The Art of the Helicopter John Watkinson - Karatunov.net

Engines and transmissions 249
Fig. 6.39 (a)The ramjet operates using dynamic pressure from travelling through the air at high speed to
achieve compression. It thus has no moving parts. (b)The pulse jet uses a non-return valve at the inlet to contain
combustion pressure.
Hiller buried the mechanism of a pulse jet inside the length of a rotor blade with the
fuel inlet and valves at the hub and an angled nozzle at the tip. This was known as a
powerblade. Hiller also developed valveless pulse jets in which the gases in the engines
resonated like organ pipes. This overcame the tendency of the valves to malfunction
because of the high-g environment. Needless to say pulse jets and sonic engines also
produce phenomenally high sound levels that preclude their use today.
In the pressure jet system the hull contains an air compressor. This may be driven
by piston engine, or it may be the compressor stage of a modified turbine engine. The
compressed air is fed through a rotating seal into the rotor and ejected rearwards at the
tips. This makes much less noise than tip-burning or tip-mounted motors. The French
SNCASO Djinn used this system and reached production.
An alternative approach to pressure jet drive is to obtain the jet pressure chemically.
Machines have been built which are propelled by the catalytic decomposition of hydrogen
peroxide. If liquid hydrogen peroxide is passed over a catalyst such as calcium
permanganate or potassium chromate, it spontaneously decomposes into steam and
free oxygen. The noise level of a rotor driven in this way is very low, but the running
cost is fantastic and hydrogen peroxide is corrosive and prone to explode if impurities
are present.
The Doblhoff tip-burning helicopter used a piston engine in the hull driving a supercharger
to provide compressed air for the tip motors. Rotor power was only used for
hovering. Once forward flight was established, the machine was driven by a conventional
propeller and the rotor would windmill like an autogyro. Doblhoff’s machine
mixed the fuel with the air before it entered the blades. Whilst this eliminated the need
for fuel piping in the blades, it meant that each blade was full of potentially explosive
mixture. In the Hughes XH-17 flying crane the safer route was taken and fuel was piped
to the tip burners.
The tip jet helicopter has not achieved wide use for a number of reasons. One of the
most serious of these is noise. The pulse and ramjet are never going to be silenced,
especially as allowable noise levels are falling. Pulse and ramjets are not fuel efficient at