Rotor downwash and light fixed wing
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Rotor downwash and light fixed wing
I asked this on Private Flying and was directed to ask here for more, err, accurate information...
I fly from a busy military airfield with big(gish) helicopters - Pumas & Merlins mostly. ATC often give 'caution rotor downwash' along with a clearance - e.g. on Monday I both took off and landed behind a Merlin using the runway, and before it took off the Merlin hovered mid-runway for about 20 seconds in virtually nil wind.
I'm familiar with the AIC on wake vortex but there doesn't seem to be an equivalent document for rotor downwash. A few questions which I hope you can answer...
a. I assume the rotor downwash travels out sideways after hitting the ground... how far, in still wind? Are there persistent vortices which last for seconds/minutes?
b. What difference does it make if the helo is in ground effect or not when hovering?
c. What are the dangers associated with hovering, taking off and landing helos? (and I suppose hover taxiing, too)
So far I've followed broadly the same avoidance technique as with wake vortex i.e. take off before the point where the helo did and stay above its flightpath, and land long again keeping above the helo's flightpath.
Please be gentle with my lack of understanding of helicopter principles of flight...
Tim
I fly from a busy military airfield with big(gish) helicopters - Pumas & Merlins mostly. ATC often give 'caution rotor downwash' along with a clearance - e.g. on Monday I both took off and landed behind a Merlin using the runway, and before it took off the Merlin hovered mid-runway for about 20 seconds in virtually nil wind.
I'm familiar with the AIC on wake vortex but there doesn't seem to be an equivalent document for rotor downwash. A few questions which I hope you can answer...
a. I assume the rotor downwash travels out sideways after hitting the ground... how far, in still wind? Are there persistent vortices which last for seconds/minutes?
b. What difference does it make if the helo is in ground effect or not when hovering?
c. What are the dangers associated with hovering, taking off and landing helos? (and I suppose hover taxiing, too)
So far I've followed broadly the same avoidance technique as with wake vortex i.e. take off before the point where the helo did and stay above its flightpath, and land long again keeping above the helo's flightpath.
Please be gentle with my lack of understanding of helicopter principles of flight...
Tim
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The rotor wake changes within three distinct speed ranges:
Up to 20 mph, the air moves primarily downwards, most of it descending from the outer edges of the blades, so you get a relatively calm area around the fuselage (in other words, you are in the middle of a ring, like a doughnut - you can see this by hovering over water)
Image 1
At 18-22 mph (on a Bell), the annular ring shortens in the direction of movement to become an ellipse, coinciding with translation. Above 20 mph, the annular ring disappears, and a large amount of separate, small airflows coalesce to provide an area of ill-defined downward airflow
Image 2
Above 35 mph, two distinct rotating vortices are formed from directly behind the machine (they are fully developed about 1 rotor diameter behind the mast, and can be sustained for up to 2500 feet). Each vortex starts from where the annular ring would be in the hover, and is relatively calm in the centre (in fact, the centre-to-centre distance between them is just under the rotor diameter, and slightly displaced from the centre towards the retreating blade). Regard them as large funnels extending rearward and downward, getting bigger as they go, looking rather like those from a fixed wing, but still with a downward flow.
This is what it looks like from the side:
Image 3
Phil
PS. The piccies (if you can see them) are my copyright!
Up to 20 mph, the air moves primarily downwards, most of it descending from the outer edges of the blades, so you get a relatively calm area around the fuselage (in other words, you are in the middle of a ring, like a doughnut - you can see this by hovering over water)
Image 1
At 18-22 mph (on a Bell), the annular ring shortens in the direction of movement to become an ellipse, coinciding with translation. Above 20 mph, the annular ring disappears, and a large amount of separate, small airflows coalesce to provide an area of ill-defined downward airflow
Image 2
Above 35 mph, two distinct rotating vortices are formed from directly behind the machine (they are fully developed about 1 rotor diameter behind the mast, and can be sustained for up to 2500 feet). Each vortex starts from where the annular ring would be in the hover, and is relatively calm in the centre (in fact, the centre-to-centre distance between them is just under the rotor diameter, and slightly displaced from the centre towards the retreating blade). Regard them as large funnels extending rearward and downward, getting bigger as they go, looking rather like those from a fixed wing, but still with a downward flow.
This is what it looks like from the side:
Image 3
Phil
PS. The piccies (if you can see them) are my copyright!
Last edited by Senior Pilot; 14th Jul 2010 at 21:42. Reason: Replace pics with link: too big for PPRuNe
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The rotor downwash from a helicopter can flip a light aircraft on its back, it has happened.
As for how far out does the effect go, if you can find a picture of a helicopter doing winching over the sea then you will see how far out the ripple effect goes, quite a long way.
I'm sure more detailed answers will be along shortly!
EDIT: My post crossed with Paco's.
As for how far out does the effect go, if you can find a picture of a helicopter doing winching over the sea then you will see how far out the ripple effect goes, quite a long way.
I'm sure more detailed answers will be along shortly!
EDIT: My post crossed with Paco's.
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Helicopter downwash certainly has the capability to damage light aircraft and injure or kill the occupants. On the ground you can damage sensitive bits or flip the whole thing over. If you look at the accident history you will see a steady trickle of accidents. There are a lot more cases of damage that do not get recorded as accidents. As suggested, generally, the damaging downwash goes down, but it can be reflected. If the wind is blowing the effects will be taken downwind. If the helicopter moves it leaves most of the disturbance behind it.
I believe that the "official" view in-flight is to regard relatively small helicopters (compared to airliners) as HEAVY for the purposes of separation. The Merlin would be such an example.[You can check that yourself]
In practice though, I suggest the main risk is that on the airfield helicopters will be more likely to cause problems. There are several main reasons:
- Their proximity to light aircraft at or close to the airfield.
- A helicopter requires a lot of power (i.e. downwash) to hover. Hovering or hover-taxying creates much more downwash than when ground taxying (on wheels), for example. Hover power means more downwash (more than it does to fly at a moderate speed for example).
- Downwash is dissipated less quickly in the hover too.
Perhaps the worst situation occurs at or close to take-off or landing for the light aircraft. The helicopter could be close and hovering AND the light aircraft is airborne at low level with little room for error and often not much power margin.
The impact of the helicopter is very significantly affected by the way the helicopter pilot chooses to operate it. Helicopters are very flexible and can change the way they do things more than a fixed wing.
I believe that the "official" view in-flight is to regard relatively small helicopters (compared to airliners) as HEAVY for the purposes of separation. The Merlin would be such an example.[You can check that yourself]
In practice though, I suggest the main risk is that on the airfield helicopters will be more likely to cause problems. There are several main reasons:
- Their proximity to light aircraft at or close to the airfield.
- A helicopter requires a lot of power (i.e. downwash) to hover. Hovering or hover-taxying creates much more downwash than when ground taxying (on wheels), for example. Hover power means more downwash (more than it does to fly at a moderate speed for example).
- Downwash is dissipated less quickly in the hover too.
Perhaps the worst situation occurs at or close to take-off or landing for the light aircraft. The helicopter could be close and hovering AND the light aircraft is airborne at low level with little room for error and often not much power margin.
The impact of the helicopter is very significantly affected by the way the helicopter pilot chooses to operate it. Helicopters are very flexible and can change the way they do things more than a fixed wing.
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There is some comment on this subject in the July AAIB report
Air Accidents Investigation: Piper PA-28-140 Cherokee, G-BRWO
PA28 inverted after landing after a S76
Air Accidents Investigation: Piper PA-28-140 Cherokee, G-BRWO
PA28 inverted after landing after a S76
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Thank you - some very useful replies as expected. paco - those pictures are great, thanks.
To be fair, the ac at my home base generally ground taxy, no doubt for exactly that sort of reason.
I'll go and read the AAIB report...
Tim
To be fair, the ac at my home base generally ground taxy, no doubt for exactly that sort of reason.
I'll go and read the AAIB report...
Tim
tmmoris
When you are flying in a 172 or similar and you are overtaken by large multi engined helicopter. Do not cross behind it because it will have left a wake that will be beyond the control ranges of what you are flying. It will not break your aircraft but it will have a very good try.
When you are flying in a 172 or similar and you are overtaken by large multi engined helicopter. Do not cross behind it because it will have left a wake that will be beyond the control ranges of what you are flying. It will not break your aircraft but it will have a very good try.
Here's another unfortunate accident (S-61 being flown by a friend and former colleague)
http://www.aaib.gov.uk/cms_resources...3%20G-BPJT.pdf
I once encountered the wake of an AS332 while landing in a Warrior. We rolled to about 60 degrees at 50 ft, and I think the two things that saved us were my previous aerobatic training and that the engine didn't 'rich-cut' as I slammed the throttle open!
http://www.aaib.gov.uk/cms_resources...3%20G-BPJT.pdf
I once encountered the wake of an AS332 while landing in a Warrior. We rolled to about 60 degrees at 50 ft, and I think the two things that saved us were my previous aerobatic training and that the engine didn't 'rich-cut' as I slammed the throttle open!
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Tmmorris
Ground taxi requires wheels. If you have them, ground taxi requires less gas. Your local helis ground taxi because they can and it is cheaper.
I have never undertaken ground-taxi as I fly R44 and EC120 which re both skid machines. (Well, to tell the truth I did once, inadvertently, during training when I failed to keep height in an R44 during a taxi and kissed the ground for 10m)
Ground taxi requires wheels. If you have them, ground taxi requires less gas. Your local helis ground taxi because they can and it is cheaper.
I have never undertaken ground-taxi as I fly R44 and EC120 which re both skid machines. (Well, to tell the truth I did once, inadvertently, during training when I failed to keep height in an R44 during a taxi and kissed the ground for 10m)
Last edited by John R81; 16th Jul 2010 at 11:58.
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Ground taxi requires wheels. If you have them, ground taxi requires less gas. Your local helis ground taxi because they can and it is cheaper
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Having said all that, any helicopter pilot with common sense and consideration would be careful not to allow his downwash near a light aircraft (or another helicopter), if ground taxying (or waiting for a minute) would prevent it. 
Unlike one Squirrel pilot I had words with recently, having watched him stupidly hover taxy over the starboard wing of a running Cessna 152 because it was blocking his way in to dispersal! He claimed it to be an ATC problem...
If he had waited one minute, off to one side, the Cessna could have taxied out and gone clear. His downwind, high hover taxy departure from dispersal, probably in the avoid curve, looked no better, mind... 
Unlike one Squirrel pilot I had words with recently, having watched him stupidly hover taxy over the starboard wing of a running Cessna 152 because it was blocking his way in to dispersal! He claimed it to be an ATC problem...
If he had waited one minute, off to one side, the Cessna could have taxied out and gone clear. His downwind, high hover taxy departure from dispersal, probably in the avoid curve, looked no better, mind...
Hover taxiing past light fixed wings?
A quick question to you clever rotary wing chappies:
Three times this year, whilst out and about in something plank-shaped, I've found myself desperately hanging onto aeroplane / controls whilst somebody has decided to hover taxi a light to medium helicopter straight past me sending primary flying controls jumping around all over the place and on one occasion had me seriously concerned I was going to lose the aeroplane.
From my first aeronautics degree umpty years ago, where I did a lot of helicopter aerodynamics, I'm well aware of why this was really stupid on their part and that I'm fairly lucky not to have had a damaged aeroplane.
However, what I don't know is what is taught to helicopter pilots on the subject? Can anybody enlighten me? - better still give me a reference that I can print onto good stiff cardboard and shove somewhere into the next helicopter pilot to do this.
G
N.B. Irritatingly, each time it's been just after I shut down, and whilst the helicopter pilot was taxiing out so my opportunities were not there to "discuss" this with the helicopter pilots in question. And before anybody asks, yes each time they had a clearly visible alternate taxi route well clear of parked fixed wings - I hadn't just been a prat and parked right next to a helicopter.
Three times this year, whilst out and about in something plank-shaped, I've found myself desperately hanging onto aeroplane / controls whilst somebody has decided to hover taxi a light to medium helicopter straight past me sending primary flying controls jumping around all over the place and on one occasion had me seriously concerned I was going to lose the aeroplane.
From my first aeronautics degree umpty years ago, where I did a lot of helicopter aerodynamics, I'm well aware of why this was really stupid on their part and that I'm fairly lucky not to have had a damaged aeroplane.
However, what I don't know is what is taught to helicopter pilots on the subject? Can anybody enlighten me? - better still give me a reference that I can print onto good stiff cardboard and shove somewhere into the next helicopter pilot to do this.
G
N.B. Irritatingly, each time it's been just after I shut down, and whilst the helicopter pilot was taxiing out so my opportunities were not there to "discuss" this with the helicopter pilots in question. And before anybody asks, yes each time they had a clearly visible alternate taxi route well clear of parked fixed wings - I hadn't just been a prat and parked right next to a helicopter.
Hovering AND talking
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A quick question to you clever rotary wing chappies
I suspect it's down to the conscientiousness of the instructor when teaching airmanship and also some knowledge on the part of the instructor who may not, themselves, be aware of the problem.
Some of us have never flown in, let alone been at the controls, of a light fixed-wing aircraft and therefore may not be appreciate the difficulties concerned.
I was taught to avoid the planks and, with respect to the two main places from which I fly, we're kept a goodly distance apart anyway. Not everyone may be taught that as not everyone may be learning somewhere where the problem arises.
Maybe have a kindly chat with the local helicopter school if the aircraft came from that school.
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Whirls
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Sometimes the airfields don't help. I've lost count of the times I've been taxiing in for fuel to find light F/W parked near the area I have to go to. A quick check with ATC seems to get a response of "It'll be alright" which I know it may not be. At my home base at the moment airfield management have seen fit to park a double row of light F/W right next to the parking area of a load of medium sized rotary, To see them bouncing on their tie-downs as a Dauphin hovers past worries me sick.
I own a light F/W and would be bloody annoyed if it were to be damaged due to the actions of airfield management and/or a careless heli-pilot. Certainly I was taught during my training to give great respect and avoidance to anything I might move or damage with my down-wash. However, looking at the standard of training applied to some PPL holders at the moment I wonder what they are taught apart from hovering and GPS programming.
I own a light F/W and would be bloody annoyed if it were to be damaged due to the actions of airfield management and/or a careless heli-pilot. Certainly I was taught during my training to give great respect and avoidance to anything I might move or damage with my down-wash. However, looking at the standard of training applied to some PPL holders at the moment I wonder what they are taught apart from hovering and GPS programming.
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Not a lot of consideration taught these days, as most instructors, down under anyway, have never been in anything bigger than a 44. That said it doesn't help to leave your home builts, and super light aircraft, including small helicopters surrounding the only jet pump within flying distance. If you are dumb enough to park there I'm dumb enough to hover over them to get to it.
I have left more than my share of bumps in the grass and marks on the conc, dragging my skids along the ground to limit downwash as much as pos, but if there isn't a clear path to taxi I will get my gas some how... Sorry
I have left more than my share of bumps in the grass and marks on the conc, dragging my skids along the ground to limit downwash as much as pos, but if there isn't a clear path to taxi I will get my gas some how... Sorry
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Rotor downwash can be a serious problem for pilots of light fixed wing aircraft. I've been bounced on the ground in a light twin by an A109 which lifted from the fuel pumps 30 metres away and banked to the right, blasting me with downwash before climbing out and I've been "flicked" sideways on short final when an errant pilot took off in a Twin Squirrel across my path (I took control from my student and executed a Go Around, he had an engine failure and executed a "run-on landing" - or whatever you call it - interesting one that!). Even the lightest of helicopters can catch you out if you're downwind of them within 30 metres or so. There does seem to be a lack of awareness amongst ATCOs/FISOs and the area around the fuel pumps is the most dangerous. I've heard of a microlight pilot sustaining hand/arm injuries whilst holding onto his machine as a heli passed by, and the story of an S61 lifting an Islander as it air-taxied past. When parking the aircraft, I advise my students to put in the control locks at all times, even if the wind is flat calm, just in case a heli passes by.