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The Staff of SOH
Supercritical airfoil question
- Thread starter Landman
- Start date
srgalahad
Charter Member 2022
Okay. I recently purchased the A2A civilian mustang and I was reading in its manual about how the laminar flow wing delayed the onset of compressibility allowing for a higher TAS before mach buffet and breakup of the airflow occurred. It make me think about the supercritical wings used on Airbuses and other aircraft.
magoo
SOH-CM-2013
I think simplistically one could say that the theory of laminar flow is in controling surface turbulence and drag across the airfoil. The supercritical moment is what happens at higher speed, moving the high speed shockwave away from the wing's surface, perhaps the step above and beyond the basics of laminar flow. It becomes sort of a different element of the science.
Here's a good article describing laminar flow:
http://www.aviation-history.com/theory/lam-flow.htm
Here's a good article describing laminar flow:
http://www.aviation-history.com/theory/lam-flow.htm
I used to work where I was up close and personal with airliners' wings, and I remember how perplexed I was by the supercritical wing profile. It seems counter-intuitive to how I was raised to think an airfoil should look. As you work your way from tip to root the lower surface becomes progressively thicker and more sharply curved until at the root their shape is quite extreme. Someone tried to explain to me how it worked, but all I really got was that it allows airliners to cruise at higher speeds with lower drag.
I suppose that if the supercritical airfoil is designed to maintain laminar flow at high airspeeds that would be a good way of reducing drag.
I suppose that if the supercritical airfoil is designed to maintain laminar flow at high airspeeds that would be a good way of reducing drag.
Grafmann
Members +
Answer
Landman, to answer your initial question in two words, basically yes.
There are several things of interest with supercritical airfoils:
1) Every airfoil has a critical mach number, the speed the airplane has to be going in order for the local flow (on just one area of the airfoil) to exceed the speed of sound. A shockwave forms & a bubble like region of supersonic flow forms on top of the wing. The faster you go the bigger the bubble gets until it eventually covers the whole surface of the wing from front to back. In terms of the intial forming of the shockwave it is desirable to have this occur as far back on the wing as possible. The airflow tends to follow the shockwave & separate from the wing surface. So when this happens near the leading edge only the front part of the wing is creating lift. The rest just goes along for the ride, & generally creates drag.
2) Supercritcal shapes facilitate the formation of the shockwave much further aft from the leading edge than more conventional shapes. The laminar flow reduces drag & results in better fuel economy - extremely important for airlines.
3) On big airliners the thick profile doesn't hurt the way it would on a high speed fighter. It provides internal volume to store all that fuel they need, & is structurally more efficient than a thinner wing would be, basically a strong, stiff wing built from relatively less metal.
4) So in terms of "delaying the onset of compressibility" there is a delay in where it occurs (further back on the wing), but doesn't so much change the speed you have to be going when a shockwave forms. All supercritical shapes are optimized for this to happen at high subsonic speeds (allows airliners to travel farther, faster, without the drag & fuel economy penalties of flying supersonically).
5) When the airplane does fly fast enough for a shockwave to form (and the flow to separate), it causes an abrupt shift in the location of the center of lift. This changes the moment arm (leverage) relative to the center of gravity, causing the aircraft to pitch (mach tuck). The severity of the tuck depends on the distribution of weight throughout the aircraft, & the compensating control forces that come from moving the elevators and or stabilators. Thus the shape of the airfoil isn't really designed to "reduce mach tuck"; that is more a function of the size of the horizontal tail control surfaces, the power of the control system, & the flight management computer. The supercrital shape is somewhat designed to minimize adverse effects resulting from wing pitching moments, but priority is given as to how that affects drag (resulting from a tilted wing moving thru the air), rather than controling if, or how much, the whole airplane will tuck.
The P-38 was one of the first American warplanes to encounter "compressibilty". Being a heavy aircraft, and aerodynamically clean, it really built up speed in a dive. It had a conventional teardrop shaped airfoil not particularly optimized for laminar airflow. Thus when speed built up in a dive, the airflow over the more curvy parts of the wing accelerated to supersonic speeds, a shockwave formed, the aircraft tucked under, & continued to accelerate - Bad news! By the way, there is also a lot of buffeting going on both from disturbed airflow over the tail, & as different regions of the wing go supersonic, & shockwaves form & move around. Anyhow, the disturbed airflow over the tail, & altered leverage due to the shift in the areodynamic center combined, & basically the tail was no longer effective enough to pull the airplane of the dive. The solution was to install dive brakes, rectangular plates that extended downward from the underside of the wing (like a backwards checkmark). They acted like spoilers to slow the airplane, & affected the overall airflow around the wing in a way which facilitated pitching the nose up again, & recovering from the dive. Thus they were called "dive recovery flaps" instead of the more familiar "dive brakes" (which merely stabilized an aircraft in a dive (dive bombers) & kept them at a reasonable speed to reduce stress on plane & pilot when pulling out - it was still pretty severe though).
Cheers,
Grafmann
Landman, to answer your initial question in two words, basically yes.
There are several things of interest with supercritical airfoils:
1) Every airfoil has a critical mach number, the speed the airplane has to be going in order for the local flow (on just one area of the airfoil) to exceed the speed of sound. A shockwave forms & a bubble like region of supersonic flow forms on top of the wing. The faster you go the bigger the bubble gets until it eventually covers the whole surface of the wing from front to back. In terms of the intial forming of the shockwave it is desirable to have this occur as far back on the wing as possible. The airflow tends to follow the shockwave & separate from the wing surface. So when this happens near the leading edge only the front part of the wing is creating lift. The rest just goes along for the ride, & generally creates drag.
2) Supercritcal shapes facilitate the formation of the shockwave much further aft from the leading edge than more conventional shapes. The laminar flow reduces drag & results in better fuel economy - extremely important for airlines.
3) On big airliners the thick profile doesn't hurt the way it would on a high speed fighter. It provides internal volume to store all that fuel they need, & is structurally more efficient than a thinner wing would be, basically a strong, stiff wing built from relatively less metal.
4) So in terms of "delaying the onset of compressibility" there is a delay in where it occurs (further back on the wing), but doesn't so much change the speed you have to be going when a shockwave forms. All supercritical shapes are optimized for this to happen at high subsonic speeds (allows airliners to travel farther, faster, without the drag & fuel economy penalties of flying supersonically).
5) When the airplane does fly fast enough for a shockwave to form (and the flow to separate), it causes an abrupt shift in the location of the center of lift. This changes the moment arm (leverage) relative to the center of gravity, causing the aircraft to pitch (mach tuck). The severity of the tuck depends on the distribution of weight throughout the aircraft, & the compensating control forces that come from moving the elevators and or stabilators. Thus the shape of the airfoil isn't really designed to "reduce mach tuck"; that is more a function of the size of the horizontal tail control surfaces, the power of the control system, & the flight management computer. The supercrital shape is somewhat designed to minimize adverse effects resulting from wing pitching moments, but priority is given as to how that affects drag (resulting from a tilted wing moving thru the air), rather than controling if, or how much, the whole airplane will tuck.
The P-38 was one of the first American warplanes to encounter "compressibilty". Being a heavy aircraft, and aerodynamically clean, it really built up speed in a dive. It had a conventional teardrop shaped airfoil not particularly optimized for laminar airflow. Thus when speed built up in a dive, the airflow over the more curvy parts of the wing accelerated to supersonic speeds, a shockwave formed, the aircraft tucked under, & continued to accelerate - Bad news! By the way, there is also a lot of buffeting going on both from disturbed airflow over the tail, & as different regions of the wing go supersonic, & shockwaves form & move around. Anyhow, the disturbed airflow over the tail, & altered leverage due to the shift in the areodynamic center combined, & basically the tail was no longer effective enough to pull the airplane of the dive. The solution was to install dive brakes, rectangular plates that extended downward from the underside of the wing (like a backwards checkmark). They acted like spoilers to slow the airplane, & affected the overall airflow around the wing in a way which facilitated pitching the nose up again, & recovering from the dive. Thus they were called "dive recovery flaps" instead of the more familiar "dive brakes" (which merely stabilized an aircraft in a dive (dive bombers) & kept them at a reasonable speed to reduce stress on plane & pilot when pulling out - it was still pretty severe though).
Cheers,
Grafmann
Wow! there are some really knowledgeable people here. Thanks for the replies. Talking about the need to keep control in a dive I noticed recently that the SBD Dauntless has openings in the wing leading edge that duct airflow to the top of the wing and these ducts are in line with the ailerons. No doubt to maintain the airflow over the top of the wing and allow for better control in a dive while lining up on target.
Quite the opposite. The leading edge slots energize the airflow at high angles of attack only and hence delay stalling of the outboard wing section.