

Thanks
Albert

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[Darryl Conway]

Besides the gear legs, toe in-out, etc. already mentioned, is the tail wheel itself properly mounted, as in is the main bolt for the wheel forks vertical instead of being angled to the ground? The main attachment needs to be perpendicular to the ground for proper tracking.

Darryl




________________________________
From: Mike Davis <mike.davis@dcsol.com>
To: rebel-builders@dcsol.com
Sent: Mon, January 18, 2010 11:24:54 AM
Subject: [rebel-builders] Fw: Instability



----- Original Message -----
From: "Rebel" <rebel@dcsol.com>
To: <mike.davis@dcsol.com>
Sent: Monday, January 18, 2010 6:15 AM
Subject: Instability

Hi
I own a Rebel series 615-R, equipped with 2.2 liter Subaru engine, with
serious stability problems while on ground.
-During taxing, tends to move to the sides.
-While on take-off run, at full engine, the instability diminishes and its
possible to take-off without problems.
-But, if during the take-off run the power is cut, the plane becomes
extremely unstable and it becomes impossible to keep it going staight on
the
runway. Generally it ends up off the runway looking to where we were
coming
from.
-To sump it up, if the plane is at speed but without traction it becomes
completely unstable while at ground.
-Ive got experience with planes of conventional layout, but its the firs
time
Ive met such an unstable one. The rear wheel is in perfect conditions and
has
been recently lubricated.

Is there a special aligning for the wheels, some specific tire pressure or
some
other element I should check or modify ??

Thanks
Albert

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[Ken]

Hello Albert

Well you have taildragger time so lets rule out pilot self induced
excitement. As they say "if the rudder is not moving -it is about to get
exciting".

So the problem is only when the tail is on the ground? Or both?
Conventional gear?, bungees?, die springs? or cessna type spring gear?

I seem to have better control if my rudder to tailwheel springs are not
loose but not everyone agrees. I have MAM's screen door compression
springs not the Scott tension springs. Cutting power with the tail in
the air on takeoff is very challenging and exactly why so many high
speed taxi tests end badly. I don't recommend doing it intentionally. Do
you have the recommended 25* of rudder throw in each direction? My rebel
is not at all squirrelly like yours seems to be. It is very rare that I
need a touch of brake to keep it straight.

If tailwheel shimmy is an issue I also find a vertical pivot helps but
you didn't say anything about shimmy.

Excessive toe in or toe out can do the crazy things you refer to by
extending and compressing the suspension. A bit like a pogo stick.
Raising the tail also changes the toe if the camber is not zero (axle
not horizontal). Changing the length of the drag strut if you have one
certainly also changes the toe. So suggest you measure toe with tail on
ground and also with tail raised.

Tire pressure doesn't make much difference for me other than when I
don't have any. I'm up to 5 flats now with the crappy Carlisle bent stem
inner tubes... Usually it is one of the little manufacturing
irregularities that starts leaking for no obvious reason other than poor
quality.

Ken

Mike Davis wrote:

----- Original Message -----
From: "Rebel" <rebel@dcsol.com>
To: <mike.davis@dcsol.com>
Sent: Monday, January 18, 2010 6:15 AM
Subject: Instability

Hi
I own a Rebel series 615-R, equipped with 2.2 liter Subaru engine, with
serious stability problems while on ground.
-During taxing, tends to move to the sides.
-While on take-off run, at full engine, the instability diminishes and its
possible to take-off without problems.
-But, if during the take-off run the power is cut, the plane becomes
extremely unstable and it becomes impossible to keep it going staight on
the
runway. Generally it ends up off the runway looking to where we were
coming
from.
-To sump it up, if the plane is at speed but without traction it becomes
completely unstable while at ground.
-Ive got experience with planes of conventional layout, but its the firs
time
Ive met such an unstable one. The rear wheel is in perfect conditions and
has
been recently lubricated.

Is there a special aligning for the wheels, some specific tire pressure or
some
other element I should check or modify ??

Thanks
Albert

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