Coffin Corner
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Coffin Corner
Have had this mind boggling question on my mind for months, and I can't seem to get to the bottom of it.
Coffin corner comes about at high levels as Vstall approaches/meets MMO. My understanding is that as the aircraft climbs, MMO translates itself into a lesser IAS value since the temperature gets colder and therefore the airspeed value of Mach 1 itself decreases. This explains the high speed limitation.
I also understand that as the aircraft climbs, it needs a higher true airspeed to sustain lift. Therefore, stall speed in TAS increases.
What i'm not getting is that whilst stall speed in TAS increases, in straight accelerated flight, stall speed in IAS should remain the same provided that the aircraft maintains the same configuration. Therefore, why does the low speed limit on the ASI (which indicates IAS!) increase at high levels?
Any ideas?
Coffin corner comes about at high levels as Vstall approaches/meets MMO. My understanding is that as the aircraft climbs, MMO translates itself into a lesser IAS value since the temperature gets colder and therefore the airspeed value of Mach 1 itself decreases. This explains the high speed limitation.
I also understand that as the aircraft climbs, it needs a higher true airspeed to sustain lift. Therefore, stall speed in TAS increases.
What i'm not getting is that whilst stall speed in TAS increases, in straight accelerated flight, stall speed in IAS should remain the same provided that the aircraft maintains the same configuration. Therefore, why does the low speed limit on the ASI (which indicates IAS!) increase at high levels?
Any ideas?
A C Kermode:
"People are often surprised to hear that the speed of sound depends on temperature alone. As a matter of fact, it doesn't! But the other properties, such as density, on which it also depends, are so related that the temperature is the controlling factor."
TAS = 39M x Sqrt (OAT C +273.15)
"People are often surprised to hear that the speed of sound depends on temperature alone. As a matter of fact, it doesn't! But the other properties, such as density, on which it also depends, are so related that the temperature is the controlling factor."
TAS = 39M x Sqrt (OAT C +273.15)
gearlever is pointing out that the IAS/MN relationship is independant of temperature - it is dependant purely on Flight level.
The OP's comment "MMO translates itself into a lesser IAS value since the temperature gets colder and therefore the airspeed value of Mach 1 itself decreases." is incorrect.
The OP's comment "MMO translates itself into a lesser IAS value since the temperature gets colder and therefore the airspeed value of Mach 1 itself decreases." is incorrect.
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But at the high altitudes and speeds that jets fly at, the effects of Mach and Reynolds numbers start being significant enough to lower critical AOA and increase IAS stall speed noticeably.
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The Following is taken from a Naval Aviation Student guide and quotes from Aerodynamics for Naval Aviators:
STALL SPEED
As angle of attack increases, up to CLmax AOA, true airspeed decreases in level flight. SinceCL decreases beyond CLmax AOA, true airspeed cannot be decreased any further. Therefore the minimum airspeed required for level flight occurs at CLmax AOA. Stall speed (VS) is the minimum true airspeed required to maintain level flight at CLmax AOA. Although the stall speed
may vary, the stalling AOA remains constant for a given airfoil. Since lift and weight are equal in equilibrium flight, weight (W) can be substituted for lift (L) in the lift equation. By solving for velocity (V), we derive a basic equation for stall speed. max
By substituting the stall speed equation into the true airspeed equation and solving for indicated airspeed, we derive the equation for the indicated stall speed (IASS). Weight, altitude, power, maneuvering, and configuration greatly affect an airplane’s stall speed. Maneuvering will increase stall speed, but will not be discussed until the lesson that deals with turning flight.
As airplane weight decreases stall speed decreases because the amount of lift required to maintain level flight decreases. When an airplane burns fuel or drops ordnance, stall speeds decrease. Carrier pilots often dump fuel before shipboard landings in order to reduce stall speed and approach speed.
A comparison of two identical airplanes at different altitudes illustrates the effect of altitude on stall speed. The airplane at a higher altitude encounters fewer air molecules. In order to create sufficient dynamic pressure to produce the required lift, it must fly at a higher velocity
(TAS). Therefore, an increase in altitude will increase stall speed. Since ρ0 is constant, indicated stall speed will not change as altitude changes.
The stall speed discussed up to this point assumes that aircraft engines are at idle, and is called power-off stall speed. Power-on stall speed will be less than power-off stall speed because at high pitch attitudes, part of the weight of the airplane is actually being supported by the vertical component of the thrust vector. For propeller driven airplanes the portion of the wing immediately behind the propeller produces more lift because the air is being accelerated by the propeller. Power-on stall speed in the T-34C is approximately 9 knots less than poweroff stall speed.
http://www.netc.navy.mil/nascweb/api...April_2008.pdf
Sorry the formulas do not translate into this post window.
STALL SPEED
As angle of attack increases, up to CLmax AOA, true airspeed decreases in level flight. SinceCL decreases beyond CLmax AOA, true airspeed cannot be decreased any further. Therefore the minimum airspeed required for level flight occurs at CLmax AOA. Stall speed (VS) is the minimum true airspeed required to maintain level flight at CLmax AOA. Although the stall speed
may vary, the stalling AOA remains constant for a given airfoil. Since lift and weight are equal in equilibrium flight, weight (W) can be substituted for lift (L) in the lift equation. By solving for velocity (V), we derive a basic equation for stall speed. max
By substituting the stall speed equation into the true airspeed equation and solving for indicated airspeed, we derive the equation for the indicated stall speed (IASS). Weight, altitude, power, maneuvering, and configuration greatly affect an airplane’s stall speed. Maneuvering will increase stall speed, but will not be discussed until the lesson that deals with turning flight.
As airplane weight decreases stall speed decreases because the amount of lift required to maintain level flight decreases. When an airplane burns fuel or drops ordnance, stall speeds decrease. Carrier pilots often dump fuel before shipboard landings in order to reduce stall speed and approach speed.
A comparison of two identical airplanes at different altitudes illustrates the effect of altitude on stall speed. The airplane at a higher altitude encounters fewer air molecules. In order to create sufficient dynamic pressure to produce the required lift, it must fly at a higher velocity
(TAS). Therefore, an increase in altitude will increase stall speed. Since ρ0 is constant, indicated stall speed will not change as altitude changes.
The stall speed discussed up to this point assumes that aircraft engines are at idle, and is called power-off stall speed. Power-on stall speed will be less than power-off stall speed because at high pitch attitudes, part of the weight of the airplane is actually being supported by the vertical component of the thrust vector. For propeller driven airplanes the portion of the wing immediately behind the propeller produces more lift because the air is being accelerated by the propeller. Power-on stall speed in the T-34C is approximately 9 knots less than poweroff stall speed.
http://www.netc.navy.mil/nascweb/api...April_2008.pdf
Sorry the formulas do not translate into this post window.
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The stall AOA decreases with very high altitude, and the IAS stall speed increases.
Altitude-IAS graph of U2
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Ok I see that the U2's stall speed increases by some 5 knots when comparing sea level to FL750.
Does this still explain the 50 knots+ increase in the low speed boundary that a B737 experiences between low and high levels? Considering such a big discrepancy, I feel that I am still missing something!
Does this still explain the 50 knots+ increase in the low speed boundary that a B737 experiences between low and high levels? Considering such a big discrepancy, I feel that I am still missing something!
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Min speed at high altitude is not necessarily defined by stall, but also by low speed buffet onset - i.e. at high Mach number, a decrease of speed and resultant AoA increase produces Mach buffet similar to that at high speed.
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Here are some numbers. A few days ago above Paris at FL400 our Vma was 206 kts and Vmo was the equivalent of 249 kts. Had we been heavier the Vma would have been greater and higher the Vmo would have been less. At sea level, the values would have been 140 or so and 320. For us, the change between Mach and Vno typically occurs just below FL290. Given enough poke you can get to where Vma is greater than Vmo. Possibly interesting to go there but without passengers and a good supply of clean underpants.
Psychophysiological entity
Decades ago, the calculation for the Speed of Sound 'proved' that the speed of sound was proportional to the temperature absolute. It was the only equation simple pilots had to derive in the entire process of becoming a pilot.
Originally Posted by Needles Crossed
Does this still explain the 50 knots+ increase in the low speed boundary that a B737 experiences between low and high levels? Considering such a big discrepancy, I feel that I am still missing something!
At 4,000ft, Vmin was 193, V Sticker Shaker was 160.
FL330 Vmin (bottom foot) was 235. V Sticker Shaker was 183.
So my sticker shaker speed went from 160 to 183.
The difference in the actual stall speeds would be even less.
As to why, I found this, which seems to be plausible (could be a lie told to children!):
From here: https://www.quora.com/Why-does-stall...tude-increases
Answer from Magnar Nordal, half way down the page:
The Mach number is important, because it defines how the air will flow around the leading edge of the wing, and especially how it will behave in front of the wing.
On the figure can you see that the airflow ahead of the wing is starting to split before it reaches the wing. At low Mach numbers is the distance from the point where the air starts to slip and to the wing’s leading edge relatively long.
At higher Mach numbers is the distance shorter. (At Mach 1.0 is the distance zero.) This results in a higher angle of attack, because the airflow has less time to split and flow around the leading edge of the wing. Indicated stall speed is therefore higher.
From here: https://www.quora.com/Why-does-stall...tude-increases
Answer from Magnar Nordal, half way down the page.
When I was a very junior copilot on VC10s, the training captain, on a high altitude training detail, took the aircraft up to a little over 46,000ft to demonstrate the high and low speed buffets and he let me try it too. As I remember it, the high speed Mach buffet was of a slightly higher frequency and lower amplitude than the low speed buffet. There were only a couple of knots between the two, and gentle stick pressure either way induced one or the other. We didn't press it far into the buffet regimes so it was fairly mild. But one knew one was very near the edge of the envelope.
I cannot vouch for other types.
I cannot vouch for other types.
