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Old 08-09-2010, 18:38   #16
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Quote:
Originally Posted by Astrid View Post
Of course, to feather properly, most designs I am aware of require the shaft to be locked (except for hydraulic, manually controlled props.
Well said. It looks like when one has one of those transmissions that should not be locked by reversing then a shaft lock is a must to feather.

b.
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Old 08-09-2010, 18:49   #17
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I dont remember where but I read that it is better to lock the shaft and not let the prop spin. Think of it this way does a rotating prop on a helocopter create more drag spinning or stoped?? Helos are able to safely land without power because of the drag of the spinning prop but drop like a rock when it stops, less drag. There is also less wear and tear on your cutless and packing gland if it is stoped.
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Old 08-09-2010, 19:01   #18
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More drag spinning...

For those who believe that there is LESS drag when the prop is spinning, please consider and explain why the following is true,

1) Helicopters that have lost all power will fall at 1100 feet per minute maximum when the prop is turning. This is called a forced rotation. You land really hard but will most likely live. You might break a skid but the helicopter will fly again.

2) Helicopters that have lost all power will drop like a rock when the prop is stopped. You land REALLY REALLY hard and will most likely die in a mangled firey crash. The helicopter will never fly again.
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Old 08-09-2010, 19:57   #19
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Not really quite the same thing. The prop is not acting like the drag of a parachute canopy. Actually, with a still spinning prop, a helicopter with a dead engine maintains its stability provided the windmilling prop keeps the engine turning, which in turn keeps the hydraulic pump turning and the tail rotor. The windmilling blades are also generating a certain amount of lift, insufficient naturally to keep airborne, but enough to slow descent. This it is not drag from the blades slowing descent, but residual lift being generated. It is necessary, though, for the pilot to keep the nose down and maintain forward momentum, as this momentum provides thrust to rotate the main rotor. If the pilot does not keep the nose down to keep forward momentum regular, the blades will stall and the helo will fall out of the sky.

One can see a similar situation in the old auto gyro of the 1930s. The auto gyro had an engine with airplane-type propeller and rotating helo type blades. The main difference was the helo blades were not powered and they had a fixed angle of attack. Thrust was by the engine's prop. The helo blades would start to turn as the auto gyro moved forward, generating lift and allowing it to get airborne. Stabilization and a control was by means of an airplane like tail and unit instead of a tail rotor and hydraulic angle of attack controls for the helo blades. If the engine quit, the auto gyro pilot could still make a controlled forced landing provided he maintained forward momentum by descending with a nose down attitude.
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Old 08-09-2010, 20:04   #20
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Good analysis, Astrid. Do they teach aerodynamics in veterinary School?

I have a shaft brake. Running the gears of the transmission for nothing in the way of minimized drag, seems needlessly risky.
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Old 08-09-2010, 20:26   #21
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Lol, unfortunately they don't teach that at veterinary school. I had to learn that the other way, by getting a pilot's license.
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Old 08-09-2010, 20:36   #22
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Helicopter Autorotation is a very different thing to a boat propeller.

For a start the Heli has to vary the pitch of the propeller. At the right point the airflow through the centre area of the blade will spin it, while the outer portion of the blade will be providing lift.

So the Heli is actually harnessing energy from falling and turning it into lift. Right before the ground the pitch is changed, turning stored kinetic enegery in the blade into lift and softening the landing.

Without pitch control a falling heli blade will stop spinning. This creates maximum drag but no lift. Aerodynamics forces will try and spin the blade in reverse but the engine is unlikely to allow this. If the blade was allowed to spin freely in reverse the fall speed would increase.


Think of it this way.

Your boat is sailing at 5 knots. At 2000 RPM your engine can also drive your boat at 5 knots. You engage the motor at 2800 RPM and do 7 knots. At this point the propeller is providing thrust. You then back the engine down to 2000 RPM. The propeller is spinning through the water, neither providing thrust or creating drag. You drop the motor speed down to 1000 RPM. The propeller now creates drag. You stop the motor. The prop creates even more drag. You run the motor in reverse and you get even more drag (reverse thrust).

Consider the following -

Thrust, no drag, minimum drag, some drag, maximum drag, reverse thrust.

If free wheeling creates more drag, how would you order these?

Thrust, no drag, maximum drag, some drag, minimum drag, reverse thrust?

How is it that your propeller goes from no drag to maximum drag directly?
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Old 08-09-2010, 20:43   #23
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Go sailing, and keeping all things constant, see what happens to your speed when the prop is spinning and conversely, when you have it locked. That will answer your question!

This being said, I always stop the shaft mechanically when sailing - there is no point in causing unnecessary wear.
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Old 08-09-2010, 21:04   #24
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Astrid - You also learned in pilot training that after an engine failure, you slow to best glide and if it has not stopped on it's own you raise the nose to stop the prop.

Airplanes with variable pitch propellers immediately feather the prop.

There are some studies on boat props. I have read them. I choose to interpret the data as inconclusive.

The boat's propeller has a completely differently aspect ratio and wetted area than an airplane propeller.

There is also the gear train and shaft wear to consider with a boat.

My personal conclusion is:

1 - Follow the manufacturers advice
2 - If no advice - Stop the prop
3 - Best bet is a folding/feathering propeller

And anecdotally - I have never been on a race boat that didn't stop the prop.

This is an energy equation - To turn the gear takes energy. That energy comes from the water passing by, that energy comes from the sails making power - therefore the boat must sail slower.

Drag is non-linear - A stopped prop creates a drag curve that is probably exponential. i.e. the faster the boat goes the more drag is induced.

If the drag curve for the mechanical gear crosses the unduced drag curve of the stationary prop there is a crossover where freewheeling the prop makes sense if you don't consider the cost of wear and tear on the system.

The hypothesis is that each boat, gear and propeller combination is different.

Now if you had a little electric motor powering the shaft at zero thrust at all speed there is in theory no drag. The energy to turn the prop comes from stored battery power.
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Old 08-09-2010, 21:30   #25
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Quote:
Astrid - You also learned in pilot training that after an engine failure, you slow to best glide and if it has not stopped on it's own you raise the nose to stop the prop.

Airplanes with variable pitch propellers immediately feather the prop.
Of course, in the case of a fixed wing aircraft, not a helicopter, the rotating engine prop acts as a brake, and also makes control more difficult as the pilot has to counteract the gyroscopic effect with rudder and ailerons, and this in turn adds even more drag. As you say, the best solution if you have a variable pitch prop is to feather it and greatly reduce drag. In my plane, which has a fixed pitch prop, one has to adjust the angle of descent so that airflow over the prop drops to the point where it will not turn against the engine's compression. And of course you want to turn off the fuel and magnetos to minimize fire risk. You still want to maintain a slight nose down attitude to retain lift and keep the wings from stalling.

With a helicopter or auto gyro, you still need to maintain some blade rotation to maintain minmal lift (though in an auto gyro you also have to keep the engine prop from rotating) to keep from stalling out.
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Old 08-09-2010, 21:56   #26
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The (unpowered) helicopter trades speed (kinetic energy) & altitude (potential energy) into rotation of the rotor (kinetic energy) during descent by using the drag of the blades to create rotation.

The blade angle is actually very fine during descent. The pilot is not trying to achieve lift he is trying to achieve a target speed on rotor rpm. He is also spending parts of that energy in directional control by adjusting cyclic and tail rotor to fly to a suitable landing spot.

He arrives close to the ground with lots of rotor rpm and little lift. By applying the collective he increases the pitch of the rotor extracting all the kinetic energy he can and converts it into lift and hopefully when all the energy is expended he is only a couple of feet above the ground with a zero descent rate.
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Old 08-09-2010, 22:08   #27
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One afternoon I was trundling along at < 3knots in slightly confused seas and light winds. Letting the prop spin made for between 0.5 and 1.0 knots difference. I repeated the test several times on the same beat in the same conditions with the same result.

Gearbox manual says leave it locked. No exclamation marks, bold type or other forms of highlighting are involved in the statement so I figure a bit of leeway is involved.

Generally now I'll leave it locked unless rolling around wishing for that extra speed....
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Old 08-09-2010, 22:17   #28
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Some transmission on larger engines use hydraulic fluid to cool the gears through a heat exchanger. Usually in these engines the manual will advise against allowing a free-wheeling propeller due to lack of cooling flow. A purely mechanical transmission on smaller engines is probably not affected by free-wheeling.
- - As to "feathering" a MaxProp here is the comments in one of their manuals: [emphasis added about in/out of gear]
>>>5) PROPELLER USE: The Max-prop works automatically. By putting the engine in gear
the blades will engage in either forward or reverse. The best way to feather the
propeller is:
• Power at 2 to 3 knots in forward.
• Kill the engine while still engaged in forward.
• When the engine has stopped, if the shaft is still spinning engage the transmission
in reverse to stop the freewheeling.
You can check to see if the propeller is feathered or not by taking the engine out of
gear. If the propeller is not feathered the shaft will freewheel like with a fixed blade
propeller.
In that case start the engine again and repeat the three steps. If your propeller has
been greased properly it will feather in a fraction of a second as soon as you stop the
shaft from freewheeling. Once the prop is feathered, you can either leave the
transmission in gear or out of gear, it does not matter.
DO NOT kill the engine while in
reverse. In this case the blades will be in the reverse position and will not feather.You
can actually use this feature to drive a shaft alternator.
<<<<<<
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Old 08-09-2010, 22:34   #29
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Sound

I don't have a working knotmeter so I love the sound of my prop spinning. It's the sound of wind converted into my vessels forward motion
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Old 08-09-2010, 23:03   #30
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Quote:
The (unpowered) helicopter trades speed (kinetic energy) & altitude (potential energy) into rotation of the rotor (kinetic energy) during descent by using the drag of the blades to create rotation.

The blade angle is actually very fine during descent. The pilot is not trying to achieve lift he is trying to achieve a target speed on rotor rpm. He is also spending parts of that energy in directional control by adjusting cyclic and tail rotor to fly to a suitable landing spot.

He arrives close to the ground with lots of rotor rpm and little lift. By applying the collective he increases the pitch of the rotor extracting all the kinetic energy he can and converts it into lift and hopefully when all the energy is expended he is only a couple of feet above the ground with a zero descent rate.
That makes sense. I am not a licensed to fly a helicopter and I am guessing that you are. I think we can agree that trying to equate a freewheeling helicopter rotor to a boat prop is as futile as comparing black knight to white bishop?
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