Tuesday, January 23, 2007

Although the aircraft to the right is just a model of an airplane, this shows the Dehaviland Dash 8 I flew in to start work yesterday.

Have you flown on one of these "puddle jumpers?" BTW, I don't like this term. It seems very disparaging of both the hardworking pilots and aircraft. Both propeller airplanes and their crews are overall very skilled and love their jobs, just like the commercial jet pilots.

Some people seated behind me were discussing why it was so dern loud while we were climbing out, but quieter when we were in cruise. Here is why:

A propeller is a device which transmits power by converting it into thrust for propulsion of a vehicle such as an aircraft, ship, or submarine though a fluid such as water or air, by rotating two or more twisted blades about a central shaft, in a manner analogous to rotating a screw through a solid. The blades of a propeller act as rotating wings, and produce force through application of both Bernoulli's principle and Newton's third law, generating a difference in pressure between the forward and rear surfaces of the airfoil-shaped blades.

Okay, so now that you know what a propeller does...

When rolling down the runway and climbing out to a safe (should an engine fail, an altitude to give you options to manuever back down for a safe landing), or cruising altitude, the props are set to give maximum lift. A consequence of this is maximum drag. Think of it like swimming laps in a pool. If you do your best crawlstroke, really grabbing at the water to move yourself forward, you will, but you'll tire yourself out. Better would be to do a more comfortable stroke, leaving energy to swim longer. It's a balance between energy and output.

So, it's most important to speed up and climb! despite the noise and wear and tear. When you reach cruising altitude, the propellers can be reduced in pitch and lessening the noise.

Ok, so that's pretty basic and doesn't give much real info. How about this more scientific explanation:

Changes to a propeller's efficiency are produced by a number of factors, notably adjustments to the helix angle, the angle between the resultant relative velocity and the blade rotation direction, and to blade pitch. Very small pitch and helix angles give a good performance against resistance but provide little thrust, while larger angles have the opposite effect. The best helix angle is when the blade is acting as wing producing much more lift than drag.

However due to the shape of the propeller, only part of the blade can actually be operating at peak efficiency. The outer part of the blade produces the most thrust and so the blade is positioned at a pitch that gives optimum angle to that portion. Since a large portion of the blade is therefore at an inefficient angle, the inboard ends of the blade are hidden by a streamlined spinner to reduce the resistance torque that would otherwise be created. (Actually, most propeller blades have been twisted, to compensate for this - lm)

Most propellers are fitted with mechanisms to allow variable pitch – coarse pitch for high speed flight and fine pitch for climbing or accelerating from lower speeds. Early pitch control settings were pilot operated and so limited to only three or so settings; later systems were automatic. Later still, variable pitch was replaced with the constant speed unit. In some aircraft the pilot can manually override the constant speed mechanism to reverse the blade pitch angle, and thus the thrust of the engine. This allows the aircraft to back up on its own, at unimproved airfields when aircraft tractors are unavailable. (Though most operators frown on their pilots doing this at any time -- it's too easy to rock the airplane back on its tail! - lm)

A constant-speed (also known as variable-pitch) propeller enables the pilot to set a desired propeller rpm (within a given allowable range). A propeller governor acts as an open-loop control system to vary propeller pitch angle (and therefore drag) as required to maintain the commanded rpm. In many aircraft this system is hydraulic, with engine oil serving as the hydraulic fluid. The purpose of varying propeller pitch angle with a variable pitch propeller is to maintain an optimal angle of attack (maximum lift to drag ratio) on the propeller blades as aircraft speed varies.

Most information from Wikipedia.

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