Tail rotor position

[nickel]

Hello,

I have been looking at some helicopter photos and I've noticed that some helicopters (usually military such as UH60) have tail rotors pretty above the center of gravity (CG position guessed by eye... ). Some others (usually civillian helis) have it close to the CG.

I can understand the logic of having the tail rotor close the CG level. In this position it would not create a turning moment around the longitudinal axis. How is this compensated in other helis? With digital control systems? Does for example a Uh60 have a serious rolling moment due to tail rotor if the control system is disabled?

thanks in advance

[Tail-take-off]

Actually the better designs have the tail rotor above the longitudanal C of G.

It is difficult to explain (& probably harder to invisage) without diagrams but I will try.

The tail rotor applies a lateral force to counter the torque reaction caused by driving the main rotors. The trouble is this force then causes the helicopter to drift in the direction of that force. This is known as tail rotor drift. To counter the drift a lateral stick in put is applied. The aircraft has now stopped drifting but is sitting one skid/wheel low due to constant lateral input.

The designers overcome this but placing the tail rotor centre of thrust above the longitudanal C of G so that it know has a rolling moment which corrects the skid low situation.

Hope this helps.:ok:

[Peter-RB]

TTO
I understood what you mean,

Nickel ,

Yes I think you are right, ... any heli would have a tad moment of indecision if tail rotor controls were disabled!

Peter RB

[Role1a]

It was always explained to me as
If a helicopter was designed to hover (Anti sub or SAR etc.) then the Tail Rotor was high to minimise Tail Rotor Roll in the hover,(in line with the C of G ish), the down side being Tail Rotor Roll in Fwd Flt.(Reverse wing stabilator used to combat this)
A cruise helicopter would have an in line Tail Rotor minimising Tail Rotor Roll in the cruise, the down side being TRR in the Hover.
I also think there is a weight and power issue as well as it seems that only the little racing helo's have the in line tail rotor.
Happy to be corrected (Where's Crab):sad:
Hope this helps.

[IFMU]

Obstacle clearance is another good reason for a high tail rotor.

-- IFMU

[yellowbird135]

Check the archive for more hot discussions on this subject. These include everything you need (or need not:uhoh:) to know

cheers YB

[MightyGem]

The rolling couple is caused, not by the c of g, but, by the difference in height between the tail rotor and the main rotor.

If the t/r was at the same height as the m/r when in the hover, the aircraft would hover skids level, but would fly one skid low when in the cruise. As it's preferable to be skids level in the cruise, the t/r is positioned appropriately. Consequently the aircraft sits one skid low in the hover.

http://img.photobucket.com/albums/v465/MightyGem/TRRoll.jpg

[RVDT]

Ouch! Better dust off the fire suit there MG! Incoming.................................................... ..................................

(Psst: You theory is 100% wrong) :rolleyes:

[Farmer 1]

Yes, post No. 2 has a good explanation.

[Bernouli's what?]

Good use of colour, though! Who's going to volunteer to do the vector diagram?:confused:

[Graviman]

For cruise it depends also on vertical stabiliser, which is generally in a high position. Main function of tail rotor is to provide yaw control, so it is best off at same height as cg. For lateral trim TR is best off as close as possible to rotor disk, which in the cruise is tilted forwards with the fuselage. It's thus a compromise.

Don't forget any development team is trying to find the most cost effective way to deliver a reliable helicopter design. A high tail rotor is aerodynamically ideal, but requires an extra gearbox. This new gearbox requires new castings, extra machining and an additional driveshaft to the tail rotor gearbox. Unless skid low hover is an operational problem, best to avoid the complexity.

There is also the problem that the horizontal stabiliser is best off out of the downwash, particularly to avoid trim changes with forward flight. A half tail plane adds weight through torsional loads in the tail. Sometimes it's easier to put tail rotor below a full horizontal stabiliser.

[[email protected]]

MG, the position of the C of G, both vertically and laterally is important here becuase any force (tail rotor thrust, main rotor thrust or weight) acts about the C of G to produce a moment.

Your QHI course diagram omits this vital information - the horizontal component of main rotor thrust and the tail rotor thrust act about the C of G to produce a rolling couple; the vertical component of rotor thrust and the weight also act about the C of G to produce a rolling couple.

The magnitude of the forces and their relative distances from the C of G determine the eventual attitude of the aircraft when they are all in balance.

But as role 1a says the positioning of the TR is partly a function of the role of the aircraft - an aircraft that will spend a lot of time in the hover is more comfortable if the cabin floor is level and as such a high TR position is often chosen. This is not a hard and fast rule as there are many other factors that a designer way have to take into account.

Most helicopters use a horizontal stabiliser to keep the fuselage relatively level longitudinally in the cruise so the TR position is less vital.

[MightyGem]

I agree. The heavier a particular aircraft is loaded, the less the amount of roll, but it is still the vertical distance between the m/r and t/r that produces the skid low attitude. If the t/r was at the same level as the m/r then that roll would be minimal. Increase the distance apart and the amount of roll will increase.

[NickLappos]

Mighty Gem is unfortunately in strong disagreement with Physics when he says TR height to MR head has ANYTHING to do with roll in a hover. Please pruners, don't try that at home!

The issue has been discussed here several times before, I have absolutely no idea why physics works for everything else on the helicopter but not for the roll attitude!

Why is this that hard, folks?

[MightyGem]

Nick, I bow to your greater knowledge. Someone obviously taught me wrong all those years ago.

[MightyGem]

Having said that, this would appear to support my argument.http://www.tc.gc.ca/CivilAviation/general/Flttrain/Planes/Pubs/TP9982/images/Exercise8b.jpg

From
HERE. (http://www.tc.gc.ca/CivilAviation/general/Flttrain/Planes/Pubs/TP9982/Exercise8.htm)

[Vertical_Reference]

MG, that's exactly how I teach. Lifting the tail rotor will produce a smaller couple between the two vectors (one pulling left and one pulling right).
It's the same when in forward flight. If you fly unbalanced your one skid is much lower than the other. That's why in the Squirrel we fly it half a ball out to keep the skids level.

[NickLappos]

You can teach it anyway you wish, and even transport Canada has it wrong.

The forces and moments act at the CG, and that is where one measures the resulting accelerations from. Because you explanation works, it seems ok (so did the ancient scheme where the planets spun around the earth!)

Having helped design the Comanche tail rotor/fantail, and evaluated where to put it based on the REAL physics (balancing left bank in a hover against tail drive shaft clearance from main blade strike) I can assure you that designers would toss their cookies at Mighty Gem's cartoon.

Where is Shawn Coyle to explain how Canada got it wrong?

Oh Shawn!

[Role1a]

Must remember that one from VR for the next CAT with Crab
Excuse 21a:)

[Choppie]

I imagine the machine lateral cg was also tweaked

I think that's what they did to the EC120 by offsetting the engine to the left side if I'm not mistaken.

[Graviman]

Not doubting that for an instant, Um... lifting...

So the flat Earth goes around the Sun, eh? :}

[Role1a]

Just to expand the argument, why does the Chinook hover left wheel low?

[Choppie]

If you look at helicopter like the CH-53. The tailrotor is mounted almost inline with the mainrotor. So why do they offset the vertical stabilizer to the left tilting the tailrotor to one side and almost looks like it could create a small amount of lift in the same direction as the mainrotor?

[[email protected]]

Choppie - because that is exactly what it does

[NickLappos]

Choppie,
Crab is right, the canted tail rotor on the CH-53E does provide significant lift. It serves to offset the aft CG of the baseline aircraft, since the E model added an aft engine and a big cabin plug as compared with the D model, which had a normal tail rotor configuration.

[Graviman]

Interesting. I would guess that splitting the CH-53E cabin plug between fore and aft was considered, but rejected due to need to redesign control system linkage. I guess that's not such an issue on a fixed wing with cables or hydraulics.


Just to expand the argument, why does the Chinook hover left wheel low?

I'm guessing Chook cg is a little port of centreline.

[Choppie]

I can imagine with a huge tailrotor like that it can create quite a bit of upward thrust.

[NickLappos]

Choppie, with a 20 degree cant angle, the upward thrust is about 1200 lbs, at the tail. This is pure lift, but also helps shift the lift balance to offset 4000 lbs just aft of the mast.

Please note that the tail lift acts as pure lift at the CG, as well as a nose down rotation due to the strong moment from the tail lift. So much for the "stick the moment anywhere you'd like" school of bizarro-physics.

[Matthew Parsons]

Love these arguments where everyone looks to find who's right when, depending on your point of view, the truth is either everybody's right or nobody's right.

Demanding a degree in physics prior to learning how to fly a helicopter is impractical, yet understanding why things happen the way the do with helicopters is necessary. To make the concepts understandable, answers are diluted quite a bit in most cases, and in some cases are complete lies. But they still get the point across.

This idea of looking at the difference in height between the tail rotor thrust and the horizontal component of main rotor thrust has huge merit. If they act at the same height, then when there is no net lateral force, there is also no net rolling moment. Thus you hover with skids level. (At least within the many approximations that have to be made.)

Is that accurate with physics? Yes, but it is far from the whole story. Just knowing the difference in height doesn't help much in designing helicopters, but it does help in understanding a small part of helicopter flight.

So is the right answer that those two forces create a moment about the center of gravity and the vector sum of those moments is the net? Well, that is a right answer, but there are others. Because of all the approximations still being made, it would be valid to call this a wrong answer as well.

What's right really depends on why you need the information. If you're designing helicopters you need way fewer lies and much more accuracy. If you're trying to learn to fly helicopters, a general understanding is sufficient.

[[email protected]]

Mainly true Matthew, the P of F that is taught in UK is essentially a convenient explantion of something we know happens but usually isn't the truth, the whole truth and nothing but the truth.

However useful simple concepts are for teaching, there is no point letting people try to convince others that these simple concepts are the whole story.

[Graviman]

Correct me if wrong, but what Nick is getting at is that for helicopter to hover (or cruise) total lift must balance total weight. Ship rotates about it's cg until tail rotor thrust and main rotor lift vector sum to a vertical force above the cg. All the while the pilot is trimming the cyclic to keep references/string/ball in position. It's as Matthew says, a different perspective.

At a guess Comanche low tail rotor would hover left wing low, but with high vertical stabiliser would cruise right wing low. That powerfull bearingless rotor would then sort out any pitch attitude changes, from tail rotor cant.

Is it fair to assume centre of drag acts at similar position to cg?

Actually, designers often use tricks to get quick answers.
I'll revise my earlier posts appropriately...

[Choppie]

I have flown a few hours on the EC130 (didnt finish my conversion though), but what I did notice is that you fly with quite a bit of left pedal when in the cruise. Is this then the same because of the huge vertical stablizer like the Comanche? But I can't say that I felt the helicopter fly left skid low. Maybe I didn't notice it.

[Bravo73]

Choppie,

Generally, yes. Same reason why you fly with a bit of right pedal whilst in the cruise in a B206. (ie the vertical stab is doing the work of the TR.)

[Choppie]

Yes that I know. But in the EC130 it actually feels strange because it's so much left pedal. What I would like to know is does it cruise with the left skid low or level?

[delta3]

It looks to me that some scientific based reasoning would probably have cut short many of these discussions. Over and over again some topics resurface...

If you argue that simplification is OK for a pilot that -just- has to fly the thing, then that is OK with me, there are more important things to learn. As long as these pilots know that when they start making great theories, they may be way out of bounds off from what they have been thought.


d3

[maxtork]

So what hppens when you use this tail rotor position?
http://uk.youtube.com/watch?v=Yu2CwHwxJYA
Max

[Graviman]

Well, this thread has knocked my thinking into shape. Read "Cyclic & Collective" by Shawn Coyle, in particular P220 covering tail rotors in advanced helicopter aerodynamics. Good read.

An important tail rotor function is to provide yaw control. If it is above or below the cg then you get a yaw-roll coupling. The vertical stabiliser can be positioned to perfectly counteract main rotor torque, but tail rotor is by necessity more of a compromise. I've stuck in a line in my earlier post.

Choppie, fly with string centred and see what ball says about trim.
Maxtork, interesting i wondered what was happening on this. 177kias is a good start, i imagine power limits level flight to 165kias. The only information i can find on X2 development is this frustratingly slow update web vid:
http://uk.youtube.com/watch?v=E-rDl8lbVcc

[heated ice detector]

Sorry for the slight change in thread!
I was wondering what determines the length of moment arm for the tail rotor, If the tail rotor was further away surely you would need less thrust to counteract the main rotor coupled with less TR drift, I am aware a heli would look pretty silly with an extra long boom but you guys seem to know what you are talking about.
Hope it has not been covered before

[Bravo73]

I am aware a heli would look pretty silly with an extra long boom

You would also need extra long hangars...! ;)

[Camp Freddie]

AW139 tail rotor is angled 11 degrees from vertical to provide lift to offset aft CG which is pronounced in this aircraft.

In north sea fit with 2 pilots and with help from tail tail rotor not much more than 1000 kg of fuel can be carried without ballast in cabin, bearing in mind that it can carry I think 1676 kg with aux tank !

CF

[Graviman]

h.i.d, not so sure i do! It is just a case of packaging the tail rotor to suit the main rotor diam (which is generally as large as practical). TR will become more efficient in forward flight, but in hover often uses MR downwash to improve efficiency and avoid TR vortex ring state (forward upward rotation diverts some MR downwash over the TR).

[BJC]

Hhmm, Graviman, I'm not sure I agree with anything you just posted. Perhaps I am misunderstanding you. The tail rotor doesn't become more efficient in forward flight, the vertical fin does though, which reduces the force required to be produced by the tail rotor, resulting in a reduction in pedal required. I was pretty sure that m/r downwash is also often a major cause of loss of tail rotor effectiveness in the hover (which can often be due to tail rotor VRS), which obviously doesn't improve its efficiency.

[puntosaurus]

Can't see why the tail rotor should be exempt from the same physics that give you translational lift from the main rotor, althought the fin effect is there also. A lot of the difficulties students have in learning transitions away are the onset of translational lift in the main and tail rotors at the same time giving a balloon and yaw left, followed by a fishtailing departure up in the avoid curve.

[Graviman]

BJC, i have no numbers, and can only go on what i've read elsewhere - perhaps i am in error. I gather that in many Sikorsky designs the MR downwash catches an advancing blade on the tail rotor. Prouty comments that vortex ring has affected some tail rotor designs in yaw against MR direction, and that bottom blade forward rotation helps. Translation effect must also affect TR.

I'm here to learn.

[BJC]

I'll freely admit that I'm no expert either, probably didn't help that my first helo aerodynamics lesson consisted of "it's all black magic, as long as you believe in magic the helo will keep flying". I know every helo design is different but I have seen wind tunnel videos of the 206 where the main rotor vortices impinge on the t/r essentially causing it to enter vrs, which I believe is the suspected cause of LTE in some wind azimuths in that series. So in that case it sure doesn't improve its efficiency. Since my background is in mostly Bells that has clouded my opinion since.
I can't wait until I get to work and Matthew can expain how my intrepretation of the laws of physics is all right or all wrong. I would love to have been there when the aero instructor used the black magic line on him (I think we had the same instructor for that) since he had the benefit of a physics degree! I was more then happy to accept the IP's description just as long as I still passed the exam and got my wings! Prouty has since cost me more then a few sleepless nights while I try to digest what he's saying.

[Graviman]

BJC, that's what i like about the heli community - there just are no putdowns. I suspect that's because the heli is the most versatile machine ever invented, so heli pilots always end up approachable. It makes this forum a great place to be.

Interesting about the Bell tail rotor. I could see that in flight slipping to port the downwash might increase flow against tail rotor outwash, which could potentially set up VRS. I'll keep an eye out for any info i see on that.

I studied aerodynamics, and came to the conclusion that anything beyond simple models are a black art. In the powertrains side of the auto industry i was exposed to CFD for various flows in and around engines, and realised that beyond simple predictions you couldn't second guess the computer. I'm guessing Matthew just got scared of the Schrodinger equation, and figured helis were easier. ;)

[Um... lifting...]

These were the azimuths we taught back in the good ol' TH-57 (B206). These are far better pictures than the ones I and my students drew on the white boards (except the one guy who was an art major... his were gorgeous).
While undergoing familiarization, the students had the dubious pleasure of doing 360 degree clearing turns after every landing (several hundred each) unless the winds were higher than I believe 15 knots. This was less for actually clearing around the helicopter than it was for improving low-level handling ability. I suspect I've orbited in that thing counter-clockwise (always counterclockwise) somewhere on the north side of 10,000 times. You get a lot of opportunity to observe the effects of wind azimuth on the tail rotor going round-and-round about a point in this environment, as you probably can imagine.
I found these azimuths to be quite accurate, and if one looks to see where the red regions of the various diagrams superimpose themselves on one another, one might suspect that the effects were additive in those regions. One would be correct.
The article from which I extracted these diagrams is available here:
http://www.dynamicflight.com/aerodynamics/loss_tail_eff/
http://www.dynamicflight.com/aerodynamics/loss_tail_eff/diskvortexint.gifhttp://www.dynamicflight.com/aerodynamics/loss_tail_eff/weathercock.gifhttp://www.dynamicflight.com/aerodynamics/loss_tail_eff/tailrotorvrs.gif

[[email protected]]

Given the choice, the best way for the TR to rotate is so that the advancing blade goes from the bottom up into the MR vortex rather than from the top down through it. The idea is that the TR blade sees the vortex and the downwash as a headwind rather than a tailwind thereby getting more Vsquared and thus more lift.

Tail rotors do gain translational lift and they also flapback - initially they become more efficient as IAS increases but eventually drag reduces this efficiency.

On models where the vertical stabiliser is designed to produce all the anti-torque thrust in the cruise, the net force is still lateral and TR drift exists in forward flight just as it does in the hover (inherent sideslip is, I believe, the correct name for it). Inherent sideslip exists regardless of whether you have a big vertical stab or not.

[IFMU]

The only information i can find on X2 development is this frustratingly slow update web vid:
Wow, was that a video of the X2 flying? Cool! I hadn't heard they had gotten that far.

Too bad they didn't show any hover, would have been interesting to see if it hovered left or right wheel low.

-- IFMU

[Phil77]

Um...lifting:
Remember the FAA's Rotorcraft Flying Manual?
These azimuth' are still beeing tought these days:
http://www.faa.gov/library/manuals/aircraft/media/faa-h-8083-21.pdf

I don't know who "adapted" his drawings from whom... but in the section about LTE the FAA uses pretty much the same pictures.
Not trying to be nitpicking, just found it curious how similar those drawings look?!

[Um... lifting...]

I'm pretty sure the diagrams in my Naval FTIs are identical as well, though I'm not at home to check them. My guess is that the FAA or some other agency, via calculated data from Bell or somebody came up with those diagrams. I would venture to guess they're at least 35 years old and the hand that drew them had no idea they'd still be kicking around today.

[Bravo73]

Wow, was that a video of the X2 flying? Cool! I hadn't heard they had gotten that far.


Nope. I'm afraid that it's only a CGI-rendered video. :8

[Graviman]

Interesting read, Um.. Lifting.

So the TR VRS is caused by high right yaw, which can be caused by MR vortex interference requiring potentially large pedal inputs. I'm starting to see the benefits of using yaw stability augmentation designed into the machine, to limit yaw rate.

IFMU, X2 is another internal Sikorsky project that looks set to fly sometime in 2008...

[[email protected]]

I think Nick might point out to you that those azimuth graphs for the 206 were simply a result of putting a 'big girl's blouse' of a TR on it instead of a proper manly one.

Any helicopter that struggles with a 15 kt crosswind should never have been certified in the first place.

[Choppie]

The R22 and R44 Raven II's tailrotor blades spin in opposite directions and you can feel a difference regarding stability in the hover.


Any helicopter that struggles with a 15 kt crosswind should never have been certified in the first place.


All helicopter can struggle with a 15kts wind if not used correctly. If you had to do a fast right pedal turn in a B206L the chances of getting LTE/VRS is quite big.

But what is the max wind for say the R22? I haven't looked through their manual for a while but I think is something like 30 or 40kts?

[Choppie]

where the main rotor vortices impinge on the t/r essentially causing it to enter vrs, which I believe is the suspected cause of LTE


Thanks BJC, I must remember that excuse next time I can't seem to hover properly. "I've got VRS in my TR due to MR vortices" :)

[[email protected]]

Choppie - the point is that a decent helicopter should not suffer from TR handling problems (much as I hate the R22, it seems to have a more than adequate TR). Bell invented LTE to cover up the fact that theirs was the only helo to suffer from it, simply because it wasn't man enough for the job.

[Choppie]

I do agree with you Crab, I've flown quite a bit on 206's and Squirrels and there is a huge difference between the two. I don't think I've ever run out of pedal on the Squirrel. But what I'm trying to say is that you can introduce LTE on any helicopter if you had to do the wrong maneuver at the wrong time.

[Bravo73]

But what I'm trying to say is that you can introduce LTE on any helicopter if you had to do the wrong maneuver at the wrong time.

Choppie,

Don't confuse LTE with LTA! The two are similar but quite different.

[Hippolite]

IFMU and Graviman

The Sikorsky X2 Technology Demonstrator is currently scheduled to fly late in 2007.

[[email protected]]

Choppie - as Bravo 73 says - getting to the pedal stops in extreme manoeuvres is usually Loss of Tailrotor Authority, LTA, (it is working fine but you can't apply any more pitch to it).

If you get to the stops doing normal manoeuvres then chances are you have Loss of Tailrotor Effectiveness,LTE, but you are unlikely to experience this in anything but a 206 - the TR is just not big or powerful enough for the aircraft.

[Graviman]

Great news, Hippolite. I'm definately a fan. :ok:

[BJC]

Crab, you're right:
"Any helicopter that struggles with a 15 kt crosswind should never have been certified in the first place."

Everyone knows that the dividing line between good and non-certifiable helicopters is at 17 kts :confused:

"A wind velocity of not less than 17 knots must be established in which the rotorcraft can be operated without loss of control on or near the ground in any maneuver appropriate to the type (such as crosswind takeoffs, sideward flight, and rearward flight)" (FAR 27.143)

I've flown at least two non-Bell turbines that were maxed out in 18-20 kt xwinds at their critical azimuths, and they were duly certified.

[rjsquirrel]

Crab and BJC,

I agree with your points about LTE vs LTA, but regret to inform you that an entire class of helicopters have been certified with near-zero crosswind capability. For Cat B operations, 9 passengers or less, Bell has managed to certify the 407, 412, 212 with "wind + 45 degrees from the nose" and NO demonstrated cross wind pedal margin!

Look in AC 29-2, para .143 to see how to pull off this trick. It is a real shame, and an invitation to LTE for unsuspecting people.

The crummy tail rotors are the Cat B limiting element, and basically, the new interpretation of FAR allows them to operate above the weight at which the tail rotor has 17 knots of capability. According to the FAA, you can operate to limit pedal (just touching the stops) while landing, as long as when you are against the stop, the yaw rate is in the direction of the pedal.

This trick buys them about 5% more payload. The aircraft in question have TWO hover charts, one for "wind from any azimuth" and one for "wind + 45 degrees from the nose". The LTE hover chart has more payload, of course.

No wonder EC is eating their lunch.

[BJC]

Interesting, here's a key phrase:
"Rotorcraft certificated prior to Amendment 29-24 can update their certification basis to take advantage of this provision."
I imagine therefore that Bell won't be the only company to take advantage of this as FAR ammendments are generally accepted at face value by other certification authorities. No one willing to moan that Sikorsky or MD are also taking advantage of this, or can MD not play because they don't have tail rotors? This can only be good news for companies as they get increased capacity because they know that pilots are always able to land with the wind on the nose, right? :uhoh:

[NickLappos]

Interesting points raised!

BJC, I can assure you that Sikorsky has no interest in raising the MGW to swamp the tail rotor. In fact, the hover performance at max engine/transmission power leaves enough tail rotor thrust to achieve 35 to 50 knots sideward flight speed in the entire current Sikorsky fleet.

[BJC]

NickLappos,
Nice to see an OEM who doesn't design to the minimum FAR requirements for controllability! So, are you saying that since pedal margin wasn't limited at 17 kts that Sikorsky is unable to take advantage of this loop hole because the aircraft are engine performance limited instead?
No where did I see that smilie that shows a guy baiting a fishing hook and casting it....
All kidding aside (and because I don't want to get pounced by the Sikorsky fan club members in the forum) what an insane AC. I first got bit by this in a civil certified helicopter (I'm a military pilot) when I landed into wind and then tried to turn xwind to taxi to the ramp. Quite a surprise to encounter LTA in a brand new helicopter model in only moderate winds, even more surprising was it happening in proximity to other aircraft. I wish an instructor some where had pointed out the significance of this cert requirement before I had the opportunity to scare myself. I don't count encountering it in a 206/OH-58 as a surprise because you know it is coming...

[NickLappos]

BJC,

The TR design is made to meet the needs of the mission/customer. For the S-76, we chose 35 knots of side flight with 10% remaining pedal travel, built a TR that could do that and met that handily. At 10,300 lbs, the S76 can go 51 knots sideways, at 11,700, it can go 35 knots.

For the Black Hawk, the design point was 45 knots sideward at 16,700 lbs at 4000 feet and 95 degrees F (7300' DA). That gives almost 60 knots sideways at sea level (I used to use the Doppler at 105Km/Hr to prove the point!)

The idea is to design each system to do the mission, and try to not use a flight manual caution as a safety device.

[rjsquirrel]

So, Nick, what you are saying is that that wind/main rotor wash diagram is really what happens on any helicopter, but only marginal ones lose control as a result?

[BJC]

I feel a little better when I read Prouty saying repeatedly "for reasons I do not understand" when he is describing tail rotor vortices and LTE. A good description of the phenomenon (in his words) is at this link:
http://www.aviationtoday.com/rw/issue/departments/engineering/13637.html
NickLappos, too bad using a "flight manual caution as a safety device" is actually an accepted practice though. You probably never had to argue that point as a test pilot with management though :)
If you can honestly say that wasn't an issue then working in West Palm Beach just became my dream post-military flying job!

[FH1100 Pilot]

Prouty's repeated "for reasons I don't understand" in that R&W article is both amusing and enlightening. If the acknowledged guru of aerodynamics doesn't understand something, how are we mere mortals going to make heads or tails of it?

My own opinion is that the airflow interaction between the main and tail rotors is so chaotic as to be completely unpredictable in an absolute sense for every phase of flight every time. Witness Prouty's report of the Cobra performing a maneuver that the Apache could not (if I'm reading that passage correctly - it was worded strangely).

I have had a few pilots of Bells tell me that they prefer to land with right crosswinds as opposed to left because of their fear of LTE. They base this on the three wind diagrams "Umm...lifting" posted above in his post. I think it's funny. If you look at the first and third diagram in that post and sure enough, looks like left cross-winds can be dangerous!

But looking more closely, the real "danger" comes at wind speeds of above ten knots. I don't know about the rest of you, but in my experience with the 206, weathercock stability trumps tail rotor power. With a 10- or 15-knot left crosswind, the pilot may find himself with substantial right pedal applied. No way is the nose going to come around further to the right - at least, not until you get into a situation like Diagram #2 (which I don't think is *quite* that wide, but oh well). With a direct left crosswind, you don't have to worry about a spinning-wildly, out-of-control LTE situation. Keep full left pedal in and it probably wouldn't even go around one full turn, settling down into the wind. Still, it "concerns" a lot of pilots I've spoken with because of those dang diagrams.

If those diagrams are true, they're true for all helicopters, not just 206's.

For sure, left crosswinds in Bells and such make you work harder, dancing on the pedals as the t/r momentarily goes in and out of vortex ring state. The BO105 is particularly awful in this regard, and its t/r is way up high! Seems to me that in these conditions the airflow into the t/r is "confused" (would Prouty approve of that aerodynamical term?).

And finally, let me admit that all of my 206 experience (and I've got quite a bit) is at sea level or close to it. I've flown 206's with big and little tail rotors, and cannot recall a specific instance when I ran out of left pedal (touched the stop). It may have happened (the memory goes with age), but it was evidently a non-event. I've never spun, never had that edge-of-the-cliff feeling of impending disaster. Perhaps the JR suffers more at high altitudes. But all of these sky-is-falling people who condemn the 206 and claim that it has a "weak" or "insufficient" tail rotor must either never have flown one or are doing stranger things than this charter line dog has been doing for the last 25 years.

(Nick is quite vocal in his criticisms of the 206. He has, what, only about 6-or-7,000 r/w hours total? And I wonder how much of that time is actually in 206's? My logbook shows 6,600 hours of 206 time alone, increasing every day. Not that that makes me a Prouty - only that I have a lot of make/model time and I haven't died in an LTE accident. Yet. For reasons I don't understand.)

And finally, let me talk about the FH1100: Pilot friend of mine and I were out in one recently in a "fairly strong" wind (15-20 knots) and we could do nothing but hover into the wind. No left, no right, no nothing but straight or nearly so. Admittedly, with its Huey-like tailboom and fin the 1100 has more weathercock stability than a 206. Even so, I was surprised that we could not bring the nose around to the right without getting to full right pedal before getting much past 45 degrees. Odd.

[MightyGem]

Prouty's repeated "for reasons I don't understand"
I remember talking to a rotary wing test pilot at Boscombe Down, many years ago, and he said that there were things about rotary POF that happened, but they didn't really know why.

[Matthew Parsons]

I haven't been reading this thread for a while. It's taken an interesting turn.

Could someone help me out with what the argument actually is? Are we arguing if the 206 is a bad helicopter, if the 206 has a bad tail rotor, if LTE is a valid term, or is this something completely different.

In anticipation, here are my responses to the first three

- 206 might not be as competitive now, but it is definitely one of the more successful helicopters.

- Since the 206 is a successful helicopter, its hard to say it has an inherent fault. Sure a relatively low crosswind limitation is not desirable, but as NickL said, "The idea is to design each system to do the mission...", and the 206 has been able to do its job quite well, with the questionable tail rotor.

- LTE a valid term? Whatever you call it, it happens. If you don't give it a name someone else will. If the explanation of why things happen is wrong, that's bad. If you want to use an easy term to help pilots keep important points in their heads, I think thats a good idea.


Matthew.

[delta3]

From Prouty

"A model helicopter mounted on a turntable in a low-speed wind tunnel permitted simulation of flight at all wind azimuths. The tail rotor was mounted on a separate support to investigate its performance at several positions with respect to the main rotor. Mounted far behind the main rotor, it suffered a thrust loss in left sideward flight due to the vortex ring state. But a position close to the main rotor produced no thrust loss — for reasons I do not understand."


Just as it is necessary to escape VRS in the main rotor by schedding it away through forward flight, the MR downwash provides for enough translational flow relative to the TR to reduce TR-VRS significantly.

delta3

[NickLappos]

The MR wash into the TR has nothing to do with TR Vortex Ring State, they occur at vastly different conditions.

The MR wash into the TR does cost about 5% of TR thrust, which is a blip on the HQ diagram for a helo with a right-sized TR and a disaster for a helo with a marginal TR. The issue is not the mystery of what happens, and "If I cant explain it, I can't control it." The issue is how the designer builds enough moxie in the design to handle the predictable effects that the real world provides. Like jack stall, LTE is predictable and preventable.

Evidence the fact that the OH-58D had LTE when the US Army first tested it, and they walked away from the aircraft. A few weeks later, a bigger TR was fitted, and the OH-58D has not had an LTE event since.

Mysteries like this are like picking your ears with a hammer and nail, if you are confounded by the blood, maybe the fault is not random circumstance.

For the record, IMHO the B-206 is a classic helo, and rightfully goes down in history as a great one. Like the S-61 however, just because it was great does not mean the waterline for safety is permanently set at the level that we have had to live with. If we mistakenly agree that the weaknesses of yesterday's helos is all that we ever need in future helos, we condemn all future pilots and passengers to suffer that way our fathers did. And for that we should be roasted slowly in hell.

[delta3]

Nick,

The point that I am making is that the MR downwash creates a 'benificiary' flow in the plane paralell to the TR. This benificiary flow has a positive effect on reducing the chance of a TR-VRS.
In the case of a light american style heli (I am referring to calculations of induced flows we made in a thread quite a long time ago) VRS will happen in the 10-15 knts wind from the left range, certainly if -as Prouty states- the disk is far away from the main rotor so that we get no interference and hence a 'clean' vortex.
The Mean Rotor down wash, washes that vortex away just in the same way as forward speed washes a MR vortex ring away. As you stated over and over again, even with small forward speed in case of the MR VRS the danger zone goes away. Well similarly even small components of beneficial MR downwash make the TR-VRS danger zone go away.

My point has -as far as I am concerned- no bearing on the question of sizing the B-206 tail rotor. Just as a MR-VRS has no bearing on lack or not of MR and engine power, the TR-VRS has no bearing on the power capability, it just depends on induced velocities. TR-power of course is needed in other TR-operating regimes.

delta3

[NickLappos]

delta,

If the TR gets to VRS at 15 knots, it means that the TR passes through the zero thrust point at this paltry speed. In other words, the helo needs no TR thrust because the weathercock stability is so strong at 15 knots that the fin provides all the anti-torque. I find this hard to believe.

Most TRs that I have analyzed reach the VRS point at 25 knots or more (30 to 35 knots for the S76, for instance.)

Also, it is my opinion that LTE is not achieved when the helo is at 15 knots, because at 15 knots the MR torque is low, since the MR is near/above ETL. LTE occurs when the MR torque comes rushing in as the speed decays to near zero. The concept of LTE as a mystery and caused by wondrous aerodynamic effects is a necessary component of the myth that single rotor helos can all get LTE, and the accompanying myth that we can't design to get away from LTE. Both myths are simply wrong.

I have a 1970 Russian helo aerodynamics text that shows the 5% decay in TR thrust available when the MR wake finds the TR. No big mystery, IMHO. Also, let me state that any TR that runs out of poop at 17 knots IGE will cause LTE if the helo is maneuvered near OGE under the same conditions, since MR torque is the biggest cause of LTE, because when MR torque is high, the TR thrust is fully consumed.

[delta3]

Nick,

All depends on the loading of the heli and the size of the fin.
As you know I did all calculating experimenting with a R44 I and II.

Very light it requires little tail rotor, so the danger zone is in the low end.
Heavier it will require more tail rotor so the induced speed zone goes up in the ranges you indicate. (I did not check the precise speed here and now, and wish not to make a point of discord of that, if I have time I could dig into the archives, but that is not the point I am trying to make)

It now can look synical but a small tail rotor will have TR-VRS in a higher speed zone because its induced velocity will be higher (higher disk loading), so the under-powered B-206 will encounter it at higher left wind speeds....

There is by the way a big difference in my personal experience between R44-I and II MR-VRS risks. A light R44-II with the bigger MR is much more prone to MR-VRS because of the low disk loading. I would say 100% (my statistic, never had one in the R44-I but I had many in R44-II, so I always keep positive vario in that case)


d3

[Phil77]

Nick,
you said:


The TR design is made to meet the needs of the mission/customer. For the S-76, we chose 35 knots of side flight with 10% remaining pedal travel, built a TR that could do that and met that handily. At 10,300 lbs, the S76 can go 51 knots sideways, at 11,700, it can go 35 knots.


and then later:


Most TRs that I have analyzed reach the VRS point at 25 knots or more (30 to 35 knots for the S76, for instance.)


how does that work?
you would need a whole lot of negative thrust I presume but in order to attempt that exercise (the sideways flight) you would have to transition through VRS somehow?

Philip

[NickLappos]

Phil,
Yes, in racing sideward to the left at terrific speeds, you pass through VRS and then go to a windmill brake state on the TR (it is actually autorotating!) It is obviously where the TR downwash equals the sideward speed, in an S76 that is about 30 to 35 knots, in an Agusta 109 it is about 25 knots.

Pilots often see this in big winds in a pedal turn, when they experience a big pedal shift, usually when the aircraft behaves like it is jumpy and nervous in yaw.

delta3,
Sounds like you are doing very nicely with the simulation. Thanks for the insights!
Nick