ASH-25E + Rotax 275

On Sun, 18 Apr 2010 09:34:34 +0200, Willy VINKEN
wrote:

I've seen (lucky) pilots rely on the 'natural' stop position after
switching off.

Some brakes on the flywheel seem really efficient, some are slippery.
Anyone has bright ideas?

I have a DG600M, so a different system, built around the same engine.
The propeller must be installed in such a way that one of the blades
finds the max. piston compression a few degrees (10-20°) before the
ideal vertical position. It's impossible to have both blades set for
the same condition. You will have to paint or "mark" the reference
blade.

The propeller brake works quite well, provided the lining material and
the metal disc surface (flywheel) are both oil-free.
If necessary, renew or resurface the lining after accurate degreasing
of the flywheel's braking surface.

Aldo

On Mon, 19 Apr 2010 14:24:21 +0200, cernauta wrote:

On Sun, 18 Apr 2010 09:34:34 +0200, Willy VINKEN
wrote:

I've seen (lucky) pilots rely on the 'natural' stop position after
switching off.

Some brakes on the flywheel seem really efficient, some are slippery.
Anyone has bright ideas?

I have a DG600M, so a different system, built around the same engine.
The propeller must be installed in such a way that one of the blades
finds the max. piston compression a few degrees (10-20°) before the
ideal vertical position. It's impossible to have both blades set for
the same condition. You will have to paint or "mark" the reference
blade.

The propeller brake works quite well, provided the lining material and
the metal disc surface (flywheel) are both oil-free.
If necessary, renew or resurface the lining after accurate degreasing
of the flywheel's braking surface.

Aldo


Fine, Aldo.
I tried to work it out with the angles, but I don't have the precise
reductor ratio.
Say we have a 3:1 reduction: the full compression (TDC) will be
encountered only once every 3 revolutions, right?
I guess the TDC+~15° reference is chosen because it's easier to feel
than near BDC.
Which means the propeller would stop after another 180°, when the
piston is at BDC+~15°, and compression starts to build up.
It's difficult for me to rely on the assumption that the propeller
will always stop turning near (enough) this precise position.
Does yours?
Also, it should be hazardous to check it on the ground, because
there's no relative wind...
And if my above assumption is correct, I guess the brake's only task
is to prevent the low compression to turn back the propeller when it
is lowered out of wind pressure?

Willy

On Mon, 19 Apr 2010 17:07:25 +0200, Willy VINKEN
wrote:

Fine, Aldo.
I tried to work it out with the angles, but I don't have the precise
reductor ratio.
Say we have a 3:1 reduction:

that's correct, at least on the 600. I'm pretty sure it's the same for
the 25.

the full compression (TDC) will be
encountered only once every 3 revolutions, right?

3 rev of the engine, 1 rev of the propeller.

I guess the TDC+~15° reference is chosen because it's easier to feel
than near BDC.

correct. At or near TDC, you feel the compression as you move the
propeller by hand.

Which means the propeller would stop after another 180°, when the
piston is at BDC+~15°, and compression starts to build up.

I'm not following you in this.
The propeller will stop (or, slow down to almost a stop) at the
beginning of a compression cycle. That has a 1 out of 3 chance of
being the desired, near vertical position.

It's difficult for me to rely on the assumption that the propeller
will always stop turning near (enough) this precise position.
Does yours?

1 of the 3 positions. Then you just wait for it to pass along the
wrong compression zone(s), and as it approaches the one very close to
vertical, you must be ready to pull firmly on the propeller brake.
I guess you have a rear view mirror.

Also, it should be hazardous to check it on the ground, because
there's no relative wind...

On the ground, you just need to move the prop by hand, and make sure
one blade (in my case, the one related to the magnetic proximity
sensor) hits compression about 10-20 cm before vertical (measured at
the tip).
The proximity sensor gives "consensus" to the electric retraction
system, provided the main switch is in "normal operation mode". In
Manual mode, I can retract the engine in whichever prop position
(accepting high risk of damage to the prop and the engine-bay lids)

And if my above assumption is correct, I guess the brake's only task
is to prevent the low compression to turn back the propeller when it
is lowered out of wind pressure?

The brake prevents the propeller to move, though slowly, due to the
relative wind. Without the brake, at 90 km/h, the prop will win over
the compression in a few seconds.

Aldo




Willy