Variable intake/Exhaust Ducts for Ducted fans.

Hey Guys,

I know that ducted fans are somewhat the sophomoric siren of amateur
aircraft design... Such tantalizing performance benefits (No PSRU/
higher fan/prop speeds, much better control of thrust airfoil
environment, greater propulsion mass for small fan diameter, etc) only
problem is, they almost never work out right- The ducted fan
efficiency increase can only be seen over a very narrow range of
operating parameters (input speed and pressure being the most
notable), and the efficiency drops outside of that range are
tremendous...

So, I go into this question knowing that I'm wrong, I'm just curious
as to why

Imagine for a moment a ducted fan, with a simple streamlined body in
front of and behind the fan which houses the engine components (for my
simple example, lets say its a small profile motor, say a Mazda rotary
turning the fan at shaft speed in a 36" fan.

Now, the streamlined body/Duct shape is designed for optimal static
thrust (simple convergent/divergent area).

However, this particular streamlined body also has a thick, inflatable
rubber coating on both the intake and exhaust sides of the fan, hooked
up to a simple pressure sensor within the duct.

As the forward airpspeed increases and the pressure in the duct drops
out of optimal, the rubber sleeves around the body begin to inflate,
"choking" the duct. The sleeves are designed to continue to produce a
streamlined shape as they are inflated, by by increasing the
circumferance of the central body, the input/fan/output ratios can be
varied nearly infinitely to produce a consistant flow and pressure to
the fan, keeping it in its optimum operating range and bringing the
efficiancy gains of the ducting to the entire operating range. In
essence, this is kinda a 'reverse-variable-pitch-prop', although
instead of adjusting the fan to vary with the forward airflow, you
adjust the intake airflow to always provide optimum stream to the fan.

There is always the matter of added parasitic drag from the duct, but
in the system I envision, the entire fan/duct system actually replaces
the aerodynamic function of the Rear tail surface in a 3 lifting
surface craft (moving the craft's aerodynamic center rearwards and
providing the moment necessary to change the craft's yaw and AOA).
Thus, while the duct is adding parasitic drag, it is replacing another
structure which produces parasitic drag.

I know this may sound a bit rube-goldberg-ish, but I see the system as
being no more complicated than a basic CS prop...

Which means there is something else big that I'm missing... I'm sure
I'm not the first person to think of this...

Any thoughts?

On Mar 1, 12:44 pm, "EridanMan" wrote:
Hey Guys,

I know that ducted fans are somewhat the sophomoric siren of amateur
aircraft design... Such tantalizing performance benefits (No PSRU/
higher fan/prop speeds, much better control of thrust airfoil
environment, greater propulsion mass for small fan diameter, etc) only
problem is, they almost never work out right- The ducted fan
efficiency increase can only be seen over a very narrow range of
operating parameters (input speed and pressure being the most
notable), and the efficiency drops outside of that range are
tremendous...

So, I go into this question knowing that I'm wrong, I'm just curious
as to why

Imagine for a moment a ducted fan, with a simple streamlined body in
front of and behind the fan which houses the engine components (for my
simple example, lets say its a small profile motor, say a Mazda rotary
turning the fan at shaft speed in a 36" fan.

Now, the streamlined body/Duct shape is designed for optimal static
thrust (simple convergent/divergent area).

However, this particular streamlined body also has a thick, inflatable
rubber coating on both the intake and exhaust sides of the fan, hooked
up to a simple pressure sensor within the duct.

As the forward airpspeed increases and the pressure in the duct drops
out of optimal, the rubber sleeves around the body begin to inflate,
"choking" the duct. The sleeves are designed to continue to produce a
streamlined shape as they are inflated, by by increasing the
circumferance of the central body, the input/fan/output ratios can be
varied nearly infinitely to produce a consistant flow and pressure to
the fan, keeping it in its optimum operating range and bringing the
efficiancy gains of the ducting to the entire operating range. In
essence, this is kinda a 'reverse-variable-pitch-prop', although
instead of adjusting the fan to vary with the forward airflow, you
adjust the intake airflow to always provide optimum stream to the fan.

There is always the matter of added parasitic drag from the duct, but
in the system I envision, the entire fan/duct system actually replaces
the aerodynamic function of the Rear tail surface in a 3 lifting
surface craft (moving the craft's aerodynamic center rearwards and
providing the moment necessary to change the craft's yaw and AOA).
Thus, while the duct is adding parasitic drag, it is replacing another
structure which produces parasitic drag.

I know this may sound a bit rube-goldberg-ish, but I see the system as
being no more complicated than a basic CS prop...

Which means there is something else big that I'm missing... I'm sure
I'm not the first person to think of this...

Any thoughts?

Why not just use a propeller that is properly designed to minimize the
tip loss; it exists, doncha know!

EridanMan wrote:

I know that ducted fans are somewhat the sophomoric siren of amateur
aircraft design...

Imagine for a moment a ducted fan, with a simple streamlined body in
front of and behind the fan which houses the engine components (for my
simple example, lets say its a small profile motor, say a Mazda rotary
turning the fan at shaft speed in a 36" fan.
...
Any thoughts?


FYI, there are two Yahoo groups dedicated to ducted fans:

http://groups.yahoo.com/group/ductedfanaircraft/
http://groups.yahoo.com/group/ductfan

The first one is about the Jethawk 2, powered by a Mazda rotary.

Roberto Waltman

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