Helicopters are undeniably slow compared to aeroplanes. There is no pure helicopter that flies faster than 400km/h (250mph), yet aeroplanes have been going much faster for a long time, and even some trains and cars can go faster. The Short Skyvan, a propeller driven aircraft with fixed landing gear nicknamed “The Shed” by pilots, has about the same top speed as the world’s fastest pure helicopter.
What do helicopters do differently?
Helicopters have rotors instead of wings. This one difference leads to big changes because, unlike an aeroplane wing, a helicopter rotor has parts moving at different speeds even when it is hovering or on the ground:
- The outer part of a blade is moving faster than the inner part.
- Blades on opposite sides of the rotor are travelling in opposite directions. On one side of the helicopter, the blades moves towards the front; on the other side towards the rear
Once the helicopter starts moving forwards, the differences are amplified:
- Where the blades move forward into the oncoming air (advancing side), the airflow over the blades is faster than before.
- Where the blades move backwards with the oncoming air (retreating side), the airflow over the blades is slower than before.
How does this make helicopters slow?
As the helicopter moves forward faster, two effects combine to reduce the lift on the retreating side of the rotor:
- The airflow over the retreating blades becomes progressively slower, since the blade is moving in air that is already moving in the same direction. This reduces the total lift produced by the blade.
- Near the centre of the rotor the air starts to flow over the blade back-to-front, since the blade is rotating slower than the air around it is moving past. This part of the blade produces no lift at all, or even downforce.
The advancing blades cannot be used to compensate for any reduction in lift on the retreating side because both sides of the rotor must produce similar amounts of lift to keep the helicopter balanced and level. Therefore, at a high enough forward speed, the retreating blades cannot produce enough lift to keep the helicopter in the air. This is the helicopter’s speed limit and the reason helicopters are so slow; rotors produce less lift as speed increases.
There are a number of other factors that may limit a helicopter’s speed, such as the vibration response at higher speed, transonic effects on the advancing blades or simply the total vehicle drag. However these effects are not fundamental consequences of the helicopter concept and can be overcome with careful design when needed.
The speed limit can be pushed back by using an oversized or a very fast spinning rotor; however these solutions bring their own problems and increased fuel consumption. Design and economic realities therefore limit a helicopter to having a rotor reasonably matched to its size and weight, and this gives a top speed of around 400km/h (250mph) in practice.