Keeping your distance

[Kermit 180]

Greetings rotorheads. I am a fixed wing pilot who recently had the unfortunate experience of being taxied overhead by a Hughes 300 (269?) helicopter (what's the difference between these?).

The control wheel in my little C152 went all over the place, the ailerons were slammed from side to side. This preventable incident distressed my passenger. The helicopter pilot said later they did not see my aeroplane under them. I'm not looking for a debate over aeroplanes-vs-helicopters, but this incident has made me think about rotor-wash (is this the correct term?).

Can anyone tell me what the approximate distance is that an aeroplane should keep from a taxying or hovering helicopter to avoid 'rotor-wash'. There is a lot of info out there about wake turbulence from fixed-wing aeroplanes, but nothing that I can find on helicopter air disturbance.

Kermie (Helicopter illiterate) :confused:

[baranfin]

The AIM on page 7-3-5 says that the rotor wash extends out approx. 3x the diameter of the rotor. I know in all of my training and limited real world experience(still being a student pilot) I have not yet been in a situation where i HAD to overfly another aircraft. As a matter of fact the school has a policy of not overflying other A/C and when a fixed wing is approaching to land and i am operating on the side of the runway or taxiway i usually go flat pitch(on the ground not pulling any power) to minimize any rotor wash.

[The Nr Fairy]

Kermit :

I do know of an accident in the UK some years ago, although I can't find the accident report on the AAIB web site.

Essentially, an S61 in a hover-taxi at heavy weights disturbed the ar so much that a light aircraft which encountered the downwash was overturned, with fatal results.

It is at least a courtesy thing for small helicopter pilots to avoid incidents such as yours which, while upsetting, don't have the potential to cause problems as above.

[Kermit 180]

Thanks for your input Bara and Nr. So 3 times the width of the rotor is a good safe distance. Cheers, I'll pass the info on to other less-helicopter orientated people as well.

Kermie :)

PS: What is the rotor diameter on a small helicopter like the Hughes 300/269(?)? :confused:

[piloteddy]

Kermit,

The diameter of the 300's (or 269's) main rotor is 10.8m

[Devil 49]

Additional consideration in avoiding rotorwash- wind. Obvious to a helo guy, but I've had ittie bittie airplanes taxi down wind of me, park and complain of being blown around.
If a helo's sitting there with rotors turning, avoid the down wind side. It'll shut down or depart shortly. Even a 4 pax type helo and good stiff breeze can put out a tremendous air movement- sufficient to push an identical helo sideways on the skids several feet, until it weathervaned. We were working close (60 feet), but that kind of energy doesn't go away in 3 rotor diameters.

[Nick Lappos]

Some comments on rotor wash:

1) the average downwash velocity is roughly determined by how heavily the rotor is loaded, in pounds per square foot for example. A small helicopter might have 3 pounds per square foot, and a large one might have 10 or so. A tilt rotor will have from 15 to 22 pounds per square foot.

The velocity would be from 25 knots for a small helo to 45 knots for a large one, and from 50 to 65 knots for a tilt rotor.

This velocity passes down from the rotor and then spreads out away after hitting the ground. The velocity quickly reduces because the wash is rapidly increasing in area, and the speed reduces proportionately. The 3 rotor disk estimate is a good one for general purposes.

2) A helicopter produces this downwash only when it is producing thrust, so when it is "flat pitch" on the ground, the rotor wash is very low, almost non-existant. Almost the same for a wheeled helo while taxiing, as long as the pilot keeps the collective and power low.

3) In a wind, the hover rotorwash will collect and get blown downwind in a more cohesive core that can be more bothersome than if no wind were blowing. This means that we rotorheads must watch what is downwind from us, especially upon landing.

4) when a helicopter is in forward flight, the wash is much more like an airplane's, and mush less bothersome. It extends behind and below the aircraft, and rolls like a shed wing vortex from each rotor tip.

[Shackman]

You should see the downwash from a heavily laden Chinook!!!! It can move some awfully big and heavy items.

Seriously though, as a QHI I spend a goodly proportion of my time with students trying to make them think of just where their downwash is going and what damage it could be doing. Difficult admittedly when they are having major problems with the hover taxy in the early days. However, the bottom line it is our responsibility as professional pilots to consider every other user, be he fixed wing or rotary.

[Arm out the window]

Kermit 180,

There used to be a long-standing Australian Aeronautical Information Circular regarding wake turbulence which you may have seen, which had some information regarding helicopters as well as fixed wing.
It was removed some time ago, but probably became an advisory publication.
Nick Lappos' post hits the nail on the head pretty well anyway.

By the way, I like the way you started your post - 'Greetings Rotorheads'; sounds a little like something from a Gary Larson cartoon where the alien greets the earthlings (or should that be the other way around?)
:D

[Kermit 180]

Thanks very much for helping me out with my query, Rotorheads. :D

All this talk about rotors has got me thinking why I dont just go and start a helicopter PPL myself.

Then reality hits and I remember I'm a broke fixed wing instructor. Oh well. :(

Kermie :D

[Harry Peacock]

NR's accident was I think the RN Sea King which inverted an empty parked lightie at Goodwood in about 1990ish, rather expensive(But may be another).

Downwash though needs care, an idea of its area of effect can be got from watching helo's hover over water/dust/snow. 9 tons of Sea King has to move a lot of fresh air to stay away from the ground. You don't stand next to one on take off from a flight deck.

By the way 3x rotor for a SK is 186' :p

[Tee]

I heard that a marshaller was blown away recently by a large helicopter at Edinburgh Airport.

[The Nr Fairy]

HP :

Not that accident - the one I read about was at Oxford, on one of the Silverstone days. I've had a scan through the AAIB pages, but can't find it again.

[Harry Peacock]

NR:

Same problem different events!!

Same Sqn as above also had problems with some tents on the Scillies the same year.....

..... well no problems for the Sea King though! :D :D

[vorticey]

in australia the heli pilot in comand must ensure suffishent distance from any thing not to cause any hard bufferting(not exact words) and the helo in question would have been in deadmans curve anyway, obviosly not acting as a comercial pilot!
taxiing is genealy done within ground cushion :confused:

[collective bias]

What the hell is a helicopter doing 'taxing' over a 150? Regardless of the rotorwash considerations this is a poor command decision. :confused:

[Kermit 180]

At least it seems most rotor heads are aware of what they can do with their rotor-wash, and I'd also like to add that most of you appear very professional when you fly. Unfortunately theres always one or two who let the side down with bad decisions.

Thanks for your replies, I am educating my fellow fixed-wing pilots.

Kermie :)