Perry Mick wrote:
It takes 2200 RPM to taxi with the duct on, 2800 with duct off.
With the duct on and a 10 knot headwind, it took 1000' to reach 70 knots
airspeed, was airborne at 1500'. It usually takes me 1800' with no headwind.
So sorry guys. I guess you just don't understand what that duct is doing
for me. As far as I can tell there is just no way a 38" fan turning 5000
RPM will ever get off the ground in a reasonable manner unless you have
about 400 HP available and maybe a variable pitch mechanism. I think
it's a combination of capturing the tip losses and the venturi duct
shape that makes it a success at more than doubling the thrust of
stand-alone fan.
I still think you need to broad band that duct. Make a more streamlined
lower drag duct such as found on turbo fan jets with a smaller diameter opening
and use the variable geometry opening. I still think it can be done with
an inflatable rubber lip.
Marc de since you are on here now perhaps you can help Perry extend the speed
range of his ducted fan. In other words reduce the high speed drag from the duct
with out losing too much low speed thrust.
Paul Lamar
Marc de Piolenc wrote:
The way to broaden the speed range of a duct is to provide variable EXIT
area, which allows a duct that is otherwise optimised for cruise to
develop high mass flow under static conditions. I don't know any easy
way to do this so you're on your own where actual construction is
concerned. If your configuration happens to require a transition from a
circular section at the fan plane to a rectangular section at the exit,
all kinds of things become relatively easy to do - thrust vectoring,
variable exit area, thrust reversal... But I'm begging the question
because there are losses involved in making the change in cross-section
and, although I have a pretty good idea how to mitigate them, I don't
know how to quantify them.
Of course you can't completely ignore the inlet - you still have to
provide enough lip camber and radius to ensure that flow doesn't
separate under extreme conditions.
Best,
Marc
The usual way of doing that is to use a movable cone as in this ME109 rad duct.
Paul Lamar
Marc de Piolenc wrote:
Works in theory, but it's very difficult to make the numbers come out
right for this application.
Marc
That duct has the wrong shape, it is the high speed shape (fanjet). With
a duct like mine with exit area greater than fan plane area, I'll have
to think if something like that in the exit could be made to work.
Perry
I think you would better off in the long run to do away the weight and high
speed drag of the duct and focus on designing an effective blade for
an unducted fan.
BTW did you adjust the CG when you removed the duct?
Paul Lamar
If I were doing this Perry I would obtain a copy of these references
and design an unducted fan blade. Particularly the Smith paper.
Paul Lamar
I was planning to fly (if it would have), so yes I did adjust the CG.
Taking that duct off made the nose very heavy as you can imagine. So all
I did was remove the 10 lb. weather radar antenna that is mounted inside
the nose cone. That made the nose lighter, but not quite as light as it
is normally. Net result was over 40 lbs. removed from the plane.
Then when I put the duct back on, it would tip backward if I didn't hold
the nose down, so had to put the wx antenna back on.
I agree it would be nice to get rid of the weight and drag, but as far
as unducted fans go, I think I will have to leave that to someone else.
I'm not really disappointed in my performance. I don't think it's that
bad to go 160 mph on 7.5 gph when the engine is running rich. If I can
somehow add mixture control and get it leaned in cruise I will get
better economy. There are times now when I'd like to go faster, but I
think I can do that with a PPort engine. I've given up on putting a
turbo on a LEZ, there is just no room for the additional plumbing and
intercooler, and it really cooks things under the cowl. With more HP
from a PPort, I will probably have to build a new duct and fan, but I'm
sure I could make the next duct lower drag and lower weight.
Perry
I also think the ducted fan blades could be optimized better. The same
principles apply. If the cord is not wide enough or not at the right
angle of attack along the radius you could be losing thrust efficiency
when the tips are going supersonic. In theory and in the worst case it is
possible to build a prop or a fan blade that will absorb all the HP
yet generate no thrust. I think this is just as important as the shape
of the duct.
Paul Lamar
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