Constant Speed Propeller

Hi RW,

"Hi all, the answer is simple.  Peak torque is at 5200.  Below that you are behind the power curve.  Above that you are on the right side to have good throttle response.  Anything below that you have to climb the pressure point to get it over the hump for acceleration.  Overpitch an engine and it is not good for performance and also it is difficult to respond to acceleration.  Flying with a constant speed should be following the manual to not overload the engine. 

You can see all the power curves in the operators manual from Rotax and their recommendation on setting a "normal" constant speed.  I believe the airmaster is similar but has only steps and not an infinite range.  Should be fine when set correctly. "

Reminder of Airmaster Controller options:

Semi Automatic - Allows the pilot to switch/select between TO/Climb/Cruise, depending on stage of flight. Pre set RPM limits can be set eg T0 5750, Climb 5450, Cruise (the question) 5000 rpm?

Manual - Rocker switch for Coarse & Fine adjustment, similar in result to a GA (LyCon) arrangement. Pilot can select any prop pitch/(engine) speed between max Fine / min Coarse limits.

 # I like Glens(above) rational - what think you?

 # What of Airmaster contention that a 5000 rpm Cruise, is what Rotax recomend?

 # What might be the thinking behind  a 5000 rpm Cruise?

 # Are you recommending a 5200 rpm cruise setting? (I understand your rationale)

Rotax doesn't provide enough information to get a completely rational answer, Lycoming does. The rpm for 75% power depends on manifold pressure! Max available MP depends on altitude. Max allowable MP depends on the engine limits and rpm. Fuel flow at a given % power depends on mixture and the combination of RPM and MP chosen via the prop and throttle setting. From the information available from Lycoming for an IO-360 200 hp engine, I made this chart. Take a close look. Except for max power, there are multiple RPM/throttle combinations to achieve that power. Low RPM/high MP (within limits) gives the best fuel efficiency. What's "best" for the engine? People will argue endlessly. Somewhere in the middle perhaps, but Rotax makes it impossible to figure out where that is.

Hi Jeffry,

Yes! Way back 30+ years ago, I did my CS endorsement on a Lycoming (I think) so I understand how your chart works and the potential for multiple combinations of RPM/MAP.

The Airmaster system is similar,  in Manual mode, in that the pilot can select (rocker switch) rpm followed by power/throttle to a desired MAP.

In Automatic mode, the pilot selects (switch) according to flight stage, between TO/Climb/Cruise.https://www.propellor.com/

All Auto stages are rpm limited - mine is 50 rpm below max, as in TO 5750 rpm, Climb 5450 rpm - Crusie ?- work in progress hence the questions.

Airmaster recomend  ( Rotax 912 ULS), but do not require, that I set Auto Cruise for 5000 rpm, then select power (MAP). Unfortunately they seem to be vague as to why the recommendation - again hence my questions.

As a farmer (retired) I have always run my engines above max torque, for the very reasons RW mentioned in his response. However I am aware, when I drive my lightly loaded car on the motorway (high speed gentle gradients), I can get away with being at or just below, max torque and that may give me better /lower fuel consumption (change down to accelerate or climb).

A car at cruise, supported by the ground, is not quite like an aircraft, in the same mode , supported by wings. Once a car is at cruise speed, on level terrain, it requires little power/effort to continue at a given speed. An aircraft, even in level flight, must continually fight the pull of gravity (it's always going up hill). 

In some ways a plane in Cruise, is a bit like a tractor working with a ripper/plough in the ground - for much of the time the load is constant, every now and again a hard/sticky spot is encountered and the load increases.  If the tractor operator has selected the correct rpm (above max torque) for the anticipated work - As the load increases, the rpm drops and the torque (ability to do work) goes up. The tractor does not stall  and the engine is not damaged. The tractor continues on past the hard spot, the load decreases and the rpm rises again to the preset point.

My question is about getting to that optimum Auto set rpm for the (Cruise) work - is it 5000 rpm, as suggested by Airmaster or what I think RW (& you) might be recommending, 5200 rpm?

Airmaster uses it for their cruise setting also as stated in my Airmaster manual.

Hi Sam,

"Airmaster uses it for their cruise setting also as stated in my Airmaster manual."

I know where the recommendation emanates from, just not what the rationale for it, is.

Check out RW's comment above (Max torque is 5200 rpm).

Logicly an engine speed at or above 5200 should be selected.  

Glenn is at 3500 rpm - with his 912 iS.

As mentioned, Airmaster is being coy, sending me down unlikly rabbit holes