static thrust

[Aerosan]

any takers how much thrust one will get with the plane tied down? I intend useing a daw scale as a test between two props. now the next question; if you set the pitch up for max thrust on the ground what will happen when you start rolling?!?! just some food for thought.

[Arnulf]

Hi,
Demon wrote:

so be careful not to make the pitch too fine

That is what probably will happen with the "draw scale method"

Unfortunately a fixed pitch prop will only give you optimal power at a certain speed/rpm. Say you had a Rotax 582 Motor, with a max of 6800 rpm, and you fitted a prop with such a fine pitch, that it would rev 6800 rpm at full throttle, with the aerie standing still. You would now get the maximum "pull" on the drawscale. However this engine/prop combination would be quite useless, because the moment you start to move, your engine will rev higher than 6800 rpm, and you would have to throttle back during take-off, probably not so clever. The prop you eventually select will be a compromise between a cruise prop with a relatively coarse pitch, or a finer climb pitch. If you need the power for takeoff, possibly consider a prop that gives you close to max engine rpm at takeoff speed. If you need a prop for good cruise performance, I would look at the torque curve of the motor, and then try find a prop that sits pretty much in the top of the curve for cruise. However at the end, it will be a compromise. To find this compromise, you probably would have to do some experimenting with both props.

Regards,
Arnulf

[Tjoek]

I find with 582 that the difference between static and flying max is about 300rpm. I normally set my prop for static rpm of 300 below the point of max torque.

[Cloud Warrior]

Jeees guys - this is way too scientific for me - I will stick with letting Piet De Necker make all those kind of decisions. I will go as far as torquing the bolts properly and getting the tracking right!

[Aerosan]

Thanx Guys
Im aware that the draw scale method is flawed so it is purely a means of measure and the test will be done extensively and there will be run`s etc. We were just interested to see what we will get. Also to see where on the rpm scale, useing Piet`s prop as a measure, we will get the same pull (newtons for duck ) as I said purely for academic reasons.

san

[Arnulf]

Would you please be so kind and let us have the results.

And please be careful when fiddling around an aerrie thats got the motor running at high power

[Aerosan]

yp Im sure if Im not carefull Ill get it chop chop!! No problem as soon as we do it I will make al public. It just depends on Duck an when he has time.

San