Business Jet Design: The Supercritical Airfoil
Breaking the sound barrier isn’t the biggest priority when it comes to how most of us travel. Efficiency is. Whether you are a business jet or airliner, you settle in to cruise around 0.65 to 0.85 Mach. For non business jet design people, this is a fancy way of saying 65% to 85% the speed of sound.
Efficiency at this speed is thanks to a breakthough within the bast 50 years – the supercritical wing. Once you start looking, you’ll see it everywhere. Yet as a shape, and cross section, it is a hidden gem that few of us fully appreciate since they have become so ubiquitous. And yet they look very different than the wing you see on your cousin’s Piper Archer, Cessna 172 or Super Cub.
And were it not for the inquisitive young minds at squashdrive.org I would not have been so inspired to write about the largely untold story of this breakthrough in the subsonic flight regime.
Last week I helped lead a gaggle of budding young East Oakland squash players through the Oakland Aviation Museum and toured the Vietnam era jets. One such jet, the Vought F-8 Crusader, sat retired with its wings folded up, carrier style to conserve space, allowing us to see the cross section of the wing.
That image reminded me of another carrier borne aircraft with a super critical wing – the Lockheed Viking S-3, which sat a few miles away, on the deck of the USS Hornet. Both of these aircraft were young children of the 1970s and employed the hottest thing at the time – a new shape to combat difficulties more conventional airfoils faced as the airflow around the wing neared the speed of sound.
Looking at the picture above (of the S-3 Vikings wing innards and cross section), you may notice that the bottom of the airfoil is noticeably not flat (i.e. it has a bulge), yet the top has some flat portion. To the classic general aviation person, this might cause a bit of concern: “How can it be flat on top and curved on the bottom? Doesn’t this cause anti-lift?”
A surprising fact for many aerophiles is that not all wings follow the same rules for cross sections. The rules change according to the expectation we have of the wing, and the regime we’ll ask it to fly in.
The F-104 (and its ilk of pointy fast things) had a thin wing. So did Chuck Yeager on the X-1 – with those thin wings that weren’t even swept. Thin, however, made it difficult to store fuel, and didn’t solve other issues for those relegated to slower speeds below Mach 1. (Imagine a passenger airliner with drop tanks, or just a big outside tank like the Hustler.)
At speeds at or near the speed of sound (Mach 1), our traditional looking wing (flat on bottom and curved on top) does funny stuff. And by “funny stuff” I mean, a taxing and problematic shock wave.
Fatter wings force air to move out of the way. This is due to the longer distance the air travels over the top of the wing. That greater distance (in same amount of time) translates to more relative speed. That air may go supersonic when the plane itself is flying 30% slower.
With the speed of sound comes a shock wave and a host of related problems. Much of “getting through the sound barrier” was about learning how to get rid of this shock wave and its unpleasant aerodynamic accoutrement.
The jet age, swept wings, the Jetsons (!) and all of the shapes that we saw enter our culture in the 1950s and 60s were all about speed. But they didn’t necessarily address the critical mach speed problem of our subsonic airliners.
The Area Rule, Sears Haack bodies and Tin Tin’s spaceship had shape driven purposeful design: “Look at me – I’m shaped to go fast!” But getting a little more granular into airfoil shape highlights the subtlety in aeronautical design. Small changes, at speed, pay big dividends. And, lucky for us, the very same person that brought us the Area Rule was instrumental in making it happen – Richard Whitcomb.
In short, the “Critical Mach” number (the lowest Mach number at which the airflow over some point of the aircraft reaches, but does not exceed, the speed of sound, ) needed to be tempered. But how?
Enter The Super Critical Wing
A discussion of the supercritical wing is timely, as the Crusader was one of the original test bed aircraft when NASA began seriously looking into the transonic speed range. The development of the supercritical wing led to a host of civilian (and military) adaptations that allow us to fly closer to the speed of sound, problem free and with more efficiency. 1972 was the year many clean sheet designs went from the conventional airfoil to the supercritical one featured in the lower half of the image below:
Fixing the Problem
The jet engine brought us the benefit of power and speed for new business jet design. Yet we were pushing the same old shapes faster and faster through the air. When these legacy or conventional airfoils went faster, a wing’s critical Mach number was exceeded and people got hurt. (As the strong shock wave in the top part of the image above got pushed back – by even more speed – the center of lift moved aft, causing the aircraft to pitch down, more speed, more shock wave creep backward, more nose down… and so on.)
To fix the problem of the shock wave’s strength and characteristics, airfoil shape had to be explored in order to increase the critical Mach number of a wing. This “crit Mach” number establishes when the shock wave “shows up” due to the increased compressibility of air at the speed of sound. While the picture of the wing in image “1” below is optimized for speed (more so than say a Super Cub’s wing which will be flatter on the bottom on more curved on top) it still has a problem when the lifting “center of pressure” begins to experience supersonic speeds, a.k.a. its “critical Mach number.”
The design challenge was to diminish the severity of the red spiky line. Secondly, it was to explore delaying its onset or mitigate it entirely. [The most dangerous thing in image “1” below – shown in blue wavy lines – is flow separation. In other words, any airflow in that area could be potentially going over control surfaces that are no longer effective. In other words, airflow can lead to loss of control.]
As civilians, what we see above is pretty much what we had to swallow up until work on the supercritical wing design entered business jet design. And we lived happily there with little excess power to take us farther and the knowledge that “too fast” could hurt you. Some aircarft (like the Spitfire in WWII) had a naturally thin wing design that allowed it to flirt with the speed of sound without major drama and loss of control, but such design was the exception. Too fast brought a host of problems: Mach tuck, flow separation, wave drag, and boundary separation.
With the age of the jet engine came these four “shock wave” related characteristics creating real problems. Aircraft began consistently bumping up against the 600 mph regime where things changed as the air accelerated along the top of our wings. The military could solve this by making wings thinner (and putting fuel somewhere else), sweeping them more and / or turning them into “delta” configurations. But civilian designs didn’t have this luxury as jet engines were strapped to them.
As we went faster and faster, swept wings were explored (and successfully woven into design) but designers still wanted to solve the issue of shape and how it induced or mitigated problems in the transonic regime.
The solution was to solve what happened to the shock wave on the top of the wing, by flattening this area to reduce the speed relative to the remaining airfoil. Other mathematically derived characteristics minimized flow separation as the shock wave was moved aft. Keep the airflow attached longer and you’ll minimize separation forces as seen by our less dramatic red spike in image “2” below:
The elegance of the supercritical airfoil lies partially in the fact that it breaks all the rules we’re given as primary aviators. (“The wing is curved on top, that’s what makes it fly!”) To get that curve (when needed) the supercritical wing of today employs slats, flaps and a host of other aides to make aircraft such as the C-17 a solid low speed / STOL (Short Take Off and Landing) performer, yet also efficient at Mach 0.74 or 515 mph.
While the analogy is far from perfect, you might say this was aviation’s “golf ball” moment. The “dimpling” of the surface of an airfoil (or even employing thousands of suction holes, which was also experimented with) was tested. The end result of the supercritical airfoil is that keeps its supersonic shock wave subdued and airflow attached longer. Put another way, consider the journey the air is making relative to the rest of the moving body, and delay that differential, as long as possible.
While WWII Germany had explored the design, it wasn’t until the 1970s that experimental work by NASA helped it enter mainstream transonic aircraft design. The supercritical wing made its debut in business aviation with aircraft such as the Beechjet 400A (née the Mitsubishi MU-300) and in commercial airlines with the Airbus A300 around 1972.
People, Planes and Power
Ultimately the supercritical wing story is one of powerplants. More powerful jet engines met our Cold War needs to go faster, and airplanes falling apart at high speed led us to apply mathematical solutions to air flow over our airfoils.
What is worth noting about supercritical airfoils (and future design hurdles we’ll face) is that their successful implementation shows that evolving shapes help us perform amazingly in the take off and landing regime (90 to 140 mph) and yet be happy, smooth and stable in the transonic regime.
The next time you see a goose landing on a lake, or any “cupping” shape of a slowly decelerating bird, keep in mind that slats, flaps and all manner of “wing warping / bending” are fine (and necessary) to get us to touch down elegantly at airport speeds. What the supercritical wing offers is something entirely different at speeds that the goose will never attain where we time-pressed humans live, in the transonic regime.