Variable pitch propeller

Hi there.
Does anyone know about variable pitch propellers for air propulsion? I know such propellers exist for boats but shovel water....
I want to use that on a hovercraft, so negative thrust is also welcome.
Hiran
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| Does anyone know about variable pitch propellers for air propulsion? | I know such propellers exist for boats but shovel water....
They do exist. You'll find them on things like helicopters.
But if you're looking for something smaller, like would be used in an R/C plane, it's tricky to make something that small that can give you a variable pitch prop without weighing or costing too much, or being too unreliable.
| I want to use that on a hovercraft, so negative thrust is also welcome.
If you used an electric motor, you could just use a reversible ESC and get negative thrust that way. It probably wouldn't provide that much thrust, as typical props are much less efficient when run the wrong way (and you'd have the same problem with a variable pitch prop) but it would provide some.
They do make props that are variable pitch, but you can only adjust them on the ground, with some tools -- not in use. I doubt this is what you're looking for, however.
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I reckon a heli's tail rotor pitch slider would give you ideas for use with folding props.
Hi there.
Does anyone know about variable pitch propellers for air propulsion? I know such propellers exist for boats but shovel water....
I want to use that on a hovercraft, so negative thrust is also welcome.
Hiran
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"shovel water"?
Hi there.
Does anyone know about variable pitch propellers for air propulsion? I know such propellers exist for boats but shovel water....
I want to use that on a hovercraft, so negative thrust is also welcome.
Hiran
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Hi, AirMan.
I meant to say those boat propellors have variable pitch but are built for water propulsion.
Hiran

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It would be much simpler to simply vary the rpm rather than fool around varying the pitch.
Ed
Hiran Chaudhuri wrote:

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Fullscale aircraft often use variable-pitch props to overcome the limitations inherent in fixed-pitch props. A fixed prop is a bit like having only one gear in your car, and it becomes a compromise between takeoff/climb performance and cruise performance. The fixed-pitch prop blades are stalled during the initial takeoff roll in the inboard areas due to the large pitch angle, and the angle of attack in cruise becomes too small to allow the pilot to use all the engine's power without going over redline RPM. The variable-pitch prop is usually a constant-speed setup controlled by a governor that the pilot sets using RPM as the reference. It's hydraulically controlled with engine oil boosted to a high pressure. The oil is fed through the hollow crankshaft nose into the prop hub. For models this hydaulic thing isn't workable. There were some props made many years ago that used a small electric motor in the hub to drive gears to adjust pitch, and others that had a sliding sleeve on the shaft behind the prop that controlled the pitch, similar to the helicopter setups seen today. No governor; directly controlled by the pilot. This is the one mechanism that might work for models, but would be expensive and not really strong. It could be controlled by a servo, one more thing to look after and one more thing to be forever adjusting. Not worth it.
Dan
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On 14 Jan 2005 15:51:45 -0800, Dan_Thomas snipped-for-privacy@yahoo.com wrote:

The Harvard seems to have a setup that relies on wheights that are centrifugally moved when the rpm changes, increasing the pitch to stabilize the rpm by increasing the load. I do not know exactly how this works, but seems doable for somebody with fine-mechanical aptitude. The prop becomes it's own gouvernor so to speak.
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It's still a governor-controlled, hydraulically-actuated prop. In that type, the weights pull the prop to a higher pitch, and the hydraulic pressure works against the weights to decrease the pitch. All controllable props have to have some sort of forces to move the blades both ways to high or low pitch. Some use hydraulic pressure both ways, some use hydraulic pressure one way and a strong spring, gas pressure or centrifugal weights the other. A prop that used only the weights would be useless, since it would limit RPM to some compromise and therefore limit horsepower. The only two self-governing props I know of included a single-bladed affair that used centrifugal force, thrust forces and an offset pivot to achieve lower rpm and higher speed in cruise; it never saw production. The other is a European design that has an air-driven hydraulic pump inside the hub that senses forward speed and increases pitch. The spinner rotates the pump, and has angled vanes on it to drive it.
Dan
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On 15 Jan 2005 16:43:58 -0800, Dan_Thomas snipped-for-privacy@yahoo.com wrote:

Thanks for your explanation. I have no knowledge about how it *really* works, but I am eager to learn.
(my dad used to fly these planes, as such I have some extra interest in the principle at work here)
Analog to CVT transmissions in scooters and some cars, setting a fixed RPM (the RPM where torque or power is maximal) to drive the car at different speeds seems to work pretty well.
I could imagine that the same could apply to planes as well - Increasing the throttle would eventually change the pitch of the blades to "absorb" more power @ same RPM. Appearently the Harvard had its prop speed regulated to an rpm where the blade tips got very close to supersonic - hence the characteristic sound of the plane. Still it was appearently possible to change the RPM (showing you must be right), to allow for economy / full war boost performance?
If you could mention a site where the actual "nuts & bolts" working principle is explained - I would like to study it!
--
- Ren

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Here it is: http://www.mccauley.textron.com/pro-sup/prosupframeset.html
McCauley does a good job of explaining the process. The Harvard's noisy prop would be heard during takeoff and climb, where max RPM is needed to get max horsepower. That high RPM drives the prop tips above 600 MPH, where the noise starts. They don't actually have to get supersonic to be noisy, and in fact going too fast only contributes drag, not thrust, as the air can't flow properly over the blade. In cruise the RPM can be lower, as max HP is no longer necessary, and the RPM has to drop to allow fro the airplane's forward speed, which contributes to tip speed. Dan
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Hi, all.
Thank you for your thoughts. I already tried reversing the electric motor with a normal air fan but it takes a long time and reverse thrust is very low compared to forward thrust. This is not really nice for braking. I've seen models that use reverse thrust for normal operation to have more power for braking.
Going for an RC helicopter's tail rotor sounds like a good idea. I'll give it a try.
Hiran
Hi there.
Does anyone know about variable pitch propellers for air propulsion? I know such propellers exist for boats but shovel water....
I want to use that on a hovercraft, so negative thrust is also welcome.
Hiran
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See the February issue of Fly RC, last page, for an article on someone who did this. The Cliff Notes version is that he has an AIRPLANE that hovers NOSE DOWN and torque rolls.
--
Charlie Funk
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How does he manage to control the attitude without airflow over the surfaces? Sounds like it would take a fine touch. Dan
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