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
***** BIG SNIP *****
Paul,
I started off thinking that the "stupid ole sections" like the ClarkY
just couldn't be as good as a really great new modern low drag foil.
NACA even created a whole series of foils esp for props, and to
work well in the high subsonic range. Only problem is they built
a pair of "identical" constant speed aluminum props - one with the
16 series special prop airfoils and one with the Clark Y. They only
tested them in the wind tunnel and at low speeds, but there wasn't
a dime's worth of difference.
As I worked on the prop design and looked and literally hundreds
of runs, with the 16 series and other foils, it became clear that there
is so much "off design point" operation of any prop that a high
CL max and a moderate drag at stall and post stall are extremely
important. The 24XX series is similar to the Clark Y, but there is
a LOT more solid wind tunnel data for a range of section percent
thicknesses. This is why I went with the 24XX foils. The NACA
16 series should be slightly better at the tips at very high speeds, but
I couldn't find much benefit and the 16 series has lower L/D max
which is important.
Short answer: Might be able to beat the Clark Y, but not easily and
not by much.
Bill
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