GREAT POST Sticky! Much better! Keep it up!

Grantland

(Peter Stickney) wrote:

In article ,
Greg Hennessy writes:
On 03 Jul 2004 04:42:25 GMT, nt (Krztalizer) wrote:


56. All Squadrons have been informed that the F6F-3 propellor is
interchangeable with the F4U-1 and that this paddle blade design will improve
the performance of the F4U-1.

Improved by how much I wonder ?

I _thought_ I'd posted to this thread yesterday, with a few fidbits on
propellers, but the record shows that it didn't make it, for some
reason.

Anyway - The amount of power that a propeller can absorb (and turn
into thrust) is goverened more htna anything else by the total blade
area, and its Activity Factor (Essentially its Solidity - the ratio of
the area of the ptopeller blades to the total area of the propeller
disk.) The higher the solidity, the more power you can absorb.

The Efficiency of the propeller, (The amount of Shaft Horsepower that
it turns into Thrust) is driven by the Advance Ratio (A product of the
propeller's rotational speed vs. the airplane's forward speed), and
the blade angle. With a Constant Speed propeller, the blade angle
factor is removed, within the limits of the propeller's pitch stops,
because the propeller governor will always select the most efficient
blade angle to absorb the shaft horsepower. The propeller's pitch
range is such that, for any power level normally encountered in
flight, the propeller blades are not on the stops, and we can safely
ignore it for the purposes of this post, and only consider Advance
Ratio. Efficiency drops off at both low and high Advance Ratios, so
you can't just shoot for the highest one you can reach. (And, in the
WW 2 fighter case, with geared propeller drives, you start to run into
efficiency losses at high speed as the propeller blades start going
transonic. Other airplanes can get the propeller tips transonic at
lower airspeeds. One of the big reasons for a T-6/SNJ's irritating
snarl is that, with its ungeared R1340, the prop tips are transonic.)

Also, there are purely mechanical considerations - the propeller had
to be able to stay together while its working. Propeller blades are
thin, but they aren't light, and the forces both along their length
(ccentrifugal), and across the propeller disk (Thrust) are high. All
the force is concentrated at the blade root at the hub, so not only
are there high forces, but they've got long lever arms to work with.

You can add blade area in 3 ways - you can increase the propeller
diameter, you can increase the area per prop blade, or you can
increase the number of prop blades. There are tradeoffs for all three
options. Increasing the diameter of the propeller works well
aerodynamically at most Advance Ratios, but the tip speeds are high.
since most of the thrust of a propeller is generated at the tips, it
also puts huge loads on the propeller blade & hub. (And it has to fit
on the airplane).
Increasing the blade area of each propeller blade ("Paddle Blades")
will increase the efficiency at low Advance Ratios, giving better
takeoff and climb performance, but at a cost in high Advance Ratio
performance, and with added problems with structural strength.
It also allows the diamter to be more carefully controlled, with some
benefits at high speed.
Increasing the number of propeller blades also allows the diameter to
be controlled, and doesn't requires as much structural strength per
blade as the other two options. It does decrease overall efficiency.
(A good rule of thumb is 2-3% per blade) This is due to interference
effects of a propeller blade tip running into the vortices of the
blade ahead of it.
So, in general, the order of merit of the three options is this:
1) Increase the propeller diameter, if practicable.
2) Increase teh propeller blade area without increasing the diameter
(Paddle blade)
3) Increase the number of propeller blades.

As to the Hellcat propeller being fitted to the Corsair - it was
certainly done - for example, the tests performed by the USN of a
P-51B vs 2 hotted up Corsairs used Hellcat propellers for both
aircraft. (The preort can be found online at:
http://www.geocities.com/slakergmb/id95.htm

As to how much it would affect performance - The Hellcat propeller
would mose likely by about 2% more efficient at the F4U's climb speed,
and about the same efficency at maximum speed. At climb speed, the 2%
greater efficency would give 40 more Thrust Horsepower at Sea Level
(2000 SHP), and about 34 more Thrust HP at high altitudes. (This is
because the Corsair's geared supercharger delivered less Shaft
Horepower at high altitudes, since more power was diverted to the
supercharger). At teh Corsair's best climb speed of 135 Kts at Sea
Level, that's 95# more excess thrust. it doesn't sound like much, but
since it's all excess, it gives about 108 ft/min more climb. At
22,000', the excess thrust would be about 58#, giving a improved rate
of climb of 95 ft/minute. It doesn't seem like much, but it makes a
difference.

--
Pete Stickney
A strong conviction that something must be done is the parent of many
bad measures. -- Daniel Webster