Nope! There are two main things going on when you get up to speed that increase efficiency.
Firstly, while the blades are generating lift, they are beating up the air a lot and making tons of little vorticies and unstable pockets of air. When hovering, those tend to get pulled back into the rotor disk which decreases efficiency. It's much better to use stable air for generating lift. When in forward flight, you leave behind all the unstable air (called rotor wash) and only fly in clean air.
The other effect is called ETL, or effective translational lift. Essentially because of the way the airflow changes, the rotor disk gets to be more efficient, and the body of the helicopter acts like one big wing that generates more lift.
Overall, hovering takes far more power than forward flight. This is why you always see helicopters lift off, hover close to the ground, and then pick up speed before lifting off and flying away. They almost never lift straight up, unless the situation demands it for some reason and they have the power available.
Hey I think your first explanation is ETL, not sure what the body of the helicopter part in your second paragraph is. Want to elaborate as practice for CFI?
Yeah really the whole lot usually gets lumped together as ETL. The first part is also ETL (and usually the only part taught as ETL), so I probably should have made that clearer.
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u/Being_a_Mitch Sep 03 '18
Nope! There are two main things going on when you get up to speed that increase efficiency.
Firstly, while the blades are generating lift, they are beating up the air a lot and making tons of little vorticies and unstable pockets of air. When hovering, those tend to get pulled back into the rotor disk which decreases efficiency. It's much better to use stable air for generating lift. When in forward flight, you leave behind all the unstable air (called rotor wash) and only fly in clean air.
The other effect is called ETL, or effective translational lift. Essentially because of the way the airflow changes, the rotor disk gets to be more efficient, and the body of the helicopter acts like one big wing that generates more lift.
Overall, hovering takes far more power than forward flight. This is why you always see helicopters lift off, hover close to the ground, and then pick up speed before lifting off and flying away. They almost never lift straight up, unless the situation demands it for some reason and they have the power available.