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Hovering Downwind

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Hovering Downwind

Old 31st Dec 2004, 08:29
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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.

Last edited by boomerangben; 31st Dec 2004 at 14:50.
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Old 31st Dec 2004, 09:04
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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.

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.
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Old 31st Dec 2004, 09:20
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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?
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Old 31st Dec 2004, 11:19
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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!
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Old 31st Dec 2004, 11:39
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Good comeback Boomerang. . .

I think you have it in a nutshell
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Old 31st Dec 2004, 13:41
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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.
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Old 31st Dec 2004, 14:10
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Thanks Boomerangben, exactly what I was trying to say but somewhat more succinctly and scientifically put.
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Old 31st Dec 2004, 14:59
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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?
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Old 31st Dec 2004, 17:21
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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
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Old 1st Jan 2005, 16:38
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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.
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Old 1st Jan 2005, 20:00
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I assume that is the minimum regulatory control requirement at any direction, any CG at 17 knots of airspeed?
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Old 2nd Jan 2005, 16:53
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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.
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Old 2nd Jan 2005, 19:26
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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
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Old 2nd Jan 2005, 19:28
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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.
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Old 2nd Jan 2005, 20:46
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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!
 
Old 2nd Jan 2005, 21:21
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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
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Old 2nd Jan 2005, 22:54
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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
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Old 3rd Jan 2005, 00:42
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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/g...literature.pdf
and a great Ray Prouty article:
http://safecopter.arc.nasa.gov/Pages...BULENT_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.

Last edited by NickLappos; 3rd Jan 2005 at 01:26.
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Old 3rd Jan 2005, 03:31
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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.
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Old 3rd Jan 2005, 03:46
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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?
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