05-04-2010, 06:43 PM
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INTRODUCTION
The Apache helicopter is a revolutionary development in the history of war. It is essentially a flying tank -- a helicopter designed to survive heavy attack and inflict massive damage. It can zero in on specific targets, day or night, even in terrible weather. As you might expect, it is a terrifying machine to ground forces.
In this topic, we'll look at the Apache's amazing flight systems, weapons systems, sensor systems and armor systems. Individually, these components are remarkable pieces of technology. Combined together, they make up an unbelievable fighting machine -- the most lethal helicopter ever created.
The Apache is the primary attack helicopter in the U.S. arsenal. Other countries, including the United Kingdom, Israel and Saudi Arabia, have also added Apaches to their fleet.
The first series of Apaches, developed by Hughes Helicopters in the 1970s, went into active service in 1985. The U.S. military is gradually replacing this original design, known as the AH-64A Apache, with the more advanced AH-64D Apache Longbow. In 1984, McDonnell Douglas purchased Hughes Helicopters, and in 1997, Boeing merged with McDonnell Douglas. Today, Boeing manufactures Apache helicopters, and the UK-based GKN Westland Helicopters manufacturers the English version of the Apache, the
WAH-64.
HELICOPTER BASICS
Helicopters are the most versatile flying machines in existence today.
This versatility gives the pilot complete access to three-dimensional space in a way that no airplane can.
The amazing flexibility of helicopters means that they can fly almost anywhere. However, it also means that flying the machines is complicated. The pilot has to think in three dimensions and must use both arms and both legs constantly to keep a helicopter in the air! Piloting a helicopter requires a great deal of training and skill, as well as continuous attention to the machine.
To understand how helicopters work and also why they are so complicated to fly, it is helpful to compare the abilities of a helicopter with those of trains, cars and airplanes. There are only two directions that a train can travel in forward and reverse. A car, of course, can go forward and backward like a train. While you are traveling in either direction you can also turn left or right:
A plane can move forward and turn left or right. It also adds the ability to go up and down. HA helicopter can do three things that an airplane cannot:
¢ A helicopter can fly backwards.
¢ The entire aircraft can rotate in the air.
¢ A helicopter can hover motionless in the air.
In a car or a plane, the vehicle must be moving in order to turn. In a helicopter, you can move laterally in any direction or you can rotate 360 degrees. These extra degrees of freedom and the skill you must have to master them is what makes helicopters so exciting, but it also makes them complex.
To control a helicopter, one hand grasps a control called the cyclic, which controls the lateral direction of the helicopter (including forward, backward, left and right). The other hand grasps a control called the collective, which controls the up and down motion of the helicopter (and also controls engine speed). The pilot's feet rest on pedals that control the tail rotor, which allows the helicopter to rotate in either direction on its axis. It takes both hands and both feet to fly a helicopter!
Imagine that we would like to create a machine that can simply fly straight upward. Let's not even worry about getting back down for the moment -- up is all that matters. If you are going to provide the upward force with a wing, then the wing has to be in motion in order to create lift. Wings create lift by deflecting air downward and benefiting from the equal and opposite reaction that results straight upward.
A rotary motion is the easiest way to keep a wing in continuous motion. So you can mount two or more wings on a central shaft and spin the shaft, much like the blades on a ceiling fan. The rotating wings of a helicopter are shaped just like the airfoils of an airplane wing, but generally the wings on a helicopter's rotor are narrow and thin because they must spin so quickly. The helicopter's rotating wing assembly is normally called the main rotor. If you give the main rotor wings a slight angle of attack on the shaft and spin the shaft, the wings start to develop lift.
In order to spin the shaft with enough force to lift a human being and the vehicle, you need an engine of some sort. Reciprocating gasoline engines and gas turbine engines are the most common types. The engine's drive shaft can connect through a transmission to the main rotor shaft. This arrangement works really well until the moment the vehicle leaves the ground. At that moment, there is nothing to keep the engine (and therefore the body of the vehicle) from spinning just like the main rotor does. So, in the absence of anything to stop it, the body will spin in an opposite direction to the main rotor. To keep the body from spinning, you need to apply a force to it.
The usual way to provide a force to the body of the vehicle is to attach another set of rotating wings to a long boom. These wings are known as the tail rotor. The tail rotor produces thrust just like an airplane's propeller does. By producing thrust in a sideways direction, counteracting the engine's desire to spin the body, the tail rotor keeps the body of the helicopter from spinning. Normally, the tail rotor is driven by a long drive shaft that runs from the main rotor's transmission back through the tail boom to a small transmission at the tail rotor. What you end up with is a vehicle that looks something like this:
A helicopter's main rotor is the most important part of the vehicle. It provides the lift that allows the helicopter to fly, as well as the control that allows the helicopter to move laterally, make turns and change altitude. The
Landing Skids
The Fundamental Parts of any Helicopter
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adjustability of the tail rotor is straightforward -- what you want is the ability to change the angle of attack on the tail rotor wings so that you can use the tail rotor to rotate the helicopter on the drive shaft's axis. To handle all of these tasks, the rotor must first be incredibly strong. It must also be able to adjust the angle of the rotor blades with each revolution of the hub. The adjustability is provided by a device called the swash plate assembly. The main rotor hub, where the rotor's drive shaft and blades connect, has to be extremely strong as well as highly adjustable. The swash plate assembly is the component that provides the adjustability.
The swash plate assembly has two primary roles:
¢ Under the direction of the collective control, the swash plate assembly can change the angle of both blades simultaneously. Doing this increases or decreases the lift that the main rotor supplies to the vehicle, allowing the helicopter to gain or lose altitude.
¢ Under the direction of the cyclic control, the swash plate assembly can change the angle of the blades individually as they revolve. This allows the helicopter to move in any direction around a 360-degree circle, including forward, backward, left and right.
POWER AND FLIGHT
At its core, an Apache works pretty much the same way as any other helicopter. It has two rotors that spin several blades. A blade is a tilted airfoil, just like an airplane wing. As it speeds through the air, each blade generates
lift.
The main rotor, attached to the top of the helicopter, spins four 20-foot (6-meter) blades. The pilot maneuvers the helicopter by adjusting a swash plate mechanism. The swash plate changes each blade's pitch (tilt) to increase lift. Adjusting the pitch equally for all blades lifts the helicopter straight up and down. Changing the pitch as the blades make their way around the rotation cycle creates uneven lift, causing the helicopter to tilt and fly in a particular direction. As the main rotor spins, it exerts a rotation force on the entire helicopter. The rear rotor blades work against this force -- they push the tail boom in the opposite direction. By changing the pitch of the rear blades, the pilot can rotate the helicopter in either direction or keep it from turning at all. An Apache has double tail rotors, each with two blades.
The newest Apache sports twin General Electric T700-GE-701C turboshaft engines, boasting about 1,700 horsepower each. Each engine turns a drive shaft, which is connected to a simple gearbox. The gearbox shifts the angle of rotation about 90 degrees and passes the power on to the transmission. The transmission transmits the power to the main rotor assembly and a long shaft leading to the tail rotor. The rotor is optimized to provide much greater agility than you find in a typical helicopter.
The core structure of each blade consists of five stainless steel arms, called spars, which are surrounded by a fiberglass skeleton. The trailing edge of each blade is covered with a sturdy graphite composite material, while the leading edge is made of titanium. The titanium is strong enough to withstand brushes with trees and other minor obstacles, which is helpful in "nap-of-the-earth" flying (zipping along just above the contours of the ground). Apaches need to fly this way to sneak up on targets and to avoid attack. The rear tail wing helps stabilize the helicopter during nap-of-the-earth flight as well as during hovering.
You could say, based on all this information, that the Apache is just a high-end helicopter. But that would be like calling James Bond's Aston Martin just a high-end car. As we'll see in the next few sections, the Apache's advanced weaponry puts it in an entirely different class.
HELLFIRE MISSILES
The Apache's chief function is to take out heavily armored ground targets, such as tanks and bunkers. To inflict this kind of damage, you need some heavy firepower, and to do it from a helicopter, you need an extremely sophisticated targeting system.
The Apache's primary weapon, the Hellfire missile, meets these demands. Each missile is a miniature aircraft, complete with its own guidance computer, steering control and propulsion system. The payload is a high-explosive, copper-lined-charge warhead powerful enough to burn through the heaviest tank armor in existence.
The Apache carries the missiles on four firing rails attached to pylons mounted to its wings. There are two pylons on each wing, and each pylon can support four missiles, so the Apache can carry as many as 16 missiles at a time. Before launching, each missile receives instructions directly from the helicopter's computer. When the computer transmits the fire signal, the missile sets off the propellant. Once the burning propellant generates about 500 pounds of force, the missile breaks free of the rail. As the missile speeds up, the force of acceleration triggers the arming mechanism. When the missile makes contact with the target, an impact sensor sets off the warhead.
Fires two Hellfire missiles in a training exercise
The original Hellfire design uses a laser guidance system to hit its mark. In this system, the Apache gunner aims a high-intensity laser beam at the target (in some situations, ground forces might operate the laser instead). The laser pulses on and off in a particular coded pattern.
Before giving the firing signal, the Apache computer tells the missile's control system the specific pulse pattern of the laser. The missile has a laser seeker on its nose that detects the laser light reflecting off the target. In this way, the missile can see where the target is. The guidance system calculates which way the missile needs to turn in order to head straight for the reflected laser light. To change course, the guidance system moves the missile's flight fins. This is basically the same way an airplane steers.
Holds four Hellfire missiles.
The laser-guided Hellfire system is highly effective, but it has some significant
drawbacks:
¢ Cloud cover or obstacles can block the laser beam so it never makes it to the target.
¢ If the missile passes through a cloud, it can lose sight of the target.
¢ The helicopter (or a ground targeting crew) has to keep the laser fixed on the target until the missile makes contact. This means the helicopter has to be out in the open, vulnerable to attack.
The Hellfire II, used in Apache Longbow helicopters, corrects these flaws. Instead of a laser-seeking system, the missile has a radar seeker. The helicopter's radar locates the target, and the missiles zero in on it. Since radio waves aren't obscured by clouds or obstacles, the missile is more likely to find its target. Since it doesn't have to keep the laser focused on the target, the helicopter can fire the missile and immediately find cover.
ROCKETS & CHAIN GUNS
Apaches usually fly with two Hydra rocket launchers in place of two of the Hellfire missile sets. Each rocket launcher carries 19 folding-fin 2.75-inch aerial rockets, secured in launching tubes. To fire the rockets, the launcher triggers an igniter at the rear end of the tube. The Apache gunner can fire one rocket at a time or launch them in groups. The flight fins unfold to stabilize the rocket once it leaves the launcher.
The rockets work with a variety of warhead designs. For example, they might be armed with high-power explosives or just smoke-producing materials. In one configuration, the warhead delivers several sub munitions, small bombs that separate from the rocket in the air and fall on targets below.
The gunner engages close-range targets with an M230 30-mm automatic cannon attached to a turret under the helicopter's nose. The gunner aims the gun using a sophisticated computer system in the cockpit. The computer controls hydraulics that swings the turret from side to side and up and down.
The M-230A1 30-mm automatic cannon on an AH-64A Apache
The automatic cannon is a chain gun design, powered by an electric motor. The motor rotates the chain, which slides the bolt assembly back and forth to load, fire, extract and eject cartridges. This is different from an ordinary machine gun, which uses the force of the cartridge explosion or flying bullet to move the bolt.
The cartridges travel from a magazine above the gun down a feed chute to the chamber. The magazine holds a maximum of 1,200 rounds, and the gun can fire 600 to 650 rounds a minute. The cannon fires high-explosive rounds designed to pierce light armor.
CONTROLS & SENSERS
The Apache cockpit is divided into two sections, one directly behind the other. The pilot sits in the rear section, and the co-pilot/gunner sits in the front section. As you might expect, the pilot maneuvers the helicopter and the gunner aims and fires the weapons. Both sections of the cockpit include flight and firing controls in case one pilot needs to take over full operation.
The pilot flies the Apache using collective and cyclic controls, similar to ones you would find in any other helicopter. The controls manipulate the rotors using both a mechanical hydraulic system and a digital stabilization system. The digital stabilization system fine-tunes the powerful hydraulic system to keep the helicopter flying smoothly. The stabilization system can also keep the helicopter in an automatic hovering position for short periods of time.
On the Longbow Apache, three display panels provide the pilot with most navigation and flight information. These digital displays are much easier to read than traditional instrument dials. The pilot simply presses buttons on the side of the display to find the information he or she needs.
Inside the Apache Longbow cockpit
One of the coolest things about the Apache is its sophisticated sensor equipment. The Longbow Apache detects surrounding ground forces, aircraft
and buildings using a radar dome mounted to the mast. The radar dome uses millimeter radio waves that can make out the shape of anything in range. The radar signal processor compares these shapes to a database of tanks, trucks, other aircraft and equipment to identify the general class of each potential target. The computer pinpoints these targets on the pilot's and gunner's display panels.
The pilot and the gunner both use night vision sensors for night operations. The night vision sensors work on the forward-looking infrared (FLIR) system, which detects the infrared light released by heated objects. The pilot's night vision sensor is attached to a rotating turret on top of the Apache's nose. The gunner's night vision sensor is attached to a separate turret on the underside of the nose. The lower turret also supports a normal video camera and a telescope, which the gunner uses during the day.
The computer transmits the night vision or video picture to a small display unit in each pilot's helmet. The video display projects the image onto a monocular lens in front of the pilot's right eye. Infrared sensors in the cockpit track how the pilot positions the helmet and relay this information to the turret control system. Each pilot can aim the sensors by simply moving his or her head! Manual controls are also available, of course.
EVASION & ARMOUR
The Apache's first line of defense against attack is keeping out of range. As we saw earlier, the helicopter is specifically designed to fly low to the ground, hiding behind cover whenever possible. The Apache is also designed to evade enemy radar scanning. If the pilots pick up radar signals with the onboard scanner, they can activate a radar jammer to confuse the enemy.
The Apache is also designed to evade heat-seeking missiles by reducing its infrared signature (the heat energy it releases). The Black Hole infrared suppression system dissipates the heat of the engine exhaust by mixing it with air flowing around the helicopter. The cooled exhaust then passes through a special filter, which absorbs more heat. The Longbow also has an infrared jammer, which generates infrared energy of varying frequencies to confuse heat-seeking missiles.
The Apache is heavily armored on all sides. Some areas are also surrounded by Kevlar soft armor for extra protection. The cockpit is protected by layers of reinforced armor and bulletproof glass. According to Boeing, every part of the helicopter can survive 12.7-mm rounds, and vital engine and rotor components can withstand 23-mm fire.
The area surrounding the cockpit is designed to deform during collision, but the cockpit canopy is extremely rigid. In a crash, the deformation areas work like the crumple zones in a car they absorb a lot of the impact force, so the collision isn't as hard on the crew. The pilot and gunner seats are outfitted with heavy Kevlar armor, which also absorbs the force of impact. With these advanced systems, the crew has an excellent chance of surviving a crash.
Flying an Apache into battle is extremely dangerous, to be sure, but with all its weapons, armor and sensor equipment, it is a formidable opponent to almost everything else on the battlefield. It is a deadly combination of strength, agility and firepower.
APACHE HELICOPTER AERODYNAMIC FORCES
We take a look at four basic aerodynamic forces: lift, weight, thrust and drag.
Straight and Level Flight
In order for an airplane to fly straight and level, the following relationships must be true:
¢ Thrust = Drag . Lift = Weight
If, for any reason, the amount of drag becomes larger than the amount of thrust, the plane will slow down. If the thrust is increased so that it is greater than the drag, the plane will speed up. Similarly, if the amount of lift drops below the weight of the airplane, the plane will descend. By increasing the lift, the pilot can make the airplane climb.
APACHE HELICOPTER THRUST
Thrust is an aerodynamic force that must be created by an airplane in order to overcome the drag (notice that thrust and drag act in opposite directions in the figure above). Airplanes create thrust using propellers, jet engines or rockets. In the figure above, the thrust is being created with a propeller, which acts like a very powerful version of a household fan, pulling air past the blades.
DRAG
Drag is an aerodynamic force that resists the motion of an object moving through a fluid (air and water are both fluids). It acts opposite to thrust.
WEIGHT
This one is the easiest. Every object on earth has weight (including air).
LIFT
Lift is the aerodynamic force that holds an airplane in the air, and is probably the trickiest of the four aerodynamic forces to explain without using a lot of math. On airplanes, most of the lift required to keep the plane aloft is created by the wings (although some is created by other parts of the structure).
APACHE HELICOPTER CONCLUSION
With the design of the apache the very concept of helicopter itself has changed all over the world. Many countries like Russia, Germany etc. have rolled over their versions of attack helicopters. They replaced the main drawbacks of apache. But it can be surely emphasized that the Apache is the pioneer in the attack helicopter family. In this seminar I've tried to put forward some of the design features of the same.
REFERENCE
howstuffworks.com
answers.com
google.com
wikiepedia.org
helicopters.com
apachehelicopters.com
