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A Supersonic Propeller Driven Aircraft, Possible?  
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Posted (7 years 3 months 4 days 19 hours ago) and read 22819 times:

I just had this random thought about prop planes and speed records, and I was wondering if it was feasible, with current techonlogy, to have a propeller driven aircraft (would have to be a turboprop obviously) flying at the speed of sound, if not more. The current record holder for fastest Turboprop according to the FAI is the Tu-114 at 871.38 km/h (about 545 mph). This record was set way back in 1960 by the Russians (or should I say Soviets? Big grin ), back when the cold war was getting pretty warm.



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The one major problem is of course, the props would be going supersonic before the plane ever left the ground, and eventually there will be a speed where the incoming air on the prop would be so fast that the prop would have an effective AoA of 0, among many other things.


Now what could be done to reach supersonic flight with a prop? How about extremely swept scimitar blades? Would you want 2 blades or 8? And while we're at it, are counterroating props faster than normal props?

Every and any insight welcome, specially those of you who know plenty about fluid dynamics and other hocus pocus stuff like that. Big grin

I mean, there's got to be a way to do it right?  eyebrow 

Thnaks in advance  wave 

20 replies: All unread, jump to last
 
User currently offlineSCAT15F From United States of America, joined Feb 2007, 402 posts, RR: 0
Reply 1, posted (7 years 3 months 4 days 17 hours ago) and read 22808 times:

The Tu-114 was significantly slower than the Tu-95 primarily because of the much wider fuselage and heavier weight. The "normal" top speed published for the Tu-95 was about 920 km/h or 575 mph, and it holds the official record for Tuboprop aircraft at that speed (575 mph) over a closed circuit.

I read that the Tu-95 prototype hit 992 km/h or 620 mph in level flight and that was only with the 12,000 shp version of the NK-12 turboprops that would eventually be uprated to 15,000 shp. But production versions of the Tu-95 were also a lot heavier)

The XF-88 tested supersonic propeller designs in the 1950's that showed very good efficiency numbers at .95 mach (ultra thin airfoil sections) They used 4 blade designs.

The infamous Republic XF-84H "Thunderscreech" had a 3-blade prop that operated at supersonic tip speed throughout the flight range. Problems with the troublesome Allison T-40 engine prevented top speed exploration, but the design speed was 670 mph or mach 1.0. It was to be fitted with an afterburner as well in order to exceed the speed of sound (the world's only turboprop with an afterburner!) I heard that in testing it hit only 530 mph because of the engine issues. In theory, with a better engine, it could have worked, or at least got real darn close!

Counterrotating props offer higher speed potential for a given power output because they convert torque to power, eliminate torque steer, and give twice the blade area (that's my guess, so don't quote me on that)


User currently offlineFlipdewaf From United Kingdom, joined Jul 2006, 1562 posts, RR: 1
Reply 2, posted (7 years 3 months 4 days 2 hours ago) and read 22667 times:
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I have often wondered this as well, il ask my aerodynamics lecturer about it tomorrow. I know that soome funny things have to happen to a jet nozzle and the air slowed to subsonic speeds before entering. I think it could be done with an impellar if the air was slowed before entering and then it was to exit through a convergent-divergant nozzle.

Fred


User currently offlineN231YE From , joined Dec 1969, posts, RR:
Reply 3, posted (7 years 3 months 4 days 1 hour ago) and read 22629 times:

I thought I read somewhere that propellers loose their efficiency dramatically after 450 mph (could be wrong on the numbers)...

I guess I will have to do some research and look around for where I found this.


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 4, posted (7 years 3 months 3 days 20 hours ago) and read 22544 times:

How does that work (thunderscreech), doesn't the blade have to have a subsonic top-side in order to have suction (as in to suck the air through and accelerate it out the back)? Because if the flow is supersonic the curve over the top would produce a positive pressure rather than a lower.

Am I missing something?


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 5, posted (7 years 3 months 3 days 18 hours ago) and read 22506 times:

A prop blade is an airfoil, just like a wing. In order to have a propeller work at supersonic blade speeds, you have to make a propeller blade with an aerofoil section which works at supersonic speeds. If you can make a supersonic aircraft fly, you can make a propeller produce thrust even though the propeller blades (and possibly the entire aircraft) is supersonic.

Probably won't be the most effective propeller in the world at those speeds though.

As for the AoA thing, you might want to draw that situation on a piece of paper.  Smile



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineThirtyEcho From United States of America, joined Dec 2001, 1643 posts, RR: 1
Reply 6, posted (7 years 3 months 3 days 15 hours ago) and read 22469 times:

You could make a cheeseburger go supersonic if you reintered it from near-earth orbit.

As for prop-driven airplanes, rumors abound concerning the supersonic adventures of the Lockheed P-38 during WWII. These were, basically, full power dives from very high altitudes in which the airplane encountered, and passed, what was known as "compressibility." The aircraft were, most certainly, somewhere in the transonic range and may have been supersonic for a short time. The same thing is said of the P-51.

These would have been out-of-control maneuvers, with a frozen stick, that were recovered only by reaching more dense air at lower altitudes.


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 7, posted (7 years 3 months 3 days 7 hours ago) and read 22369 times:

Not to mention the first nibbles at the transonic region in P38s... and the discovery of Mach tuck. Ouch.  Sad

Rumors has it a certain bucket of nuts, whose name currently escapes me, pushed a Mossie past M1 during WW2. As with most such rumors, it's not confirmed.

It would surprise me a lot if there wasn't someone, somewhere in some aircraft who went supersonic and lived during the war years. However, if it ain't on record it didn't happen. That's the bitter truth of it.



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 8, posted (7 years 3 months 3 days 5 hours ago) and read 22326 times:

The part that makes me think a truly supersonic prop is not possible is because the blade depends on it's ability to suck air towards the blade, and then accelerate it to a higher velocity.

If the blade is supersonic (totally) the convex-curved portion on the front of the blades (which produce the suction which pulls the air towards itself) would be producing negative lift as the curve would cause a pressure increase. Maybe I'm having a problem conceptually understanding, but a prop at supersonic speeds would blow air forward rather than suck it back.

I'm not sure if a reverse pitch would work either since the leading edge of the blade would produce a shockwave which would cover both the front side and the rear side increasing the pressure on both, although admittedly more on the rear side. I'm not sure if the shear spinning of the blade (with inverse camber or reverse pitch) would be able to force air through from front to back especially with positive pressure on the front.

Am I right or wrong?

Andrea Kent


User currently offlineFredT From United Kingdom, joined Feb 2002, 2185 posts, RR: 26
Reply 9, posted (7 years 3 months 3 days 5 hours ago) and read 22319 times:

Can a wing generate lift when supersonic?

Yes, it can.

How does it do it?

Action/reaction. It creates a downward force on the air, thus pushing the air down while the equal and opposed force pushes the wing up.

----------------------

Can a propeller blade generate thrust when supersonic?

Yes, it can.

How does it do it?

Action/reaction. It creates a rearward force on the air, thus pushing the air aft while the equal and opposed force pushes the propeller blade forward.

------------------

Have another look at supersonic airfoils?



I thought I was doing good trying to avoid those airport hotels... and look at me now.
User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 10, posted (7 years 3 months 3 days 5 hours ago) and read 22317 times:

I'm no aerodynamic expert!

Andrea Kent


User currently offlineAvt007 From Canada, joined Jul 2000, 2132 posts, RR: 5
Reply 11, posted (7 years 3 months 3 days 4 hours ago) and read 22293 times:

Quoting ThirtyEcho (Reply 6):
You could make a cheeseburger go supersonic if you reintered it from near-earth orbit.

 Silly
Between this and the STOL SST thread, why not ask for a sub-orbital helicopter?  Big grin


User currently offlineBlackbird From , joined Dec 1969, posts, RR:
Reply 12, posted (7 years 3 months 3 days 2 hours ago) and read 22262 times:

You know what I'm wondering? How come someone in the US during the time turboprops came around think of using a shrouded turboprop?

They would have reduced a lot of vibration, and that was one of the biggest problems with propeller planes. And if they used counter-rotating props, the torque, and prop-wash issue would be neutralized.

Andrea Kent


User currently offlineJerald01 From United States of America, joined Oct 2006, 161 posts, RR: 2
Reply 13, posted (7 years 3 months 3 days 1 hour ago) and read 22239 times:

When a propeller blade goes supersonic, it produces a supersonic compression zone (shockwave) just in front of it. That shockwave then travels over the propeller airfoil (from leading edge to trailing edge at any point along the chord of the propeller) when the propeller is moving (rotationally) faster than the shockwave. Additionally, if the aircraft continues to accelerate to the point that the wings go supersonic, the propeller is subjected to a second shockwave - that of the air rushing at it from IN FRONT of the aircraft. This "longitidunal-aspect" shockwave is separate from the "rotational-aspect" shockwave that is created by the rotation of the propeller blade through the plane of the propeller disk.

A designer can overcome the drag from the rotational-aspect shockwave simply by increasing the propeller RPM at the moment the leading edge goes supersonic. What is extremely hard to overcome is the longitidunal aspect shockwave that is hitting the "top" surface of the propeller airfoil (i.e., that air which is coming directly at the front of the engine, 90-degrees to the plane of the propeller blade rotation.) When THAT shockwave encounters the rotational-aspect shockwave somewhere along the "top" of the propeller airfoil, the resultant interaction creates some very tricky, if possibly unmanageable, forces on the air the propeller is trying to push aft.

This interaction of two shockwaves colliding with each other along the "top" of the propeller airfoil produces many different effects, one of them being a sudden "overpressure zone". That zone spreads from it's initial start point along the airfoil chord all the way to the trailing edge of the propeller, where it meets the high-pressure zone from the "bottom" side of the airfoil. Extreme turbulence at this juncture then creates high-low-high pressure zones at the aft edge of the propeller, which then propogate somewhat forward onto the "top" of the propeller airfoil.

These high-low-high pressure zones then set up harmonic vibrations along the propeller airfoil chord that can be so great as to totally destroy the propeller in seconds. This is not exactly the same as what was happening when all the Lockheed Electras were crashing during the late '50's and early '60's, but it is close. The propellers on those aircraft actually "went supersonic" in two separate axis (longitudinal to the aircraft fuselage, and rotational to the plane of the propeller disk). The props / engines / engine mounts / wings were not designed to handle the ensuing harmonic vibrations, so things flew apart.



"There may be old pilots, and there may be bold pilots, but there are darn few green cows"
User currently offlineFLY2HMO From , joined Dec 1969, posts, RR:
Reply 14, posted (7 years 3 months 3 days ago) and read 22211 times:

Quoting Blackbird (Reply 12):
How come someone in the US during the time turboprops came around think of using a shrouded turboprop?

Wouldn't that make it a turbofan then?  eyebrow 

Quoting SCAT15F (Reply 1):
The infamous Republic XF-84H "Thunderscreech" had a 3-blade prop that operated at supersonic tip speed throughout the flight range.

I reasearched a bit and couldn't find an answer. Do you know if it was a constant speed prop? From pics of it, it looks like it at least has variable pitch blades.

Quoting Jerald01 (Reply 13):
A designer can overcome the drag from the rotational-aspect shockwave simply by increasing the propeller RPM at the moment the leading edge goes supersonic.

So is that why the prop was always spinning at supersonic rates in the XF-84H?


Thx for the insight  Smile


User currently offlineLehpron From United States of America, joined Jul 2001, 7028 posts, RR: 21
Reply 15, posted (7 years 3 months 20 hours ago) and read 21970 times:

Quoting Avt007 (Reply 11):
Between this and the STOL SST thread, why not ask for a sub-orbital helicopter?

It's been done, well, as a research proposal I think, I don't recall if it has been tested. The idea was to use wind-milling blades (like helo blades) to slow a supersonic vehicle from reentry as opposed to parachutes, therefore they could be reuseable and easier to stow away. Note: these blades are not meant to deploy hypersonically. I remember my first year in college, a senior design team proposed a deisgn that involved two sets of blades, forward and back, counter-rotating, for stability I suppose.

Quoting Blackbird (Reply 10):
I'm no aerodynamic expert!

As long as you are learning.



The meaning of life is curiosity; we were put on this planet to explore opportunities.
User currently offline747400sp From United States of America, joined Aug 2003, 3493 posts, RR: 2
Reply 16, posted (7 years 3 months ago) and read 21825 times:

Quoting SCAT15F (Reply 1):
The infamous Republic XF-84H "Thunderscreech"

Was this the aircraft that was so loud it made people sick?


User currently offlineSCAT15F From United States of America, joined Feb 2007, 402 posts, RR: 0
Reply 17, posted (7 years 2 months 4 weeks 1 day 7 hours ago) and read 21738 times:

Quoting FLY2HMO (Reply 14):
I reasearched a bit and couldn't find an answer. Do you know if it was a constant speed prop? From pics of it, it looks like it at least has variable pitch blades.

According to Wikipedia the prop was constant speed variable pitch, turning at 3000 rpm and the tip speed was mach 1.18. The XF-84H hit a (unofficial) speed of 623 mph in testing, making it (unofficially) the fastest propeller driven aircraft ever flown. That speed was reached without using the installed afterburner, so theoretically, it might have actually gone mach one, but I think it would have been more successful if they had used a counterrotating setup.


User currently offlineContnlEliteCMH From United States of America, joined Mar 2005, 1455 posts, RR: 44
Reply 18, posted (7 years 2 months 4 weeks 1 day 3 hours ago) and read 21692 times:

Quoting Blackbird (Reply 12):
You know what I'm wondering? How come someone in the US during the time turboprops came around think of using a shrouded turboprop?

They would have reduced a lot of vibration, and that was one of the biggest problems with propeller planes. And if they used counter-rotating props, the torque, and prop-wash issue would be neutralized.

They did. What you have described is, to a lesser extent, the modern day turbofan. The fan is driven by enthalpy taken out of the exhaust stream, but unlike a turboprop, there is no geared reduction (though P&W is trying to build a geared turbofan.)

There are other dissimilarities. The fan blade pitch is constant and the fan velocity generally only changes as a function of power setting. And of course, there are far more blades on the fan disk than you'll find on a propeller.



Christianity. Islam. Hinduism. Anthropogenic Global Warming. All are matters of faith!
User currently offlineTimz From United States of America, joined Sep 1999, 6790 posts, RR: 7
Reply 19, posted (7 years 2 months 4 weeks 22 hours ago) and read 21626 times:

Quoting SCAT15F (Reply 1):
it holds the official record for Tuboprop aircraft at that speed (575 mph) over a closed circuit.

When, and how far?

Quoting FLY2HMO (Thread starter):
a propeller driven aircraft (would have to be a turboprop obviously) flying at the speed of sound

Didn't they hope to go supersonic with the (piston-engined) Pond Racer?


User currently offlineSCAT15F From United States of America, joined Feb 2007, 402 posts, RR: 0
Reply 20, posted (7 years 2 months 4 weeks 6 hours ago) and read 21485 times:

Quoting Timz (Reply 19):
When, and how far?

Honestly, I can't find the source anymore- it was in a book.

Quoting Timz (Reply 19):
Didn't they hope to go supersonic with the (piston-engined) Pond Racer?

I remember the goal for the Pond Racer being to break 600 mph.


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