Mach Number on Stall-Speed
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From: 3rd Rock, #29B
the simplified answer has mainly been... CP shift but that is only partially true and does not move in the right direction to what mach tuck is classically described as.
At MCD for the airfoil, flow over the wing achieves above mach 1 and as the flow at the trailing edge of the wing is usually the second Kutta condition, a stagnant point, then the flow must decelerate, depending on the velocity profile that the profile of the section results in. Shortly thereafter, or sometime quite a lot later, MDD occurs where there is a drag divergence condition that is significant in drag. Looking at the pitching moment, Cm, there is not a great change in moment, and that is curious, we are taught otherwise... but... hang on... what happens when you continue to accelerate and push the airfoil along faster? at a point at higher speeds, due to a relatively low AOA to maintain level flight conditions (or 1g... whatever floats yer boat), shocks start to develop on the underside of the wing, (surprise!!!) and that results in a collapse in CL, an increase in Cd, and, yet not much Cm change. However from the little boys room at the pointy end, stuff happens, as the loss of CL for the AOA changes the trim state, and the aircraft now has a flight path that will diverge from desired for the static trim case. take out lift and the flight path will degrade quickly. what happened to the Cm, not much, what about the stab inflow change? not much change, but the nose will drop quite happily. The concern in a severe overspeed is that the condition is divergent, lower pitch attitude results in greater shock involvement, and loss of CL component.
Whenever I have flown Mdive I do not enter with anything other than a high power setting, to allow an opportunity to decelerate by thrust reduction. In this case the Cm has not caused the nose gown pitch, the rapid loss of CL component has caused the steepening pitch attitude. Speed brakes mostly will cause a pitch up, and will alleviate the lower shock condition, except on a Learjet, where that will open up your eyes promptly. In the end, it is just a wing and a moment arm from the AC, if you have a variable incidence tailplane. High mach overspeed with a fixed horizontal stab/elevator are quite a lot more sporty.
At MCD for the airfoil, flow over the wing achieves above mach 1 and as the flow at the trailing edge of the wing is usually the second Kutta condition, a stagnant point, then the flow must decelerate, depending on the velocity profile that the profile of the section results in. Shortly thereafter, or sometime quite a lot later, MDD occurs where there is a drag divergence condition that is significant in drag. Looking at the pitching moment, Cm, there is not a great change in moment, and that is curious, we are taught otherwise... but... hang on... what happens when you continue to accelerate and push the airfoil along faster? at a point at higher speeds, due to a relatively low AOA to maintain level flight conditions (or 1g... whatever floats yer boat), shocks start to develop on the underside of the wing, (surprise!!!) and that results in a collapse in CL, an increase in Cd, and, yet not much Cm change. However from the little boys room at the pointy end, stuff happens, as the loss of CL for the AOA changes the trim state, and the aircraft now has a flight path that will diverge from desired for the static trim case. take out lift and the flight path will degrade quickly. what happened to the Cm, not much, what about the stab inflow change? not much change, but the nose will drop quite happily. The concern in a severe overspeed is that the condition is divergent, lower pitch attitude results in greater shock involvement, and loss of CL component.
Whenever I have flown Mdive I do not enter with anything other than a high power setting, to allow an opportunity to decelerate by thrust reduction. In this case the Cm has not caused the nose gown pitch, the rapid loss of CL component has caused the steepening pitch attitude. Speed brakes mostly will cause a pitch up, and will alleviate the lower shock condition, except on a Learjet, where that will open up your eyes promptly. In the end, it is just a wing and a moment arm from the AC, if you have a variable incidence tailplane. High mach overspeed with a fixed horizontal stab/elevator are quite a lot more sporty.
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From: Cyprus
Mach Number on Stall- Speed
FDR: " Little boys in the pointy end"-? Ouch. Spoiled an otherwise valuable post.
I am unaware of any heavy-jet transports falling out of the sky from coffin corner because the little boys in the pointy end didn't know what to do.
Most of us are bright enough, educated enough, trained enough to face any uncomfortable situation with a simple-"I'm outa here !".
I also, as a real little boy, enjoying a black & white movie about tests flying or space-re-entrystuff where the pilot, entering a dive, speed increasing, instead of ejecting as ordered, pushed the nose forward. "No,no" me & my mates yelled but were silenced as the speed dropped and Hollywood ace saved the aircraft and the day.
Of the six of us, three wound up Senior Commanders of very heavy transport jets.
I am unaware of any heavy-jet transports falling out of the sky from coffin corner because the little boys in the pointy end didn't know what to do.
Most of us are bright enough, educated enough, trained enough to face any uncomfortable situation with a simple-"I'm outa here !".
I also, as a real little boy, enjoying a black & white movie about tests flying or space-re-entrystuff where the pilot, entering a dive, speed increasing, instead of ejecting as ordered, pushed the nose forward. "No,no" me & my mates yelled but were silenced as the speed dropped and Hollywood ace saved the aircraft and the day.
Of the six of us, three wound up Senior Commanders of very heavy transport jets.
Thread Starter
Joined: Oct 2023
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From: NY & LA
Firstly, I apologize for the enormous amount of time that's elapsed between my replies (I had a bunch of stuff going on).
Can I contact the admin/moderators and ask them to place this into the tech-log?
BTW: I put it in the military aviation forum because I figured this area of flight (high subsonic and above) would be an area that the military aviation folks were most familiar with.
Moved from Mil Forum
Senior Pilot
Can I contact the admin/moderators and ask them to place this into the tech-log?
BTW: I put it in the military aviation forum because I figured this area of flight (high subsonic and above) would be an area that the military aviation folks were most familiar with.
Moved from Mil Forum
Senior Pilot
Thread Starter
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From: NY & LA
Senior Pilot Thank you
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From: Where the Quaboag River flows, USA
To give a simple answer and in the subsonic region, the wing stall at a given AoA which equates at 1G to a specific equivalent airspeed (EAS). At low Mach numbers, that is low altitude, IAS will be equal to the EAS. As we climb, that EAS remains, but compressibility requires a higher INDICATED airspeed to Produce that EQUIVALENT airspeed over the wing. As the pilot, we only see IAS, in knots, but because we need more IAS to have the same EAS at stall, it appears the stall has increased. Other than an E6B, I don’t have a rule of thumb other than….KEEP THE MACH UP.
My E6B says if your wing stalls at 145KEAS at F410, you’ll see 150 KIAS at stall.
My E6B says if your wing stalls at 145KEAS at F410, you’ll see 150 KIAS at stall.
Fleet Manager
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From: various places .....
Some dated light bedtime reading ... Effect of Mach and Reynolds Numbers on Maximum Lift Coefficient - NASA Technical Reports Server (NTRS)
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From: australia
The DH Vampire was of interesting in terms of pitch characteristics at high mach numbers, not really mach tuck as understood today. The RAAF early single seat aircraft had auxiliary engine air intakes (elephant ears) mounted on the top of the fuselage. A number of these aircraft crashed from high speed dives after pitching down at high mach. The fix was to change the position of the auxiliary intakes to the lower fuselage. While I don't know the aerodynamics involved, it would appear that the wake of the upper aux intakes would impinge on the tailplane at high mach.
The two seat Vampire Mk35 had a marked pitch up at compressibility buffet onset. The onset of compressibility was quite sudden, the pitch up was attributed to sudden shock stall of the wing markedly reducing the nose down lift/weight couple, with no change in the tailplane down load.
I acknowledge that the stability and control characteristics of these ancient aircraft should not be compared with even the first generation of swept wing aircraft, although these aircraft had their own handling problems.
The two seat Vampire Mk35 had a marked pitch up at compressibility buffet onset. The onset of compressibility was quite sudden, the pitch up was attributed to sudden shock stall of the wing markedly reducing the nose down lift/weight couple, with no change in the tailplane down load.
I acknowledge that the stability and control characteristics of these ancient aircraft should not be compared with even the first generation of swept wing aircraft, although these aircraft had their own handling problems.
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From: NY & LA
That makes sense: It looks like it would have caused the local mach-number over those intakes to go up, which then produced stronger shockwaves, and the resulting turbulent air, which interfered with the airflow over the tail.
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From: UK
Compressibility error (CAS - EAS difference) has an effect, but more significant is the effect of mach number on the CL - alpha curve.
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40
Last edited by Rivet gun; 21st June 2024 at 11:49.
Joined: Jun 2004
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From: UK
AFIK, the Eclipse business jet has its PFD speed tape calibrated to read KEAS.
Last edited by Rivet gun; 21st June 2024 at 12:23.
Thread Starter
Joined: Oct 2023
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From: NY & LA
Compressibility error (CAS - EAS difference) has an effect, but more significant is the effect of mach number on the CL - alpha curve.
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40
Joined: Dec 2002
Aviation Qualifications: ATP+Mil
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From: Where the Quaboag River flows, USA
Compressibility error (CAS - EAS difference) has an effect, but more significant is the effect of mach number on the CL - alpha curve.
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40
https://cfapp.icao.int/tools/videos/...ALL_VL_MIX.mp4
The relevant bit starts at 3:40
Joined: Mar 2005
Aviation Qualifications: Military
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From: Aus
AFIK, the Eclipse business jet has its PFD speed tape calibrated to read KEAS
Minimum airspeed 310KEAS when supersonic, 300KEAS when subsonic above FL250, 145KIAS below FL250 unless angle of attack limit would be exceeded
Design Mach 3.2, Maximum scheduled cruise Mach 3.17, Mach 3.3 may be flown if approved by Commander if compressor inlet temperature 427°C is not exceeded
Maximum airspeed for normal operation 450KEAS, design limit speed 500KEAS
