A World Speed Record
In the context of advanced rotor blade design as discussed in Chapter 6, and as an example of realized performance, it is of interest to record the capture of the world speed record for
helicopters by a Westland Lynx aircraft in August 1986. The incentive to make the attempt was provided by the results of a programme of test flying on the Lynx fitted with an experimental set of blades in which lift-enhancing aerofoil sections of the RAE ‘96’ Series (Section 6.2) were used throughout the length of the blade, together with the Westland tip design (Section 6.3) combining a sweepback benefit on local Mach number with delaying the tip stall. The tests showed the flight envelope to be improved by the equivalent of 35-40% increase in blade area and made it clear that level flight speeds beyond the existing record were achievable.
Different aerofoil sections were used for the inboard, mid-span and tip parts of the blade, chosen in relation to the local speed conditions and lift requirements. The section used for the tip was thinner than the other two. The blade was built in glass fibre with a single spar, special construction methods being employed.
The aircraft was a standard Lynx (Utility version) with a skid undercarriage, in which protuberance drag had been reduced to a minimum and an attempt had been made to reduce rotor head drag by fairings. The engine power was enhanced by water-methanol injection. The purpose of these measures was to ensure that, given a large alleviation in the flight envelope, the aircraft would not then be power limited unnecessarily.
For the record attempt, the course of 15 km was flown at 150 m above ground over the Somerset Levels, this being well within the altitude band officially required. The mean speed of two runs in opposite directions was 400.87 km/h (216 knots), exceeding the previous record by 33 km/h. The aircraft also had an extraordinarily good rate of climb near the bucket speed (80-100 knots), this being well over 20 m/s (4000 ft/min) – exceeding the capacity of the indicator instrument – and generally exhibited excellent flying characteristics. Figure 7.7 shows a photograph of the aircraft in flight and Figure 7.8 presents a spectacular view of the rotor blade.
Figure 7.8 BERP rotor blade on world speed record helicopter (G-LYNX) (Courtesy Agusta Westland)