Re: Rotax 912 prop indexing and dynamic balancing

[Induna]

Hi all,

In my quest to get a super smooth engine I was wondering if indexing a 2 blade prop on a Rotax 912 makes a difference, and if so, what should the prop position be in relation to cyl1 TDC to get the best results?

Also, how effective is dynamic balancing of the prop?

I am sure there has been a lot of experimenting with this.

Regards

Jacques

[Jean Crous]

Induna, on a direct drive engine it is relatively easy, however on a geared engine it will prove a bit more of a headache . Try moving the prop one bolt hole at a time, go fly and record your figures and repeat till you find the sweet-spot.
Just my 2c worth
Jean.

[Boet]

Do not waste your time. No "sweet spot" with the 2.43 gear ratio, ever. None needed. Go figure.

[Induna]

Haha, ekt so gedink! Ek dink al wat nou nog kort is daai mooi spinner en n dynamic balance... Sal iemand naby die kaap moet kry om dit te doen... Moet nog net uitvind wie dit kan doen teen n bilikke prys.

Ek is baie beindruk hoe glad die prop van oom piet is

J

[Goose]

. Moet nog net uitvind wie dit kan doen teen n bilikke prys.

J

Bring hom nylstroom tailwheel flyinn toe en ek doen dit vir jou verniet

[Induna]

Dis baie gaaf!!! Maar nylstroom is dan nog lank in die toekoms...!

[Boet]

Boetman, die spinner is met sorg gemaak. Jy kan hom maar net aansit, en gaan vlieg lekker.

[Dobbs]

This is the first time I have heard of the position of a propellor blade relative to the piston position being of any relevance - or am I reading the question of Induna incorrectly?

Surely whether it is a direct drive or geared engine, the position of the blade relative to any piston position is totally immaterial and arbitary?

[justin.schoeman]

This is the first time I have heard of the position of a propellor blade relative to the piston position being of any relevance - or am I reading the question of Induna incorrectly?

Surely whether it is a direct drive or geared engine, the position of the blade relative to any piston position is totally immaterial and arbitary?

The crank doesn't rotate at a constant rate. When a piston is on a power stroke, the crank is accelerating, and the prop is moving slightly faster. You generally want a prop blade to be near horizontal when it is turning fastest, as this is where most of the blade is exposed, and least interference with the slip stream.

The difference is slight, but it is there.

Once you have a geared prop - especially with an uneven ratio, the prop is in a different position for each power stroke, and then you are looking at averages - much more difficult to determine.

[Dobbs]

Thanks Justin - still doing research on the www to try and get a feel for the changes in rotational velocity of a typical 4 cyl engine.

I would think that with a composite prop, with their inherent flexibility, the tip speed of the prop does not vary much?