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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? |
#2
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Variable intake/Exhaust Ducts for Ducted fans.
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! |
#3
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Variable intake/Exhaust Ducts for Ducted fans.
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 [ Please reply to the group, return address is invalid ] |
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