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Left or Right Crosswind? Let The Fun Begin!


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#1 Nearly Retired

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Posted 16 May 2019 - 21:41

It always amazes me when pilots get into the "left or right crosswind" question regarding American-type helicopters.  Over on the "bad" forum, idiots are arguing it out right now.  Clearly some of them literaly do not know how to fly.

 

So let me 'splain it to you...

 

People who advocate using a *right* crosswind on approach often cite the FAA diagram that shows that a crosswind from the left will produce "roughness due to tail rotor vortex ring state."  The ill- or uninformed take this to mean that if you put the wind off the left, the tail rotor will immediately go into VRS and the helicopter will spin out of control and crash.

 

Nonsense.

 

Despite what some pilots may think, the tail rotor never "stalls."  It's ALWAYS working.  It may get overwhelmed by the weathercock stability of the helicopter, but as long as it's turning, the tail rotor never signs off and loses thrust.  That said, it's output thrust may vary, but that's what those things under your shoes are for: You can adjust them and keep the tail rotor thrust more or less constant.

 

If you get an uncommanded or unanticipated right yaw, just push FULL left pedal if you need to and hold it.  At some point (probably before 180 degrees of rotation) the tail rotor will get a clean "bite" again and you're back in business.

 

Now, a left crosswind surely will cause roughness as the tail rotor momentarily goes in and out of VRS.  But it never goes into complete VRS like a main rotor can.  Because as soon as the tail moves even slightly, the relative wind into the tail rotor changes and it's out of VRS.  "VRS" as it applies to a tail rotor is a momentary, transitory thing.  As soon as you feel the nose do something you didn't ask it to do, push the left pedal until it stops!  Push it to the stop if you have to.  And hold it there.

 

Not only is the t/r going in and out of VRS, but in a front-left crosswind the main rotor downwash can cause interference as well.  Your feet will be BUSY!  You have to be on your toes, pardon the pun.  But again, repeat after me, the tail rotor never stops working.  ...Unless it stops turning.  Then you're screwed.

 

But here's the important thing: A *left* crosswind will cause the nose of the helicopter to want to yaw to the left.  This will reduce the need for left (power) pedal.  Get it?  The less left pedal you have to push, the less power the tail rotor is absorbing, the more power is available for the main rotor!  In a strong left crosswind hover, you might not need any left pedal at all!  Super!  

 

Now, a *right* crosswind will cause the nose of the helicopoter to yaw to the right, necessitating more left pedal to compensate.  And left pedal is bad, mm'kay?  If you're heavy, and using a lot of torque on the landing (like up near the limit), pushing hard on the left pedal is just not a good idea.  So if my ship is heavy, I'm going to choose the LEFT crosswind option if I can and accept the fact that I'm going to be working those pedals to keep the nose straight.  But I do not fear that the t/r will go into VRS and cause me to lose control.  Because that's not what happens.

 

You guys who think that a right crosswind is better on approach have never flown marginally-powered helicopters with weak tail rotors.  Because if you had, you'd NEVER accept a right crosswind if you could avoid it.  It's just dumb.

 

Those damn diagrams the FAA put out have sure scared the hell out of a lot of pilots.  They equate T/R-VRS with tail-rotor stall, and that is simply not so.

 

The whole discussion on the "bad" forum was spurred by the video of that 206L pilot who crashed into the Hudson River by the W30th Street Heliport.  He came to a big, high, downwind OGE hover, and then lost control of it, ending up in the water.  He was the sole occupant, so we know that he was light.  But he discovered that 206's like to point their nose into the wind, and they don't like it when you try and point their tail to the wind.

 

I've got a lot of time in LongRangers...a *lot* of time.  And I've flown "straight" L-models in and out of W30th Street.  A lot.  I guarantee you...GUARANTEE that if he had lowered the collective just a smidge, and pushed FULL left pedal (and held it), and leaned the cyclic either forward or into the rotation a bit, he could have flown out of that uncommanded right yaw situation that he found himself in.  But I believe he panicked (or simply went into brain-overload), assumed that his tail rotor had stalled, probably didn't push full left pedal, and crashed into the water.

 

I always say this: As we first start flying, we think that helicopters are hard to fly.  As we gain experience, we come to realize that helicopters are pretty easy to fly.  But they are also very easy to crash.  And that never changes.  No matter how much experience we gain, the helicopter never stops being easy to crash.


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#2 Nearly Retired

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Posted 17 May 2019 - 17:53

Aaaaaand...let me add something.  You have to be aware of what the helicopter is doing.  SWP and VRS or T/R VRS don't just sneak up and surprise us.  There are indicators/cues that you're getting close to a performance limit, and you better be attuned to those signs!

 

If you're doing something and you find that your left foot is extended and you think to yourself, "Gee, I wonder how much pedal travel I have left?" then you're probably damn close to an LTE encounter and should be asking yourself how you got into this mess?  If you already have all of the left pedal applied and the dang thing still does an uncommanded yaw to the right, pretty much the ONLY way out of that situation is to reduce power.  How much?  Well, first thing I would do would be to reduce the collective *slightly* - reducing the torque and seeing if I got some yaw control back. 

 

If not, then a larger reduction is warranted providing you have the altitude.  And you better.   Because if you're mucking about down low and downwind and you're *NOT* ready for an LTE encounter, shame on you!  This larger reduction in torque must be accompanied by getting some airflow past the cabin to help with the streamlining.

 

You CANNOT let a VRS encounter surprise you.  You have to be ready for it and have a plan as to what you're going to do should the application of full left pedal not stop the yaw.  Me, any time I get near a control limit, I get hyper-aware of what's going on.


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#3 TomPPL

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Posted Yesterday, 04:56

Wow. What a great post. It's the best response I've read to date that explains it like that.



#4 helonorth

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Posted Yesterday, 10:24

I've got a fair amount of time in the 206 also, but I've never noticed a difference between left and right crosswinds. The lesson here is don't mess around with high power settings with a tailwind in a out of ground effect hover. The guy basically did a rapid decel with a tailwind. Recipe for disaster.



#5 octagon

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Posted Yesterday, 14:07

I hate to ruin a great thread with a stupid question, so apologies in advance. But I am puzzled by the effects of LTE.

 

I read AC-90-95 and it makes perfect sense to me, intuitively. In that AC they make a special note that it's normal to operate in conditions that are conducive to LTE, and that the danger lies in not being prepared for it and allowing an uncommanded right yaw rate to develop into a loss of control situation. But the procedure they recommend also states that you may need to enter autorotation to stop the yaw. What I don't really grasp is how LTE can develop into a situation that can't be stabilized except by entering autorotation.

 

I was thinking along the lines of what nearly retired was saying: with a right crosswind not only is there a right yaw moment due to weathercocking, but the airflow from the right also acts to decrease the angle of attack of the tail rotor blades. Both of these conditions require additional tail rotor pitch to counteract. And conversely, a left crosswind both induces a yaw to the left by weathercocking, and also increases the angle of attack of the tail rotor blades, both of which allow the pilot to use less left pedal.

 

But when a right yaw is allowed to develop in a LTE situation the right yaw rate should act like a left crosswind as far as the tail rotor is concerned, making the tail rotor more efficient, and the higher the yaw rate the greater the gain in efficiency for the tail rotor. Additionally, it seems to me that with a significant yaw rate the tail rotor would not stay in vortex ring state, as it would be moving into fresh air constantly. This dynamic stability makes it seem to me that one can always recover from a runaway right yaw without entering autorotation, even in the fully developed case, given enough time.

 

But there is also the issue of torque being lost to rotating the fuselage, which i'd imagine would result in a decreasing main rotor RPM relative to the air (obviously the governor would try to maintain NR relative to the fuselage but the fuselage itself is rotating in this case).

 

It seems pretty obvious to me that the last thing anyone would ever want to do would be to pull pitch in an LTE situation where you have a fully developed right yaw, so you'd need to accept losing some altitude. Is this why so many crashes are caused by LTE, just not enough altitude for the pilot to recover? If so, I wonder how much altitude you'd need?


Edited by octagon, Yesterday, 14:16.


#6 octagon

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Posted Yesterday, 14:33

Also, since LTE happens in high power, low airspeed environments it seems like it makes entering autorotation kind of a moot point, unless you are operating outside the shaded area of the height-velocity diagram. This seems extremely unlikely unless you're in a 500' hover or something.



#7 iChris

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Posted Yesterday, 18:44

Myths about Crosswinds. They’ve taught that there’s a good side and a bad side to be avoided. That’s a generalization and oversimplification of the actual flight characteristics. Both left and right crosswinds present their own unique flight characteristics. Once you learn those characteristics you’ll see it’s sometimes a pick your own poison situation (left or right crosswind).

 

Winds from the right require more tail rotor thrust (more left pedal) to maintain heading due to impingement of main rotor wake on the tail boom, main rotor torque, and the wind speed increase from the right trying to also turn the nose right into the wind; however, that relative wind will not cause much if any undesirable tail rotor thrust variations; therefore, very little handling issues with respect to increased pedal activity are required.

Winds from the left require less tail rotor thrust (less left pedal) to maintain heading due to the main rotor torque being countered by the wind speed increase from the left attempting to turn the nose left into the relative wind. However, relative winds for the left are undesirable as they oppose the tail rotor induces flow velocity and lead to main rotor vortex interference, tail rotor vortex ring state, along with handling issues that require increased pedal activity. The pilot ends up dancing on the pedals, swing left and right, trying to settle down a consistent heading.

 

These handling issues with winds from 225º - 330º lead to the wild pedal swings shown in the figure below. These swings can and will give the pilot the false appearance of Loss of Tail Rotor Effectiveness (LTE) as they try to correct for these unanticipated yaw swings.

 

Though unlikely in most cases, LTE is possible when the helicopter is operating at low airspeeds OGE, especially at a gross weight under the crosswind conditions above when the margin between power available and power required narrows to near zero.

 

That’s what’s being explained in this safety notice.

 

Quote
Safety Notice SN-42

Issued: May 2013

UNANTICIPATED YAW

A pilot's failure to apply proper pedal inputs in response to strong or gusty winds during hover or low-speed flight may result in an unanticipated yaw. Some pilots mistakenly attribute this yaw to loss of tail rotor effectiveness (LTE), implying that the tail rotor stalled or was unable to provide adequate thrust. Tail rotors on Robinson helicopters are designed to have more authority than many other helicopters and are unlikely to experience LTE.

To avoid unanticipated yaw, pilots should be aware of conditions (a left crosswind, for example) that may require large or rapid pedal inputs. Practicing slow, steady-rate hovering pedal turns will help maintain proficiency in controlling yaw. Hover training with a qualified instructor in varying wind conditions may also be helpful.

 

click photo to enlarge

Scan1_zps51617644.jpg

Scan2_zps6d62fc27.jpg

 

Also, see prior post below on Crosswinds:

Myths and Crosswinds and Know-It-Alls Started by Nearly Retired, Jun 22, 2013

 

 

 


Edited by iChris, Yesterday, 19:52.

Regards,

Chris

#8 Nearly Retired

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Posted Yesterday, 22:18

Octagon, I think the reason there are so many "LTE" crashes is that pilots do not fully understand what's happening to their helicopter.  Remember, the tail rotor never "stalls" - even at full left pedal.  And as you note, once the tail has rotated even a tiny bit, it is now in clean air (or at least, different air).  You get it.  But also remember that there is inertia working against you too.  Once you get a yaw rate going, it's hard to stop it.  Much better to, as Barney Fife would say, nip it in the bud!  DO NOT let a nose-right yaw rate build up.  STOP IT.  And if you can't stop it with full pedal, lower the nose and fly away, bubba.

 

I think if you asked 100% of all pilots who've had "LTE" accidents, you'd find that they believe that they used FULL left pedal and that it was ineffective.  I kinda/sorta doubt that.  Our memories play tricks on us.  A long time ago, an ENG 206B crashed in Van Nuys, California as the pilot was trying to land on a dolly.  It went 'round and 'round and ended up on its side.  The pilot *swore* to the FAA that he got into LTE and he had full left pedal stuck in.  But the cameraman in the back seat caught a glimpse of the pilot's legs and feet as the ship went around.  Sure enough, he was at neutral pedals!  I think the LongRanger pilot in NYC will tell his Chief Pilot and the FAA that he too was using full left pedal and it did not stop the spin.  That's what I'd say too ;-)  And sure enough, this latest accident will just add to the legendary "weak-ass 206 tail rotor" myth.

 

Octagon, you say that pulling pitch would be the *last* thing you'd do?  Err...maybe you should rethink that.  If we watch the 206L video, we see that the rate of rotation is not particularly high - certainly not as high as it would be if he'd had a tail rotor failure.  As long as you can keep some sort of orientation, climbing/spinning up a bit and *then* reducing torque and flying out of the situation *might* save the day.  You'd have to think and act quickly, but that's what we get paid the big bucks for, right?

 

As iChris points out, sometimes  you might choose a termination with a right crosswind.  It all depends on the circumstances.  You have to know the characteristics of your ship.  W30th Street is a great example of this.  With the wind blowing straight across the Hudson River, you'd want to make your final approach parallel with the Manhattan shoreline so you won't terminate with a tailwind but rather with a crosswind of some sort.  Although my experience makes me normally choose a left crosswind, I *might* choose a right crosswind, knowing that the ship might yaw into the wind at the bottom as we come to a hover if I run out of power or pedal.  It just depends.

 

I have done some funky, funky stuff just to land on drilling rigs out in the Gulf of Mexico where the heliport was situated in the exact WRONG place for the wind that day.  Shell Oil's first deepwater tension-leg platform (called the Auger) had the heliport in a horrible place when the wind was out of certain directions.  It could be ball-clenchingly tense getting in there with a heavy BO-105 on a hot day - no easy way in.

 

If you really screw up and get the thing a-spinnin' down low and can't get it to stop even with full left pedal, then your only viable option might be to chop the throttle and suffer the consequences.

 

Finally, I know there will be some guys who claim to have experienced LTE while doing high-altitude stuff and not having enough left pedal (i.e. tail rotor thrust) to keep the nose straight.  That's really LTA (loss of tail rotor authority) and yes, the 206 can experience that, for sure.

 

These wacky machines can be awfully confusing to fly.



#9 iChris

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Posted Yesterday, 22:35

Not sure why he slowed so high to an OGE hover.

 

In his statement: I could see them  taking video of us (we were that close) I waved to them. next thing, the helicopter started spinning slowly to the left a few times and then violently to the right. 

 

 

 

 


Edited by iChris, Yesterday, 23:24.

Regards,

Chris




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