Propellors vs Rotors

Don W wrote:
Smitty Two wrote:

So what am I missing in this? I can see there might be some cost, ground
clearance, and possibly vibration issues involved in putting a twelve
foot diameter prop on my RV. But hypothetically speaking, are there
other, perhaps more important, reasons not to make the prop bigger and
the engine smaller and slower?

Smitty,

You just asked the $24 question that I am dying to find out the answer
to. Seems to me that thrust is thrust, and thrust is what makes
airplanes go fast, but I'll bet I'm missing something here, and someone
will set me straight.

Don W.

A larger, slower prop will provide better takeoff and
climb, but unless its blades are variable in pitch (as in a
constant-speed setup) your forward velocity will be lower in cruise.
You can't swing a large prop anywhere near fast enough to allow the
engine to develop its horses unless that prop has a low pitch, while a
smaller, higher-pitched prop will "slip" more easily at low forward
speeds. The smaller fixed-pitch prop compromises more easily.
WW2 fighters like the Corsair had huge props (14 feet or
so?) that turned at 1300 for takeoff and around 900 in cruise. The low
cruise RPM was necessary to keep the tip speeds within reason. A (tip
speed) squared plus B (forward speed) squared equals C (total tip
speed) squared. The prop's blade angles at the tip in cruise were more
than 45 degrees to maintain a workable AOA.

Dan