Carter copter

The airplane won't amount to a damn until they get a machine that will act like a hummingbird-go straight up, go forward, go backward, come straight down and alight like a hummingbird.

-Thomas A. Edison

Mr. Edison was not quite correct in this prediction, as we all know. Airplanes that need large runways have revolutionized travel and are an integral part of the modern world. The primary advantage of flight over other forms of transportation is speed. Planes also have the added advantage that they can go in a straight line-point to point-and ignore such inconveniences as mountains, rivers and even oceans.

Unfortunately, as every modern traveler knows, the slow part of the trip happens on the ground, not in the air. Edison was right to the extent that he foresaw that shackling airplanes to airports introduced severe limitations on their usefulness. In a perfect world, we would be able to take off from home, fly fast and then land at or near our destination. To do this requires a machine that can take off and land "like a hummingbird."

The dream of efficient VTOL flight remains elusive. We have helicopters that are good at vertical flight and hover but are slow and inefficient in cruise. We have VTOL jet aircraft such as the Harrier that are fast but pay an enormous cost in range and payload for their ability to hover inefficiently on the blast of their jet engines.

Many concepts have been tried, and the history of VTOL flight is rich with interesting machines and frustrated designers. The latest attempt, the V-22 Osprey tiltrotor is the result of a decades-long attempt to develop an airplane/helicopter hybrid. The V-22 has had a long and difficult history, and it is not yet clear that it will be successful. Even if it does succeed, the tilt-rotor is a very expensive, complicated machine.

Sometimes, the key to progress is to ignore what "everybody knows" and see what really happens when you try to do something common knowledge says is impossible. In the skies over Texas, a futuristic composite rotorcraft is trying to do just that. Jay Carter Jr. dreams of a craft that can take off vertically, cruise cross-country at speeds comparable to airplanes and land vertically at the end of the flight. The CarterCopter Technology Demonstrator, or CCTD, currently being flight-tested, could be the first step to achieving this age-old dream.

DISC LOADING

The reason craft with rotors are so attractive for VTOL flight is that the key to efficient hover is low disc loading. A large-diameter rotor can produce a lot of lift for each unit of horsepower that goes into turning the rotor. This is why small helicopters can fly well on relatively low horsepower.

It is also the problem with the "direct-lift" approach to VTOL seen in the Harrier. The effective rotor diameter of the Harrier is approximately the diameter of the engine face. The Harrier and airplanes like it use a huge amount of energy to generate a relatively small amount of lift. It works, but it's so inefficient that it only pays for relatively short-range military missions.

FORWARD FLIGHT

A rotor is a very efficient way to generate lift in hover but quite inefficient in forward flight. The lift-to-drag ratio of a good rotor in cruise is less than one-third that of a low-technology wing. Even at relatively low airspeeds, a pure helicopter or autogyro is a relatively high-drag machine.

A traditional rotor also suffers from a more fundamental limitation. As the aircraft moves faster, the airspeed on the advancing blade increases, and the airspeed on the retreating blade decreases. To keep generating lift, the retreating blade has to move backward faster than the rotorcraft is moving forward. The airflow in the inner portion of the retreating blade is then reversed and would flow from the trailing edge of the blade to the leading edge. As airspeed increases, the area of reversed flow on the retreating blade gets bigger, and the airspeed of the rest of the blade keeps getting smaller. At some point, the retreating blade cannot make enough lift to keep the helicopter flying because its airspeed is too low. The helicopter is caught between the need to keep positive airspeed on the retreating blade and the need to keep the tip of the advancing blade below Mach 1. These two requirements limit the theoretical maximum speed of a helicopter to significantly below Mach 0.5, or half the speed of sound. This is why the current world helicopter speed record is only 249.10mph (400.87km/h).

Every realm of flight has its barriers. For every class of flying machine, there is one parameter that dominates the designers' thoughts. For fast jets, it is Mach number and the once-dreaded "sound barrier." For sailplanes, it is the lift-to-drag ratio (L/D), which measures pure aerodynamic efficiency.

For rotorcraft, the world is dominated by the Greek letter (mu). Mu is the tip-speed ratio: the ratio between the overall airspeed of the machine and speed of the rotor blade tips caused by the rotors' rotation. In hover, (mu) is equal to zero. As the rotorcraft flies faster, (mu) increases. The airspeed of the advancing blade increases and the airspeed of the retreating blade decreases. When (mu) reaches a value of 1.0, the retreating blade of the rotor has reversed airflow over its entire length. It has long been believed that above a certain tip-speed ratio, somewhere below 1, a rotor will become unstable. The highest tip-speed ratio ever achieved by a helicopter was 0.8 on the Lockheed Cheyenne compound attack helicopter prototype. The McDonnell XV-1 compound autogyro is the current (mu) champion. It achieved a (mu) of 0.92 during flight tests. No rotorcraft has ever sustained flight at a tip-speed ratio greater than 1.0.