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All about Rotary Wing Aircraft

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"All about Rotary Wing Aircraft"
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Achieving heavier than air flight was a triumph in engineering and aerodynamics. But it was only a baby step down the path toward a successful helicopter. Thirty-six years would pass before Igor Sikorski made the first tethered flight in his VS 300. Sikorski had an uncanny ability to solve the problems of the rotary wing aircraft. He insisted on personally flight testing each of his breakthroughs. Being the first, Sikorski had to learn to fly what is still the most difficult and dangerous type of aircraft.

Designing the rotor blade was the easy part, as earlier research into airfoils (wings) and angle of attack directly applied. Sikorski only needed to make them smaller and devise the mechanism of the rotor head that would spin them. Understanding how the spinning rotor disk behaved and how to control it was a very different matter.

Airflow moves past a wing in one direction. Not so for a helicopter rotor, because this is only true for part of the path the blade follows. As the blade turns toward the rear, it then faces backwards. This changes the lift production of each blade, causing the blades to rise and fall each rotation. As drag increases with increased lift, each blade also retreats and advances with each rotation. The tendency of each blade to rise, fall, advance and retreat hundreds of times per minute was mechanically disastrous. Only the invention of "drag" hinges and "flap" hinges would solve these issues.

Sikorski had to solve all these problems before his machine could be flown and controlled. Each lesson was learned the hard way, with several crashes. It's amazing (and lucky for us) that he wasn't killed.

As if these weren't enough problems, the gyroscopic effect has to be taken into account. Any change in input to the spinning rotor disk happens 90 degrees later. Sikorski had to devise his control system to adjust the rotor head 90 degrees earlier to account for this.

The spinning rotors cause torque to be applied to the airframe. The tail rotor cancels this effect but adds weight, complexity and uses about one quarter of the horsepower. Failure of this system guarantees disaster. Twin, counter rotating rotor helicopters like the CH-47 Chinook avoid wasting this power. Some Russian helicopters stack two counter-acting rotors. More efficient Fennistrom rotors are contained in a shroud, reducing wasteful tip vortexes. Notar (no tail rotor) helicopters use directed engine thrust. A helicopter pilot lets his machine turn on the vertical axis by adjusting the pitch of the tail rotor blades. This lift adjustment allows the main rotor torque to affect a turn and is controlled by the foot pedals.

Engine power is adjusted to maintain a constant rotor rpm, (between about 385 - 470 rpm at the blade tip). It's vital the blade tip does not go supersonic, otherwise it will stop generating lift. As the tail rotor is much shorter it will spin about ten times faster.

Articulated rotor heads control the pitch of all the blades collectively with yet another hinge, hence the name of the "collective" control stick. As the pilot adjusts altitude with the collective, a change in direction and speed of travel is performed with the "cyclic" control. The cyclic adjusts the plane of the rotor disk in one position only. These adjustments are translated to the rotor disk via the actuating rods to the "swash plate", which is attached to the rotor mast.

While not the safest way to fly, an engine malfunction in a helicopter is often easy for a pilot to deal with. As it descends, energy is stored in the spinning blades. With skill and timing, the blades can be flared near the ground for a very nice landing. Any failure involving the integrity of the main or tail rotors is unfortunately disastrous. Flying a helicopter is compared to balancing a beach ball on a pool cue. For this reason, pilot error is responsible for more accidents than mechanical failure.

War has always been an enormous catalyst to aircraft development. The contribution made by Sikorski came too late to influence World War II. But the wars in Korea and Vietnam saw incredible advances in design and capabilities.

The Bell 47, an icon of the Korean War, completely changed the survival rate of injured soldiers. The power to weight ratio of piston engines severely limited the role of these early machines. The military could only dream of using them to transport troops and supplies.

The development of light and powerful gas turbine engines changed everything. The Vietnam War saw incredible advances in performance. The war began with the Vertol H-21 "flying banana". It had a small payload and could be brought down by a single rifle bullet. The enemy was no doubt horrified to see CH-47 Chinooks disgorge their cargo of troops, artillery and vehicles. The lightweight UH-1 Iroquois could exchange the wounded for fresh soldiers and supplies, while delivering a hail of .30 calibre gun fire and rockets. The AH-64 Apache gunship would provide even more punishing firepower. And the Sikorski family of heavy lift Army, Marine and Air Force helicopters would make Igor very proud.

Continuous improvements in electronics, materials and engines provide us with amazing helicopters such as the triple engine CH-101. Not to be left out, the former Soviet Union produced staggering numbers of exceptionally capable helicopters. The eight-bladed Mil Mi-26 has an incredible maximum takeoff weight of over 123,000 pounds!

While the majority of helicopter development is driven by military needs, commercial and civilian use eventually enjoy the rewards. Emergency medivac, law enforcement, fire fighting and search and rescue are excellent examples. My own son was rescued by a helicopter borne infra-red device only hours before he would have perished. Heavy industry such as heli-logging, oil and gas exploration, and mineral exploration/mining make extensive use of them. Tourism and heli-skiing, while expensive, are very popular uses.

The Robinson R22 and R44 (two and four passenger) helicopters are fulfilling the dream of personal helicopter ownership. Light-weight, piston engine powered and relatively inexpensive, these are outselling all others.

A combination of fixed wing and rotary wing aircraft is another dream that is represented by the V-22 Osprey. Given the number of ways it can malfunction and crash, I always expected it to be abandoned. Perseverance seems to be improving its reliability and it might end up being a big success.

No form of transportation offers more freedom and versatility than a helicopter. The confines of other modes of travel, such as roads, bridges and runways, are of no concern to those flying in a helicopter. To those who money is of no concern, there is no other way to travel. Excluding the Space Shuttle, it is unfortunately the costliest way to move around. But in situations where cost is irrelevant, or when there just isn't an alternative, they are completely indispensable.

More about this author: Dave Jackson

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