Post
by larry cottrill » Tue Mar 30, 2004 3:41 pm
The plane is a counterclockwise-running U-control swept-wing design of conventional configuration, but with a ‘profile’ [flat slab] fuselage. The fuselage comprises a sandwich of two sheets of 1/8-inch plywood around a balsa truss interior. Also in the interior are hardwood engine mounts for Maggie Muggs at the tail and a .60 cu in piston engine at the front. The horizontal tail plane is mounted well above the airstream around Maggie. Tricycle gear with large, narrow-profile tires is provided for takeoff and landing on mown grass. Wingspan is only about 40 inches, since this is meant to be a fast sucker. The rudder is canted slightly to the outside of the circle, to ensure control line tension, in the usual manner.
The ‘launch engine’ swings a 12-inch diameter, 6- or 8-inch pitch propeller. This engine is mounted with the cylinder to the outside of the flight circle, keeping both the main engine mass and the oily exhaust stream to the outside. The fuel tank for this engine is sized small, so that it will only serve to keep the engine running for launch and flight acceleration. The prop is set up to be approximately vertical [aligned with the plane of the fuselage] when it comes to rest against piston compression.
Maggie is mounted on the INSIDE surface at the tail – usually not good practice, but suitable in this case because of Maggie’s fairly low mass compared to the piston mill at the front. Maggie’s fuel supply consists of an aluminum cylinder of pressurized butane, oriented at a severe angle to the thrust line so that vapor will be available at the outlet [i.e. the cylinder will fit THROUGH the fuselage in a slot beneath the wing, so its outlet will be forward and somewhat to the INSIDE of the flight circle; centrifugal action will keep the liquid toward the OUTSIDE]. The valve will be depressed by quickly snapping the cylinder into place within a 1/8-inch plywood frame after the propeller mill is up and running.
A fairly roomy duct will run along the inside fuselage surface from immediately behind the propeller ‘disk’ to immediately in front of Maggie’s intake mouth. This duct can be a simple box of thin plywood or even balsa, just slightly tapering from front to rear. It captures a large portion of the propstream and funnels it directly to Maggie, so that ‘static’ running is achieved at launch. Since this stream has a velocity of at least a couple of hundred ft/sec, this is adequate for a short run on the ground.
Pre-launch consists of fueling the launch engine and setting the butane cylinder in place [without snapping it all the way in], and connecting up the battery to the launch engine glow plug and the HV leads to Maggie’s spark plug. Needle valves are adjusted for start, and the launch engine is started by propping until it goes; then it is adjusted for best torque and the glow plug lead pulled. Oil-laden exhaust passes the fuselage on the outside, with clean air flowing through the duct into and around the jet intake. Now the spark to Maggie is started and the butane cylinder is snapped in place, causing fuel vapor to flow. Maggie starts at a low but stable volume of combustion. The spark is killed and the leads are pulled, and Maggie is verified as running in a stable condition, although this will not be perfectly smooth, due to the pulsating character of the prop-driven air. This is fairly rich running, due to the limited air and the fact that at this point, fuel pressure is at maximum.
The pilot signals he is ready for launch, and the model is released. The prop engine accelerates the plane until liftoff, and more rapidly thereafter. As speed is attained, Maggie is hit with smoother air impinging on the duct, and propeller air becomes less important. Maggie’s fuel mixture becomes leaner, and the burn becomes smoother. Soon, the launch engine is out of fuel and quits, with the prop coming to rest in a vertical orientation clear of the duct inlet. Maggie is now providing 100% of available thrust. Acceleration continues until the total airframe and control line drag equals Maggie’s net thrust at speed. More than adequate air remains available for both combustion and cooling.
Sooner than we might expect, the fuel is essentially exhausted, fuel pressure drops radically and combustion dies out. During the glide and landing, Maggie is continuously cooled down by uninterrupted airflow inside and out. The ship is flared out for landing on the tricycle gear, and rolled out uneventfully.
In a universe of perfect justice, that’s how it ought to work. In a universe of physics, chemistry and imperfect human understanding, well, maybe not.
Hey, a guy might as well dream . . .
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