An adjustable pitch 3-blade propeller has certain advantages over a fixed pitch 2-blade propeller, but also a few disadvantages.
Amongst the advantages we can point out:
.It is more versatile, as you have the possibility of adjusting the pitch to suite your personal requirements, such as improving impulse for take-off or reducing consumption at cruising speed, etc.
.It is equivalent to a larger 2-blade and can be useful for ultralights with limited space for the propeller because it passes near to the ground or the tail tube. This problem can be solved by fitting a 3-blade or 4-blade if space is very limited.
.It works more smoothly and is better balanced (indeed, the majority of aircraft manufactured mount 3-blade propellers). As the masses are better distributed on the propeller hub, the in-flight sensations are much smoother.
On the other hand, there are a few disadvantages:
.The pitch is less "elastic" than with a 2-blade, and they lose performance when speed is increased, especially above 150 Km/h.
.During the pitch adjustment, when the blades turn not only is the pitch altered, but at the same time it is degraded. All the sections of the blade are turned at the same angle, whereas for a change of pitch the blade should turn a certain angle on the section up to 50% of the blade length, and only half of that angle from here to the propeller tip (100% of the radius).
.There is the risk of adjusting the pitch incorrectly. This would reduce propeller performance.
.In their search for the best possible performance, there are some pilots who make frequent changes without ever finding the best point, either because their objective is unobtainable, or because they make random changes in differing atmospheric conditions, etc.. Only a person with adequate experience and knowledge can achieve success with just a couple of tries, but we can offer the following general advice:
- If there are noise restrictions in your country, this will establish the maximum RPM during take-off without exceeding the maximum allowed noise.
- For the final adjustments you need to measure the maximum RPM during take-off, when the aircraft is moving quickly and about to take off and NOT when the aircraft is stationary.
- Normally the possible adjustments of RPM during take-off will be within 10% below the nominal RPM for maximum engine power. For example, if the maximum engine power is 100 CV at 5,500 RPM, the maximum RPM during take-off should be between 4,900 RPM in aircraft with a long cruising speed of 200 Km/h or more, and 5,400 RPM optimised for short take-offs and climbing.
- Especially in 2 stroke engines it is important that any excessive propeller pitch does not prevent the engine from "breathing" properly, as although it may be possible to make the engine work with 200 - 400 RPM less at cruising speed, if the engine is choked it cannot work well and consequently will consume much more fuel. If this happens, then the plugs will be black when taken out, a sure sign if this has only occurred after the propeller pitch has been increased.
Once you have studied the above, you will be in a position to decide if it is suitable for you to use a variable pitch 3-blade or 4-blade propeller.