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Al Gietzen wrote:
A prop is a wing that flies in a circle. Right? The most efficient
wings (the ones that put on gliders), are very long and thin to get a
high aspect ratio. Right? Given this premise, it only stands to reason
that the best prop would be the longest one that would leave some ground
clearance, and then absorbs any remaining Hp by increasing the chord.
That's my story, and I'm sticking to it (unless I'm wrong).
Nice try, Ernest; but the term ‘gross over-simplification’ comes to mind J.
No doubt. But we're talking rule of thumb as a guide to choosing a prop for a light plane. Unless we're building our own, oversimplification can be useful, even necessary to the selection process. You first oversimplify to get a basic idea of the appropriate area to be looking in, and then iteratively add complication to narrow the selections down to one.
With this wing; every increment is traveling at a different speed, has a different AOA; and it’s always making a sharp turn.
Nearly all of the methods I've seen for dealing with this complication involve dividing the wing into sections and calculating incidence, AoA, lift, etc, for each. The technique is just an approximation, but the approximation become very accurate as the number of sections approaches 8. It isn't an extremely DIFFICULT thing to do (after completing a refresher course in trigonometry), but it is an extremely TEDIOUS process. Thank God for computers and spreadsheets.
Not to mention various turbulence factors, material stress, etc; but then I know you knew all that.
Granted. I supposed it would be possible to get a prop so long on a tall taildragger that the blades would simply bend forward instead of pushing air back. For most of us, though, ground clearance will be the first limitation, followed closely by tip speed.
Another big factor is going to be what is behind or in front of the prop. I think Tracy demonstrated that pretty conclusively with the prop he was using that was designed for a pusher on his RV. The air is much slower at the center section of the prop on a pusher, and the prop designer had taken that into account. When put onto a tractor configuration, the center of his prop was seeing a much different AoA than what it was designed for. It is easily seen if the conditions are known and just the sections near the hub are analyzed.
But then we're not at the first stages of the selection process anymore. My story is that for our category of airplanes, length is the primary determinant of efficiency and should be the first criteria to use in selecting a blade.
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