Quote:
Originally Posted by Crumpp
Hi Blackberry,
You have drawn some of the right conclusions but there is some work required still.
First of all, these are constant speed propellers. They change pitch as required. I am sure you got confused looking at that single F4U graph but it is a fact, you cannot compare CSP propellers at different advance ratios.
The advance ratio does not tell you a thing except in the context of that specific pitch angle. Now what you are doing is how that pitch stops are determined. A good propeller design will keep the polar at the flat area on the top as the pitch of the blade changes throughout the flight envelope.
This is what a complete CSP efficiency over advance ratio graph looks like:
The best aircraft/engine for this propeller will achieve Vmax at ~2.2 advance ratio and the propeller will have the stops at 15 degrees and 45 degrees.
That is the advantage of a CSP, you maintain peak efficiency over a wide range of velocities.
The F4U graph looks like it comparing airfoil selection at a specific velocity. |
Thanks for correcting me, you are right, the NACA16 vs Clark-Y diagram is at a specific velocity=640km/h TAS, and at a specific altitude= 6000m(19500ft)
advance ratio=J= V/(n*D)
V=177 m/s, D=4.1m(13.5ft), when J varies from 1.0 to 2.5, the propeller's rpm is from 2590 to 1036 respectively, and engine rpm is between 5180 and 2072rpm(reduction ratio=0.5:1). But 5158rpm is far more than engine's max. rpm, Thus the working range of 3-blade 4.1m diameter CSP is the "red curve", other part of curve is just the calculated result.
Larger prop. will always benifit from lower advance ratio when other things being equal.
12.JPG
3-blade CSP diagram
a.JPG
4-blade CSP diagram
b.JPG
It seems that 4-blade CSP with larger diameter prop, is FAR MORE efficient than 3-blade with smaller size, especially when advance ratio is very high(diving).
Assume 3-blade diagram is Fw190A8, 2700rpm engine , reduction 0.54:1, 1458rpm for 3.3m Propeller
Assume 4-blade diagram is P47D, 2700rpm engine , reduction 0.5:1 or 0.56:1, 1350rpm or 1518rpm for 4.0m Propeller
When both Fw190A8 and P47D dive to 6000m altitude @ 950km/h TAS=264m/s TAS=680km/h IAS=421m.p.h IAS. This speed is within Tempest MKV's permitted dive limit.
Propeller efficiency for P47D:82%-85% , advance ratio=3 or 2.6
Propeller efficiency for Fw190A8:0%. advance ratio=3.3
What a surprise! It's very wise for allied to drag fw190A to such a high speed, and make fw190a lost its power!
The most important is whether il2 4.11m models detailed prop. efficiency curve? Is there Mach number in il2's FM code? Crumpp Do you know?