I have refined and added to my simulation of a dual three
rotor ducted fan aircraft. I have also corrected one
or two mistakes.
The column labled Drag is the aero drag
due to a flate plate equavalent area of two square feet.
The column labled Acc. is the acceleration of the aircraft
in G's based on a gross weight of 4000 pounds ignoring the
smaller amount of drag you have at less than rotation speed.
1000 pounds for fuel, 1000 pounds for engines, 1000 pounds
for people and 1000 pound for structure as a first guess.
A lighter structure sounds possible so perhaps more fuel
could be carried.
As you can see despite the very poor efficiency of
the ducts at low speeds the airplane has more than
enough acceleration to take off due to the 1000 HP
available.
There are some over simplifications here. This
is not meant as an exact simulation. It is merely intended
to show the qualatative characteristics of an internal
combustion engine driven ducted fan airplane. Further
refinements in the simulation are possible.
As before the column labled Theoretical Thrust is
that thrust you would get with a 100 percent efficient
duct. The column labled Real Thrust is based on an
assumption of flat thrust through the speed range.
Many plots of actual ducted fan thrust show this characteristic.
Actually most are not quite flat with a gradual drop off
in thrust with speed. If this were done the simulation
would show better acceleration for takeoff.
Perry's accleration data shows a drop off to near
zero at 100 MPH. This I think is because his
duct design is skewed towards low speed with large
intake lips and his fan design is such that the engine
cannot run over 3500 RPM or so which is way off the
HP peak. A better blade design and perhaps a lower
pitch would allow Perry's engine to run faster and
generate more HP. IMHO his fan blades are going
transonic with a brick wall drag rise limiting
his HP. More HP will help but not much.
In this simulation it is assumed the two rotary
engines are running at the 500 HP peak at 6500 RPM.
It may take a variable pitch fan to do this.
Theoretical Real Duct
MPH Thrust Thrust Eff. Drag Acc.
25 15000 750 5 3 0.187
50 7500 750 10 13 0.184
75 5000 750 15 29 0.180
100 3750 750 20 52 0.174
125 3000 750 25 81 0.167
150 2500 750 30 117 0.158
175 2143 750 35 159 0.148
200 1875 750 40 208 0.135
225 1667 750 45 263 0.122
250 1500 750 50 325 0.106
275 1364 750 55 393 0.089
300 1250 750 60 468 0.070
325 1154 750 65 549 0.050
350 1071 750 70 637 0.028
375 1000 750 75 731 0.005
400 938 750 80 832 -0.021
Now here is the downside. At a cruise speed of 350 MPH
and a total HP out of the engines of 750 HP the fuel
burn at .5 BSFC would be 375 pounds per hour or at
six pounds per gallon 62.5 gallons per hour or 5.6
MPG. Not good. A good design experimental or homebuilt
prop driven airplane at 250 MPH could get 15 to 20 MPG.
This is at sea level. Altitude would give better MPG.
Speed cost money. How fast do you want to go?
I don't know what a Lear Jet gets in MPG. I would
settle for pounds per hour. Anybody know?
Paul Lamar
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