> > Perry Mick replied:
> >
> > > Actually the ducted fan had great static and low speed thrust,
it
> > would push me back in the seat when applying throttle for
takeoff,
> while the prop is very sluggish until about 40 knots on the IAS. The
problem was high speed. The ducted fan is probably a good
> > > choice for the airplane if it is a slow plane, like a Light
Sport Aircraft.
>
>
> > Actually, I mis-spoke. Getting a ducted fan to work well at low
speed
> (for their STOL objective) and high speed (140 kts max cruise speed
spec.) could be a challenge.
> >
> > Russell Kent
> >
> >
> > Try impossible :) Not unless you can devise a retractable duct.
> >
> > There is some engineer at NASA that is fond of ducted fans but
> obviously he has never read Kuchemann & Webers book on the
> Aerodynamics of Propulsion. There is a very good reason for not
using ducted fans in high speed aircraft. The high speed drag of the
duct itself far outweighs any low speed advantages. I had a run in
with the guy at OSH. He is not even a pilot.
> >
> > Sport plane must be cheap. Nobody wants to pay $100,000 to get
them around the pattern a couple of times. They can get an
ultralight
> for that. Cross country transportation airplanes need to go fast.
Probably at least 200 mph and 300 mph would be better. If it is much
less than 200 MPH and you have a head wind you might as well drive.
No ducted fan is going to be efficient enough to compete with a
simple prop in this speed range.
> >
> > Paul Lamar ...No rotor no motor.
> >
> > That is correct. The ducted fan or shrouded prop has good low
speed
> and static thrust qualities but as speed increases the drag of the
shroud becomes a major impedement. Some blimps use shrouded props
because they can. Very few piston aircraft have benefited because
speeds are too high. However. the A10, 747, etc. etc. etc. do
benefit because the high power overcomes the drag penalty of the
shroud.
> >
> > BY
> >
> > Yes in the 400 MPH and up speed range ducted fans become the most
> efficient way of doing it.
> >
> > IMHO NASA has no business designing GA aircraft. Typical
government
> attitude. We are smarter than private industry and we are going to
show you how to do it. What they should be doing is fundamental
applied research that is too expensive and therefore too risky for
private enterprise. Namely
> > things like turbo compound, airfoils and bottoming cycle engines.
> >
> > Paul Lamar ...No rotor no motor.
> >
> >
> > Looking at the duct in the drawing I can see where it may not be
> effective. When NACA tried ducts early on they got poor results.
August Raspet reversed the duct shape and now it would work. Raspet
did a lot of work on shrouded props, or ducted fans, with some
success. Unfortunately I think he was killed while flying one of his
shrouded aircraft. I am no sure.
> >
> > BY
> >
> > Perry Mick gathered all the info he could find and spent 13 years
as
> I recall flying a ducted fan airplane. He built at least three
different ducts and tried at least three different fans or props.
Never got over 150 MPH as I recall. All ducted fan aircraft in the
150 to 350 MPH speed range have failed in the market place to
perform to expectations.
> >
> >
> > Paul Lamar
>
>
>
> Seems like a duct helps a small diameter propeller look like a
larger one. Increasing prop diameter appears to help low speed
performance, all else being equal. Its the whole thing about
whether a propulsion system accelarates a small mass of air by a
large amount (turbojet, scramjet) or a large volume of air by a
small amount (conventional propeller).
>
> An interesting article:
>
> http://en.wikipedia.org/wiki/Unducted_fan
>
> If your prop diameter is radically constrained by either physical
mounting location (on a pylon of fixed length, ground strike
problems like on pushers), or by output RPM (which would otherwise
require a reduction unit to keep blade tips subsonic), a duct unit
may be the way to go.
>
>
> Matt-
>
> The jury is still out on that one. Perry was running 5000 RPM direct
drive.
>
> What is boils down to is this: if you have enough HP to go 450 MPH a
duct is worth looking into :)
>
> Paul Lamar ...No rotor no motor.
>
> ---------------------------------
>
One point that is often skipped is that a duct is in some ways very much
like an extra airfoil from an aerodynamic point of view. It has
stability and control issues depending on where it is mounted and may
vary stability with throttle setting. Even a moderate diameter ducted
fan, say 24 inches diameter, with a short chord, say 12 inches, is the
equivilent of a surface over six feet long with a twelve inch chord.
This is additive to the sum of the drag surfaces of the other portions
of the aircraft.
Raspet recognized
this and attempted to use an integrated ducted fan aft. I say
integrated because he was trying to use the surface area of the duct as
a kind of ring tail surface. He also incorported integral elevator and
rudder controls. (Jim Miller tried this on his Texas Gem sport/racing
plane in the 70's and dropped it because it was slower than the unducted
competition). In general, it was highly effective at higher speeds. The
problem was low speed control. Even though the movable surfaces were
essentially "blown" behind the fan, this effect diminished with reduced
throttle. The SV-11A attempted to integrate the rear ring tail with
laminar suction control of the boundary layer on the wing. This was in
fact achieved (they claimed demonstrated lift coefficients in excess of
3), but the method used to achieve it was very susceptable to
degradation with dust, moisture, etc.
Jim Bede
attempted to use this work by Raspet in the design of a twin-engine
ducted prop 5-seat executive aircraft. A prototype was actually built
and flown. It now resides in the EAA Museum. Those planning on trying
on an exercise in ducted fan design might want to be a bit conservative
in their fan design.
As many exparimenters have discovered, the limiting tip speed for
the fan is frequently not the "sonic" or structural limit - it is the
sound limit. High speed fans are REALLY loud! At least one firm spent
hundreds of thousands of dollars trying to deal with this "sirening" in
the 70's and ended up using a gearbox and larger diameter fan.
From a technical point of
view, a low-speed ducted fan LSA is perfectly feasible. But there is an
economic question regarding the required tolerences for the fan and its
impact on overall cost. I know someone who built a really interesting
fan about 10 years ago and remember him saying it would cost about
$8,000 each in 1995/96 dollars. I honestly don't see the LSA crowd
going for something like this.
George Wright
Thanks George. That was very interesting, and confirms other thoughts I
have had about ducts.
Matt-
You want to plug your book on ducted fans George?
Now is your chance.
Paul Lamar ...No rotor no motor.
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