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nj4ck

Any engineers/ engineering students/ aerodynamics experts here?

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Posted · Original PosterOP

I need to know something that for the life of me I haven't been able to find out just by googling:

 

is it possible for any type of fan to accelerate air to supersonic speed? Or in different words, is it possible for a propeller to produce enough thrust to propel an aircraft well beyond Mach 1?


      

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seems like yes but its extremely inefficient

 


According to Wikipedia: "The tip of the propeller on many early aircraft may reach supersonic speeds, producing a noticeable buzz that differentiates such aircraft. This is particularly noticeable on the Stearman, and noticeable on the North American T-6 Texan when it enters a sharp-breaking turn. This is undesirable, as the transonic air movement creates disruptive shock waves and turbulence. It is due to these effects that propellers are known to suffer from dramatically decreased performance as they approach the speed of sound. It is easy to demonstrate that the power needed to improve performance is so great that the weight of the required engine grows faster than the power output of the propeller can compensate. This problem was one that led to early research into jet engines..."

So basically the performance drops so badly that you better switch to jet engines. In fact when designing the blades, a max tip Mach of ~.9 is a hard limit of design. The text continues to say the nevertheless some propeller aircrafts were able to approach speed of sound in a dive.


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doesn't get to a point in which the propellers are basically trying to create thrust but they cant due to low pressure/vacuum ish. Don't know probably chatting shit. 


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Yes, it's why the chopper makes that sound iirc. (no not the bike :))

 

I answered that question wrong; 

No it's not. You have a certain amount of thrust a propellor can generate. It does not scale in a linear fashion to infinity, even hypothetically speaking and you could create a powersource that propelled it infinitely fast. Because at a certain point you get the point of diminishing returns (turbulence, delamination). So no, not ANY fan can propel ANY object to supersonic. You have to be specific about which fan and which object.

 

Also, mach is not a speed, it's a ratio. Speeds of higher than mach 1 are usually achieved at high altitudes due to air density.

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Any type of fan? Sure.

 

There is a large difference between the speed of the air moving through an aircrafts engines and the speed of the aircraft. 


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Posted · Original PosterOP

Any type of fan? Sure.

 

There is a large difference between the speed of the air moving through an aircrafts engines and the speed of the aircraft. 

Yes, but a supersonic aircraft's engines work by expelling expanding hot gases that produce thrust. The air inside the compressor never exceeds supersonic speed, as you said. My question is, would it be possible for a fan itself (without combustion, say its driven by an electric motor with infinite hypothetical power) to accelerate air to supersonic speed?


      

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Posted · Original PosterOP

Also, mach is not a speed, it's a ratio. Speeds of higher than mach 1 are usually achieved at high altitudes due to air density.

I know. We're talking about supersonic speed, the altitude can be anywhere within the earth's atmosphere. What's important to me are the physics of air moving at supersonic speed.


      

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Yes, but a supersonic aircraft's engines work by expelling expanding hot gases that produce thrust. The air inside the compressor never exceeds supersonic speed, as you said. My question is, would it be possible for a fan itself (without combustion, say its driven by an electric motor with infinite hypothetical power) to accelerate air to supersonic speed?

Lol You dont have to explain the working of a turbine engine to me. I am a mechanical engineer who specializes in fluid flow, however all of my work has been on much lower velocities.

 

The more I think about it the more I think without the pressure differentials created by some form of combustion I dont think you will be able to impart the amount of work needed to get a column of air to supersonic speeds. In theory you should be able to but with the inefficiencies of fans and the rates at which that column of air bleeds off velocity I doubt its realistically possible. 


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Posted · Original PosterOP

Lol You dont have to explain the working of a turbine engine to me. I am a mechanical engineer who specializes in fluid flow, however all of my work has been on much lower velocities.

 

The more I think about it the more I think without the pressure differentials created by some form of combustion I dont think you will be able to impart the amount of work needed to get a column of air to supersonic speeds. In theory you should be able to but with the inefficiencies of fans and the rates at which that column of air bleeds off velocity I doubt its realistically possible. 

Yeah, that's what I thought, I'm just looking for some form of absolute confirmation. And btw sorry, I had no way of knowing what you do for a living  ;)


      

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Yeah, that's what I thought, I'm just looking for some form of absolute confirmation. And btw sorry, I had no way of knowing what you do for a living  ;)

It's cool I'm not that easily insulted :)

 

You almost certain that you could (theoretically at least) get a column of air moving at supersonic speeds with an axial fan, or compressor if it was contained in a tube to isolate it from the static air in the open environment, however I think that as soon as that column of supersonic air hits the wall of static air in the atmosphere it will loose its energy extremely quickly. So yes you could do it, but not (at least anything more than instantaneously) in an open environment. But the amount of energy you would have to put into an electrically driven fan to achieve this would be extremely high, probably tens of thousands of horsepower. 

 

Edit: small emendation from some quick head math, based on the power draw of fans that I have worked with.


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Posted · Original PosterOP

It's cool I'm not that easily insulted :)

 

You almost certain that you could (theoretically at least) get a column of air moving at supersonic speeds with an axial fan, or compressor if it was contained in a tube to isolate it from the static air in the open environment, however I think that as soon as that column of supersonic air hits the wall of static air in the atmosphere it will loose its energy extremely quickly. So yes you could do it, but not (at least anything more than instantaneously) in an open environment. But the amount of energy you would have to put into an electrically driven fan to achieve this would be extremely high, probably tens of thousands of horsepower. 

 

Edit: small emendation from some quick head math, based on the power draw of fans that I have worked with.

OK, that's all I need to know. I was just looking for hard scientific facts to disprove a ridiculous concept the Kremlin has been "planning to have in service by 2024", a supersonic transport plane with a 200-ton payload capable of reaching Mach 1.8 using electrically driven fans instead of turbofans. The entire design screams "I don't fly!", but the idea of electric fans pushing it past the sound barrier sounded like the easiest way to prove that it is propaganda. I've got some pretty gullible friends  :lol:


      

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OK, that's all I need to know. I was just looking for hard scientific facts to disprove a ridiculous concept the Kremlin has been "planning to have in service by 2024", a supersonic transport plane with a 200-ton payload capable of reaching Mach 1.8 using electrically driven fans instead of turbofans. The entire design screams "I don't fly!", but the idea of electric fans pushing it past the sound barrier sounded like the easiest way to prove that it is propaganda. I've got some pretty gullible friends  :lol:

that makes things little different then, turbofans are ducted fans powered by a turbine engine. In theory, you could have the air exiting the duct at mach powered by an electric motor. The fan is there to build up pressure and because the entrance to the ducted fan would be much larger than the exit, the exit velocity will be much higher than just the fan alone could move. This same theory works very well in rc aircraft to attain high speeds compared to standard props. Not sure how well this would work in the real world at the speed of sound though.


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Posted · Original PosterOP

that makes things little different then, turbofans are ducted fans powered by a turbine engine. In theory, you could have the air exiting the duct at mach powered by an electric motor. The fan is there to build up pressure and because the entrance to the ducted fan would be much larger than the exit, the exit velocity will be much higher than just the fan alone could move. This same theory works very well in rc aircraft to attain high speeds compared to standard props. Not sure how well this would work in the real world at the speed of sound though.

yeah, its pretty much a ducted fan. OK, lets say it uses a huge intake and a narrow exhaust nozzle to accelerate air past mach. Does that mean it can create the necessary thrust to push an aircraft (especially one with a 200-ton payload) through the sound barrier? I'd imagine that the pressure building up in the duct would be very high and the amount of air exiting at supersonic speed would be very low, creating only little thrust. Btw, here's a video of this thing, the wing design, nose section and intake ducts don't exactly look like its meant to go supersonic either.

 


      

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