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From the Ray Prouty Archives: Designing for High Speed

By Ray Prouty | November 17, 2017

In August 1986, Westland Helicopters did what no one had done for almost a decade — it set a new helicopter speed record. For eight years prior, the record had stood at 198.7 kts, set by the Russians with a souped-up Mil Hind.

To set the new mark, the Westland team flew its modified Lynx both ways on a 15-km course at an average speed of 216.3 kts. The fact that the old record had held for so long may be taken as an indication that 200 kts is about as fast as helicopters are meant to fly.

What was the British advantage? It appears to lie in two different areas: an innovative use of engine power and a new and usual shape for the main rotor blade.

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The Westland Lynx first flew in 2971. At the time of service introduction, its Rolls-Royce Gem engines were rated at a total of 1,800 hp on a standard day (59 F at sea level). As has been done before, the transmission was derated; that is, it was qualified to only 1400 hp. That saved transmission weight and was based on the assumption that most operations would be at combinations of temperatures and altitudes where the engines could not develop their full-rated power.

Excess power

During the years, the Gems were improved so that with a little special attention, two of them could put out 3,200 hp. The transmission was also upgraded and, at the time of the record attempt, could be trusted to absorb 2,500 hp — still 700 short of the engine capability.

What can you do with an extra 700 hp that you can’t put into the rotor drive system? One possibility is to power a 700-hp propeller through a special auxiliary gearbox. The other possibility — and the one Westland chose — is to put the exhaust system to work.

Most helicopters are designed to hover effectively. This means getting the most engine power to drive the rotors. One way to do this is to minimize the engine backpressure by providing expanding exhaust nozzles so that nothing restricts the escape of the gases behind the power turbine. This results in a relatively low exhaust velocity that is good for hover but might actually generate a drag force at high speed.

Designers of turboprop airplanes have a different philosophy. They use convergent nozzles with high exhaust velocities that, while reducing available power at low speed, provide significant jet thrust at high speed. This was the approach used by Westland. By experimenting with different exhaust configurations, they found that by squeezing down the exit area to about 40% of what is on the conventional Lynx, it would be possible to convert the extra 700 hp into nearly 600 pounds of usable jet thrust.

This and an extensive drag cleanup combined to reduce the equivalent parasite drag to about half that of the basic Lynx. In addition to reducing the required rotor power, the auxiliary thrust allowed the main rotor to operate at a lower forward tilt, producing a more benign angle-of-attach environment on the retreating side.

The record was set in the “pure” helicopter category. Had the 600 pounds of auxiliary thrust come from a propeller or a jet engine, the aircraft would have been rated as a compound helicopter for which the unofficial record is 274 kts, set by a jet-engine-powered Bell Helicopter UH-1 way back in 1964.

Designing For High Speed: The BERP blade

The other innovation was the paddle-tip blade that was developed during the British Experimental Rotor Program (BERP). Modifying the outboard 15% of the blade tips helped in both places where a rotor gets into trouble when flying fast — on the advancing and retreating tips.

The advancing tip is exposed to high Mach numbers that can result in noise, drag rise and high pitching moments. Two geometric factors can be used to reduce these compressibility effects: an airfoil with a small thickness-to-chord ratio, and a swept leading edge. Westland used both. Some thinning came as a result of extending the chord both fore and aft of the original 12% tick blade. The rest was achieved by progressively thinning the composite structure toward the tip until a ratio of about 6% was obtained.

Air is fooled into thinking the Mach number is lower than it really is by leading edge sweep — whether it be fore of aft. The BERP tip has both, since the lading edge first sweeps forward and then aft in a curve to give more and more sweepback as the local Mach number increases. The result is that the aircraft could be flown on the record flight with an advancing tip going 97.7% of the speed of sound — undoubtedly noisy, but permissible for this special occasion.

On the other side

As forward speed increases, the angles of attack on the retreating side eventually become high enough tot cause stall, as that blade tries to lift in a region of low relative velocity. Depending on the airfoil section used, most blade tips cannot go beyond 12 deg or 16 deg before stalling.

To obtain as high a stall angle as possible, it is desirable to have a thick, highly cambered section in contrast to the thin, flat section desired on the advancing side. Westland’s paddle tip introduces a unique solution to stalling due to a vortex that is generated at the notch shown in figure 55-3. This energizes the flow over the tip, much as happens on delta-wing airplanes, and keeps the boundary layer attached up to high angles of attack.

From a theoretical standpoint, the extra blade area in the paddle tips alone should have helped by allowing the same thrust to be generated at reduced average angles of attack. Test results published by Westland, however, indicate that this particular advantage did not, in fact, materialize. At thrust levels and speeds free of blade stall, there was no measurable decrease in collective or cyclic pitch from the baseline Lynx rotor. For conditions in which blade stall existed on the baseline rotor, however, the BERP rotor did the same job with lower control angles.

Tailoring the blade

Compared to metal blades, one advantage that composite blades bring to the aerodynamicist is the east with which airfoil and planform shapes can be changed along the blade. This is because the composite blade is formed by compressing material in a mold that can be made with any reasonable arbitrary shape. Westland took advantage of this capability to not only form the paddle tip but to also vary the airfoil section along the basic blade as shown in Figure 55-4. From the 70% radius station to just inboard of the Mach-sensitive tip, they chose a 12% thick, cambered airfoil for its good lifting capability on the retreating side.

Such an airfoil, while having a desirable, high stalling angle, also has an undesirable, nosedown pitching moment that can lead to high control loads. This was counteracted by using an airfoil with a reflexed trailing edge inboard of the 70% radius station. The reflex produces a noseup pitching moment in this portion to balance the nosedown moment on the rest.

The stall-resistant tip, the airfoil tailoring and the more benign angle-of-attack environment due to auxiliary propulsion, all made it possible to fly at a tip speed ratio (the ratio of forward speed to tip speed) of 0.51 — just to the high side of the 0/5 value usually quoted as a maximum for pure helicopters.

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