# Does the rotation direction of intermeshing rotors have any noticeable effect?

Consider the Kaman K-Max, which has intermeshing (side-by-side) rotors that rotate as indicated below (when viewed from above, the rotor on the right (green) rotates clockwise, the left one (red) counterclockwise). I was wondering, does this particular choice has any effect on handling or aerodynamics, or is it more or less arbitrary, and just influenced by the mechanical design?

Or in other words, if we had two identical helicopters but one with a reversed rotation: Would a pilot notice any difference between those?

Naively I would have thought that it doesn't matter at all, but I got curious and couldn't find any information on that design choice.

I was wondering, does this particular choice has any effect on handling or aerodynamics?

Yes it does and it has to do with the fact that the two rotors are tilted sideward for obvious reasons of clearance between the two rotor heads.

Seen from above, the right rotor rotates clockwise and the left rotor counterclockwise but due to the sideward tilt, the rotors rotate also when seen from the side.

To simply picture it, just imagine them as being tilted sideward not just a few degrees but a full 90°: now they can be clearly seen rotating from the side. So, because of the sideward tilt, the rotors rotate both as seen from above and from the side.

In particular, from the side they rotate with the blade going up when leaving the tail and down when approaching the nose, as can be seen in the following picture¹ (yellow arrow):

As usual with helicopters, whenever a rotor rotates in one direction then the fuselage wants to rotate in the opposite direction (action-reaction law aka Newton's third law). In our case, since the two rotors rotate with the blades going down when approaching the nose then the fuselage wants to rotate nose-up (blue arrow):

(Obviously it would have been the other way around if the rotors had rotated in the opposite direction).

Now, somehow this blue nose-up tendency of the fuselage has to be opposed and the easiest way to do it is by tilting the rotors a bit forward in order to create a horizontal force (horizontal green arrow in the next picture) which generates a pitching-down moment in respect to the CG. Then, the thrust of the rotors not only lifts the helicopter (vertical green arrow) but produces also a pitching-down moment compensating the blue arrow:

Why are all those arrows good when pointing like that and not the other way around? Because this increases the longitudinal stability of the helicopter: let's suppose that the helicopter is invested by a vertical upgust. This upgust pushes the helicopter upward and increases the AoA of the rotors therefore increasing its thrust as well. In this condition a helicopter is stable if it reacts against these effects i.e. if it reacts pitching-down. And this is exactly what happen when all the arrows are how they are: the gust increases the thrust which increases the horizontal green arrow which increases its pitching-down moment opposing the gust and stabilising the helicopter longitudinally.

Another example² is a loss of power in hover: the stabilising effect results in a pitch down moment generating forward descent, which is a better starting point for an autorotation than a backwards nose up descent.

So, the rotors in an intermeshing design rotate as they rotate because this increases the longitudinal stability of the helicopter.

¹ all pictures from Wikipedia, modified by me. ² thanks to @qqjkztd.

• Probably, an important issue is the retreating blade stall. Commented Nov 29, 2022 at 20:15
• Thanks for the excellent explanation! These are all points I had not considered! Commented Nov 30, 2022 at 21:37
• @xxavier Can you elaborate how this is an issue here? I was under the impression that this was less of an issue for a helicopter with counterrotating rotors? Commented Nov 30, 2022 at 21:38
• @flawr In an intermeshing-rotor helicopter, like the Flettner Kolibri or the Kamans, it makes a big difference if the 'outer blades' move forwards or backwards, because, if they move backwards, the lateral overturning moment in the case of retreating-blade stall could be severe... Commented Dec 1, 2022 at 7:30
• @xxavier yes there is either a pair of (shorter wingspan) inner advancing blades forming dihedral together, or a pair of (longer wingspan) outer advancing blades forming anhedral. No source for this, but during forward flight one or the other option should induce different evolution of roll stability too, as horizontal speed increases.
– user21228
Commented Dec 1, 2022 at 12:16

I don't know exactly how that intermeshing rotors work, but I assume that the animation in this video is representing the actual situation which the engine turn the helical gear of the right rotor (left or right is determined by the pilot's arm when sitting the seat). The engine creates torque to the left side of the tail, but then it is cancelled by the interconnecting spur gear. By so, the net torque is zero. That's why there is no tail rotor as in normally helicopter. But however, there is gyroscopic precession created by each rotor. Both the rotors will push the nose down (pitched down). So, the helicopter must be already designed to slightly tend to the nose so the thrust they crated will cancel the gyroscopic precession. So to answer your question, will be any noticeable effect if one an identical intermeshing-helicopter but the rotors are rotated in the opposite direction, then my answer is: there is, as the helicopter will be pushed the tail down (pitch up), just the opposite of another one.

• The two rotors rotate in opposite directions so the gyroscopic precession should be one the opposite of the other one and should cancel each other out. Commented Nov 30, 2022 at 12:18