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-


Thank you Paul and Matt...

GK

Quite the slippery little ship you have in mind - vmaxprobe + NA
rotary?  turbo?
At 950fps prop tip efficiency has already started to fall off.
The prop efficiency loss at 350knots will be low.  A correctly pitched
prop will be more critical.
You should have enough HP/thrust to get off the ground even with the 28"
prop.
Probably easier to go with a 40# 2.17/2.85 redrive and a 3.5-5' prop.
Cheers
Cary

Paul Lamar



Just came from a recent effort of getting a better prop for my aircraft
and had some talks with a couple of "propeller gurus".

Talked with a guy name Jack Norris, he's been writing a book about
propellers, and I just happened to ask him that particular question, the
advantages or disadvantages of a two versus three bladed prop. His
answer was that in a comparison between two completely optimized
propellers for the application, a three bladed will be about 1% less
efficient than a two bladed.

A three bladed will have bigger thrust in the beginning of the speed
window, ( thus good for take offs) but it's advantages will bleed off
the closer to the top speed you're getting.

A three bladed prop will have each blade slightly lesser efficient
compared with the same propeller surface needed for a two bladed prop,
doing the same thing.

I was also told that there has lately been a lot of developments by a
lot of people in prop efficiency, and he mentioned Mr Lippe, and Mr
Hertzler as two of the guys that have their heads straight in this
business.... he did seem to have somewhat of a preference for Mr Hetzler

He especially pointed out for me that the improvements are basically in
the shape, he explained that it has to be a long teardrop shape for best
efficiency, then there was some improvements of the propeller tip and
propeller root.

Paddle blades , or blunt tips , or squared tips are seemingly noise and
vortice makers, while a pointed tip seems to be the ticket for an
effective prop.

Regarding the propeller root, he urged me to go to a propeller maker
that knows how to calculate the air flow, that will separate before the
aircraft, and at the propeller root ( on a tractor...pushers will remain
a mess) ... get the angles right. He further urged me to get a propeller
that will have a helix going all the way into the spinner, or as close
as you can. as he described, at the spinner , you can have either a back
pressure..and drag, or stagnant air and drag, or a working airfoil that
will do some work...and drag, so as there will always be drag either
way, let it do some work.

Now, a good propeller, is somethings that the pros need to deal with, as
this is a complex and completely different science, all I can say is
from the benefit from my last conversation when I talked with these people.

I did learn though, that the efficiency has gone up on the fixed props,
to the degree that the advantages of an in flight adjustable, or a
constant speed are from both a cost, weight and complexity stand point,
starting to get a bit into question.

A person in the Quickie community I was in contact with about 2 months
ago , had ordered a new prop, he had ordered his at Cloudcar in Texas,
it was cut according to the latest prop gurus advice. the prop didn't
look like anything else he ever have had. The prop looked almost little
like an S. The leading edge curved very drastically, in one long curve,
and the trailing edge curved with it slightly. The blade was a surface
that slowly went smaller and smaller out to the tip, the tip was almost
a point.

He had gone through a few props earlier, but this one gave him, right
out of the box 10% more oomp in climb and 10-11 mph better speeed.(
Usually you have one but no the other)

What I am saying here, is to research the subject, there are some good
props getting out now, that are seemingly a good step better than the
plain vanilla stuff.

Again, this is all second hand information from people I have talked
with, but it does indicate that it will pay out good, in digging deeper
into the subject, if you are out shopping for a prop.

Roger

Yes I know Jack. He is a local. I have his book. (books?) I did not
comprehend a word of it :) I'll have to read it another ten times
unless I die first :)

Paul Lamar