Peter wrote:
Not a valid comparison
Agree. It will give you a wrong intuition to think like that. With manually operated pitch, this is somewhat correct though. In practice you would only use two settings, climb and cruise, which will correspond to first and second gear in a car (could be continuously adjustable, infinite number of gears, but that’s beside the point).
With a constant speed prop, there’s a governor controlling the rpm by adjusting the pitch, but the rpm is set manually. I can’t think of anything in a car that is similar. A car with cruise control and automatic transmission driving in hilly terrain is perhaps something close, but yet so far away it will be confusing. A constant speed prop is much simpler than that anyway.
I don’t think there’s any way to get a correct intuition without understanding torque and power of an engine for varying rpm and throttle settings, how a governor works, and how a propeller works with varying air speed, pitch and rpm.
Using those 3 setting from the POH will work all the time anyway 
RobertL18C wrote:
at typical cruising altitudes, say 8,000 feet, it probably is worth varying the RPM occasionally to keep the governor oil warm and refreshed. I tend to operate between 2400 and 2500 RPM and like yourself only go full fine for take off.
I have never found it necessary, but would not object to the practice. I fly using the KISS method (Keep it simple, stupid). I don’t use flaps on an instrument approach until the runway is made and even then am likely to land without flaps. We were taught to never go over square, after takeoff and gear up, reduce the MP to 25, RPM to 2500, add MP in the climb until full throttle, set the cruise RPM to set power, set mixture by leaning to peak and either enrichen +50 or -50, enrichen mixture as you descend, add partial approach flap setting, go to full RPM on landing for the possible go around, if go around, pitch up, power up, positive rate gear up, then retract approach flaps. I have simplified that to just one RPM change, not using partial flaps, setting power by adjusting the mixture to a LOP fuel flow setting.
NCYankee wrote:
I don’t use flaps on an instrument approach until the runway is made and even then am likely to land without flaps.
Indeed much more stable and easier on the manual or autotrim I also make it simple for a potential go around …the mixture is set full rich/RPM for potential GA …no flaps. just push the GA button clean up the airframe..make sure your in RWY HDG mode after missed. In most cases ATC will give you vectors anyway after climb to XXX on RWY heading. I do use full rich during the GA exercise though but aggressively lean once back level e.g. but on a very low power setting 19" max 20"to stay out of the red box. Don’t have tuned injectors…
Worth also mentioning that a CS prop supposedly gives you better pitch stability, because if you pitch down with a fixed prop the revs go up and it produces more thrust.
if you pitch down with a fixed prop the revs go up and it produces more thrust
Muddy waters here… yes, with the same throttle position the revs will go up, but that will not necessarily equate to more thrust (mass flow rate through propeller against velocity change through the propulsion system) as such. Precession will increase though
I think engine power does go up because if you pitch down, you speed up, the airflow through the prop speeds up, revs go up, so the engine – at a fixed throttle opening – is pumping more air, and whatever fuel metering system you have (carb or FI) will deliver more fuel.
Half of the engine is just an air pump and is quite separate from the fuel burning half.
I wonder if anyone who has flown behind a fixed prop and then flown the same plane behind a CS prop, has noticed any change.
I remember after emergency landing a Schweizer 300 in a field (another story), we got picked up by the chief flight instructor in the schools Bell JetRanger. As he pulled us up into a hover, I remember him saying, “wow, that used a lot of torque” as the torque meter almost pegged. Turbines often come with torque or HP, often measured via oil pressure or other sensors. It was the first I ever heard of torque in aviation.
Years later when I moved to CS props etc, I always thought of MP as simply torque. In my mind, it made it easier to understand everything. You can increase torque two ways only – give more “gas pedal” (MP), or increase rpm. Or both.
Years later when I moved to CS props etc, I always thought of MP as simply torque. In my mind, it made it easier to understand everything. You can increase torque two ways only – give more “gas pedal” (MP), or increase rpm. Or both.
This is a good model except that you can’t increase torque with RPM, so there appears to be a typo here. You can increase horsepower two ways only – more MP (torque) or more RPM, or both.
I always think of it this way.
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.
AdamFrisch wrote:
Turbines often come with torque or HP, often measured via oil pressure or other sensors
Way, way back I had some courses in this. The usual method (from what I remember) is to have two pickups positioned on a shaft (somewhere between the turbine and the prop/rotor). Torque becomes a linear function of the phase shift between those two pickups, due to twisting of the shaft.
Thinking about it, a turbine engine is a diesel engine (thermodynamically). It always have “full MP” (no butterfly valve). In those diesels on Diamonds or from Continental, do they still use MP, or something else ? They are all turbo diesels, they also have MP of course, technically speaking, but it’s more (almost exclusively) of a function of the turbo.
