Pulse Jet History & Technical


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Pulse Jet History & Technical

These Jet Speed pages are a reproduction of a website first created in 2002 by John Adams, and maintained later by Rob Fry. Circumstances changed and Rob was unable to host the website, so it disappeared.

Since the Nowra Nationals in April 2000 a group of Pulse Jet enthusiasts from around Australia have been working together to form an interest group to promote the Jet scene and really get things moving. Their main interests are in Control line models but also support the use of Pulse Jets with Radio Control models as well.

At the Albury Nationals end of 2001 the Pulse Jet Aeromodellers of Australia (PJAA) were formally created and were incorporated as an association in May 2002.

First task has been to propose a set of rule changes in the Control line speed class for the controlling body for Aeromodelling, ‘MAAA’, rules conference in 2001. The proposed rule changes were forwarded to the Chairman of the C/L Technical Subcommittee and have been accepted by the MAAA and published in the Australia rules, C/L section.

In August 2002 PJAA were recognised as a National Special Interest Group with regard to the use of Pulse jets in model aircraft by the MAAA.

We hope to encourage more modellers into flying Pulse jets and to develop a strong competition class through sharing our knowledge and experience with Jets.

Over recent years many advances and improvements have been made with Pulse Jets and some excellent products are now available from the United States.

Quite often when Jets are being flown there will always be one or two people who talk about knowing someone who owns a Pulse Jet but never uses it. We are looking for these people as well as those who would like to have a Jet but think it is too complicated or difficult to purchase, set up or use.

The Pulse Jet Aeromodellers of Australia Inc. have representatives in most states that can be contacted for information about Pulse Jets.

The following list of people can be contacted for more information.

Western Australia – Bob Fry

Victoria – David Axon

New South Wales – Stan Pilgrim

Queensland – John Taylor

Pulse Jet Technical

Since the mechanism of a Pulse Jet engine is different from that of other engines a knowledge of its nomenclature is necessary to understand its operation.

This nomenclature of a typical Dyna Jet engine follows:

VALVE HEAD: This is the finned aluminium part at the front of the engine. It contains the ten valve ports and serves as the mounting for the flowjector as well as for the valve and valve retainer.

COMBUSTION TUBE: This is the welded stainless steel tube forming the body of the engine. The enlarged forward section is known as the combustion chamber. The slender remainder is known as the tail pipe.

FLOWJECTOR: The assembly which is screwed into the venturi tube air intake of the valve head acts as a combined carburetor and starting device. Fuel for starting and operating the engine enters through the metering jet screwed into the outer end of the flowjector. Pressurised air for starting is introduced through the blowpipe which is attached by a strut at an angle to the main flowjector stem. The blowpipe has a threaded end for attaching an ordinary tire pump for starting.

VALVE: The thin spring steel part having ten petals, one covering each valve port.

VALVE RETAINER: The aluminium radiused disc which clamps the valve against the valve head.

OPERATION: Pulse Jet engines operate on the pulse or resonant jet cycle, like the German V1 “Buzz Bomb.” This engine is much more efficient, however, having a very high cycle frequency, When air under pressure is supplied through the blowpipe of the flowjector, a metered charge of fuel is drawn from the fuel tank, through the metering jet, and then through the spray orifices or openings. This metered charge is mixed with air as it passes through the intake venturi and valve ports to the valve. The resulting combustible mixture of fuel and air under pressure then opens the valve petals and passes into the combustion chamber. Here it is fired by the shark plug which receives its electric impulse from a small spark coil. The pressure resulting from combustion closes the valve and sets up a pressure wave of hot gas in the tail pipe of the engine. Once the initial charge has been inducted and fired as above, the operation is automatic. No further forced air supply or spark is required.

The pressure wave set up in the exhaust pipe by the firing of the initial charge is of a cyclical nature. It first becomes a negative pressure (below atmospheric) drawing another metered charge of fuel gasoline and air through the valve and into the combustion chamber. It next becomes a positive pressure (above atmosphere) closing the valve and firing the new charge with the aid of hot gases remaining from the previous combustion. The process is repeated automatically at the rate of 260-280 times per second so long as fuel and air are supplied.

In order for this automatic operation to take place, all parts of the engine must be in balanced design. Alterations of any one element calls for alteration of all other elements in order that harmonic balance may be maintained.

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