3
$\begingroup$

enter image description here

My question stems from the fact that, although this is a very fierce looking, and formidable aircraft with a very powerful engine, Ka-52 Alligator has not seen many export success. Its export is very limited as only Egypt uses this type of helicopters as a foreign customer.

Are there disadvantages of coaxial rotor on attack helicopters?

$\endgroup$
2
$\begingroup$

The unique configuration of twin coaxial rotors presents both advantages and disadvantages, and the relative weight of each must be considered in light of the aircraft's mission.

I am going to assume the reader has a basic understanding of helicopter controls (cyclic, collective, antitorque).

Disadvantages

Aeromechanical complexity

A coaxial rotor has a much more complex hub than a regular one. It does not just need to invert the rotation direction, it has two sets of cyclic and collective controls that also mimic the effect of a tail rotor.

Since the lower rotor operates in the down-wash of the upper one, its collective pitch needs to be higher to produce the same lift (and thus torque). This is important because a torque imbalance will make the aircraft yaw uncommanded. In fact, this is how yaw is achieved when the pilot steps on the pedals, by letting the rotors generate differential lift, but ensuring their combined total stays the same so the aircraft retains level flight.

The cyclic controls suffer the same effects, that is the least of their problems, see below.

Rotor self-intersection

A single rotor in forward flight will assume a slightly conical shape due to the lifting force on the blades. This cone will also be tilted sideways with respect to the direction of flight because the blades see different airspeeds at different points in their cycle. That is to say, the blades will generate more lift while they advance into the direction of flight, and less while they retreat on the opposite side of the helicopter. The higher the forward speed, the larger will this effect be (In fact, there will usually be an area on the rotor where the blades do not produce lift because the airflow over them is reversed: the forward speed of the helicopter is greater than the local speed of the blade).

In a coaxial design, both rotors turn in opposite directions, so their tilts will be in opposite directions too: the clockwise-turning one will tilt right, and the anticlockwise-turning one will tilt left (the actual locations of maximum bending are not quite as clear cut because of blade inertia, but this serves as a basic explanation).

The resultant problem is pretty obvious: the two rotors can intersect each other. This is a Bad Thing(TM) and will lead to all sorts of issues like rapid unplanned dissasembly of the rest of the rotor blades. A pilot in this situation will most likely make use of the conveniently installed ejection mechanism.

The actual forward speed at which self-intersection will occur is not published (to the best of my knowledge), but is probably somewhere above 250km/h. This is only taking forward flight into account... sudden collective or cyclic inputs at high but not critical airpeeds may have the same effect.

The overall effect is that coaxial rotor helicopters are more limited in their flight envelopes than conventional ones. Although this is not a fatal drawback, agility is highly valued for attack helicopters.

Yaw Response

This is somewhat speculative, but the lack of a dedicated tail rotor may limit the yaw rate at low forward airspeeds, where most of the yawing toque would come from the tail rotor. At high speeds the weathervaning effect of the tail becomes dominant anyway, making this a moot point.

Advantages

Simpler tail design

The lack of a tail rotor frees the helicopter from having to transmit torque to it, thus heavily simplifying the boom design. In fact, some coaxial designs make a particular point of exploiting this advantage, having very short boom, like the Ka-25 and Ka-27, which are navy helicopters meant to fit into the tight confines of a shipboard hangar.

Less weakpoints

The loss of a tail rotor is not, like many movies would suggest, an unrecoverable situation. A helicopter can be stabilised with sufficient forward speed if it has enough tail volume.

That being said, losing a tail rotor in a combat situation will most likely result in a mission kill. And it's not just the rotor that's vulnerable, the boom contains the transmission rods for it, so any damage along that structure can be potentially fatal.

For a coaxial helicopter,the loss of the tail will lead to some stability issues, but nothing catastrophic.


Whether the advantages of the design outweigh its drawbacks is ultimately a matter of preference and of the fit between the aircraft and its intended mission. The adoption rate of a design is not necessarily an indicative of its merits, especially when military procurement is involved, as there are countless other factors at play.

$\endgroup$
  • $\begingroup$ Yaw response is typically better for coaxial designs, so it should be in the 'Advantages' section. In fact, Ka-50 has, form what I know, by far the best yaw response of all comparable helicopters. This, in particular, allowed to use a more powerful gun that is (almost) fixed in horizontal plane: it is just as easy to aim the whole helicopter, even at a significant forward speed. $\endgroup$ – Zeus Apr 27 '18 at 1:05
  • $\begingroup$ @Zeus If you can find an source on the yaw response, I'll gladly add it. As of now I have heard claims in both directions from supposedly authoritative sources so I'd rather ditch that paragraph altogether if I can't support it better. $\endgroup$ – AEhere Apr 27 '18 at 11:09
  • $\begingroup$ Another disadvantage for attack helicopters is the higher rotor which makes hiding behind obstacles harder. Also, self-intersection is not only a matter of speed but mostly of load factor. $\endgroup$ – Peter Kämpf Dec 24 '18 at 23:42
  • $\begingroup$ @PeterKämpf Fair point on the load factor, but I am unsold on the hiding behind obstacles part: the difference in height is small and the heat sources are on the fuselage, so most of the hiding can be accomplished without obscuring the rotor head. In fact the mast-mounted radar on the AH-64D suggests this same line of thought. $\endgroup$ – AEhere Jan 7 at 12:36
  • $\begingroup$ @AEhere … but the mast-mounted sight makes much less noise. Unhidden and moving noise sources are far easier to spot. Also, the difference in height is substantial in order to allow different disk loadings (for turns). Otherwise the blades might collide. $\endgroup$ – Peter Kämpf Jan 7 at 14:30
2
$\begingroup$

The main real disadvantage, as mentioned in all other answers, is complexity, and, specifically for combat, vulnerability of the hub (which can't be shielded). On the other hand, it could be argued, the tail boom of a 'normal' helicopter is just as critical and vulnerable.

Another disadvantage is the aerodynamic drag of the hub, which can make up to a half of total drag in some cases. Due to the inherent danger of rotor self-intersection, explained by AEhere, the hub must be quite tall, especially for high-speed and/or manoeuvrable machines. And again, it can't be shielded, and very little can be done about it.

This rotor self-intersection itself is often quoted as a disadvantage, but in fact it simply results in other design compromises, such as the taller hub mentioned above, or the flight envelope restrictions. In a well designed system, the danger of it happening is no greater than the danger of cutting off the tail boom by the traditional rotor.

Pretty much everything else is an advantage.

With regards to attack helicopters specifically, the main one is perhaps simplicity of control and manoeuvrability. Although the coaxial design is not perfectly symmetric, it is orders of magnitude more symmetric than the classic design, with very little cross-coupling, and is consequently much easier to fly. This allowed to design such an oddity as a single-seater combat helicopter, the Ka-50. There are manoeuvres that only a coaxial helicopter can realistically perform, and they are arguably more combat-relevant than super-manoeuvrability of modern airplane fighters.

Such handling qualities may even result in a certain complacency on pilot's part. The helicopter controls so well that it is easy to push it beyond its intended envelope. In the case of Ka-50/52, all the cases of rotors collisions (at least one of them fatal) were ultimately attributed to that.

As for the export success, the arms export is much more political than technical, such that the nuances we are discussing here are almost irrelevant.

$\endgroup$
1
$\begingroup$

Added complexity in any aircraft is generally considered a disadvantage. You have increased failure points, more part to service, very generally speaking a higher cost to service and maintain and more failure scenarios should something go wrong in the hub/rotor assembly. In this specific case you would also introduce an added drag penalty.

I cant read Russian but wiki links this article under the reference for the disadvantage section, chrome will translate if you are so inclined to check it out.

$\endgroup$
  • $\begingroup$ That article says that military sources suggested an uncommanded activation of the ejection system as reason for the crash, so I'm unsure why Wiki links it where it does. $\endgroup$ – AEhere Jan 7 at 14:19
0
$\begingroup$

Turning (yawing) a coaxial at high power settings will be very powerful. (assume a 10% difference in torque between upper and lower rotor) At low power a 10% differential between rotors will be correspondingly low. At autorotation, the higher pitched rotor will attempt to SPEED UP, this is because it is being asked to carry more weight compared to the other rotor. This is not comparable with a single rotor system, where an increase in pitch slows the rotor. However, the flaring (increasing load) of this system will SPEED the rotor. Decreasing load by cyclic (reduced G) will slow the rotor. If we consider both these single rotor LOAD changes in opposite directions on a coaxial system, the control reversal becomes apparent. In the hover, a coaxial has very good yaw control in both directions, wind from either quarter causing no control limitation. In autorotation the rudder system is the prime means of yaw control, (a run on landing being
normal I believe) If power can be regained before touch down, good torque control is resumed.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy