Perhaps the very pinnacle of British aero-engineering was the superlative Handley-Page Victor nuclear bomber of 1952. The world’s greatest medium bomber, the Victor was far superior to rival designs. Key to its superiority was its distinctive crescent wing. Considering the excellence of the wing we wondered why this design solution is dead. We turned to Jim Smith to find out.

Hush-Kit asked me about the Victor crescent wing, and why it had not been more widely used. The answer needs a bit of discussion about what was being sought, and the problems of flight at high altitude and high transonic speed.

The unusual Victor wing design is extremely interesting. Like all aircraft wing designs, it must balance the competing needs of required lift, altitude and speed capability, while carrying the powerplants and minimising structural weight – without falling off.

So… why the crescent shape?

This primarily derives from the desire to cruise at high subsonic speed and high altitude over long distances, going right back to the original requirement as a strategic nuclear bomber. This requirement means that it will be helpful to have good internal volume for fuel, in an aerodynamic shape that has low drag at speeds close to the transonic drag rise.

What’s that the transonic drag wave?

The transonic drag rise is the increase in drag of an aerofoil or wing as speed is increased towards the speed of sound. The rise in drag is typically due to the formation of shock waves in the flow as areas of locally supersonic flow develop over the wing. Bear in mind we are aiming for high altitude, so the wing will be having to generate reasonable lift coefficients (a daunting term that simply means the effectiveness of an aircraft wing) in the cruise, and those are generated by suction due to increased local air speed. Hence areas of locally supersonic flow may develop, which may result in the formation of shock waves and increases in drag. The speed at which shock waves first appear in the flow is called the critical Mach number, and it is at around this Mach number that potentially performance-limiting drag rise occurs.

Not only that, the shock waves that form are likely to interfere with the flow over the wing, and badly affect handling. For a conventional, straight-tapered, swept wing (Sabre, for example) at a given incidence, subsonic, the greatest lift coefficient will be at perhaps 70% span, and this area is likely to be where shock waves first appear in the flow as speed is increased. Because a shock wave is essentially a sudden jump in pressure in the flow, it can, and does, greatly affect the flow close to the wing surface in the boundary layer. At high speed and high altitude, this can cause the flow to separate, resulting in a drastic loss of lift, and a phenomenon called transonic pitch up.

All of the above can be delayed by a combination of wing sweep (which reduces local Mach number), and low thickness-chord ratio, which reduces local suction, hence delays formation of shock waves.

Now, to the Victor. The Victor was designed to achieve the same critical Mach number across the whole span of the wing. The wing design has a very large inboard wing chord, with high sweep, and this allows it both to have sufficient depth to accommodate the engines (more on this shortly), while still allowing a high critical Mach number. Outboard, the wing tapers, and reduces significantly in thickness, and moderately in sweep, these two factors resulting in a constant critical Mach number, and no tendency to transonic pitch up. Overall, the result is a max cruise speed of Mach 0.92 at 55,000 ft, which is a fairly remarkable achievement for an aircraft which first flew in 1952.

So why doesn’t every aircraft look like this? Well, there are two short answers to this – one, because they don’t need to, and two, because of the disadvantages of the engine installation. The Victor is a great package, but you don’t really want to bury the engines in the wings if you can avoid it, notwithstanding a certain post-war British fascination with doing just that.
If you bury the engines, you will have to redesign the wing if you choose to upgrade the engines – see Victor, Nimrod, Nimrod MRA4 for example, quite apart from the added time and cost of routine maintenance or engine changes. Intake design also turns out to be tricky, because leading edge intakes next to the fuselage will see substantial changes in flow with varying incidence and lift. In addition, there are structural benefits to distributing the engines across the span, due to something called inertia bending moment relief, which results in lower stresses at the wing root, and hence lighter wing structure. However, podded solutions at these high cruise Mach numbers will also be tricky to design, as they may well reduce critical Mach number.

Today’s airliners are among the most efficient aircraft ever made, and this has been achieved by not needing to do some of the things the Victor could do. If you do not need to travel at such a high cruise speed, you can go for structurally-efficient podded engines, and gain a bonus in upgradeability and maintenance costs, as well as lighter wing weights. The wings can be lighter as the weight of an engine on the wings gives relief from wing bending.

Additionally, at lower cruise speeds, and with modern aerofoil design methods, lower sweep and thicker wing sections can be used, and higher local Mach numbers can be tolerated without shock waves causing flow separations. All of this, and the use of new materials, result in lighter and more aerodynamically and structurally efficient wings.

The Victor and the Vulcan both resulted from a desire to cruise fast, high and for long-distances. With the exception of the B-2, the subsonic military transport and bomber aircraft of today are essentially transports, and are designed like transport aircraft. These aircraft are all vulnerable to advanced anti-air weapons, which is why the strategic capability now generally resides with submarines rather than aircraft. The B-2 (and B-21) are pursuing a different survivability route (stealth), which imposes its own constraints and compromises.

Supersonic aircraft tend to punch through the difficult transonic area at low lift coefficient, and are driven to completely different configuration solutions depending on their particular requirements.

Today only one or two business jets operate in the difficult transonic cruise environment, aided by advanced aerofoil design, and generally rear-mounted podded engines with integrated design of the fuselage, wings and engine pods to reduce drag. Respect is due to any aircraft capable of cruising at Mach 0.9+ for long distances, even though the Victor and Vulcan have passed into history.

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