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UAL747
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Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 2:27 am

Mach 1 is 661.7 knots...

Why does aircraft design have to change so drastically from flying at Mach .86 to only go up .14 mach? It's like flying becomes a totally different being.

I'm seriously no physics expert, and would like to know.

Thnks.

UAL
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legoguy
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RE: Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 2:47 am

I believe the mach number varies with temperature. The higher you go, the colder it gets and the lower the velocity required to achieve the speed of sound. An aircraft can go Mach 1 at sea level around 761.2 mph whilst can achieve mach 1 at 11000m when travelling at 654.6 mph (data from Wikipedia).

Secondly, an aircraft approaching the speed of sound will experience shock waves associated with mach 1. I'm not an expert though and now I'm starting to realise I shouldn't have skipped half my aerodynamics classes this year! I'll hand over to someone with more knowledge.
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comorin
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RE: Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 3:54 am

There are far more qualified experts on this forum, but I'll give you a simple explanation:

Mach 1 is the speed at which a disturbance travels in air. It varies with the density of air, so Mach 1 at sea level is faster than Mach 1 at flight levels.

Now when a plane is flying , the air in front of the wing 'knows' the wing is coming as the motion is transmitted forward at the speed of sound. In the water, if you look at the front of a boat, you can see this bow wave. This helps with the smooth flow of air around the airfoil.

If you start approaching Mach 1, the wing is traveling just as fast as the disturbance in front, so the air becomes 'harder' - shock wave is formed. This affects airflow characteristics, so a subsonic wing is not optimal once you go transonic past Mach 1.

Drag also increases at supersonic speed because the air is no longer slipping by you; instead you are dragging along a shock wave beyond its natural speed.

I hope this is helpful from an intuitive sense - it's been a while - others please correct me if I'm wrong .
 
mpdpilot
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RE: Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 3:56 am

Legoguy was very much on the right track. To expand a little further:

airplanes actually start to create shock waves for traveling near the speed of sound at around .70MACH. This number can vary but that is about where it is. This occurs because the air moving over the top of the wing moves faster than the air under the wing creating that difference in pressure that is called life. The increase in speed on the top of the wing approaches 1MACH at a true airspeed around .70MACH. This is referred to as the Critical Mach Number. Exceeding this speed requires much more thrust because of the increased drag from the shock waves that are forming. One might say that airliners typically cruise in the .80MACH range so why. Well one can increase the Critical Mach Number by sweeping the wings. By sweeping the wings the actual airspeed of the relative wind perpendicular to the wing's leading edge drops to around .70MACH. This is why the most efficient speed is in that neighborhood of .80MACH. Airplanes with a more swept wings typically cruise faster. Basically flying transonic range (.80MACH-1.20MACH) is the speed that requires the most power. Getting through that range the drag falls off again as you move into the supersonic range. Just to clarify it doesn't fall back to subsonic efficiency but it does subside some.
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tdscanuck
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RE: Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 4:25 am



Quoting UAL747 (Thread starter):
Why does aircraft design have to change so drastically from flying at Mach .86 to only go up .14 mach? It's like flying becomes a totally different being.

I'm seriously no physics expert, and would like to know.

The stuff above is all correct. It's important to note why shockwave formation actually causes drag. There are two big reasons:
1 - The shockwave is not isentropic...that is, entropy increases through the shockwave. This shows up as a drop in total pressure, which is basically a measure of the air's ability to do work. That's energy that's expended inside the shockwave, and that energy has to come from the thing generating the shock. Bigger energy loss = more drag.

2 - Shockwaves screw up boundary layers. For the lowest drag, you want laminar flow but that's not practical in most commercial jet applications except a couple of special cases (some noses, nacelles, etc.). Next best is turbulent flow. If the boundary layer comes right off the body then you have separated flow, which is horrendously draggy because you end up with big pressure differentials going the wrong way. Shockwaves tend to cause boundary layers to separate, so they create separation where it didn't exist before, or make it worse.

Tom.
 
gregarious119
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RE: Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 4:45 am

I think my head just exploded...




From a shockwave no less.


 cool 
 
DfwRevolution
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RE: Why Is 661.7kts The Magic Number?

Sun Dec 14, 2008 5:04 am



Quoting UAL747 (Thread starter):
Mach 1 is 661.7 knots...

In a certain fluid at a certain temperature.

Quoting UAL747 (Thread starter):
Why does aircraft design have to change so drastically from flying at Mach .86 to only go up .14 mach? It's like flying becomes a totally different being.

I'm seriously no physics expert, and would like to know.

Don't feel too bad if the above answers went over your head. The question you are asking is toward the upper-end of undergraduate physics or engineering. If it helps you grasp the concept, fluid flow behaves in distinct "modes" between subsonic and sonic velocities. The change over just 0.14 Mach is extreme, but so is the difference in state of water at 33 degrees and water at 32 degrees. Just like the transition of water from liquid to solid, there is a dramatic and abrupt change in the boundary layer of air (or any fluid) between subsonic and supersonic flow.
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cptspeaking
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RE: Why Is 661.7kts The Magic Number?

Thu Dec 18, 2008 4:28 pm

A couple different reasons...

Airflow over the wings is a big one, but also the airflow into the engines.

Turbine engines don't do well with supersonic air. As such, supersonic jets have to have a way of reducing the velocity of the intake air into the engines to subsonic velocities in order to be able to keep running. Several military jets do this by having a moving "flap" at the front of the engine that changes the direction, amount, and speed of the airflow entering the nacelle.

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wingscrubber
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RE: Why Is 661.7kts The Magic Number?

Fri Dec 19, 2008 1:22 am

cptspeaking is right, all jet powered aircraft designed to cruise above Mach 1 usually have a variable geometry intake in the form of a moving ramp or cone which creates a shockwave to 'slow' the air. (its not quite that simple, but basically...) a traditional air breathing turbojet can only process subsonic air, hence the need to slow it. Engines such as that used on the SR-71 get into ramjet territory, and then beyond those are the pure ramjets and scramjets.
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rwessel
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RE: Why Is 661.7kts The Magic Number?

Fri Dec 19, 2008 2:15 am



Quoting Wingscrubber (Reply 8):
cptspeaking is right, all jet powered aircraft designed to cruise above Mach 1 usually have a variable geometry intake in the form of a moving ramp or cone which creates a shockwave to 'slow' the air. (its not quite that simple, but basically...) a traditional air breathing turbojet can only process subsonic air, hence the need to slow it. Engines such as that used on the SR-71 get into ramjet territory, and then beyond those are the pure ramjets and scramjets.

That would seem to indicate a problem with the F-22...  Wink

But the need for a variable geometry inlet varies with speed. The SR-71, for example, needs to carefully manage the shockwave to get maximum recovery without choking off the inlet. For slower aircraft, like the F-16 and F-22, a simple fixed inlet, which generates the shockwave slowing down the incoming air in a fixed position, is usually good enough, but you're basically optimizing the inlet for a particular speed. I'd expect any civilian SST that stayed below about M1.8 to also have a fixed inlet optimized for the nominal cruise speed.

But the basic point is correct: there *is* an inlet system to slow the airflow down, and you can’t just expose the front fan or compressor to the airflow. It’s just not always a *variable* inlet.

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