Russian PAK-TA concept question 1

[nj4ck]

Hi all,

first off, I am NOT an engineer (hope that's not a problem). But I do have a question, in case any of you could spare a minute or two to explain something to me.

Is it physically possible for a fan (any type of fan), to produce supersonic airflow? I'm by no means an expert on this, but for some reason I feel like this shouldn't be possible. If the intake ducts on a supersonic jet need to be designed to slow the incoming air to subsonic speeds so that the compressor can handle it, it doesn't seem logical to me that any fan by itself should be able to accelerate air to supersonic speed.

The reason I'm asking is because of the recently announced russian "PAK-TA" transport aircraft. There are many things that seem illogical and borderline ridiculous about this "concept" (apparently expected to enter service within 10 years... or not), but the one that struck me as the most asinine was that it is apparently supposed to use electrically driven fans to produce the necessary thrust to reach Mach 1.8... with a 200-ton payload. That and I'm pretty sure that the wing tips would collide with the shock wave coming off the nose at supersonic speed. It seems only logical to me that the entire concept is merely propaganda and will never get past the stage of being a 3D model, but I'd like to have some sort of scientific proof to back this up.

Can any of you help me out?

 

[moon161]

They usually don't. I don't know if they can or not.

http://www.grc.nasa.gov/

http://WWW/k-12/airplane/nozzled.html

The SR-71, for instance, used variable inlets and nozzles to decelerate incoming air from Mach 3 to subsonic speed, heat it up, and spit it back out, at a net acceleration by the time it left the nozzle.

http://www.enginehistory.org/Convention/2005/Presentations/LawPete/SR-71Propulsion2.pdf

Details? Ask someone who got further than undergrad aerodynamics.

The mach cone at 1.8 (1200 mph ground speed) Mach would I think be 41 degrees each side, so included angle of 82 degrees.

I think it looks cool, but pop sci still slams it like a propaganda rag.

http://www.popsci.com/look-ridiculous-russian-supersonic-transport-concept

 

[GregLocock]

"Is it physically possible for a fan (any type of fan), to produce supersonic airflow?"


Well, I'm going to take a punt and say yes, because an airfoil can operate supersonically

Cheers

Greg Locock


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[nj4ck]

"Well, I'm going to take a punt and say yes, because an airfoil can operate supersonically"

sure, a propeller can operate with the tips travelling at supersonic speed, but can it accelerate the air it moves past Mach 1?

 

[GregLocock]

Sorry, that is a much better point, I think you are right, air will not move supersonically in a free field, so as soon as the duct ends, shock waves will form, and the flow will decelerate. If you define fan loosely enough, yes it could entrain some air at supersonic speed, but as soon as the air gets into the stream it'll decelerate

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

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[IRstuff]

The video I've seen of the PAK-TA shows the exhaust to be looking more like a turbofan output, but that could just be artistic license.

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Of course I can. I can do anything. I can do absolutely anything. I'm an expert!
There is a homework forum hosted by engineering.com:

http://www.engineering.com/AskForum/aff/32.aspx

 

[nj4ck]

There's two types of exhaust, the one for engine 2 is similar to a standard turbofan exhaust, the ones on engine 1 and 3 remind me of the exhausts on the B-2 and supposedly feature thrust vectoring (which so close to the center of mass makes no sense, as far as I can tell). I suppose it could be possible for the air to be accelerated to supersonic speed while being forced through a duct with gradually reduced diameter, but I can't imagine that being an efficient way of producing thrust.

 

[btrueblood]

Yes, that a fan operating at supersonic speeds (at least near its tips) can accelerate the airflow to produce an exhaust moving faster than it came in (the basic requirement for net thrust, based on momentum law). Proof is in

http://en.wikipedia.org/wiki/General_Electric_GE36,

though even modern turbofan engines have tip speeds well above mach 1.

" I suppose it could be possible for the air to be accelerated to supersonic speed while being forced through a duct with gradually reduced diameter, but I can't imagine that being an efficient way of producing thrust."

Pretty much every jet engine uses the deLaval nozzle principle to accelerate airflow to supersonic speed

 

[rb1957]

I'm also not an engine guy, but I can't see why you couldn't have a supersonic exhaust. as for "the wing tips would collide with the shock wave coming off the nose at supersonic speed", well that's just part of supersonic aircraft design.

another day in paradise, or is paradise one day closer ?