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Guys,
I've been pondering the hoverhawk.com prop speed
calculator (which in turn is from the pponk.com aviation site).
While I think it's okay for hovercraft (hoverhawk's
goal), I'm wondering if it doesn't contain an error for our
purposes.
The hoverhawk calculator calculates the tip
velocity of user-input prop diameters, adjusted for temperature and
rpm. They recommend tip velocities of .88 to .92 mach, which really cranks up
the RPMs if you look at it.
However, isn't the tip velocity the sum of both the
rotational tip speed (i.e., the number they calculate) PLUS the forward vector?
Don't you need both to figure out how fast the tip is going?
You know that the forward velocity component
is not in there because the forward speed is not a user-input variable in
the equation.
So while the hoverhawk calculator would be
perfectly applicable to a relatively slow-moving hovercraft, wouldn't it
substantially understate the mach speed of an airplane's propellor carving a
forward-moving helix in the air at the rate of 150 to 200 mph? After all, that
forward vector itself could add 20-25% to the mach number if my theory is
correct.
That would mean that the rpm they calculate would
be too high for an aircraft (except for the first five seconds
of takeoff) and the prop length recommended would be longer than an
aircraft could use because the longer tip could go supersonic when the forward
velocity is added in.
Anyone want to tackle this or straighten me
out?
Thanks.
Barry Gardner
Wheaton, IL
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