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Here it is straight from P&W.
Reverse flow, inlet is at rear of engine exhaust is at front. i.e. PT-6
The air enters the engine through the inlet screen; it is then compressed by a multi-stage compressor and fed to the combustion chamber where it is mixed with fuel and ignited.
The hot gas expands through three turbine stages; the first drives the compressor and the accessories; the other two mechanically independent from the first, drive the propeller shaft by means of a reduction gearbox. Finally, the hot gas is discharged through the exhaust ducts.
A free spinning turbine is one that has no direct linkage to the rest of the engine core. High pressure gases moving through the turbine section flow into a separate area, commonly called the power section, and turn an additional turbine which is linked to the propeller with its own independent shaft. This is different from a direct drive engine, in which the propeller is connected directly to the compressor shaft through a series of reduction gears.
The reverse command that I believe you're referring to is the "region of reverse command", or more commonly being "behind the power curve". Basically this occurs at a particular point, usually on final approach when the aircraft is configured for landing, when the amount of drag being produced overcomes the lift when the aircraft is flown at too slow an airspeed. Essentially lift is just a component of angle of attack times velocity. When you get "low and slow" on final with gear and flaps extended you must add more and more power in order to maintain a specific glidepath or altitude. There comes a point when the total drag will exceed the power available to maintain this altitude. This is called being on the backside of the power curve or in the region of reverse command. The only way to increase the lift is to reduce pitch and allow the airspeed to build and "nurse it" back, so to speak. Retracting the gear, if applicable, might help the situation, but a sudden flap retraction will decrease lift and make a bad situation worse. As you can see, this is not a desirable situation to be in. It has been awhile since I've thought about this stuff in any great detail, so maybe somebody else can chime in and expand a bit.
Normally (in cruise) we control altitude with pitch and airspeed with power.
As the previous poster explained (due to the fact that at approach airspeeds we're operating on the back side of the power curve [drag curve]) the region of reverse command dictates that now we control altitude with power and airspeed with pitch.
I know this will start an arguement; it always does. Have at it, guys I'm outta of this discussion now.
An easy way to tell if an engine is direct-drive or a free-turbine when it's shut down:
The free turbine is always feathered because otherwise the propeller will windmill and without any oil pressure the bearings would wear.
The direct drive is always flat pitch so as to create less drag during engine start (thus keeping EGTs low).
You're experiencing the Region of Reversed Command when you are flying the airplane at critically slow airspeed, i.e. minimum controllable airspeed. Your instructor probably had you try this on your second or third flight, when he/she had you slow the airplane to flap range, deploy all of the flaps and add power to maintain flight. Your airspeed was just above Vso, the bottom of the green arc (I believe the Private PTS standard is Vso + 5). You probably found that you had to lower angle of attack to gain airspeed.
Look at the lift-to-drag curve in your textbook to learn more about region of reversed command.
I, too, will opt out of the pitch v. power debate on controlling airspeed, except to say that you should ask a Lear driver his/her opinion.
Yes, ma'am, you're on point. I don't want to launch into that debate, except to say that pitch controls altitude and power controls airspeed. Add power and what does the airplane do? Go faster. Reduce power? It goes slower. Point the nose up? It climbs.
Sure, for lightplanes, if you add power they will climb. Reduce power and they will decend. But the idea is we train pilots to fly airplanes, not just light aircraft. Remembering what controls what will work in any airplane.
The learjet pitches in response to the addition of power because the longitudinal axis of the engine is canted down and aft, and is not in line with the long axis of the airplane. Application of power will cause a downward pitching moment because of the thrust vector. It has nothing to do with being ahead of, or behind the power curve.
The reason is because the two are ultimately linked together. You can choose to see them as opposites, pitch and power controlling airspeed and altitude, or we can see the big picture. The airplane is flying in a state of motion (hopefully) in an airmass and any change in one will necessitate a change in the other. If you pitch down you will increase airspeed and lose altitude. If you reduce power you will slow down if you make the resulting pitch change necessary to hold altitude. (if you don't, the aircraft should continue in a flight path to achieve the airspeed it was trimmed at before the power reduction probably resulting in a descent) and so on. Let's look at them for what they are... both working in harmony to create a state of equilibrium for our aircraft.
Obviously there are different techniques we want to use for different flight situations and configurations.
I'm sorry if anyone thought that I was discussing aerodynamics with respect to the L/D curve.....I even stated 250K (last time I checked it wasn't near the "area of reverse command" or the "back side of the power curve" or whatever linguistics are being utilized).
The Learjet does have interesting pitching moments due to the thrust of the engines and is easily demonstratable.