F4U inverted gull wings

In article PyBFc.18031$%_6.5403@attbi_s01,
"John R Weiss" writes:
Very interesting! The only part I question is:

"Peter Stickney" wrote...

Increasing the diameter of the propeller works well
aerodynamically at most Advance Ratios, but the tip speeds are high.
since most of the thrust of a propeller is generated at the tips, it
also puts huge loads on the propeller blade & hub.

How can "most of the thrust" be generated at the tips -- given the
combination of shorter chord, lower AOA, and [vortex] airflow around the
tips -- compared with the midspan of the blade?

Ah, you caught me oversimplifying/underexplaining. Thanks.

Because the airspeed of the propeller is much higher at the tips,
most of the thrust produced by the propeller is higher there. A
simple rectangular prop with a constant chord would, all other things
being equal, be developing a tremendouds amount of force at the tips,
and hardly any near the hub. This sets up a severe structural problem
- a simple propeller shape will have a tremendous bending moment both
at the point where the blade joins the hub, and along the length of
the blade. (As an aside, you also don't want the propeller blades to
be too heavy - a WW 2 fighter propeller blade typically weighed about
100#/45 Kilos). Building strong, lightweight blades that could take
these forces and not have, say, problems with resonance, was a
difficult and involved process, fraught with danger - It wasn't
unusual for a disintegrating propeller to destroy the test cells at
Hamilton Standard and Wright Pat, let alone be something that could be
trusted in the air - so propeller shapes, before WW 2, were tweaked to
provide a fairly constant (at some particular design point wrt prop
pitch and airspeed) distribution of forces. This included reducing
the area at the tips (Which also decreased the strucural issues),
changing the propeller pitch across the propeller blade's length, to
keep the lift coefficient the same, and making the shank of the blade
completely round. (For greater strength, and for making the
pitch-change mechanisms easier to design & build.)
using a progressive pitch ditribution (propeller twist) allows for
more thrust at low speeds.

A typical early WW 2 propeller, with a relatvely narrow propeller
blade chord and a fiarly pointed tip produced its maximum thrust in a
region between 80 and 90% or the propeller radius.
(Ref: NACA Report No. 712, "Propeller Analysis From Experimental
Data", Stickles & Crigler, 1940.)
As structural techniqes improved during the war, it became possible to
make wider chord propeller tips, moving the point of maximum thrust
further out along the blade radius. It still doesn't peak at the
tips, though, because of the tip vortices. It's still a serious load
on the propeller shank.
Compare, for example, the early and later CUrtiss Electric propellers
used on P-47s - the early propeller blades are "toothpicks", while the
later "paddle" blades are nearly rectangular, with the same maximum
chord, but held for a much longer section of the blade radius.
Or the propeller of a P-40 or P-51A to that of a P-51B.


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