Helicopter Take Off

[Rwy in Sight]

Following an Augusta Power 109 in Greece on Monday June 17th at 2:15 am during an EMS I have a question regarding take off: Do helicopters need to fly into the wind for take off like fixed wing aircraft or they can take off with a significant cross wind component.

[B.Loser]

I may get stepped on pretty hard for this, but simplistically speaking, and considering one will remain within design limitations, helicopters can take off with a significant cross wind component (downwind also). It takes more work, more horsepower and maybe not the best choice if the other option were available.

[Grainger]

More to the point we're not constrained to runway direction so you can keep the relative wind on the nose even if transitioning away at an angle, and that gives you the best of both worlds.

[Nick Lappos]

The actual performance of the rotor is better in a crosswind than in zero wind, so a crosswind takeoff is just fine. The distances, climb rates and safety are easily better than a zero wind day, although not as good as the help a pure headwind would give.

Downwind is more of an issue, but even then, the performance of the helicopter is no worse than that on a no wind day, but the exposure to a long, dragged out takeoff is much greater because you must accelerate through zero knots before you can accelerate to climb speed. If the aircraft is loaded to the max while hovering downwind, and then you take off, you will probably settle during the portion where you have zero airspeed, and you will touch the ground while moving downwind at the wind's speed.

For military and civil SAR operations, with sufficient power, and foreknowledge of your aircraft's performance, upwind, downwind, crosswind, you can do it all, if operationally important. For low time pilots, with marginal single engined aircraft, don't mess with Mother Nature, use the head wind!

[Weight and Balance]

To rephrase part of Nick's excellent post:

A head wind component will increase your climb angle relative to the ground, or in other words improve your obstacle clearance ability. If obstacles are not a problem, any wind is better than no wind - up to a point.

Nick, I'm surprised you didn't discuss control margins during hovering with higher winds from certain directions. The little European helicopters I was familiar with all had a "bad" wind direction, usually from somewhere behind you, where hovering near the posted wind limit would have the cyclic and/or the pedals near the stops. We always avoided those directions, just in case the wind picked up, or we had to dodge something in a hurry.

I know most of your Sikorsky products have very high cross wind capabilities (in comparison, anyway). Does this mean you don't have to worry as much about control margins in high wind hovers?

[Nick Lappos]

W & B,

Thanks for the clarification, you are right on.

With most modern military helicopters, crosswind yaw control is quite good, usually measured as the demonstarted crosswind capability. For Sikorskys, this is 35 to 45 knots, meaning that in winds up to that speed, a full hover turn, and trim in any direction is possible without worrying about hitting the pedal stops.

For light helicopters, the designs are more marginal, and the capability can be 17 knots or even less. For those machines, while main rotor performance does not depend on the wind direction, tail rotor authority can be compromised in crosswind takeoff. The critical crosswind is that which uses increased anti-torque pedal, right crosswind for a US machine, left crosswind in France.

"Weight and Balance" also describes another effect that should be discussed. Many helicopters experience an increase in downwind hover power when the tail rotor wash strikes the main rotor tips in a cross wind hover. And many helicopters experience reduction in crosswind capability when the main rotor shed vortex strikes the tail rotor during quartering headwind operations. Some flight manuals are helpful in these areas, but most machines leave it to experience to find these regimes.

[The Nr Fairy]

If I remember correctly, and I'll bow to Nick on this, I think the 17 knot limit is a certification limit which manufacturers must show controllability up to, and not necessarily beyond.

I interpret as meaning that the FAA says "show controllability up to a minimum of 17kts" so manufacturers stop there if the design complies. To go further would mean spending more money and time certifiying to a higher limit. When I get to see my copy of the POH I'll see if it's in the limitations or performance section.

[Nick Lappos]

The Nr Fairy,

The 17 knots is required for most helicopters under part 27 and 29, but a new class of approvals have been made for Bells like the 212, 412 and 430 where they are approved as "Catagory B, 9 passengers or less" with no crosswind aurthority at all (!!) This is the reason why there are several sets of hover data for those aircraft, with some charts labeled "no wind or wind within 45 degrees of the nose" written across the top of the chart. They actually load the aircraft until there is virtually no pedal margin in a hover, and show that you can sneak the aircraft into flight without any LTE events.

To my knowledge, no other approvals but those have been made, so most helicopter brands have been certified to at least 17 knots of crosswind.

The last time I described this, someone disputed it, I will be gald to post the performance charts from these Bells for those who wish to see what to look out for.

Nick

[comedyjock]

Another consideration is what happens if something goes wrong whilst gaining airspeed particularly in a single engine helicopter.
Should you need to land or even worse have to enter autorotation you really want to be pointing into wind. Low height and airspeed leaves little room to manoeuver and get yourself out of trouble so into wind t/o and landing is probably best.

[Q max]

I thought Grainger's point was good though disturbingly not picked up upon:

"More to the point we're not constrained to runway direction so you can keep the relative wind on the nose even if transitioning away at an angle, and that gives you the best of both worlds."

You can land T/o 'into wind' in a helicopter through about 300 degrees.

Sad about these people killing themselves though....

[rightpedalRIGHTPEDAL]

Hover in winds from any direction up to a certain velocity you will find in the performance section.
Note that although they substantiate hover to the 17 or 18 knots for the entire operating envelope, a note is also made that at max gross at sea level, hover has also been substantiated at up to 30 knot winds any direction. (AS 350 Flight Manual)
Remember - it is not a limit.