I got the CL and CD curves for NACA 2424, 2418,
2415, and 2409 added into the simulation
spreadsheet.
If you remember, it was originally all 2412. I
suspect
the drag numbers for the 2409 are high, and it is
from
a different source than the other data, non-NACA.
This
should make the spreadsheet underestimate thrust,
which
is good.
In any case, I decided to examine the effects of
varying
the airspeed, adjusting the prop overall blade
angle to
keep the hp constant like a CS governor would do.
Here
are the plots. Very interesting. It appears that
the 1.0
chord (developed from Theodorsen, and selecting a
design CL) is slightly less efficient at cruise
(250 HP @
2500 RPM, 8Kft MSL) than the 1.25X chord (all
chords
multiplied by 1.25). The differences at cruise
are pretty
slight, like 299 lbs to 302 lbs thrust - see
"crthrust.gif".
At sea level it acts a bit wierd, but the 1.25X is
still OK.
The real eye opener is what happens to the TO &
climb
thrust when the chord is increased. At constant
power, this
forces the blade angle of attack to decrease,
apparently changing
the whole blade angle so that the inner part
unstalls, GREATLY
improving the low speed thrust with only a very
small change
in the thrust at cruise, like 1%. The payoff is
that you gain
30-40% thrust at climb airspeeds by unstalling the
blade sooner.
See "chord8kt.gif" and "chordslt.gif" for the
thrust variation with
chord factor at 8K ft MSL and sea level.
The overall prop efficiency changes dramatically
by unstalling
the inner portion of the blades earlier due to the
lower angle of
attack necessary to keep the HP at 250 with a
wider blade
chord. See "chord8ke.gif" and "chordsle.gif".
Let me know what you think.
Bill
Looking good Bill. Keep up the good work.
There does indeed appear to be trend towards wider
blades in the
GA field.
Paul Lamar
