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A question came up as to which wing will require more power to stay
up. I'm not concerned with covering miles, just staying airborne. First,
to stay up the longest on any given wing, fly at the speed that gives
the best sink rate when power off. For paragliders, that's usually with
about brake pressure one or two (about the weight of your arms) but go
with what the manual says.
How about comparing wings, though? There are many variables, but
here's one way to look at it. Thanks to Dana Hague for contributing this
morsel.
For the same hands-up airspeed (we'll assume glide speed=level flight
speed; a very close approximation), power required for level flight
equals sink rate times all up weight. You must have enough power to
overcome the sink and fly level. That's the same as excess power
required to climb at that rate.
For actual numbers the units must match: sink rate in feet per second
(ft/s) and power in lb-ft/s. One hp = 550 lb-ft/s.
Lets take a 250 pound PPG (all up weight) that sinks at 300 feet per
minute or 5 ft/s. You would need 250 x 5 or 1250 lb-ft/s of power
required for level flight—2.27hp. If that sounds absurdly low, remember
our props have maybe about 30% efficiency so the motor would need to be
7.5hp. Again, that's just to maintain level flight. Now figure you want
300fpm climb; that doubles the power required to 15hp which is typical.
We can also use this to figure thrust since power equals thrust times
velocity. Assuming 30 ft/sec (20.5mph), 1250/30 = 41.7 lbs thrust for
level flight or 84 lbs of thrust for 300fpm climb. Again, these numbers
seem low, but that could be (1) a reflection of thrust dropoff
from a prop optimized for static thrust, 2) the effects of "dirty air"
(turbulence) behind the pilot, or 3) manufacturer's overly optimistic
claims.
Another observation you can safely make from this discussion is that
larger wings, having a lower power-off sink rate, will require less
power to remain aloft given the same efficiency.
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