These things are unique to helicopters. Well, most everything else that was already stated was unique to helicopters, but these are more things unique to helicopters. Now, some may argue that NOE (Nap of the Earth) flight can be performed in airplanes, but nothing like the way it can be done in a helicopter. I used to take the Airforce pilots out during survival training exercises low and fast over the trees. They would all comment that they never had experienced any kind of flying like that. The lowest they ever got was 500 FT AGL. Yes, they went much faster than we ever did in a helicopter, but the sensation of speed and the margin of error at 500 FT is much different than it is at 5 FT. In the summer, we would fly with the doors taken off the aircraft. When you are flying 100 Knots at 5 FT with the doors off, it becomes a very exciting and unique experience.
ETL is a state of flight where the aircraft leaves the cushion of ground effect, and starts to fly forward into "Clean Air". It is reached at approximately 15-20 knots of forward airspeed, and is noticeable to the pilot as he/she must change pedal inputs from heavy left pedal inputs at a hover to a more neutral setting. The pilot will also feel a shuttering in the rotor system as the aircraft begins to fly out of the recycled rotor wash generated from the aircraft rotor system. The cyclic will try to move backward in the pilots hand as the rotor system wants to "Blow Back". The front of the rotor system will try to rise and slow the aircraft automatically. This happens since the front of the rotor system is flying in clean air first, and the aft portion of the rotor system is still not in clean air quite yet. It is then that the pilot will further induce forward cyclic inputs to keep the aircraft moving ahead to gain airspeed. ETL can best be characterized as the transition from a hover to forward flight.
Nap of the earth flight (NOE) is a mode of flight in which the pilot must keep the aircraft very close to the ground, following the contours of hills, streams, canyons, and all other land features. Often, the aircraft is within a few feet of power lines, trees, grass, rocks, and other obstacles. This is usually a form of combat maneuvering where you try to limit the enemy's view of the aircraft. If they can't see it, they usually can't shoot at it. It was developed as an answer to the problem of being shot down by heat seeking missiles during the Viet Nam conflict. Pilots used to fly very high so they were out of the range of small arms fire. Once the deployment of heat seeking shoulder fired missiles had taken place, the helicopter pilots needed to develop a new tactic for routine flying to limit the effectiveness of these portable and very lethal weapons. By putting trees, hills and other ground contours between the person with the missile and the aircraft, the pilots found themselves in a lot safer environment. Also, small arms fire was still not as much of a problem since the aircraft was directly over the shooter for a very short period, and they could not sight in on the target in so little time. By the time a shooter could get his weapon raised, the aircraft was usually out of sight or there would be some obstacle between the aircraft and the shooter. In NOE flight the pilot must be in complete control of the aircraft. All obstacles must be avoided, and navigation must be performed with fewer visual queues for the pilot to work with. Things look very different at such a low altitude. Ground navigation techniques must be employed as normal flight navigation is almost useless at these low altitudes. Aircraft limitations must not be exceeded, radio calls must be performed, and time schedules must be met. NOE requires an instant division of attention, and quick reflex actions. If you start taking fire from the ground, an alternate route must be established immediately. If obstacles become a problem, evasive maneuvers must be initiated without hesitation. At maximum airspeed, and minimal altitude, instantaneous decisions must be made at all times, or the aircraft could become a smoking hole in the ground in a matter of seconds. NOE flight can be the most exciting, rewarding, and physically exhausting flying anyone could do in a helicopter.
On the front of most U.S. Army and many civil helicopters you may notice a knife like fixture on the top of the cockpit, and one on the bottom of the aircraft near the chin bubbles. These are not antennae for radios like most people believe. They are part of the Wire Strike Protection System (WSPS).
The WSPS is made up of several components to protect the helicopter from high wire strikes. It was developed because of the increased risk of wire strikes while flying at NOE altitudes. If a helicopter hits a power line (Telephone line, electrical line, guy wire for a tower, or any other wire obstacle), the rotor system may become entangled with the wire, and catastrophic failure of the rotor system could lead to total destruction of the aircraft.
The WSPS was developed to reduce the severity of a wire obstacle collision by diverting the wire into the cutter blade assemblies. The cutter blades affixed to the top and bottom of the frontal area of the aircraft will usually cut the wire and eliminate the hazard. On UH - 1 Huey helicopters, a set of bars will carry the wire over external parts of the windshield wipers. On OH-58 helicopters, the center section of the windshield has an abrasive cutting strip (Built into the windshield deflector) to score the wire and weaken it before it comes in contact with the WSPS cutters. The WSPS system protects 90% of the frontal area of the helicopter, and reduces the hazard from most wire strikes. With the WSPS, the pilot has a 95% chance of surviving a single wire strike. The odds of survival decrease as the number of wires increases. 2 wires will reduce the chances to 75%, 3 wires to 50%, and 4 wires to about 25%. Although the WSPS system is quite effective, care must still be used to avoid all wire obstacles. Apache, Cobra, and Blackhawk helicopters all have a smaller, less noticeable WSPS system on them. They can usually be seen just above the cockpit, and near landing gear struts. Chinooks do not have WSPS systems.
Here is an excerpt from an e-mail I received (January 2009):
I work for (Company) where I am responsible for the marketing of the Wire Strike Protection System (WSPS). My concern is that the following information relating to the WSPS is incorrect:
"The WSPS system protects 90% of the frontal area of the helicopter, and reduces the hazard from most wire strikes. With the WSPS, the pilot has a 95% chance of surviving a single wire strike. The odds of survival decrease as the number of wires increases. 2 wires will reduce the chances to 75%, 3 wires to 50%, and 4 wires to about 25%."
No study has ever been conducted to determine chance of survival percentage. There are too many variables that effect the chance of survival (velocity, yaw, type of cable, type of helicopter, etc) to be able to assign percentages. If you have data to support this claim please share with me. I'd be interested to see it. In addition, the 90% protection of the frontal area of a helicopter varies from model to model. Some helicopters are higher than 90% coverage and some are much lower. I would appreciate if you could make the necessary changes to reflect these concerns.
(End of E-mail excerpt)
The original information posted here came from the U.S. Army Aviation Center (Unclassified) literature concerning WSPS systems on Army helicopters. There are always two sides to just about every story, and this seems to be no exception. Now, in the interest of fairness and a better understanding, you have both.
Most people think that a helicopter will fall like a rock and the rotor system will stop once the engine fails. This is a totally false assumption. A helicopter can continue to fly without any power from the engine. "Autorotation" is the term used for "Gliding" a helicopter down after the engine fails or the throttle is retarded to the idle position.
If you look at a rotor blade from the tip of the blade toward the root, you will see it will twist laterally. At the tip of the blade, the leading edge may point down while at the root of the blade, the leading edge may point up. This allows different regions of the blade to perform different tasks, one of which is Autorotation. The outer portion of the blade, when the collective is lowered all the way to what is called "Flat Pitch", will drive the rotor system as the aircraft glides downward, increasing or maintaining the speed of the rotor system. The rotor system is driven normally by a centrifugal clutch which is positively engaged while the engine supplies power, but disengages when power is removed. The rotor system "Free Wheels", and continues to spin. The air traveling upward through the rotor system continues to drive the system and maintain rotor RPM.
The aircraft descends rather rapidly, but with a high rotor RPM, the aircraft can be cushioned to the ground effectively and landed without incident. Additional weights are housed in the tips of the rotor blades to increase the inertia of the rotor system, and aid in autorotation. The procedure for autorotation is to lower the collective immediately and put in full right pedal, and enter a steady state of autorotation. Full right pedal must be put in because the torque has stopped from the lack of engine power, and the tail rotor thrust is only necessary at this point to control aircraft trim. (By putting in right pedal, you effectivly neutralize the tail rotor, and it provides no thrust). The pilot must find a suitable landing area, and maneuver as necessary to make the intended landing area, making certain that the rotor RPM is within limits. At approximately 100 FT AGL (Above Ground Level) start a progressive deceleration to decrease forward airspeed, and about 15 FT AGL, lift the collective in a quick jerking motion to cushion the aircraft initially as a vertical brake. The initial collective pitch pull should be enough to retard the descent, and the rest of the collective pitch should be pulled in gradually and continually as the aircraft settles to the ground. You should land with little or no forward airspeed, and the landing should be relatively soft, depending on the surface you are landing to. A safer autorotational approach depends on where you land. If you land in a field where forward movement would be dangerous, you should plan for a shorter landing run. This requires a more vertical drop in the last part of the autorotation. If you have the room to slide, then a more shallow approach can be made and a longer ground run should be allowed. A more vertical drop is harder to accomplish and your timing needs to be a lot more precise, where a shallower drop is more forgiving and you need to be less precise on your timing.
In a Chinook, the rear wheel locks are electrically operated. During autorotation, If the rotor RPM decreases below a certain value, the generators will fall off line, and the wheel locks will disengage. If this happens, the aircraft will most likely land aft wheels first, and without swivel locks engaged on the rear wheels, it could make for a very interesting ride.