Perry Mick wrote:
Paul wanted me to summarize a short conversation we had earlier
today about ducted fans.
***
Paul wrote:
Just took another look at your web site Perry.
It occured to me you may need some stators behind that prop.
Dan Grey showed me this styro foam P80 jet model with a ducted
electric fan that flys great. Its an RC toy for $100 to $200
Has about a two foot wing span and has what looks like a Mikita
portable drill battery. I think you need to see one.
It has stators and puts out a bunch of thrust.
***
Comment from Perry:
Back in '95 when I was first looking into this and didn't know
anything (don't know much more now) about ducted fans
I bought a Dynamax model, five inches in diameter, 11 blades, 4
stator blades. I still have it, but have never fired it up. Need
to buy a model airplane engine to drive it.
As usual the model airplane guys are leading the way. It has
been like that for the last 100 years. It is much cheaper to
build and test ideas and things if they are small :-) PL
***
Later, Paul wrote:
I suddenly had this great insight.
You know what is at the core of a vortex?
LOW PRESSURE!!!! DRAG!!!!
That is why your rad works so well.
Most of the HP is going into generating a huge vortex with
low pressure and drag at its core.
The low pressure is reflected forward in the far field
causing drag canceling some of the thrust!
What happens is the air molecules are flung outwards due
to the centrifugal force drastically lowering the pressure
inside the vortex. This is used on many modern day fighter
planes to delay the stall at high angles of atack.
they generate their lifting vortices over the wing by sharp
strakes in front of the wing roots. Another example
of a low pressure inside a vortex is a tornado. A house
can literally explode because the pressure is so low inside
a tornado and outside the house. PL
I now understand why stators are necessary and why Moller
is using counter rotating fans.
The counter rotating blades in a duct is an ideal
situation. The front and rear fan pitch can be adjusted
to optimise the overall thrust. A stator has some loss
associated with it so it is not ideal.
Counter rotating blades outside
of a duct are extremely noisy. The Russian Bear long range
turbo prop bomber is an example.
A long duct behind the fan
can contain the vortex from expanding, consoladate the
pressure and diminish the rotation due skin friction. Perhaps
with a net increase in thrust but it is not a sure thing. PL
Perry replied:
Great insights, Paul! I'm sure there's a lot going on back there
that I don't understand yet. I had stators on Duct1, intend to
put them on duct2, but may not since I'm starting to think about
duct3. (http://www.mickaircraft.com/duct3.htm).
When I get what I consider a final configuration I'll put on
stators. I think they do help some, especially as the diameter
gets smaller and the rpm higher, you definitely get a loss in the
swirl energy. Marc covers that.
Flew the 13th flight yesterday morning, 9.5 hours in the air now.
All is well except the vibration, and the speed. But boy is it fun to
fly.
Just think, I'm flying with no shock mounts at all! Try that with
a four cylinder piston engine. I'm still trying to figure out how
to analyze the accelerometer output, hoping I can still improve
on the dynamic balance.
***
Later, Perry wrote:
forgot to mention, based on the fact that I usually only add about 5
gallons of gas each time I fly for an hour, I'm probably not burning
more than 5-6 gallons per hour. So I'm operating pretty low on the HP
curve at 4500 rpm.
***
Paul replied:
We know that Perry by the modest RPM's your turning :-)
BTW I picked up one of those model ducted fans for $30
and a special electric motor for another thirty.
I was highly impressed by the amount of air those things
pump out.
I will measure the static thrust and the HP consumption
and report back. Some of the features. There are four
thin stators with lots of camber on the leading edge and
with a very wide 24 mm cord. There are five 18 MM wide cord
very thin fan blades with sharp leading edges.
The ID is about 28 MM and the OD is 75 mm.
The pitch is about 45 degrees with not a lot of twist.
They look very much like a ducted fan in a large
turbofan except for the number of blades.
More when I get a chance to measure a few things.
I will have to figure the reynolds number to see if it
is anywhere near close to your configuration.
--
Perry J. Mick
Mick Duckt N7XR
http://www.mickaircraft.com/duckt.htm
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