Understanding drag?
I struggle to get my head around a subject. Drag versus efficiency. Let me explain.
We all know that the bigger the propeller, the more efficient the propulsion is at subsonic speeds. Or the less horsepower you need. I will ignore the weight of the bigger prop for now. But obviously, the drag from a bigger propeller is higher. However, as long as the prop is powering the craft forward, can this drag be ignored, i.e. will it only come in to play when it's windmilling? If not, when does the size become greater than the efficiencies from increasing diameter (ignoring weight once again)?
Reason I'm asking is that fuel efficiency seems to be very linked to fanjet size/prop size. That's why they keep getting bigger and bigger. In helicopters, it's a well known fact that the bigger the rotor is, the less hp's it need to hover or takeoff. But at some point the drag increase must 'catch' up with the oversize, unless that can be ignored as mentioned above.
Thank you.