How Fast Do Helicopters Fly?

Most helicopters cruise between 110 and 140 knots — 200 to 260 km/h, or 125 to 160 mph. Airbus gives the H125 a fast cruise of 136 knots, 252 km/h, on its technical information page for the type. That is the aircraft doing most of the flying in Nepal's Khumbu, out of Tenzing–Hillary Airport at Lukla.

None of those figures is what a passenger experiences, and the reason is not the one most accounts give. Weight and height move every performance number a helicopter has — that is the subject of how helicopters perform in nepal — but speed is the number they move least.

Between about 110 and 140 knots, for almost anything with one main rotor.

That band is narrow, and it has been narrow for fifty years. A helicopter's forward speed is set by what happens at the tips of its rotor blades rather than by how much power it has, so a bigger engine does not widen it.

The three types this platform lists for Nepal all sit inside it. Airbus publishes a fast cruise of 136 knots, 252 km/h, for the H125. GlobalAir and Airforce Technology both give the Bell 412EP a maximum cruise of 122 knots, 226 km/h, and a never-exceed speed of 140 knots, 259 km/h. The AS350 B3e on the same list is the same airframe as the H125 under its earlier name.

One of five, and they are not interchangeable.

Cruise speed is what the aircraft holds in level flight at a normal power setting. Fast cruise is the same thing at a higher setting, and it is the figure manufacturers prefer to publish. Maximum speed is the fastest the aircraft will go in level flight. Never-exceed speed is not a performance at all but a limit, the speed past which the aircraft must not be flown.

Ground speed is the fifth, and it is the only one a passenger can feel. It is the aircraft's speed through the air plus or minus the wind, and it is what decides when you land.

Most articles answering this question give one number without saying which of the five it is. That is why their answers disagree with each other by forty knots and none of them is wrong.

400.87 km/h, or 249.09 mph, set on 11 August 1986 and never beaten.

John Trevor Egginton flew a Westland Lynx demonstrator, G-LYNX, over Glastonbury in Somerset, with Derek Clews beside him. Guinness World Records, Leonardo and the Fédération Aéronautique Internationale all carry the same figure.

The aircraft was not a production machine. Leonardo's own account of the flight describes experimental BERP main rotor blades and two Rolls-Royce Gem 60 engines with water-methanol injection, each rated at 1,345 shaft horsepower.

Set that beside the H125's 136 knots and the shape of the problem appears. The fastest helicopter ever flown managed roughly one and a half times the cruise of an ordinary utility single, and it needed a bespoke rotor to do it.

The limit is aerodynamic rather than mechanical. In forward flight one blade advances into the airflow and the opposite blade retreats from it. Pilot Institute puts the advancing tip of a typical turbine helicopter at about 295 metres a second at 150 knots of forward flight, against a sea-level speed of sound near 340 metres a second. Push the aircraft faster and that tip goes transonic, and drag climbs faster than any engine answers it. Slow the rotor to avoid that and the retreating blade runs out of airflow and stalls instead.

So rotor speed is a compromise pinned between two limits, and neither of them moves when a larger engine is fitted. Forty years of engine development have produced no challenger because engines were never the constraint.

Faster over the ground than the airspeed indicator shows, and the gap widens with height.

An airspeed indicator does not measure speed. It measures the pressure of the air arriving at a probe, and thin air arrives with less of it. The same needle reading therefore means a higher real speed the higher the aircraft is.

The rule of thumb pilots use is about two per cent per thousand feet. AOPA prints it in those words and Boldmethod gives the same figure.

Lukla stands at 2,845 m, 9,334 ft. At that height the rule puts 100 knots indicated at roughly 119 knots true. At Kala Patthar's 5,545 m — 18,192 ft — the same 100 knots indicated is about 136 knots true.

This is the half of the subject the general articles leave out. They say, correctly, that altitude costs a helicopter speed, and they mean the indicated limit, which does fall as the aircraft climbs. They do not say that every indicated knot buys more ground up there, which is the effect pulling the other way. Neither cancels the other, and at the heights a Khumbu landing involves the falling limit is the larger of the two.

Both are smaller than the wind. A Himalayan valley in the middle of the day moves air faster than either correction, which is why the morning matters more to a departure time than any figure on a specification sheet.

Lift, long before it takes speed.

A rotor and a turbine both work on the mass of air passing through them. Under the International Standard Atmosphere, air at Lukla's 2,845 m holds about 71 per cent of sea-level pressure and air at Kala Patthar's 5,545 m about half — the figures this site publishes on its altitude pages. Temperature counts the same way, because warm air is thinner air, so a warm afternoon at a given elevation behaves like a higher one.

Airbus gives the H125 a maximum flight altitude of 7,010 m and a hover ceiling out of ground effect, at maximum take-off weight, of 3,840 m. The second figure is 55 per cent of the first, and it is the one that governs a landing.

Everest Base Camp at 5,364 m and Kala Patthar at 5,545 m are both above that hover ceiling. So weight comes out of the aircraft before it goes. Speed is not treated the same way, because it is not the binding constraint — nobody offloads anything to gain knots.

That is the rule worth carrying out of this. Altitude in Nepal is paid for in weight, and the speed figures barely move. It is close to the opposite of what a general article on helicopter speed implies, and it is why an operator asked about a high landing answers with a load sheet rather than a cruise speed.