Hovering vs Orbiting?
- Thread starter Minuteman
- Start date
Takes more power-required to hover. When hovering, the rotor system is fully dependent on the power available from the engine. With forward speed, generally beginning about 15 knots, the rotor system begins to benefit from the relative wind that it’s traveling through, which provides lift and thus reduces the power required by the engine. This area is known as where Effective Translational Lift occurs. Also can be seen in lower speeds to nearly a hover, if strong enough headwinds are encountered, as the rotor system doesn’t know the difference.
To add, for anti-torque purposes, the tail rotor if affected in the exact same way in terms of ETL, just oriented in a yaw direction as opposed to vertical direction.
Lawman
Well-Known Member
Helicopters are not Hover machines, they are the worlds best STOVL design grown from generations of “converta plane” efforts that simply were too heavy or too flimsy until Igor figured it out.
Hover is a byproduct of that requirement.
To give an idea of power required to hover at OGE vs fly at approx max endurance, power is effectively double and fuel burn is about 1.5 times for the same aircraft. Flight envelope for a conventional helicopter is usually somewhere in the 55-95 knot window for max endurance/power available and scales exponentially at both ends above and below.
Cruise chart attached, as you can see it’s basically a total drag chart turned in its side.
Attachments
Agreed. The amount of fuel burned hovering around below ETL, doing things like long-line work or fire bucket work, really spikes the fuel burn, and places a lot of stress on the tail rotor system and it’s constantly loaded and constantly having to “work”, as well as severely increases the transmission temps, both getting no respite from some forward flight time to relieve their workload with aerodynamic help as well as cool them down. Hence why a common accident scenario of logging helos or fire bucket helos, is failure of the tail rotor gearbox, either seizure or catastrophic destruction of
Lawman
Well-Known Member
Yeah for the fixed wing pilots in the room observing there is an entire region in performance charts called the, “avoid region,” which is entirely discussing below transitional lift/hover profiles. The best analog I can think of is imagine flying at the edge of the stall envelope intentionally…. It’s only ok if you have what’s needed to recover and hovering below about 300 feet AGL is probably gonna end badly which is to Mike’s point where a lot of helicopter work lives.
Essentially the avoid region is a deceleration that no matter what you do…. This thing is touching dirt and the best case is minor crash with minor injury based off reaction time to the scenario. Hover speeds to ~25 knots at lower altitudes (40-250 AGL) are usually the shaded part of the chart where it ceases to be any chance of flying away in a dual engine aircraft in case of an engine failure and we are looking at developing a similar chart to discuss loss of tail rotor control authority because those numbers would be different. We have a directional control margin chart which talks of conditions to avoid and maximum control authority envelops, but doesn’t associate any specific envelope to recover if exceeded.
Also forward flight translates to the tail vertical stab taking on the load the tail rotor handles so forward airspeed gains power available simply by negating that parasitic loss. Another reason to avoid hovering when able.
For the single engine helos, the avoid region also accounts for the ability to successfully complete an auto if the engine fails in a hover. With many light helos that have low interia rotor systems, there ain’t much room for error. The Astar, the chart says 8 AGL below ETL, above that is the avoid region. Even 8 AGL below ETL, if the engine fails, you’d have to perfectly time a collective full down lowering, and then near immediately bring collective back in, in order to not deplete Nr too far or worse, balloon back up with too much pull. The margin for screwing that up is razor thin, especially considering reaction time and control inputs both being smooth when startled with no time. At best, the skids will be spread flat on landing, at worst the seats will stroke and the fuselage crunch. On the flip side is the Huey, so high inertia of a rotor system, that you can cushion a landing, pick back up, move about 20 yards or so, do a 180, and land again, and still have residual Nr.