Hovering Downwind
 

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 :-)

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.

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.

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

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.

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.

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

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.

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

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.

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

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.

Hovering Downwind
 

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

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.

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.

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!

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

Cyclic authority ?

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.

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