I have found out upon researching jet engine fan blades that they can vary from 16 to 34. Why do they vary, I mean, does decreasing or increasing the number of fan blades have any performance impact? Does this have any scientific/technical reason?
This might not be a super satisfying answer as I'm not a fan expert, but since no one else has answered, let me tell you what I know. There are many competing and often mutually exclusive constraints on the fan module design. This include, but are not limited to:
- Overall weight
- Thrust generated
- Stall margin
- Aeromechanics (i.e. flutter or high cycle fatigue issues)
- Ability to resist a bird strike
- Ability to contain a fan blade out event
- Manufacturing cost
- Maintainability / Repair cost
(many existing answers on this site about most of this topics if you are not familiar with them).
The number of fan blades is one knob that the fan designer can play with in order to optimize all of the above. For some of them more blades is better, and for others more blades is worse. But number of blades is not the only knob, there are many more. Other variables include, but are not limited to:
- Fan diameter
- Nozzle exit area
- Fan blade material
- Stagger angle of the blade
In short, there is a complicated optimization problem, where you are trying to turn 5-10 knobs in order to find the best combination of 5-10 desired quantities. In order to find the best configuration, Designers will use experimental wind tunnel measurements, 3D computational fluid dynamics, as well as experience on previous engines.
— Rolls-Royce via leehamnews.com
The trend has been fewer blades as shown above in the evolution of R-R's RB211, and they're even fewer nowadays for the same thrust class – you can check the trend of the two main GE90 versions and its derived models.
From the standpoint of thrust generation, just like propellers, the fewer the blades the more efficient. But the individual bigger wide-chord blade would be heavier, i.e. higher centrifugal forces, and also subject to more forward bending.
Advances in material engineering and blade design make this trend possible. In broad strokes, the use of hollow titanium blades, and then the even bigger twisty blades where composites are used.
The number of fan blades in a jet engine decides the amount of energy that can be transferred from the shaft onto the incoming air. Fan blades are like little wings and those create something like lift. More wings means more lift. Some engines have no fan blades at all. They rely on the incoming airflow to result entirely from the aircraft's airspeed.
Many fan blades don't really make the air go any faster than fewer fan blades, but it does provide more control over the thrust.
There is other reasons to have fewer fan blades, so for every jet engine, size and purpose, there is an optimal number.