Bill,
If you ever get time I would love to have the specs for a fixed pitch prop optimised for a 70" length, 225 HP and 200 to 225 MPH.  I have prop carving on my LONG list of things to learn.

Ken Powell

Ken Powell
kenpowell@aristotle.net

Perry is building something close to that Ken for test with the LAEV
hub as we speak.

Paul Lamar

You did not specify shaft rpm for that power level, so I assumed prop rpm of 2700.
Inputting a design top speed of 210 mph , with your 70 inch diameter, I got the following
recommended size. The program assumes a flat bottm clark Y type foil, with pitch
measured to this surface. The lift coeff for this foil at that zero angle is roughly 0.4
and drag for typical prop aspect ratio about  0.02. The program picks a true helical pitch
for that angle of attack - relative to disk flow thru velocity, and comes up with a tad over
87 inch pitch. The disk delta V at that point is only about 19 ft per second. tip airflow
speed is about 887 fps (faster tan a 45 slug, and about 10 percent above recommended
for a wood prop.predicted  thrust is about 340 lbsf for a bit over 83 percent eff.
Proposed dimensions would be
      radius            blade width       blade angle        eff air velocity
       0.9 R                   4.9                  23.7                   811
       0.7R                    6.0                  29.5                   663
       0.5R                    7.7                  38.4                   526
       0.3R                     8.0*               52.9                   410
* the 0.3 radius zone  is usually highly blocked, and even theoretical  contribution
to thrust is less than 10 percent. The only prop I have seen that carries the width
for constant HP per square foot  this far in is the prop for the Carter Copter.

This is a theoretical undisturbed inlet prop - pushers need more pitch
to compensate for the velocity accelleration around the fuselage.
Berkuts routinely use over 90 inches of pitch. Also for this to be valid
- you really need to have a plane clean enough to go 210 with 225 HP
Vance

Vance,
Thanks for taking the time to run this thru your program.  I plan to turn my 13b up to 7400 RPM which equals 2,597 prop RPM.  I have the prop program spreadsheet by John Ronze (sp?? - the guy of Longez canard and RV-9 wing fame) that I have used to run the numbers but that data is at work and I do not have the data (or the program) before me or I would compare the data points.  I think that his program also used the Clark Y airfoil.  I was also interested in what airfoil would really be best suited to this flight envelope - is the Ckark Y really still the best?

Here is Vance's program I think Ken. You will need QB or GW Basic to run it.

Paul Lamar

I will freely admit to being a Neanderthal in many ways - still using GWBASIC for one thing.
The Clark Y foil is still frequently used for prop blade sections, and while it is probably
not the absolute best, it is still better in this case than it is given credit for, and it is easy
to use and measure. Props run at low Reynolds numbers (narrow chord) so the drag
bucket characteristics of the more modern foils are generally not obtained. The Dowty Rotol
people have used under cambered foils in some cases, but I think that may not be the
best thing at the off design operation of fixed pitch props. Near the tips the foils should be very
thin, and probably with relatively sharp leading edges and max thickness further back, and
defiantly not undercamberd in my opinion.. Vance

 
The AirCraft Rotary Engine NewsLetter.              Powered by Linux.
ACRE NL web site.
http://home.earthlink.net/~rotaryeng/            http://www.linux.org