Myths and Crosswinds and Know-It-Alls

[ridethisbike]

Troubleshooting a problem? Over the water? Yea, right... and I'm the Dalai Lama!

[aeroscout]

I always wondered what the final determination of the cause of that incident was.

[Retreating Brain Stall]

Thats another area of difference between piston and turbine engines, the effect of atmospheric conditions on power available. The power available from a piston engine depends upon density altitude, whereas the power available from a turbine engine depends upon pressure altitude and mainly temperature.

 

This is confusing to some since pressure altitude and temperature makeup density altitude; however, that same density altitude can be made from a variety of pressure altitudes and temperatures, just take a look at any density altitude chart.

 

Put another way, the thermodynamics of a turbine engine are affected more by the inlet air temperature than by the density of the air, whereas a piston engine is more affected by the overall air density.

 

Study the torque available chart for a TH-67 (Bell 206) below. The torque available is actually higher at 10,000 PA@-10C (9,380 DA) than for 5,000 PA@40C (8,860 DA). This increase in torque available (From 87% to 95%) at the higher PA and DA is mainly due to the decrease in inlet air temperature.

 

Very nice iChris (I always enjoy your posts- thought provoking)

 

To add to this for another way to twist and comprehend this statement, remember maximum predicted torque is going to correspond to your TOT limit. 100% torque as your max for the day means you will be experiencing an aircraft limit (normally transmission capability). Below 100% torque you will be experiencing an ambient limitation of the aircraft- hence a corresponding torque value (predictaded) will be the torque at which your TOT hits its highest value. The engine is rated to provide a lot of power and in some ambient conditions it provides so much it will chew up the aircafts transmission which can't handle that much power. Then you increase the ambient air and you will start to find ambient limits on TOT rather than 100% torque max, you will never be able to reach that high of a torque value before exceeding the TOT limits.

 

Correct me anyone if there is any flaw in what I have learned and understand.

Edited June 26, 2013 by HeliFun

[Wally]

The 206 tail rotor has less excess authority than many contemporary helicopters. That's not good or bad, it just is- the aircraft was designed with hover lift the priority over hover yaw. The real argument is whether it has an adequate amount of antitorque authority. It's the most successful single engine helicopter ever built...

 

It's possible to VRS a rotor, main or tail. It's possible to put yourself in a situation that demands more thrust from a tail rotor than it can deliver, true for all airframes and especially true in an airframe known for comparatively limited antitorque authority. It's logical that the limited authority would more easily allow the conditions to develop VRS/LTE in the tail rotor. Don't casually put yourself in the situation where that's likely to develop, even if the odds are that it won't happen. That's been my experience, the odds are it won't happen because it's really difficult to get the conditions exactly right.

 

Comparing the 206 to the 350, no question the Astar has more TR. But it's a squirrel at the bottom, whereas the 206 is a stable, stable, stable. Can't speak to Robinsons.

[Nearly Retired]

So a 206 went down while cherry-drying the other day. Flipped it over and crashed inverted into an orchard. Pilot luckily not hurt too badly. First reports said he got caught up in the bird-netting. Second reports said it might have been something else TBD. The crash was discussed on the Nick Henderson's "Helicopter Pilots" Facebook page.

 

Then some chick chimes in with a big dissertation about cherry drying. She said she just got done drying for 3.0 in her ship (a 206B) and then proceeded to tell us all how to do the job, blah blah blah, yadda yadda yadda, oh and avoid left crosswinds in a 206.

 

I nearly sh*t myself. I mean, really?

 

So...ever-tactful me, I posted a condescending, sexist response to her response saying basically, "Sweety, the only pilots who advise against a left crosswind in a 206 are those who've never flown one or those who have very little time in one. Which are you?"

 

This "Left Crosswind Causes T/R LTE" bullshit has got to go.

 

In my job...that being helicopter pilot cherry-dryer, I spend my entire life up in an OGE hover. With our 56 foot rotor diameter, we hover high and slow. It's like mowing an endless lawn. With a self-propelled push-mower.

 

Now, we have nine fields in "our" orchard, split between me and one other S-55...about 250 acres of cherries. We don't get to choose the way they're aligned - and they're all aligned differently. And the wind is always changing, even from field to field. You might be drying with a wind off the river in one field, and have the wind coming down the mountain in another. Others might be calm. Sometimes you take your best guess as to which way it's blowing as you approach the field, only to find that you were wrong. I keep my door slid back and I watch my downwash 50+ feet below.

 

We also don't get to choose the wind direction or velocity.

 

I spend a lot of time hovering in the wind. Headwinds, tailwinds, right crosswinds and...(drum roll)...LEFT crosswinds! Sometimes the wind is blowing strongly across the rows, making for a right crosswind going thisaway, and a left crosswind going thataway. I am here to tell you that the helicopter does not spontaneously combust when you encounter a left crosswind. My left leg just gets a rest when the wind is off the left. If the wind is really whipping from the left, my right leg will start to get a workout. (Or I'll just let it yaw to the left and hover sideways for a while.)

 

Oh, and the "Avoid Left Crosswinds!" advice is not limited to the 206. *ANY* helicopter with a clockwise rotor (as viewed from the top) will be susceptible to that diagram the feds put out that just happens to show a 206 in the center.

 

Only...they're not. Like I told the little girl 206 pilot, weathervaning tendency will ALWAYS be there. LTE from a left crosswind is a *POSSIBILITY* but one without a guarantee. Not only that, but the phenomenon will DISAPPEAR as soon as the nose yaws even a little. The tail rotor will not stay in sideways VRS as it yaws around past ten degrees or so. It is a myth to think that the t/r will go into VRS and stay there through 360 degrees of rotation. Just doesn't happen.

 

So the girl 206 pilot and I had some good back-and-forth...at least I think we did. She may now think (like many do) that I am the biggest A-hole in all of aviation. I don't care. Pilots must not be afraid of having a left crosswind in a helicopter. (Not unless you're landing way up high in the moutains where total tail rotor thrust will be lessened by the thin air.)

[aeroscout]

NR, I am having this same conversation with another VR member. My contention is that the thrust of the tail rotor as diminished by Higher PAs and DAs will be proportional to the loss encountered by the main rotor's thrust. The ratio that I recall is that tail rotors take up around 7% of the power generated by the engine(s). That 7% ratio should be the same across the PA/DA spectrum.

[Wally]

NR, I am having this same conversation with another VR member. My contention is that the thrust of the tail rotor as diminished by Higher PAs and DAs will be proportional to the loss encountered by the main rotor's thrust. The ratio that I recall is that tail rotors take up around 7% of the power generated by the engine(s). That 7% ratio should be the same across the PA/DA spectrum.

 

The percentage would remain the same if both rotors had the same pitch and thrust capabilities. But they don't, and that affects those with less TR authority when operated without proper planning. If you were using all the TR pitch available at lower DAs and still had collective pitch before your power limit, then the TR would hit the pitch stop at higher DA if you had engine power to lift the same load. Demanding more TR thrust than available won't VRS the TR, but that limit could start the sequence that allows the VRS elements to be in place at the TR.

 

I've never had an inadvertent VRS of any sort. I found it difficult to demonstrate a main rotor VRS, and have never had a tail rotor VRS demonstrated except in a simulator. Just because it hasn't happened to me doesn't mean it won't, so I try to anticipate and minimize.

[Hand_Grenade_Pilot]

NR, I am having this same conversation with another VR member. My contention is that the thrust of the tail rotor as diminished by Higher PAs and DAs will be proportional to the loss encountered by the main rotor's thrust. The ratio that I recall is that tail rotors take up around 7% of the power generated by the engine(s). That 7% ratio should be the same across the PA/DA spectrum.

 

Firstly, you and I are having a completely different conversation from what NR has just described. I am talking about T/R authority at high DA. NR is talking about a 206 pilot who doesn't understand basic aerodynamic principals and how crosswinds affect yaw control.

 

So, on the topic of T/R authority and DA-

 

To compensate for thin air, the pilot must maintain a higher collective pitch setting to produce the same lift as they would at a low DA. Meanwhile, as collective pitch increases, you inherently need more power pedal (left pedal) to counteract torque.

 

-However-

 

The increase in power pedal (left pedal) toward full travel is not proportional to the increase in collective pitch toward full travel. Simply put, you will reach the mechanical stop on the pedal before the collective. The only thing that would limit an increase in collective pitch would be powerplant/torque limitations.

 

Your original post stated that T/R authority does not diminish with an increase in DA because there is a proportional decrease in torque- which is incorrect. This lead to the discussion of how torque is created in a piston and turboshaft engine.

 

Even with a piston engine, where torque is directly affected by DA (vs. the turboshaft engine, which is primarily affected by temp) the decrease in torque is not proportional to the loss of thrust. A piston helicopter with a derated engine is a perfect example. You could be operating at the same horsepower/torque output at a higher DA because it is being artificially limited at lower DAs. Meanwhile, the T/R is still creating less thrust.

 

No matter how you try to pick it apart, you will lose T/R authority as DA increases.

 

Edit: I also suggest you re-read the last part of the last paragraph that NR wrote.

Edited June 28, 2013 by Hand_Grenade_Pilot

[SBuzzkill]

I will agree on that and have seen it. Flying at 8,000 feet DA required a significant increase in left pedal to maintain heading in an IGE hover. Much more than the OGE hovers near sea level that I was used to. Similar torque numbers.

 

Pretty close to the stop was required when hovering up at 10,000 feet DA. Still plenty of authority left, but my left foot was far enough forward that it felt awkward.

Edited June 28, 2013 by SBuzzkill

[Rupert]

I don't know about LTE. Instead, I think in terms of Not-Enough-Tail-Rotor-Authority-For-The-Conditions.

 

Yeah, the dreaded NETRAFTC.

 

Not the end of the world. Push right pedal and cork-screw up and away. Done it. It works. Seriously. Do not think emergency; do not reduce power; and, do not autorotate. A helicopter with full right pedal does not spin so fast you can't stay oriented and fly it. Just use all that extra horsepower transferred from the tail rotor to the main rotor to get far, far away from the earth before you reduce power and stop the spin.

 

Which brings us to the S55T mentioned earlier in someone's post: a beautiful machine from the inside and ugly from the outside. A sweetheart! Don't turn it down because of its looks.

[Eric Hunt]

Wow, Rupert, you are far braver than me. Full right pedal? Corkscrew away?

 

In my 44 years of aviation, I have never heard anything so outright stupid.

 

But then, I don't live in your country. Maybe such a wonderful technique just hasn't filtered through the recommended procedures and flight manuals and simulator sessions to get this far.

[Hand_Grenade_Pilot]

I don't know about LTE. Instead, I think in terms of Not-Enough-Tail-Rotor-Authority-For-The-Conditions.

 

Yeah, the dreaded NETRAFTC.

 

Not the end of the world. Push right pedal and cork-screw up and away. Done it. It works. Seriously. Do not think emergency; do not reduce power; and, do not autorotate. A helicopter with full right pedal does not spin so fast you can't stay oriented and fly it. Just use all that extra horsepower transferred from the tail rotor to the main rotor to get far, far away from the earth before you reduce power and stop the spin.

 

Which brings us to the S55T mentioned earlier in someone's post: a beautiful machine from the inside and ugly from the outside. A sweetheart! Don't turn it down because of its looks.

 

That is absolutely stupid and reckless. How on earth could you think that this is a legitimate technique?

 

First, you are not reducing as much load on the engine as you think. Full non-power pedal (right pedal) puts you at negative pitch, which requires power from the engine.

 

Even if you had the pedals set correctly, that method is still ridiculous. If tail rotor vortex ring state begins to occur, and full anti-torque is applied, the rate of turn will initially be slow enough that the pilot can increase forward or lateral speed to regain control. Upon reaching ETL, there should be enough increased lift to reduce power without losing altitude and enough of a weathervane effect to re-establish yaw control.

 

You are taking something that is beginning to turn bad, and turning it into the absolute worst case scenario. By putting the helicopter in a rapid spin, you are not maintaining control. Imagine doing this at night, with limited visual references. I would bet money that 99% of the pilots stupid enough to do this would become disoriented.

 

Even if you don't become disoriented, you now have a substantially increased momentum. If the tail rotor wasn't able to manage torque with no additional momentum, how do you factor in the helicopter spinning violently?

 

The fact that you have 'tested' this and now think that it is an acceptable recovery method is a dangerous thing. Your logic is not sound, and the fact that you did not crash is not testament to it being acceptable.

 

The correct method is to maintain full anti-torque to prevent the rate of yaw from becoming excessive. If altitude permits, reduce collective pitch slightly. If there is no room to reduce power, then maintain it. Use the cyclic to establish forward or lateral movement until the aircraft transitions through ETL. At that point, you will be able to reduce collective pitch as needed and regain total control.

 

Edit:

Additionally-

If you are not able to recover before a spin develops, maintaining a constant direction of movement can be difficult. Cyclic inputs tilt the disk relative to the direction that the aircraft facing. If you are facing 180deg away from your original heading, you would be applying aft cyclic to move in the same direction. At 270deg, you are applying right lateral cyclic. In a spin, you are making what could be described as circular movements with the cyclic to continue tracking in the same direction.

 

With full antitorque applied, the spin is slow enough that the pilot can keep up with the constant change in relative direction and keep the aircraft moving along the desired track (the Erickson Skycrane video is a good example).

 

Now imagine trying to do this with full right pedal... it no longer becomes possible. The relative direction is moving so quickly that you would have to rapidly move the cyclic like a mixing spoon. Your only hope at that point is a significant reduction in collective to reduce torque and to establish airspeed by descending.

Edited June 30, 2013 by Hand_Grenade_Pilot

[iChris]

 

Not the end of the world. Push right pedal and corkscrew up and away. Done it. It works. Seriously.

 

A helicopter with full right pedal does not spin so fast you can't stay oriented and fly it. Just use all that extra horsepower transferred from the tail rotor to the main rotor to get far, far away from the earth before you reduce power and stop the spin.

 

 

You're correct, the so-called corkscrew technique does work; however, your description of the technique and its usage is a gross misrepresentation. The maneuverer should not be used as a recovery technique or solution to any tail rotor effectiveness deficiencies.

 

It was a technique used in logging and logline operations. It was a soft and easy “finesse” technique used to rob Peter (tail rotor) to pay Paul (main rotor). With regard to lifting capability the tail rotor absorbs 10% to 20% of the engine power that otherwise could be used by the main rotor.

 

So, the technique was to pull the power in with collective up to the limit. Then slow, soft, and easy, allow the helicopter to yaw nose right by relaxing left pedal and taking up the slack with slight up collective, as allowed for by the reduction in tail rotor thrust. The slow and easy is important because if done too rapidly the turbo-shaft engine’s pneumatic-mechanical fuel control system (Governor) will respond with a decrease in fuel flow as it sees the event as an unloading or over-speed condition.

 

If done with finesse you were normally able to left a load 10 to 15 feet clear of the ground (just about the time the nose passes 90º to the right) then you cyclic forward into and through ETL, flying away with a load you normally wouldn’t be able to lift.

You’ll need the 10 feet of ground clearance since you’ll normally lose about 6 feet before you pass through ETL. All in all it needs to be done with finesse. There’s no full right pedal, tight radius turns, or 360º spinning involved. So, the goaled is to make the transition through ETL and gain as much forward airspeed as possible during the reduction in tail rotor thrust.

For example, as a rule of thumb in the Bell 206, 1% torque equals about 30 pounds of lifting capacity. You’re lifting an old chiller unit off a roof one morning and you’ve calculated with reserve 850 pounds maximum based on the customer’s estimation. However, when you try to lift the unit it will not quite lift free. Because of rust and water residue inside the unit it ends up at 1,000 pounds. However, by using this technique you’re able to rob Peter (tail rotor) and free up 5% torque to give to Paul (main rotor).

 

The 5% X 30 lbs. makes up for the additional 150 pounds and allows you to lift the unit off the roof and fly it down to the parking lot. That’s a 17.5% increase in lifting capacity over your calculated 850 pounds.

Edited July 1, 2013 by iChris

[Eric Hunt]

Chris said

 

flying away with a load you normally wouldn’t be able to lift.

 

So the pilot looks at his performance charts and calculates that he can only just (LEGALLY) lift 850kg, with the usual safety margins factored into these charts to allow for pilot stuff-ups, fickle breezes etc.

 

Then Chris says "Throw on an extra 150kg, I will be able to lift that using the corkscrew spin-away technique."

 

How will you defend that in the court case, assuming you survived the crash, after the boss has disowned any responsibility and dropped you into the deep doo-doo by telling the court that you used to do this regularly, even though it was not a flight manual technique?

[iChris]

Chris said

So the pilot looks at his performance charts and calculates that he can only just (LEGALLY) lift 850kg, with the usual safety margins factored into these charts to allow for pilot stuff-ups, fickle breezes etc.

 

Then Chris says "Throw on an extra 150kg, I will be able to lift that using the corkscrew spin-away technique."

 

How will you defend that in the court case, assuming you survived the crash, after the boss has disowned any responsibility and dropped you into the deep doo-doo by telling the court that you used to do this regularly, even though it was not a flight manual technique?

 

I’ve heard that from some before, that the performance data constitutes an aircraft limitation, in this case, it does not.

 

Performance data provides information to be used in flight planning. It’s up to the pilot to make the final decision based on the existing conditions. In some cases you may not even be able to meet the performance calculated and sometimes you’ll be able to do a little more than calculated. RFM performance is based on flight test data under controlled and standardized conditions.

 

The legal limitations are as follows:

 

As long as the maximum external gross weight, maximum hook weight, torque, TOT, and N1 are within limits the operation is LEGAL. It’s all done within the limits. It’s not illegal to get the maximum allowable from your aircraft; however, this type of work is not for all pilots. Everyone needs to know his or her personal limitations as well.

 

SECTION 4 PERFORMANCE:

 

The Bell 206B Jet Ranger III performance data are contained in this section. The data listed on the graphs are derived from actual flight tests and are intended to provide information to be used in conducting flight operations. The performance data contained herein is applicable to the 250-C20B/C20J engine.

 

 

MD HELICOPTERS RFM (Page Fi)

THE FAA APPROVED ROTORCRAFT FLIGHT MANUAL CONSISTS OF THE FOLLOWING SECTIONS.

SECTION II - LIMITATIONS

SECTION III - EMERGENCY PROCEDURES

SECTION IV - NORMAL PROCEDURES

SECTION V - PERFORMANCE DATA

SECTION IX OPTIONAL EQUIPMENT

THE HELICOPTER MUST BE OPERATED IN COMPLIANCE WITH THE OPERATING LIMITATIONS AS SET FORTH IN SECTION II OF THIS MANUAL AND ANY ADDITIONAL LIMITATIONS FROM SECTION IX AS A RESULT OF AN INSTALLED OPTIONAL EQUIPMENT ITEM.

SECTIONS III, IV, AND V CONTAIN RECOMMENDED PROCEDURES AND DATA AND ARE FAA APPROVED.

Edited July 1, 2013 by iChris

[500F]

Spot on iChris.

Performance data, HIGE & HOGE Charts, HV curve, etc. (in part 27 helicopters at least) is performance data, not limitations.

[Gomer Pylot]

I've used the corkscrew technique, as have many others, in the UH1 to get out of tight LZs. We had no idea what our gross weight was, we just knew we weren't leaving anybody there, and we had to get up and away. iChris is correct, it's totally a finesse technique, done slowly and precisely, and only long enough to get above the obstacles. It's not a technique I would use in normal civilian operations, but it can work if you're really desperate to up. I'm seldom that desperate these days.

[cburg]

(Cross-posting just to reactivate the thread)

 

 

Unanticipated Right Yaw In Helicopters AC:

 

 

http://www.faa.gov/documentLibrary/media/Advisory_Circular/ac90-95.pdf

[aeroscout]

iChris linked it to our most current LTE conversation.