The exact airfoil sections are not known (though it would probably be possible to find out for the Il-76 with some digging...), but we do know one (the Y-20) has modern supercritical airfoils while the other (Il-76) does not.
Wings with supercritical airfoil sections have a characteristic appearance from certain angles if you know what to look for*. It became obvious that the Y-20 has supercritical airfoils based on photos pretty early on, whereas the Il-76 certainly does not (quite apart from anything else its design simply pre-dates the introduction of this technology - I think the first Soviet aircraft was the An-124).
* Supercritical airfoils reduce drag due to the formation of shocks on the upper surface of the wing at transonic speed. Air flows faster over the top of the wing than the aircraft's forward speed in order to generate lift, so in Mach 0.8 cruise the local flow velocity will be >Mach 1.0. In fact, in some STOL aircraft with elaborate slotted flap systems it can be comfortably supersonic during final approach to landing, which may be a source of considerable community noise!
Conventional airfoil shapes generally produce an extremely pronounced "suction peak" (overspeed) not far aft of the leading edge, which (at transonic speed) terminates with a strong shock that causes a lot of drag. A supercritical airfoil spreads its suction out further aft along the wing chord (they are sometimes called "back loaded" for this reason) and has a weaker terminating shock as a result. This is done by having the site of maximum airfoil thickness further to the rear which creates a marked concave "dent" on the lower surface of the wing that is pretty obvious on some Y-20 photos.
https://www.sinodefenceforum.com/ch...heavy-lift-aircraft.t197/page-326#post-490381
https://www.sinodefenceforum.com/ch...heavy-lift-aircraft.t197/page-327#post-490383
Trivia: a very similar approach is used in laminar airfoils for subsonic aircraft, the first of which famously was the P-51. However, mass production surface finish in WWII was nowhere near good enough to take advantage of laminar flow, in fact the performance benefit in the Mustang came from an unintentional "supercritical effect" at high speeds.
Wings with supercritical airfoil sections have a characteristic appearance from certain angles if you know what to look for*. It became obvious that the Y-20 has supercritical airfoils based on photos pretty early on, whereas the Il-76 certainly does not (quite apart from anything else its design simply pre-dates the introduction of this technology - I think the first Soviet aircraft was the An-124).
* Supercritical airfoils reduce drag due to the formation of shocks on the upper surface of the wing at transonic speed. Air flows faster over the top of the wing than the aircraft's forward speed in order to generate lift, so in Mach 0.8 cruise the local flow velocity will be >Mach 1.0. In fact, in some STOL aircraft with elaborate slotted flap systems it can be comfortably supersonic during final approach to landing, which may be a source of considerable community noise!
Conventional airfoil shapes generally produce an extremely pronounced "suction peak" (overspeed) not far aft of the leading edge, which (at transonic speed) terminates with a strong shock that causes a lot of drag. A supercritical airfoil spreads its suction out further aft along the wing chord (they are sometimes called "back loaded" for this reason) and has a weaker terminating shock as a result. This is done by having the site of maximum airfoil thickness further to the rear which creates a marked concave "dent" on the lower surface of the wing that is pretty obvious on some Y-20 photos.
https://www.sinodefenceforum.com/ch...heavy-lift-aircraft.t197/page-326#post-490381
https://www.sinodefenceforum.com/ch...heavy-lift-aircraft.t197/page-327#post-490383
Trivia: a very similar approach is used in laminar airfoils for subsonic aircraft, the first of which famously was the P-51. However, mass production surface finish in WWII was nowhere near good enough to take advantage of laminar flow, in fact the performance benefit in the Mustang came from an unintentional "supercritical effect" at high speeds.
