Jets and the Toledo Show

Model Airplane News, Aug 1998 by Leu, George

ETS HAVE ALWAYS intrigued me as scale subjects because of the wide variety of kits available; that they fly like pattern ships is also a plus. While interest in ducted fans continues to grow, jet models aren't considered mainstream and probably will never be part of every modeler's inventory. This is too bad. If you haven't thought of a jet as a possible future project, maybe a little more information will help generate some interest.

DUCTED FANS 101

I love jets and have a fair number of them in my workshop. The power system is fairly simple and in principle, is the same for all ducted-fan models. The power system consists of an air inlet and duct, a fan unit, the engine/tuned pipe and a thrust tube. The fan unit is composed of an outer shroud, impeller blades and hub attached to the engine, and stator blades just behind the impeller. As the air is drawn into the inlet, guided through the impeller and then out through the thrust tube, the highvelocity air exiting the model generates the thrust.

Although all the parts of a ductedfan model are important, the design of the impeller blades is most critical. The fan shroud helps direct the airflow rearward and prevents "tip losses," i.e., losses in efficiency caused by vortices generated at the impeller tips. The fixed stator blades at the rear of the unit straighten the swirling airflow from the impeller. If the impeller is too small for your engine or doesn't have enough pitch, the engine may exceed its maximum rpm limit. This may damage the engine, and the connecting rod usually fails when this happens.

Conversely, an impeller that doesn't reach peak rpm won't generate its rated thrust. Ducted-fan engines produce very high rpm and, when properly set up, do an excellent job powering jet models. Tuned pipes are the norm for these engines.

Over the years, several popular ducted-fan units have been developed, including the Bob Violett Models* Viojett, the Jet Model Products* Dynamax, the Jet Hangar Hobbies* Turbax and the Byron Originals* Byrojet. In the Byrojet, the engine, exhaust header and tuned pipe all sit in front of the fan unit, and this is called a "pusher" design. In the other units, the engines and exhaust systems are behind the fans, and this is called a "tractor" design.

The idea behind a pusher design is that by keeping the airflow through the thrust tube undisturbed, more exhaust velocity is sent out as thrust. On the other hand, tractor setups rely on air flowing smoothly through an intake duct before it enters the fan unit to increase efficiency and thrust velocity. Various fiberglass fairings cover the engine and pipe in the Viojett, Turbax and Dynamax units to minimize air drag around the engine and exhaust components.

Planes designed around the Byrojet often don't have inlet ducting at all because the engine/header/pipe assembly is in front of the fan unit. Some Byrojet-powered models have auxiliary air intakes (cheater holes) on the underside of the fuselage to supplement the airflow into the impeller.

The area of the 6-inch-diameter Byrojet is approximately 43 percent larger than the area of the 5-inchdiameter Turbax, Dynamax or Viojett units, and the Byrojet's takeoff acceleration is very good. If you fly from grass fields, the Byrojet will probably get your jet airborne faster. However, experience with the Byrojet shows that once you achieve about 135 to 150mph flight speed, your aircraft is hard-pressed to go much faster.

In contrast, well-designed, front-mounted inlet designs like those of the Dynamax, Turbax and Viojett units channel air smoothly into the fan, even at higher flight speeds. Though initial acceleration may be slow, once airborne, these units really excel. Like the inlets, the thrust tubes are made with very smooth internal surfaces to keep airflow velocity up and drag down as the air travels through the system. Typically, the diameter of the thrust tube tapers down to about 90 percent of the fan's diameter.

Keep in mind that ducted-fan technology is continually changing, and I have only scratched the surface here. Don't hesitate to contact these jet manufacturers for more information. SCALE JETS

Jet models do very well at the Nats, Scale Masters Championship and the Top Gun Scale Invitational. Speaking from experience, I know jets have an advantage in competition. They fly like pattern ships as a result of high flight speeds and very little engine torque. The relatively small size of the impeller produces less torque than that of a model with a large propeller. There is no air blast from a prop over the elevator and rudder, so there is less control available until you increase airspeed. One hundred- to 200-foot takeoff runs are the norm for jets. One positive note is that jets usually go where they're pointed during takeoff, and the typical tricycle landinggear arrangement makes ground handling a breeze.

Flying maneuvers at higher airspeeds is another advantage. A jet doing a 170mph fly-past maneuver will cover 200 feet in only a few seconds. Compared to a WW I biplane, which will probably cover that distance in about 30 seconds, the jet simply experiences fewer corrections during its short time in the judging window.