Jetset (oz5518)

About this Plan

Jetset. 28in span Jetex free flight model by Len Ranson. Free plan with Aeromodeller April 1972.

Quote: "A lightweight 28 in span free-flight design for use with the Jetex Paa-loader 150 motor unit. Jetset, by Len Ranson

IT IS PERHAPS one of the disappointments of the model flying movement that, because of the technical complexity of the turbo jet unit, it has never been possible to reproduce it in a simple, miniaturised form. This, in a way, has kept the model aircraft a critical step behind full-size development, particularly in the area of scale verisimilitude. True, a pulse jet engine was available for some years after the war. and made an exciting impact on the model scene, but the only really acceptable form of jet-type flight has been afforded through the simple, solid fuel cylinder, working on a gas expansion principle. Now popularly known by its trade name, Jetex, it has proved over the years, a safe and relatively cheap method of jet or rocket propulsion.

The writers experience of Jetex power goes back to the early 1950s, when there was a whole range of motors to choose from, of which the most popular were the famous 50 unit, still in production, and the larger 'Jetmaster'. The latter was capable of powering a duration model of some 30 inch wingspan, albeit that the model had to be built extremely light in order to achieve a good rate of climb. The models however, were exciting to fly and the units quite cheap to operate. A number of Jetex contests, flown on a power run overall flight time ratio, were organised during the 1950s both by the SMAE and the manufacturers.

The Paa-loader is an obvious development of the old Jetmaster, taking identical pellets and giving a similar performance. It differs, however, in two respects: the casing is of a harder substance and instead of a jet nozzle the gas escape is through a domed outlet in the end cap. Loading is simple, but the utmost care must be taken not to disturb the rather brittle ignition wick from its copper core. The minute fractures which can occur when ramming home the gauze or feeding the wick through the end cap are the most common cause of ignition failure. These failures are not only irritating but rather expensive. Also frustrating arc the ignition failures due to damp fuel pellets; it is essential that the pellets be kept in a dry place (the wick too) and the lid of the tin replaced immediately upon the removal of pellets.

I found the unit to be very consistent in its power output - that is, from flight to flight. This was due to the incidence of leakage from the cylinder/end cap joint being almost negligible - the Paa-loader being better in this respect than the early Jetmaster. My only criticism here is of the tendency of the asbestos scaling washers to disintegrate, sometimes after only one firing. Duration of thrust per pellet I found to be 6-7 seconds, and approximately 13 seconds for two pellets. I did not fire three pellets, as I felt that two pellets per flight was quite expensive enough, say five new pence, and also given a reasonable model, a two-pellet flight was sufficient for small field.

Taking up this question of cost per flight, the figure of five new pence per two-pellet firing is not perhaps excessive if the model is used just for competition but could be considered high for sport flying, particularly for the younger modeller. Personally, I would welcome a larger, cheaper pellet, and a more reliable wick (indeed, the latter is now improved, the latest sample from the distributors, KeilKraft, overcoming all the short-comings of the previous item).

Models designed for the Paa-loader most necessarily be efficient and highly consistent if the best use is to be made of the limited thrust time available. This means that the model must attain a fair height on each flight in order to economically convert fuel expenditure into useful flying time. Unlike an orthodox airscrew engine you cannot make up for poor performance by adding an extra drop of fuel; short flights and non flights use the same amount of fuel as a long flight. Then again, you must fly from the start on all-out power: you cannot throttle back, as it were. However trimming is helped by the absence of torque and gyroscopic effects, and given that you are testing the model over fairly long grass, the very light hies models are not highly subject to damage.

The Paa-loader has been reviewed here from the point of view of an average modeller looking for a diversionary interest, and here the unit can give a tremendous amount of flying field pleasure.

The Model: In order to make more practical use of the motor, a suitable model was then designed and built - hence 'Jetset'. Naturally, a good flight must incorporate a good glide, which means that your Jetex model should be as large as possible consistent with a high rate of climb. Previous experience of such models would indicate a 28 to 30 in span for the Paa-loader unit, built to a weight approximately equal to that of the unit: 1-1/2 ounces. It is not too difficult to build down to this weight, at least not for the experienced modeller, provided that care is taken with the selection of the balsa. However, the model must be strong enough to take an occasional full-power dive in, and if damaged, should be so constructed that repair is easily effected.

This fuselage is triangular in section, the base and upright being of 1/16 medium sheet and the angle fillets of 1/32 medium sheet. After cementing the upright to the base, the fillets can be measured and cut in. This is a cleaner, more accurate method than tracing each former piece from a plan. In the same way the wing support struts should be measured and cut after the wing platforms have been cemented in place and correctly aligned. The tail platforms do not require such supporting struts.

It is possible to build the whole fuselage of 1/32 sheet, perhaps desirable if you have competition flying in mind. For this, really good quality sheet is essential; very soft or very hard woods must be avoided. If the fin is of 1/32 outline cover only one side otherwise severe warping may result.

The motor mount is securely screwed to a strip of 1/16 plywood cemented on the underside of the fuselage.

Wing and tailplane construction is slightly unconventional in that they are not built into a set of notched ribs but rather built up on a simple jig consisting of two spanwise supporting strips. The leading and trailing edges should be shaped and notched (but not at tips) before being pinned down over the undercamber support strips. The ribs can then be inserted into the prepared notches to the depth indicated on the plan. Naturally the outer panels must be constructed separately, chamfering the spars to form the dihedral angle. Note that the tip pieces of 3/32 sheet should be slightly oversize, and then cut in to fit. Finally, sand down ribs to the prescribed airfoil shape, and clean up the whole assembly.

The tailplane is constructed in a similar way to the wing. Epoxy the DT hooks in position as shown.

Dope the whole structure and lightly sandpaper before covering with lightweight Modelspan. Lightly watershrink, and then, remembering you are dealing with a very light structure, dope with a thinned down solution, adding up to 50 per cent thinners. This is most important, as a too strong application of dope can warp the model out of all recognition.

Flying: Adjust fore and aft trim by moving the motor unit back and forth. It may be that the model is still over-elevated when the motor is in the most forward position - in this case do not adjust the flying surfaces, but add small amount of plasticene to the nose - on no account pack up the tailplane or alter the wing incidence.

Take time with hand glides, as the Jetex engine is flat out on power from the word go. See that the glide is flat with little or no tendency to turn. Jetex models tend to 'twizzle' up as the momentum builds, and in this way appear to find their own natural turn; any degree of induced turn can lead to a disastrous spiral. Altoether, a flat, straight hand glide makes for a safe first flight. Glide turn, if insufficient, can be increased by tail tilt.

Two pellets should be adequate for reasonable flight performance on this model, though for a really high ceiling three must be used. One pellet should be sufficient for your test flights."

Direct submission to Outerzone.

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