Mach Crit for transport aircraft
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Hmm! I'm glad I don't have a twisted mind like you, good Dr!
Thing us a thong
Does your friend have any similar friends..........?
..........and just to clarify - are we to be less acquainted with lamda feet or your friend's.................?
Thing us a thong
Does your friend have any similar friends..........?
..........and just to clarify - are we to be less acquainted with lamda feet or your friend's.................?
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I can echo Thunderball's experience of having seen a shockwave dancing on a 737 wing en-route from LAX to Las Vegas many years ago.
The wing was up-sun, and I was perched (as usual) right over the point of maximum camber with little to look at but metal. The key seemed to be the polaroid sunglasses I was wearing, because these produced the image of a shock wave about 3 feet away, as clear as any Schlieren picture that I ever produced in a lab (BOAC will vouch for that). Took off the glasses - gone - put them back on again for the show of a lifetime.
My old papers (!) reveal that I subsequently asked the crew the cruise Mach No. - it was 0.76. When a light chop was encountered, the shock wave moved fore and aft by an inch or so, co-ordinated exactly with the "g" of the chop. When the chop turned more severe the aircraft decelerated (Rough Air Speed on a 737-200?) and the shock simultaneously reduced in intensity, slid forward towards the leading edge and disappeared.
I hope I successfully hid my excitement but, belatedly, thank you Western Airlines.
The wing was up-sun, and I was perched (as usual) right over the point of maximum camber with little to look at but metal. The key seemed to be the polaroid sunglasses I was wearing, because these produced the image of a shock wave about 3 feet away, as clear as any Schlieren picture that I ever produced in a lab (BOAC will vouch for that). Took off the glasses - gone - put them back on again for the show of a lifetime.
My old papers (!) reveal that I subsequently asked the crew the cruise Mach No. - it was 0.76. When a light chop was encountered, the shock wave moved fore and aft by an inch or so, co-ordinated exactly with the "g" of the chop. When the chop turned more severe the aircraft decelerated (Rough Air Speed on a 737-200?) and the shock simultaneously reduced in intensity, slid forward towards the leading edge and disappeared.
I hope I successfully hid my excitement but, belatedly, thank you Western Airlines.
Last edited by D120A; 23rd Mar 2003 at 23:19.
Thunderball!!
I, too have seen what you have seen. I was sitting in window seat right neat the no.1 eng of a UAL DC10. It appeared over the engine, and I would best describe it as having the appearance of a very hot jet of air blowing in a straight line (as we were moving about M0.8x, obvioulsy any air escaping would've had a curved appearance). The "jet" would have only been around 5 mm in diameter. Truly amazing stuff to see! I have not seen it since.
ODL
P.S. Very different to the enveloping cloud of mist seen in many photos, as it was the very first are of supersonic flow {Mcrit}. Therefore I can only assume that Mcrit on a DC10 is below crusiing speed.
I, too have seen what you have seen. I was sitting in window seat right neat the no.1 eng of a UAL DC10. It appeared over the engine, and I would best describe it as having the appearance of a very hot jet of air blowing in a straight line (as we were moving about M0.8x, obvioulsy any air escaping would've had a curved appearance). The "jet" would have only been around 5 mm in diameter. Truly amazing stuff to see! I have not seen it since.
ODL
P.S. Very different to the enveloping cloud of mist seen in many photos, as it was the very first are of supersonic flow {Mcrit}. Therefore I can only assume that Mcrit on a DC10 is below crusiing speed.
Thunderball,
In conditions of high humidity but where condensation into water droplets has not taken place in free air, any small drop in pressure will cause local cooling below dewpoint and visible condensation will take place.
You can see the resulting water droplets in the wingtip vortices, flap edge vortices, and spanwise in the area of reduced pressure above the wings.
I wonder if this is what you saw?
ps: what was the date of your flight Ovda - Gatwick?
pps: Inadvertently replied before readind p2 of postings. All v interesting esp observation by D120A.
In conditions of high humidity but where condensation into water droplets has not taken place in free air, any small drop in pressure will cause local cooling below dewpoint and visible condensation will take place.
You can see the resulting water droplets in the wingtip vortices, flap edge vortices, and spanwise in the area of reduced pressure above the wings.
I wonder if this is what you saw?
ps: what was the date of your flight Ovda - Gatwick?
pps: Inadvertently replied before readind p2 of postings. All v interesting esp observation by D120A.
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Basil. I agree with you! I have seen the same halo of condensation around a Buccaneer in flight and that was definately not at Mcrit.
Remember that the point of minimum pressure will also condense the water vapour under the crrrect conditions, and although that is where the shock waves will eventually form, they have not necessarily formed just because there is condensation.
Remember that the point of minimum pressure will also condense the water vapour under the crrrect conditions, and although that is where the shock waves will eventually form, they have not necessarily formed just because there is condensation.
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Basil, OneDotLow,
Thanks. The date of the KT B757 flight from Ovda to LGW, by the way, was Monday 22nd March, 1993. How do I remember that? Don't ask.
But I must emphasize that what I saw on that B757 was very clearly a refraction phenomenon, related to a dramatic and highly-localised change of air density (presumably), rather than any hint of condensation. As I mentioned before, it looked almost like a transparent plastic washing line stretched between the tip the root of the wing which constantly moved chord-wise, forward and aft, but only a few inches.
I'd had one small bottle of red wine, maybe two at the most.
Thanks. The date of the KT B757 flight from Ovda to LGW, by the way, was Monday 22nd March, 1993. How do I remember that? Don't ask.
But I must emphasize that what I saw on that B757 was very clearly a refraction phenomenon, related to a dramatic and highly-localised change of air density (presumably), rather than any hint of condensation. As I mentioned before, it looked almost like a transparent plastic washing line stretched between the tip the root of the wing which constantly moved chord-wise, forward and aft, but only a few inches.
I'd had one small bottle of red wine, maybe two at the most.
Ok guys! The crew rest was U/S the other day so i went to sit down in C Zone (B744). I was sitting on the right hand side watching a movie ("The Hot Chick"....dont bother!), when i decided to open the shade and take a look outside. As we were in light chop, the wing was bending and I could see the refraction (appearance as i described in my post above as "hot jet of air" like apperance) moving inboard then outboard with the bending of the wing.
I had not had any bottles of wine...
Hope you all get to see it... truly amazing.
ODL
I had not had any bottles of wine...
Hope you all get to see it... truly amazing.
ODL
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Just for interest. I eventually got an answer from Boeing.
Critical Mach number is a term aerodynamicists use to quickly compare the "drag rise" or "compressibility drag" of several wing design alternatives. At Boeing, we define critical Mach number as the Mach number, at a constant lift coefficient, where the drag coefficient is 20 counts greater than the drag coefficient at the incompressible Mach number.
Typically, a wing does not have a critical Mach number. Rather, critical Mach number of a wing varies with lift coefficient. Generally, as lift coefficient increases, critical mach number decreases.
Wing designers can change critical Mach number by changing wing sweep, airfoil thickness or airfoil technology.
There are two other speeds that characterize the integrated airplane (as opposed to just the wing). These speeds are "Max Range Cruise speed" and "Long Range Cruise speed".
They continue on about the calculations for Max range cruise speed and LRC speed. Basically they are more worried about MCDR as that affects their economy, and as you see they refer to that as “critical Mach number” As they allow a margin of safety when they calculate their Max range cruise speed, which is calculated at a given weight and therefore lift co-efficient, I would estimate the “aerodynamic” MCRIT to be in the vicinity of M 0.80 under the same conditions on a B737.
[
Critical Mach number is a term aerodynamicists use to quickly compare the "drag rise" or "compressibility drag" of several wing design alternatives. At Boeing, we define critical Mach number as the Mach number, at a constant lift coefficient, where the drag coefficient is 20 counts greater than the drag coefficient at the incompressible Mach number.
Typically, a wing does not have a critical Mach number. Rather, critical Mach number of a wing varies with lift coefficient. Generally, as lift coefficient increases, critical mach number decreases.
Wing designers can change critical Mach number by changing wing sweep, airfoil thickness or airfoil technology.
There are two other speeds that characterize the integrated airplane (as opposed to just the wing). These speeds are "Max Range Cruise speed" and "Long Range Cruise speed".
They continue on about the calculations for Max range cruise speed and LRC speed. Basically they are more worried about MCDR as that affects their economy, and as you see they refer to that as “critical Mach number” As they allow a margin of safety when they calculate their Max range cruise speed, which is calculated at a given weight and therefore lift co-efficient, I would estimate the “aerodynamic” MCRIT to be in the vicinity of M 0.80 under the same conditions on a B737.
[
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In my 737 experience, up to 0.78M cruise the overall fuel consumption for a trip varies little (due to the lower deck angle at higher speeds giving lower form drag) but above 0.78M it starts to 'guzzle'!
