"Vertical horizons" (Flight International, 12-18 August) listed the numerous problems of rotorcraft, many of which have persisted despite 50 years of development. Is it not time to return to square one and use benefits of new technology to circumvent these problems?
For instance, turbofan engines can now have bypass ratios of 10:1 or greater, and their exhaust streams have temperatures of around 65°C (140°F) and pressure ratios of around 1.7:1. Such conditions are ideal for ejector thrust augmentors as they support the subsonic flow conditions that make ejectors efficient. Ejector use would be limited to take-off or hover and translation when the overall propulsion system would have an effective bypass ratio of 50:1. Although this ratio is less than that of standard helicopters it must be remembered that the downwash would be more effective since it contains no rotational component. Furthermore, without large rotors, torque reaction on the aircraft is eliminated so that tailrotors, complex mechanical components and attendant weight penalties are completely avoided. The wings needed for lift during horizontal flight also ensure a safe aircraft and can be used for short take-off and landing operation when required.
Ejectors have been tried and then abandoned in the past because they were fed from gases at higher pressures and temperatures; thrust augmentation was then disappointing because of compressibility problems. The new turbofan engines enable a new concept in flying machine that will be quiet, simple, vibration free, low cost and efficient.
Richard Adkins Square One Consultants Child Okeford, Dorset, UK
Source: Flight International
