It depends a lot on the prop design, and there is such a thing as relative efficency. I remember an article a few years ago about new research in turboprops because they would be more fuel efficient, and this page:

http://www.aerospaceweb.org/question...s/q0031b.shtml

seems to back that up. From there:

Despite the promising results of NACA's research, the rapid development of the jet engine doomed the supersonic propeller and the project was abandoned by 1949. However, rising fuel costs have generated renewed interest in high-speed propellers since turboprops are more fuel efficient than turbojet and turbofan engines, so we may not have heard the last of the supersonic propeller.


I recall some things about methods to reduce the noise, but that (not efficiency) seems to be one of the key items holding them back on commercial airliners.

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The Leif Ericson Cruiser

I am familiar with the Cl/Cd , Cl/alpha, Cd/alpha curves which also count for propellors (replace lift with thrust, alpha is then a product of forward velocity and blade velocity), but what still isn't clear to me from this discussion: is it theoretically possible to make a propellor plane that flies supersonic? I can't see why not, but only in theory.

What I can understand, is that even the most efficient propellor comes at a point where his drag increases faster than his thrust somewhere before mach 1, and therefore will never reach mach 1, even in dives. This increase in drag happens -hence the "sound barrier" so you'd need a propellor blade that still is an amazingly nice part of its CD/alpha curve and at an incredibly high Cl/alpha point at that time in order to have a lowze total drag (including the suddenly rising wave drag) than lift, something that seems rather impossible to me.

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One thing that should be noted is that it's one thing to have a supersonic propeller (a prop that is turning that fast) and another to have it on a plane that is moving forward supersonically. From this page, Transonic and Supersonic Propellers

After the war, with the growing reality of the turbo-propeller, the prospect of using propellers at speeds well beyond 500 mph, upward to.transonic and even low supersonic flight speeds, seemed likely. [snip]A small propeller of this kind has supersonic conditions over virtually the entire exposed blade and is thus referred to as a "supersonic propeller" even though the design forward speed may be still subsonic.

The aerodynamic criteria for design of transonic or "supersonic" propellers with low profile losses were clear: use the thinnest possible blade sections, sharp or very small-radius leading edges, and little if any camber. The first two of these criteria had in fact been obvious for some 25 or 30 years-since the thin propeller tests of Reed, and the section tests of Briggs and Dryden. Propellers tapering from about 5-percent thickness ratio at the base to 2-percent at the tip with either zero or very small cambers yielded efficiencies of 75 to 80 percent at a forward Mach number of 0.9. At Mach 1, peak efficiencies as high as 0.75 were obtained (refs. 155, 158). The recovery in lift and 1/d observed in airfoil section tests at high supercritical speeds where the separated flow disappears (fig. 4) is also seen in the propeller tests; curves of peak efficiency against flight speed level out and may rise slightly at speeds beyond about Mach 0.9 (ref. 155).

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Can a prop plane exceed the sound barrier?

Of course it can.

You just need two things:

1. A deaf pilot.
2. An overengineered prop/engine combination that produces thrust well in excess of Mach 1. To do so most aeronautical engineers would probably have the prop enclosed so the airflow would be controllable. Plus the source of rotary motion might be some kind of radial engine which eliminated the drawbacks of reciprocating motors, maybe some kind of turbine. Then there should be no limit on the number of props, which would not only provide thrust but would also load the turbine engine with air and fuel such that the thrust levels needed could be obtained. Advanced alloys would be needed to withstand the high temperatures that such a device would operate under. Finally, the airframe and control surfaces would have to be beefed up to withstand the unusual conditions and stresses that such increased thrust and aerodynamic loading would create.

I wonder if this was ever attempted?

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I vaguely recall such an attempt by Whittle during WWII. Lead to nothing.

Well, except for it being the invention of the jet engine and jet aircraft...

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Minor point: I'm pretty sure you meant "radial" engine to mean turbine engine but a radial engine does not necessarily have to be a turbine. Most were piston (reciprocating) engines with the cylinders radially mounted. Some had the radial cylinders mounted solid and the crank was attached to the prop - others had the crank mounted solidly and the radial engine was attached to the prop, which spun with the prop.

Minor point: I'm pretty sure you meant "radial" engine to mean turbine engine but a radial engine does not necessarily have to be a turbine. Most were piston (reciprocating) engines with the cylinders radially mounted. Some had the radial cylinders mounted solid and the crank was attached to the prop - others had the crank mounted solidly and the radial engine was attached to the prop, which spun with the prop.

The second type you described (where the crankshaft was bolted to the firewall and the cylinders spun with the propellor) was called a rotary engine. While wierd, they were very widely used in WWI by both sides. IIRC, the rotary engine was invented by the French Le Rhone company who licensed it to German and British companies. The engines were used in fighter planes such as the British Sopwith Camel, several French designs, and the Fokker E-1 (Eindecker) and Dr-1 (Dridecker triplane) flown by German pilots.

Rotary engines had some unusual operating characteristics. They couldn't be throttled very well so they had a kill switch meaning the engine was either full throttle or windmilling. They used castor oil as a lubricant so the pilots had to drink some strange brews to keep from having the trots. They also had huge gyroscopic precession issues especially at low airspeeds such as on takeoff. A turn to one direction would cause the nose to pitch up while a turn to the opposite direction caused the nose to pitch down.

You're right of course. But when I see the term "rotary" engine I think Wankel:

From wiki:

Yes, it is called a jet engine.

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This design has always amazed me. Who would have thought to anchor the crankshaft and let the engine spin around it?

Even jet engines don't normally operate in a supersonic airstream. They use specially designed inlets to slow the airstream to subsonic speeds before it hits the compressor blades. Even the SR-71's engines operated with a subsonic airflow at the compressor blades. One of the many genius details of the plane was that the inlets themselves generated over 50% of the thrust at cruise speeds.

I thought he was doing that on purpose to be funny.

Nick

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Yeah, I threw that in to cover as many kinds of engines as possible. I had "rotary engine" in there too, but edited it out, since I had already used the term "rotary motion" and it interrupted the flow.

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You got it!

Of course, MentalAvenger was just being tongue-in-cheek, I'm sure. At least that seemed the main thrust of his post.

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Of course. Actually, I was impressed with the way that Maksutov was able to subtly transform a rotary engine into a jet engine in a single paragraph. Excellent, no, superior literary craftsmanship. Well done.

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Indeed, all jet engines (AFAIK) operate at subsonic speeds from at least the point of entering compressor stage 1. If a plane flies supersonic, the inlet must slow down the air to subsonic speeds before arriving at compressor stage 1. The inlet acts as a static compressor here.

The only exception is the scramjet. Here, there is no compressor (not the dynamic, rotating kind at least). The inlet compresses and slows down the air, but it is still supersonic when entering the combustion chamber and remains supersonic throughout.

You say the inlet of the SR-71 generated 50% of thrust. How must I interpret this? I cannot see how just an inlet would generate net thrust. In contrary, I would say that just an inlet would generate drag only. What I can understand, is that it takes X% ofcompression into account, which in the engine as a whole results in 50% of the thrust generated (the other 50% due to the compressor stage). Can you elaborate?

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It is nice to see apparently my Whittle post was invisible to others

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Frankly, I saw it.

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Anybody here ever been over flown by a Tu-95?

They are incredible aircraft. All the ones that ever buzzed me in the Indian Ocean were shiney stainless steel monsters that would get your jaw to drop as you watched it bank and the wingspan keeps getting bigger and bigger, then you see how tiny the Tomcat escorts are. Another amazing feature is it's almost all steel and heavy as hell yet still the fastest prop job in the world. I believe the ones that I knew were based in North Vietnam.

I used to have a whole bunch of pictures of Tu-95's but they got lost during a move years ago. Some of the better ones were from photo recon birds that were flying along side the Bear and taking pictures of the crew through those big side blisters. (Russians play cards it seems, just like real people.) Since we helped the Photogragher's mates torment their supervisor they let us have copies of cool pictures when we went to the photoshack on business.

(Yes all the pictures were confiscated once during a health and wellfare inspection, and yes, I got them all back after they were reviewed.)

As a side note one of the funniest things the photogs ever did to thier boss was when they altered one of the decorations on his desk. A "poop storm" insued that was felt throughout the squadron. Now thier boss wasn't a bad guy or even a bad chief petty officer. He just had the same problem a lot of guys in intel have and that is they take their job waay too seriously. AND was high strung.

Anyway, we were out at sea for a while, where boredom can turn the most serious risks into light entertainment, they did this:

You know those half spheres of glass that some folks have that you put pictures under so they get magnified? Well, the photog chief had one that he put a picture of a pair of "bosoms" from a nudie magazine under and he kept on the near left side as you looked at him. (Far right to his perspective) I saw it many times as I had to deal with the phototrogs due to my lack of seniority. *

Well, while the studio audience was out they replaced the "bosoms" with another set of organs altogether, from a completely different gender. The perps who did this thought at first that when the chief came back from lunch, he would see the switch, have a fit, they would have a laugh and get some extra duty and that would be the end of it.

Except he didn't notice.

Now this led to incremental increases in the hilarity factor over the next several days as part of his job included daily debriefings at his desk with senior airwing staff and ship's company. The height of which was when the XO of the ship, (the second in command of the whole shebang) came down to talk to him and did a double take on the offending artifact.

I was let in on it on day three. I went to the Photoshack as usual and noticed the swap after about 5 seconds. When I started to bring this to the chief's attention one of the E-5's caught my eye with the ol' hand wave behind a binder trick and filled me in.

When the chief finally caught on it was about five days later. He completely and utterly lost it when he found out how long it had been like that. He pulled down all sorts of heinious punishments on his men and even tried to deny them liberty at the next import in the Phillipines but was overridden by the master chief and the skipper (of our squadron) who thought all the dicsiplinary actions of the previous two weeks mitigated their misdeeds. No need to go overboard on sailors who were doing their jobs well otherwise.

And of course after that "funny" magazines with all male casts began to show up in his mail and his rack. Just to see if he would blow another wingnut. It did the first two times then it got old.

* (Photogragher's mates were all painfully nerdish, highly cloistered in a airconditioned space and you practically had to threaten them at gunpoint to get one of them on the flightdeck during air ops. You actually had to give a verbal password to get into their shop, which had a door that required a conbination to get in or out of. Oh yeah, and some of them had bathing issues. They felt that since they worked in an airconditioned space they didn't have to bath as often. They were wrong and had to be taken to task on it.)

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I had to check that out myself as, like you, I thought that was strange at first glance. Also, when I took turbomachinery, I seem to remember doing some calculations on the shock waves inside the SR-71's engines but I don't remember thrust. But then, I seriously doubt we got into the full details of what was happening too.
Wikipedia says 58%, I've seen 54% elsewhere.

You can read about the SR-71 inlets from the pilot's manual, starting at page 31. I don't pretend to understand how they did it but a coworker of mine who was an SR-71 backseater for 5 years confirms it's true.

Ah like that, the inlet has -of course- a positive pressure gradient, pushing the structure forward. Check.

On a rather unrelated note, calculations after the war showed that the heat exchanger of the P-51 Mustang actually acted as a mini jet engine, providing 3% of thrust. This explained why the calculations done on the Mustang during design always were off with tests.

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Well, I did have this idea of putting a Cessna atop an R-7... http://en.wikipedia.org/wiki/R-7_Semyorka

"Wait! stop! whoa!!!!"


HELLLLP!


"You're a SR-71 driver, whattaya mean you were passed by a Skylane!"

It was in pieces sir!

Even at the height of the Cold War, you had to watch out for Cessnas. Remember Mathias Rust? Who needs stealth bombers when a Cessna 172B proved it could penetrate Soviet airspace all the way to Red Square?

It's probably no different today - with the US or Russia.