Autorotation - Helicopter

[MyMeowCat]

I was going to take helicopter lessons many years back and pondered the differences between fixed wing and rotary wing.

One question I had was -- what happens if a helicopter engine quits?? Well you are supposed to "auto-rotate" down to safety.

Then I hear of choppers going down killing all passengers. I heard news stories of how Chinooks ( with two main sets of blades ) go down -- killing all onboard (you would think that with two sets of blades) it ought to autorotate better.

Anyways, coming from a guy who took some fixed wing flying lessons -- can any rotary wing guys out there explain autorotation? Souns like if an engine were to quit on an aircraft you might do better gliding in a Cessna than a bell Ranger.

[grimey]

Read the TSB reports. There are very few fatalities caused by an engine failure in a helicopter. Most fatalities are the result of some other incident, or due to having an engine failure in harsh conditions (ie, the crash is survived, but the people die of exposure before rescue).

[bmc]

You should post this on the helicopter forum.

[Hedley]

An engine failure in a helicopter is like an
engine failure in an airplane - a bit of pilot
skill is required to make it come out ok

The theory behind an auto-rotation is
simple - the helicopter pilot allows the
powerless helicopter to descend, which
uses the energy from his height (mgh)
to speed up his main rotor blades.

As he nears the ground, the helicopter
pilot increases the angle of attack of
the windmilling main rotor blades, which
increases lift.

If he times it right, he gets a nice
soft landing. If he doesn't time it right,
he gets a hard landing.

Keep in mind that a successful forced
landing in a helicopter requires a LOT less
real estate than for an airplane.

Over a corn field, I suppose a crappy
pilot would be best off in a fixed-wing,
because he could just keep his wings
level and wait.

But over a city, a skilled pilot would likely
do far better in a helicopter.

[sky's the limit]

Autorotation is an interesting thing.


The rotor system on helicopters is designed with two areas - the first providing lift when the blades are engine driven, the second providing Rotor RPM, ergo lift, when as Hedley points out, you exchange altitude for Rotor RPM. The upflowing air as yo decent turns the blades, and provides energy to the rotor system, allowing the pilot to "fly" the machine as required, forwards, backwards, side to side, turns and zero airspeed - all within the framework of a decent.

The approximate target airspeed for minimum rate of decent in most light/intermediate/medium type machines is about 60kts give or take. This produces a glide angle that does vary between types, and Gross Weights. When you get to the "bottom," roughly 70-100ft, you need to exchange your airspeed for lift, again as Hedley points out. This "flaring" on the machine reduces the decent rate to zero, you can even climb (not recommended..), and the machine comes to a point roughly 20-30ft above the ground at zero airspeed - which is a good thing - but with LOTS of energy in the rotor, meaning the pilot can pull the Collective (increase blade pitch) and cushion the machine onto the ground. Et viola.

After thousands of hours in fixed wing aircraft, all things being equal, I'd take an engine failure in a single engined helicopter ANY DAY over one in an airplane. The one major caveat to that is: We use helicopters to work in terrain where you can't use anything else.... This often means that even with a perfect "Auto," the machine will end up in some form of distress given the ground underneath. There are other situations where autorotation is extremely difficult, for example at the top of a 100ft long line and zero airspeed. (which is where I spend most of my time.) This falls in the Height Velocity curve, in short meaning successful autorotation is not assured/possible.

As evidenced by the tragic Goose accident this week, the terrain under ANY aircraft suffering a power outage is a HUGE factor in survivability.

You do not often hear about the successful engine out incidents that happen. While they are infrequent these days, a well trained pilot stands a very good chance of getting the machine safely onto the ground.

Hope that helps.

stl