HeliSim flight model

HeliSim is a separate mod, HeliSim Core, by BradMick. The Apache supplies its aircraft data: weights, rotor, engines, transmission, fuel, hydraulics and electrics.

What it simulates

• Rotor and airframe. Lift, drag and power required come from the rotor and the fuselage, wings and stabilator. Weight, altitude and temperature change the power you need. Ground effect, translational lift and vortex ring state come from the rotor model.
• Engines. Two turboshaft engines with a gas generator (Ng), a free power turbine (Np) and a turbine temperature (TGT), governed to keep the rotor at 101%. They start, run and fail as engines do: see Engines and rotor.
• Transmission and gearboxes. Engine torque passes through a nose gearbox on each side into the main transmission. Too much torque damages them.
• Systems. Battery, generators and rectifiers, the APU, primary and utility hydraulics with an accumulator, the fuel cells and pumps, and the flight management computer.
• Weight and balance. Fuel, ammunition, missiles, rockets, external tanks, the radar and the crew all add mass and move the centre of gravity. As you burn fuel and fire weapons the aircraft gets lighter.

Limits

Exceeding a limit is not just a warning. HeliSim counts the time spent above each band and damages the engine or drivetrain when the allowance runs out.

Torque

Both engines	Allowed
Up to 100%	Continuous
100% to 115%	6 seconds
Above 115%	None: damage at once

One engine	Allowed
Up to 110%	Continuous
110% to 122%	2.5 minutes
122% to 125%	6 seconds
Above 125%	None: damage at once

The aircraft counts as single-engine when one engine’s torque falls below about half of the other’s.

Rotor speed

Nr
95% to 105%	Normal
Below 95%, airborne	LOW RTR warning
106% and above	HIGH RTR warning
110%	Maximum

Temperature

The engines limit their own TGT to 867 °C, or 896 °C with one engine. TGT above 810 °C is time-limited; keep it there only as long as you need the power.

TGT, both engines	Allowed
Up to 810 °C	Continuous
810 to 870 °C	30 minutes
870 to 878 °C	10 minutes
Above 878 °C	None: damage at once

TGT, one engine	Allowed
Up to 810 °C	Continuous
810 to 870 °C	30 minutes
870 to 878 °C	10 minutes
878 to 896 °C	2.5 minutes
896 to 949 °C	12 seconds
Above 949 °C	None: damage at once

Engine speeds

Each engine	Allowed
Ng up to 102.2%	Continuous
Ng 102.2% to 105.1%	12 seconds
Ng above 105.1%	None: damage at once
Np up to 105%	Continuous
Np 105% to 121%	12 seconds
Np above 121%	None: damage at once

Each band’s allowance starts again from full whenever you enter it. The ENG page shows the time left.

Weight

Empty	13,920 lb (6,314 kg)
Maximum gross	23,000 lb (10,433 kg)

A full load of Hellfires, rockets, 1,200 rounds and full fuel is heavy. On a hot day or at altitude you may not be able to hover out of ground effect. The PERF page tells you what you can do at your weight in the conditions; the mission planner does the same before you load the aircraft.

If you cannot get airborne, or can only hover in ground effect, take fuel or weapons off. The flight model is not broken: the aircraft is too heavy for the day.

Environment

The engines and rotor see the base altitude and temperature set in Aircraft Environmental Settings (Addon settings). The Middle East and Central Asia - Summer settings in particular cost a lot of power. Wind from the mission’s weather is included unless a tester has turned it off.

Vortex ring state

Descending slowly and nearly vertically under power lets the rotor settle into its own downwash. Lift falls away, and adding collective makes it worse. To avoid it, keep forward speed when you descend steeply, and keep the descent rate low below about 20 kt. To recover, move the aircraft out of the column of air: forward or sideways cyclic, then collective once the rotor bites again. Turn on Enable VRS Warning in the HeliSim settings for a warning as it develops.