ASH-25E + Rotax 275

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