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T/O Downwind -v- power available ?

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T/O Downwind -v- power available ?

Old 6th Sep 2001, 07:56
  #21 (permalink)  
Nick Lappos
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Don't want to make a big deal out of it, but for tgrendl, heedm and acorn your method of thinking about the power needs is earth-centric, so you picture the power needs as somehow "knowing" what the ground speed is. The aircraft is a beast driven by aerodynamic forces, and its power needs are highest at 0 knots. If we did the maneuver in the dark, and simply decelerated from + 50 to -15 knots while holding a constant angle, the power would be seen to be at a maximum at the 0 knot point, and that power would equal the power of the still air approach we discussed. There is no -1 foot per second descent or such. I ask you to try it on a quiet day with a moderate wind and see. I have tested helicopters to rearward speeds up to about 80 knots in Comanche, and 55 knots in the S-75 and Black Hawk. There is no mystery, guys, but it is a place you don't often go, so I understand your questioning.

Regarding a rule of thumb to need +5% over OGE hover power, acorn, you should keep that rule if it works for you, but the cost in payload is large in most climates and conditions, if not in your helo in UK. In 33 years of professional helo flying, I can count on one hand the times I had that much extra power (read it another way, that much too little payload).

Your UK scenario is a good one where you feel that you do not lose payload until 3000 feet or so, but try that at 3000 feet in an equitorial climate and you will find your rule of thumb has cost you all the reason why you are flying to begin with.

Rather than use power as the way out for downwind approaches, I suggest the barrier clearance issues, and forced landing issues are much more the safety drivers.

When a US Army crew is inserting a SOF team in the black of night, they do not calculate the wind, I assure you, they simply fly carefully, and place their aircraft where the mission dictates. Similarly, when a helo minesweeper is tied to a sled and sweeping precise lanes at night, it goes where it has to, and lets the wind do what it will.

I do not advocate downwind operations blithely, but I do believe, like vfrpilot's instructor in the original post, that they are part of our bag of tricks.
 
Old 6th Sep 2001, 20:52
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EESDLI'm surprised that you're surprised! No offence taken.
I must ask though...you state better to take off down wind using a clear area than take off into wind over something nasty. Um....why not back track the clear area & then take off into wind? or use helipad departure?
I'm not a single engine pilot so...is there such a thing as helipad in singles?
I'm no wise man & certainly open to others views. I've been taught down wind techniques, but everyone who has had an input into my training....Navy, North Sea & Police have emphasised (is that spelt right?) Why you shouldn'tdo down wind approaches.
So yes...to reiterate....I will only do a down wind approach if I absolutely have to. &, as I stated, in 12 years I haven't had to; except for training puposes.
Obviously I'm in a minority group...fine...I also believe in each to their own.
But my OPC's every six months do not include down wind approaches & take offs! I'm fairly sure that if it was anticipated I'd need to do down wind approaches, the CAA would have written it into my Base Checks?
Hand on heart I can see very little reason to do one.
That said there are some amazing facts & figures flying around on this thread & I shall continue to follow it with interest!
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Old 6th Sep 2001, 22:03
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Cool

To all of you, thank you for your answers, I have learnt quite a lot from reading,and re-reading the answers as you people see it, today I have very carefully with a CFI sat at my side, plus two other passengers undertaken what was suggested, to see what the power would be like whilst only hovering down wind, the R44 without hydraulics, ran out of rear cyclic in a 7/8 knt wind at a hover height of around 8/10ft, I was forced to turn into wind to stop the movement of the A/C, the power being pulled was 25", had I gone beyond that I would have been in the area of overpitching, so after reading all that you have put into this thread, and doing this little rather gentle test, I can see that the D/W operation should be treated with great caution, I also see what Roofus is saying, but there must sometimes, be a time when this sort of take of is the only one availble, with the only alternative of staying on the ground. Thank you for sharing your thoughts and experience with us who need to understand more.
My Regards
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Old 6th Sep 2001, 23:50
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Nick,
I was led to believe that there is a slight decrease in power required in a no-wind hover due to a 'cushioning effect' that is lost when a few knots are attained. Could it be that those on this forum who are arguing that the power required is not the same for a downwind take off and a zero-wind take off are referring to this? Assuming there is such an effect, it would seem that the power required to take off from a zero wind speed/positive groundspeed hover would be the same as that for a zero wind speed hover. However, taking off or hovering with a finite, yet small wind speed would in this case take slightly more power than hovering or taking off with zero airspeed.

I realize this explanation is a bit garbled, but perhaps this is at the crux of the disagreement.

P.S. Got your msg. Thanks. Will do..
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Old 7th Sep 2001, 01:23
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Nick, I was not referring to the sterile laboratory that you fly in. I was referring to a real world comparison between actual zero wind and downwind approaches. In both I assumed starting with the same airspeed, and taking the approach to landing, thus to zero groundspeed.

I know that slowing from one airspeed to another does not depend on the wind.

The power needs don't "know" what the groundspeed is, but they are based on decelerating the helicopter and controlling it's descent. To fly the same approach path with a tailwind requires more decelerating. QED

As far as this not being "a place I don't often go", wrong. I don't fly comanches backwards at 80kts but I fly a heavy underpowered machine in the rocky mountains, frequently in the worst weather conditions (SAR). Mountain approaches sometimes are dictated by terrain and weather rather than winds. Considering downwind safety and power requirements is a place I frequently go.

Matthew.

[ 06 September 2001: Message edited by: heedm ]
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Old 7th Sep 2001, 07:04
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Matthew,
Sorry if I tweeked a nerve, all opinions are valid around here, don't take my post the wrong way, please.

Regarding the time to decel the extra few knots, that is all relative. If you do the typical approach that holds constant airspeed until the appearant groundspeed gets to the old proverbial brisk walk, you will atuomatically adjust the downwind approach to start at the right ground speed point, and that will have nil effect on power. Besides all this interesting theory, why not just try it?

Kyrilian,

The cushion is not lost at forward speed, contrary to pilot lore. It has less effect, but only because ground cushion acts to reduce the induced power by about 15%. But since induced power falls off rapidly with airspeed, so that profile power begins to predominate, the power savings shrinks with speed.

The proof of how ground cushion is independent of speed (and how much it depends on induced power) is to recall how a big airplane gains ground effect at high-for-helicopter speeds as it comes within 1 wing span of the ground plane.

[ 07 September 2001: Message edited by: Nick Lappos ]
 
Old 7th Sep 2001, 08:33
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Nick, I'd love to try this, but we have a new baby so I'm on parental leave, and I can't fly the FS2000 206 well enough to do the experiment.

I think maybe we're talking about different things. My position is that two approaches flown identically, but one in zero wind and one downwind will have different power requirements. In practice, downwind approaches are extended and shallowed so the power requirement could be made identical. I believe this may be your position.

I think my argument is one to consider because the ideal approach over flat land would keep the energy state of the helicopter ideal for engine failure. To take that approach and shallow it due to being downwind would put you in a less than ideal energy state during your approach. I agree then that a downwind approach doesn't have any added power demand, if flown appropriately, but that comes with a tradeoff.

I took a mountain flying course with Canadian Helicopters, who teach techniques based on decades of mountain flying. They teach a flat or nearly flat approach (depending on terrain) to keep power requirements as low as possible. The safety is there because the approaches are flown well above terrain, until the short strokes.

As far as taking things personally, I respect your knowledge and look forward to your posts, but in that one I did feel a bit of condescension. I should have responded with wit rather than rapier. Sorry.

Matthew.
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Old 7th Sep 2001, 08:56
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hi guys,

I have gotta go with Nick on this one. While the AC speed relative to the ground is higher when the airspeed hits zero, once you you have gone below translation whether you are into wind or out of wind is irrelevant in regards to the power required. Given a cautious approach profile I will go with a clear obstacle free approach downwind over the alternative 99 times out of 100.

Regarding takeoff's over obstacles... when youre heavy and theres the option of a large clear area downwind of the take-off point, again I will generally go with the downwind option. I figure that if the engine quits or there are any problems its preferable to try and sort these out over a clear area downwind, rather than an obstacle ridden environment into wind. Falling through those trees can really hurt!

The key to all this I guess is good technique and plenty of practice with people who are confident in this environment.

Food for thought maybe?
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Old 7th Sep 2001, 17:14
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Roofus
Thanks for the info.
I'm now off to take my instructor to court for all those dw approaches he made me do at Chetwynd/Shawbury/Ternhill!!
Joking aside, appreciate the differences between mil and civvy.
Civvy pressures that I have come across so far have all been commercially based, funny old thing. Would dearly love to back track the clear area but the extra time spent backtracking (269m I think) would not make the flying financially viable (ie Pleasure Flying)
Roughly, allow 1 minute for hover taxi/turn, 60 trips a day = 1 hour of flight!
The good book wants you to be at 100' over the obstacle on departure, my logic tends me to try and remove the obstacle completely.
Loads of ways to skin a cat, and if your engine is going to fail, it will fail at the most awkward time!!
You are just showing off if you have 2 engines:-)
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Old 7th Sep 2001, 22:38
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Talking

Is it better to do a towering (Helipad? Profile) takeoff to clear obstacles or downwind over open area, Id say downwind as you are you dont need to go inside the HV curve,whereas upwind towering you do.And its far easier to run it on than drop vertically from 150 feet! but id be interested in everyones more experienced views.

Regards
Hover Bover

[ 07 September 2001: Message edited by: hoverbover ]
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Old 8th Sep 2001, 02:07
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Hoverlover,
I think you are right on, a towering takeoff is a prescription for a bruising if an engine has a problem, downwind over reasonably good ground is a much better bet.

Matthew,
For condesention, just spray Rain-ex in my eyes.
 
Old 8th Sep 2001, 06:40
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Thou Shall Not Hover(Takeoff) Downwind or Else The Ground Will Rise Up and Smite Thee! And you can take that to the Bank.
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Old 8th Sep 2001, 07:53
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Gutless,

Bring that to where I work and the only thing you'll take to the bank is your unemployment check. You need to learn where, when and how to do them.

[QUOTE]Thou Shall Not Hover(Takeoff) Downwind or Else The Ground Will Rise Up and Smite Thee! And you can take that to the Bank.
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Old 8th Sep 2001, 11:43
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Now I'm intrigued!
Firstly...are we talking singles or twins?? I'm talking twins! I've only got 200hrs in singles & as such I'm not aufait with the Cat A profiles for singles.
'A helipad take off into wind isn't as safe as a down wind departure' did I understand you guys right??
Helipad approaches & departures are taught to allow you to safely get in & out of areas much smaller in area than the proverbial 'Clear Area'.
'Towering' take offs are not a recognised CAT A profile (at least not in the helo's I fly).
A donk stopping during a Helipad allows for a safe reject....that's what the profile is designed to cater for!
Even in a very fat Squirrel you can reject post donk failure back into your site without too much grief.
Of course it does mean that you have to fly the profile well & accurately! Surely that isn't a problem?
I am presently too stunned at the answer 'commercial pressure' to allow coherent thought. I cannot believe that commercial pressure forces pilots to ignore into wind profiles.
I therefore am pleased to say that I work for a commercially liberated company in an ideal world!
Oh...by the way....there are no Cat A performance profiles, Cat A restricted performance profiles or Cat B performance profiles in the Flight Manuals for the two helo's I presently fly that allow for downwind T/O's or landings.
Obviously flying under an AOC or a PAOC, an aircraft captain has to satisfy himself that his aircraft can meet the relevant performance criteria.
Perhaps that is why this all comes as such a shock to me.

[ 08 September 2001: Message edited by: Roofus ]
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Old 8th Sep 2001, 13:26
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Talking

Roofus,

I agree hat a twin is a whole different ball game, I believe a helipad profile allows you to reject back to the pad if you suffer OEI prior to CDP, am I thinking correctly ? (only being a PPL on singles such things are for the future !) But the same profile in a single if you go OEI (OEI inthat case is ONLY engine inoperative) Just puts you slap bang in the middle of the HV curve and only allows you to crash on the pad from a great height, rather than the downwind TO that keeps you within it.

Regards

Hover Bover
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Old 8th Sep 2001, 18:26
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Roofus,
You point out the extreme difference between airline type Cat A operations and utility operations. Cat A is a regimented method of assuring single engine getaway for every millisecond of the journey, a very nice thing. It is achieved at the expense of the flexibility of doing what the machine is capable of doing physically.

There are towering Cat A takeoffs (which involve whistling down Cat A rejects!).

Certainly, any points we were making in these posts were not to condone inventing Cat A ops. I know of no downwind Cat ops, although some Bells and EU's have you back up while climbing vertically to keep the small heliport in sight during thier heliport Cat A procedures. (oh goodness, that means you fly in rearward flight downwind in the first segment of the takeoff, I wish I'd remembered that while we were discussing this earlier!)

The issue is not single or twins, otherwise the advantage of allowing single engine fly out would become a liability if you always had to fly Cat A. Sometimes the thicker fellows in the FAA forget that. There will be times that the tiny risk of engine failure is acceptable when balanced against the ability to do the job. BTW, tiny is correct, data from the North Sea tells us that engine failure during the first seconds of a rig (or towering) takeoff is likely to occur about once each 100,000,000 takeoffs.

In commercial operations under Cat A operations (as dictated in the Company's Ops Manual), you must follow published procedures, upwind or downwind as they are written, of course.

[ 08 September 2001: Message edited by: Nick Lappos ]
 
Old 8th Sep 2001, 20:54
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Nick,

To declare my hand, I was an unquestioning fan but now I am concerned.

The downwind scene is something I am forced to live with in aeroplanes because the poor bluddy things need runways and lots of factors cause me to take an undesirable solution.

In helicopters, however, there are generally two distinct scenes: one - military and two - commercial.

In my military role, I may well be forced to operate downwind due to tactical imperatives. History has forced miltary machines to be specified with the specific problem in mind. In my commercial role, I can usually avoid it because it does have significant problems. The machinery is rarely specified to cope.

In any event, it should not be underestimated and the risks should not be diminished.

I am disappointed that the posts thus far are in my view a touch cavalier. If I had my bum strapped to a Commanche, I certainly wouldn't be overly concerned about tail rotor power and critical azimuths or downwind autorotations or a sh*tload of other things. If I happen to be in a Robbo or a H269 or similar, then I sure as hell am not going to be so comfortable.

For my sins, I have swapped ends with pedals on the stops and have shuddered to a halt with severe pylon rock and little lateral control and I have investigated three vortex ring/settling with power accidents that have resulted from downwind landings. I have also put my skids through the trees on marginal power take-offs when the wind stopped being my friend.

Roofus - you get my vote.

As for performance margins, I am again a little disappointed. Nick, forgive me for being cynical but I am old and no longer bold - your job is to produce performance figures that sell helicopters and how much your experience/conscience interferes with that commercial imperative is not known to me. I am not for one second suggesting that you lack any desirable personal attributes - my emphasis was only on the unknown - but there are any number of junior helo drivers out there who may read this thread and feel supremely confident that downwind operations are OK and no big deal.

It is a fact of life that they will be operating machines of very limited capability in performance and control power. I just hope that they do not get confused between what you can do and what they (and I) cannot!!
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Old 9th Sep 2001, 00:05
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Well said, 4dogs,
The limits of each machine are very important and must be part of the equation. I don't condone cavalier downwind operations, and I think the whole thread so far has been about "What are the issues to deal with"

The one place we have concentrated on is the detail the possibility of more power being needed for downwind operations, several posters say yes, I say no.

We all say to perform downwind operations judiciously, and if one operates outside of his machine's limits, he is foolish.

To believe that a Robinson can be handled like a Black Hawk is suicidal. To believe that a Black Hawk must be limited to a Robinson's capabilities is as wrong, but at least it is a safe belief, if not a productive one.
 
Old 9th Sep 2001, 00:08
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hoverbover thanks for pointing out differences in helipad singles v twins. Agreed helipads in singles are out!
But without Helipad profiles how do you guys get into the more restrictive landing sites?

Nick Are we calling Helipad profiles Towering? Towering to me means vertical. Helipad profiles are not vertical.

IAW the ANO any aircraft flying whilst carry pax must conform to Performance Profiles as laid down in the Flight Manual.

Yep...they restrict you. Yep...they ensure single engine climb...um...that's exactly the point! That is why they are there!

To say not to use the published profiles to allow you flexibility, is shocking! (fingers crossed I misread that!)

Professional pilots have a responsibility to those they carry. As such the published profiles must be used...thus giving the greatest chance of a safe conclusion to any 'mishap'.

To use commercial pressures as justification for going against the aircraft Flight Manual is equally shocking!

I stand by my earlier statements.
If you're considering downwind approaches or T/Os ask WHY???
Look for an alternative....or use the statement 'I can't get in there'

So...I also stick by my earlier statement.... I fly Police & will only consider a downwind approach & T/O if it is Life or Death & I have no other alternative.

Yes helicopters can take off & land downwind, yes instructors teach you these techniques, NO they shouldn't be taken lightly.
I'm not saying they can't be done...they can But not as safely as into wind & in some cases not legally.

I'd merely like to think that upcoming helo pilots would use the same caution when considering it as I was taught to.

I've only been flying 12yrs, a tiny amount compared to some, but in that time I've yet to have to do a downwind departure or approach.

Play nicely & Fly Safely

Edited 'cos I just noticed I've past 100 posts Is that good or bad?

[ 08 September 2001: Message edited by: Roofus ]
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Old 9th Sep 2001, 01:13
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Nick,

The one place we have concentrated on is the detail the possibility of more power being needed for downwind operations, several
posters say yes, I say no.

How about a downwind approach at or steeper than the FAA shallow approach? Seems to me that you will reach zero airspeed OGE or at least at the very limits of IGE. The critical part of this approach is to replace the loss of ETL with engine power before a higher than normal rate of decent is established. It takes a LOT more OGE than IGE.
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