Hover Height
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Hover Height
When we do a blade track and balance in the hover, we hover at about 50 feet. The idea behind this is to get out of ground effect (?). My question is wouldn't it be safer to hover high enough to recover from an engine failure? What do others do?
On this vein, when hoisting into trees we tend to hover at treetop level. With only 255' of cable, sometimes that is our only option. Would anyone consider it safer to hover higher than this or is treetop level the best?
Matthew.
On this vein, when hoisting into trees we tend to hover at treetop level. With only 255' of cable, sometimes that is our only option. Would anyone consider it safer to hover higher than this or is treetop level the best?
Matthew.
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Heedm,
I believe the equipment you are usings program will be set up for a particular hover height.
I don't think that the Rads and chadwick 8500c and above care too much about 5 feet to 50 feet but I am not too sure.
If you can take a run at the desired height and then another at fifty feet it would be interesting to know if there's any difference in the solution provided. If a track and balance comes up for me soon I will check and let you know. If it's the same keep me at five feet.
Does anyone remember tracking flags, grease pencils on a stick and fishing weights?
On point 2 that's how Columbia did it when I worked there, no other option.
They did have a string of tags in small wood that ended up being around 500 feet though
I believe the equipment you are usings program will be set up for a particular hover height.
I don't think that the Rads and chadwick 8500c and above care too much about 5 feet to 50 feet but I am not too sure.
If you can take a run at the desired height and then another at fifty feet it would be interesting to know if there's any difference in the solution provided. If a track and balance comes up for me soon I will check and let you know. If it's the same keep me at five feet.
Does anyone remember tracking flags, grease pencils on a stick and fishing weights?
On point 2 that's how Columbia did it when I worked there, no other option.
They did have a string of tags in small wood that ended up being around 500 feet though
tgrendl,
I didn't see it myself but some of the maintainers at my first squadron talked about using an inked eraser (rubber to us Aussies) lockwired onto a stick for tail rotor tracking on the UH-1.
They would position themselves on the other side of the tailboom and gingerly feed the stick in, pool cue style, using the stinger as a rest, until they got a touch.
Then they knew the blade with ink on it would be the one that was flying closest to the tail boom.
Heedm, regarding the hover height for hoisting, I guess it's good to be near the treetops for a good hover reference and less chance of fouling the cable by moving around inadvertently.
I didn't see it myself but some of the maintainers at my first squadron talked about using an inked eraser (rubber to us Aussies) lockwired onto a stick for tail rotor tracking on the UH-1.
They would position themselves on the other side of the tailboom and gingerly feed the stick in, pool cue style, using the stinger as a rest, until they got a touch.
Then they knew the blade with ink on it would be the one that was flying closest to the tail boom.
Heedm, regarding the hover height for hoisting, I guess it's good to be near the treetops for a good hover reference and less chance of fouling the cable by moving around inadvertently.
heedm,
Why do you wish to track and balance out of grd effect ? Normally most helis are hovering in grd effect not only for safety but all the other associated reasons.
If you study all maintenance manuals you first start with flat pitch at 100% adjusting pitch links to get a base. Then take the aircraft into a hover to again adjust pitch links. Tabs are normally adjusted for the fwd flight regeime. I can only imagine you are hovering at 50 ft in the assumption that the rotor will react diffently oge. I don't see what difference it would make to the track or balance if you are oge or ige ?? Perhaps an expert such as Nic could help here. Personally I prefer to stay out of the avoid curve as long as possible. Plus in longlining I have not noticed any difference in track and balance at 5 ft or 100 ft.
I shudder to ask if Lu has any ideas here, but as I say from a practical ponit of view there seems no difference
Have a safe one
Why do you wish to track and balance out of grd effect ? Normally most helis are hovering in grd effect not only for safety but all the other associated reasons.
If you study all maintenance manuals you first start with flat pitch at 100% adjusting pitch links to get a base. Then take the aircraft into a hover to again adjust pitch links. Tabs are normally adjusted for the fwd flight regeime. I can only imagine you are hovering at 50 ft in the assumption that the rotor will react diffently oge. I don't see what difference it would make to the track or balance if you are oge or ige ?? Perhaps an expert such as Nic could help here. Personally I prefer to stay out of the avoid curve as long as possible. Plus in longlining I have not noticed any difference in track and balance at 5 ft or 100 ft.
I shudder to ask if Lu has any ideas here, but as I say from a practical ponit of view there seems no difference
Have a safe one
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I'm not convinced that being OGE is all that important, but that's what I've been told. It may be a lesson learned from the past that cleaner air is found higher up. The procedure is passed down but not the why.
If there is a significant difference in the track between OGE and IGE, then we would probably be best tracking OGE...that's where most of our hovering is done.
Matthew.
If there is a significant difference in the track between OGE and IGE, then we would probably be best tracking OGE...that's where most of our hovering is done.
Matthew.
If the amplitude and phase of the vibration were significantly different between OGE and IGE it would be necessary to smooth both conditions. In fact the difference is negligible, and if it isn’t then you have other problems which no amount of pitch link, hub weight, product balance or tab will help. The hover coefficients for the RADS and Chadwick programs for Bell Helicopters are all developed IGE, I can't speak for the Lab.
As for your hoisting question, I vote for the trees. Since we are in the business of risk management, we compare the chance of an engine failure with that of losing good reference. I suspect (hope) the latter would pose the greater risk. Additionally, the shorter hoist distance would also limit the exposure time to an engine failure at a critical moment. Just a thought…….
As for your hoisting question, I vote for the trees. Since we are in the business of risk management, we compare the chance of an engine failure with that of losing good reference. I suspect (hope) the latter would pose the greater risk. Additionally, the shorter hoist distance would also limit the exposure time to an engine failure at a critical moment. Just a thought…….
I'll go out on a bit of a limb here and say that you probably get a more consistent track OGE because of the lack of ground effect pressure changes under the disc - the thing that makes a Squirrel dance around like a bastard over a concrete pad.
You're not that far out on the limb, but it's not significant in most aircraft. Even the 350's IGE instability would not seriously effect coefficients, as the measurements are averaged over time. Besides, the slight cyclic inputs do little to change the amplitude and phase of the vibration, nor the track of each blade in relation to the others, which is essentially what you're measuring.
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We do our T&B as heavy as possible from many attempts and finding that it may show good when Tracked light but be rough heavy, but if you Track it heavy it rarely feels rough when light.
Light singles.
About the hoisting: always close to the trees for best ground/tree reference and minimal time on the line.
Light singles.
About the hoisting: always close to the trees for best ground/tree reference and minimal time on the line.
Food for thought. I've developed the habit when tracking my BK to go to a high hover, since the subconscious desire to hold a really tight hover at low level was inducing sufficient inputs to throw the T & B. Up higher (about 70 - 80 ft) the sight picture changes, and a drift of 6" or so doesn't crave an immediate cyclic input.
We use the Chadwick 8500C+ with optical tracker, and it is certainly sensitive enough to notice these movements on a rigid rotor system. But we also get ips down below 0.07
up to 140kts. Conversely, the A109 with its SAS is so stable that a low hover isn't a problem, whilst the good old 206B is somewhere in between.
As to winching, the lower the better - less distance for the poor old sod on the winch to fall (it's happened before, and it'll happen again), and less time on task trying to get the cable to the survivor. Just keep the tailwheel out of the water
We use the Chadwick 8500C+ with optical tracker, and it is certainly sensitive enough to notice these movements on a rigid rotor system. But we also get ips down below 0.07
up to 140kts. Conversely, the A109 with its SAS is so stable that a low hover isn't a problem, whilst the good old 206B is somewhere in between.As to winching, the lower the better - less distance for the poor old sod on the winch to fall (it's happened before, and it'll happen again), and less time on task trying to get the cable to the survivor. Just keep the tailwheel out of the water
John, you are correct that the aircraft seems unstable to the pilot, so we carry that perception in our thinking. The rotor however, is quite happy and the small inputs by the pilot are relatively transparent to your measuring equipment.
Let's look at how a typical system works, and I’ll use RADS as an example because it’s the system I know best.
There are two basic components which provide data input…..an optical tracker, a mag pick-up, and a series of accelerometers, which together measure amplitude and phase of the vibration. The tracker looks at the relationship between the blades with respect to each other, averaged over 50 revolutions. This relationship does not change significantly with slight control input (unless there is another underlying problem). Furthermore, track carries very little weight in the calculations of hover mode corrections, it’s factored in more heavily on the ground and in flight.
The accelerometers average 1/rev and x/rev (depending on type) over the same 50 revolutions. Unless you were to time and sustain your cyclic inputs perfectly to 1/rev, your moving the controls in a hover will be averaged out by the system. Any disturbance that was recorded would not factor significantly into corrections.
Bottom line…..IGE or OGE will feel different to the pilot at times, but these differences do not find their way into rotor smoothing calculations (unless they are exaggerated). So, do what’s comfortable for you.
On the other point, seems unanimous that low is the way to go for winching.
[ 12 September 2001: Message edited by: CTD ]
Let's look at how a typical system works, and I’ll use RADS as an example because it’s the system I know best.
There are two basic components which provide data input…..an optical tracker, a mag pick-up, and a series of accelerometers, which together measure amplitude and phase of the vibration. The tracker looks at the relationship between the blades with respect to each other, averaged over 50 revolutions. This relationship does not change significantly with slight control input (unless there is another underlying problem). Furthermore, track carries very little weight in the calculations of hover mode corrections, it’s factored in more heavily on the ground and in flight.
The accelerometers average 1/rev and x/rev (depending on type) over the same 50 revolutions. Unless you were to time and sustain your cyclic inputs perfectly to 1/rev, your moving the controls in a hover will be averaged out by the system. Any disturbance that was recorded would not factor significantly into corrections.
Bottom line…..IGE or OGE will feel different to the pilot at times, but these differences do not find their way into rotor smoothing calculations (unless they are exaggerated). So, do what’s comfortable for you.
On the other point, seems unanimous that low is the way to go for winching.
[ 12 September 2001: Message edited by: CTD ]
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To sum up, it seems that most are saying that an IGE track should be fine, and of course is safer. If for some reason this is not possible with our machine/tracking equipment, where would you hover OGE? 50', 200', or how high?
As far as hoisting into the trees, close to treetop is what I prefer, just wanted to see what everyone else thought. If possible I stay a little above the trees since the nearby treetops are so affected by rotorwash that they are unusable reference anyhow. About 20-30' above and there is a bit of altitude to play with for recognition of problem and to accelerate for the overshoot.
For the time hoisting, 30 feet of cable takes 6 seconds up and 6 seconds down, not a substantial increase. The 30' gives time to peel out/cut cable as necessary and to manoeuvre away from the guys on the ground or towards a better place (overshoot?).
Anyone agree with this?
Matthew.
As far as hoisting into the trees, close to treetop is what I prefer, just wanted to see what everyone else thought. If possible I stay a little above the trees since the nearby treetops are so affected by rotorwash that they are unusable reference anyhow. About 20-30' above and there is a bit of altitude to play with for recognition of problem and to accelerate for the overshoot.
For the time hoisting, 30 feet of cable takes 6 seconds up and 6 seconds down, not a substantial increase. The 30' gives time to peel out/cut cable as necessary and to manoeuvre away from the guys on the ground or towards a better place (overshoot?).
Anyone agree with this?
Matthew.
Heedm,
You don't indicate what helicopter you are operating, but I'm assuming a twin, maybe a Bell? If you're going for an OGE hover for tracking, sometimes higher is better, giving you more opportunity to get some forward motion, and maybe Vyse, if you have a donk failure. I tend to around 150' in the BK, but it's just about SE hover capable at tracking weight, anyway (it's better tracked light, not heavy). It also gets me away from the hangar turbulence, if I can't move somewhere else on the field.
Re your over tree winching, another 30' of cable is another 30' of pendulum motion to contend with. If the treetops moving in your downwash is a problem, should you be referencing away from them, maybe further down into the trunk, or move your sight picture further out? Just a thought
You don't indicate what helicopter you are operating, but I'm assuming a twin, maybe a Bell? If you're going for an OGE hover for tracking, sometimes higher is better, giving you more opportunity to get some forward motion, and maybe Vyse, if you have a donk failure. I tend to around 150' in the BK, but it's just about SE hover capable at tracking weight, anyway (it's better tracked light, not heavy). It also gets me away from the hangar turbulence, if I can't move somewhere else on the field.
Re your over tree winching, another 30' of cable is another 30' of pendulum motion to contend with. If the treetops moving in your downwash is a problem, should you be referencing away from them, maybe further down into the trunk, or move your sight picture further out? Just a thought
I'll have to climb out on the limb with Arm out the window: I think the T/B is checked OGE only because it is a more "ideal" condition - i.e. less ground effect interference, less tendency for the pilot to over control, and an increased pitch angle and conning angle. Not to say it is required to be OGE, just that it is technically more ideal. If you want to do it OGE, then it doesn’t matter how high – and getting out of the dead man’s curve is a good idea.
The dead man’s curve is also my reason for hoisting close to the trees in most aircraft – it all depends upon weight. In singles there is no dilemma: I always choose close to the trees to reduce time in dead man’s curve. If able to hover OEI in a twin I will choose a height dependant upon factors like pendulum moments and downwash effects on survivors due foliage stripping, dust, etc. As few twins are OEI OGE capable, I am sitting close to the trees in these machines. Few twins are even capable of fly away from OGE hover either because there is no upward momentum (like when you are taking off), you are concentrating on the winch and seldom react quickly enough to an OEI situation before you get bleed, etc.
Besides, a winch of say 220 ft will place you only 150 ft above 70 ft trees and may be even less above surrounding terrain. The 220 ft winch will take about 5 mins of winching time at 100 ft/min (even more considering hook up at bottom and reduced speed in last 20 or so ft). 5 Minutes during which the chances of flyaway in an operational weight (EMS) B412, B212, BK-117, and even UH-60 is pretty slim from a 150ft hover with a cable out. So give me tree top level usually, but I will come higher if I could make a clear area nearby.
The dead man’s curve is also my reason for hoisting close to the trees in most aircraft – it all depends upon weight. In singles there is no dilemma: I always choose close to the trees to reduce time in dead man’s curve. If able to hover OEI in a twin I will choose a height dependant upon factors like pendulum moments and downwash effects on survivors due foliage stripping, dust, etc. As few twins are OEI OGE capable, I am sitting close to the trees in these machines. Few twins are even capable of fly away from OGE hover either because there is no upward momentum (like when you are taking off), you are concentrating on the winch and seldom react quickly enough to an OEI situation before you get bleed, etc.
Besides, a winch of say 220 ft will place you only 150 ft above 70 ft trees and may be even less above surrounding terrain. The 220 ft winch will take about 5 mins of winching time at 100 ft/min (even more considering hook up at bottom and reduced speed in last 20 or so ft). 5 Minutes during which the chances of flyaway in an operational weight (EMS) B412, B212, BK-117, and even UH-60 is pretty slim from a 150ft hover with a cable out. So give me tree top level usually, but I will come higher if I could make a clear area nearby.
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John, I fly the Boeing Vertol H46. In BC we often are hovering in trees that are 150'+ high. The mass of the helicopter and the skill of the pilots are enough to keep the cable movement down.
I agree that making a 70' hoist into a 220' hoist is making things worse. However, I think that making it into a 100' hoist may just give you better options. BTW, we have a 300 fpm hoist...may change things for some.
Engine failure in any hover, we have no recourse. If we're 150' above the ground with trees between or 180' doesn't make much difference to me.
Matthew.
are enough to keep the cable movement down.I agree that making a 70' hoist into a 220' hoist is making things worse. However, I think that making it into a 100' hoist may just give you better options. BTW, we have a 300 fpm hoist...may change things for some.
Engine failure in any hover, we have no recourse. If we're 150' above the ground with trees between or 180' doesn't make much difference to me.
Matthew.
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From memory the 8500c program for the UH-1H asks for an OGE hover height of 80 feet (could be wrong but I do remember doing them at that height).
I also remember the maint Test flight paperwork asking for the pilot to note any unusual vibes at an OGE hover height of 80 feet and an operating N2 of 6400 (normally 6600) I don't think I ever flew one in this config that didn't have a lot of unusual vibes!!
I also remember the maint Test flight paperwork asking for the pilot to note any unusual vibes at an OGE hover height of 80 feet and an operating N2 of 6400 (normally 6600) I don't think I ever flew one in this config that didn't have a lot of unusual vibes!!
