Helicopter Limitations and Capabilities
A helicopter cannot start while falling. Without rotor RPM, the blades would snap off or bend beyond use. Helicopters have limits on the wind speeds when starting. The rotor blades can act like wings, ensuring enough lift for the helicopter to glide until safe landing. The rotors of many helicopters dip within 6 feet of the ground, risking contact with people or objects. Even light rotor contact may permanently damage the aircraft. A Sikorsky S-76 cruises around 178 mph. It would cover 100 miles in just over 30 minutes.
In freefall, 007 starts the engine at the last moment, arresting the fall to fly at ground level. But no pilot could gain control and restart falling from thousands of feet, either saving the helicopter with power or the standard emergency autorotation technique.
Autorotation works because the spinning rotor. You cannot autorotate from zero RPM because the rotor stalls when starting. In autorotation, the engine disengages and the rotor is driven by upward airflow. Emergency helicopter landings are possible without the tail rotor using the autorotation procedure.
According to crash data, helicopters are more dangerous than planes. Light helicopters cruise around 140 mph; larger ones 170-200 mph. At low rotor RPM, the blades would snap off or bend from centrifugal forces at operating speeds.
When helicopter blades spin, they create lift allowing the craft to rise. Unlike fixed-wing planes, helicopters can move up, down, hover and land almost anywhere that fits. Their hanging rotor system makes them act like a pendulum, quickly seeming out of control. Helicopters fly because their spinning rotors generate "lift" to move them upwards.
Helicopter Operating Altitudes
Turbine-engined helicopters can reach around 25,000 feet. But the maximum height at which a helicopter can hover is much lower – a high performance helicopter can hover at 10,400 feet. Helicopters rely on air density and oxygen levels. The top of Mount Everest has one-third the sea level’s atmospheric pressure. The oxygen levels at the Everest base camp itself drop 50%.
Higher temperatures make it harder for helicopters to fly as hot air is less dense than cold air. Thinner, hot air provides less oxygen for the combustion of fuel in a helicopter’s engine, reducing power and lift-off capability. Helicopters can fly over the ocean. Helicopters can air refuel. Air to air refueling reinforces the superior capabilities of the HH-60W Jolly Green II and its ability to perform long-range missions.
The airfoils on the rotor blades generate lift that overcomes the weight, pushing the craft up. Some military helicopters refuel in flight from tankers.
When the engine fails, the pilot can change the rotor blade angle using the thrust lever and glide down steadily. The gliding helicopter descends slower than it falls, allowing a safe autorotation landing.
Gliders cannot hover since they use no engine thrust. They require an initial external force like a plane tow or cliff launch. Gliders descend slowly, but locate rising air to gain altitude. Their potential energy keeps them airborne longer.
Turbine helicopters reach 25,000 feet, but maximum hover height is under 11,000 feet. A helicopter can catch a parachuting payload in mid-air using a hook. This risky technique requires precise aircraft control in the right conditions.
Can a Helicopter Restart Midair?
A helicopter in free fall cannot restart. Without rotor RPM, centrifugal force would not hold the blades out. The blades generate enough lift for a controlled descent. Spinning rotors allow "autorotation" landings without power. The pilot can still maneuver the blades’ angle, trading altitude for push.
With power, an expert pilot could perhaps gain some control and arrest a falling helicopter. But without autorotation, restarting level from a high-altitude free fall is unrealistic.