F4U inverted gull wings

In article ,
George Ruch wrote:

hobo wrote:

In article ,
"Geoffrey Sinclair" wrote:

Corsair propeller diameter 13 feet 4 inches, ground clearance
9.1 inches, engine R-2800-8.

The corsair used a 3-blade prop. Why didn't they use a smaller 4-blade
prop if ground clearance was such an issue?

Check again, please: http://www.warbirdalley.com/f4u.htm

The link you provided has no textual information regarding the prop, but
there is a picture, dated 2001, of a surviving Corsair with a 4 blade
prop. This prop may not be the original factory issue.

When this question was first posted the first website on the Corsair I
found was this:
http://www.nasm.si.edu/research/aero...t/voughtf4.htm

This is the Smithsonian's website and has a photo of a Corsair with a 3
bladed prop and this text: "The R-2800 radial air-cooled engine
developed 1,850 horsepower and it turned a three-blade Hamilton Standard
Hydromatic propeller with solid aluminum blades spanning 13 feet 1 inch."

This website was my sole source for the claim that the Corsair had a 3
blade prop. Perhaps a 4 blade was later added, but it seems odd that a 3
blade was ever used if ground clearance was so pivotal to the whole
design.
--

Peter Stickney wrote:


It's all a balancing act - but in ggeneral, you're best off going with
the largest diameter propeller with the fewest number of blades that
you can practically manage.


i) I'm sure I remember seeing, years ago, a picture of a Noorduyn
Norseman with a single-bladed prop. Since you seem to know what you are
talking about (more than I do, anyway), what factors would drive a
manufacturer to adopt such a radical solution?

ii) Radical solutions such as the Unducted Fan proposals mooted a few
years ago, had many curved blades - any idea what gain they were seeking
that justified the loss in efficiency?

iii) How does this work with contraprops? On the face of it they must
interfere with each other horribly, but they seem to fly quite well.
Can you point me in the direction of some clues?

hobo wrote:

Why were 2 and 3 blade props used anyway? Is there some engineering
tradeoff favoring fewer blades in certain situations?

A two or 3 bladed prop was easier to balance than a four bladed one
was. They could also be built faster than the more bladed props.

John Lansford
--
The unofficial I-26 Construction Webpage:
http://users.vnet.net/lansford/a10/

hobo wrote:

Why were 2 and 3 blade props used anyway? Is there some engineering
tradeoff favoring fewer blades in certain situations?

A two or 3 bladed prop was easier to balance than a four bladed one
was. They could also be built faster than the more bladed props.

John Lansford
--
The unofficial I-26 Construction Webpage:
http://users.vnet.net/lansford/a10/

In article ,
Alan Dicey writes:
Peter Stickney wrote:


It's all a balancing act - but in ggeneral, you're best off going with
the largest diameter propeller with the fewest number of blades that
you can practically manage.


i) I'm sure I remember seeing, years ago, a picture of a Noorduyn
Norseman with a single-bladed prop. Since you seem to know what you are
talking about (more than I do, anyway), what factors would drive a
manufacturer to adopt such a radical solution?

In a word, efficiency. Note that many of the model airplanes used in
free-flight competitions, (Escpecially the rubber powered ones, where
the judges issue you your engine (So many strands of Pirelli rubber,
of some particular length) and "fuel" it up for you (So many turns of
the rubber bands)) where getting the absolute most out of the limited
omount of energy you've got means the difference between winning and
losing, use very wide chord single-bladed propellers. The downside is
that you need a fairly large diameter. That's not much of a problem
in a hand-launched model airplane, but it doesn't work so well in Full
Scale stuff.


ii) Radical solutions such as the Unducted Fan proposals mooted a few
years ago, had many curved blades - any idea what gain they were seeking
that justified the loss in efficiency?

In tha case, what they're trying to do is reduce the effects of the
shockwaves that form on the propeller blades as they fly further and
firther into the transonic region. It's not unlike sweeping a wing
back to delay the Mach Number that the drag rise occurs at, and the
magnitude of the drag rise. Above about Mach 0.65, the efficiency of
a straight propeller drops off alarmingly. At typical airliner cruise
speeds, (Mach 0.78-0.85) efficiency would be down around 60% at teh
low end of the speed range, and 50% at the high end. That's not very
useful at all - there are some measures that you can do to cut the tip
speed down - for example, the Tu-95 Bear (Russian turboprop transonic
bomber) uses a very high step-down gearing from the engines to the
propellers - the props rotate at 750 RPM, vs, say, 1500 or so for that
of a P-51, and a very clever variable pressure ratio compressor system
in its engines that essentially "supercharges" them to deliver sea
level power at 40,000'. (About 3 times what you'd get from a typical
turboprop). The swept propeller blades supply efficiencies in the
Mach 0.78-0.85 range of between 75% and 70%. Using many blades allows
the diameter to be cut down from, say, 22 ft for our notional
conventional propeller to 13 ft. This gives a lower airspeed at the
propeller tip than a large diameter propeller, thus delaying the
transonic effects.
(Note that the entire propeller doesn't go transonic - the airspeed at
the propeller blade is a product of the propeller's rotational speed,
and teh forward speed of the airplane. The rotational speed of the
propeller in ft/sec or m/sec increases as you move outward along the
propeller blade. So, a propeller will start having supersonic flow
appear at the tips, with the supersonic flow field moving inward as
speed increases. A smaller diameter and a slower rotational speed are
helpful in delaying the formation of these shock waves.
(transonic/supersonic flow). You do lose efficiency in the lower
speed ranges, but you get big gains at what your desired cruise speeds
are.


iii) How does this work with contraprops? On the face of it they must
interfere with each other horribly, but they seem to fly quite well.
Can you point me in the direction of some clues?

A contraprop does lose some efficiency by placing one propeller behind
the other, and it requires a more complex drive system. (Which gave
fits on several early U.S. contraprop-equipped aircraft, most notably
the XB-35 Flying Wing, where they never got the contraprops doped out,
and the Hughes XF-11 recon machine (looked like a hyperthyroid
P-38). which was lost on its first flight becasue the aft bladeset in
one of the contraprops went into reverse pitch at low altitude. (This
is the crash that nearly killed Howard Hughes, and led to his drug
addiction (painkillers) and fear of infection.)

What you gain is a greater ability for a propeller of a particular
diameter to absorb power, adn the elimination of torque and P-factor
(destabilization of the airframe due to the rotating airflow from the
propeller affecting the airframe). P-Factor is a Big Deal, with a
high-powered airplane. For example, with a P-51 or a Corsair, you
have to be careful with throttle movement at low speeds, or on
takeoff. If you jam the throttle to it too fast, you'll either swing
off the runway or roll the airplane inverted.

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

Peter Stickney wrote:
In article ,
Alan Dicey writes:

Peter Stickney wrote:


It's all a balancing act - but in ggeneral, you're best off going with
the largest diameter propeller with the fewest number of blades that
you can practically manage.


a Noorduyn Norseman with a single-bladed prop: what factors would drive a
manufacturer to adopt such a radical solution?

In a word, efficiency.

Hmm. Efficiency in the sense of translating engine power to thrust? I
can't see it being aimed at top speed, so I guess it would give more
range for a given fuel load?


ii) Radical solutions such as the Unducted Fan proposals mooted a few
years ago, had many curved blades - any idea what gain they were seeking
that justified the loss in efficiency?


In tha case, what they're trying to do is reduce the effects of the
shockwaves that form on the propeller blades as they fly further and
firther into the transonic region. It's not unlike sweeping a wing
back to delay the Mach Number that the drag rise occurs at, and the
magnitude of the drag rise. [...] You do lose efficiency in the lower
speed ranges, but you get big gains at what your desired cruise speeds
are.

Of course - tip speed and transonic drag rise. To get more airscrew in
the airflow /and/ keep the tip speed suitably subsonic,the only answer
is more blades - with sweepback to delay the drag rise. I should have
remebered that from the discussions at the time. None of the Unducted
Fan experiments seem to have made it into a production implementation. I
guess the aim was a cheaper powerplant - propellors being cheaper than
ducted fans - but the loss of efficiency was too great.


iii) How does this work with contraprops? On the face of it they must
interfere with each other horribly, but they seem to fly quite well.


What you gain is a greater ability for a propeller of a particular
diameter to absorb power, adn the elimination of torque and P-factor
(destabilization of the airframe due to the rotating airflow from the
propeller affecting the airframe).


So, for an increase in power turned into thrust there's an improvement
in flyability and the ability to make the airframe lighter because it
doesn't have to absorb the stresses - they're balanced out at the
source. That explains to me how the Fairey Gannet was able to shut off
one half of the Double Mamba powerplant, feather one half of the
contraprop and achieve better endurance at patrol speed.

Thanks very much for taking the time to give me some pointers.

Do you do this for a living? :-)

In article ,
hobo writes:
The link you provided has no textual information regarding the prop, but
there is a picture, dated 2001, of a surviving Corsair with a 4 blade
prop. This prop may not be the original factory issue.

When this question was first posted the first website on the Corsair I
found was this:
http://www.nasm.si.edu/research/aero...t/voughtf4.htm

This is the Smithsonian's website and has a photo of a Corsair with a 3
bladed prop and this text: "The R-2800 radial air-cooled engine
developed 1,850 horsepower and it turned a three-blade Hamilton Standard
Hydromatic propeller with solid aluminum blades spanning 13 feet 1 inch."

This website was my sole source for the claim that the Corsair had a 3
blade prop. Perhaps a 4 blade was later added, but it seems odd that a 3
blade was ever used if ground clearance was so pivotal to the whole
design.

Hobo,
All the F4U-1 models had 3-blade propellers, with a 13'1" diameter.
The later production models, the F4U-4 and F4U-5, with higher-powered
engines, had 4 blade props with a 13'2" diameter, to absorb the extra
power. (More than 800 HP in some versions.)

So, its fair to say that they didn't go to a 4-blade prop to decrease
ground clearance.

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

I'm not sure if it is the answer - but fitting gull wings (whether
inverted or not) means that the wing root joins the fuselage at approx
90 deg - therebye eliminating the need for a large, drag-producing
wing-to-fuselage fillet.

Uhhh no. The fillets were there to DECREASE drag.

You only need fillets on high or low-winged a/c

Fillets are used to smooth out airflow and thus decrease drag. The air
over a wing is moving at a higher velocity than the air over the
fuselage, and when the streams mix you get turbulence and drag. The
fillets work to counteract this interaction and the drag it causes.

In article ,
John Lansford wrote:


A two or 3 bladed prop was easier to balance than a four bladed one
was. They could also be built faster than the more bladed props.



Considering the few inches of ground clearance any of those props
provided, does anyone have a good idea of how often the propeller was
damaged by (bad) landings?

In article ,
Alan Dicey writes:
Peter Stickney wrote:
In article ,
Alan Dicey writes:

Peter Stickney wrote:

a Noorduyn Norseman with a single-bladed prop: what factors would drive a
manufacturer to adopt such a radical solution?

In a word, efficiency.

Hmm. Efficiency in the sense of translating engine power to thrust? I
can't see it being aimed at top speed, so I guess it would give more
range for a given fuel load?

Efficiency in the sense of power to thrust. You're right - it won't
work so well at higher speeds.

ii) Radical solutions such as the Unducted Fan proposals mooted a few
years ago, had many curved blades - any idea what gain they were seeking
that justified the loss in efficiency?


In tha case, what they're trying to do is reduce the effects of the
shockwaves that form on the propeller blades as they fly further and
firther into the transonic region. It's not unlike sweeping a wing
back to delay the Mach Number that the drag rise occurs at, and the
magnitude of the drag rise. [...] You do lose efficiency in the lower
speed ranges, but you get big gains at what your desired cruise speeds
are.

Of course - tip speed and transonic drag rise. To get more airscrew in
the airflow /and/ keep the tip speed suitably subsonic,the only answer
is more blades - with sweepback to delay the drag rise. I should have
remebered that from the discussions at the time. None of the Unducted
Fan experiments seem to have made it into a production implementation. I
guess the aim was a cheaper powerplant - propellors being cheaper than
ducted fans - but the loss of efficiency was too great.

They're also complicated and heavy. UDFs, like propellers on
high-powered aircraft, have to be variable pitch to operate halfway
decently across their speed & altitude range. The pitch change
mechanism and, for that matter, the structure of the blade itself
aren't simple problems. For teh lower end of teh airliner cruise
range they may be somewhat more efficient - but they'll also have a
shorter Time Between Overhauls, and overhaul costs aren't going to be
cheap. Fuel prices would have to go up a _lot_ more than they have to
make it worth the extra overall cost.


iii) How does this work with contraprops? On the face of it they must
interfere with each other horribly, but they seem to fly quite well.


What you gain is a greater ability for a propeller of a particular
diameter to absorb power, adn the elimination of torque and P-factor
(destabilization of the airframe due to the rotating airflow from the
propeller affecting the airframe).


So, for an increase in power turned into thrust there's an improvement
in flyability and the ability to make the airframe lighter because it
doesn't have to absorb the stresses - they're balanced out at the
source. That explains to me how the Fairey Gannet was able to shut off
one half of the Double Mamba powerplant, feather one half of the
contraprop and achieve better endurance at patrol speed.

Right. Another example would be the Griffon engined Seafires. With a
single rotation prop, the Griffon Seafires had 5-bladed single
rotation propellers, and were limited to roughly 66% power on takeoff.
This was becasue of 2 reasons - the Torque/P-Factor would drag the
airplane right into the carrier's island. (A bad idea), and trying to
hold it straight was overstressing the tire sidewalls, forcing tire
changes after only a couple of flights. It's tough when you've got to
explain that you need to pull your ship out of the battle becasue you
ran out of tires, rather than gas, bullets, or bombs. The contraprop
used on the later Seafire 47s (6 blades, 3 per bank) allowed more
power to be used without the swing, and better propeller clearance.

The same basic engine allowed the development of teh Avro Lincoln into
teh Shackleton - you could hang Griffons with contraprops in the same
wing center section without changing the location of the engine
mounts. That's basically a Lancaster wing, so they got a lot of
stretch out of it.

Thanks very much for taking the time to give me some pointers.

Do you do this for a living? :-)

Sometimes. I'm an Engineering Mathemetician/Computer Scientist type,
and a Certified Wing Nut and Gearhead.

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