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
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!!!!
CRAN