Friday, May 24, 2013

Wing Spotting

In today's edition of Trendvector we learn about the supercritical airfoil. Now, this is not super critical, like some a fabled mother in law, in this case the super means fast. 

 Here we are at flight level 400 (40,000 ft) going Mach .82.  Notice how wide the operating envelope is, even up here.  So much for "coffin corner" - it's more like the entire end of the room!
You can just start to see the little yellow area at the bottom of the airspeed indicator (around 209 kts) that is the mimimum selectable speed for autothrust, stall is farther below that. 
The maximum speed (Mach .86) is the barber pole at the top, but that's due more to other aerodynamic load reasons than Mach buffet.

In order to allow for high cruise airspeeds, and avoid the effects of Mach buffet, airplanes of the A330's generation employ a “supercritical” airfoil. These airfoils typically have a larger leading edge radius, a flatter upper surface, and a rather distinctive cusp at the trailing edge.

The supercritical airfoil is not “extra critical”, but one with a high critical Mach number (super, meaning high). It allows efficient cruise speeds at relatively high Mach numbers before incurring a large increase in drag due to shock wave formation. 

Check out this A330 wing. Notice the cusp near the underside trailing edge.

Compare it with the 747-400 wing below. Notice the flat bottom, and the sharp leading edge. The 747 is a fast airplane, and without a supercritical wing, it achieves its high mach cruise with a large amount of wing sweep (which, of course, makes it look extra cool).

Modern aircraft with supercritical wing profiles offer numerous advantages, which include:
  • Improved aircraft control characteristics at high speed.
  • The profile can be thicker, and  requires less sweep, which can allow for a lighter structure.
  • The large leading edge also makes for great takeoff and landing performance. 
  • The position of the aerodynamic center is virtually stable for supercritical profiles, and therefore less susceptible to adverse high Mach effects such as Mach tuck.
  • The increase in drag above a given speed is so great that it is extremely unlikely, or even impossible, to fly faster than the demonstrated speeds that ensure the absence of flutter (VD/MD) dive speed.
  • Therefore, the threat of loss of control due to an overspeed is much less than in older generation aircraft.
On some types of airplanes (Airbus A320, for example), because of the aerodynamic characteristics in the approach to stall, the stall warning threshold is often independent of Mach.
On the A330 and other airplanes of its generation, despite the flight school admonition that an airplane always stalls at the same angle of attack, the stall and stall warning angle of attack is greatly influenced by Mach number. 

In the case of Air France 447's high altitude stall. Though the stall margin was initially quite tight (only a few degrees), as they lost airspeed, their stall angle-of-attack increased along with their actual angle of attack, preventing them from stalling until they had climbed nearly 3000 feet and lost 90 knots of airspeed. But at least one of the pilots thought that the buffeting they felt was due to high speed, instead of stall buffet. 

You can learn more about this and many other aspects of that accident and the A-330 in my upcoming book on the Air France 447 accident. 
Sign up on this page, to receive notice when it or excerpts are available. 


Karlene Petitt said...

Bill, this is fascinating stuff. I'm really looking forward to the final edition of your AF447 book. I know everyone will enjoy the read...and learn something too. Keep these tidbits coming... fascinating!

flyaway said...

I am fascinated by 447 and am looking forward to your book.