A tractor is built for torque and not speed
Whereas a Ferrari is built for speed with very little torque. Even if they had the same HP engine.
That’s a terrible analogy 
For a Ferrari to do 200mph it needs a huge amount of torque at the wheels.
Power = rpm x torque (you need to pick suitable units if you want HP to come out).
So you will always have both.
Without one or the other, nothing moves.
To get back on topic, the right way to think of a CS prop is that it varies the blade pitch to absorb the engine power as best as possible, over a range of airflow velocities. It does it automatically (some manual setups excepted) and that is why one doesn’t need a PhD to operate such an aircraft. As posted above, you could literally just set max rpm (blue lever all the way forward) and fly everywhere like that. Nothing bad will happen; you just waste a bit of juice, and make more noise.
Try towing a tractor up a steep hill with a Ferrari and you will likely blow the engine.
Tow a Ferrari up a hill with a tractor is no problem.
The thing pulling an aircraft forward is the propeller. If set to full fine it uses more of the available power to produce more torque.
After the climb, in the cruise you don’t need so much torque so you can set the prop blades more coarse, this in turn will use the available power to produce less torque and more speed. If you want to climb again, you add a bit of torque by pushing the props finer which leads to less speed.
I didn’t say the tractor/Ferrari analogy was a good one, just the one I use to help me think what I am trying to do when using a VP prop.
The reason the Ferrari makes its power with RPM
Is that RPM doesn’t weight anything, if you have a gearbox in any case. The gearbox then turns RPM into torque at nearly constant horsepower (1% loss per mesh). A turbine helicopter engine is a more extreme example of the same thing.
The problem with towing something with a vehicle having little torque is that the engine typically has high minimum speed, not much power at that speed, and the gearbox doesn’t have a super-high ratio. So the resulting torque at low speed isn’t enough to do the job. An electric motor is good in this application as it may make full torque at zero speed.
Yeh it is just my way of envisioning the VP prop levers. In my head I move from tractor to ferrari and back again..It just helps me remember or automate the process in my head, when it comes to when to push prop levers forward first and when to move throttle levers first. It works for me, other pilots I am sure find their own memory triggers. It’s like time, turn, twist, talk or identify, verify, feather.🙂
@Silvaire – you are correct, my bad. HP.
LeSving wrote:
In those diesels on Diamonds or from Continental, do they still use MP, or something else ?
There is no actual throttle in injected turbo-diesels. MP is directly coming from the turbo (so mainly controlled by exhaust gas temp and RPM), and the rest is managed by injection (this means that you can run an extremely poor mixture at idle, but the whole compression-ignition cycle is very different from gasoline engines).
Most of these aircrafts use a single-lever FADEC (all Thielert / Continental, Autro, not sure about SMA but that covers all Diamonds and factory diesel 172), and the pilot only aims for % power (sometimes RPM too, but indirectly via power %). Pitch control (RPM) is managed by the ECU to always be at best efficiency for all power levels.
Values in POH are specified as: takeoff 100% power, MCP/climb 92%, economy cruise 65%, normal cruise 75-85% etc.
