Tip jet


A tip jet is a jet nozzle at the tip of some helicopter rotor blades, used to spin the rotor, much like a Catherine wheel firework. Tip jets replace the normal shaft drive and have the advantage of placing no torque on the airframe, so no tail rotor is required. Some simple monocopters are composed of nothing but a single blade with a tip rocket.
Tip jets can use compressed air, provided by a separate engine, to create jet thrust. Other types use a system that functions similarly to the afterburner on a conventional jet engine, except that instead of reheating a gas jet, they serve as the primary heater, creating greater thrust than the flow of pre-compressed air alone; the best description of this is thrust augmentation. Other designs includes ramjets or even a complete turbojet engine. Some, known as Rocket On Rotor systems, involve placing rockets on the tips of the rotor blades that are fueled from an onboard fuel tank.
If the helicopter's engine fails, the tip jets on the rotor increase the moment of inertia, hence permitting it to store energy, which makes performing a successful autorotation landing somewhat easier. However, the tip jet also typically generates significant extra air drag, which demands a higher sink rate and means that a very sudden transition to the landing flare must occur for survival, with little room for error.

Origins

Austrian Ludwig Wittgenstein investigated the use of tip jets to drive an aircraft propeller while studying aeronautical engineering at Manchester University, UK and patented his ideas. He soon left engineering and would become better known as a philosopher.
The Italian designer V Isacco built "Helicogyres" in the 1920s. Although they used piston engines at the ends of the rotary wing, he foresaw that they might be replaceable by jets.
During the Second World War, German engineer Friedrich von Doblhoff suggested powering a helicopter with ramjets on the rotor tips. The first tip jet-powered helicopter was the WNF 342 V1 in 1943. After the war two WNF 342 prototypes ended up with the Americans and Doblhoff joined McDonnell Douglas who subsequently produced the McDonnell XV-1. The engineer who had actually produced the tip jet engines, August Stephan, joined the Fairey Aviation company of the United Kingdom which used them in their Fairey Jet Gyrodyne and Fairey Rotodyne aircraft first flying in 1954 and 1957 respectively.
Eugene Michael Gluhareff was an early pioneer of tip jets.

Rotorcraft using tip jets

Cold tip jets

The compressed air in cold tip jets generally exited at quite high temperatures due to compression-heating effects, but they are referred to as "cold" jets to differentiate them from jets that burn fuel to heat the air for greater thrust; similar to the difference between the "cold" and "hot" exhausts on the Harrier "Jump Jet", which uses "cold" air heated to several hundred degrees by compression inside the low-pressure compressor of the Pegasus engine.)