Can anyone tell me the biggest disadvantages of
running a 28 inch multibladed [3, 4, or 5 blades]
constant speed or in-flight adjustable prop at 6K?

My calculations yield that prop tip speed of 950 fps,
forward velocity of 350 knots at 6000 rpm gives a
prop diameter of 28.41 inches.

prop dia [feet] = sqrt( [(V tip of 950 fps)^2 minus (forward speed of
350 knots=591fps)^2]/[(100rev/s)^2(pi)^2] )

I understand why moving larger numbers of air molecules a little
slower is more efficient that moving fewer numbers of air molecules a little
faster, but say I really want to avoid a large prop disc, and have enough power
to be a little inefficient.  I won't be using ducts, however.  Any
takers?

Gene Kahn

Well I can't really say why other than the factor you just stated.
However the worlds fastest prop airplanes have huge props running at low
RPMs. P51, Corsairs, Bearcat, etc., etc.. The latest example is the Osprey V22.

I would say another huge drawback will be the noise. The un ducted
fan was abandoned for the reason.

I would say the largest drawback would be take off performance or
the lack thereof.

Paul Lamar

A larger portion of the prop span will be affected by tip and hub losses than one with longer blades.

The hub will have to be more complex and consequently heavier than one with fewer blades.

The airflow around the engine compartment will be more critical because the short blades won't tolerate very much turbulence at the root without an intolerable loss of efficiency.


Matt-