Interesting article on U2, but something...

[expat]

I don't understand.

http://www.bbc.com/future/story/20140224-flying-at-the-edge-of-space

"How do you stay alert for 12 hours isolated in a tiny cockpit high above the planet?
The aircraft flies on autopilot for most of the flight and that’s good because the margin at altitude is very narrow between the maximum Mach that the aircraft can fly before it breaks up and the stall speed. It’s about 10 knots."

I can understand the Aircraft being ripped apart (by air pressure)if it exceeds it Maximum Mach, But at that altitude, with barely enough air flowing over the control surfaces to control the plane, How could that be so?

 

[A_Harman]

In the cold temperatures at 70,000 feet, the speed of sound is much lower than it is at sea level. Think of the U2 as a powered glider. It has long, high aspect ratio wings that are very flexible. Any kind of aero-elastic fluttering that is set up by shock waves forming around the wings will cause them to fail. So if the flight Mach number gets too high, and the wings start to flutter, the pilot must slow the aircraft. But if he slows too much, the wings will go into stall because the air density at 70,000 feet is so low that a certain minimum airspeed must be maintained to get enough lift so the airplane can fly.

 

[Reg# 43897]

Amazing...

 

[Astro14]

The mach # is easy to understand - compressibility happens as a function of speed...and the plane is going relatively fast up there...about 0.72 Mach, the cruise speed of a Boeing 737. The air has to get around the plane as the plane flies through it, but the plane is at .72 MN, so, some of that air that flows across (not over) the wing, or moves out to go around the fuselage is having to get pretty close to the speed of sound to do it. So, without having any parts of the plane get supersonic air flow (the plane as a whole is clearly subsonic), you have to stay about .72 MN.

Now, here's where it gets tricky...the Indicated Air Speed is really low at that altitude for a given True Air Speed. True Air Speed is how fast the airplane is moving through the air mass itself (pretty fast in this case, see above), but IAS is a measure of dynamic pressure acting on the airplane. It's measured by little probes on the airplane that measure the difference between static pressure (the undisturbed air pressure) and the dynamic pressure (pressure that happens from the motion of the plane). So, lift is generated by IAS - the pressure of the air acting on the wing...but the air is really, really thin up there, so while it is moving fast, not too many air molecules are hitting the airplane, so not much pressure is being created...so, the airplane is near stall because there isn't enough air for it to get good lift.

At sea level, .72 IMN would give you about 450 Knots of IAS...because that's about how fast your true airspeed is, and the air is nice and thick...lots of molecules hitting the plane and creating lift (and drag and thrust, etc.). But at 70,000 feet, your IAS at that speed will be down around 100 IAS (guessing, I am certain that you can look it up)...so while the plane is moving fast, it is really close to stalling because the air is so thin. It's flying at a moderately high angle of attack, just to generate any lift from those few, spread out, air molecules flowing over it.

As a data point: at 60,000 feet in an F-14, I was flying at 1.5 IMN. 1.5 times the speed of sound, or about 900 Knots true airspeed. But the plane was mushy on the controls because the indicated airspeed was about 300 Knots, or 1/3 of the pressure that you would expect because the air was so thin, and with teh wings tucked back at 68 degrees (for drag), it wasn't making a whole lot of extra lift...now...the U2 can't get anywhere near supersonic (for structural reasons), so it flies along at a point where the minimum IAS is right next to the maximum TAS...it's known as the "coffin corner"...because you can't speed up and you can't slow down...

 

[CourierDriver]

coffin corner,,,aah,,my ex wife stays there....

 

[JetStar]

I worked on the production of the analog F118 engine control for the U-2S.

 

[Reg# 43897]

Thanks for the aeronautics lesson Astro. Always appreciated.

John.

 

[expat]

Thank you.
I guess I have a problem understanding compressibility.
Or, how it can be a problem when there really isn't much [air] to be compressed.

 

[Shannow]

expat,
speed of sound is calculated from

k*R*SQRT(T), where
k is the ratio of specific heats (Cp/CV, where Cp is specific heat at constant pressure, Cv, specific heat at constant volume, and R is the Ideal Gas Constant...T is in Kelvin).

As the temperature drops, the speed of sound drops, so you run into it earlier (speed wise).

k, a dimensionless number, doesn't change a great deal, so A Harman's statement is demonstrated.

Astro14's is yet another case where he can bring his own real life into stuff that the rest of us find in theory...thanks Astro.

 

[A_Harman]

Thanks Astro. Great stuff.

 

[expat]

Sorry Guys, please bare with me.

My understanding problem area is: compressibility in the rarefied atmosphere of 60,000'

I understand that sound will travel faster through a more Dense atmosphere than a Thin one (I can imagine the air molecules close together, bumping each other and forming a wave)

I understand sound NOT traveling through a vacuum for the same reasons.

But when it comes to a moving body (ie. an Aircraft)
I understand that in a dense atmosphere a pressure wave will build up at the leading edge (like a Bow wave on a Boat) And, at a certain speed (Mach1) that wave will be compressed to a Shock wave.

But where I get things wrong is:
To my mind,
In a rarefied atmosphere (60,000')a moving body would have to travel FASTER (than it would need to at Sea level. in terms of MPH) to compress what little air there is, before a shock wave would be created.

But that appears NOT to be.
Mach1 is attained at a HIGHER speed at Sea level than at high altitude

Help!

 

[Shannow]

expat,

Consider the the shockwave is sort of like an extreme doppler shift, when the vehicle reaches a speed that all of the soundwaves are hit at the same time, it's not the number of atoms (density), but the speed at which sound propogates through the medium.

Sound travels adiabatically, and doing the calculus gets the equation that I posted...for an ideal gas, the Cp/Cv ratio doesn't change a lot with density or pressure, meaning that the speed of sound is related predominantly to temperature.

Don't know if it helps any, but if you have for example an air reciever, and drill a hole in it, you can have "choked flow" where a shockwave is produced which limits the leakage through the hole to the sonic velocity x cross sectional area, regardless of the upstream and downstream pressure...doesn't matter whether the pressure is 30psi, or 300psi, the same amount of air will come out of the hole anywhere from sea level to space...as long as temperature is constant...

 

[Astro14]

What Shannow is saying is the tricky part, where physics doesn't follow intuition: The speed of sound isn't dependent on density.

Sound (compression/rarefaction) travels through a given medium at, pretty much, a given speed. That's quite true in air.

So, it doesn't really matter if a billion molecules of air are hitting the airplane every second, or only a million, those molecules have an equally difficult time getting out of the way, and shock fronts form, at the same speed in either case.

So our intrepid U2 is feeling the effects of those air molecules getting compressed despite the thinner air up there at 70,000 feet...

 

[expat]

Thank you

Now I get it.


I just had this image stuck in my mind that Mach 1 would be faster, the thinner the atmosphere became. right up till, when you would be traveling in (almost) Vacuum, and Mach1 would be at (almost) infinite speed.

So where does the transition take place?
Obviously there is no speed of sound in vacuum. But 60,000' must be pretty darn close.

When I was a kid my father took me to the Science Museum in London. One interactive display had an Electric bell in a glass Bell Jar. A push button would cause the bell to ring, another button would activate a vacuum pump that would evacuate air from the Bell Jar.
It was interesting to hear the bell fade to near silence as the air was evacuated from the Jar.

I guess to my mind it was easy to imagine the sound waves were just getting 'slower'

But that (I think) would have caused a Doppler shift. Which did not happen.

 

[Astro14]

As far as transition, I couldn't tell you. For vehicles transitioning into space (rockets, space shuttles, etc.), the pressure on the vehicle is key, and is a function of both Mach number and Indicated Airspeed. I am not a rocket guy, but the pressure is referred to as "Q", and it peaks somewhere in the Stratosphere. From there, the mach number is increasing, but the pressure is decreasing with the faster decrease in density...

Interestingly, there are shock fronts in interstellar space. They're found in gas clouds and are thought to play a role in stellar formation. Even though those gas clouds would qualify as a vacuum here on Earth...

As far as the ringing bell - what a great hands-on exhibit, I love those kinds of things (like the two-story vacuum tube in which feathers fall as fast as a metal ingot...an image that will forever remain with me). The decline in volume was simply a decline in amplitude, not speed, as the air got less dense...

 

[Astro14]

One other note, as hinted in the article, 50,000 feet is pretty high in an airplane, and a pressure loss at that altitude can have some serious consequences. So, 50,000 feet was both the flight manual restriction in the F-14 and the OPNAV (overall regulatory guidance) limit on flight without a pressure suit...but 50,000 feet clearly wasn't an airframe/powerplant limit...and the sky above you is awfully dark up there at 60,000. I imagine that at 70,000, it does feel like you're in space...

 

[expat]

I remember something about wing tip thrusters on an experimental, high altitude aircraft (X15?), as at peak altitude, there would not be enough air flowing over the wing to maintain stability.

There were also incidents of 'Pinwheeling' (inertia Coupling?)
due to lack of airflow over the control surfaces.

I think I'll keep my flying to a maximum 35,000'

 

[Shannow]

I love the bell in the jar...and that it didn't change in pitch, meaning that my daughter's flute is in tune up here in the Mountains (well our excuse for mountains, 93% of sea level air pressure), as it is in the Sydney Opera House.

(put her in a room of helium, and it's different)...I think

 

[expat]

Originally Posted By: Shannow
I love the bell in the jar...and that it didn't change in pitch, meaning that my daughter's flute is in tune up here in the Mountains (well our excuse for mountains, 93% of sea level air pressure), as it is in the Sydney Opera House.

(put her in a room of helium, and it's different)...I think


Well i don't remember it changing pitch, but that was not really the point of the experiment.
Besides, I'm pretty Tone Deaf.

 

[john_pifer]

Originally Posted By: expat
I remember something about wing tip thrusters on an experimental, high altitude aircraft (X15?), as at peak altitude, there would not be enough air flowing over the wing to maintain stability.

There were also incidents of 'Pinwheeling' (inertia Coupling?)
due to lack of airflow over the control surfaces.

I think I'll keep my flying to a maximum 35,000'

Yep, the X-15 had them, as did the NF-104 like Chuck Yeager flew in the altitude record attempt, as depicted in the movie The Right Stuff.