# Was the B727's wing supercritical (even if not so designed)?

The Boeing 727's wing upper surface appears unusually flat, especially at the root (Albeit at an angle). Refer to the images linked below:

airlinereporter.com

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Was the B727's wing designed as supercritical? Did the B727's wing behave in a supercritical fashion, even if it wasn't explicitly designed as such?

• Also notice the difference in AOA between wing and horizontal stabilizer in first picture! Keep in mind they probably did not need a lot of airfoil lift at cruise, and would reconfigure for landing, so one may not see a "classic" airfoil shape here. – Robert DiGiovanni Nov 3 '18 at 16:05

I've got an aged book on pre-design of aircraft, which states that the B727 wing thickness was 13% at root and 9% at tip, average thickness 11% chord. $M_{M0}$ = 0.9, first flight of prototype is noted as 1963.

The 727 was a derivative of the 707: first flight 1957, average thickness 10% chord, $M_{M0}$ = 0.9. So the 707 wing was even thinner than B727, Mach number is the same. Profile of both airliners is not listed as a NACA profile.

The book references the first research paper on supercritical profiles as T.G. Ayers: Supercritical Aerodynamics Worthwhile Over A Range Of Speeds, published in 1972, nine years after the B727 prototype flew. The profiles depicted in the report seem to be thicker than those of the B727.

The photos in the question focus on the wing root, however the shape of the wing root is not indicative of that of the rest of the wing, but determined by a lot of structural and aerodynamic factors: placing of landing gear, fairings for aerodynamic interference etc. Furthermore, the overwing exit is there and evacuation of passengers should not be too difficult, caused for instance by a bulgy shaped cross section. It would make sense if this bit of the wing was designed mainly with evacuation in mind.

So it appears from the published date of the research work on Supercritical wing design, and from the similarity with the much older B707: no, the B727 did not have supercritical wing profiles.

Update

The main advantage of a supercritical wing is that it enables reducing the wing sweep. There was some progress between 707 and 727, because wing sweep was reduced from 35º to 32º on the 727. However, the A320 has a comparable $M_{M0}$ and has wing sweep of 25º. So again without knowing what exact wing profile was used and based on order-of-magnitude checks, it does not look like the 727 had supercritical wing profiles.

The profiles of supercritical wings I've seen are flat on top indeed, and horizontal and thickish and have a characteristic curve at the end. Flatness per se does not indicate that the profile is supercritical, for example NACA 23015 isn't.

The horizontal flat bit on top allows the supersonic area to expand calmly again, but it needs to be horizontal, like indicated in this figure from E. Torenbeek, Synthesis Of Subsonic Airplane Design:

Update 2

A fascinating film explaining how high subsonic speed was achieved before supercritical rear loading profiles were implemented on aircraft. Discoveries like these are what I love about the Aviation SE site. From this question & answer

• The characteristic curve at the end is called rear loading and contributes most of the lift at maximum Mach. By the way, the first work on supercritical airfoils is from 1940 by Kawalki and the first airliner to use a supercritical airfoil is the Vickers VC10; however, at those times the airfoils were not called supercritical. – Peter Kämpf Jul 3 '17 at 18:11