Logs / Route 006
The helicopter got deleted
The old helicopter flew on fake forces and aimed at the mouse cursor. In one week in September it was replaced by 24 little wings, a turbine, a governor and blades that snap off. How it works, and the ways it tried to kill its pilot.
The first helicopter in this project arrived on 11 January, when the game was still a shooter. Its own code described it as “top-down arcade controls: WASD movement, mouse aiming”. It turned to face the cursor at 360° a second, had a hard top speed of 50 m/s, and carried weapons with lock-on. For eight months, that was the helicopter.
On 15 September it was deleted, about 2,800 lines of it, and rebuilt from the parts a real helicopter has. A week later it had blades that flap, a turbine with a governor, a tail rotor that costs power, and blades that can hit things and break off. This post is about that week.
Twenty-four little wings
A main rotor is four long, thin wings going round very fast. Each blade is split into six strips, which gives 24 airfoil sections, skipping the inner 15% of the blade where it’s really just a mounting. Every tick, each strip works out what air it’s meeting (the helicopter’s motion, the blade’s spin, the air the rotor pulls down through itself) and produces lift and drag. Add all 24 up and there’s the rotor’s thrust and its pull in every direction.
The travel helicopter’s rotor is 10.8 m across and turns at 395 rpm, which puts the tips at about 223 m/s. That’s roughly two thirds of the speed of sound, going round over the pilot’s head.
Real blades are hinged and flap up and down as they turn, which tilts the whole rotor disc in response to the controls and the wind. The first version didn’t. It was fine in a hover and departed violently at 30 m/s, rolling to 71°. Flapping went in on 19 September.
The ground helps, the tail costs
Close to the ground the downwash has nowhere to go, so hovering takes less power. It’s strongest sitting on the skids and has mostly gone a rotor’s width up.
The tail rotor exists to stop the fuselage spinning the opposite way to the main rotor, and it isn’t free: in a hover it takes about 25.6 kW, around 7% of the power. It also nudges the helicopter sideways a little, which the rigging cancels with a small built-in roll.
The engine
All three helicopters have a 550 kW turboshaft. Starting it is a process: the starter spins the turbine to 15%, then it spools for about 20 seconds until the rotor is up to speed. After that a governor holds rotor rpm steady while the collective moves, right up until the pilot asks for more than the engine has, at which point rpm droops and so does lift.
If the engine stops, the rotor keeps turning on the air flowing up through it. That’s autorotation, and in the game it’s the same physics with the fuel cut off. Lower the collective to keep the rotor spinning, glide down, and spend that spin on one last cushion of lift at the bottom.
Once it’s on the ground and shut down, a rotor brake stops the rotor in about 24 seconds. It used to coast for over three minutes.
Flying it without a licence
A real helicopter is unstable. Let go of the stick and it wanders off. So there’s a stability assist, with limited authority. It holds the attitude at the moment of letting go, holds heading at low speed and coordinates turns at speed, but the pilot’s input always reaches the rotor. The collective defaults to a climb-rate mode: centre the lever and it holds height, push it for up to 4 m/s up or down.
The first version of that assist was not limited. The bug report on 16 September reads “the auto center logic from SAS is so aggressive we can’t fly”. It fought every input, and got rebuilt that week as something that helps rather than argues. Same week: holding a key on the keyboard for a third of a second commanded the full blade pitch, which is why keyboard cyclic is now capped at half.
Blades that break
From 22 September the rotor can hit things. Every tick the game checks the rotor disc against rocks, terrain, buildings, vehicles, tree trunks, treetops and water, and each blade pass takes damage according to what it hit. A rock is a big hit. A treetop barely tickles. People are never struck, on purpose.
A damaged blade loses lift and flies slightly out of pitch, so there’s a once-per-revolution thump through the airframe. Hits near the tip erode it. A blade that runs out of strength, or takes one hard enough hit, comes off and flies away as debris at about 100 m/s. The tail rotor is much more fragile: a light strike can take a blade or shear its drive shaft, and then the helicopter is spinning. Stopping the main rotor too suddenly damages the engine too.
Wash you can see
Hover over a dry landing site and the downwash throws up dust: 1,024 particles following an outward jet along the ground that curls up into a cloud and gets sucked back into the rotor. Over grass and trees, everything bends away underneath, the treetops by up to 2.2 m.
The fleet
| Travel | Police | Rescue | |
|---|---|---|---|
| Mass | 2,400 kg | 2,550 kg | 2,600 kg |
| Price | 480,000 | 620,000 | 650,000 |
Same rotor, engine, 540 litres of fuel and 260 km/h never-exceed speed on all three. The police lights and the rescue hoist are coming later.
It needed a voice
The old helicopter borrowed a car engine sound, tuned for 700 to 2,000 rpm. The rotor turns at 395, so it never got into range, and the only thing audible was the starter. Now the sound is synthesised in five layers: turbine whine, starter, main rotor, tail rotor and wash. The main rotor’s beat is 26 Hz, the one felt in the chest more than heard. Each helicopter is detuned by up to 1%, so two of them in the air together beat against each other like real ones do.
The ways it tried to kill its pilot
Full right pedal switched the tail rotor off. There was a cliff in the tail rotor’s lift curve, so maximum pedal stalled it and the helicopter spun nose-left at 325° a second.
A test helicopter that had thrown all its blades into the sea spun its bare hub up to about 1,200 rpm. Nothing in the drivetrain was losing any energy, so there was nothing to stop it. Real friction losses and a proper flameout fixed that, and the rotor brake came the same day.
And it would roll left happily but fought going right. The fix wasn’t in the assist at all. It was the small roll mix in the rigging that cancels the tail rotor’s push.
Filed under helicopters, physics, deep-dive