- This article was first published in 2006.
I take no responsibility for the initial idea to build a pulsejet powered model – my good friend Bryan Passey is to blame for the inspiration. Always reminiscing about flying pulsejet powered control line speed models, he tried to get me to build and fly one. Frankly, though, a 1.5cc diesel is enough to get me feeling a bit giddy, so I took the pulsejet idea in a slightly different direction.
Now, the only commercially available engine that could easily be obtained produced only 9 lb of thrust. The Z-models Z23 Pulso from Italy had been used by many modellers such as the late Roy Lever in his Vulcan and John Greenfield of the Ghost Squadron. Though I knew the engine to be sound and reliable, 9 lb of thrust just didn’t really cut it. The answer? Use two!
THE SCIENCE BIT
So, how does a pulsejet work? Well, to start with it has only one moving part, a reed made of high tensile steel. Air enters the front of the engine and passes over a venturi (a basic carburettor), which introduces the fuel. This fuel and air mixture carries on past the valve and enters the combustion chamber. Ignition occurs due to residual heat of the previous combustion, creating a massive pulse of positive pressure. As the valve is one way, only letting air into the engine from the front, this pulse has just one direction to travel – rearwards down the exhaust pipe, thus producing forward thrust. This pipe works in the same way as a tuned pipe would on an i.c. engine, but instead of reducing noise it seems to have the opposite effect, with noise levels believed to be around 120db at 7m – not good for your average club field!
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The basic installation of a pulsejet engine is very simple, requiring just a fuel feed and a mechanical fuel shut off valve, so fitting two engines wasn’t going to be a problem. They’re also quite light at just over 9oz (85g) each; the problem comes when you want to start them as with no airflow over the outside of the engine whilst stationary, things can get a little hot!
DESIGN & BUILD
With the engines having arrived from Italy, I began designing the airframe. I wanted a sleek, canard delta that offered good slow-speed handing and minimal drag for high-speed flight. Fixed canards seemed a good idea to keep things simple; if operational they’d only really be useful for low speed manoeuvrability, low speed being something that’s a little tricky without a throttle!
The wing was cut from white foam, using a fulcrum point just beyond the wing tip, to which one end of the hot wire bow was attached. This, incidentally, resulted in only one root rib template being needed. A twin 1/8” ply main spar (which incorporates the Unitract air retracts and engine mounts) was glued into the wing before skinning it in 1/8” balsa. The rest of the model is built up from balsa and ply in a conventional manner, finishing being by way of sprayed Japlac enamel over tissue and dope covering.
Futaba Radio is used throughout comprising a PCM Rx (principally for the fail-safe function), 6V 1500mAh battery and 6 servos employed thus:
Elevons x2 6kg pull (non digital)
Throttle cut-off: x2 micro
Nose wheel steering: x1 micro
Retract selector: x1 micro
With such a radical aerodynamic shape, working out the C of G was always going to be a fairly complicated affair, so to help I studied the many publications written by Alistair Sutherland, and therein found a procedure that seemed reasonable! Having worked out the neutral point I located the undercarriage in this position, the theory being that as long as the model would sit on its nose wheel then the C of G must be in front of the neutral point, and therefore I must have a stable aircraft! In fact, for the first flight I decided to play safe and put an additional few ounces of lead in the nose.
With limited space inside the model I placed the fuel tanks externally on the C of G, and in line with the jets. Some i.c. engines struggle with inverted flight (mixture going rich or lean) if the fuel tanks aren’t in line with the engine, and as I’d never operated a pulsejet before I didn’t want to take any chances. Being out in the breeze, I pondered how I might make the fuel tanks streamlined and aesthetically pleasing, but I needn’t have worried. Bryan Passey jumped in at this point and arranged for a plug and vacuum formed moulds to be made. From these I laid up two carbon fibre shells within which a baffle was installed, the latter also acting as a support for the fuel pick-up. This took a bit of thinking about as the engine is behind the tank rather than in front, and I didn’t want external tubes, pipes etc. With the tanks complete I simply attached them to the wing using brass straps (two each). This mounting method has the added safety feature of allowing the fuel tanks to part company with the airframe should the unthinkable happen, bouncing away from the engines in close formation behind them!
The build was completed just in time for the Autumn Machrihanish Fun Fly held near the Mull of Kintyre. This event attracts modellers from all over the UK, enjoying about one square mile of hard standing that’s suitable for any type of model you can think of. Oh, and noise isn’t a problem either… which is just as well!
With the car suitably packed I set off on the 5-hour drive, collecting Bryan on the way. The forecast was reasonable and spirits were high, in anticipation of a successful weekend. In fact Saturday ended up a bit too blustery for a first flight so we elected to just run the engines and prepare ourselves for Sunday.
Sunday dawned a much better day. Though still a little breezy it was bright, clear and a balmy 15°C! All the paraphernalia was carried to the take-off point, and with everyone in place the start sequence was initiated. With the first engine started it’s vital that the team (4 members!) are swift and efficient in starting the second engine and getting into place to release the model. As luck would have it, the second engine started immediately and I jumped up and grabbed the Tx. With a firm nod of the head the two ground crew restraining the model gave a countdown from three, and she was away.
Swift acceleration, followed by a gentle application of up elevon soon had the model airborne. It was quickly apparent that the C of G was certainly on the forward limit! At this point I lusted for a throttle, as I had no chance to take my fingers off the sticks to trim! After only three circuits one engine decided it had had enough and went quiet – the model seemed perfectly positioned for a dead-stick approach, so I cut the other. With airspeed rapidly decreasing the elevator authority was somewhat lacking and she touched down nose first. Leaving the nose wheel behind the rest of the model came to a halt with no other damage. This first flight was declared a distinct success!
The only damage sustained was the forced extraction of the self-tappers that hold the nose wheel retract in place. With this quickly fixed and with a reasonable chunk of lead removed to get the C of G back, the model was lined up for a second go.
Having suspected a slightly rich mixture on the previously failed engine, the acceleration was even smarter this time. Rotation occurred just as before, but it was obvious that the C of G was still too far forward. Feeling a little more comfortable with the model and getting a chance to trim a little elevator, I tried a few manoeuvres. Rolls, Immelmans and reversals were all smooth, tidy and precise, pretty much as you’d expect from a highly loaded, high-powered aircraft. After a couple of passes for the camera I set the Aurora up for landing. Travelling down wind in a circuit pattern I stopped both engines and, though better than the previous flight, the glide still resembled the proverbial brick and she touched down well short of my planned area. At least the nose wheel was still intact this time!
Since that flight I’ve removed all the lead from the nose, bringing the C of G back by a further inch (25mm). All the little touch-up jobs have been completed and we’re waiting for another opportunity to visit Machrihanish to complete the test flying. All being well I hope to take the model to a few select shows this year; it will certainly be at the Machrihanish Fun fly event, held every spring and autumn.
Aircraft type: Sports delta
Designed by: Steve Rickett
Wingspan: 46'' (1168mm)
Fuselage length: 79'' (2007mm)
Wing area: 5sq. ft.
All-up weight: 12 lb (5.4kg)
Wing loading: 38oz / sq. ft.
Engines: 2 x Zanin Z23 pulsejets (9 lb thrust each)
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