ORIGINAL: pimmnz
Sorry Alisdair, I don't buy it.
so long as positive pitch is on that prop, there will be thrust. [NO it depends on airspeed. go fast enough and prop Thrust reduces to zero then becomes prop drag]
I think what you meant to say was that at some RPM and pitch, the disc drag of the prop was zero. [YES?] The prop might not be adding any extra T to the airplane, but there will be some. [?????]Perhaps thats what the dive thing was meant to show, at some speed, with gravity assistance, the prop is neither providing all the T, nor is the prop disc providing any drag. That is not anything like T = 0.
Again, I am unschooled, and might be wrong...
That is not anything like T = 0.
disc drag
Pimmnz, You would need to explain what you mean by that, or better still take some more schoolin and learn what the rest of us mean.
Thrust is the force exerted by the air on the prop, in line with the propshaft. It depends on airspeed. At zero airspeedT is at a maximum. As airspeed increases the angle of attack of the blades reduces and so Thrust reduces. And as airspeed increases, Thrust just keeps on reducing (for an aeroplane in level flight) until Thrust equals Drag. In the diagram attached T is the thrust of the prop, D is the total drag of the airframe. The airspeed at which they cross is the maximum speed at which the aeroplane can fly in steadylevel flight. It won't go any faster, unless you dive. In a slight dive most prop planes will pick up some speed, the Thrust will reduce but now gravity is helping maintain the speed.
Gravity is powerful stuff, and if you use quite a lot of it (yes, quite a steep dive) the Thrust can go right down the curve and even below the line to a negative value. That is when we would call it Drag. It is additional to the airframe drag, but it is holding the plane back sonegative Thrust IS an addition toDrag.
This is not a regular part of flying, it is a bit extreme and probably beyond Vne. But it is part of what is possible, theoretically.
I should mention that a very coarse prop will have its blades stalled at low airspeed. At low airspeed the thrust will be poor, the purple line in the diagram, but once the blades are working properly with a bit of airspeed through them they will keep thrusting to a higher airspeed and produce a higher top speed. The only problem is taking off (and overshoot) and that's why variable pitch props were invented.