For Helicopters, why is the Power Required curve not the same as the Total Drag curve?

Question

I am currently getting my CPL and we have learned today about these concepts.

I understand the Power Graph and the Drag Curve Graph in isolation, however when you put the minimum power required point on the drag graph, it is not at minimum drag. My googling hasn’t given me a satisfactory explanation.

After the minimum power required point, power need slowly goes up with TAS, despite drag continuing to go down to the minimum drag point. Why is power need rising if drag is still decreasing?

Cheers.

In: 1

Answer 1

I’m guessing you’re looking at a curve like this:

[http://www.copters.com/aero/drag.html](http://www.copters.com/aero/drag.html)

Power required is the drag *times the airspeed*. So you don’t just look at the minimum drag point, you’re looking for the minimum drag*airspeed point. In general, that won’t be at the minimum drag point because the drag curve is pretty flat near the minimum so going a little slower means not much more drag but enough less airspeed that the product of drag*airspeed is lower.

For example, suppose the minimum drag point is at 100 knots and that minimum drag is 1 (units don’t mater here, we’re just comparing relative values). Our power required is 100*1 = 100.

If we go 5 knots faster, drag might only go up by 1% (parasite & form drag goes up but induced drag goes down), so the power required is now 105*1.01 = ~106.

If we go 5 knots slower, drag might go up by 1% (parasite & form go down but induced goes up), and the power required is now 95*1.01 = ~96…lower than the power we needed at the minimum drag point because the drop in airspeed was proportionally bigger than the increase in drag.