I am very familiar with the relationship of mass flow and
velocity
in the momentum equation - however, the divergent section
cannot
increase the mass flow (basic continuity). If the velocity at
the
fan plane is 200 mph then it would seem that terminating the
duct
at that point would be the best answer - you would have that
mass flow at the higher exit velocity. - Vance
Vance,
The problem is the fan performance curve. You are dealing with
a shaft power (constant HP) problem. Unlike a jet (constant
thrust). If you wish to discharge the same mass flow rate at
200 mph, you will have to raise the pressure ratio on the fan.
This will increase power required. To maintain the same power
absorption and get a 200 mph discharge would require reducing
mass flow. No way around this. It is possible to increase
thrust by reducing the discharge speed as long as it moves the
fan down the curve to a place where it flows more air while
consuming the same power. This works theoretically until the
discharge speed is the same as the aircraft speed Ve-Va>1. If
you increase mass flow while decreasing discharge speed you
increase propulsion efficiency (turbo-jet vs. turbo-prop). For
jets, the most efficient cruise is found for Ve=2Va (for
constant mass flow). My prediction is a shaft powered ducted
fan is similar to a prop in that the propulsion efficiency is
higher for greater mass flow and smaller difference between Ve
and Va.
Monty
Jets are not constant thrust devices. it is just that their working
area is
so small, and the delta V so high that they do not drop off with
forward speed as fast as a classic prop. Fans are also not constant
shaft power devices, the input power varies related to the amount
of pump work. It is possible that your divergent area does add some
mass flow by ejector action on the clearance between your fan tips
and the duct inner wall, The big increase with the duct is at
slow/near static operation where they pump down a lifting surface
(the inlet bell) This advantage goes away pretty rapidly with
forward speed. I think you would improve the overall peformance by
terminating the duct very shortly behind the prop, and permit free
expansion instead of forcing the expansion which is only optimum at
single, fairly narrow point. As you indicated this might require a
different size prop to match engine power. So far as I know, there
is no specific best point for jet operation (they stink everywhere,
which is why they are almost extinct) - Thrust is MV power is
MVsqared/2 which is why for max thust per hp - move a lot of mass
a little bit for best use of hp - props are only 100 percent
efficient with mass
infinate and delta V zero. For most EAA class airplanes ducts are a
waste of time
unless you want to look like a jet. The design goal of the fanjet
trainer was to acquaint
student pilots with the disadvantages of the jets at lower cost. -
Vance
There are a lot of slow EAA applications that would benefit from a
ducted fan. Sport class most definitely, speed limit is 115 knots. If
I'm running a rotary in a sport class airplane I would rather have a
duct with no moving parts than a gearbox, and a prop with extreme
limits
on the pitch to keep the speed down.
I will just post this from http://www.bridgingworlds.com/raspet.htm:
EAA Letter About Raspet
A reader of this webpage faxed me this letter from the December 1997
EAA
Experimenter Magazine:
...The discussion of propellers brings to mind another bit of
knowledge
on shrouded propellers. The old NACA explored shrouded propellers, and
found little difference between them and unshrouded ones. The shroud
reduced the losses at the tip of the propeller, but the drag of the
shroud offset the performance improvement so that the overall
performance was about the same. The late Professor August Raspet of
Mississippi State University had the idea of making the duct in the
shape of an airfoil, with the curvature on the inside of the duct. I
have drawn the two shrouded propeller systems below, the NACA system
on
the left, the Raspet system on the right:
Raspet's system had two major advantages:
1. The propeller in the Raspet system was smaller, thus allowing a
higher engine rpm, and yet the system performed as well as an
unshrouded
propeller with a diameter equal to the diameter of the outlet of the
duct. The duct was performing the function of a Propeller Speed
Reduction Unit (PSRU) by reducing the propeller tip speed and also
increasing the propulsive efficiency by affecting a larger cylinder
of air.
2. Raspet's system also increased the static thrust because the inflow
of air into the duct when it was not moving forward caused a suction
over the forward part of the airfoil. In tests with the Anderson-
Greenwood airplane, Raspet was able to double the static thrust by
adding a properly shaped shroud and a propeller designed for the flow
velocities within the duct. The performance improvement decreased with
increasing forward speed until at about 200 mph the performance was
back
to that of an unshrouded propeller. The duct system was then
performing
the function of a variable-pitch propeller with no moving parts.
Mississippi State got an Army contract to build the XFV-!!A Marvel
Aircraft using this shrouded propeller system and a bunch of other
advanced ideas. Unfortunately, Raspet was killed before the airplane
flew. It was completed, but when flown with a free-stream propeller in
the duct (instead of the intended one with the twist matched to the
local flow velocities within the duct) the mismatch resulted in very
poor performance and the project was shelved. All this is documented
in
reports by Mississippi State during the period 1955 to 1965.
The Anderson-Greenwood would be a better airplane with the Raspet
shroud
system. Also, all of the gyrocopter, trike and powered parachute
designs
would benefit from a Raspet shroud. I cannot understand why people do
not use it. Apparently no one has read Raspet's reports but me. If I
were more of a "doer" and less of a "thinker," I would have built a
Raspet shroud long ago.
Sincerely,
John E. Jenista, EAA 3470
A good part of the problem is the duct weight increase. A hollow
carbon fiber
duct would mitigate some of that. Not available when Raspet was alive.
Here again is the data from K&M Aerodynamics of Propulsion with some
of the
surrounding text. Note the drag contribution of the duct to the overall
performance. The data is plotted relative to the advance ratio on the
x axis.
Very typical for propellors. A turbo fan on the other hand would have
advance ratios of well over one at low speeds and that is kind of what
K&M
are referring to when they speak of disk loading. Also note that
opening up the
exit area (the right side chart) increased the efficiency plotted on
the y axis.
That is what Perry verified.
The DRAG of the duct DROPPED the OVERALL efficiency almost 30% at
advance
ratios over .4 (high speed).
Note also the inverted airfoil shape of the duct. I am sure Raspet had
this
book since it was published in 1953.
IMHO with a much larger radius on an inflated lip such as I am
suggesting
low speed thrust would be greatly increased and the duct could then be
made
thinner or smaller in diameter with less high speed drag. A smaller
diameter
duct with a higher advance ratio, higher disc loading fan could allow
the
rotary engine to rev higher and generate more HP and still have the
same
low speed thrust advantages. In other words it would become more like
a turbo fan but have much better low speed thrust.
I also think with a pito tube placed behind the fan the consequent air
pressure might inflate the lip. Normal inflow into the duct would also
help inflate the lip. A two way valve could shut off the pito
tube pressure and vent the lip volume to the atmosphere collapsing the
lip. Better yet to a source of low pressure forcing the lip to
collapse.
No compressor required. Patent applied for :)
BTW if you make the duct too short there will be reverse re
circulating flow on the
out side skin of the duct drastically reducing the low speed thrust.
Some experimental work done by Bell Helicopter indicates that you don't
actually need as much duct length as a lot of theory indicates, they
were getting big efficiency gains with what amounted to a ring around
the tail rotor. I think that the overriding factor in Perry's case is
the efficiency loss with the reduced diameter (cube function), which he
makes up at lower speeds with the duct, but it kills him at higher
speeds. When he gets some PP power that will make a huge difference.
Perhaps a small duct about 6 inches long 1/2 thick might have most of
the low speed thrust and less drag at higher speeds.
I am amazed by his bravery flying with stock 2nd gen RX-7 electronics
which in my experience are prone to constant problems like they were
built in Great Britain or something.
The re circulation problem is well known with a short duct. The
helicopter flow field around the tail rotor is entirely different.
For one thing it is embedded in a vertical stabilizer that effectively
blocks re circulation.
Paul Lamar
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