Is fixed wing more efficient than rotary wing?
If yes, why is that?
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Helicopters use more fuel than airplanes (considering everything else being equal).
When helicopters are traveling slow, they need more energy to hold their weight. When traveling fast (according to their standards), the rotor drag is very high. This is the primary reason they need more power. More Power = More Fuel.
Conventional aircraft have much less drag at a given weight and speed than a helicopter.
Helicopters travel slow. When they burn more fuel per hour and travel slowly, fuel consumption per mile is much worse. They also stay low. Most have turbine engines, turbines burn lots of fuel at low altitudes.
Wikipedia has a good article about energy efficiency in different vehicles.
- Two-seater and four-seater flying at 250 km/h with old generation engines can burn 25 to 40 liters per flight hour, 3 to 5 liters per 100 passenger km.
- The Sikorsky S-76C++ twin turbine helicopter gets about 1.65 mpg-US (143 L/100 km; 1.98 mpg-imp) at 140 knots (260 km/h; 160 mph) and carries 12 for about 19.8 passenger-miles per gallon (11.9 litres per 100 passenger km).
Fixed wing aircraft are generally much more efficient than rotary aircraft. This is because of the difference in how they generate lift.
For fixed wing aircraft, they use an engine to keep the plane moving forward. The air flow over the wings generates lift. The engine must only overcome the drag of the airplane (in level flight). This drag is both due to the form drag of the plane and the generation of lift from the wings.
For a rotary aircraft, the engine keeps the rotors moving. The rotation of the rotors generates lift, but this is also the main force used to move the aircraft forward. So in this case, the rotors not only provide the lift to keep the aircraft in the air, but also the thrust to keep it moving forward.
In a fixed wing aircraft, the propeller can be made to be efficient at providing forward thrust, and the wing can be made efficient at providing lift. In a rotary aircraft, the rotors must serve in both roles, and result in a compromise.
When a helicopter enters a hover, it is using all of its power to provide lift, while a fixed wing aircraft can enter a holding pattern with still only enough power to offset the drag.
Comparing the efficiency of lift versus drag of different aircraft shows the lower efficiency of a helicopter. A Cessna 150 has an L/D (lift to drag ratio) of 7 in cruise, while a helicopter would only have around 4.5 in cruise. Other sources show that small aircraft can achieve an L/D of over 10.
There is also a speed limit for helicopters that prevents them from going as fast as most turboprop or jet aircraft. This is because the tips of the rotors should stay at subsonic speeds. Current helicopters like the UH-60 are limited to around 200 knots. Newer designs such as the Eurocopter X3 and Sikorsky X2 are capable of up to 250 knots, but are still in development.
For comparison, a small helicopter and small fixed wing plane. Even though the PA-31 (fixed wing) is heavier than the 206 (helicopter) and can carry more passengers, it can fly faster and further than the helicopter. The "fuel economy" of nautical miles per gallon is based on the fuel capacity and max range, so it's not an exact figure of actual fuel burn.
Also, the PA-31 ended production in 1984, while the 206B-L4 is the latest version still in production.
Opinion; The lift to drag ratio of fixed wing versus rotary wing by logic costs the rotary wing greater fuel because energy is expended in lift, where that is not required in a fixed wing. The most efficient practical fixed wing situation is " wing in ground effect" where less energy is required to generate lift due to air compression beneath the wing. Here, the better efficiency is obtained with a larger wing by adjusting speed to maintain lift.