Jetex Radio??? You crazy?
Well, maybe, but why not? ... just airlift it X-15 style, fuse it off and cut it free at high altitude. Try one!
See dimensioned construction plan at end of article
The "Drop Out"
by Don McGovern
The "Drop Out" is a handy term used by educators to label the many misguided students who take the easy road and leave school before graduation for one reason or another. The term "Drop-Out" has come to stand for a large number of such youth who generally find the choice a rocky downhill road in our fast moving world where education is a necessity.
"Drop-Out" pretty well describes this aircraft design also, though about the only similarity you will find between a poor student and this unique Radio Jetex craft is that they both start out in life and flight on a downhill descent path.
A Jetex powered Radio craft sounds pretty unlikely we'll admit. However, it is a very nice arrangement if you think it out past its obvious drawbacks. On the negative side of the ledger, Jetex is a short running type engine, developing thrust for 12 to 16 seconds. It is a very low-powered type engine, developing just a few ounces of thrust in the larger sized "Scorpion", and fractional ounces in the smaller sizes. Thus, it is difficult to think in terms of lifting additional ounces of payload in radio equipment, and even if you succeeded in becoming airborne, it would no sooner take-off than it would fizzle out. About the same deal as rubber-radio. A nutty dream project barely possible, but not very practical. Or is it? It is!!! It becomes possible, and practical when we re-adjust our thinking and de- sign around the noble Jetex engine's obvious limitations. As most modelers know, the Jetex engines come in a variety of sizes, from the small Jetex 50 units which will fly a hand-launch glider sized affair, to the Jetex 150, which is used for Contest Rocket flying, lifting ships of about 30" span to thermal-hunting heights. The largest Jetex engine available is the "Scorpion", developing some six ounces of static thrust, and capable of flying a moderate-sized aircraft. This unit is of course the most practical for our needs, but not necessarily the only one we can use. A pair of Jetex 150's for instance will offer the possibility of firing one after the other for a lower-powered, but longer power run. Or, the smaller engines could be ganged into a cluster and fired together if you had reason to do so. The "Scorpion" however remains the best choice for the powerplant. While Jetex thrust is nicely suited to contest free-flight endurance events where thermals take-over after a power run, it leaves a radio design with but a brief few seconds of usable thrust. And, not a lot of power at that when com- pared to the thrust generated by a conventional glo engine swinging a prop. Yet, it does have advantages easily overlooked ... It is ready-to-go power, already stored, needing only a fuse to touch it off, and so can be fired in mid flight at high altitude. It is a torque- less form of power, which will not whip you into a spin to the left. And, it is compact, streamlined, without the en- cumbrance of a large drag creating prop to mar a glide once power is off. "Drop-Out" is the answer. As the student drops out of school, so can a Jetex design be dropped from a larger mother plane, much the same as the "X-15" research aircraft is dropped from a B-52 many thousands of feet high. Actually, the X-15 and the "Drop- Out" have the same problem in common, that of preserving limited rocket fuel for the portion of flight where it will do the most good. A brief few minutes of powered flight would be wasted on a conventional take-off and climb to altitude, for the B-52 can do this for the X-15, with no cost in rocket fuel. It's flight can then start at 40,000 feet with the aircraft in a clean con- figuration, and make every drop of fuel pay off in performance.
Our Jetex counterpart "drops away" from a larger mother ship in much the same fashion, with its Jetex fuse ignited from a larger dethermalizer type fuse an instant before separation. Thus, our impossible Jetex-Radio design finds itself at perhaps 500 to 1,000 feet, in flight altitude, and the thrust just starting to develop. And this thrust is not to be sneezed at. There is a big difference between the output of a Rocket engine at launch and zero speed, and that at the end of the power run. While a conventional engine develops rather constant thrust throughout a flight, these rocket type engines keep accelerating the model all through the power run. As our "Drop-Out" is released at near flying speed, after a loss of just a few feet, the aircraft is in full flight and capable of gaining a modest amount of additional altitude under Jetex thrust. This altitude gain is not really necessary however, and actually the model can turn in its flight performance without the unit even being lit. It just makes it more fun to have the thrust available for whatever you wish to do with it, be it a climb to additional altitude or some wild stunt flying, with gravity helping out to attain speed. The "Drop-Out" craft itself is simple to build, rugged for its size, compact for airlifting requirements, and needs only slight rudder control to trim it for its gliding flight. The span of wing and stab is variable, and can be shortened further for higher speeds, or increased somewhat for a more floating glide. It should be modified with care and an eye on the capabilities and flight speeds of the mother craft which will airlift it. A fast multi for instance might create excessive strains on an enlarged "Drop- Out" wing, creating a flutter in flight. Too small an aircraft may have difficulties handling the weight of the "Drop-Out", causing all kinds of troubles. Most aircraft from a .19 engine size on up will be capable enough for the task, but it is best to select a design that can be hand-launched, so that the "Drop-Out" can be slung beneath it. An old boxy single channel crate with about a 56" span or so should be about ideal, and will possess the slow flight characteristics that are desired. Needless to say, the radio equipment chosen for the "Drop-Out" should be of the lightweight miniature variety, and on a different frequency from the mother craft, which will still be in flight as you start to feed the "Drop- Out" signals to trim its flight. Elevator trim is possible as an optional feature if you like, and will of course add to the stunt characteristics of the model. Bear in mind that the Jetex thrust is but a temporary thing, however gravity will take its place, and you can achieve whatever flight speeds you desire, at a cost of altitude, for looping or spinning, or whatever.
The design hangs the engine at the C.G., to keep the aircraft in trim whether the engine is fueled up, or burnt out. It gives the engine a mounting with good air-cooling characteristics, and it should be slung under the C.G. of your mother craft. If this is not practical with the design at hand, you might consider mounting it above the mother craft. If the speed of the mother ship is greater than the "Drop- Out's", it will tend to zoom away from it. If the craft airlifting it is a slower type design, better to hand launch, with the "Drop-Out" suspended beneath. In either case, the model should be rigidly mounted to prevent shifting incidences in flight, as well as shifting rudders conflicting with each other. A single point of release controlled by the burning fuse is your most reliable way to launch. By splicing the smaller Jetex fuse into the strands of the D/T fuse material, the Jetex can be ignited at the same instant the model is released, or a few seconds before the moment of release if you prefer.
It is also nice to know about when the models will part in mid-air. The same fuse in burning could release a bit of chaff or colored tissue about 30 seconds prior to release as a warning. This would advise you visually to run the models upwind and make-ready to hand the transmitter of the mother aircraft to a buddy while you jump on the problems of your itty-bitty Jetex glide bomb. That's about what it is, though it does possess a reasonably flat glide capability. It is possible it might play a bit with an occasional thermal, but it would have to be a moderate boomer to give you good lift. This drop-glider is small in span, weighted with Jetex, and hits a pretty good clip. Get your kicks from its speedy characteristics rather than comparing it to a thermal soaring glider which it is not.
As on all radio craft, pay attention to the installation of the electronics, cushioning them well in foam rubber padding. The escapement will only need a relatively short length of rubber for the winds, and all torque rods and linkages etc. must be free-working to avoid foul-ups.
In mounting the "Drop-Out" on the mother craft, it is best to follow biplane design, rigging the design so that the "Drop-Out" has about 1 degree less incidence than the mother craft, when slung underneath, and a shade more incidence than the larger craft if carried above the model. In this case, try to mount it reasonably high above the wing of the larger ship, to lessen air disturbances etc. which could cause a loss of lift and a bad stall.
Trim out both ships individually in flight as well as you can before linking them together. The larger craft should be flown under its own power, while the "Drop-Out" can be glide- tested from a hillside or other elevated launching spot. Balance -carefully, and trim for straight flight. The rudder control will compensate for most other trim when necessary. It responds quickly to rudder, so don't hang on the button for more than a moment. A touch of up-elevator tied into the rudder motion will lessen the tendency to enter into a diving turn. Try to gain all the altitude you can for initial flights, timing the separation when the craft are upwind of the field. Concentrate on just your "Drop-Out", as watching both in the air is most confusing. Let a buddy handle the powered mother ship, while you keep the "Drop-Out" under control. First flights should be made without the Jetex engine being ignited, to check it out in the glide. This will give you a better feel of the model, as you can trim it out for any stalling, diving or turning characteristics, with- out the complications of the Scorpion engine's thrust, which is quite a bit. Once the powered Jetex flights are at- tempted, the corrections for trim which might be required will be in the thrust- line of the Jetex engine itself, much as you might alter the thrustline of a conventional engine.
As the power developed by the Scorpion engine is considerable to a model already in flight, it might be a good idea to make initial flights with a Jetex 150 engine first. Possibly two of these engines could then be fired for further flights before installing the larger and more powerful "Scorpion" Jetex engine. If you choose to cluster two Jetex 150 units, try to fuse them simultaneously and attach them close together to minimize asymmetrical flight effects should one misfire in flight. Rudder throw will compensate somewhat for this if mounted side by side, while if suspended one beneath the other, the model might require only elevator correction if one fails to ignite. Actually this is a safer method of positioning the engines, as if only one engine develops thrust, it is not likely to loop with half the power, nor to dive as it is already trimmed for the gliding flight. It will just soar around for the duration of the fuel charge. If mounted side by side, one engine developing full thrust paired with a dead engine will tend to force a turn toward the inactive engine, which could develop into a spin, in spite of opposite rudder commands. Every model favors a turn in one direction or the other, and little warps and conditions of the moment cans add up to splinters all over the landscape.
One further word of caution: Don't fly in tinder dry areas where a hot engine can trigger a brush fire. If in doubt, rig a shield under the units to allow air cooling, but deflect dry weeds. It is not the normal flight which causes trouble, as it cools in the air. Rather the unexpected dive under power when the engine is still hot. Have fun.
