SAR Helicopters

[Dave81]

Hello,

I'm not actually involved in the aviation business but have a interest in the mechanics of heli's and I was hoping for some information. I'm a keen watcher of the Highland Emergency program and frequently they seem to be unable to maintain a hover to winch due to a lack of power, an alarm is heard going off and the pilots quoting power over 100%. In my limited knowledge of helicopter flight characteristics i'm assumming this is as most of the power is going into keeping the heli steady in the wind and theirs not enough to maintain the height or is it the other way around in that the wind actually helps and a drop in wind can be the problem?

Thanks,
Dave.

[Mike Rosewhich]

It's usually the lack of a steady wind that causes the problem. Helicopters use less power in forward flight (or hovering into wind) than in a still air hover. It's called Translational lift, or it was the last time I read a book on the subject.

The alarm is the height warner.

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In the mountains, the problem is made worse by downdraughting air which requires more power to overcome. Normally more wind (horizontally) means less power required to hover but unless that wind is steady, requires constant power changes in order to maintain a steady hover. Updraughting is generally great (unless it pushes you up into cloud) and, where possible, we do try to operate on the updraughting side of a feature.

[Dave81]

Thanks for all the replies. The downdraughting part is mentioned quite alot, once when R177 came within a few feet of hitting the ground and the pilot mentioned when caught in it more power would'nt make a difference.

Out of interest, how long does the start up process take on the Sea King?

Thanks,
David

[Thone1]

Really pushing it? IŽd say 2-3 mins was the time down at 771 NAS, if I remember correctly.

Not pre-cocking anything on a SAR job weŽre talking about maybe 4-5 mins.

Thomas