Re: Prop Question (again). (beware on-topic post)
- From: frank <famvburg@xxxxxxxxxxx>
- Date: Sat, 19 Apr 2008 06:44:14 -0700 (PDT)
Actually, you're throwing in jets, or else WWII era USAAF photo
a/c, F-4 & F-6. Correctly, the Corsair is & always will be, except for
non-Vought versions, F4U, not F-4. The Hellcat is & always will be
F6F, not an F-6.
On Apr 19, 7:50 am, "John Carrier" <j...@xxxxxxxxxxx> wrote:
"Daryl Hunt" <dh...@xxxxxxxxxxxxxxxxx> wrote in message
news:4fb58$4808d5dd$10739@xxxxxxxxxxxxxxxxxxxx
"Don McIntyre" <DDon...@xxxxxxx> wrote in message
news:5b75e748-3666-4866-9db2-84cf027a7bc6@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
IIRC from our last discussion on 3- versus 4-bladed props, one of the
main resons was to absorb the power (torque) of the engine and make
the aircraft more manageable (for lack of a better term). If that's
the case, with similar power outputs (R-2800 variants), why is it that
the P-47 more ore less from the beginning used a 4-bladed prop, and
the F4U versions from the F4U-4, on used 4-bladed props, but the F6F
Hellcat stayed with 3-bladers for it's entire service? Might it have
had to do with the lighter wing loading on the F6F?
Good question.
The most efficient Prop at sea level is a one bladed prop. (try and
balance that sucker). At sea level, the 4 bladed has more drag than the 3
bladed so the 3 bladed outproduces it. Now, start climbing up. As you go
up, the 3 bladed starts to starve out since there is less air for it to
grab. At some point, the 4 bladed becomes more efficient since it can
grab more of the thin air.
While a single blade prop can be counterbalanced, it's thrust loading will
be assymetrical and raise hell with drive shaft, reduction gear, etc that
connect it to the engine.
The P-47 was designed for hight altitude to combat the German Fighters
going after the Bombers. Since it had so much excess power it also was
fair at lower altitudes. But at low altitude, the lowly P-40 was quite a
handful for any enemy fighter since it had plenty of power at sea level
and a 3 bladed prop.
While the P-40 enjoyed success in China, it was primarily through superior
tactics. While its Allison engine + 3-bladed prop were less disadvantaged
at low altitude, it was still inferior to the enemy fighters it encountered.
The F4-U was designed for support. That means low altitude. Power was
best used at low altitude as well. The 4 bladed would have overloaded the
engine so the 3 bladed was the best answer. As the power of the engine
goes up, the more blades you need to utilize the power and the higher
altitude it can operate at. Hence the F4U-4.
Untrue. The F-4 was designed as a high speed fighter (it predates the P-47).
The CAS mission came later.
Meanwhile, the Hellcat was designed primarily as a ground attack and used
against the Japanese who didn't usually operate at the extreme altitudes
that the Germans did. Hence the 3 bladed prop for efficiency.
Utterly false. Hellcat was designed as an air superiority fighter. So much
so that it incorporated "lessons learned" technology from the Wildcat vs
Zero experience in early WW2.
Both F-4 and F-6 met US Navy requirements for a mid-altitude fighter
mission. As the surface threat was reduced in the later war years, but the
kamikaze threat increased, air wings became more fighter heavy. When the
need for CAS arose, the fighters were equipped with bombs and rockets.
Case in point in a more modern setting. The C-130A (allison -7) versus
the C-130B with almost the same engine. The A used less runway for
takeoff with almost the same loaded weight than the B. But was slower at
altitude. The A carried a 3 bladed Prop while the B carried a 4 bladed.
The As were modified later to accept a 4 bladed prop but the originals
made from 1953 to 1956 were all originally 3 bladed.
Now, enter the C-130D. While it was essentially an A with skis, it was
built from the ground up to accept the additional plumbing it required.
The D model was NEVER a converted A. They could have chosen the new 4
bladed prop that was just coming out but they kept the 3 bladed. Since
sea level power for takeoff was the most important fact, the 3 bladed
utilized the engines power the best. Besides, when it's 40 below, you
engine power is off the scale and it's quite easy to overtorgue. The 4
bladed wasn't needed while the 3 was.
Your basic theory is sound. The most efficient prop would be a single blade
of very large disk diameter at relatively low rpm. Given the mechanical
issues with such a device, a two-blade prop would be the ideal comromise.
This runs into one aero and one mechanical brick wall. A very large disk
encounters tip speed limits (supersonic is bad) and becomes an installation
problem regarding ground clearance and blade structural integrity.
Typically disk diameter becomes the limiter first and given only a certain
amount of power can be transmitted through each blade, an increase in blade
count can increase power transfer (and eventually thrust) through the prop
disk.
A three blade is a better choice than a four blade only when it is capable
of absorbing all the available power. If it is not, even though four blades
exacts a drag penalty, it is the better choice.
R / John- Hide quoted text -
- Show quoted text -
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- References:
- Prop Question (again)… (beware on-topic post)
- From: Don McIntyre
- Re: Prop Question (again). (beware on-topic post)
- From: Daryl Hunt
- Re: Prop Question (again). (beware on-topic post)
- From: John Carrier
- Prop Question (again)… (beware on-topic post)
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