Tail Rotors

[Speedbird252]

G`day all. I was watching a rather interesting programme last night on the Super Puma, and I noticed that the tail rotor was on the starboard side of the tail boom. I have noticed this on other helo`s, but the port side seems to be the more common.

My question, is there any design or performance reasons in having the rotor one side or the other? I assume that power induced yaw would be opposite to that produced if the rotor was the other side, but thats as far as my fixed wing brain will go.

And an IFR approach into a Oil Platform Heli Pad? I dont know what you guys earn but it aint enough.....

Regards,

Speedy

:confused:

[PPRUNE FAN#1]

Speedbird, there rages a controversy about which side of the airframe to mount the tail rotor. A noted helicopter authority (and renowned teetotaler) Sir Nicholas of Lappos maintains that it doesn't matter as long as the tail rotor itself is stout enough.

Realize though, that a propellor can either push or pull, (see: Cessna 337) depending on which way you've aligned those vane type thingees that go 'round and 'round. Torque from the helicopter's engines will always be causing the fuselage to turn in the same direction (depending on which way the main rotor turns). In the case of the Puma, the fuselage will always desire to rotate nose-left/tail-right. Which means that the tail rotor must always produce a force to counteract that. Whether the tail rotor pulls the tail or pushes the tail matters not, really.

Some will say that a so-called "tractor" type (puller) of tail rotor is less powerful or effective than a "pusher" or vice-versa, blah blah blah. The Bell UH-1 started off with the tail rotor on one side, but evolution occured and it migrated over to the other as time went on, sort of like a flounder's eyes.

But there's more to it than that. The horizontal cyclone produced by the tail rotor interacts and is affected by the downwash from the main rotor. This interaction is different in forward flight from hovering. Obviously, hovering is where you'd like the tail rotor efficiency to be optimized, as the fuselage will more or less streamline in cruise with the help of whatever vertical fin surfaces are back there.

Engineers may find that it's not so much a case of which side of the aircraft the tail rotor is on, but which way the rotor (and its resulting airflow) rotates, like the boys at Enstrom did. It's complicated. But since the tail rotor driveshaft is always going to spin in the same direction, the easiest and cheapest way to change the direction of t/r rotation is to simply flop it over from one side of the fuselage to the other.

Clear as mud? I thought so. For an explanation more in-depth than that, you'd need to speak to an aerodynamacist. And I ain't one.

[Thomas coupling]

Type in "tail rotor direction" in the search facility:
12th september entry should tell you all.

[GLSNightPilot]

And an IFR approach into a Oil Platform Heli Pad? I dont know what you guys earn but it aint enough.....

Please tell that to my employer. :(

Actually, it isn't that bad. The weather minimums are only 200'-3/4 mi. At night it's only 300'-1mi. We obviously don't deserve much compensation for that. ;)

[Nick Lappos]

Some thoughts:

The effects of tractor/trailor, direction of rotation, and wind into tail rotor all limit the maximum thrust that a given tail rotor can produce. All these factors serve as excuses for design teams once the tail rotor is determined to be marginal, none are reasons for it to be marginal, because it could/should have been made large enough to produce the required thrust, regardless of its details. That is why Pprune Fan #1 said, "A noted helicopter authority (and renowned teetotaler) Sir Nicholas of Lappos maintains that it doesn't matter as long as the tail rotor itself is stout enough." Saying that a tail rotor has poor performance because it (insert one-) is on the left/right, rotates the wrong way, has main wake impingement) is like saying after your ear was bleeding because you picked it with a nail, "I hate when that happens!" In other words, no matter what the basic design features, it could have the required thrust, if you built up some other feature(s). If the direction of rototion will rob some thrust, just make the rotor a bit larger, or have it spin a bit faster, or build in some more pitch range.

A tail rotor that blows its high velocity wind onto a close surface is slightly less efficient that one that blows away from the fin.

A tail rotor that spins so its bottom blade moves toward the main rotor is slightly more powerful than otherwise.

A tail rotor that receives some main rotor wake during high thrust operations will produce slightly less thrust than otherwise.

A tail rotor that is too small will become inadequate if any of the above are also true. It could have been made larger, and we would not discuss how any of the above was a factor, because only when it is too marginal to begin with do we discover the small aerodynamic effect that drives it over the edge.

For this reason, LTE is the product of inadequate tail thrust, not a mysterious aerodynamic problem that swamps healthy tail rotors. That is why it affects only one type of helicopter.

And last, but not least, I am not a tea-totaler, in spite of SASless's misdirected assertions otherwise. Should any Ppruner come past Stratford CT, I would be glad to conduct a demonstration to prove this fact (at the expense of the doubter, of course!):D

[PPRUNE FAN#1]

Call the Enquirer! Hold Page Six! Rumour debunked!

Famed former boy-band member Nicky Lappos strongly protested the characterization of him as a "tea-totaler," and challenges any other doubting Thomas coupling or he-who-is-without-SAS to a drinking contest worthy of a college frat party in some bourg called Stratford in the (city? county?) of New Connecticut, wherever that is - (must be some American "Spring Break" town of ill-repute with drunken helo pilots in every gutter - oops, that would be New Orleans).

"I am not a tea-totaler!" Lappos raged, sounding as if he'd already consumed many pints, "...in spite of SASless's misdirected assertions otherwise. Should any Ppruner come past Stratford CT, I would be glad to conduct a demonstration to prove this fact.

Eek! Jolly good show that SASless didn't make a disparaging comment on Mssr. Lappos' manhood! Lord only knows the challenge that might have been issued forth. But he didn't, thankfully, so you Ppruner's who with to travel to this "Stratford" need only bring your favorite beer glass, not a magnifying one. ;)


Ahh but seriously, I do have a question with regard to tail rotors and vertical fins. Nick sez:
A tail rotor that blows its high velocity wind onto a close surface is slightly less efficient that one that blows away from the fin.

I don't get this. Isn't it the same thing? The tail rotor doesn't magically create air. That high velocity wind has to originate somewhere, i.e. upstream of the vertical fin.

I would imagine that a big vertical fin (see: UH-1, BH206, A-109) would impinge on both the outflow and the "inflow" of a tail rotor, depending on which side the vertical fin was mounted on, no? People act like the mere act of flopping the tail rotor on the Huey made the vertical fin disappear. Did it (aerodynamically)? How come a big vertical fin that blocks the entry of air "into" the tail rotor does not also reduce its efficiency?

Did I miss this part in Helicopter Aerodynamics 101?

[John Bicker]

Does drag still increase with the square of the speed given the different velocities?

I think the TR performance increase on the UH1 was a little more to it than just "flopped" to the other side. I think this never occurred on any single engined UH-1 production models. The UH1-N would be the only one. It was done on the 205 when it became an A1 with the 212 TR. Little bit more span and chord involved here. Like about 35-40% bigger chordwise. I think that would account for most of the benefit received.

Yes removing large chunks of fin/pylon helps enormously. S76A's versus B's and C's. A109A and C versus K2 and E.

[Nick Lappos]

Pprune Fan#1:

You seem to enjoy your own words so very much, look to them for the answers. I give up.

Nick

[Speedbird252]

Cheers for the replies guys, and thanks Nick and pprunefan#1 for the explanations, saw Nick in the Commanche the other night, slick bit of kit or what......

And GLSNightPilot, yeh I saw the viz improved once you had transitioned thru the gloom, I was a bit more worried about the quoted wind speeds, in excess of 60knts over the deck? Can that be correct?

If I put down full flap in the kind of headwind id hover.......

Now thats got me thinkin`.....


Thanks again.

Speedy.

:cool: ;)

[Dave Jackson]

PRUNE FAN #1

The streamtube velocity of the air after the rotor is faster than its velocity was before the rotor. A vertical fin, which is located on the discharge side of the tail-rotor will be subjected to a higher force and therefor will be more restrictive to the actions of the tail-rotor.

A Fenestron significantly reduces this impediment, and of course it goes without saying :), exclusion of the tail-rotor totally eliminates these problems.

Dave J.

[PPRUNE FAN#1]

Dave Jackson:
The streamtube velocity of the air after the rotor is faster than its velocity was before the rotor. A vertical fin, which is located on the discharge side of the tail-rotor will be subjected to a higher force and therefor will be more restrictive to the actions of the tail-rotor.

Thank you, Dave! Glad to see there's at least one gentleman among you lot. Learn something new every day.

[GLSNightPilot]

Speedy, I've never flown in the North Sea & didn't see the program, so I don't know about the windspeeds, but 60 kts certainly isn't unknown offshore, there or over here. As for the minima, I was being a little sarcastic, but I don't know the emoticon for that. ;)

[Arm out the window]

Emoticon? Is that the term for those little smiley or angry faces you can put in your posts?
Sounds more like a science fiction torture device:
"Alright Zorg, if you won't talk, you're going straight to the Emoticon!!"

Tail rotors, very handy, wouldn't be without it.

I hope I've made a useful contribution to the debate.

[Thomas coupling]

:D

[john du'pruyting]

Remember. you don't need a tail rotor, try an MD 902, the aircraft that uses a kazoo for anti torque control:)

[GLSNightPilot]

j d'p, splain me sumthin'. Never having been to the northern parts of the Empire, I'm ignorant, I suppose. But what are 'handysnaks'? :p

[john du'pruyting]

They are something you eat when you're drinking from shandykans:D

[BrianG]

Nicky Lappos - Forgive my fixed-wing ignorance (I will change when the bank manager agrees), but which is the one type of helicopter that is affected by LTE.

Speedybird - if you were flying a Slepcev Storch you wouldn't need anything like 60kts wind to hover.

[Cron]

Let me see, have I got this right? Teetotal means tail rotor on right and vice versa?

[Speedbird252]

Hey BrianG,

Wanna tell me what a Slepcev Storch is?

Sounds like wrestling move....


Speedybird

[The Nr Fairy]

BrianG:

Bell 206.

Speedy :

Slepcev Storch is an Australian's copy of the German WWII Fiesler Storch.

[BrianG]

Speedy,

NR is correct on the Storch. The link for the Storch site is:

http://www.nor.com.au/business/storch/

The current issue of Australian Flying magazine ( which has a rotor section and has an article on mountain flying in helos) reports that the Qantas (or former Ansett, I can't remember) captain who holds a number of records for a round the world flight he did in a Lancair iV he built has sold the Lancair and is currently building a Storch. Talk about opposites!

NR,

Thanks for the info.

[Nick Lappos]

Brian G,
I did a study about 4 years ago, and found that fully 95% of all "LTE" incidents in two different data bases happened to Bell helicopters, and 90% were Bell 206's. I usually raise irate posts from loyal Bell guys, but the data speaks for itself (and with over 1500 hours in Bells, 1000 in combat, I refuse to be labeled an anti-Bell guy).

It is my ernest belief that an adequate tail rotor cannot be forced into "LTE" in any normal flight conditions. The whole idea of LTE masks the true cause - weak tail rotors- and makes it seem that all single rotor helos can suddenly get consumed with the LTE disease and start spinning.

What consumes tail rotor anti-torque capability and makes LTE occur is when main torque is raised too high during otherwise marginal maneuvers. For example, this is when you terminate an approach and raise the collective too high.

Most helos are tested in level, sideward flight to higher and higher speeds, until the tail rotor hits its pedal stops. I believe a more effective way to judge adequacy is to trim to a sideward speed and then command some transient torque increase (a brief collective pitch rise) to see if adequate margin exists.

[Capn Notarious]

"Remember. you don't need a tail rotor, try an MD 902, the aircraft that uses a kazoo for anti torque control."



SO HERE IS MY SILLY QUESTION, THAT SHOULD AMUSE THE PPRUNER'S

Does a German ADAC Explorer kazoo Beethoven
A U/S Coastguard Explorer kazoo 'The stars and Stripes'
Ecetera Ecetera
And if they; the Explorers: do perform music accordding to the country of operation and residence.
Does this mean the Mesa boys&girls make the most musical helicopter?

Answers to [email protected][/email]::D

[CyclicRick]

MD902 a Kazoo? I thought it was a hair dryer!
You don't really need a main rotor either, look at Mr.Kaman, he put two tail rotors on top and bob's your uncle!

Nick: Your'e right about 206 tail rotors, very weak, especially on the L's

[low n' slow]

Hi all!

Something on the "Blackhawk" just caught my interest:

Why is the tailrotor ofset from vertical (so that the slipstream is pointed slightly downward)?

I'm only guessing here, but is it to lift the tail because of CG being aft of the main rotor shaft? Does this make things more complicated when you fly regarding alterations in rotor speed or power?

regards/lns

[Dave Jackson]

It's a tentative move by Sikorsky, toward twin rotors and away from tail rotors. :eek: ;)

[rotorboy]

The canted TR provides lift. I believe the figure is 3% of total lift.

Is that correct Nick?

RB