helicopters q's: rotor blade integrity/strength and autorotation

[Machine_Elf]

I first saw this video several years ago. It shows a Sea Stallion helicopter attempting to connect with a Stratotanker for mid-air refueling. The heli pilot gets a little wild with his stick inputs as he chases the fuel dispenser around, and at some point the rotor blades dip down in front of the canopy and slice off his own refueling boom. Fortunately for the pilot, the refueling boom loses the contest, and the rotor blades remain intact.

Question: Is the refueling boom on that helicopter (and others capable of mid-air refueling) designed to break like that, or was it just dumb luck that it broke and the rotor blade didn’t?

Second question concerns autorotation in the event of engine failure. Is it necessary for a helicopter to have forward velocity all the way until touchdown, or is it possible to touch down relatively gently with zero forward velocity?

[Xema]

@Machine_Elf

Is the refueling boom on that helicopter (and others capable of mid-air refueling) designed to break like that

The refueling probe needs to be strong enough to stand the weight of fuel flowing along it: not a severe load near its midpoint. So there’s no reason to make it extremely strong - and indeed good reason (the desire to keep the helicopter’s weight down) to make it no stronger than necessary.

The fact that it broke without seriously damaging a rotor blade could well be a consequence of this, rather than reflecting a specific design aim.

[Xema]

@Machine_Elf

is it possible to touch down relatively gently with zero forward velocity?

Here’s a video of an autorotation landing that ends with close to zero forward velocity. But note that a headwind could contribute to this.

[Johnny_L.A]

@Machine_Elf

Second question concerns autorotation in the event of engine failure. Is it necessary for a helicopter to have forward velocity all the way until touchdown, or is it possible to touch down relatively gently with zero forward velocity?

Here is a video showing a zero-airspeed autorotation. The airspeed indicator is the second dial from the right on the top row. You can watch the needle as the pilot slows down and comes into an OGE hover before initiating autorotation.

This page says:

Most autorotations are performed with forward airspeed. For simplicity, the following aerodynamic explanation is based on a vertical autorotative descent (no forward airspeed) in still air. Under these conditions, the forces that cause the blades to turn are similar for all blades regardless of their position in the plane of rotation. Dissymmetry of lift resulting from helicopter airspeed is therefore not a factor, but will be discussed later.

I’ve never seen anyone autorotate straight down; there’s always forward movement. The quote above seems to indicate it’s possible though. FWIW, I’ve always autorotated at about 70 kts.

As far as the touchdown goes, you can touch down with zero forward velocity. A headwind helps, but you can do it without one. Usually though, you’ll slide on the skids a little bit. Practice and technique. A nicely performed autorotation landing can be just as gentle as a normal landing.

[GusNSpot]

Long time ago in a different world:

An old W4 Officer used to auto rotate backwards, with the engine shut down completely, all the way to touchdown in H-34’s. 1964 at Ft. Bragg.

The brass fussed a lot but he had 34 years in service, was highly decorated blah blah. I think they were secretly proud of him.

Got a lot of stories about this guy from when I did a lot of sling load hookups under his aircraft.

[rsat3acr]

When I was flying in the Army our practice autorotations usually were done while moving forward. The instructor pilots would sometimes roll the throttle off while a a hover so no forward airspeed, but an altitude of only three feet. Can’t imagine that from altitude you’d want to go straight down as you can’t see below you.

[Johnny_L.A]

Incidentally, this is a good reference site for helicopter aerodynamics.