psru

[896tr]

OK, I really don't know why an engine needs a psru. I'm going to run a 140 hp turbo Subaru and full horse power is at 4800 rpm. As long as I don't jab the throttle into the back of the engine (like one of my early instructors thought was the way to get an airplane into the air) but ease it in so as not to stress the crank do I really need one? I am going to use a 72 inch Sensenich prop. Ideas and recommendations wanted and appreciated.
Thanks
Steve

[t j]

Thats too many RPMs for a kitfox friendly prop. You will have ro run a very short prop on it. No more than 48 inches to keep the tip speed under .92 mach. See the prop tip speed calulator.
http://www.pponk.com/HTML%20PAGES/propcalc.html

[HighWing]

Steve,
From all the forum talk during the NSI Subaru support era, the answer to your question is because of prop tip speed. With a 72" prop, you will never get to use your 140 HP because as you approach the RPMs necessary, your prop tips will go supersonic. There is a prop tip calculator at http://www.warpdriveprops.com/propspd2.html
I put your numbers in and at Springtime air temps (72°) your tip speed will exceed 1000 mph which is 1.33 mach - not good, actually really bad. According to the calculator you should be looking at a 48" prop, if you will ever want to use your max HP. This is one of the reasons the NSI engine was not a good choice with the early Kitfoxes as the round cowl blanketed much of the shorter propellor's diameter. In the Kitfox they worked, but not impressively, but they seemed to work pretty well in airplanes like the Sonex as the cowl at the prop was much sleeker. This is also a factor with the Volkswagen in direct drive as it also will need a short prop to be able to use its HP.
Lowell

[Dorsal]

My comments from another thread.

http://www.teamkitfox.com/Forums/showpost.php?p=27092&postcount=15

[896tr]

Thanks to all of you, now I know.