Question about what will happen to the angle of attack of the propeller blades

[gerardflyagain]

The answer to the below question is "it will decrease". Can anyone explain why please?
"As a fixed pitch propeller aeroplane climbs in ISA conditions with constant indicated airspeed and constant RPM, the true airspeed increases. What will happen to the angle of attack of the propeller blades?"

[Chris Martyr]

Hey Gerard,
They are called , 'Centrifugal Twisting Moments' , it is a massive force exerted on the prop. blades when turning . It will have the effect of trying to decrease the AoA of the blade .

[ good old prop. theory notes from Chelsea College many years ago]

[PDR1]

Erm...no!

Angle of attack of the prop blades is a function of rpm and true airspeed. It increases with rpm and decreases with airspeed, so if the RPM is constant and the airspeed increases the AoA must reduce.

If you want to visualise it you can think about the (horribly empirical) concept of "pitch speed", which is the speed at which the pitch times the rpm is equal to the airspeed. The empirical theory would suggest that at this speed the blade AoA is zero and the prop is just freewheeling (this isn't true, but it's a handy way of visualising what's going on). So that would suggest the AoA is at a maximum when stationary and decreases as true airspeed increases. Does that help?

PDR

[BEagle]

The clue is in the increase in TAS and constant blade rotation speed.

Although blade angle is the angle between the blade and the plane of rotation, blade angle of attack depends on the relative airflow. The angle of the relative airflow is dependent upon the interaction between the forward speed (TAS!) and rotational velocity. As TAS increases, the angle between the relative airflow and the blade will reduce if the RPM remains constant.

[Haraka]

Assuming the angle of incidence of the blade to the hub remains constant, as the aircraft accelerates under constant R.P.M. and true airspeed increases then the relative positive angle of attack of the blades to the airflow will decrease -it has to. Indeed in a dive it could theoretically go to zero - or even go negative and act as a brake if the R.PM. .stayed constant .

[gerardflyagain]

Thanks all.