s-altpp.htm
One of the difficulties of the Small-Pogo project will be finding engines to power the larger models. "Micro" jet engines are available starting at $2,200 US. This is out of my economic reach. Further, obtaining some of the military engines may be politically impossible.
With that in mind and a request from a friend to come up with low-cost ways to power homebuilt aircraft the side project of searching for alternative power plants schemes began.
Besides the obvious turbine engines, such as the "micro" jets, there are three very simple types of engines that can be built in a garage. These are the Pressurejet, Ramjet, and Pulsejet. For all of these the thrust is obtained by compressing air into a chamber, adding heat, and exhausting it at higher speed than it entered. In general, the higher the compression the higher the efficiency. There are many textbooks and internet explanations of the details.
The pressure jet relies on an external source of compressed air. The air is ducted into the combustion chamber, mixed with fuel, the fuel-air mixture is burned, and the air is exhausted out a nozzle to produce thrust. The pressurized inlet air can come from a variety of sources. One example is to have a small gas engine-driven compressor provide the inlet air, though this could be very heavy. A pressure jet powered helicopter could use the pressure differential inside hollow rotor blades to provide the compression. While not usually highly efficient, pressure jets are very simple to build and operate and provide thrust at all speeds (given a constant inlet air pressure.
The ramjet is similar to the pressure jet except that the inlet air comes from the freestream air. A simple subsonic ramjet could look much like a football with the front two inches and back three inches cut off. As the ramjet travels through the air, the air enters the small inlet and expands as the interior of the engine (football shape) expands. As it expands it slows down and compresses. Inside the engine fuel is burned heating the air. The heated air is allowed to expand toward the exhaust end thereby accelerating. It is this acceleration that causes the thrust.
The major drawback for ramjets is that they produce no thrust at zero speed. In fact, they produce little thrust and are very inefficient until they get up to high subsonic speeds. On the other hand, they have been operated as low as 250 mph (ref Casamasa & Bent, _Jet_Aircraft_Power_Systems_, 1965, McGraw-Hill. The early Bomarc ramjet was rated as operating between Mach 0.7 and 1.5 (ref _Janes'_All_The_World's_Aircraft_"). Thrust and efficiency increase as speed increases and peak between Mach 3.5 and and 4.5. Later Bomarc engines weighed 78 lbs and were rated at over 12,000 lbs thrust.
A pulsejet differs from ramjets and pressurejets in a couple of significant ways. The pulsejet has a combustion chamber, similar to that of a ramjet, but with a reed-valve assembly on the front and a long tube connected to the exhaust. The reed-valve assembly allows air to enter but not exit the front, similar to the valves in a piston engine but less complex. The operation of a pulsejet starts with a fuel-air mixture in the combustion chamber. This mixture is ignited and explodes subsonically (deflagrates) bringing it up to high temperature and pressure almost instantly. This high pressure gas then accelerates down the exhaust tube providing thrust until the pressure inside equals that outside. However, these gasses have momentum and continue to exit producing a partial vacuum inside the combustion chamber. In theory, air is then pulled in through the inlet due to the partial vacuum. In practice, much of the air comes in through the exhaust tube. In any event, fuel is injected into the combustion chamber, ignited, and the cycle continues.
Pulsejets are estimated to be viable power plants up to about 450 mph. They are more complex than ramjets but can produce thrust at zero speeds and are lighter than pressurejets (when the pressure source is taken into account).
The rimfire engine basic concept is to use the three non-turbine types of jets in unconventional ways to enhance their good features while overcoming their poor features.
By placing an inlet cover with reed valves, like a pulse jet, in front of the ramjet the engine can be operated at zero speed. The jet would operate in pulsejet mode until high enough speed is gained to operate the ramjet. At that time the hinged inlet cover/reedvalve assembly is retracted up and out of the way to allow free stream air flow through the ramjet. The acceleration at zero speed would be slower than a tuned pulsejet but the top speed would be much much higher than a pulsejet alone.
|\ | \ | \ | \----------------------------------------------------\___ | waverider aircraft \___ |________________________________________________________________\ \ > : / hinged exhaust \ flameholders > : / reed-valve \ > :/ inlet cover \---------------------------- pulsejet-ramjet
There are many "microjet" engines available on the market starting at about $2,200 US. There are also plans for making your own. These use the compressors, approximately one inch diameter, from car and truck turbochargers. Because these engines are small they appear to be difficult to build and are limited to a few pounds of thrust, though some get up to 40 lbf of thrust.
Reference "Radio Control Jet International," Traplet Publications, Worcestershire, England. Their web site is (http://www.traplet.co.uk/traplet/).
Reference _Model_Jet_Engines_, by Thomas Kamps, Traplet Publications, 96 pp, il, 1995, $24.95 US.
At one point we looked at using the thrust produced by commercially available leaf blowers. These use the same engines as the weedeaters but are very heavy compared to their thrust, mostly to provide ease on use. On closer inspection though, there may be a way to use the compressors from the leaf blowers like the compressors from the turbochargers.
The turbocharger jet engines are about 1 inch in diameter and operate in the 80,000-100,000 rpm range (about 283,000 inch/min tip speed). The leaf blower compressors are about 8 inches in diameter and operate in the 8,000 rpm range (about 201,000 inch/min tip speed). This means that it may be possible to use these light weight and inexpensive leaf blower compressors (high production, stamped plastic or aluminum) in homemade jet engines. Such engines should have higher thrust, due to the higher air flow, and be easier to work with, due to their larger size. They could also be used for multi-stage compressors thereby increasing the efficiency of the engine. They could also be used for homebuilt aircraft engines.
Another possible use for leaf blowers is as starters for pressurejet rimfire engines (see below). The leafblower output could be ducted through the hub of a helicopter rotor assembly or a rimfire engine and down through the blades to ressurejets mounted at the ends of the blades. These pressurejets would operate at low efficiency at first but would increase as the blades picked up speed. This is due to the centrifugal forces compressing the air within the blades. The leaf blower would act as the first stage of compression and the centrifugal force the second stage.
This Page Last Updated 17 Nov 97