Jeff Spitzer wrote:

At 09:51 AM 2/3/99 -0800, you wrote:

If you are building or planning on building a ducted fan
airplane this is why you need plenty of excess power.
As you can see the airplane is not going to take off
and climb very well unless you do. If you have enough
power to go 300 or 400 MPH then you probably have enough
power to take off as long as your wing area and gross
weights are reasonably.

This chart is from Perkins & Hage Airplane Performance
Stability and  Control John Wiley & Sons.

Paul Lamar

Wow!  according to that our slow flying airplanes should only have 35%
propulsive efficiency with a prop at 70 KIAS.  It's a wonder we get those
suckers off the ground ;)  I think the point this chart is trying to make
is correct (e.g. ducted fans are mostly suited to high subsonic/transonic
speeds) but I question the data.

Jeff Spitzer

Yes.. but since constant HP is thrust times speed you have a lot of
thrust at low speed :-) 



Just pulling your leg Jeff. There is a lot more
to it than that of course. I am surprised the prop is so bad in this
chart 
as well. Most turbo prop engines  have variable pitch props so the eff.
curve 
should be more rectangular. Rise more steeply and be more or less flat
on top
for awhile. This looks like a chart for an over pitched fixed pitch
prop.


Paul Lamar
 
The Aircraft Rotary Engine Newsletter.              Powered by Linux.

http://home.earthlink.net/~rotaryeng/            http://www.linux.org