I was just wondering why there are no supersonic propellor aircraft? is it a question of cost, or that jets are more efficient, or technology? what would the forces be on a propellor as it goes through the sound barrier? I always thought that the propellors are pretty much transonic, so there wouldnt be that much diference would there?
Also, why is the latest trend in aircraft seem to be curved propeller blades? I'm thinking specifically C-130-J and the Airbus Future Military Transport (can't remember exact designationm, sorry)
I'm sure someone will be able to give you a more detailed answer, but from memory the problem with props is the speed of the tip of the propellor.
It is the product of the angular velocity of the prop shaft and the radius from the shaft to the tip of the blade. if the velocity of the tip of the blade gets supersonic, shockwaves will form as a result and cause the flow over the prop blades to seperate. This reduces the efficiencey of the propellor due to increased drag. Also stresses can be experienced in the blades due to the supersonic shocks on them. This can cause the blades to fracture, and disintigrate. This limits conventional propellers to airspeeds under 300kts. I'm sure someone will correct me if I've got it wrong.
Can't help you with the second question though.
Last edited by Monitor .9; 15th Apr 2003 at 08:06.
Below is G-NESU, the North East Spotter Unit which is a Britten-Norman BN-2B-20 Islander. Based at Teesside Airport, it acts just as the helicopter does. They are currently evaluating scimitar-shaped Hartzell propellors whcih are developed by NASA! They are being used to reduce the Islander's noise footprint.
The blades are highly twisted from root to tip which is a characteristic of most higher performance propellers allowing designers to optimize cruise while incurring the smallest possible drag penalties.
Howard Hughes developed a piston-powered (R-4360's) counter-rotating propellor aircraft that was designed for 470 knots....but unfortunately he crashed it on the Riviera golf course in Los Angeles...and spent a long time in hospital as a result..
I have a figure of 477kts for the Bear, but that's max ('dash speed') not cruise. Fastest ever prop was the XF-84H Thunderscreech at 450kts cruise, but the noise from the engines and props incapacitated the ground crew, even with ear plugs
I remember my dad who worked for Vickers and the RAE telling me that it can be done but not very effectively. Basicaly Its fare to say Props are best when used < 300 / 350 odd Knots, and is much easer to use a Jet. As early as 1944 a Photo Recon. Mk XIV Spitfire was traced on Radar to be doing M 0.84 in an emergency dive following a pressurization failure. Shortly after the war the late Mark Spitfires and Mustangs were used for sonic Tests. The Mustang while very fast in the subsonic range was a dead loss in a sonic dive due to its smaller Tail fin area and Broad chord 4-blade prop. The Spitfires did better despite the drag from those massive rad-cowls (under the wings). With all it's armour removed and it's much larger deristricted 2650 - 3000bhp Rolls Royce Griffon. The Spit could be driven to insaine speeds! The wings have a very high M-crit, (the critical speed at which airflow can become sonic / shockwaves forming) The late type 5 blade 'sabre' shape prop delay M-crit, and the late type large Tail fin keeps the plane stable (er). But as Mach 1 is hit things have a tendency to drop off! and though it was done, the fatality rate was unacceptable!
Last edited by Duck-U-Suckerz; 16th Apr 2003 at 01:08.
Even quite slow aircraft can have supersonic propeller tips. The Harvard trainer does, and the noise is very annoying to the boring groundborn types with no sense of the dramatic. The speed at the propeller tip will be the vector sum of the forward speed of the aircraft and the rotational tip speed, which itself is the product of RPM and blade length. The Tupolev Bear certainly had supersonic tips - intercepting Lightning pilots could hear it miles away as a result. It had to have them go supersonic because there would be no way that it could go fast, have the props go round fast enough to generate thrust and still have a vector sum which was subsonic. The snag is that in addition to the noise the prop is generating wave drag and hence operating less efficiently, just as a supersonic aircraft generates wave drag and hence needs more fuel to get from A to B.
Been there, done it, heard it! Impressive noise, too, Not, I must say, from 'miles' away, more like yards, but the unmistakable noise certainly came through into the pressurised cockpit - and through the bone-dome.