# Hover Performance in Helicopter dynamics

What do we mean by “good” hover performance in Rotor dynamics? If I want to analyse hover performance of a helicopter, then What parameters should I monitor?

• – user14897
Oct 13, 2021 at 6:16

You may want to check out the "figure of merit." It's not perfect, but is a useful metric of hover performance. A figure of merit of 0.8 is considered good according to some sources.

Figure of merit is defined as the ratio of ideal hover power (for a given thrust T, air density $$\rho$$ and rotor area A) to actual power required (P), i.e.

$$F = \frac{ T \sqrt{ \frac{T}{2 \rho A} }}{P}$$

F=1 indicates that a rotor is perfectly efficient; it operates at ideal power. If the real power is larger—the rotor is less efficient—the figure of merit decreases.

Since real, practical rotors have blades with drag, nonuniform inflow, etc they will have F<1. An efficient rotor could have F as high as 0.8.

For a given helicopter, F can change based on the flight condition. For example, when hovering closer to the ground—in ground effect (IGE)—F will be larger/better. Also, F may increase with gross weight (more passengers, cargo). This is because thrust will increase to counter the added weight, which means blades operate at larger angle of attack, typically with larger lift to drag ratio (L/D).

A common mistake people make with F is to associate high values with good helicopter design. There are reasons why low F can be better. First, a helicopter with a larger rotor and smaller F can require less power than a helicopter with a smaller rotor and higher F (at the same thrust). This is because F won't give you bonus points for the larger rotor, it's already factored out that advantage.

Also, a high F rotor may be less capable of high speed flight, maneuverability or other desirable characteristics. A rotor may have low F because other design goals were more important, e.g. max speed.

For more information here are some slides by Prof. Sankar (see slide 45 ~20min). There's also a good discussion in this article on helicopter performance.