Supersonic Prop Aircraft

[MMEMatty]

Hi All

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)


Matty

[Monitor .9]

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.

[seacue]

Golly gee uncle Monitor, a number of common piston / prop planes did over 300 mph.

Some Constellation models had a max speed of 377 mph. The DC-7C could do 406 mph.

The Lockheed P-38L had a max speed of 414 mph. The Spitfire Mk VA could do 369 mph. The Republic P-47D could do 433 mph. All in the mid 1940s.

The turboprop P-3D Orion can do 473 mph and the similar Electra airliner 404 mph. The Britainia could do 397 mph.

[Mad (Flt) Scientist]

Don't forget the Tu-95/-142 Bear when thinking of speedy propellor aircraft. Props and a swept wing!:o

A couple of reasons for "scimitar props":
Props on an Islander

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.

Hartzell Props PR

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.

[411A]

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..

[PaperTiger]

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

[Duck-U-Suckerz]

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!

[Onan the Clumsy]

We're talking about two things here. One is tip speed which is a factor of the radius and angular velocity of the prop. Theoretically tip speed could be supersonic with the aircraft not even moving.

The other thing is forward speed of the a/c through the air which is a different matter altogether and something I know very little about.

[northwing]

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.

[Hilico]

What? Somebody sitting in a Lightning could hear it? Bl**dy H*ll!

[BOAC]

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.

[sycamore]

fast t-props

P-T,et al,
There is a bit on the XF-84H on Google,claims it`s top speed was supposed to be 570kts, 2 prototypes,only did 12 flights between`55-`56,and the aircrew suffered nausea as well!

[PaperTiger]

Bear .wav file: http://lsss.homestead.com/tu95.html

Lots of other sounds on there too
http://lsss.homestead.com/AircraftSoundIndex.html

[PaperTiger]

Good XF-84H article in the current issue of Air & Space magazine.

[compressor stall]

a humble Cessna 210 can has supersonic prop tips.