This is kind of a rough approximation of the ducted fan
situation given 1000 HP and a constant thrust of 937
pounds. It is assumed the duct is optimized for max
eff. at 400 MPH.
Ducted fans give more or less constant thrust
with speed. Most curves of thrust verses speed for
turbofans show these characteristics. I have used
that assumption to calculate
the duct eff. It is not quite this bad as the thrust
does fall off somewhat with speed in the real world
but hopefully this will give you a feel for what is
going on. I have also assumed the duct is 80% efficient
at 400 MPH which it might not be that good.
BTW the T.Thrust column is what you would get
if all the HP was converted to thrust with
the engines putting out the full 500 HP each.
Perhaps the RPM/fans would bog down at low speeds
and in that case the HP would be way lower
than 500 HP and of course so too the real thrust
would be way less than shown here.
This should give some idea of what one is up against
with ducted fans.
The prop eff. curve approximates a quarter sine curve.
One could use that assumption to chart the thrust
from a 1000 HP engine using a prop for comparison
with this chart.
MPH FPS T. Thrust Real Thrust Duct Eff.
25 37 15000 937 5
50 73 7500 937 10
75 110 5000 937 15
100 147 3750 937 20
125 183 3000 937 25
150 220 2500 937 30
175 257 2143 937 35
200 293 1875 937 40
225 330 1667 937 45
250 367 1500 937 50
275 403 1364 937 55
300 440 1250 937 60
325 477 1154 937 65
350 513 1071 937 70
375 550 1000 937 75
400 587 937 937 80
Paul Lamar
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