The Indian Light Utility Helicopter (LUH) program is progressing towards completion. Four helicopters from four different nations and commercial conglomerations are in the bidding for this lucrative contract. In addition to the Bell 407, the Eurocopter Fennec and the Kamov Ka-226T, HAL’s own design, known by the generic title LUH, is in the fight to win the contract. The winner of the contract will provide the next generation helicopter to replace the ageing fleet of IAF Cheetah and Chetak helicopters in the coming years. And while most of the competitors bidding for this contract are basing their hopes on preexisting designs from their industries, the HAL proposal is new and untested. It has the advantage of following the highly successful Light Combat Helicopter (LCH) program. And the HAL design teams have certainly benefitted significantly from the experience, which they will now bring to bear on the LUH effort. But where does the LUH stand amongst its competitors? For that matter, where do the competitors stand amongst themselves?
The procurement of any military aircraft or helicopter type is a complicated process. And this analysis will not attempt to cover all possible areas pertaining to geo-politics, economics or the like. Instead, the focus of this analysis is on a preliminary aerodynamic and propulsive standpoint, especially for the extremely high-altitude conditions encountered in the Himalayan Mountains. The analysis is done using simulation tools that integrate payload capacities and typical rate-of-climb requirements with a preliminary rotary aerodynamics model and a simple propulsion module. When coupled with an atmospheric simulator for the Himalayas, the performance of each helicopter type can be predicted and compared. The rotary-aerodynamics module is advanced enough to predict the different performances of a single main rotor plus tail rotor system (such as that being used in the Bell, HAL and Eurocopter designs) or a contra-rotating rotor system as found for the Kamov design. Furthermore, the models allow for the performance analysis in ground effect conditions. Ground effect conditions are encountered when the helicopters are hovering very close to the ground and serves to work as a performance multiplier for power required in lifting a certain payload.
The analysis models used here do not compensate for transmission limitations for the power, which means that the analysis is idealized wherein power generated is power available. This works well for high-altitude conditions where power available is almost always less than transmission limits. The rate-of-climb (ROC; measured here in meters/second) is a true measure of the maneuvering capability of a helicopter. Typically, a ROC of 0.5 m/sec is used to evaluate service ceiling conditions. A ROC of 2.5 m/sec is typically the bare minimum for combat conditions. Of course, beyond a certain altitude, the helicopter may not be able to fly with the required payload, let alone providing additional power for high ROC. Rate-of-climb performance with a usable payload is of higher importance within the context of a light-utility helicopter than it is for medium and large transport helicopters. A utility helicopter is expected to perform a variety of roles under tough conditions where the maximum payload is less important than the maneuverability in the vertical plane.
Design philosophies for the Himalayas
Of the four helicopter types involved, three belong to the single main-rotor design concept. These are the Fennec, LUH and Bell-407GT. All of these designs feature a main rotor and a tail rotor. The tail rotor designs all have a major power/aerodynamics drawback in that the tail rotor does not correspond to lifting payload and yet draws power away from the engines. This power requirement can vary in the range of 10-15% of total available power in some designs. The Ka-226T belongs to the contra-rotating design model and overcomes the tail rotor by having two contra-rotating rotors that cancel net rotor torque. Since both rotors contribute to vertical thrust, the losses from the tail rotor are theoretically recovered. However, two contra-rotating sets of blades in close proximity contribute to other rotor interference losses that serve to negate the advantages. Current analysis suggests that this loss is almost the same as tail rotor losses. However, the lack of the tail rotor serves additional practical advantages including a compact design, removal of a vulnerable boom, easier entry and exit of passengers (with less risk) and overall increase in maneuverability.
From the power standpoint, the LUH’s power-plant and drivetrain is the biggest variable at the time of writing of this article. While the Bell, Eurocopter and Kamov designs are essentially “stabilized” from a design standpoint, the HAL design remains a mystery in terms of performance. The first prototype has not yet flown. And varying sources at different times have quoted different power and weight numbers. HAL has been quoted in one of their presentations to state that the power limitations on the transmission of the LUH will be 750 KW at sea-level whereas the Shakti engine’s rated power is ideally 1,067 KW. The Shakti engine power output is well ahead of any of the equivalent engines in the competition, but the powertrain restrictions will decide how much potential of the engine has been extracted.
Similarly, another area of focus will be the overall weight of the LUH design. Numbers provided by the HAL during its presentations at Aero India 2015 point to an empty mass of the LUH to be 1,910 kg. When compared with the empty masses of its competitors, 1,220 kg (Fennec), 1,700 kg (Ka-226T) and 1,210 kg (Bell-407GT), the weight of the LUH is an immediate area of concern. One possibility is that the weight is a result of a much more powerful power system (Shakti engine) in the LUH. However, this is only balanced out if the resulting power from the engines transmitted to the rotors is much higher than the other designs. If only a 750 KW powertrain is extracted despite the 1,910 kg empty weight of the helicopter (as quoted by HAL in its official presentations), the resulting performance can be expected to be dismal. The HAL design team, drawing experience from the ALH and LCH efforts, will have to undergo a similar effort in weight-trimming and in improving the power-train restrictions of the LUH design.
Performance in the high mountains
Two sets of hover performance numbers have been evaluated for the contending LUH designs. The first set is for conditions where the helicopter is hovering out of the ground effects (OGE) and the other set is for hover performance in ground effect conditions (IGE). The IGE performance is evaluated for the various contenders for a hover altitude of 2 meters. The performance is evaluated for all the helicopters at empty weight conditions (no fuel and no passengers) and the maximum allowable payload is restricted to 1,000 kg (internal or external). The rate of climb is evaluated for a given altitude based on available power once the maximum amount of payload has been lifted for that altitude. In other words, when a helicopter is able to lift the specified payload, all remaining power is assumed to be directed towards the rate of climb. In doing so, we focus more on payload lifting capacity rather than rate of climb. Consequently, if the helicopter is unable to lift the specified payload, then it will be assumed to have zero rate of climb potential while it is carrying what maximum payload it can carry for that altitude. The hover performance is evaluated at altitudes varying from 0 feet (sea-level) to 25,000 feet. Altitudes in the Himalayan Mountains regularly require flights above 10,000 feet and can be up to 22,000 feet in the Siachen Glacier.
The Bell-407 and Fennec perform similarly, which is not unexpected considering similar designs. But the Bell design is found to perform slightly better at higher altitudes (greater than 10,000 ft.) and this is attributed to its better rotor blade design. For example, at 20,000 feet altitude, the Bell-407 can lift ~100 kg more payload than the Fennec. Both these helicopters have a similar loss in performance versus altitude as ascertained from the slopes of their payload-altitude curves. When the requisite crew and fuel masses are added, the Fennec is unable to fly beyond 19,000 feet altitude. Similar limit for the Bell-407 is at 22,000 feet. The Bell-407 also has a better rate of climb compared with the Fennec and maintains that difference at higher altitudes. The Bell design also loses that ability at higher altitude limits than the Fennec (12,500 feet versus 10,000 feet, respectively). At sea-level, the Bell-407 can reach about 4.5 m/sec vertical rate of climb compared with 3.0 m/sec for the Fennec.
The Kamov model has visibly different performance owing to its fundamentally different design. A combination of high empty mass, contra-rotating rotors and higher available power means that the tail-off in performance for this design is different from the Bell and Eurocopter models. The performance of the Kamov design generally tails-off faster than its competitors at higher altitudes. This is attributed to the high rotor interference effects that increase the amount of power required to maintain a given payload. There is a substantial difference in hover performance between the Kamov design and others beyond 15,000 feet altitude and this difference only increases. When similar crew mass, fuel mass and rate-of-climb effects are added, the Kamov design’s payload capacity is negligible beyond 16,000 feet, which is much lower than that of the Fennec (19,000 feet) and the Bell-407 (22,000 feet). At sea-level, the Kamov design also has the lowest rate of climb power available and is unable to exceed 2.5 m/sec compared with much higher numbers for the Bell, Eurocopter and HAL designs. It also loses all available power for rate of climb at 9,000 feet altitude.
The HAL design’s performance varies between outstanding or dismal depending on what power transmission numbers are assumed. If the Shakti engine is fully utilized for its power, then the performance of the HAL design exceeds that of its competitors and can haul usable payloads up to an altitude of ~21,000 feet despite the much higher empty weight. It also has the highest rate of climb at sea-level (5.8 m/sec). The only other helicopter in the competition that comes close to it is the Bell-407 with a rate of climb of 4.5 m/sec. The trail-off in the available power for rate of climb is also the highest for the HAL design at 13,500 feet altitude.
The ground effect factor
Ground effect multipliers for the various designs are also different and offer differing improvements for the four designs. Once again, the Bell and Fennec designs are near identical in their IGE performance. The LUH, with a slightly higher main-rotor blade radius performs better. The Kamov design gains the best effects in IGE conditions on account of its twin-rotor system with large blades. It is the result of this improved performance in IGE that allows the Kamov design to match the Bell and Fennec designs in IGE hover up to an altitude of 17,000 ft. Beyond 20,000 ft, the IGE performance gains for the Kamov design are lost and leads to a general performance below that of the other competitors. All four helicopter designs perform significantly better in the ground effect conditions, with the HAL design performing best and the Bell-407 staying close to it. The Ka-226T and Fennec perform somewhat similar in ground effect and are behind the HAL and Bell designs. For example, at 20,000 feet altitude, the Fennec can lift ~340 kg more in ground effect than out of ground effect. Similarly, the Bell-407 can lift ~410 kg in ground effect than out of ground effect and the HAL design can lift almost 550 kg more in ground effect!
