The design of a given helicopter mass for hover conditions with three main rotor blades instead of two can be approached in two different ways. A first solution is obtained by maintaining the blade radius as it is with a 2-blade main rotor and thus reducing the collective pitch. A second solution is obtained by reducing the blade radius from the value of the 2-bladed solution with a constant blade pitch. In both cases, the total required power for the helicopter will increase when switching to the 3-bladed option.

In the first case, with a constant Gross Weight (GW) and a constant main rotor tip radius (Rt), the main rotor induced power (Pi) will be kept constant and there will be just a change for the main rotor profile power (Pp,s), that will increase. This negative change of Pp,s will be due to two opposite effects :

• a higher number of blades that implies an increase of Pp,s by a factor 3/2;
• a priori, a reduced value of the profile drag coefficient CD of each blade but this effect will be limited to a few percents (like 10 or 15 %) for
  CD and then also for Pp,s .

In the second case, as the Rt is decreased, Pi will increase, proportionally to the Rt decrease. The term Pp,s will also increase due to the higher number of blades, an effect that will be slightly, maybe, compensated by the slightly lower collective pitch.

In both cases, the total main rotor power PMR will be increased
(PMR = Pi + Pp,s where Pi is representing about 65-70 % of PMR).
As a direct consequence, the tail rotor power PTR will also increase and therefore the total required power for the whole helicopter. Indeed PTR is proportional to the tail rotor thrust TTR to the power 1.5 and this TTR is directly proportional to PMR.

Of course, if a higher power is available from the propulsion system, the
3-blade design has the capability to produce a higher main rotor thrust TMR
(= GW) and then to equip a heavier helicopter. But on the other side, the
3-bladed rotor will also be heavier and more complicated from a main rotor hub construction point of view.

Patrick Hendrick
Professor
Université Libre de Bruxelles

Conclusion

According to all the arguments that were developed by Professor Patrick Hendrick, we decided for the moment to keep developing a 2-blades helicopter, and not a 3-blades one.
A 195 hp turbine in the future will permit us to develop the Winner T150
(3 blades) with much better flight performances. Unfortunately, this modification will have an effect on the current selling price.
This modification is estimated at € 20.000.

The important question we have to ask ourselves is the following : is it really worth it ? Thanks for giving us your invaluable opinion.