Ducted Fan Mach Sound Barrier

[noobflyer92]

If the technology ever evolved in to large enough scale with enough thrust, could an electric or gas ducted fan, not jet, ever breach the sound barrier? Or would it stall out due to compression? My guess is it would not because it is sucking in air.

[vintage1]

Not sure, but I think it could.

If a wing can work supersonic, a fan blade should..

[jkettu]

I believe it could be done, but not easily. Intake ducting would need to convert the flow into subsonic, just the way it is done on full-scale.

[AndrewBurns]

Fan blades CAN work transonic but you generally really don't want to. In large scale applications supersonic air is shocked down to subsonic speeds over an intake ramp, energy is added via the engine and then the air is expanded through a supersonic nozzle. I'm not sure how well it would work with a ducted fan though, I think you'd have trouble adding enough energy to the airflow to make net thrust after the drag of shocking it subsonic and accelerating it supersonic again.

[noobflyer92]

interesting. thanks for your response guys. maybe some day we will see high tech edf airliners breaking sound barrier in level flight. That'd be cool. Does that mean the ducting would need to be changed from a typical edf to something more streamlined and more squeezed at the end? I dunno.

[Cory]

Isn't a high bypass turbofan just a JTDF (Jet Turbine Powered Ducted Fan), almost the same in principle to an EDF or glow powered DF? The jet in the core does produce thrust, but it is a small percentage of the total thrust. It's pretty much the same as the the difference between a prop plane with a reciprocating engine and a turboprop. The turbine is advantageous because it's a lighter, more efficient power source for the prop. Same goes for a high bypass turbofan. It could be done by a recip, but not as efficiently. Has a high bypass turbofan gone supersonic?

[Brandano]

Not really. most supersonic aircrafts are supersonic only when afterburning. Those that can "supercruise" rely on low bypass turbofans, or plain turbojets. Some marks of the AV8B harrier can be supersonic at high altitude and in a dive, but I don't think that counts.

[Cory]

I'll bet noobflyer92 would be excited with an EDF that could make a boom in a dive!

[BMatthews]

I think you'll find that the small sizes we use are inefficient enough that it won't be happening.

To reach supersonic speeds you need to throw the exhaust out at more than supersonic speeds backwards. Probably quite a bit more than supersonic. IE; to generate useable thrust there needs to be an acceleration of the air in this case. It's not enough to just screw your way through it like a propeller does. So to reach supersonic speeds the tail exhaust needs to be pushed backwards at a rate and mass that is high enough to produce the thrust needed to push the aircraft forward at that speed and against the very high drag that is occuring.

[noobflyer92]

Right. Isn't that essentially what an edf does, though? It makes exit velocity faster than intake velocity?

[atmk]

while entirely possible to make a ducted fan go supersonic, with the above mentioned ducts and nozzles to make the airflow through the fan subsonic, the real problem is power. the insane amounts power generated by gas turbines simply cannot be matched my anything of the same weight (rockets excepted). and only the highest performance airplanes (with gas turbines that make up large portions of the entire airplane) can break the sound barrier.

i don't know if this was about rc airplanes or not but the sound barrier is all but impossible on our scale because of scaling laws (drag being roughly proportional to surface area and power to volume) models have a hard enough time reaching 200mph

[vintage1]

...
i don't know if this was about rc airplanes or not but the sound barrier is all but impossible on our scale because of scaling laws (drag being roughly proportional to surface area and power to volume) models have a hard enough time reaching 200mph

Yep. The problem with a model, as I discovered, is that a wing area large enough to have the model able to both turn within visible range, and have some chance of landing without destroying itself has too much drag..

In short what seems to be the equation is that power to weight relates to top speed divided by stall speed. You can get to 500mph at 200W/lb if you don't mind the stall speed being around 130mph. And flying in circles two miles in diameter.

Which is why we haven't, since WWII, been able to operate off grass strips..for fast military aircraft. Harriers excepted.


As to whether an EDF is a good way to go..well, no it isn't. You probably could get one to work by expanding the airflow to a large fan diameter from a small intake, slowing the flow down, then squeezing it again at the tailpipe to get it to speed up again.

Hi bypass turbofans are designed specifically for subsonic efficiency. Low bypass are efficient in the transonic region, but to go seriously faster you need straight jets and probably with RAM or SCRAM technology.

If you want to run a jet engine off electricity rather than fuel, you might find that striking an arc in the combustion chamber would be a better way to go than stirring it with a fan..

Or using ion drive or MHD maybe?

[Brandano]

what about an afterburning EDF? containing the efflux plume might be a bit of a problem, not to mention the need to have both batteries and fuel aboard the same plane...

[noobflyer92]

yes this was meant for if the day ever came, where there are electric or fuel cell airliners, would they be edf because who knows how much longer the gas turbine can stay on an airliner. think 50 years out from now. What will be pushing airliners, bomber, or fighters past the sound barrier? I hope still not jet fuel.

[Brandano]

Most likely it will still be jet fuel. Or hydrogen. Or methanol... we got plenty of chemical sources of energy, all of them ultimately coming from the sun. Most people fail to realize that the net carbon content of earth is constant (ok, sometimes a comet or a carbon rich meteorite slams into it, but it's minimal confronted with what's already here). The problem we have now is that we are taking carbon from underground and pushing it into the atmosphere faster than the speed at which plants can take it out of the atmosphere and put it back underground. if you want to reclaim carbon, you ought to make enormous fields of quick growing crops and bury their product before it can be decomposed. Given enough time it will become fossile fuel again.. though the aim then is to avoid using it. So oddly enough, chopping down trees to turn them into paper is actually better for he environment than recycling paper, as long as you plant new trees to replace them.

[noobflyer92]

wow. i would have thought we could advance more in 50 years.

[gtfreeflyer]

I couldn't imagine a supersonic rc plane being any fun to fly. Listening to the boom... yes that would be worth it. At supersonic speeds you'll never get to fly straight and level as your model will be out of sight in no time. You'll constantly be in a turn pulling many g's just to keep it within sight/radio range. On the positive side, just imagine how great those crash videoes are going to be :)

[JetPlaneFlyer]

At supersonic speeds you'll never get to fly straight and level as your model will be out of sight in no time.

Yes and the radius of the turns you could pull at supersonic speeds would be simply enormous. For instance if you could maintain a 10g constant rate of turn while flying supersonic the circle radius over 1.1 km which would mean that the model was permanently out of sight... if you did a straight run you would cover over 1 km in 3 seconds and that's not including the 1 km radius turns at either end :eek: .
Such speeds are never going to be practical for RC models that need to be kept in visual range regardless of power source or theoretical top speed capability.

Steve

[vintage1]

yes this was meant for if the day ever came, where there are electric or fuel cell airliners, would they be edf because who knows how much longer the gas turbine can stay on an airliner. think 50 years out from now. What will be pushing airliners, bomber, or fighters past the sound barrier? I hope still not jet fuel.

The problem is a very simple one. Energy density.

You need to carry some form of energy on the plane..unless its flting attached to a cable :-)

Electrochemical energy is fairly low energy density. Liquid hydrocarbon fuel is better, Nuclear is terrific.The physics shows that we wont ever be flying across the Atlantic carrying passengers on LIPO cells.Or any orther kind of battery.

Hydrogen is good weight wise, but is terribly bulky and very dangerous. Kerosene is a nice balance between weight and drag of the tanks its held in.

I suspect that actually air traffic will decline as carbon fuel gets more and more expensive. And be replaced by ultra high speed rail transport. It should be possible to crack 500 mph or so in an electric train. Apart from the Bering straits, is is just about possible to connect Europe, Asia, Africa and the Americas by rail if we wanted to, and the Bering straits themselves are not an insurmountable barrier.

That just leaves Australasia and the pacific as the last bastion of air travel ;-)

You could synthesise hydrocarbon fuel with surplus output from the 500 or so nuclear power stations the US will need to run all that ;)

Trains are nice. Weight is not an issue really. So you can have comfortable seats and decent catering. And no need to park 5 miles from the runway either, or spend time getting scanned for metal objects..or waiting in a stack..

[noobflyer92]

Electrochemical energy is fairly low energy density. Liquid hydrocarbon fuel is better, Nuclear is terrific.The physics shows that we wont ever be flying across the Atlantic carrying passengers on LIPO cells.Or any orther kind of battery.[QUOTE]
Yes lipos dont carry enough energy for their density. I think we need new battery technology to power edf. I've seen some edfs that darn well near perform as well as turbines, but lack flighttime. If we can get a battery that could run an edf large enough for a passenger airliner, fuel cell to power the battery, or a safe nuclear powerplant to be recharging the battery in flight, I think that'd be great.

[noobflyer92]

I also enjoy trains. I wouldn't mind traveling on a comfy train for a long trip. But right now, it does seem like the jet turbine is unmatched in power and efficiency. I would like to see a cad cam design of what kind of ducted fan could compress air correctly to break the sound barrier.

[vintage1]

I also enjoy trains. I wouldn't mind traveling on a comfy train for a long trip. But right now, it does seem like the jet turbine is unmatched in power and efficiency. I would like to see a cad cam design of what kind of ducted fan could compress air correctly to break the sound barrier.

Mm. Jet engines are neither the most powerful, nor the most efficient way to propel things. They are very COMPACT though.

AS far as flying planes go, given a sufficiently powerful source of electricity, a ducted fan aint the way to go. A linear motor acting on ionised air would probably be better.

Sort of scram jet ion drive..

[Accu157]

Time to break out the wind tunnel. Even if it isn't feasible technology wise, make a fan unit the size necessary to make a ducted fan break the sound barrier. If possible. I like the idea of ionized air, however I think that might require very high altitudes to get the air thin enough.

http://www.youtube.com/watch?v=d-ym2aQjvqI&feature=related

I think these things are less efficient than even the most advanced steam fired tether speed boat (1%-2%... massive waste heat to get that speed)
This is steam powered believe it or not! Flash boiler
http://www.youtube.com/watch?v=hVN8iHfQl3s&fmt=18

Maybe if I have the time this coming weekend I'll run some numbers on that ion drive idea.

[vintage1]

I cant remember the exact principle, but MHD used something lie seawater in a tunnel, through which you passed a current, and then a magnet outside acted on the current to spit water out the back.

Here's some fun stuff.
http://en.wikipedia.org/wiki/Ion_thruster

[BMatthews]

There one huge difference in an EDF and a turbojet. The EDF acts to directly accelerate the air using nothing more than the fan blades. Even with multiple stages of fan blades instead of the single stage we modelers use there's still going to be a lmit to how fast the air will be ejected and thus the forward speed that is achievable. In effect you're using an air compressor to compress the air and shoot it out the back.

A turbojet that uses fuel is going to achieve far higher exhaust velocities for two reasons. First is that you're adding the mass of the fuel to the mass of the air. Now on it's own this is fairly negligable. But where the real power comes in is the expansion of volume and pressure provided by the heat of combustion. This increases the pressure a HUGE amount and results in extremely high exit velocity. And since the thrust is a product of the mass x velocity the fuel burning turbojet brings with it a huge advantage over a simple ducted fan that has the exhaust coming out at the same temperature as the inlet.

Adding an afterburner is all about the ability to even further increase the temperature and pressure of the exhaust flow.

Add an afterburner to the EDF? Yeah, it's possible and it's been done. Campini-Caproni did a piston engine ducted fan that forced air through an afterburner open duct like this. The airplane flew in under this system in 1940. And the Coanda biplane did a sort of similar process back in 1910 but using his oddball method that I'm not sure qualifies as a ducted fan and afterburner system but is the same principle of compressing air and then injecting fuel to burn and thus increase the rearward exhaust velocity.

[JetPlaneFlyer]

Adding an afterburner is all about the ability to even further increase the temperature and pressure of the exhaust flow.

I believe that afterburners don’t really work on ducted fans... The reason being that the efficiency of an afterburner is proportional to the compression ratio of the air entering the burner. EDF's produce negligible compression therefore an afterburner is largely ineffective the marginal thrust increase produces is probably negated by the increased weight of the afterburner systems. I think Campini-Caproni discovered the same thing, their aircraft did fly but it was largely on thrust from the engine driven fan rather than the 'afterburner'.

There was such a system being marketed for EDF quite recently, I don’t have the link.. It may still be in production. The consensus was it was great for wow! factor but of no practical benefit as far as performance was concerned.

Steve

[noobflyer92]

would it be inefficient to have a nuclear reactor constantly charging a high power battery pack for long flights? giggity.

[Brandano]

No point in doing that. Just use the nuclear reactor as a source of steam or compressed superheated air to drive the turbines directly. http://en.wikipedia.org/wiki/Aircraft_Nuclear_Propulsion

[Accu157]

There are a few videos on the history/TLC/military/aircraft/discovery channel documenting the development of directly-nuclear powered aircraft. Pretty crazy ideas. Might have actually seen one fly if it weren't for ICBM's. They had issues with radiation exposure to pilots... massive protection from the radiation, but it was only effective through line of sight. I can't remember if they tested either in flight.

[noobflyer92]

the ion thruster looks cool, but is supposedly low power and would not be enough to escape the atmosphere. And that steam engine boat was brutally fast, but couldn't it be refined to be more efficient?

[Cory]

A couple of the nuclear jet engines are on display in the parking lot of EBR-1 out at the Idaho National Laboratory about an hour's drive west of me. The massive hangar for the nuclear plane is a bit north of there at TAN (Test Area North). The project was cancelled before the runway was built. My dad briefly worked on the project as an intern while he was studying engineering at UCLA, years before he ever thought of moving up here. I believe one of the nuclear jet engines was flown on a B-36 for radiation shielding testing. The project was cancelled as a result of that testing.

[JetPlaneFlyer]

wow. i would have thought we could advance more in 50 years.

Hmmm.. Look how far we have gone in the last 50 years. In 1956 we already had 4 engine jet airliners such as the Boeing 707 flying on engine technology that is virtually identical to today's. All we have done in the last 50 years is tweak turbojet engine design to improve efficiency.
In fact in some respects we have gone backward in the period... 30 years ago you could buy a commercial airline ticket to fly supersonic over the Atlantic.

Steve

[vintage1]

Hmmm.. Look how far we have gone in the last 50 years. In 1956 we already had 4 engine jet airliners such as the Boeing 707 flying on engine technology that is virtually identical to today's. All we have done in the last 50 years is tweak turbojet engine design to improve efficiency.
In fact in some respects we have gone backward in the period... 30 years ago you could buy a commercial airline ticket to fly supersonic over the Atlantic.

Steve

All technology is like that. A basic design represents a breakthrough, after that it is simply adapting it to work better.

The wheel has been around a fair time..

[markschaffin]

On the surface the technology of today might look like that of 1956, but it isn't. Efficiencies have been improved substantially in every area. Structures have become more efficient and lighter, engines more efficient, both cruise and high-lift wing technologies are far more advanced than what was around back then, and don't forget about safety. The basic layout of the aircraft hasn't changed, but that's because that was one of the easiest things to get right in the first place.

[JetPlaneFlyer]

On the surface the technology of today might look like that of 1956, but it isn't. Efficiencies have been improved substantially in every area. Structures have become more efficient and lighter, engines more efficient, both cruise and high-lift wing technologies are far more advanced than what was around back then, and don't forget about safety. The basic layout of the aircraft hasn't changed, but that's because that was one of the easiest things to get right in the first place.

Yes, there have been lots of advances in the details that add up to pretty big differences overall .. However (other than in the electronic systems) the basics are pretty much the same, .. 4 x turbojet power, the engines themselves being largely similar in basic design. Same fuel. Very similar airframe, same basic materials of construction. Overall layout almost identical.. etc. etc.

Compare the advances in the last 50 years to the previous 50 :rolleyes:

I fully appreciate that this is the same way any mature technology progresses.. You could say the same for automobile development. The point I was trying to make is that there is really no reason to expect that there will be a ‘quantum leap’ forward in aviation within the next 50 years, when there has not been any more than detail development the last 50. My guess is that the aircraft of 2059 will look pretty similar to those of today. I do hope I’m wrong though as this makes it all pretty boring.

Steve

[noobflyer92]

i guess we have refined our technology, but no major breakthroughs. I am kinda bored of the tube and wing airliner. Boeing should follow through on a blended wing body or something. More efficient?

[Brandano]

harder to stretch, though

[noobflyer92]

but with fans like stumax and what not, are we coming closer to the technology that propels planes through the sound barrier?

[JetPlaneFlyer]

but with fans like stumax and what not, are we coming closer to the technology that propels planes through the sound barrier?
Nope, not even close and never will be. A jet or a rocket is the only propulsion method that's going to get you anywher close to supersonic with a model. Military unmanned jet and rocket drone/UAV aircraft have cracked the sound barrier and then some, but the are flown by onboard computer or perhaps by video link.. Keeping a supersonic model in visual range from the ground would be impossible.

Steve

[BMatthews]

i guess we have refined our technology, but no major breakthroughs. I am kinda bored of the tube and wing airliner. Boeing should follow through on a blended wing body or something. More efficient?

On that front there may be something to be gained but one reason this idea has not progressed is the need to still be able to taxi the aircraft up to a terminal to let all the nice people out and the other nice people get back on. The spanloader and other unique formats may have some interesting possibilities but they would require massive changes to the airports of the world. Not something that would be accepted easily or without a massive payback in lower aircraft and flying cost. Likely the airlines themselves in connection with the company that was to make such craft would need to fork up the cash for the changes.

Sometimes you just can't get around the practicalities of the real world.

[AndrewBurns]

Another few problems with the blended wing-body design:

- Big complex cabin shape is not as structurally efficient a pressure vessel as a tube.
- A large percentage of the passengers (everyone not flying along the edge of the cabin) will have no view out a window, lots of people like looking out a window and I think a lot of people would be uncomfortable flying if they couldn't even see a window from far away.
- Having people farther away from the lateral center of the aircraft means they experience higher g-forces when rolling. This means the aircraft can't roll into and out of turns as fast and has to take much larger turn radii, which is a problem for what will be increasingly cluttered airspace.

[vintage1]

I'm bored with round wheels. Let's make them square..

Prizes to the first person to make a paddle powered airplane. ;)
Flash steam powered is acceptable..

[BMatthews]

There was a flash steam engine powered ornithopter I saw on the 'net a few years back. Is that oddball enough for ya? Flapping wings ARE the airborne version of paddles after all! ! ! !

:D

[Hughbert]

There's a huge cost associated with proving new technology is safe enough to fly - practically all modern aircraft manufacturers don't have the capitol to make the next 'leap' so they develop slowly. That's why we keep getting more of the same.

When you're talking about a supersonic EDF do you mean the flow is supersonic inside the fan? Even in modern jet engines, the flow isn't usually supersonic until the outlet nozzle because supersonic flow in the compressors/turbines means shockwaves, aerodynamic inefficiencies and generally bad news.

As a side note a BWB is being developed for GA - http://www.wingco.com/genesis_of_aircraft.htm, by NASA http://www.ac2030.de/main_e.html and by Hamburg University - http://www.ac2030.de/main_e.html but neither Airbus nor Boeing have announced plans for one.

[noobflyer92]

I mean whatever source of electric ducted fan propulsion it will take to push an airplane, rc or full scale, past mach without stalling out due to compression.

[JetPlaneFlyer]

I mean whatever source of electric ducted fan propulsion it will take to push an airplane, rc or full scale, past mach without stalling out due to compression.

Aint gonna happen, forget it.. You want to crack the sound barrier with a model than stick a rocket in it or develop a new model size jet engine with an afterburner. Then figure out a way to control the model when it's out of sight.

[Hughbert]

I'm with JetPlaneFlyer on this one - I can't see an EDF, or any other 'driven compressor' ever generating enough thrust to go supersonic without some other source of energy as well.

I wouldn't be too surprised if they made a real aircraft powered by an EDF (several people have made electric-motor powered full-size planes, all of them small and with varying degrees of success) but there's no way it would break the sound barrier.

[noobflyer92]

http://www.youtube.com/watch?v=OqH3QiNzkUk
so a fan unit like this couldn't be scaled large enough to give supersonic thrust?

[JetPlaneFlyer]

so a fan unit like this couldn't be scaled large enough to give supersonic thrust?

In a word.. No.
That model was pretty fast for an EDF, maybe over 150MPH.. but that's still less than one quarter of supersonic speed.

There are at least two fundamental problems:
1. Fans and props don’t work with supersonic airflow. If you approach supersonic with a fan you get an effect called 'stonewall' that is you can add more and more power but flow won’t increase. Even in supersonic jet engines the air has to be made to slow below supersonic before it enters the compressor. I'm pretty sure that even in the power turbine stage of a jet supersonic flow is avoided. Only in the re-heat section (where fitted) can the flow go supersonic, or you may be able to get just about supersonic by exploiting the high pressure developed by a jet and forcing through a converging rear nozzle.
2. Even if you could design a sub sonic multi stage high compression electric turbine with converging outlet to accelerate the air above mach (which would look nothing like today’s EDFs) then the huge power required would mean that the plane was too heavy to fly and the shear bulk off batteries and motor would cause too much drag anyway. The very fastest EDF currently approach 200mph in level flight. In round figures you would need to go about 4 x faster to break the sound barrier.

Here's some physics...
Drag increases in proportion to velocity squared so thrust has also to increase by velocity squared to ballance with drag. So to go four times faster 16 x more thrust would be required..
But it's worse than that.. Because power is equal to force x velocity then power required increase would be 16 x 4 .. So you would need about 64 x more power to go supersonic compared to current fastest EDF. If todays fastest models use 10 Lipo cells then you would need 640 LiPo cells and a motor to match :eek:
But its still worse All of above assumes that the model weight and size is the same as the 200mph model. Clearly with 640 LiPo cells to accomodate it's going to be a much bigger and much MUCH heavier model. Power would have to increase at something over the square of the linear size increase. So if the model was twice as big then you would need at least four times the power, thats 2560 Lipo cells:rolleyes: ... Ain't gonna happen, not with anything resembling current battery and motor technology!

EDF power is great.. It's clean, convenient, quiet. It will fly a jet model at scale speed and sometimes higher... But it cannot and almost certainly never will power a model through the sound barrier.. It's just the wrong technology for that.

Steve

[Cory]

Could you possibly imagine a way to out-do the Thunderscreech at RC model size? http://en.wikipedia.org/wiki/XF-84H_Thunderscreech It wasn't supersonic, even with some thrust from it's turbine exhaust and it used a prop which is more efficient that a fan. Notice in the article how far away it could be heard. That would be a good way to loose RC airfields because of all the complaints! :eek:

[harveyweewax]

Could you possibly imagine a way to out-do the Thunderscreech at RC model size? http://en.wikipedia.org/wiki/XF-84H_Thunderscreech It wasn't supersonic, even with some thrust from it's turbine exhaust and it used a prop which is more efficient that a fan. Notice in the article how far away it could be heard. That would be a good way to loose RC airfields because of all the complaints! :eek:

I would doubt it. I really don't think that you could ever get prop driven aircraft to fly supersonic (other than out of control and in a dive) due to the wave drag effects. I thought the limit for a conventional prop plane was around 450-500 MPH.

However, GE and Pratt & Whitney were both working new designs for turboprops that used swept propellor blades (I believe GE called theirs an "un-ducted fan jet"). IIRC, the whole purpose of the un-ducted fan engine was to increase the fuel economy of the engine. I don't think the goal was ever to push a prop plane supersonic.

The internet is a wonderful thing :) :Here's an article telling all you need to know about prop fans. http://www.airspacemag.com/history-of-flight/prop-fan.html

Regards,

Sven

[Hughbert]

The open-rotor turbofan is an attempt to increase fuel efficiency for commercial & transport aircraft; if/when it comes into service it will probably have a lower operating speed than the high-bypass turbofans now in service, but burn less fuel.

The open-rotor will theoretically have a larger bypass ratio since the open-rotor will affect more air than just in the rotor disk - but the high tip speeds can cause a lot of noise, meaning difficulties with regulations.

[Brandano]

I believe the fastest prop driven plane that was ever built as a production model and is still flying is the TU 95 Bear (http://en.wikipedia.org/wiki/Tupolev_Tu-95). The speed seems transonic (900 km/h) but it's hard to say without knowledge of the altitude at which it's achieved. I expect the prop blades will at least be partially supersonic anyway

[NoMatta]

In regards to the power that might be necessary, a little bit of Googling found this link and the following info regarding power requirements. Based upon my uninformed opinion that a wind tunnel is basically a ground bound ducted fan???

Interesting reading.

Quoting from the site below:
Power requirements
The power required to run a supersonic windtunnel is enormous, of the order of 50 MW per square meter of test section. For this reason most wind tunnels operate intermittently using energy stored in high-pressure tanks. These windtunnels are also called intermittent supersonic blowdown wind tunnels (of which a schematic preview is given below). Another way of achieving the huge power output is with the use of a vacuum storage tank. These tunnels are called indraft supersonic wind tunnels. Other problems operating a supersonic wind tunnel include:
enough supply of dry air
wall interference effects
fast instruments needed for intermittent measurements
UnQuote.

http://www.absoluteastronomy.com/topics/Supersonic_wind_tunnel