The main criterion for selecting the number of blades is to effectively take away engine power and convert that power into propeller thrust. In principle, the fewer the number of blades, the more aerodynamically efficient the propeller is, but in practice, due to the nature of paragliding propulsion systems, which generate an aerodynamic shadow that is very destructive to the correct operation of the propeller, this does not always work.

In addition, there are several important factors that affect the sense of using more shovels:

Limitation of the diameter of the basket and the propeller:
A properly designed 3-blade propeller with a smaller diameter, in a sense, is a replacement for a 2-blade propeller with a larger diameter. To a good approximation, replacing a 140 cm diameter 2-blade propeller with a 130 cm diameter 3-blade propeller will give the same performance. It is as if the area, pitch and elongation of a 2-blade propeller, were spread over 3 blades. In practice, you will sometimes find that 2-blade propellers have a thicker profile, a larger profile area and chord, and usually a larger pitch. The analogous 3-blade propeller is figuratively speaking slimmer. Sometimes, however, the differences are not “visible to the naked eye” because they are small changes in the angles or pitch of the propeller.

Engine power:
For engines above 20 hp, it already usually pays to use a 3-blade propeller. Especially for small basket diameters (for pilots/pilots with smaller height and arm span), when this natural limitation does not allow the use of large propeller diameters, 3 blades can be useful.
For higher horsepower engines, like 30 hp or more, 3 blades can be considered strongly advisable, as a 2-blade propeller would have to have an unfavorably large pitch or diameter.

Reduction of engine speed:
Here a lot depends on the profile of a given propeller. Each profile has its own optimum angles of attack, which are the resultant of propeller speed and forward speed. Leaving aside the complex nuances of this issue, it can be generally assumed that engines with higher reduction (lower propeller rpm relative to engine rpm), will usually require more blades, sometimes even more than three.

Static and dynamic thrust:
The derivative of the above points is what is actually at stake, i.e. what thrust and under what conditions the engine-propeller combination achieves. FleXer propellers are designed primarily to achieve the best possible performance in flight, so we will omit the less important parameter of static thrust, which, although a certain analogy, does not capture the essence of the matter. Any measurements conducted in flight or on a dynamic dynamometer, usually show higher efficiency, the performance of 3-blade propellers by an average of 5% for the same diameters.

Vibration and flight comfort:
Here, definitely more blades give benefits, although at the same time the propeller’s sheer mass, flexibility and correct balance and installation are extremely important.
Any propeller is a source of unavoidable vibration, as it operates under a huge aerodynamic shadow from the pilot himself, the propulsion components (tank, harness, stock, resonance tube, etc.). 3-blade propellers convert all these vibrations into perceptibly smaller ones by changing the frequency, i.e. the vibrations are more frequent but of smaller amplitude. Also, any imperfections in balance are distributed more favorably (provided the propeller is relatively light)

Noise limitation and the nature of the sound the drive makes:
The vast majority of noise in our drives is caused by propeller operation. The parameters involved are difficult to measure, and often the perception of something as comfort or annoyance here depends on the preference of the pilot or bystanders.
A factor that may be relevant here is the fact that 3-blade propellers, as a rule, operate at smaller angles of attack, which may translate, into smaller pressure spikes of the air flowing around them and, as a result, affect the measured decibels. The rest is purely a matter of preference, as some prefer a louder but lower-frequency sound and others a quieter but kind of higher-pitched sound (sometimes described as more racy, airplane-like). On the other hand, due to the purely physical nature of sound wave propagation, it is important to keep in mind that lower tones will be heard from further distances, which can also affect the overall annoyance to the environment.

Aesthetics. Is it important?
For many pilots YES, so as an “icing on the cake” you can add just pure aesthetics and so-called racial appearance. Simply 3-blade propellers look better! And such, which by their very appearance bend the space-time continuum, it is already cosmos :)

So in our approach, we prefer 3-blade propellers, considering them more efficient in the vast majority of cases.