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|Reprinted from Model Airplane News, Nov. 1948, pp. 27, 42-43
This model is powered by a simple and safe jet unit – results are excellent
The Swish is a free flight jet-propelled airplane developed especially for the new Jetex power unit recently invented in England and now becoming available in this country. For the writer, this ship proved the most interesting and unusual project yet encountered – stick around and see what we mean!
The Jetex motor is manufactured by Wilmot, Mansour & Company Ltd., Salisbury Road, Totton, Hants, England, and a fiendishly clever gadget it is! It is at present available in two sizes: Jetex 100 for models of 18" to 30" span, and Jetex 200 for ships of 24" to 42". The smaller unit has a thrust of 1 oz. lasting for 20 seconds; the larger unit a thrust of 1 3/4 to 2 oz. for 20-40 seconds. In appearance the unit is, roughly speaking, an aluminum cylinder of about 1 inch diameter, with a removable end held firmly in place by three strong spring-loaded clamps. The fuel is a 3/4" long solid, cylindrical in shape to slide into the cylinder, and hard and brown, resembling a slice from a wood dowel. The small Jetex takes one of these charges, the bigger unit two charges. A fuse is coiled to fit against the charge inside the cylinder with one end protruding through a small hole in the center of the removable end of the cylinder. The escaping gases passing through this hole provide the thrust.
A lot of questions will spring to mind. How does the fuel burn? Is it safe? Does the unit get hot? How strong is that thrust when it comes to flying a model? First of all, the fuel was found quite safe and simple to handle according to directions in the instruction booklet. It burns with a gentle hissing sound, more like a swish when the ship is in flight (from whence the name “Swish”). The unit does get hot but not dangerously so. As long as the metal of the unit does not come into direct contact with paper or wood there is no danger of the heated metal igniting the structure. While it does get too hot to handle with the naked hand, the heat is far less than one would expect. As to thrust, the Jetex seems incomparably better than using a C02 capsule as a straight jet, but not as “hot” in our estimation (at least in this size model) as the performance of a good C02 engine. It has, however, ample thrust for good sport flying and definitely is not a disappointment. This observation is based on the performance of the Jetex 100; the larger unit was not available at the time of these experiments.
A point that should be made immediately is that the first model followed closely the suggestions of the Jetex booklet; it is possible to develop other sized models – for example, it may be possible to fly this same size ship with a larger unit, or to experiment further with different size and weight ships.
In designing a ship for the Jetex it is essential to keep in mind the proper cooling and ventilation of the powerplant. A minimum space of 1/16" must be left between the engine and any cowling or side of a fuselage. A good air flow seems essential for the development of full thrust, although in the dozen test flights made thus far this aspect had not been too closely observed. One thing that does pose a threat to the model is the fuse which flares up when first ignited. After a near fire on the first flight, the length of fuse sticking out of the unit was restricted to less than the recommended 1/4" and the cowl was cut back a little from the fuse station. The fuse occasionally throws fine sparks and it might be a good idea to protect the tissue wing covering back of the fuse on this particular design; a piece of bond paper would be sufficient.
The design problem is an interesting one. The most important consideration is weight, which should be held down to a maximum of 3 oz. including motor, for the small powerplant, and 5 1/2 oz. for the big unit. This weight, incidentally, corresponds to that of the structure (no motor) of a well designed rubber model. The model will fly at a greater weight but presumably performance suffers. After much head scratching one comes up with the impression that a Jetex-powered free flighter is essentially a towline type of model. The aspect ratio of course will be much less, and other proportions may differ somewhat, but you end up with a ship that is apt to fly like a towliner and to need nose ballast.
In the case of the Swish, a straight wing doubtless would have been located far back toward the tail to get gliding trim. The fuselage forward represents practically no weight useful in balancing or getting the e.g. where it should be. It was decided to employ a sweptback wing at no incidence and to limit it to the position on the plans for the sake of appearance. To trim, solder or clay must be added to the nose.
A study was made of various configurations suggested by the Jetex booklet and the Swish is based on one of their force diagrams. It was felt that the high thrust line position would be approximately at the center of resistance of the ’ship, minimizing any climbing or diving tendencies under power, and any difference between power-on and power-off trim. The c.g. threatened to be high, but with no torque to contend with it was believed that the ship would exhibit no real vices.
Construction is orthodox throughout, the fuselage being a box, and the wing a two-spar affair. The front spar runs through the center of the ribs – a la Hank Cole’s wings – an arrangement we found both lighter and stronger than a wing with the same sized spars on either top or bottom surface. It is cleaner aerodynamically and resists warping to a great degree. Probably the greatest lesson learned from this ship was that construction must be absolutely true with all flying surfaces squarely in alignment, both when viewed from above or from the front. Brother, when you see a free flight model under power without torque you appreciate the effects of every little twist.
Walt Schroder joined us for the test flying. The ship was trimmed for glide and even given mild tow launches by putting the towline around the hooked end of the landing skid. Then came the big moment. What was going to happen? Did the thing go off with a bang? Would the ship burn up? What would it do under power? How fast would it go?
Knowing so little about such a revolutionary development gives one a bad time on the first power launch!
A lighted cigarette was put to the fuse which immediately went to town. Now, the Jetex booklet tell you to hang on until you are sure the unit is “jetting”, but until you have “teethed” on a few flights you don't realize how long this takes. At any rate Walt launched the ship, which, lacking full power, settled in front of him. Picking it up quickly he gave it one of those Brooklyn discus heaves. Maybe he figured it should go up like a rocket, but three quick launches were followed by three quick stalls and power dives. All the while the doting designer (yours truly) was screaming at him to hang on and try a gentle rubber-type launch. In the confusion the neighbor's sheep stampeded, baaing and bleating across the field.
By the time the second and third flights had been put in we had the hang of the thing. After the fuse is ignited and burns quickly into the cylinder with a pronounced hiss, all action seems to die down for a moment. A very gentle hissing follows, increasing slowly in volume for a second or two, followed quickly by a real fast build-up in thrust. You feel this thrust build-up as you continue holding the model and then gently launch it almost letting it fly from the hand like a rubber model.
Now we were getting gentle stalls, with the ship falling off into spirals. Since the model was trimmed to glide well, this meant the problem of thrust adjustments had to be solved. The clamp that holds the unit in place is held onto this model by two screws (all parts being part of the powerplant kit). By loosening the screws, shims of paper match covers could be placed under either end of the clamp to vary the thrust. Adding shims under the rear would increase the downthrust in the usual sense. Elongated holes in the clip permits sliding the clamp sideways for offset thrust when the screws are loosened.
The theory is that on pushers you use what would normally be down thrust to get up thrust and vice versa. After a flight consisting of a series of wicked stalls, conventional adjustments were resorted to. Thrust adjustments are fairly positive and the addition of one paper shim makes quite a difference in the power flight. The position of the Jetex unit on the plan has been changed to suit the adjustments worked out on the experimental model.
Once adjusted for good flights the Swish is quite impressive. If it should turn sharply, it wheels much like a glider. It is suggested that the glide be adjusted for straight ahead flight or with very slight turn. A turn in the glide tends to build up under power. Should the model spiral under power it probably will not continue all the way down to the ground, but even if it strikes the ground no damage results, provided the surface is grass, weeds, etc. The turn imparted to the model at the moment of launching seems to govern the direction and degree of bank in the power flight. After a really good power flight the model glides exceptionally well, and on one occasion during an evening's flying indulged in an impressive bit of ridge soaring. This glide after power should be in a wide moderate circle; if power turns are too sharp, make final, adjustments with off-set thrust.
Based on the experience of twelve flights – all that the fuel charges in the motor kit would permit! (extra charges can be purchased) – it would seem that a smaller model than the 30" Swish should exhibit fairly spectacular power performance. The powerplant might be mounted under the fuselage and well forward toward the nose. This would lessen the amount of ballast and, if the nose were long . enough, might eliminate ballast completely. Use of a lifting type tail should also eliminate ballast and offer some compensation for the low mounting of the thrust line. Downthrust should permit adequate power trimming.
But make no mistake, the Swish will put up good flights. It is capable of a minimum of 75 feet altitude under power without soaring (which it does very well) and a duration of at least 1 1/2 min.
|swish 100 jetex bill winter
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