Power Availableby Dave Hernandez
Winter is upon us, and with that we usually notice a marked increase in performance in our helicopters. Whereas in the heat of summer it takes 36 or even 38 inches of manifold pressure to hover and taxi at a given weight, now it may only take 34, and sometimes even as low as 30! We know this is largely due to ambient conditions — colder air is denser and can hold less moisture, which is incredibly helpful. Resultant density altitudes can be as low as -1000 feet. So when you find yourself needing to lower the collective more to get a small rate of descent, it is partly because in these colder conditions there is a larger amount of excess power available, where in the summer there is not. Thus, it takes a larger reduction in power to get a descent, because less power is needed to maintain your current flight profile. Let’s look at that a little deeper.
There is one source (sometimes more, but let’s look at the engines as a singular unit) of power for the helicopter that drives both the main rotor and the tail rotor. The engine does not care what it’s powering, nor does the airframe care what’s powering it, so long as power is delivered. Different situations require more power, until reaching a point past which no more power is available. That is the finite limit of power available. Some situations, like high-density altitude, high weight, humidity, and hot temps can lower that maximum amount of power available. In the case of a tailwind, from a physics perspective, the rotor system doesn’t care where the wind is coming from (vortex generation aside). Nevertheless, we need more power with a tailwind because the tail rotor is working harder to keep directional stability. That higher demand from the tail rotor diminishes the amount of power available for the main rotor; thus, tailwind landings require substantially more power. Add in high-density altitude, max gross weight, or hot and high situations, where there is already a reduced amount of power available, and the results can be dangerous. Understanding how different maneuvers require more or less power in different conditions will help you manage your power and get the most out of your helicopter.
For more information, here are a few recommended readings on helicopter performance:
- An excellent article on the subject, “Understanding Helicopter Power Requirements: The Power Struggle,” can be found at the following link:
- Cyclic and Collective by Shawn Coyle, chapter 6
- Principles of Helicopter Flight, by W.J. Wagtendonk, chapter 13
- Helicopter Flying Handbook, chapter 7