<< Page 9 Page 10: Page 11 >>   Thrust (Airplane Engines) Thrust can also be understood in terms of Newton’s Third Law (That’s the great thing about these laws, they can describe just about anything you can see… except for the causes behind gravity, and other fundamental questions like that). Engines move air backwards so that the air pushes those engines (and the airplanes attached to them) forwards. The question is, how is the air moved back? How do different types of engines work? Let’s look at different aero engine types chronologically. Piston engines In-Line Radial Rotary Jet Engines Turbojets Turbofans Turboprops Ramjets and Scramjets Pulsejets Exotic Rocket Engines Electric Airplanes Human-Powered Flight   Here's a summary of what you will read about airplane engines over the next few pages:   - - - - - - - - - - - - - - - - - -   Summary:Thrust -- Piston engines: In line or V (cylinders in a row or two, along crankshaft), then Radial (cylinders in a “flower” around crankshaft), then Rotary (not the Wankel kind – cylinders in a flower, but they spin around the crankshaft rather than the crankshaft spinning in them), then back to Radial and Back to In-Line. Liquid Cooling allows for more power in a smaller engine (more fuel burnt per cylinder before the engine melts), as does supercharging/turbocharging (more air into each piston, so more fuel can be burnt).   -- Jet engines: Compressor-burner-turbine (turbojet), first with centrifugal compression, then axial flow (air always moving in same direction). Shaft can be connected to a big piston through a transmission (turboprop) or to a big series of blades in a cowling (turbofan). In the nozzle, more fuel can be burnt after the turbine – afterburning, or augmentation. If the compression is done by sonic shockwaves at the inlet (or just by the inlet shape), then it’s a ramjet. If the air is not slowed down to below supersonic in the combustor, then it’s a scramjet. Turbine blades melt if incoming air is compressed too much, so a turbojet can only swallow so much air at a time, so turbojets have a top speed of about MACH 3. Ramjets still have to slow the air down to below subsonic on the inside, and this is impractical above MACH 5 or so. So the only way to do MACH 7 or above is a scramjet, a tube where air is going through it faster than sound throughout, even in the combustor. VERY hard to achieve. Turbofans are more fuel-efficient and quieter, the higher their bypass ratio (a turbojet is essentially a turbofan with a bypass ratio of zero), i.e. the wider the fan at the front.   -- Miscellaneous types: Rocket (not air-breathing; carries oxidizer internally), pulsejet (a tube where bouncing pressure waves or pulses or shockwaves burn fuel), solar/electric or human powered (must be very slow, extremely light (i.e. flimsy) and with huge wings) (Paul McCready).   Different kinds of engines have different “speed limits”: --Prop engines (piston engines, turboprops, electric or human-powered) cannot pull/push a plane faster than sound because the propeller blades’ “lift” is messed up by the sonic shockwaves. --Turbojets take in more air the faster they go, so the air gets compressed more in order to be able to fit through the engine, so the air gets hotter, and at one point (around MACH 3 at the most) it gets hot enough to melt the blades in the turbine. --If you get rid of the blades (no turbine, no compressor, just a tube’s aerodynamics), you have a ramjet. But in order for the burner to work, the air must be slowed down to below MACH 1 as it enters the engine (or shockwaves would mess up the combustion in the burner). This is very hard to do above MACH 4 or 5. --Of course, if you can pull off supersonic combustion (scramjet), you can go pretty much as fast as you want, theoretically (like with a rocket engine), and the limiting factors become drag and heat, not thrust.