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stryperStalker

I need some suggestions

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I get to sign a 120 ft. wind turbine blade. I've got writer's block. The wife already claimed her signature..."Who broke wind?"

 

I know the fine folks in the Tavern can come up with some good suggestions, so fire away.

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I couldn't think of anything better than 1.21 gigawatts, so that's what I put down. Everyone else just wrote their name, except for one kid who added "2011" after his name and then crossed it out and wrote "2012" underneath it. Part of me thinks the town will just powerwash all of it off tomorrow...which our taxes will also pay for.

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That's why these are better in my opinion, all sizes for different needs;

 

Notice the fuel tanks next to it to run the turbine when the wind does not cooperate.

 

 

Wind turbines are a hoax, I think, all varieties of them.

 

 

 

Dennis

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Start-up wind speed is only 2.5 mph. Most of the ones presently in use are 5 mph.

Other variables are, when was it installed. They've been improved aver the last few years.

A hoax in what way ?

 

Jim

 

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" Sorry to anyone near this when she fails"

To much wind brought this one down. :laugh:

 

Dennis:)

I thought governors were standard equipment that feathered the blades to prevent overspeed accidents like that. Yes? No?

 

 

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Found this; we don't know the specs of that turbine so there's no real answer.

 

In normal operations, you're familiar with pushing the power up (increasing, or opening the throttle) to see an RPM increase, and pulling it back to decrease RPM. RPM follows the throttle movement. Additionally, pulling back on the stick causes the airplane to climb, causes airspeed to decrease, and consequently causes engine/propeller RPM to decrease as well. Pushing forward on the stick (decreasing angle of attack) causes the airplane to pitch down, altitude loss, airspeed increase, and RPM increase. So much for the obvious.

 

A constant speed propeller varies the blade angle to maintain a constant RPM, just as the name implies. If one flies straight and level and increases the throttle setting, RPM doesn't change, but propeller blade angle does. An additional power instrument is added that you don't have with a fixed pitch propeller installation, and that's the manifold pressure gauge. Whereas with a fixed pitch propeller, you can set power by simply setting RPM, you need more information (or rather, have more information) when setting power with a constant speed propeller. This is where the manifold pressure gauge comes in.

 

Manifold pressure is the air pressure in the induction manifold. When sitting on the ground without the engine running, it's the ambient barometric pressure...at sea level on a standard day, a manifold pressure gauge will read 29.92" of mercury (equal to 1013 mb), sitting on the ramp or apron, at rest. The way it works is much like a vacuum cleaner. The engine acts as an air pump, pushing air out the exhaust, drawing air in through the induction. Just like a vacuum cleaner, if you put your hand over the hose to the vacuum, you'll cause a decrease in air pressure in the induction or the vacuum cleaner hose. The throttle works the same as your hand; close the throttle while the engine is running, and you'll see a drop in manifold pressure...typically about 12" while the engine is running at idle.

 

Throttle position, then, doesn't affect the engine RPM in much of it's range, just the manifold pressure by varying the opening to the engine induction system (the throttle plate). In a normally-aspirated engine (a non-boosted or non-turbocharged engine), this manifold pressure varies all the way up until one reaches the ambient barometric pressure. This pressure naturally decreases about 1" per thousand feet of altitude increase...which is to say that it's normally about thirty inches at sea level, and about twenty five inches of manifold pressure at five thousand feet. One can't increase manifold pressure more than this value by opening the throttle more, in a normally aspirated engine once one has reached the ambient barometric pressure.

 

As a side note, in a turbocharged engine, although off-topic, one can keep increasing manifold pressure above barometric...but in that case the RPM remains the same with a constant speed propeller.

 

A constant speed propeller only maintains RPM when in the "governing range," or an RPM range in which the propeller speed is held constant by the propeller governor (a hydromechanical device that admits engine oil under pressure to the propeller. This oil pressure moves a piston and components in the propeller hub which change blade angle, in order to maintain a constant RPM). The propeller maintains a constant RPM by varying blade pitch...increase pitch (angle of attack), and just like pulling back on the stick where airspeed decreases...propeller RPM decreases as blade angle is increased. This happens simply because when the propeller takes a bigger "bite" of the air with a larger angle or attack, it experiences a drag increase, helping prevent the engine from going any faster.

 

In order for the propeller governor to increase RPM, it needs to decrease blade angle (similiar to pushing forward on the stick when you fly); this decreases drag, and the engine can turn faster. However, there's only so far that the angle may be decreased in the propeller. When it's at rest on the ramp in a typical piston installation such as a Cessna 182, the propeller pitch is as flat or decreased as it can go...the blades are resting on mechanical stops known as the low pitch stops. What this means is that there will come a point in the engine operation when the propeller governor can no longer decrease blade angle any further, because it's mechanically limited...and as the throttle is retarded then the RPM will fall with the manifold pressure. The point at which the RPM can no longer be maintained and it begins to decay as power is retarded is known as the bottom end of the "governing range," or the effective range in which the propeller governor can maintain a constant RPM.

 

With these basic principles in mind, let's turn to the propeller's use in flight. Two basic forces drive the propeller. With power to the propeller, the engine drives the propeller, and it varies it's blade angle to maintain a constant RPM. However, when below the governing range, when the propeller blades are as flat as they can go and resting on the low pitch stops, the slipstream in flight may be driving the propeller. This is generally considered a bad thing in a piston engine, for a number of reasons...but here the only consideration will be that the engine is no longer driving the propeller. Any time power is pulled back far enough that positive torque isn't being maintained on the propeller, the fact that it's spinning out there is largely due to the slipstream driving it. This is a big drag rise for the airplane.

 

To eliminate the drag from a windmilling propeller, one in which the slipstream is driving the propeller, we can feather certain propellers. This means that the propeller blade is rotated nearly parallel with the slipstream, to minimize drag. This may, and usually does, stop the propeller from rotating, and drastically decreases drag. As an illustration, the drag on a windmilling propeller can be, and usually is, higher than if a giant plywood disc were fixed out there in place of the propeller, and the same diameter of the propeller. This may sound sketchy, but it's true. A windmilling propeller can produce a significant amount of drag (I've been unable to maintain altitude in four engine airplanes before when a propeller wouldn't feather during an engine failure...that's with three other engines working hard to keep me flying, but one working against me...it's a LOT of drag).

 

Preventing engine overspeed in a fixed pitch installation is by propeller design; it has to be able to stay within the designated RPM range during a static runup on the ground. In flight, however, it's possible that a fixed pitch propeller might be able to overspeed, or go beyond it's approved RPM limits, especially at higher airspeeds such as in a dive. In a case like that, the only two choices to control RPM are to decrease power, and to decrease airspeed.

 

Preventing overspeed in a constant speed propeller installation is a function of the propeller governor. It's a mechanical unit which is a valve controlled by a spring. The valve moves open and closed using flyweights that respond to propeller RPM...at higher RPM's the weights pull apart, and mechanically they're linked to the valve (called a pilot valve) that admits or denies engine oil to the propeller dome (which is used to change propeller blade angle). The propeller governor prevents overspeed by increasing blade angle, thus increasing drag on the propeller blade, thus slowing down their rotation, thus controlling RPM.

 

Once an engine is feathered, it must be brought out of feather as part of the engine starting process if one intends to use the engine in flight again. To do this, the engine is rotated and either engine oil or a dedicated supply of pressurized oil from an "accumulator" is used to drive the propeller back out of the feather position.

 

If the aircraft airspeed is high or the propeller comes out of feather too quickly, it's RPM increase is driven by the slipstream, and the propeller governor may not have time or the ability right away to supply pressurized oil to govern the prop. The engine may not be producing high oil pressure yet, and as the RPM increases, the propeller may not be capable of governing the speed just yet. This may mean that the engine can overspeed. This is a function of airspeed and as a result many times an airspeed range is recommended or prescribed for engine air starts. This may include a minimum speed to provide adequate rotational assistance, and a maximum speed to help prevent overspeed.

 

An overspeed can be problematic for several reasons. A faster speed can increases stresses on propeller blades, hubs, and shanks. It can affect engine integrity, and put excess stresses on crankshaft counterweights or balances (and can detune a crankshaft), and it can exceed the aerodynamic limitations of the propeller itself. At too high a speed the prop tips experience transonic aerodynamic conditions, which can cause flexing, vibration harmonic stresses, and other problems for not only the propeller, but the engine. Furthermore, as the drag that a windmilling propeller is very high, it becomes particularly detrimental at higher engine RPM's and speeds.

 

The procedure for unfeathering an engine varies with the propeller system and installation. With some engines, the procedure may be to simply drive the propeller out of feather with respect to the blade angle, and let it windmill up to speed on it's own before introducing fuel or spark. Some engines may require a lower propeller setting until the engine speed is stable. Some may require a higher setting. Using the wrong airspeed or technique may result in an unsatisfactory start, an engine that won't come out of feather, or one that can overspeed when coming out of feather.

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That's why these are better in my opinion, all sizes for different needs;

 

from what I remember, those turbines are a lot more expensive, and actually are less efficient because of how much they weigh

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