vjaqua wrote:

I may have to take back some of the bad things that I have been saying.
My prop performance prediction program does have a rasonably
believable  provision for stalled blades, and the results were a bit
suprising. The disk area of the larger steep prop appears to
overpower the stalled losses (at least at first cut). I have to run
a lot of cases and cross plot based on the estimated power curves.

I set up cases for a 250 hp engine geared for either 2750 or 2300rpm
at peak, for an airplane that would do 215 TAS at sea level at that power

The low rpm case gave me an 84 in dia with 103 in pitch and a 72 in
diameter with 105 in pitch (and wider blades of course). the 2750rpm
case suggested a 72 in dia with an 86 in pitch (With blade widths very
similar to the slow 84 in prop) take off rpm with the low rpm system
was roughly 1700 at the prop, and roughly 2100 rpm with the higher
speed gearing, and a bit over 190 hp delivered to the prop in both cases.
very little rpm change was predicted up to 60 mph TAS, and suprisingly
small change in thrust. A wide crayon could enclose all the results, as
shown below

speed          84 X103          72 X 105        72 x 86
 0                   615#              585#                595#
30                  610#              580#                585#
60                  590#               550#               550#

No  warrantee expressed nor implied :-)    - Vance

Well! Well! If this is true looks like we will still get off the ground in 500 feet with
our 2.85:1 psru optimized for 2100 prop RPM and 190 HP cruise.
OK Tracy the ball is in your court. Sorry about all this:-)

Thanks Vance.

Paul Lamar
 
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