Comparison to Other Jet Engines The G8-2 pressure jet engine was developed primarily as a blade-tip engine for a "flying motorcycle" or one-man portable helicopter. Prior to its development, however, no suitable blade-tip engine existed. A review of the operating characteristics of various other candidate engines will put the significance of the G8-2 engine into perspective. The turbojet is not reviewed because its inherent complexity eliminates it as a potential power plant for such aircraft. Rocket Engine A rocket engine produces very high thrust for its size and weight. A rocket can also be very simple in design. But such engines have little promise as potential power plants for small aircraft, mainly due to their high specific fuel consumption. The average sfc of a rocket engine is on the order of 36 lbs/lb/hr, or 36 pounds of fuel per pound of thrust per hour of operation. High sfc is due mainly to the fact that all the reactants must be carried onboard, and therefore must be counted as part of the fuel supply. In comparison, a jet engine takes much of its reactants from the air; in the ratio of about 15 pounds of air for every pound of fuel. A rocket engine would therefore have difficulty competing with a jet on the basis of specific fuel consumption. Ramjet Engine A ramjet is considered the simplest design of all the jet engines. It is also the most hungry in terms of fuel consumption. A ramjet's efficiency and thrust are a function of ram air pressure, which in the subsonic region is marginal. Although a ramjet will operate in the subsonic speed range, fuel consumption is typically very high; on the order of 12 lbs/lb/hr. A ramjet will not, however, operate statically. It must achieve a relatively high speed before it begins to produce thrust. So a method of accelerating the engine to operating speed must be provided. Profile drag is also high in comparison to other types of jets. The advantages of a ramjet are its inherent simplicity, and that fact that is has no moving parts. Pulsejet Engine The pulsejet is a member of the jet family that operates on the explosion cycle. In other words, its thrust is the result of a series of explosions, which are controlled by sonic synchronization of the inlet valve to the resonant natural frequency of the tailpipe. The length of the tailpipe is normally equal to onequarter of the sonic wavelength.
The pulsejet operates best statically. Thrust decreases with forward speed due to its high drag and the desynchronizing effect of ram air on the valve. At mach 0.6, jet thrust is almost zero. The pulsejet is long, bulky, heavy, and it requires auxiliary starting equipment. The engine also has a limited throttle range. Thrust can be varied only within a limited range of 80% to 100% of full throttle. Average sfc is on the order of 6.0 lbs/lb/hr. Short life of the valve is also a problem. Valveless Pulsejet The valveless pulsejet is simply a pulsejet without the valve. This is accomplished by matching the natural frequency of the tailpipe to the natural frequency of the intake. Elimination of the valve does not alter the jet's operational cycle, but it improves efficiency to about 5.0 lbs/lb/hr. Overall length is increased to one-half of the sonic operating wavelength, in comparison to the one-quarter wavelength tuning of the conventional pulsejet.
Pressure Jet Engine A pressure jet engine is a ramjet to which the air for combustion is supplied under pressure. Consequently, ram air is no longer of any value. Pressurization of the combustion chamber allows the burning cycle to take place under higher pressure, which significantly improves the propulsive efficiency and reduces sfc. Average sfc of a pressure jet is on the order of 2.0 lbs/lb/hr. The burner unit is small by comparison, so profile drag is normally not as great as with the ramjet or the pulsejet. However, the compressed air requires its own compressor, which in turn implies a separate power source. As a result, overall scf ceases to be as attractive. G8-2 Pressure Jet Engine The G8-2 engine operates on the same principle as the conventional pressure jet engine, but it requires no separate power source for pressurization. Incoming air is pressurized by the sonically tuned intake system and the kinetic energy of the super-heated propane gas. As a result, sfc is superior to other types of jet engines, with the exception of the turbojet engine (not reviewed). But for its advantage in sfc, the turbojet must accept the tradeoff of significantly greater complexity, weight, and costs.
Horsepower versus Jet Thrust Most people are used to evaluating power on the basis of an engine's horsepower rating. However, an engine that produces pure thrust (rather than torque at a rotating output shaft) is rated in pounds of thrust. Jet thrust cannot be stated in horsepower, unless a specific speed is given. By calculation, the horsepower rating of any jet engine operating in a static condition is zero, regardless of the amount of thrust. It's possible, however, to make comparisons on the basis of tests performed on conventional power plants in similar applications. This would be referred to as "Equivalent Horsepower" (HP 1). For example, if tests on a conventional reciprocating engine showed that a 10 hp engine with a fixed-pitch propeller produced a static thrust of 50 pounds at full throttle, then a value of 5 pounds of thrust per horsepower might be used. Since the G8-2-40 engine develops 40 pounds of static thrust at full power, HP 1 would be 8 hp. The shortfall of this approach, however, is that the thrust of a conventional engine/propeller system declines with increased speed, whereas the thrust of the G8-2 jet increases with speed (using an air scoop). So the two power systems are not directly comparable ( click to see graph). In addition, one would have to be confident that the comparison horsepower value was actually representative of a typical ICE/propeller power system.
Equivalent Horsepower Equation Comparison Thrust:
Equivalent Horsepower:
Another approach would be to insert a value for velocity so horsepower could be calculated. With a value for velocity, one would simply multiply thrust in pounds by speed in feet per second, then divide the result by 550 to determine horsepower (HP). The equation is as follows: Horsepower Equation
HP = horsepower T = thrust in pounds V = velocity in feet per second 550 = constant If a G8-2-40 jet were moving at 400 mph (586 ft/sec), the foregoing equation would result in 42.7 hp (HP = 40 x 586/550 = 42.7). The same engine moving at 600 mph (954 ft/sec) would result in 69.5 hp (HP = 40 x 954/550 = 69.5). In order to achieve meaningful results, however, the value for velocity must be appropriate for the particular application. In other words, the velocity must be based on the result of the particular thrust acting to overcome the resistance of a particular body. The speed at which the two factors become equal (thrust and resistance) will be the maximum speed attainable in the particular application. This would be the appropriate velocity to use in the foregoing equation.
