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Hey guys,
I have heard contradictory statements on the relation between your prop pitch (fine vs. coarse) and TAS, and I would like some of your opinions.
I always thought that with a finer setting (high RPM), you will have a higher fuel consumption, higher temperatures for CHT, and a higher TAS. Is this the case?
Also, can someone explain when and why prop efficiency is greatest- low, or high RPM?
I understand that with a coarse setting (low RPM), the prop's AOA is greater and thus, the props take a "bigger" bite out of the air, implying greater distance travelled per revolution (i.e. pitch). I always thought this implied a higher TAS, but this is contradictory to the first statement I made. Am I misinterpreting something?
Any help is greatly appreciated.
Chris.
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It is probably easiest to understand this by discussing airplanes with fixed pitch props.
You have probably noticed that when you add full throttle to take off, you only get arount 2300-2400 rpm, yet your airplane may make full power at 2700 rpm. In a Piper Cadet rated at 160hp, you only get arount 145hp on take off. Rate of Climb is directly related to available horsepower. So less available horsepower = lower rate of climb.
It would be possible to reduce take off ground roll and increase the rate of climb by installing a propellor with a lower pitch. This would increase the available horespower by allowing the engine to reach full rated power in a Vy climb. The trade off is that once you reached cruising altitude, you would not be able to cruise at a high power setting, because the engine will overspeed. This results in the pilot having to reduce the throttle setting. This understandably results in a lower cruise speed.
We could fix this by installing a propellor with a higher pitch. This would solve our cruise problem. We can now cruise at a high power setting without overspeeding the engine. The only problem is that we cannot make full rated power on takeoff, causing longer ground roll and reduced rate of climb....
This is really no different than a car with a 1 speed transmission. Do you want a low gear for acceleration, or a high gear so you can drive more than 30 mph without redlining the engine?
As a result of this, most airplanes with fixed pitch props have a prop that is not optimized for climb or cruise. It is a compromise that allows decent climb performance without excessively hurting cruise speed.
There are a few aircraft that are fitted with pure climb props, but these are usually Piper Cubs, or other small STOL aircraft. The cruise penalty is around 10 kts. I do not know of any airplanes off the top of my head that have cruise props.
The adjustable pitch prop solves our delemma, by giving us a low pitch prop / high rpm for takeoff and a high pitch prop / low rpm for cruise. Since pilots are inherently lazy, the constant speed prop is even better, since we do not have to manually adjust the prop pitch every time we change our power or airspeed. The governor does it for us.
However the constant speed prop disguises what is really going on. It is true that at full throttle and high rpm an aircraft with a constant speed prop will have its highest TAS, but the angle of the prop blades at full power and cruise is greater than their angle at takeoff. As our airspeed increases, the governor increases the angle of the propellor blades to keep the prop from overspeeding.
Overall the efficiency is greatest at a low prop rpm. It is more efficient to run the engine (recip) at a higher MAP and lower rpm, and it is more efficient to turn the prop at a low rpm. In general, the most efficient way to create thrust is to move as much air as possible, as little as possible. At the speeds we operate at, propellors are more efficient than turbofans, which are in turn more efficient than the pure turbojet.
Again, look at your car for an example. It is often possible to cruise at 65 mph in several gears. Which is most efficient? Top gear, which is usually overdrive (transmission output shaft turns faster than input shaft) is most efficient.