Propeller Governor
First, some fundamentals. A propeller is essentially a set of little wings, which produce lift and are subject to drag much like a normal wing. Also like a normal wing, a propeller moves through the air with a so called angle of attack. The greater the angle of attack, the greater the lift (or thrust in the case of propellers) at the cost of increased drag, making it harder to move through the air. And again like a wing, the angle of attack is affected not only by the structural position of the propeller, but also by the speed and direction of the airflow passing over it. Finally, and once again like a wing, a propeller produces a downwash, or an induced velocity. Because the linear velocity of any propeller blade section is a function of the propeller radius (the outer blade edges of a spinning propeller move through the air faster than the inner edges), the blades are shaped such that the blade angle is progressively reduced toward the outer edges.
Early propellers were constructed with fixed blades, where the blade pitch angles cannot be changed. In a fixed-pitch propeller, as the aircraft’s speed increases, the angle of attack is reduced, which in turn reduces the thrust produced by the propeller. If airspeed continues to increase, the propeller eventually turns into a kind of airbrake, producing reverse thrust, but still demanding power from the engine to turn it. Ultimately the propeller can start to windmill, where it itself begins to turn the engine instead of the other way around.
The problem with fixed-pitch propellers is that they only work well in a narrow range of airspeeds. The pitch can be optimized for low speed, useful for maximum takeoff thrust, but then efficiency begins to drop as airspeed climbs. Conversely, the pitch can be designed for best climbing speed or higher speeds in general, but at the cost of low speed efficiency, which reduces takeoff performance.
Variable-pitch propellers were introduced to remedy this problem. The pilot could now manually control the propeller pitch angle. This was, no doubt, an exciting time for aircraft engineers. For pilots however - fighter pilots in particular - it was another headache in flight. It became much easier to break the aircraft by overstressing or overspeeding the engine as a result of mismanaging propeller pitch control. So in a spur of innovation, designers began to work on mechanisms around the 1930s that would automatically adjust propeller pitch to maintain a constant engine RPM – propeller governors. All the pilot would have to do is set a desired engine RPM and the governor would load or unload the propeller by adjusting the pitch angle to maintain this setting.
Let’s now take a look at the Hamilton Standard propeller governor system used on the P-51. Inside the prop spinner is a propeller dome, which houses a horizontal piston cylinder. The piston is surrounded by oil to either side – low pressure engine oil in the forward side and propeller governor oil, pressure-boosted by a pressure pump, in the rear side. Relative pressure of the oils to either side of the piston determines its position. As the piston moves in reaction to pressure differentials, a special mechanism translates this motion to the propeller blades to adjust their pitch.
Oil flow to and from the cylinder is controlled by a vertical pilot valve in the governor assembly. The pilot valve’s neutral position is maintained by a balance of forces between a tension spring that pushes it down and special flyweights that pull it up under the centrifugal force of spinning action when the engine is running. This balance is maintained and the propeller pitch remains constant as long as the engine RPM is stable. When RPM changes, the flyweights and the tension spring become unbalanced, moving the pilot valve to open oil lines to and from the piston cylinder. Oil moves into one side of the piston and out of the other, the piston moves in response to a pressure change, and the propeller pitch is adjusted until equilibrium is restored. Tension of the spring is controlled by the pilot’s RPM lever. As such, moving the RPM lever in the cockpit unbalances the pilot valve and again moves the piston to adjust the propeller pitch until equilibrium between the tension spring and the flyweights is restored at the set RPM value.
For example, if RPM increases, the flyweights move outward under increased centrifugal force, overcoming the tension of the spring and pulling the pilot valve up. The pilot valve opens oil lines to push high pressure governor oil into the rear side of the cylinder and engine oil out of the forward side of the cylinder. The piston moves forward and propeller pitch is increased. As propeller pitch increases, the higher drag increases the load on the engine and RPM is returned to its original value. The flyweights return to a neutral position and equilibrium is restored over the pilot valve, closing the oil lines. Conversely, if RPM is reduced, the tension spring overcomes the flyweights, moving the pilot valve down and pushing engine oil into the forward side of the cylinder and governor oil out of the rear side. The piston moves back, decreasing the propeller pitch and unloading the engine to increase RPM until equilibrium is restored.
There is a third element affecting propeller pitch angle – the centrifugal force of the propeller itself, which moves the blades toward lower pitch. It’s important to point out that in the absence of oil pressure inside the piston cylinder, the propeller will set to low pitch.
So, what does it all mean in practice?
Running at maximum RPM is very stressful for the engine, even if manifold pressure is kept down. It’s generally best to maintain the lowest RPM possible for any desired flight condition. A number of manifold pressure and RPM combinations are recommended for various parts of the flight envelope. These are provided in the manuals and graphs, but can be determined independently given a sufficient understanding of the principles involved.
A special case worth considering is an engine failure. In autorotation, the prop effectively acts as an airbrake, so assuming the governor remains functional, the RPM should be immediately set to full decrease. In this case, the aircraft might attain a glide ratio of 9-10:1. If RPM is left high, this ratio will drop by as much as a third. Worse of all is a situation where the propeller stops altogether due to a jammed or poorly turning engine. In this case, the prop’s surface area of nearly 1 square meter will reduce the glide ratio approximately by half. Luckily, getting the propeller to stop mid-flight, even with the engine turned off, is practically impossible. Although, if the oil is frozen and airspeed is low, it does become likely. As long as the engine is warm – this could only happen in a spin or maybe a complete loss of airspeed, such as at the top of a stalling vertical maneuver. Once stopped, spinning up the engine is impossible, regardless of airspeed.
Here are a couple of new screenshots where you can see the visual difference between a low and high pitch setting of the propeller:
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发表于 2012-4-28 08:12:22
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