Rotation direction of Tail Rotors: merged threads

[CyclicRick]

Hello all,

Here's a nice one:

If you stand infront of the tail rotor of most helicopters it turns in a clockwise direction ( the blade at the bottom moving towards the nose of the helicopter).

Why?

I talked to our magical chief engineer about this and we speculated on a number of theories.

1: Simple Design feature
2: Aerodynamic reasons
3: Centifugal force reasons (gyroscopic procession)
4: That the way the gearbox was made by the manufacturer!

Any body know why? or is it just a design fluke as in 1:

Going further on that theme. Why do some helicopters have left turning rotors and some right? Is there an advantage in one or the other?

[Irlandés]

In one of the books I've read, it cites studies into noise and tail rotor effectiveness as being determining factors in deciding the direction of rotation (clockwise being the preferred). And some designs of helicopter have actually had changes in direction and boom side in the course of their lives. Also interesting to note that the R22 is anti-clockwise but the R44 is clockwise (looking from the left hand side of course).

Irlandés

[GLSNightPilot]

IIRC, Prouty had a discussion of this in his column in Rotor & Wing some time back.

As for the main rotor, it seems to be just tradition. Sikorsky, in the very early models, had some one way & some the other, depending I surmise on the simplest engineering on that model. Having flown both, I can say it's a pain to switch from one to the other. Hovering is no problem, but when you turn final, drop the power to descend while rolling into a turn, & automatically push the wrong pedal, everyone aboard knows it. :D

[CRAN]

My understanding is that the direction of rotation of the tail rotor is dominated by aerodynamic considerations. The reason for the inconsistencies in design that are observed is that it is very difficult to simulate or predict the exact (and very complex) nature of the MR wake/downwash, in which the tail rotor must operate.

In forward flight the rotor wake skews backwards and at very high speeds the wake adopts many of the characteristics of the wake that could be expected to trail behind a circular wing. If we consider low forward speeds, such as encountered during transition to forward flight the wake skew angle is small and the demand on the tail rotor is high. Consider the tip vortex tailed by the main rotor as it passes from the 270-degree position (left side) to the 90-degree position (right side) over the tail. A very strong tip vortex is produced in the wake that passes over the tail boom in the same plane of rotation as the tail rotor. This vortex is happily convected back with the flow as the helicopter moves forwards. Before long this ‘mini-tornado’ is sitting right over the tail rotor and is rotating forwards at the top and back-at-the-bottom. It is easy to see from this reasoning that if the tail rotor is rotating forward–at-the-top then the rotation of the flow induced by the presence of the vortex is subtracting from the relative rotational velocity of the tail rotor motion and hence reducing its effectiveness. Rotate the tail rotor in the other direction and the effect is positive interference……NICE!

Unfortunately, it’s not that simple, because at any one time the tail rotor is being battered by multiple tip vortices of various ages. The number depending on the forward speed. All of these vortices interact with each other and the tail rotor and so the total effect is very difficult to predict. However, taking the initial simple 1-vortex concept and having the tail rotor rotate back at the top tends to work, but it’s not always the case, because many other effects have an influences that are very difficult to quantify.

This is a gross simplification of one of the aerodynamics mechanisms that makes the direction of rotation important. As with many helicopter design problems the system must be optimised for many flight conditions with conflicting requirements. The reason why some helicopters have TR's rotating the other way is because we currently cannot simulate accurately the rotor wake and so calculating what is going on becomes a very educated best guess. Inevitably, best-guesses are not always right and things have to be modified in development.

Give us 50 years to allow computer technology to reach a level where current CFD techniques can be exploited to full advantage in the commercial design environment and we will get it right first time every-time!

Hope this helps
CRAN
:cool:

One consideration in the direction of rotation for SMALL helicopters is which side of the machine you want to pilot to sit on.

Ideally you want the pilot to be sitting on the side that is hanging high, so when he flies solo, his weight levels the machine to some extent rather than making the left/right skid low problem worse. If you machine is intended for training or general flying around you would want the pilot on the right so his 'collective' hand - the one he can use in flight, is directly over the centre console/instrument panel for easy access. In this case you would want to have an american/bristish rotation of CCW from above.

If you fancy doing a bit of sling loading, so your jolly-jock is going to want to hang out of the door, then he would prefer to sit on the left hand side. In this situation the french/russian (CW from above) rotation is more sensible.

In bigger machines these considerations are not so important as the weight of the pilot is insignificant in terms of the weight of the machine.

Oooo and the rotors spin the wrong way on French helicopters because they are awkward!!!! :D :D :D

CRAN
:cool:

[Jeep]

If your helicopter firm happens to be called Westlands, you design your Lynx firstly with a counter clockwise rotating TR so that you can make your main customer purchase the midlife upgrade of a clockwise rotating TR. Amazing difference it makes too. Shame the designers didnt go for it in the first place.

Perhaps it was the flip of a coin.

[Q max]

One reason is:

For the efficient direction there is a greater Arm on the TR shaft than in the other direction.

You might notice some with one bearing and some with two - have a look at the direction of the TR and correlate!

[Thomas coupling]

With the TR in the config you suggested. Advancing blade moving up into the rotor downwash, the TR becomes more efficient by producing more lift therefore offloads TR therefore maximising TRE ;)

That's why the lynx TR was swapped to the other side.

[CyclicRick]

Thanks CRAN and Thomas Coupling.

I shall bombard my engineer with all this and baffel him to bits.
Thinking about it, the tail rotor moving forwards and up in to the MR downwash sounds totally logical to improve effectiveness but I still wonder if any gyroscopic force plays any part.

Talking of tail rotor noise. Is a CW rotating tail-rotor any quieter than a CCW tail-rotor due to the decreased speed of airflow over it?

[handyandyuk]

It may be unrelated, but during some Army reserve service in Bosnia 2000, we had some Czech Mi 17's located at the UK base at Banja Luka. One of the first things I noticed was the strange 'chook-chook' sound they make when running with lift pitch on. (Ground running, lever down; no noise).

Anyone encountered this or anything similar?

Andy

[Nick Lappos]

Thomas Coupling has it right on, I think. Igor Sikorsky was said to believe that the bottom of the tail rotor should move into the main rotor downwash, and he never designed a helicopter otherwise.
The AH-1G's that I flew did the opposite, and they were woefully short on tail rotor margin until they were retrofitted by flipping the rotor over to the other side of the pylon.

[Thomas coupling]

Driving the TR up into the MR downwash does impact significantly on the noise footprint of the helo.
Hence the evolution of staggered TR blades (in tip path plane and plan form (135 / apache / blackhawk, etc) to reduce the noise.
The following blades don't then run head on into the vortices created by the previous blade which creates a lot of noise.

I wonder if they tried it on the MR:eek: :eek: :eek:

[Igor did try a single MR blade though, didn't he - perhaps he was thinking about it then;) )

Another noise profile you should try to modify is while descending to land. Normal inconsidered arrivals lead to the MR blades slapping the vortices of the previous blades - big time. By modifying you speed ROD, it makes for far quieter approaches - though not always practical I might add! (STAR's)

[CyclicRick]

Thomas Coupling:

I was doing some pleasure flights today ( over 50 trips!)in a 206 and was doing some experiments with blade slap.

I found that it was mainly dependant on the aircraft weight. I tried steep approaches, shallow, fast, slow etc. etc. When heavy it was very hard to prevent while trying to remain in a fairly normal profile.

I know that on the 205 that at about 60kts and 500-600 ROD the blade slap is at its worst no-matter how heavy you are but the 206 is definitly different, the lighter I was the quieter and on the way back to base with only 30gals and me it was practically non-exsistant.

On our 205's the tail rotor is on the opposite side compared to a UH-1 and I'm sure they're a bit quieter

[3top]

Hi all,
On Robinsons it is recommended to keep the approach speed up to avoid too much noise. A high speed means low power on descend, maybe that does the trick, but better plan ahead, a 206 will float forever with that kind of approach! If interested I will check the actual numbers on monday in the poh!

:) 3top

[Head Turner]

I have noticed that the direction of rotation of tail rotor blades are different. Some design have the forward blade descending, otheres have the blade ascending. Why is there a difference?

[Shawn Coyle]

Long story, and it boils down to engineering preference at the time of design, followed by inertia afterwards. Some of the current designs were made in the late 50's/early 60's and have stayed the same. About that time someone did research that showed that top turning aft was the optimum direction, but those with the tail rotor top going forward haven'g changed as they'd have to re-qualify the design, and re-do the flight test - all very expensive.

[SFIM]

the R22 and R44 revolve in opposite directions to each other,
the R22 anticlockwise when viewed from the left and the R44 clockwise.

the R44 being the more conventional direction, i heard several reasons why the R22 is like it is, one was that they used off the shelf parts and thats how it was, and another that it just worked better anticlockwise

hard to know what is horse s**t and what isnt

[SFHeliguy]

At the robinson factory course in January, Tim Tucker mentioned that there was a rear transmission factor on this decision having to do with the weight of changing the direction and additional gearing.

[Thomas coupling]

Use the search button using tail rotor direction or similar, there was a huge thread on on this about a year ago.

[Dennis Kenyon]

TAIL ROTOR BLADE DIRECTION.

I know there are many aerodynamics experts out there with greater knowledge of this situation than mine so I am surprised none have provided answers to this old chesnut.

I have to return to the Enstrom marque although it is sometime since my sales history was linked to the Spoonair Enstrom distributorship.

Most readers will be aware that the early model Enstrom non turbo models (F28A, 280 Shark) circa 1970 to 1976 - rotated anti clockwise (when viewed from the left side of the tailboom.) I'll do my best to explain without diagrams and hope the words will cover the point.

It was always felt by the Enstrom R & D department that on the original 'right hand tail rotor' layout - the thrust lost by the downgoing blade in the main rotor induced downflow was offset by the upgoing blade's thrust increase. However further research revealed that there was a marked reduction in induced main rotor airflow speed occurring about the tail rotor hub centre line and further reducing as it extended past the tail towards the upgoing blade. So what was lost on the downgoing blade was not regained on the upgoing. Additionally as we all know, the thrust generated varies with the square of the speed, resulting in a further thrust loss.
The fix was simple, reverse the tail rotor gearbox through 180 degrees to become a 'left hand' tail rotor layout and hey presto, the upgoing blade is now receiving the benefit of the increased induced M/R airflow providing increased thrust and a more efficient rotor. (less tip induced drag !) This being the only drag that 'reduces' with increased speed !

Enstrom introduced the reversed 'left hand tail rotor' on all factory produced higher powered 'C' models and also offered a conversion kit for the earlier 28A's. Enstrom pilot's will still come across both 'left' and 'right' 28A types and a few 280's and should understand the lower tail rotor authority available to him on the unmodified right hand tail rotor. In addition the 'C' and later models were fitted with the wider 3.3 chord blade which further improved the situation.

With my selling boots on, a point I was always at pains to explain to prospective purchasers was the superb tail rotor control of the later layout, and usually demonstrated this by underspeeding the main rotor, (overpitching) to the point at which vertical lift was lost but even as the machine descended, the airframe could still be yawed to the left against the torque.
I hasten to mention this exercise was shown in a low hover !!!

Thanks for reading me and I now expect an 'expert' to enlarge on the dynamics of all this !!!

Fly safely chaps.

Dennis.














degrees and h9 so by the t

[MightyGem]

This may be apocryphal, but there was a story that when Westlands changed the direction of rotation of the Lynx tail rotor, they forgot about the worm drive in the gearbox that lifted the oil for lubrication. Hence it was going the wrong way and seized on the first test flight.

[[email protected]]

Mightygem - nothing about wastelands would surprise me! One thing is for sure though, changing the direction of rotation on the Lynx TR to a clockwise one stopped the regular pirouettes of the Mk 1s - I think it is almost impossible, even in display manoeuvres, to run out of TR authority in a Mk7. Even when one of the Blue Eagles display pilots got clearance to experiment with yaw rates of 120 deg/sec in hover manoeuvres, the aircraft coped admirably. I have to say I have never seen Hampshire spin past the window quite as quickly since! Mac where are you?

[Nick Lappos]

Igor Sikorsky always wanted the bottom blade to move toward the aircraft, if possible. The increase in efficiency is a good thing, but not critical, if one has a big enough tail rotor with enough collective pitch. The direction of rotation has a net influence of perhaps 5% on the available tail rotor thrust, easy to overcome if the tail rotor is powerful enough, but that 5% could be a whole bunch if the tail rotor is marginal.

On the old Cobras I flew (AH-1G's) the yaw was so poor that we ran out of tail authority in IGE hovers all the time, especially when the wind was from the left rear quadrant. Later models flipped the box over to the other side of the pylon, and the opposite rotation made things a bit better, but still not wonderful. The later model Cobras have a plot in the flight manual to show where you will run out of tail authority if you try to HOGE with a slight downwind at altitude.

[B Sousa]

Nick
Not only true on the Cobra but the various models of the Huey. The difference between the UH-1 M (204) and the UH-1H (205) was so critical that folks had to be checked out to drive the UH-1M for things other than gun missions at High DAs. Major difference was a shorter tailboom on the M Model. I remember it well and as a Unit Trainer made sure those hot dogs flying M's were well aware that the reason for the Left Pedal smashing through the chin bubble meant you were too high, too hot and fast coming into a mountainous LZ. Next was the aircraft spinning when ETL was completely lost.

Not that it matters, but lots of folks dont know that the Military Huey (UH-1D, H, V etc) has the tail rotor on the left side, wheras the Civilian 205 has a larger corded one on the right side. Much better.

[Lu Zuckerman]

To: Mighty Gem

This may be apocryphal, but there was a story that when Westlands changed the direction of rotation of the Lynx tail rotor, they forgot about the worm drive in the gearbox that lifted the oil for lubrication. Hence it was going the wrong way and seized on the first test flight.

I can't believe that Westland would make a mistake like that considering that Sikorsky did the same thing on the H-37.

When they built the first H-37 they found that there was structural interference on the tail rotor drive shaft. In order to get around it they installed another gear set which made the shaft turn in the opposite direction. They reversed the tail rotor and changed the gears in the intermediate and tail boxes but they forgot to change the Archamedes (sp) pump in the tail rotor gear box and instead of moving oil to the outer bearings the pump took oil away from the bearings.


:rolleyes:

[Head Turner]

Thanks everybody. I've learn a great deal from you, especially in regard to the Enstrom and Robinsons.

[mdw350]

Anyone know why there is a difference in the direction of rotation of the tail rotor between the R22 and the R44 ?

[Pat Cox]

Anyone know why there is a difference in the direction of rotation of the tail rotor between the R22 and the R44 ?

R22 tail rotor rotation direction was dictated by the need to save weight.

Early R22's used a cantilevered output shaft in the tail rotor gearbox (p/n A021) for a small form-factor (volume) and thus minimal weight. Cantilevering the output shaft placed the output gear on the left side of the input gear (when viewed from above), which results anti-clockwise tail rotor rotation.

To enhance serviceability, later R22 tail rotor gearboxes (p/n B021) do not use a cantilevered output shaft but still maintain anti-clockwise rotation direction to allow direct retrofitting. Advances in casting and machining methods result in no increased weight compared to the earlier A021 gearbox.

The R44 could afford a heavier tail rotor gearbox and therefore did not require a cantilevered output shaft. Since having the tail rotor's advancing blade closer to the main rotor downwash is more efficient, the R44's output gear was placed on the right side of the input gear, resulting in clockwise tail rotor rotation.

Both helicopters have significant tail rotor authority due to the use of asymmetrical tail-rotor-blade airfoils.

Google "square-cube law" for more on the importance of volume versus mass (i.e. size versus weight).

Fly safely.

[topendtorque]

Thanks Pat.
i'll put that on the coal face.
cheers tet