Mach Buffet
Thread Starter
Joined: Aug 2002
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From: Great White North
Mach Buffet
A question regarding high speed mach buffet. I know that it is a function of the speed of the airflow over the wing, and not the speed of the aircraft.
What I'm not sure of is the indications of this buffet in the cockpit.
In heavy high altitude flight below MMO, with either a gust load or high bank angle applied, what will the indications be?
I know that the aircraft might shudder.
Will you get the stick shaker like a low speed buffet?
What mechanical devices warn of high speed buffet?
Thanks
What I'm not sure of is the indications of this buffet in the cockpit.
In heavy high altitude flight below MMO, with either a gust load or high bank angle applied, what will the indications be?
I know that the aircraft might shudder.
Will you get the stick shaker like a low speed buffet?
What mechanical devices warn of high speed buffet?
Thanks
Joined: Jun 2001
Posts: 4,777
Likes: 9
From: Blighty
It is caused by the airflow on the top surface of the wing reaching supersonic speeds. The airflow behind the subsequent 'shock wave' becomes turbulent, rather like pre stall buffet. (Simple answer).
The indications are much like pre stall buffet, except perhaps felt at a higher frequency. You feel it just as you feel the low speed buffet.
Much like low speed buffet, it is a function of loading. The higher the coefficient of lift, the sooner the buffet will appear. So if you are heavy or manoeuvreing, the high speed buffet will appear earlier.
To my knowledge, no stick shake devices which warn you of high spped buffet have been fitted to aircraft. However, on one aircraft I flew, we used to bug the high speed 1.3g buffet boundary on the ASI at higher altitudes, and on another with a glass cockpit, the high speed buffet boundary was indicated by a series of red bars ('red bricks' as we knew them) much as the low speed boundary was.
The indications are much like pre stall buffet, except perhaps felt at a higher frequency. You feel it just as you feel the low speed buffet.
Much like low speed buffet, it is a function of loading. The higher the coefficient of lift, the sooner the buffet will appear. So if you are heavy or manoeuvreing, the high speed buffet will appear earlier.
To my knowledge, no stick shake devices which warn you of high spped buffet have been fitted to aircraft. However, on one aircraft I flew, we used to bug the high speed 1.3g buffet boundary on the ASI at higher altitudes, and on another with a glass cockpit, the high speed buffet boundary was indicated by a series of red bars ('red bricks' as we knew them) much as the low speed boundary was.
Thread Starter
Joined: Aug 2002
Posts: 7
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From: Great White North
+mach trimmer
I understand that aircraft have an overspeed warning or clacker. But correct me if I'm wrong...you could still have a high speed BUFFET without it going off.
Since the air is accelerated over the wing further with increased angle of attack...you could have a mach buffet well below your max mach. In other words, the overspeed sensor does not know your angle of attack, only your mach #.
High speed buffet occurs as the shockwave moves rearward when the air becomes turbulent and separtes as airflow goes from transonic to subsonic flow. (Bernoulli)
So my next question along similar lines is. Which boeing aircraft today have a mach trimmer? I believe the 747 and 727 do not.
What about Airbus?
Thanks again
Since the air is accelerated over the wing further with increased angle of attack...you could have a mach buffet well below your max mach. In other words, the overspeed sensor does not know your angle of attack, only your mach #.
High speed buffet occurs as the shockwave moves rearward when the air becomes turbulent and separtes as airflow goes from transonic to subsonic flow. (Bernoulli)
So my next question along similar lines is. Which boeing aircraft today have a mach trimmer? I believe the 747 and 727 do not.
What about Airbus?
Thanks again
Joined: Mar 2000
Posts: 8,571
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From: Arizona USA
Most (but not all) Lockheed TriStar -500 series aircraft have a maneuvering speed brake system whereby if the mach number exceeds .88, the speed brake is automatically partially deployed.
'Tis called maneuvering direct lift control (MDLC) and was fitted so that the aircraft could obtain UK CAA certification.
Works good.
'Tis called maneuvering direct lift control (MDLC) and was fitted so that the aircraft could obtain UK CAA certification.
Works good.
Joined: Jun 2003
Posts: 156
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From: Madrid
As aircraft is accelerating it´s wing´s center of pressure moves aft, this unbalance the equilibrium mantained with CG that cause an strong diving tendency (tuck under) in order to avoid that, lots of a/c are equipped with Mach trimmer. MD-80 is another good example.
I think overspeed warning doesn´t have anything to do with AoA, it will sound to avoid inadvertent excursion over Mmo. This could produce a shock induced flow separation that may result in a contol surface buzz and loss of elevator and aileron effectiveness.
I think overspeed warning doesn´t have anything to do with AoA, it will sound to avoid inadvertent excursion over Mmo. This could produce a shock induced flow separation that may result in a contol surface buzz and loss of elevator and aileron effectiveness.
Joined: Jun 2001
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From: UTC +8
Once or twice I was at the "Barber Pole" and got the usual overspeed warning, (Clacker) but no buffet at M.90.
The Lear 23 & 24 I recall from long ago had some sort of siren instead of a clacker! I remember that loud siren going off one flight and not immediately associating it with an overspeed warning. I was spooked by it and initially reached for my O2 mask as I had thought it was a pressurization problem.
The Lear 23 & 24 I recall from long ago had some sort of siren instead of a clacker! I remember that loud siren going off one flight and not immediately associating it with an overspeed warning. I was spooked by it and initially reached for my O2 mask as I had thought it was a pressurization problem.
