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Martin, Engineer
Category: General
Satisfied Customers: 4956
Experience:  i'm 41 and i never stopped studying and experimenting
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In an aircraft with propellers,whats the differance between

Customer Question

In an aircraft with propellers,what's the differance between athree blade prop. and a four? One might be a rip. and one might be a turbo shaft.
Submitted: 8 years ago.
Category: General
Expert:  Martin replied 8 years ago.
HelloCustomer welcome to Just Answer.

There is lot of choice that may advantage 3 blade over 4, or the opposite.

It's all a question of compromise.

Some things the 3 blade give.
-less weight
-cost less (less material to make them)
-can turn faster (previous bade turbulence have more time to dissipate)
-less complex to build (it is more complex if you do it by hand, but the junction to be shaft is easier to secure as there is more space for each one).

Thing the 4 blade give.
-essentially more screw power in the air.

For example if you have a glider with a small electric motor, 3, or even 2 blade might do. If you want a cargo plane, 4 blade will be better.

The more efficient blade would be a tight disk made of an infinite number of very thin blade to not be impacted by air viscosity (those would give the more torque in the air). But they can't be build because they would be too fragile and. The closest to that design are those used for wells pump in old far west movie. Another as much efficient propeller and impossible to build would be just one almost massless blade very thin but spinning close to the speed of light.

And the compromise game don't stop there, as you can then think about the pitch of the blade and it's length.
Martin and 29 other General Specialists are ready to help you
Customer: replied 8 years ago.
That was helpfull,but the info i'm lookinking for is about vietnam era AC-130's.I guess The early mod. Had Recip. engines. Glenn
Expert:  Martin replied 8 years ago.
"In 1980 the aircraft was upgraded from the original three-bladed propellers to the quieter four-bladed propellers and was eventually retired in late 1995"

Now it seem they even have 6 blade variant.

you might be interested in reading:

A second AC-130A, AF Serial No. 56-0509, named the Ultimate End, was accepted by the Air Force on February 28, 1957, and modified to the AC-130A configuration on July 27, 1970. The aircraft participated in the Vietnam conflict and the rescue of the USS Mayaguez.