“Wow, it really is curved on the bottom!” I still remember how surprised I was when I took my first look at the wing of the CRJ. The reason for my astonishment was because the wing of the CRJ was designed significantly differently than any other plane I had flown before. In fact, the CRJ series of aircraft incorporates one of the most pronounced examples in the entire fleet of transport aircraft of a specially designed wing type called the supercritical wing.
In this post, I am going to shed a little light on the supercritical wing and discuss how it works, and how it compares to a conventional wing. This mid 1970s invention, though seldom discussed, has allowed airliners to fly faster and farther, while burning less fuel. In essence, the supercritical wing has revolutionized modern air transportation.
Some history
During early testing in the 1940s and 1950s into supersonic flight, it became apparent that flight into the supersonic regime presented some serious engineering problems. It was quickly noted that when an aircraft approaches and passes the speed of sound, many unfavorable things begin to happen. Two of the most notable of these effects are the sharp increase in drag caused by the formation of a shockwave, and the rearward shift of the center of lift generated by the wing.
These challenges were eventually compensated for in the designs of supersonic military aircraft, but transport category aircraft were left orphaned with wing designs from the days Wilbur and Orville.
In the early ‘70s, when the economic demand for cheap high-speed air travel became greater, research began into a new type of wing which would perform more efficiently at near supersonic speeds. This eventually led to the development of the supercritical wing.
The modern supercritical wing was first flown on a modified F-8 Crusader during a NASA research project in the early 1970s. Since then, adaptations of supercritical wing technology have been built into nearly every transport category aircraft. Canadair and Lear were among the first aircraft manufactures to apply a supercritical wing to a commercial aircraft, which is likely why the supercritical wing on the CRJ series aircraft is more pronounced than other transport category aircraft.
So, what is a supercritical wing and how does it work?
When airflow around an aircraft exceeds the speed of sound, shockwaves are generated. These shockwaves, among other unfavorable things, greatly increase the force of drag on the aircraft. Because the drag rise caused by supersonic flow around an aircraft is so sharp, the aircraft’s engines have to produce significantly more thrust to push through the air with shockwaves than without them. This causes a much higher fuel burn for a fairly small increase in speed.
As a result, supersonic passenger travel is extremely expensive because of the huge amount of fuel supersonic aircraft have to burn to push their way through the shockwave. Thus, the economic demise of the Concorde and reason all modern airliners fly at a speed slower than then speed of sound.
Even though shockwaves begin to form when the airflow around an aircraft reaches the speed of sound, this doesn’t mean that flying just under the speed of sound will keep an aircraft shockwave free. The reason?
Some parts of an airplane, the wings in particular, have a shape which causes the air flowing around them to accelerate. This means that an aircraft can actually be flying at a speed well under the speed of sound, but have localized areas where the airflow is actually supersonic and is generating nasty fuel sipping shockwaves.
The upper surface of the wing of a conventional airfoil (a non-supercritical wing) is rounded. This helps draw air flowing over the upper surface of the wing downward generating extra lift. Unfortunately, the rounded upper surface of a conventional airfoil also greatly accelerates the air over top of it, much like a Venturi tube. This means that the airflow over the top surface of a conventional airfoil will reach supersonic speeds well before the aircraft will actually reach the speed of sound. Bummer.
So, to counteract this and delay shockwave formation, the supercritical wing employs a flatter upper surface and a more curved tail section. The flatter upper surface of the supercritical wing does not speed up the air as much as the rounded conventional airfoil. This allows an aircraft to fly faster without supersonic flow over the top of the wing. The flatter upper surface of the supercritical wing does create less downwash however, and thus less lift than a conventional rounded airfoil. To compensate for this, the supercritical wing utilizes a specially designed tail section to recover the lift lost by the flattened upper surface.
The result is an airfoil which still retains decent low speed performance while gaining the ability to fly at speeds closer to the speed of sound without generating large shockwaves. According to NASA, supercritical wings increase fuel efficiency at transonic speeds by about 15%. Supercritical wing technology has also increases the speed an aircraft can efficiently fly from about .70 times the speed of sound with a conventional wing, to faster than .84 times the speed of sound with a supercritical wing. Which means that you can get to where you need to go much faster, and theoretically pay less for a ticket. Theoretically…
Low Speed Vs. High Speed Wings
So, you may be thinking, with all of its advantages, why don’t all aircraft have supercritical wings?
Well, as with everything, there are always tradeoffs. Where the supercritical wing shines in the high speed, transonic flight regime that the airliners and business jets thrive in, it will never be seen on a small piston powered aircraft that you would find putting around the local municipal airport. The Reason?
The supercritical wing is structurally more complex than a conventional airfoil, and therefore more expensive to build. The aft section of the airfoil is particularly stressed because it produces a significant amount of lift over a fairly small portion of the wing’s structure. Because the aft section must also be thin, it means that special methods must be used in its construction.
When the supercritical wing was being developed, the structural problems associated with the heavily loaded aft section of the airfoil were eventually compensated for by incorporating a blunt trailing edge to beef up the structure. This has become a defining feature of supercritical wings, and is evident on the wing of the CRJ
Also, because supercritical airfoils generate a large portion of their lift at their curved aft section, they create larger nose down pitching moments than conventional airfoils do. This means that if all other factors are constant (CG location, weight, ect.) the tail has to work harder to keep the nose up on a plane with a supercritical wing than a plane with a conventional wing. This makes the supercritical airfoil SLIGHTLY LESS efficient at all flight regimes other than high-speed cruise because of the extra trim drag. As a result the supercritical airfoil is less desirable for smaller, slower-flying aircraft. See my post on aerodynamics for more info.
In brief, aircraft which are not designed to cruise in at high subsonic speeds like the Taylorcraft pictured above, wont reap any benefit from the supercritical wing and thus, don’t need one.
In closing
Today, there are still numerous airliners that do not have supercritical wings and are thus limited to slower subsonic cruising speeds. Supercritical wing technology is still fairly new in aviation terms and it will take time for conventional airfoils to be phased out. Soon however, the adaption of supercritical airfoils will become the industry-wide standard.
I feel pretty fortunate to be able to fly an aircraft which can comfortably cruise in the low .8 mach range. This allows me, and of course the people on board, to zip right around some of the old timers out there and get to where we are going faster and more efficiently. This means faster travel times and lower ticket prices.
And to think that all of this is due to a little-known invention from the ‘70s called the supercritical airfoil!
The Blog of Michael Soroka 