V-22 Prop Configuration, 3-vs-4 blades

"Reed Judd-Dyer" wrote in message
oups.com...

Kyle Boatright wrote:
"Don McIntyre" wrote in message
oups.com...
Why did they settle on the 3-blade configuration? Wouldn't a 4-bladed
system allow for a smaller footprint on the ground (or ship)? Or is
there too much thrust loss involved?

More blades = less efficiency.

Shorter blades = less efficiency in vertical flight.

They probably couldn't have absorbed the performance loss from both
increasing the number of blades and reducing blade length.

KB
How does more blades equal less efficiency? If that is true why do all
current Helicopters use 4-5 blades instead of two like the legacy
systems of the Huey and Cobra? Why do all modern turbo-props have more
then two blades? Somthing sounds off here. Guesses are like.... Anyone
actually involved in the engineering of the V-22 or at least privy to
some of the original evaluation documentation?
Reed

More blades equals less efficiency for several reasons. One is that the
more blades you have, the dirtier the air becomes. Rotor and prop blades
like undisturbed air best. Also, you balance the number of blades and their
diameter against the HP from the engine. More blades = less blade length =
lower aspect ratio = lower efficiency.

Early helicopters needed the most efficient blade/power system possible just
to get off the ground. That resulted in two bladed rotors with long blade
length. As engines came along with significantly more HP, designers could
trade-off aerodynamic efficiency for packaging efficiency and create a
helicopter with adequate performance and reasonable size for the delivered
performance.

Aircraft with turboprop powertrains have multiple blades because those
designers have to make compromises too. There are clearance issues to deal
with (ground clearance, fuselage clearance, prop to prop clearance). There
are tip speed issues (you don't really want to take prop tips over .9 mach
if you can help it). And there are dynamic issues where a fast turning big
prop generates tremendous forces in shear, bending, and precession.. All of
those things drive designers towards a smaller diameter prop. So, the
designer compromises those packaging needs versus aerodynamic efficiency,
and voila... Multi-bladed prop's.

If more blades was a better solution, you'd see Cessna 150's with 20 blade
prop's, because those little airplanes need all the help they can get...

KB

Kyle Boatright wrote:
"Reed Judd-Dyer" wrote in message
oups.com...

Kyle Boatright wrote:
"Don McIntyre" wrote in message
oups.com...
Why did they settle on the 3-blade configuration? Wouldn't a 4-bladed
system allow for a smaller footprint on the ground (or ship)? Or is
there too much thrust loss involved?

More blades = less efficiency.

Shorter blades = less efficiency in vertical flight.

They probably couldn't have absorbed the performance loss from both
increasing the number of blades and reducing blade length.

KB

How does more blades equal less efficiency? If that is true why do all
current Helicopters use 4-5 blades instead of two like the legacy
systems of the Huey and Cobra? Why do all modern turbo-props have more
then two blades? Somthing sounds off here. Guesses are like.... Anyone
actually involved in the engineering of the V-22 or at least privy to
some of the original evaluation documentation?
Reed

More blades equals less efficiency for several reasons. One is that the
more blades you have, the dirtier the air becomes. Rotor and prop blades
like undisturbed air best. Also, you balance the number of blades and their
diameter against the HP from the engine. More blades = less blade length =
lower aspect ratio = lower efficiency.

Early helicopters needed the most efficient blade/power system possible just
to get off the ground. That resulted in two bladed rotors with long blade
length. As engines came along with significantly more HP, designers could
trade-off aerodynamic efficiency for packaging efficiency and create a
helicopter with adequate performance and reasonable size for the delivered
performance.

Aircraft with turboprop powertrains have multiple blades because those
designers have to make compromises too. There are clearance issues to deal
with (ground clearance, fuselage clearance, prop to prop clearance). There
are tip speed issues (you don't really want to take prop tips over .9 mach
if you can help it). And there are dynamic issues where a fast turning big
prop generates tremendous forces in shear, bending, and precession.. All of
those things drive designers towards a smaller diameter prop. So, the
designer compromises those packaging needs versus aerodynamic efficiency,
and voila... Multi-bladed prop's.

If more blades was a better solution, you'd see Cessna 150's with 20 blade
prop's, because those little airplanes need all the help they can get...

KB

OK, That jives with research I was doing on ducted fan systems. The
advantage of the duct being that it reduced the "disturbed" air effect,
making them more effecient untill the drag created by the duct zeroed
out the gains. Also helps explain why a two engine tilt-rotor can
verticly lift more then a four engine tilt-prop. Thanks for the clear
answer.
Reed

Reed Judd-Dyer wrote

How does more blades equal less efficiency?

Those of us that flew rubber band powered contest model
aircraft way back ('40s-'50s) KNOW that the most efficient
propellor has only ONE blade...and a counterweight on the
other side. :-)

Bob Moore

Bob Moore wrote:

Reed Judd-Dyer wrote

How does more blades equal less efficiency?

Those of us that flew rubber band powered contest model
aircraft way back ('40s-'50s) KNOW that the most efficient
propellor has only ONE blade...and a counterweight on the
other side. :-)

Bob Moore

A Helo with one blade would have to be the most uncomfortable ride,
until after a minute or so, when it had shook itself to pieces.

Jeremy Thomson

wrote in news:1143609590.585290.233720
@u72g2000cwu.googlegroups.com:


Bob Moore wrote:

Reed Judd-Dyer wrote

How does more blades equal less efficiency?

Those of us that flew rubber band powered contest model
aircraft way back ('40s-'50s) KNOW that the most efficient
propellor has only ONE blade...and a counterweight on the
other side. :-)

Bob Moore

A Helo with one blade would have to be the most uncomfortable ride,
until after a minute or so, when it had shook itself to pieces.

Jeremy Thomson

He did say with "a counterweight on the other side", so balance is not an
issue. This works better on larger, slower turning props, though. I'd hate
to try it on a fast spinning model airplane prop.

Dave in San Diego

In article .com,
wrote:

Bob Moore wrote:

Reed Judd-Dyer wrote

How does more blades equal less efficiency?

Those of us that flew rubber band powered contest model
aircraft way back ('40s-'50s) KNOW that the most efficient
propellor has only ONE blade...and a counterweight on the
other side. :-)

Bob Moore

A Helo with one blade would have to be the most uncomfortable ride,
until after a minute or so, when it had shook itself to pieces.

quite a bit less than a minute...

The single-blade prop with counterweight actually worked pretty well in
testing.