My Pulse jets!

Quick History..

I once bought (a good ten years ago) a new (very old stock!) American Dynajet engine complete with all the 60's style instructions and advertising papers.

This little thing started easily on a bottle of compressed air, I ran it on 'Avgas' which is just good petrol, and it had a fixed fuel jet. It made loads of heat and noise! but only around 4-1/4 Lb. thrust. Due to the fuel system which relied upon the venturi effect to 'draw' fuel from the tank, the tank had to be level with the motor. If lower then it would run lean and more so under high positive G 'forces and rich under negative ones.

What it really needed was a pump fed system. A (Kavan) high pressure fuel pump and a 'bag' instead of a tank to make sure that no air got to the fuel jet. No conventional flywheel means that if even a small air bubble gets to the motor, then it simply stops! Instantly. Due to the weight of batteries and pump, the motor was a little impractical for RC operation on an aircraft, because you only have low thrust to play with. By the time you add the flight pack (radio system) etc the power would have been very marginal and so it was sold.

Then I built a number of home designed jets that were never intended to fly, far too heavy!, and discovered that almost anything will run if the small resonator part of the tube is made long enough. Most used car and truck exhaust pipe tubing and two stroke Yamaha motorcycle reed valves, oh and also one from an outboard!

Next I obtained an un-run and still boxed Italian built motor about 5 years ago. This was built by a company that was called Z-Jet or similar (Anyone know more?). They had two different pulse jets that looked very similar, but the one I have is the more powerful of the two and a little bigger in diameter. It also had a restrictive venturi, and a fuel cut off lever, and a fuel chamber mounted on the front that fed fuel from the middle, in an attempt to avoid fuel 'bubbles' in the supply. It also had a needle valve to allow mixture to be adjusted. I ran it in this configuration on the 'recommended' petrol and it ran well! And very loud! Thrust was surprisingly good but was unfortunately was not written down at the time, and I cannot remember the figures!

Fuel

This thrust was subsequently improved the following day by around 20 % due to the following mods. Fuel changed to Kerosene-70%, Propylene Oxide-3%, the remainder being Avgas (basically petrol)-27%. The Propylene oxide is purely to keep the fuel 'bags' soft. The fuel was fed from bags (no air!) via a Kavan geared pump, and an in line needle valve. This change alone caused the motor to run much more 'evenly' and seemed to reduce the pulse frequency (should have recorded it!). Also safer in a crash.

Next the restrictive venturi diameter was machined out slightly by 3mm (it is restrictive purely to allow the motor to draw its own fuel) and the strange mixture control / anti fuel bubble header tank / cut off was removed and filed in the bin. This saved weight (loads!) and allowed the motor to breath 'normally' as nothing was now in 'front' of it. The motor now can not be started from fuel tank directly as the Kerosene is not atomized or vaporized that well and it cannot draw its own fuel on start-up any longer.

Next the metal 'washer' that forms the reed valve or petal 'stops' was modified to allow the very conservative opening to be increased slightly.

Fuel is fed into the combustion chamber directly now, (after drilling through the centre of the bolt that holds the reed valve assembly through to the bolt head - not right through!) and then drilling six small holes across the bolt head flats, across the centre drilling. Fuel now sprays out of each flat inside of the combustion chamber in a sort of 'fuel plane' around 10 mm behind the reed valve petals. This arrangement works very well.

The engine was run after these mods and was noticeably lower in frequency and dead stable and smooth. In addition to the increase in thrust, the combustion chamber / tube now runs MUCH cooler and hardly glows. Due to the pressurized fuel delivery system, G 'forces can not really have any effect on mixture now. Starting is more complicated to arrange but very consistent using the following procedure.

1. Fuel up bags carefully, squeeze out all air right up to the pump!

2. Connect temporary spark plug lead (glow plug sized plug was fitted into combustion chamber). Start spark going!

3. Fire a mixture of compressed air and anesthetic either (the Dutch use 'Easy-Start' car engine aerosols!) into the intake. Due to to the wide mixture toleration of this stuff, it will run regardless of how correct the ratio is, but with very low power and sound rough.

4. Using the throttle stick turn on the fuel (pump) and the thing will run properly.

5. Discontinue Spark and air/either supply.

6. Launch it! (or put your fingers in your ears...)

Pics

The following photograph shows the motor mounted on a plane built as a flying testbed, that I never flew because of a lack of time mainly and a noise problem.. Yet!!

This plane was basically designed for far more power, and has mounting plates built in to the wing for two of these motors. However now that I've started playing with turbines, it will probably get used for testing these now.

 

No the picture is not upside down! The plane that the motor is sat on is hanging from the ceiling upside-down..

 

See ! This plane is much bigger than the photo makes it appear, as it was designed as a flying testbed, and to be slow (ish) and have low wing loading with subsequently safer handling. I suspect it may be faster than I expected it to be after seeing other similar aircraft fly. Should be no problem as my flying abilities are pretty good after years of flying pylon race type aircraft and very fast deltas of various kinds. My kind of model planes all have to leave completely straight up! This means they need to have about 2 or 3 times as much thrust as they weigh, or they slow down noticeably after a 1000 ft climb... The plaster is new, as is the room, yet to be decorated.

Interestingly the Dutch AMT gas turbine building crew also fly similar engines of comparable size and they too use a very similar direct injection fuel system, and the have discovered that their motor can be throttled smoothly from 2LB. up to 19LB. simply by controlling the pump speed using a model car speed controller, and very quickly, due to the lack of a turbine to spin up and down. (Spool time) This is something that I never though would be possible! So have never tried it. Yet! I simply ran the pump from a battery pack, controlled via a microswitch / servo arrangement, and fuel flow adjusted for maximum thrust using a needle valve in the 'cockpit'.

This plane is seriously fast, and loud but unfortunately not mine The photograph was taken at the 'Woodvale Show' in England around 13 years ago. 250 mph plus and no wheels or throttle, with 3 minutes of fuel on board. During a very fast low pass the motor went out - silence! - and the pilot continued to the other end of the very long runway, still doing a slow roll, and pulled up from about 15 ft high and vanished straight up into the clouds. Good trick with a dead motor! It inspired me to build all kinds of stuff...

This is the small Dynajet motor. Friend and fellow non club member John Spinks is stood in a dangerous place. Strangely all the local model plane clubs have banned us lot! 

This is OK by me because I always said that I would never join any club that would have me as a member... The UFO in the top of the picture is a drawing pin hole. So that's how they do it!

This thing started it all. And also nearly finished it too! email me for drawings etc - big file..

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