MotoCalc and Propeller Stalled

[WillowPtarmigan]

What does "Propellor Stalled" means in Motocalc, when you do the calculations?

[Ollie]

When the propeller stalls the load on the motor goes up sharply and the thrust produced goes down. This is caused by the airflow over the forward face of the propeller blades separating and the wake behind the blades becoming very, very turbulent. If the pitch of the propeller is reduced to better match the RPM of the motor and the airspeed of the airplane, the stalled condition will be corrected. Another way is to increase the gear ratio between the motor and propeller so that the propeller RPM is reduced. Gearing allows a larger diameter propeller to be used. Either way, the diameter to pitch ratio ratio of the propeller is increased.

[vintage1]

Two things to note in addition to that.

(i) Motocalc makes very broad assumptions that may or may not be true, to determine when a prop is stalled.


(ii) I am not so sure that a stalled prop loads the motor more. In fact it may load it less, due to the fact that it is not moving the air backwards as efficiently, and each blade may be operating more in the wake of the other, as it were. Thisd is just a hunch, and I do npot maintan that it is so, just that a few odd things I have noticed - some sound of RPM *slowing down* after launch, suggesting the 'stalled' prop is 'biting' better once airspeed is up - and of course Doppler doesn't help...

[shaneyee]

Originally posted by vintage1
Motocalc makes very broad assumptions that may or may not be true, to determine when a prop is stalled.


Thats certainly true. Motocalc assumes that whenever the prop pitch is more than 2/3 of the prop diameter, the prop will be stalled as long as there is no forward speed. That may or may not be true or accurate. In practical terms, it means you have to assume that the thrust is a lot lower at takeoff than you would otherwise expect so a hefty throw at launch is called for. Its just an advisory...

Shane

[jrb]

MotoCalc does not accurately predict pitch speed! It’s calc is based on no slip. The following shows one of the sets of measurements that I’ve made:

[jrb]

You also might find this interesting, the 4 blade is an APC 15x11, the two blade is an APC 15x11; while the equal load 2 blade is a 17x11.

By MC reasoning and most prop myth the 17x11 > 1.5 should do better than the others which are nearer to 1.3.