Hovering Downwind

[ravenx]

This may have been discussed before but here goes. Why does it take more power to hover downwind than into the wind.

With my feeble brain it would seem that the blades are rotating into the wind at the same rate wherever they are in the disc. Ok there will be differences in terms of any flapping, leading and lagging - but would this really make a difference.

My only other thought is that it might be something to do with the tail rotor being in the way and causing some disturbance.

So - what's the real answer - no doubt it will be really simple :-)

[rotornut]

Well, let me hazard a less than perfect opinion.

Could it be that when you are hovering downwind, you are using more left pedal (or right if it's French) to hold the machine steady? I seem to recall (it's been a few years) that the chopper always wanted to swing around into the wind and you had to fight to keep it facing downwind.

[NickLappos]

ravenx,

It generally does not take more power to hover downwind than up wind. What data do you have otherwise?

And it always takes less power to hover in wind from any direction than to hover in zero wind.

[TeeS]

Come on Nick, while I am happy to accept that you (and maybe half a dozen others of your calibre on the forum) can hold a downwind hover without a bit of overcontrolling on the pedals, I am sure that I and most others happily throw a few available horsepower away:ok:

Seasons Greetings

TeeS

[NickLappos]

TeeS,

The tail rotor does eat more power in some conditions, (not downwind, actually!) but the effect is quite small, and much less than the power you "gain" from having the wind. Dancing on pedals will eat some power, but it is tiny.

I am guilty of provoking another of those fun threads where we carefully puncture some "urban myths" about helos.

Hovering down, cross up, or side wind eats no more power except for some slight increase where the tail thrust is higher, which is not downwind, it is generally right side flight in conventional helos, left in french/Russian. And even so, the tail thrust required to fight the fuselage drag at wind below about 10 knots is peanuts. Pedal position is not thrust, most of the pedal shift in side flight is to overcome the inflow change, where the actual tail rotor thrust is a constant but the angle needed to make that thrust is increased when the rotor is traveling in the direction of the wind.

In fact, with virtually any wind direction, you hover better than with no wind.

[diethelm]

It would seem that eventually, as the wind speed increases, you will not be able to hover downwind when you could hover upwind.

Although at that windspeed, you may want to be doing something different.

[Vfrpilotpb]

When being taught I was taken out one roughish day to sample flight in high wind conditions, Wind was westerly and steady at 28/30 knts, the R22 with two up hovered directly into wind with 15" showing on the gauges it almost felt like god was truly embracing the Heli, so like all good CFi's I was then told to swing round and hover with the wind behind us, this I tried several times and found that the R22 was not able to hold that hover and actually was starting to run out of rear stick,

Left and right was no problem, but rear wind at that wind speed was a NO NO. I spent nearly 1.5 hours just seeing what the wind did to us on that day, some may think that boring, but I learnt a lot from that windy day!

Hope that helps you, RavenX

Peter R-B

Vfrpilotpb

[Robbo Jock]

Nick,
I would imagine that in the helos you fly, the difference in power between into-wind and downwind hovering is negligable, but in the R22 (and, I assume, other light pistons) there is quite a marked increase in power required when your bum's to the breeze.

RavenX,
I also would assume that the disk itself is unaffected by wind direction. I've always put the difference in power down to the fact that if the wind's from the front, it is gently parted and ushered around the fuselage, therefore the disk needs to generate less forwardly-force and so can remain more horizontal hence generating more upwardly-force. With the wind from behind it catches in all the dirty rough bits of the fuselage and fights its way around, thus trying to push the whole shebang with it. So you need more rear cyclic to hold position and hence more power is required to fight gravity.
Obviously, the old soft shoe shuffle on the pedals will also add to the power required.

[diethelm]

If Igor is Russian, and his helicopters are backwards, does this make him dyslexic?

[flyer43]

I think we need to expand on Nick's comment about there being little difference in power required to hover facing any direction with winds up to 10 knots.
If you wish to remain in a stable crosswind hover, as the windspeed increases, so does the profile drag associated with the large slab sided section of the helicopter. This requires more and more lateral cyclic which, in turn requires an increase in applied collective to maintain the required vertical component to maintain the hover height.

[jellycopter]

RavenX

You often require less power to hover downwind than into-wind. It depends upon the windspeed. Here are a couple of reasons why:

1. When into-wind the main rotor tip vortices (and downwash) will be blown rearwards and can interfere with the tail-rotor. This 'dirty' air can decrease the efficiency of the T/R.

2. The downwash of the rotor system pushes down on the fuselage and creates an apparent increase in weight. When hovering into wind, more of the aircraft structure is subject to downwash than when hovering downwind.

Finally, don't hover exactly downwind but allow a slight x-wind component to try to weather-cock the aircraft back into wind. Assuming you choose the correct side, you'll need a load of non-power pedal to hold the heading and consequently use less power.

Robbo Jock.

Having flown the R22 a bit, I wouldn't dream of trying to deduce what goes on with its tiddly tail rotor; I just know it doesn't behave as predictably as bigger, 'proper' helicopters when subjected to a x-wind. But that said, I know that it takes no more power to hover down-wind than it does into-wind. Now if we're talking into-wind approach versus downwind approach; that's a whole different story..........

J

[Robbo Jock]

J,

I'm afraid I disagree. As RavenX and VFR have also said, I have noticed a definite increase in power required for an out-of-wind hover compared to an into-wind one. And I'll repeat I think it's due to the profile drag of the gubbins behind me causing me to have to use more aft cyclic to stay in one place, hence having to pull more power to keep the lift component.

Into wind and downwind approaches require different amounts of power because at some point in the downwind approach you pass through zero airspeed, giving zero Translational Lift, so you need huge great gollops of heave to replace it. Through that, you've then got the wind behind you and yes, you may have Translational Lift again, but the wind's pushing you forward faster than you're comfortable with, so you need more aft stick, etc, etc, until you're in a downwind hover. Look at the power being used, spot turn and check again. In reasonable winds it'll be half an inch or more lower.

[john Armstrong]

Don't get too technical, think about where your downwash is, OK!

[GLSNightPilot]

My experience isn't quite the same as Nick's. I've done quite a few takeoffs from supertankers, where the landing area (I won't call it a helipad, because it really isn't) is very confined, and allows only one way in and out. Basically you have to land perpendicular to the ship, facing toward the center. Usually this puts the wind on your left side, with luck it's a quartering headwind, often a direct crosswind or sometimes even a quartering tailwind. With a 412, it often takes well over 90% torque to come up to a hover with a strong crosswind, even with a very light load; you can feel the ship fighting the crosswind. After coming up to a high enough hover to clear the tail from all the obstacles around, we usually turn into the wind. Almost instantly as the nose turns into the wind, the helicopter literally jumps from a bare hover to a very rapid climb, and I've seen it go from no climb at all to > 1000 ft/min just from turning into the wind, with no increase in collective at all. Normally I decrease collective by several inches with the turn to prevent such a rapid climb in the dark. The S76 doesn't like crosswinds either, but it's not as dramatic as the 412. I find it always requires more power to hover with a direct crosswind, and the higher the wind the greater the difference between a crosswind and a headwind. I'm not sure I can explain the physics, but I certainly see the effect.

[NickLappos]

I stand by my assertion, which has little to do with handling.

Who will post the manifold pressure, wind speed/azimuth and headings for a steady hover in any helicopter, to prove the assertion that the rotor knows which direction the wind is from?

Who believes the drag of the fuselage moving sideways or backwards at 6 mph is enough to actually measure by any means they have at their disposal?

The problem is that these myths exist only as we are taught them, and they die hard against the cold light of data.

Except for rare occasions where the tail rotor wake upsets the main rotor flow, or where the main rotor wake vortex rolls up into the rotor tip, a single rotor helicopter in light to moderate wind has no difference in power required for a hover, regardless of the wind direction.

Furthermore, zero wind is the worst for power required, and any wind from any direction is better for performance. In other words, a helicopter hovering downwind is better performer than one hovering in zero wind.

[bellfest]

Someone could rewrite the aerodynamics of helicopters from this thread alone. Specific wind conditions can either help or hinder power. It only needs to vary a few degrees or knots to change from an aid to the rotor efficiency to an airframe drag or vice versa. Tailwind or crosswind, but never headwind (Unless you're in a 47 & it's gusting 55kts). I'd say 9/10 of the time you want a bit of extra torque to turn downwind above 8kts(Excluding mediums and above). If there was no effect at all helicopters would fly as fast backwards as they do forwards. I've seen a lot of people try but they don't!

[Gomer Pylot]

If it makes no difference where the wind is from, why is there a limit in the RFM for hovering with crosswinds and tailwinds?

[The Nr Fairy]

Cyclic authority ?

[Thomas coupling]

Make sure you are discussing either:

1. Aerodynamic effects

Where power demands about the main rotor do not change either down or into wind. [Tail rotor effects are negligible].
To appreciate the scenario more, consider a helo with the tail cone and tail rotor removed, simply a rotor disc rotating with a lump of metal dangling underneath. The rotor doesn't know (nor cares) where the wind comes from, it's always the same result.

or,

2. Airframe effects.

Profile drag or parasite power etc. Is the entire slab side of the helo impacting the wind direction, or is it head on. Is the wind from astern impacting on the top surface of the stabilisers pushing it down? Is the astern wind pushing the downwash ahead of and away from the helo??

1: wind direction no effect.
2: wind direction major effect.

[jellycopter]

I'm with TC on this one.

However, I don't think a tailwind of, let's say, 30 kts impinging obliquely on a horizontal stabiliser will have anywhere near the same 'negative' effect as the downwash does.

Another misconception. The majority of us are taught that the rotor produces a 'mythical' column of downwash. This is of course rubbish. The downwash is more toroidal (doughnut shaped) in profile on a conventional (non BERP-type rotor). ie, there's no downwash at the hub or for a few feet outboard from it and there's minimal downwash right at the tip. The greatest downwash is just inboard from the tip which, depending upon the degree of blade washout, decreases slightly towards the hub and then drops off rapidly as it reaches the stalled area by the hub.

C'mon.......... Shoot me down in flames!:)

J

[boomerangben]

Here goes - the neck is being stuck out here, especially as I dare to disagree with the great Nick Lappos.

As a helicopter does a spot turn, the rotor turns at the same(ish) rpm and the air flow remains largely unchanged, in fact the only thing that changes is a parasite drag of the airframe. Now the torque required to maintain the hover depends on the weight of the aircraft plus the requirement to over come the drag on the airframe. This is achieved by tilting the rotor disc and applying a little more collective to counter loss of vertical component. So as the drag varies as the aircraft turns, then so will the torque. So when cross wind (huge airframe drag), more power is needed (regardless of the power required by the tail rotor). Unless the drag characteristics for fore to aft air flow is the same for aft to fore airflow, then will there not be a differing power requirement when hovering downwind?

As for downwash on the airframe - I can see the argument for having greater downward force on the airframe by virtue of a greater area, but isn't that countered by a greater area for the ground cushion pressure to act on?

Edit: Just reread Nick's last post and realised that he is talking of light winds and therefore I would have to agree that there would be little noticeable difference. However, as all Robbo jocks and no doubt drivers of many other small types have realised, more power is needed in strong winds when downwind.

[ravenx]

Ok. So naively I thought I would get about 2 responses, both agreeing and both of which explained the answer. Unfortunately I've got god knows how many and I'm still none the wiser. No wonder the examiner kind of ignored the question when I asked him. :O

Nick - not sure what you fly but I can say that I have been in an R22 with an instructor and had it demonstrated and done it myself. You sit fat dumb and happy in an into wind hover with X inches of MP required. Do a spot turn 180 degrees and hold the hover with X + at least 1" sometimes 2". I know there are a lot of myths surrounding all sorts of aviation but this does seem to have been proven to many.

I don't understand the downwash argument. Surely there is downwash all around A/C anyway - why does the wind directionn affect it. The only difference would appear to be the tail boom position relative to the tilted disc - and since the amount of tilt is hardly dramatic I'm not sure I understand why this has an affect.

The only argument that makes any sense to me at all is that the airflow over the main rotor when you are downwind is disturbed by the tail rotor. not saying it's right - just the only one I can imagine.

[muffin]

I am glad somebody asked this question again, as last time it was discussed there was never a very satisfactory explanation. I looked it up in Shawn's book but could not find the answer either.

I agree with RavenX, this definitely can be demonstrated. Yesterday afternoon I was hovering in an R22 with a 15-18 kt wind. I had one medium weight passenger and about half tanks. A stable hover into wind needed about 22 ins and with the wind behind me 24 ins. I repeated the exercise several times and the figs were repeatable. So it does occur. Why?

[boomerangben]

Muffin,

My answer would be that downwind a Robbie fuselage has barn door aerodynamics compared with into wind. Therefore you need a greater horizontal component of total rotor thrust to maintain position downwind than into wind. If you consider the still wind cyclic position as a datum, when hovering down wind, the cyclic would be further from the datum than when hovering into wind. In other words, the disc has to be tilted further towards the wind downwind to counter the greater drag on the airframe. Greater tilt means a decrease of the vertical component of rotor thrust that has to be countered with more collective.

I think that makes sense!

[Johe02]

Good comeback Boomerang. . .

I think you have it in a nutshell :ok:

[The Rotordog]

jellycopter:However, I don't think a tailwind of, let's say, 30 kts impinging obliquely on a horizontal stabiliser will have anywhere near the same 'negative' effect as the downwash does.You've obviously never flown a 206. I can assure you that if you were to turn your tail into a 30 knot wind, you'd find yourself with the nose down and the cyclic in your gut and you going "Yeeehaw!"

We all know that helicopters have strong weathervaning tendencies. Just as raindrops do not fall pointy-end-first, helicopters don't like to fly backwards. It takes more power to make them do so. Even if they are standing still over the ground.

[Robbo Jock]

Thanks Boomerangben, exactly what I was trying to say but somewhat more succinctly and scientifically put.

[Droopystop]

Nick


Furthermore, zero wind is the worst for power required, and any wind from any direction is better for performance. In other words, a helicopter hovering downwind is better performer than one hovering in zero wind.


How then does the S61N needs more power to hover IGE at 5kt (factored) headwind than in still air? Or have I misinterpreted the graphs?

[jellycopter]

Rotordog,

I'm with you when you refer to aft cyclic limits when hovering downwind with teetering rotors. Been there, done that, and even made the video, literally!

However, I think you misinterpret what TC was getting at when he referred to a tailwind on the stabiliser. If, as in a 206, the wind gets under the stab it will actually generate lift, albeit small, and this will reduce the overall power required to hover with a tailwind. I think what TC was talking about (correct me if I'm wrong TC) was when a tailwind acts on the top surface of a stab and thus produces 'negative' lift thereby increasing the power required to hover.

Not being qualified on the 206, just wondered what the out-of-wind hover limits are? 30 knots downwind seems a bit high for an old tech teetering rotor system.

Droopy,

Would the strake on the tailboom of the sea-king cause the effect in the graphs you describe? ie. In still wind the strake is acting to full effect having max downwash. Whereas in a light wind, less of the downwash will be acting on the strake and therefore the T/R has more work to do. Just my thoughts.

J

[NickLappos]

droopystop asked (good question!):
How then does the S61N needs more power to hover IGE at 5kt (factored) headwind than in still air? Or have I misinterpreted the graphs?

Nick sez:
The power required to hover is determined in flight test and we accept all data below 5 knots, so we really don't plot or measure the data below that. Thus the IGE power required charts are flat (I don't believe they show more power needed at 5 knots, BTW) up until 5 knots. The effect of the ground vortex roll-up strikes between 5 to 8 knots (depends on disk loading) and increases power a bit, which is why the helo dips a bit while accelerating during an iGE takeoff.

Here is a way to prove that the aircraft uses no more power in rearward flight at say 10 knots:

In still air, accelerate forward very gently and increase collective to keep power constant (the torque or MP will drop if you don't).

Now do the same thing but gently accelerate rearward, and note the same behavior. If you do not reduce the collective, you will take off rearward, because the power is less than the steady hover.

Many folks posted about the "barn door" and "high drag" of the fuselage as the reason why the power goes up in rearward flight. This really is not a big factor, it is virtually zero drag at 5 knots or even 10 knots, especially rearward. The drag of a square foot at 10 mph is about 3 lbs (hold a card out your car window to verify!) If the Robbie has a total flat plate area of 20 square feet (like a big triangle of 4 foot by 10 foot) it has a sideward speed drag at 10 mph of about 60 lbs. This is achieved by tilting the thrust about 2 degrees. The lift lost in this 2 degrees of lateral tilt is .06% of the thrust (cosine of 2 degrees is .9994). Peanuts.

For those who comment on the control issue with some helos, that is correct for some helos at some speeds, but all helos are approved for 17 knots of flight in any direction at any CG before they run within 10% of the control stops. Those who say they run out of cyclic at 10 knots rearward are not corrrect, but it feels that way because the stick is pretty far back in their stomachs.

[diethelm]

I assume that is the minimum regulatory control requirement at any direction, any CG at 17 knots of airspeed?

[[email protected]]

Jellycopter - most helicopter horizontal stabilisers are upside down wings designed to produce a downward force to keep the fuselage attitude more level in forward flight - the direction the air is coming from eg from the front or the back is immaterial and a tailwind would not produce more downforce than a headwind, especially at the low airspeeds we are talking about in the hover.

The strake on the Sea King is designed to disrupt lift on the port side of the tail boom and only affects pedal position and thus hover power in a crosswind - the old girl used to run out of left pedal in a right crosswind because the tail boom was creating lift horizontally to the left and more left pedal was needed to counteract it.

I am particularly amused that so many pilots with a fraction of the knowledge and experience of Nick Lappos insist on rubbishing his explanations just because they go against what they were taught at flight school. Just because your instructor knew how to teach you to hover doesn't qualify him/her as a TP - they are regurgitating the same lessons and opinions that were taught to them - Nick is trying to highlight that knowledge of helicopter aerodynamics has moved on and the old wives tales from the past are mainly fallacies.

[heights good]

If i remember my P of F its to do with retreating and advancing blades, max flap up/down etc. These all happen at specific points on the rotation of a blade. If wind is blowing from the wrong direction i.e. anywhere but the front, where helicopters are designed to accept wind from then it puts things out of position from the norm.

Not very scientific but its all i can remeber from the top of my head.

Next thing, tail rotors (when using the pedals) do use up quite a lot of power regardless of the aircraft. The deciding factor is the wind speed and direction, just the same as the main rotor problems.

Sorry but thats all i can offer just now because my notes are at work and when i get to work i will research it and give a sciency type answer :)

[Hilico]

Must admit, Crab, I got a bit nervous about that myself; except that a couple of posters are reporting the numbers they're actually seeing. People who have got beyond Ex. 8 are trying it and finding that the situation isn't as symmetrical as I, for one, might expect.

[Rotorheadmark]

This is my first time replying to any of these questions, so try not to be too hard on me lads ok!!

I was always led to believe that hovering downwind will increase the chance of getting into a settling with power situation. Pulling more power to hold the hover, more induced drag, bigger hole!!

I'm sure lots of you have tons of hours so I would love to hear your thoughts!:ok:

[Droopystop]

Nick,

Thanks for your reply.

My question was prompted by the fact that the Max AUW graphs for hovering on single/twin engine IGE show a decrease over the first 5 knots of factored wind, recovering to the zero wind values at around 10kts. Hence I assumed that this means an increase in power required for a 5kt factored (10kt actual) wind speed. I am a little puzzled over your explaination though. Can you explain what you mean by the "ground vortex roll-up". Is this aeronautics for ground cushion?

DS

[jellycopter]

Crab,

Thank you for your reply to my post. Here's a few of my thoughts which may clarify one or two things for you:

Jellycopter - most helicopter horizontal stabilisers are upside down wings designed to produce a downward force to keep the fuselage attitude more level in forward flight - the direction the air is coming from eg from the front or the back is immaterial and a tailwind would not produce more downforce than a headwind, especially at the low airspeeds we are talking about in the hover.

You're absolutely right about the upside down wing bit, but when you state "the direction the air is coming from eg from the front or the back is immaterial" you are sir, quite wrong! It's the angle of attack of the aerofoil which dictates how much lift (or down force) it produces. You'll note that many fixed geometry horizontal stabs are rigged at a negative incidence angle ('Rigger's Angle') to assist in their main role....to keep the fuselage at the optimum attitude for cruise flight (not necessarily level by the way). These stabilisers can be cambered (eg. Bell 206) to improve their effeciency and stalling characteristics at the generally low airspeeds and diverse alphas they operate at. Such a Riggers Angle can, and often does, produce the opposite effect to what is normally required when subjected to a tail wind. Hence the B206 et al running out of aft cyclic with moderate tail-winds due to undemanded nose-down pitching moments generated by the horizontal 'de-stabiliser' (?). When the stab lifts the tail up, it must be producing lift; ergo, it must reduce the hover power, albeit small and arguably immeasurable.

The strake on the Sea King is designed to disrupt lift on the port side of the tail boom and only affects pedal position and thus hover power in a crosswind - the old girl used to run out of left pedal in a right crosswind because the tail boom was creating lift horizontally to the left and more left pedal was needed to counteract it.

Not being qualified on the SK, I must accept what you say regarding the purpose of the strake. Nonetheless, does the strake not also have a secondary effect of generally reducing hover power in the same way it does on the Squirrel B2/B3?

I am particularly amused that so many pilots with a fraction of the knowledge and experience of Nick Lappos insist on rubbishing his explanations just because they go against what they were taught at flight school. Just because your instructor knew how to teach you to hover doesn't qualify him/her as a TP - they are regurgitating the same lessons and opinions that were taught to them - Nick is trying to highlight that knowledge of helicopter aerodynamics has moved on and the old wives tales from the past are mainly fallacies.

Crab. You and I were both taught at Shawbury, and we both probably taught there too (Don't know your identity so don't know for sure). Although the P of F served it's purpose, you and I both know it left many gaping holes. Notwithstanding this, I find your 'amusement' at posts disagreeing with Nick the 'Test Pilot' to be interesting. I have the greatest respect for Nick, and for many of his ilk and Rotorheads would be a much poorer place without his (and indeed your own) contributions. Neverthless, he's human and even TPs have been known to make mistakes once in a while. (Not that I have disagreed with anything he's posted on this thread by the way!) To trust a TP implicitly would be like beleiving the Eurocopter TPs when they insist the Gazelle can't get Fenestron Stall/LTE/Yaw Divergence when, after bitter experience, many of us know it can. Therefore the Robbo jocks who see 2" MP rise when they hover downwind aren't necessarily making it up just because Nick reckons it shouldn't be so. When instructing on the R22, I've seen the same effects myself, but on an equal number of occasions, I also saw the MAP drop. Why...... That's the whole point of this thread. I've just highlighted some potential contributary factors.

J

Edited for p@~s poor spellign

[NickLappos]

droopystop,

The ground vortex roll-up is the effect we see during a normal IGE takeoff, when the helo dips down at about 10 knots, and some wiggling and cyclic shft occurs. This dip is caused by the aircraft catching up with the rotor downwash (which is spreading away from the dsk at about 10 knots or so). The rotor tip in front of the helo starts to see some reingestion which affects the cyclic trim and the power required. The settling down of the helo is caused by this reduction in efficiency. Thus, the power required for 10 knots at low IGE is about the same as that needed for zero knots (this is true in any direction).

Thus, the power required curves for IGE usually show this reflex upward (or at least are shown flat until about 8 knots or so). I am looking on the net for an illustration and will post it when I find it! HERE is a paper, see Fig 2:
http://www.adl.gatech.edu/archives/groundeffect/literature.pdf
and a great Ray Prouty article:
http://safecopter.arc.nasa.gov/Pages/Columns/RayProuty/pdf_files/FLIGHT_IN_TURBULENT_AIR.pdf
Back to the subject of the thread, the power needs for the helo are dominated by the main rotor. The drag of the fuselage is simply insignificant at normal wind speeds (less that 15 knots or so) especially when going rearward, and the tail rotor does not need very much more power at these low speeds.

Remember that the tail rotor is only balancing the main rotor torque, so its power needs are almost purely driven by the MR power. The wind direction changes the pedal positon, but NOT the power the tail rotor eats.

The myth that the wind causes large power changes will not go away easily, but for those brave souls, I do suggest that you just take your helo in still air, raise the nose about 2 degrees, and let it start a takeoff backwards. Hold your speed to something below 15 knots, and note if the aircraft starts a climb and a rearward takeoff. If the "rearward flight is hell" theorists are correct, the helo will sink and you will have to significantly raise the power. Do the same thing forward, of course, to compare the behavior, since some settling at about 10 knots will happen in any direction, and some collective pitch increase is needed (collective is not power, gentlemen!)

The idea that the rotor even knows what direction it is going is wrong, and the further idea that the helo changes its power needs because of wind at 5 or 10 knots from any given angle is also wrong.

There are many wise reasons to hover nose into the wind, but preserving power is not one of them.

That being said, I cannot explain why some have viewed a small MP rise at a few knots rearward flight. I suggest they try the rearward acceleration in still air as a way to check the behavior of their aircraft.

[Gomer Pylot]

Nick, do you have the luxury of never having to hover with a crosswind of more than 10 knots? Or 20 knots? Those of us who fly in the real world certainly don't have that luxury, and the power difference there is real.

[NickLappos]

Gomer,
Please do not misread my comment. I said that hovering up down or sidewind in moderate winds is not a big power difference. This is a discussion of the basics of how the helicopter operates, not how far you can stretch my comment until it makes no sense.

Regardng luxuries, I can assure you I have spent more time at 50 knots sideward flight than you have at 10. Wanna bet?

[Gomer Pylot]

No, no bet, but you have repeatedly qualified the power requirement with 5-10 knots. Perhaps I'm missing something, but I'm wondering what the difference is between light winds and higher winds. I've seen, at least hundreds of times, the difference in power required for takeoff by having a crosswind or being able to turn into the wind, and with a 20-knot wind or higher, the difference is very noticeable, sometimes the difference between being able to take off and not. I certainly don't claim it's the main rotor that makes a difference, but something certainly does.

[NickLappos]

Gomer,

The tail rotor needs no more power to produce its thrust, regardless of the wind direction. In fact, at some cross winds, especially from the left, it needs less power and hover performance might be better than with wind off the nose.

Generally, the "extra" power needed at some azimuths in high wind is to overcome the drag of the fuselage, but the drag of the fuselage in side flight is inconsequential at relatively low speeds (say 10 to 15 knots or less, as I have previously posted). At much higher speeds, the fuselage drag begins to show itself, and power will rise in some azimuths. At 50 knots right side flight, the tail rotor power might double, for example, and at 70 knots of right side flight, the tail rotor might need almost what the main rotor does. At 50 knots left side flight, the tail rotor power is LOWER than in a hover!

Rearward is probably the lowest power point, roughly equivilent to forward, since the drag of the fuselage going backwards at speeds up to perhaps 30 or 40 knots is probably equal to the drag of that same fuselage going forward (the drag is mostly flat plate area, and a backwards helo has about the same cross section as forwards.)

The whole idea of my comments is to help folks recognize what is really happening to their machine, and to use their observations and experience in a positive way to understand the aerodynamics of the helo. I see you are an S-76 driver. Try to trim the 76 at each cardinal heading at the same hover height with a steady wind of 10 or 15 knots, and note the torque. If you want, just translate on a still day and point in different directions. As I have said, the helo never uses as much power in moderate winds from any direction as it does in still air at zero velocity, except for the slight power rise just as the ground vortex rolls into the main rotor - sometimes erroneously called ETL by some instructors.

[pa42]

How interesting, none of us have broached the subject of increased HIGE power required between 2 knots airflow-past-helicopter and about 9 knots airflow-past-helicopter!

Refer, if you will, to page 243 of Shawn's Cyclic and Collective, wherein the generic helicopter experiences an increase-in-hover-power-required as the ship moves slightly off the 'ground cushion' and the airflow gets . . . well, a little more complicated. (p. 58 also describes the effect BUT the illustration there falls short of accuracy, so use p 243).

Power required for a real-slow relative wind (head-, cross- or down-wind) is always greater than that for 0 speed simply because the ground cushion is less effective. Up until ETL (or even minor TL at speeds before it becomes "Effective") replaces the beneficial effect of a 0-speed cushion.

I suggest that indiscriminately mixing discussions of power required in "wind" without specifying HOW MUCH wind only serves to muddy the waters. We need to carefully distinguish between weak and strong winds, as they have different power-required effects depending on where we are on the curves of Shawn's p 243.

Weak winds, <9 knots, may be expected to increase hover power required (by about 8-9% if I can read the graph directly). Stronger winds (any direction) bring in ETL. For the power required in those stronger winds, I defer to the previous umpteen posts and posters on this thread!

[Gomer Pylot]

Well, as I said above, my experience is different. With a 20kt+ wind from the left, a nd a moderate load, sea level, OAT about 70F or so, many times, I've struggled to get to a high hover, with the torque at or above 90%, and then when high enough to clear the TR obstacles, turned 90 degrees left (which was very easy to do!) and as soon as the nose was within 30 degrees or so of the wind, went to an immediate 1000'/min climb, without any increase in power, from what had been a hover with little or no climb. This effect is more noticeable in a 412, but it's also present in the 76 to some extent. The 412 obviously has more fuselage drag when presented sideways to the wind, and that may account for all of what I've seen, but I'm not sure. I don't claim to know exactly why this happens, but it certainly does. I'm seldom in a level, obstacle-free environment when this happens, because flying from a large airport is a rarity. It's usually on a tanker or a drilling rig with the heliport in a bad location.

[[email protected]]

Jellycopter - yes I did instruct at Strawberry on 2 Sqn and CFS, 91-94 if that helps.
You will remember that the Gazelle hovered less nose-up when downwind than into wind, not because the stabilizer was producing upwards lift, but because the downwash from the MR was no longer pushing down on it - I suspect the stabiliser on the 206 has exactly the same problem and that neither aerofoil is capable of producing lift at 5 -15 kts in any direction.

As for the Sea King strake, the anti-clockwise (when viewed from above) swirl of the MR downwash hits the tail boom from the starboard side in a right crosswind and the air travels further, and therefore faster, around the port side creating lift in that direction. Since this right yaw (tail left, nose right) must be corrected with left (power) pedal then it is probably fair to say that the strake reduces power required in the hover only in right crosswinds. (I am not being condescending, just trying to explain for those less knowledgable than yourself)

As to Fenestron stall - it doesn't exist - all the pilots who have 'experienced' it have been used to SAS equipped aircraft and never developed the necessary feel for pedal requirement in the Gaz - try flying an Army one for a while and you realise that full application of right pedal is required sometimes - it feels unnatural but it stops the yaw just like the Aerospatiale TP showed in the video.

Gomer I think you are forgetting the premise of this thread, namely the respective power requirements when hovering intowind and downwind. The crosswind argument is muddying the waters as is the altering of the windspeeds when arguing a point. One moment we are talking about 5 kts then suddenly 20 or 30. Turning into or downwind is a different argument as you need more power to combat the weathercocking tendency of the fuselage.

Droopy - the ground vortex rollup is the reason for the burble you get just before the onset of ETL and also the reason that your maximum power demand when carrying out a level IGE transition is just before you get ETL.

[Droopystop]

Nick

Thanks for your reply, I am happy now. Like the ground vortex rollup explaination - makes more sense that the "falling off the ground cushion" that is what I was taught in my initial flying lessons.

[NickLappos]

Droopystop,
I agree. That is one of the reasons why I rather enjoy the controversy when it comes time to puncture a myth or two. Somehow knowing the real story makes us better at our game.

[boomerangben]

Crab,

I am glad that you find my education amusing. We are very luck to have the likes of Nick Lappos volunteering his knowledge to us all and I really appreciate him taking time to put us right. In a previous life I was a professional engineer and challenging the ideas of others and having your own challenged was part and parcel of getting the best solutions. As long as the likes of Nick are happy to educate us, I will continue to ask questions and present my own theories for scrutiny, however stupid they may seem. Hopefully it will make me a better aviator.

[Thomas coupling]

Heights good: I dont think we'll hold our breath until you get back to work to give us a scientific appraisal of whats going on in the MR head! You're already talking bo**ocks.

Doesn't ANYONE understand that the main rotor:

DOESN'T KNOW NOR CARE WHERE THE BLOODY WIND IS COMING FROM .....

therefore it CANNOT alter its power requirements to suit:*

Please differentiate between the two:

aerodynamics of a rotating disc,

and,

drag effects of various bits of a helo.

Mix in with this cacophany of movement, your average pilots' mishandling traits and most helo drivers will misinterpret where the extra power demands are coming from:ooh:

Gomer:
when your helo climbs away from your tanker (90 degrees out of wind) and then rotates 90 left INTO wind, the main rotor DOESN'T CARE, it's NOT INTERESTED, it will NOT cause you to increase/decrease power. What causes the change in power demand is:
change in profile drag.[From beam on in climb, to head on after the turn into wind].
change of tail rotor thrust from beam on (in the climb), to nil effect (into wind).
downwash effect (side on) to its effect (into wind).
Change in torque demand due to initial application in climb, to steady state climb.
Your imperceptable inputs into the controls.

and so on...............

In summary:

There IS an increase in power demand when pointing downwind in a helo, but it isn't because of the main rotor.

[[email protected]]

boomerangben - of course you were a professional engineer - who would claim to be an unprofessional engineer, or pilot for that matter, especially on this forum.

You are right that having the likes of Nick on this forum is of great benefit to all, it just amuses me when people flatly tell him he is wrong about things when they clearly don't have a scooby about the point they are arguing about.

[Gomer Pylot]

Thomas, I understand that, and never claimed that the main rotor is the reason for the increase in power demand - just that there is an increased demand. Perhaps I misunderstood Nick, but my understanding is that he was claiming that there is no requirement for more power to hover in any wind condition. I cannot agree with that, regardless of the reason for the increase.

[sycamore]

Nick, and all unbelievers,
I`ve just dug out a load of graphs of t/r tq, t/r pedal posn. and main rotor tq for steady state measurements on a Wessex1 at RAE in `72-73, IGE and OGE,and up to 10000ft, and in steady sideslips around each 30 deg up to 30 kts.
They do agree on all points, and show that with right sideslips, from 30 deg R to 150 deg R, there are the largest variations in pedal requirements and torque .They also show that for those conditions that, although t/r tq may have doubled from a steady hover condition, that there has been a greater (usually) main rotor tq. reduction.
The graphs also show that t/r tq increases with increase in altitude up to about 7000ft, with a similar reduction in m/r tq and then a reduction in t/r tq up to 10000-ft, with m/r tq increasing. These are for hover conditions OGE.
How do you establish a true hover OGE ? Well actually it`s done by SOTP, a cricket ball and clouds................ and leave the rest to your fertile imaginations !!:ok:

[TeeS]

Nick

My prime memory of an example of this was while teaching downwind transitions. I had demonstrated the procedure without any problem, however, when the student tried to hold a downwind hover, we were struggling for power. He was over-controlling like a goodun and I was sure this was the source of the power problem. I took control and, while it is too long ago for me to remember the actual figures, the power requirement dropped considerably.

I can't come up with any explanation for this other than power is being wasted by over-controlling, initially on the pedals followed closely by cyclic and collective.

Cheers

TeeS

[NickLappos]

Tees,

That sounds quite right to me. I think a bit of cyclic and pedal stirring can create about 5% power loss. In defense of the "other side" in this debate, the practical performance of the helo is what counts, so if the pilot uses more power due to handling issues, that detracts from the performance in a real way.

Next time I am aloft, I will carefully take some wind effect data and post it.

[Shawn Coyle]

Back from a long winter's nap...
One of the problems with the discussion here has been stated before - we're not talking about crosswinds, but headwind vs. tailwind. And exactly how much wind are people talking about?
If the wind is less than 5 to 8 knots, I'd be very surprised to see a difference in power required. Above 8 knots, you could be close to the ground vortex starting to interfere with the tail rotor (giving small handling in yaw issues) and the horizontal and vertical stabilzers.
The slight increase in power required to hover downwind may be due to overcontrolling, particularly on the pedals, but also to the change in fuselage attitude caused by horizontal stablizers getting wind over them the wrong way and generally screwing things up, as well as causing some increase in drag.
And I never thought I'd be able to catch Nick Lappos out - and it is a minor point - civil helicopters don't require 10% control margin, just 'adequate margin' for control.

[GLSNightPilot]

Control movement is certainly an issue. If you're trying to take off from an offshore platform at max weight, no wind, and 90+ degrees, you have to be very smooth. Any movement of the controls results in loss of lift, and can be seen and felt immediately, and I've seen pilots who were rough on the controls have to drop a passenger or two in order to get off. But after 35 years of flying, and something over 25,000 takeoffs, close to half of them from offshore platforms at max gross, I've learned to tell when the controls are being moved too much, and I see the effects of wind, from all angles, even if the pilot is being as smooth as possible. Anecdotal evidence points to high fuselage drag and high TR demand, but I have no actual empirical evidence to prove it. And it's not just crosswind, it's tailwind also which has an effect, albeit somewhat lower. Light winds have little effect, but higher winds have increasing effects. Given a 30+kt wind, I can hover a 206 with my feet on the floor, and it will never turn from the wind at all, even with the torque in the yellow. I've done it. The weathervaning tendency is very high, and doesn't like turning from the wind. Most models react similarly, although not all to the same extent. The main rotor may not know where the wind is, but the fuselage certainly does.

[WHK4]

I was out flying today in an R22 and decided to check the power required to hover upwind and downwind in a 15 kt wind so that I could post it on this thread tonight.

The hover height in each direction was 3 ft. skid height. The AUW was 1230 lbs.

The power required to hover downwind was the same as that required to hover upwind: 20.5 inches MP.

[NickLappos]

WHK4,

Thanks for that!

As Igor Sikorsky told junior engineers at the company years ago,
"Young gentlemen, there will come a time in your career when you discover that the facts and the theory do not agree. At all times, I urge you to respect the facts!"

[VeeAny]

On my initial instructor course i was taught to teach the increase in parasite drag means increase in power required in a downwind hover.

On a day with a constant wind I could never get it too work, on a gusty day it would.

I dropped it from the things I demonstrate until i could find some conclusive answers relaled to it.

This thread has awoken my interest so today I went out and tried albeit in a H300 not an R22.

Hovering into a 15Kt headwind 24"MAP.
Hovering downwind 15Kts again 24"MAP.

Windspeed not measured scientifically just a report from the Tower.

Skid height 3ft
with a constant heading in each direction.

I think you may be right (again) Nick.

I have however seen the increase in MAP in the R22 when downwind, and will try the same experiment next time i get in one (which may not be for some time)
.
I am not an engineer but is it possible that

a) the airflow from roughly downwind is restricting the exhaust outflow from the engine, reducing the throughput of air and increasing the MAP.

b) the R22 governor is partly to blame, on those occasions when this has been seen.

c) someone could try this, governor on and off, and let us know.
This would obviously have to be done as part of emergency procedure training as the RFM forbids governor off flight unless for this purpose.

Any thoughts ?


V.

[NickLappos]

veeAny,

thanks for the fresh info, and the understanding you help provide!

This is another example of the need to scrub our system of the old wives tales that pervade our "knowledge" of how helos operate. I do not want to be a gadfly, simply bursting bubbles, because that is both pointless and ego-centric. I do want all of us to have enough basic understanding that we do the right thing, for the right reasons, and also that we know how to get our aircraft to do what we want when we want it.

I think I would like to start a new thread - ten untrue urban myths about helicopters that "everybody knows" but that are siply untrue! Should spin a few electrons around the web!