Tue, Nov 17, 2009 — David Evans
The aviation industry as we know it today is possible because of a technology dating back to the Romans: concrete. A slab of concrete two miles or more in length makes it possible for passenger and cargo jets to get into the air and land safely.
When runways aren’t available, the flying machine is compromised, as every aspect of its design must be focused on getting into the air with nil forward momentum, and landing without benefit of bleeding off speed through brakes and/or thrust reversers.
Helicopters and their variants – tilt rotors or tilt wings – are optimized for the first and last phases of flight. They pay a penalty through reduced cruise performance.
Airliners are optimized for the cruise portion of flight, and the runway is necessarily long enough to accommodate the takeoff and landing performance needed to support a flying machine that is most “comfortable” moving through the stratosphere at more than 500 miles per hour.
The design of a modern jet has been honed to a fare thee well to cruise at 30,000 feet or higher. The jet engine sips fuel most efficiently at altitude, i.e., achieves optimum air nautical miles per pound of fuel in less dense air. The wings are angled back to provide lift while minimizing the effects of drag.
Getting airborne and down again is where design compromises and added complexity must necessarily play their role. The landing gear must be robust enough to support takeoff rolls and landing impacts, but not weigh too much. The engines’ power must provide thrust enough for takeoff and climb but then be efficient when throttled back fuel-sipping cruise. The wings must provide low speed lift for initial climb and the later approach for landing, and that entails deployment of all manner of flaps and slats; however, for cruise these “high lift devices” must be retracted or stowed so that the basic “clean” wing can do its optimized job.
Even the cockpit of a modern airliner reflects this design approach. Rather than ensconcing the pilot and co-pilot in a bubble canopy, which would provide enhanced visibility but at a cost in drag, the pilots are seated in a flight deck with limited forward visibility – just enough to control the airplane during takeoff and landing. Various electronic warning systems aid the pilots in avoiding other aircraft and the ground.
The modern jet is tailored for high altitude cruise because a couple miles of concrete at each end of its flight makes the engineering compromises acceptable.
The helicopter represent the other extreme – what design compromises must necessarily be made to allow takeoffs and landing without benefit of a long slab of concrete. Helicopters must take off and land on isolated pads, platforms, and in confined areas or, sometimes, operate from no prepared area at all. Helicopters may also be required to carry external loads, hover out of ground-effect, winch persons aboard while hovering, and land on sloping surfaces.
How is this done?
A modern jetliner’s wings begin to generate lift at about 50 knots, and at about 180 knots sufficient lift will be produced such that all the pilot has to do is pull back on the control column or sidestick and the airplane will be wing-borne. It’s almost as if the is yearning to return to its natural element.
Momentum begets lift, which enables flight.
When there is no forward momentum, the wing itself must nevertheless be moved with enough speed to produce lift. This is done by spinning the airfoils, known as the main rotor, on the helicopter. Consisting of two or more large blades, the rotor must be strong enough to support the helicopter in flight and flexible enough to meet the needs of control. As each blade moves forward, providing lift, it also moves to the rear as it rotates. The rotor disk must provide lift during this phase, as well, by having its blades change their angle of attack during each revolution. An assembly known as the swash plate enables this constant repositioning of each blade’s angle of attack in order to provide a uniform lift vector as it rotates. The blade’s incidence is reduced as it moves forward in the direction of flight (i.e., the advancing blade), and then the incidence is increased as the blade passes through the 12 o’clock (ahead) position and proceeds toward the rear.
The entire disk is tilted forward slightly, so that some of the downward vector of lift is partly converted to a rearward vector of thrust. Thus, as a helicopter lifts off, one will observe that the helicopter will angle forward – to “transition” to disk-borne flight and climb. Transition (from hovering flight) generally occurs at around 15-25 knots of forward speed and is held to occur when the helicopter is no longer being supported by the corkscrew spiral of its blade rotation, but by the moving disk acting as an elliptical airfoil.
Most helicopters have one main rotor, powered by either a single turbine engine or a pair. The engines are connected to the rotor through a transmission gearbox, which is a complex of marvelous engineering with two disadvantages: (1) the gearbox adds weight to the helicopter that is not seen in fixed wing airplanes, where the engines provide power directly, without the encumbrance of gearing and whatnot, and (2) if the helicopter gearbox should fail in flight, the result is usually catastrophic; therefore, the gearbox must designed with the strength not to fail, even if its lubrication does, thereby invoking a weight penalty.
The main rotor’s spinning momentum also tends to impart a spinning moment to the entire airframe. To prevent this, designers have a couple choices. They can create a counterforce through a small rotor mounted on a long moment arm, the tail. Alternatively, two main rotors, spinning in opposite directions, can cancel each other out.
Fine control, to turn left and right, for instance, is done through a combination of pilot adjustments to the main and tail rotors, via his collective lever (blade-angle), cyclic stick (for disk-tilt) and tail-rotor pedals (for law control).
To minimize the weight of the power train, it is located close to the base of the main rotor, on the top of the fuselage.
Fuel is located in sponsons at the base of the fuselage, or under the floorboards. This leads to a major problem – in a crash, the crew and passengers are frequently crushed or injured by heavy power train components coming down from above, and fuel escaping ruptured tanks below. In a fixed wing airplane, fuel is often carried in the wings, and tail mounted engines provide further separation and fire protection. Such an arrangement is not possible for helicopters, so the risk of fire in a crash is particularly acute.
In terms of crashes, medical evacuation helicopters, for example, have some 30 times greater accident toll than commercial airliners. There is a phrase used to waggishly describe the relative risks: “An airplane inherently wants to fly; a helicopter inherently wants to crash.” In practical terms, the helicopter is subject to such unique phenomena as retreated blade stall, mast bumping, vortex ring (“setting with power”), tail-rotor failure and, of course, engine failure. In this latter case, an engine-off autorotation can be attempted, which will result in a not necessarily catastrophic “run-on” landing. Helicopters are very vulnerable to wind-shifts when operating at high all-up weights near the ground. Helicopter pilots conducting external load operations must constantly consult W.A.T. (weight/altitude/temperature) charts. The first indication of weight troubles can be rotor RPM loss or loss of directional control in the hover. Suffice to say, there is a lot that can go awry in a helicopter.
As the helicopter takes off and lands, pilot vision is especially important. Accordingly, the pilot cockpit features large plexiglass windows with good visibility forward, above, and down. As the helicopter normally operates at lower altitudes, good visibility is essential for obstacle avoidance on approach/departure and for visual navigation. Helicopters figure much more often in wire-strike accidents than do fixed wing machines.
In the helicopter cruise, we see the greatest compromise: speed. A modern helicopter may achieve a cruise speed of 150-180 knots, but that’s it. To achieve greater speed would require some means of in-flight folding (or telescoping) of the rotors and relying on wings, a complexity deemed not worth the effort. A variant of helicopter technology known as the tilt-rotor basically takes two engines, mounted on either side of modest wings, and has them spin propellers. The engines rotate, so what the machine takes off, the engines are vertical and the propellers function as helicopter rotors. For cruise, the engines are rotated to the horizontal, and the propeller-rotor blades provide thrust. The forward speed is less than half that of a modern airliner, the mechanical complexity is daunting, and the flight characteristics are unforgiving – even though interconnected by a cross-ship shaft, the catastrophic failure of an engine, rotor, tilt assembly or transmission on one side would usually spell the loss of the entire machine. The cost of a tilt-rotor is also considerable, greater than either a helicopter or a fixed wing airplane.
Because of the many compromises necessary to enable take offs and landings without a runway, helicopters have been restricted to niche applications: offshore oil platform servicing, aerial surveys, logging, agricultural chemical applications, search and rescue, emergency medical evacuation, air tours and some corporate flying. The tasking as always over relatively short distances, at low altitudes (which puts helicopters “in the weather” more often than fixed wing operations), at slow speeds. Helicopters are also very vulnerable to icing conditions.
The modern helicopter provides an essential capability, but its ability to take off and land vertically imposes design compromises that must be recognized. There is a reason millions of people regularly fly across the Atlantic in fixed wing airplanes. One would rarely envisage doing so in a helicopter, but for servicing an offshore oil platform, the helicopter is the only way to go.