PPRuNe Forums - Tail rotor turning direction

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Tail rotor turning direction

Hi everyone.

In the most of the types, the tail rotor turns in such a way that the blades closer to the main rotor downwash go upwards, slashing into it. This is true for the most of the helos and makes sense to me specially in forward flight as of 20-30 kt where the tail rotor disc should get more effective as it is washed by the main rotor. Just like with ETL, where the main rotor becomes more efficient as the advancing blades slash into the air.

I was wondering why in a few types the tail rotor turns in such a way that the blades closer to the induced flow go downwards (thus escaping from it) like in Robinsons, S300, MD500, OH-23 and a very few more.

Thank you!

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Robinsons, S300, MD500, OH-23 and a very few more.

Designed in 50s, 60s and 70s.

I think they got smarter in more recent times. Even Bell designed the flip-flop tail mod for the 50s Huey, moving it from downwards on the left side to upwards on the right side.

The Lynx tail rotor direction was reversed on later models, quoted reason was to reduce vibration. The “top” blade was chopping forwards on earlier models but later versions had it moving backwards.

Yup, pretty much everyone figured out tail rotors should kick rocks into the fuselage... at least that's how I remember it. I'm sure the more correct answer is the advancing blade should be in cleaner air and not advancing into the wake of the main rotor in forward flight.

If I recall correctly, the R22's tailrotor spins in the "less effective direction" because the gearbox needed to spin it the other way would have been too big and heavy. The R44 on the other hand, does spin the other way.

The tail rotor is more efficient if it is turning in the opposite direction of the main rotor tip vortex. The earlier designs did not take that into effect. Bell flopped the T/R on the 212/UH-1N so they would not have to redesign the gear box to rotate the "correct" way. The Bell aircraft which were designed from the mid-sixties on (206, 222, 407, etc) had gearboxes that turned the proper direction. The AH-1G had an MWO that flopped the T/R with mods starting in 1970.

After many years in service, CFS (H) at Tern Hill 'discovered that it was possible to fit the Whirlwind tail rotor the wrong way round ! :eek: Subsequent preflight was to include checking that the lower t/r blade 'matched' the aerodynamic profile of the tail pylon. ... and, yes, the rotation was into the main rotor.

As in “retreating” blade at the top.

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it was possible to fit the Whirlwind tail rotor the wrong way round

And on the Huey, one reportedly flew for some 70 hours with comments of high freq vibes before an alert lad walking down the flight line noted that something didn't look quite right when compared to the other aircraft in the line.

Seen a Schweizer nee Hughes 300 with the tail rotor blades on backwards, 300 "B" blades an a 300 "C", H500 "C" with a "D pitch link opposite the correct "C" version (different length) - nobody mentioned anything undue from the "cyclic attendant" department although the 500 sounded in bit funny when it turned up..............

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Originally Posted by Robbiee (Post 11238681)

Yes, Tim Tucker mentions this during the Robinson safety course, along with the length limitations on the first R22 tail rotor blades - they couldn't be any longer than the width of Frank's kitchen oven!

As Ascend Charlie had made reference to a “flip flop tail rotor” on the Huey / Bell 205 in the thread regarding the recent ditching of Bell 204B VH-EQW (though not necessarily related to the ditching), it made me aware of the various Huey tail rotor configurations which I hadn't been aware of before. Rotorheads has at least two threads on this topic, including this one. Info posted on the web and in at least one text book indicates tail rotors rotating aft over the top provide better control authority and require somewhat less power.

It is interesting to see that on the UH-1Y Venom (and AH-1Z with its common drivetrain) Bell has reverted to a “Huey” with tail rotor on left side but now turning aft over the top. Perhaps this is the optimum overall configuration where the tail rotor wake doesn’t impinge on the vertical stabiliser?

Puller vs pusher TR is an old design conundrum, you can have clear airflow into or out of the TR but not both. Unless you opt for a Fenestron

H-1's tail rotor swap to the left side.

Helispotter,

The swap of the tail rotor to the left side involved the maximum clearance between the T/R blades and tailboom during maximum flapping. The right-hand side minimised this clearance while the left-hand side maximised this clearance. The aft-over-the-top direction should be described as up-into-main-rotor-downwash (same direction, actual reason for direction of rotation) for additional lift.
Ott

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Originally Posted by Otterotor (Post 11511988)

Otterotor: Are you saying the flapping consideration is what motivated a swap 'back' to left-hand side tail rotor on the UH-1Y? If so, does that relate specifically to its 4 blade tail rotor? On earlier 2-blade Hueys the flapping clearance would presumably have been same whether tail rotor was on left or right side?

I had read in one discussion that HU-1 / UH-1 tail rotor changed from right to left and back to right over its evolution, but I haven't so far found any photographic or drawing evidence of an initial right hand side placement. Anyone know more?

As for terminology for tail rotor turning direction, I noticed several ways to describe the same thing (including up into main rotor wash) but created yet another description! Twin screw ship propellers are described as being either "inwards over the top" or "outwards over the top" direction of rotation which is fairly unambiguous.

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Originally Posted by helispotter (Post 11512000)

Ray Prouty's books are a good start - yes it makes a difference empirically, but he was struggling to say exactly why, but had some fair "assumptions". Met the man a couple of times and always great conversation and extremely humble. He had time for anyone regardless.

ref: the Bell 205A LHS like a UH-1 to 205A1 RHS those were just 212 parts retrofitted.

An oldie but a very goodie from the esteemed Mr. Prouty on the topic.

Sunday, July 1, 2007
Ask Ray Prouty: Tail-Rotor Vortex Ring State

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READER JEFF FOZARD RECOUNTS uncommanded right turns at low speed ("Loss of Tail-Rotor Effectiveness," May
2007, page 9). The cause was probably a tail rotor in vortex ring state, a condition we normally associate with the
main rotor.

When a smoker blows a smoke ring, it propels itself with a constant velocity proportional to the strength of its
"circulation." A hovering rotor makes its own rings (tip vortices) that propel themselves down in the same way.
Their circulation strength can be related to disc loading.

When the pilot descends vertically by reducing collective and using partial power, his descent speed can approach
the vortices’s self-produced velocity. This happens at about 800 fpm for the Robinson Helicopter R22, with a disc
loading of 2.5 psf, and about 2,000 fpm for the Sikorsky Aircraft CH-53E (disc loading=14).
Since they are descending at the same rates, the vortices and helicopter get entangled. Erratic angles of attack at
the blade elements result, causing lift changes and buffeting.

More importantly, average thrust decreases 20-30 percent. The thrust loss is not due to high angles of attack
(which might cause stall), but low angles of attack caused by vortices in the rotor’s plane that create downflow
through the rotor disc. They induce surrounding air to be re-ingested into the disc.

This is an unstable situation. If the descent rate increases, thrust falls off even more and the helicopter descends
even faster. Increasing collective is ineffective; the tip vortices get even stronger, increasing the downflow and
down you go!

The cure, of course, is getting forward speed to leave the vortices’s influence behind. You need a descent angle of
about 30 deg. to fully succeed. Don’t do this and you continue down vertically, soon outrun the vortices, and enter a
stable region leading to vertical autorotation — at about 2,800 fpm for the R22 and a hair-raising 6,500 for the CH-
53E.

It should come as no surprise that a tail rotor can also get into vortex ring state. Depending on its disc loading, the
critical velocity is 15-30 kt as generated in a right hover turn or by left sideward flight (on American helicopters, of
course).

The U.S. Army’s desire to hover over a spot in a 35-kt wind and maintain good control while pointing in any
direction produced great interest in this phenomenon, with some surprising results.

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.

It appears the primary benefit of the main rotor proximity occurs when the tail rotor rotates so the bottom blade is
going forward. The tail rotor on Lockheed’s AH-56 Cheyenne compound helicopter rotated the other way. It was
impossible to maintain steady left sideward flight at 15 – 20 kt. This was cured dramatically when a gearbox
redesign reversed the direction of rotation — for reasons I do not understand.

You can see evidence of similar fixes on other helicopters. Bell Helicopter mounted the tail rotors on the left side of
the single-engine UH-1 and AH-1; they turn the "wrong" way. This led to some piloting problems in low-speed
maneuvers. Bell fixed this on the twin-engine versions by switching the tail rotor to the other side, reversing its
direction of rotation. (You can see the same fix on the Mil Hind. Compare photos of the prototype with the production version.)
New H-1s Bell is developing for the U.S. Marine Corps have tail rotors again on the left, which
is aerodynamically the best position. A gearbox change has them turn in the correct direction.

A number of helicopters have the "wrong" tail-rotor rotation, including the Hughes OH-6 and its descendants.
Apparently, there were not enough complaints like Mr. Fozard’s to justify a redesign. The R22 has the same. Frank
has told me he saved a few ounces and moved the center of gravity ahead a bit with this choice. The R44 has the
correct rotation.

When the Hughes Apache was designed, all this was known. The tail rotor spun in the correct way and just a few
inches separated the rotors. Did that preclude problems? No!

The Apache was tested by flying in formation with a pace car. In left sideward flight at about 15 kt, it became
increasingly difficult to maintain heading. Pedal activity was nearly stop to stop as the test pilot tried to compensate
for erratic tail-rotor thrust. A recent Army veteran current in the Cobra (the chase aircraft), he got into that, with its
wrong-way tail rotor, to repeat the tests and just sailed through-for reasons I do not understand.

This Apache prototype had its horizontal stabilator atop of the vertical tail. This "T" proved aerodynamically and
dynamically unsatisfactory; the configuration was changed to that of the Sikorsky Black Hawk. To properly clear the
stabilator, the tail rotor had to be raised 35 in., putting it closer to the main rotor. This completely solved the
problem-for reasons I do not understand.

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Originally Posted by LRP (Post 11238682)

This is the best explanation for it. The swirl from the main rotor tips rotates in the same direction as the tail rotor thus effectively stalling all the blades at once but only briefly hence the temporary unpredictable power output of the tail rotor and the pilot sees the yaw stability drop where the tip vortices get ingested. It would have been great to have heard what Prouty thought of this but it seems very logical to me. And the rotation direction is just as critical on fenestron as conventional. (The only one that goes the wrong way is Gazelle, every othe fenestron goes bottom forward).

Now if your interest in rotary aerodynamics are piqued, explain why some tail rotors have negative delta 3 hinges.....(increase in pitch with flap)