real experimenting

[dust]

OK, rotary with a psru, been done, how about getting rid of that pesky gear box?

 

Why not run the prop at 5 or 6 thousand rpm, reduce the prop size to keep it in the right tip speed arena and extend it way in back of the engine, like three or so feet for clean airflow.

 

Racing airplanes regularly run 5000 to 55oo rpm, 2700 is not a magic number, why not really expand the envelope.

[No4]

Sorry Dust but that's rubbish.

formula for kinetic energy or dynamic pressure if you like

1/2mass multiplied by velocity squared.

10kg * 1 m/s = 5 watts

1kg * 10 m/s = 50 watts

The more air you can push back the better.

Only if you were attempting speeds in excess of 300 knots would you then need to head for an unducted fan.

An ATR 72 which is a 70 seater turboprop has two 2500bhp motors turning six bladed carbon fibre props of about 16 feet with curved tips which spin at 1000rpm.

I agree 2700 is not a magic number, but slower is better, but that requires a longer blade and then ground clearance is a problem for a cozy.

[dust]

The one that continental is developing with, i think it is macauley

[LargePrime]

No4;

 

You mention 300 Knots. Why is that a magic number?

[No4]

At 300 knots the aircraft is traveling roughly at Mach 0.5.

The propeller is also rotating to provide thrust, the tips must therefore be travelling through the air at something like their rotational velocity plus the aircraft speed. eg Mach 1

Parasite Drag increases with Indicated Airspeed, thus the plane must provide more power, or climb higher to travel faster. But then when climbing the True Airspeed will increase, increasing the Mach number.

When objects start travelling above Mach 0.8, they start into supersonic flow. The speed of sound is somewhat like a wall. Lift will increase then suddenly decrease and then stabilise somewhere below it's original value. Drag inreases rapidly until supersonic, and then drops to a higher value than at 0.8M.

So a supersonic prop will lose lift, gain drag, and become less efficient, therefore requiring a large increase in horsepower for a small increase in TAS.

The sound of a big turboprop you hear is more the sonic boom from the props than the motor.

Then of course there are engineering proplems in dealing with these forces and shock waves. A Spitfire was recorded at over 600 mph in a dive, when it's constant speed unit failed removing the prop and half of the engine from the aircraft. The Douglas Skyshark had a 5000bhp turboprop, and could manage 600mph straight and level. The only problem was the noise and resonance of the contra rotating props caused the crew to involuntarily vomit and pass out, and nick name it the "Thunderscreech".

 

0 - 300 knots = propeller

300 - 500 knots = high bypass turbofan

500 - 600 knots = low bypass turbofan

600 + = turbojet

 

for some excellent aircraft sounds http://lsss.homestead.com/Aircraft_Pictures_and_Sounds.html

cheers:D

[LargePrime]

Very informative.

 

Once you are fast enough you need a very fast tip speed to even add thrust.

 

Thanks.