On Tue, 02 Mar 2004 07:32:33 -0700, Bill Daniels wrote:


The gear shifted prop was the last gasp of piston engine development
before the turbine age. Look at the Lycoming XR7755, Napier Nomad or the
Rolls Royce Crecy. These were 5000 HP+ monsters that needed every trick
in the engineers bag. Piston engines produce more HP at high RPM at the
cost of fuel consumption but deliver low fuel consumption at low RPMS.
Props produce more thrust at low RPM and most efficiency with the blades
at a single best AOA. That AOA must be maintained over a wide range of
airspeeds. Just too many variables for a CS prop to deal with alone.

The two speed gearbox isn't perfect but it does buy the engineer a bigger
range of options.

Bill Daniels


Bill,

Thanks for pointing out these fascinating engines. I had heard of all of
them, but had never really looked into the details before.

The Lycoming XR-7755 certainly was a huge, complicated monster.

http://www.aviation-history.com/engines/xr-7755.html
http://www.people.virginia.edu/~rjr/engines/

The Napier Nomad was a bizarre combination of two-stroke diesel and gas
turbine. It managed a very impressive specific fuel consumption of 0.345
lb/ehp/hr. The only reference I can find to a gear box was a variable
ratio gearbox between the gas turbine and the piston crankshaft of the
Nomad 2. Not exactly what the original poster was referring too, but
interesting non-the-less.

http://www.home.aone.net.au/shack_one/nomad.htm
http://www.yourencyclopedia.net/Napier_Nomad

The Rolls-Royce Crecy was a highly supercharged diesel, that supposedly
produced about 5,000 on the test stand. I can't find any reference to a
two-speed gear box between the engine and the prop either on-line, or in
Aero Engines, Bill Gunston, but neither have I found a detailed technical
description of the engine. So perhaps that detail was left out.

http://www.stobbe.dk/technical_literature/combustion_engines/rolls-royce/Rolls-aircraft.html

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