# How can the F-15 and Su-27 achieve a high top speed without variable wing geometry?

I watched a documentary once that explained, I believe, how the Su-27 and similar aircraft such as the F-15, can achieve a high top speed without variable wing geometry.

How does this work? How do they achieve high performance without swept wings in the F-15, Su-27, and similar aircraft?

• Everyone: could we try not to answer in the comments, please? thank you. Dec 4, 2018 at 9:07

They simply have enough thrust to do so with their given wing. There is no rule that high speeds are associated with variable sweep wings or that you need such a wing to go fast. As a matter of fact many of the testbed super sonic aircraft as well as the really high speed stuff dont have swept wings.

Wing design can effect efficiency (which may or may not be a concern in fighters) and can limit top speeds but that simply means that for a fixed shape wing it must be optimized for a desired speed range.

Another signifficant factor is because the F-15 is an airforce aircraft. So, it doesn’t have to land on a carrier. Hence it can take plenty of runway length to land, which means it can touchdown at a higher speed. So, its wings can be permanently swept at relatively high angle, because there is no need to have heavy wing pivots that enable them to sweep forward and allow lower speed flight for carrier landings.

In comparison, the F-111 had a variable geometry wing so it could fly slower, when the wings were swept forward, because it was intended as a joint aircraft, and so there was going to be a Navy version, that had to be capable of landing (and takeing off!) on carriers.

The F-111 wing sweep is from 16 to 72.5 degrees. In comparison, the F-15 leading edge of 45 deg. The wing angle is asignificant factor that helps define the mach number at which the shock cone from the nose will impinge the wing tips. See below.

If the wing tips are outside the shock cone and in the sonic flow, the handling charteristics, as well as the increased drag, is likely to be very undesirable. The formula linking mach number and shock angle is given here by NASA.

• Interesting explanation about lacking need for an airforce aircraft to land at lower speeds, and it surely justifies differences with F-14 for the navy, but in the navy actually flies the F-18 that has fixed wings but less angled... maybe because F-18 is slower than F-14 at top speed? Mar 28, 2019 at 13:35
• @LucaDetomi. Yes, I would agree that because the F-18 has a lower top speed, it doesn’t need variable sweep wings. Variable sweep adds a lot of weight, which is not good when you want a 10 g aircraft for dog fighting. Most aspects of engineering involve a comprise. If you gain somewhere, you are losing somewhere else. Cheers 🍻 Apr 3, 2019 at 12:21

All sweep does is make the air think the wing's chord to thickness ratio is higher, or you could say thinner in relation to its chord than it really is, for a given wing area. The benefit is mainly for wings with a fairly high aspect ratio, that is long with a narrow chord. Fooling the air into thinking the wing is "finer" than it really is delays shock wave formation to a higher speed than if the same wing, with the same chord and length, was straight.

Airplanes like the F-15 with a low aspect ratio, fairly thin wing don't really need a lot of sweep because their airfoils are already really thin relative to their chord.

The F104 is the most extreme example of this. Ultra low aspect ratio, thin biconvex supersonic airfoil, no sweep at all required because the benefit is negligible.

• No, sweep does a lot more. Please ask for "supersonic leading edge suction" to learn what else happens when wings are swept. Dec 3, 2018 at 23:12