Advanced AH-64E Block II Compound
The US Army's FARA (Future Attack Reconnaissance Aircraft) program is looking for a successor to the lightweight reconnaissance and combat helicopter OH-58D Kiowa Warrior. One of the requirements is extreme speed and range beyond the physical capabilities of classic helicopters. At present, only a few VTOL machines - the S-97 Raider helicopter, the European Airbus X3 and the Leonardo AW609 convertible - fulfill FARA conditions. Boeing therefore offers the FARA radically modified extremely fast AH-64 Apache (or AH-64E Guardian) with a rear pusher.
Boeing introduced the AH-64E Block II Compound, a high-speed version of the AH-64 Apache attack helicopter, at the 75th Annual Forum and Technology Display. Unveiled in October 2018 at the Vertical Flight Society’s Helicopter Military Operations Technology (HELMOT) conference in Hampton Roads, Virginia, the AH-64E Block II compound helicopter would feature a rear propulsor.
The AH-64E Block II is a response to the FVL (Future Vertical Lift) initiative, which is looking for a new generation of Vertical Take-Off and Landing (VTOL) high-speed machines. The attack requirement was not included in the Army’s Future Long Range Assault Aircraft (FLRAA) program, which was unveiled in April 2019 – Boeing says the Compound Apache could fulfill the role in the future. The AH-64E Block II high-speed attack helicopter project is designed as a temporary replacement for the arrival of an entirely new category of VTOL category high-speed battle machines. The first FVL machines will be acquired by the US Army before 2030, but Apache battle helicopters (or the newest Guardian) will carry the main weight of US Army air combat operations until 2040. The AH-64E Block II is based on the Lockheed AH-56 Cheyenne technology of the 1960s. The AH-56 managed to fly at nearly 400 km / h, but in 1972 the Army canceled the project. The official reason was the obsolescence of the then-used "analog and mechanical" weapon systems. The Army then launched the AAH (Advanced Attack Helicopter) program, which was released by the current "digital" AH-64 Apache helicopter.
Boeing offered AH-64E Block II helicopters with a rear pusher propeller that can fly a cruising speed of 345 km / h as a temporary solution to the FVL. High-speed Apache can carry up to 2676 kg of payload, but it is unclear whether only weapons (rockets, cannon ammunition), or crew and fuel are included in this value. The current AH-64D has an empty mass of 5350 kg, a normal take-off 7270 kg (a maximum of 10,105 kg), can carry 1100 kg of fuel in the inner tanks and 770 kg of weapons (possibly including cannon ammunition) under the wings on four pylons.
The dominant helicopter configuration in the present time is based on Sikorsky's basic design with a main rotor and an auxiliary tail rotor to counter torque. Said conventional helicopters show excellent hover capabilities but suffer from limitations in terms of horizontal flight speed. These limitations are associated to two aerodynamic phenomena at the main rotor: the retreating blade stall and the maximum blade tip velocity. In general terms: The lift and thrust force capabilities of a helicopter rotor decrease with increasing forward speed.
At high speeds, the retreating main rotor blades move at a speed that is lower than the helicopter's forward speed, leaving only a very low buoyancy. Under these circumstances, it is almost impossible to maintain a steady flight. The AH-64E Block 2 Compound with a classic rotor has the same problem, so it is complemented by large buoyancy wings that provide additional lift at higher speeds. stability. The S-97 Raider and SB-1 Defiant solve high-speed stability problems with counter-rotating rotors.
The compound helicopters and so-called convertiplanes are basically the most relevant concepts aiming to overcome the horizontal flight deficiencies of the dominant helicopters by introducing attributes of fixed-wing aircrafts as compromise. However, a compromise between both aircraft types has always to be conveniently adapted to the planned mission profile of the helicopter.
Compound helicopters with lift compounding, thrust compounding or a combination of both basically aim to off-load the main rotor from its simultaneous lifting and propulsive duties to allow for higher forward speeds of the compound helicopter. A lift compounding entails adding wings to a helicopter hence enabling to increase the load factor of the helicopter and to reach a higher manoeuvrability. This improves the efficiency of the helicopter at moderately high speed but at the expense of reduced efficiencies at lower forward speeds and in the hover.
A thrust compounding implies the addition of essentially horizontally oriented auxiliary propulsion units to the helicopter. This has been typically accomplished by means of a single or a pair of propellers being driven by drive shafts powered by the main turboshaft engines. The use of a pair of propulsion units has the advantage of providing for anti-torque capabilities without the need of an additional tail rotor, hence relativizing the inherent system complexity of the thrust compound configuration.
A more extended configuration of a compound helicopter includes both the addition of wings and propulsion units. The lift during cruise is simultaneously provided by the main rotor--in powered condition--usually addressed as "hybrid helicopter"--or in autorotation--"autogyro"-modus--and wings. The higher forward speed is provided by the horizontally oriented auxiliary propulsion units of the compound helicopter. The compound helicopter hence overcomes the rotor lift limits by means of the wings and the rotor thrust limits by means of the propulsion units. As a result, the benefit of a higher load factor is obtained along with potential for higher speed. The use of a pair of thrust propulsion units--opposed and both offset relative to each other and to a longitudinal axis of the helicopter--enables for a simultaneous torque correction.
A compound helicopter generally includes an airframe, a main rotor assembly, wings, a tail rotor and one or more propellers or ducted fans. The airframe has a main section, an upper section and a tail section. The main section is formed to define a cockpit that can accommodate a pilot and in some cases one or more crewmen and/or passengers. The upper section is disposed at an upper portion of the main section and the tail section is disposed to extend in the aft direction from the main section. The main rotor assembly is disposed at the upper section of the airframe and may include an upper and lower coaxial, counter-rotating rotors. The tail rotor is disposed at the tail section. The propeller or ducted fans is/are disposed at the tail section or along the side of the fuselage.
The helicopter further includes a flight control computer, an engine and a transmission. The engine is configured to generate power that can be used to drive rotations of the main rotor assembly and the propeller in order to generate lift and thrust for the helicopter. The transmission transmits the power to the main rotor assembly and the propeller. The flight control computer controls various operations of the engine and the transmission as well as the collective and cyclic operations of the main rotor assembly and the propeller(s) in accordance with pilot inputted commands and current flight conditions.
As a result of the compound helicopter including multiple propellers or ducted fans in addition to the tail rotor, the overall weight and part count of the compound helicopter can be relatively high. This can lead to performance degradation, such as reduced fuel economy to transport a given payload weight to a destination. Additionally, for the case of a coaxial compound helicopter, there are opportunities to improve the yaw control capability with respect to current coaxial pusher-prop designs.
Compound helicopters are not new, and several different approaches have been used in their design. For example, it has been proposed to use separate engines to provide power for driving the rotor and for producing horizontal propulsion. This approach has the disadvantage that during the cruise mode of operation, the engine driving the rotor is throttled back or even shut down, and then becomes dead weight, and the cruise engines do not contribute to the lift at take-off. An alternative approach, therefore, has been to make the same engine or engines perform the tasks of driving the rotor and providing forward propulsive thrust. This entails the problem of switching from one function to the other, and several different proposals have been made for doing this, none of which have yet found acceptance.
Compound helicopters such as the Cheyenne (AH-56A) have a main and tail rotors, but in addition they possess a small fixed wing and a rear propulsion propeller adjacent to the tail or antitorque rotor. The tail gearbox is driven through a drive train coupled to the main rotor drive shaft. Such aircraft are capable of both hover and high speed forward flight, but they require a large vertical lift force in hover, and a large horizontal thrust force at high forward speeds. For a winged compound helicopter which utilizes a pusher propeller for horizontal thrust, the hover and low speed propulsive forces are provided by the helicopter main rotor, with anti-torque/directional control being provided by a smaller tail rotor. In this hover and low speed flight regime both main rotor and tail rotor thrust requirements are high, with the pusher propeller rotating but producing no thrust. In the high speed forward flight regime, part of the required lift is transferred from the main rotor to the wing and directional control is obtained through normal aircraft control surfaces, thus eliminating the need for tail rotor thrust. Forward propulsive force is generated by the pusher propeller.
Generally, during high speed flight a compound helicopter requires full power to the pusher propeller, located at the extremity of the aft fuselage, with no thrust required from the tail rotor. Likewise in low speed and hover flight the tail rotor power requirement is approximately ten percent of total aircraft power with the remaining power being absorbed by the main rotor, which provides the required lift. This is generally true for all sizes of compound helicopters.
In an aerodynamically optimized tail drive propeller/rotor system it can be found that the propeller normally operates at speeds of approximately fifteen percent higher than the tail rotor. Thus in the high speed crusing flight regime almost the entire engine output is fed to the tail gearbox to drive the propulsion propeller, the main rotor absorbing only a small percentage of the lift and horsepower. As a consequence the anti-torque or tail rotor is also driven, requiring consumption of power, and producing increased drag. Insofar as I have been able to determine means are not known for stopping or feathering the tail rotor when driving the pusher propeller, or, conversely, stopping or slowing the pusher propeller, when the tail rotor is in operation.
It can be seen that present dynamic drive system design for compound aircraft requires that the three propulsive devices, the main rotor, tail rotor, and pusher propeller, be rotated continuously during all flight modes. Having either the tail rotor or pusher propeller rotating when it is not required to produce a propulsive force reduces the overall operational efficiency through decreasing power available, increasing fuel consumption, producing unnecessary aerodynamic drag, and creating unnecessary wear (life cycles).
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