>  > 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



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