A stealth helicopter does not need to have a tail rotor. Even non-stealth helicopters have been designed (decades ago) that do not have a tail rotor (called NOTAR (No Tail Rotor)). Instead, they use thrust-vectoring through a jet to achieve the same effect. Examples of such helicopters are MD 520N, MD 600N etc. Stealth aircrafts like F- 117 Nighthawk use folding or retracting antenna aerials. A rotor is much worse than an antenna from the point of radar detection. By its very design, a helicopter is agile and is able to fly low and slow. It can follow terrain more closely than a fixed-wing aircraft. Thus it is able to fly under the radar without being stealth. For low-observability, the most important modifications for a helicopter are to reduce its visual, audio and thermal signature. Radar signature can be reduced to some extent by designing a diamond-shaped fuselage but that would only be effective if the aircraft does not change its attitude (turn or climb). Additionally all the weapons like gun-turret or rocket/missile/grenade launchers have to be mounted internally. Probably it was due to same difficulties that RAH-66 Commanche program was cancelled. The R in RAH-66 stands for Reconnaissance, A for Attack and obviously H is for Helicopter.
The difficulty of making rotors stealth can be judged from the fact that till now all the stealth or even low-observable aircrafts have been powered by jet engines. No stealth propeller-powered aircraft is known to exist and for obvious reasons. Even the jet engine powered aircrafts have to hide their front compressor blades by using an S-shaped air intake duct (like used on F/A-18E Super Hornet, Dassault Rafale and Dassault Neuron) and/or a fine grill in front of the air intake (like used on F-117 Nighthawk). All this is only meant for forward sector stealth because the exhaust turbine also has blades and is visible from the nozzle side. But then the low-observable aircrafts generally do not care about back/rear sector stealth. The Lockheed F-117 on the other hand is an all-aspect stealth aircraft and therefore it has special flat exhaust nozzles to hide its turbine blades. Hiding the rotating blades of jet engines is so important that even non-stealth aircrafts like F/A-18E/F Super Hornet have to use it (intake side only). Now in a helicopter, the blades of main rotor are much bigger, totally exposed and having both forward and backward sweep, not to mention the rotor hub, blade pitch control mechanisms, big windscreens and top-mounted engines. Even incorporating composite materials in the rotor would not help much.
Now lets focus on them forward-swept elevator. Stealth designs till now have exclusively used backward swept surfaces. Even low-observable (say semi-stealth) aircrafts have employed backward sweep for all aerodynamic surfaces. A good look at the Lockheed F/A-22 Raptor would reveal that both its wings and elevators have the same degree of backward sweep. Even different degrees of sweep are not tolerated. Now using a forward-swept surface on a stealth helicopter is not justified. Coming back to the main rotor, an advancing blade of the main rotor would have a forward sweep and a retreating blade would have a backward sweep. These alternating forward and backward sweeps do not help in reducing the radar signature of a helicopter. Then we have the inevitable element of large transparent glass windows in a helicopter. A transparent glass would let all the radar waves come inside the helicopter cabin, get reflected around and some waves would go back to the radar. It is like a cavity reflector. Some treatments do exist for reducing the radar reflections from transparent glass but they can't completely eliminate the problem.
Lastly we come to then top-mounted gas turbine engines (turboshafts) of a helicopter. These turboshaft engines are mounted on the top to avoid engine damage in case of a crash landing from a few meters height and for safety from small arms fire. Now in case of two engines, we would have two air intakes, two exhausts and therefore four big holes with four rotors visible (two turbines from exhaust side and two fans or compressors from intake side). As if the issues of main and tail rotors were not enough, we have to deal with the engine rotors too. All these things make it very difficult to keep the radar cross section of a helicopter low. According to the available reports on the net, when summoned by the Judicial Commission, Air Chief Marshal Rao Qamar Suleiman of Pakistan Air Force pointed out the policy of government (USA being an ally and not declared as threat). He should also have discussed the junkyard stealth helicopter.
One thing that almost everybody forgets is the radar network of the civil aviation authority (CAA). The CAA radars at Islamabad, Peshawar, Parachinar etc should have detected the unidentified aircrafts. If there were no primary radars on these airports, then they should be installed. Pakistan can't afford to have separate civil and military radars with separate functions. Both civil and military radars should do the surveillance work. The civil aviation radars should be tuned to detect fast flying fighter jets too. But for helicopters, even the de-tuned civil radars could have easily detected them provided they were not flying very low. Historically, during such intrusions, the radars are either shut down (Iraq reactor case) or are in maintenance (Pakistan Abbottabad Raid). A city or zone is not protected by only one radar. Most of the time, radar coverage is of overlapping nature, meaning that one radar covers the outer limits of the other radar, so that there is no blind spot left. So technically, the helicopters coming into Abbottabad should have been detected, once flying over a flat terrain. The radar recording can prove whether there were any contacts or not.
After detection on radar, a contact needs to be classified as friendly or foe. This task is performed by IFF (Identification of Friend or Foe) equipment of aircrafts. For reasons that are hard to explain, Pakistan Army Aviation helicopters did not carry IFF Equipment. Even if detected, it was not possible to say that there were not Pakistan's own choppers. If there was no alert, the radar crews simply believed a friendly movement. Thats what Air Chief of the time had said, that PAF had not trained for or anticipated an attack from USA. What nobody discussed was that why Pakistan Army helicopters DID NOT carry IFF Equipment? So, most probably, the intruding helicopters reached Abbottabad safely due to lack of IFF sets on the Pakistani helicopters. NO STEALTH WAS EVER NEEDED FOR THIS MISSION. Why would you bother about Stealth when Pakistan Army Aviation has no IFF Equipment? Furthermore, it was shown that the stealth helicopter crashed over the wall of the OBL's compound but the wall was not damaged. The wall of the compound should have been damaged. Also there were no evident marks of fire on the helicopter fuselage which was shining in the sunlight. In theory, the helicopter was carrying fuel for the return trip too and a fire would have burnt the helicopter beyond recognition.
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