HeliSim – Advanced Helicopter Flight Model for Arma 3
Overview
As of version 2.2.0, the previous SFM+ flight model has been completely removed and replaced with HeliSim.
HeliSim is a complete physics replacement and evolution of SFM+, designed primarily for helicopters but adaptable for other air vehicles. It replaces Arma 3’s Simple and Advanced Flight Models, offering a comprehensive simulation of realistic helicopter aerodynamics.
Key Simulated Effects
HeliSim accurately simulates various helicopter flight behaviors, including:
- Effective Translational Lift (ETL)
- Ground Effect
- Vortex Ring State (VRS)
- Translating Tendency
- Retreating Blade Stall (planned)
- Transient Torque (planned)
Current Simulated Systems for the Apache
The AH-64D Mod includes realistic mechanical and electronic systems, such as:
- Hydraulics & Pneumatics
- Electrical Systems
- Flight Management Computer
- Stability & Command Augmentation Systems
- Non-boosted flight controls
- Auxiliary Power Unit (APU)
- Turbine Engine & Transmission
Flight Dynamics
Ground Effect
- The aircraft operates In Ground Effect (IGE) when hovering within one rotor diameter (~48 feet AGL for the AH-64D).
- IGE reduces power requirements, as rotor downwash is restricted by the proximity to the ground.
- As altitude increases, ground effect dissipates, requiring additional power for Out of Ground Effect (OGE) hovering.
Effective Translational Lift (ETL)
- Occurs between 16–24 knots, improving rotor efficiency as the aircraft moves into cleaner air.
- Pilots will experience a slight vertical vibration during this transition.
Translating Tendency
- Due to the counterclockwise main rotor rotation, increasing collective induces right yaw.
- Tail rotor thrust counters yaw but causes lateral drift to the right.
- Pilots must apply left cyclic input, resulting in an approximate 3° left roll at hover.
Main Rotor Torque Effect
- Applying collective increases rotor torque, causing right yaw.
- Pilots must apply left pedal input to counteract yaw.
Advanced Aerodynamic Effects
Vortex Ring State (VRS)
VRS occurs when the main rotor enters an aerodynamic stall due to excessive vertical descent.
For VRS to develop, all three conditions must be met simultaneously:
- Vertical descent rate exceeding 300 fpm.
- Power applied between 20–100%, but insufficient to arrest descent.
- Forward airspeed below ETL (<16 knots).
- The AH-64D Mod enters fully developed VRS at ~4,800 fpm.
- Recovery requires directional movement (forward, backward, or sideways).
- Early warning signs include progressive rotor shake.
- Collective input may worsen descent, increasing over-torque risk and rotor droop.
⚠ Pilots must monitor descent rates, especially in low-speed, high-density altitude conditions!
Settling With Power (SWP) (Different from VRS!)
SWP occurs when the rate of descent exceeds available engine power, preventing recovery.
This is not the same as VRS, though symptoms can seem similar.
SWP Example
A pilot terminates to an OGE hover after high-speed flight, requiring 95% torque—but the continuous torque limit is 100%.
- A 100 fpm descent demands ~2% torque to arrest.
- A 5% torque margin allows a maximum descent rate of ~250 fpm.
- Higher aircraft weight, altitude, and temperature worsen the situation.
⚠ Understanding helicopter performance margins is essential to prevent SWP.
Additional Flight Phenomena
Mushing
Mushing is a temporary stall condition occurring during aggressive, high-speed dive recoveries.
- The helicopter’s momentum exceeds rotor thrust, delaying recovery.
- Pilots should apply forward cyclic to regain control.
- Pulling aft cyclic worsens the stall, increasing altitude loss.
Autorotations
A power-on autorotation must be performed between 77–107 knots.
- 77 knots: Minimum rate of descent speed.
- 107 knots: Maximum glide efficiency speed.
- Unsafe above 145 knots.
- Reduce collective input until torque is <10% with both engines engaged.
Damage Modeling
HeliSim enforces realistic startup procedures.
Improper startup will result in transmission failure, leading to aircraft loss within 30 seconds if limits are exceeded.
Engine Start Parameters
| Torque (Tq) | Rotor RPM (Nr) |
|---|---|
| ≤ 30% | ≤ 50% |
| ≤ 70% | ≤ 90% |
Single Engine Torque Limits
Exceeding critical torque levels results in failure:
| Tq (%) | Time Limit |
|---|---|
| 0–110% | Normal Operation |
| 111–122% | Max 2.5 Minutes |
| 123–125% | Max 6 Seconds |
| > 125% | Immediate Failure |
Dual Engine Torque Limits
| Tq (%) | Time Limit |
|---|---|
| 0–100% | Normal Operation |
| 101–115% | Max 6 Seconds |
| > 115% | Immediate Failure |
Performance Model
HeliSim considers aircraft weight and environmental factors to ensure accurate performance calculations.
Key Aircraft Performance Figures
| Metric | Dual Engine | Single Engine |
|---|---|---|
| Max Torque | 127% | 131% |
| Max Continuous Torque | 100% | 110% |
| Max Gross Weight (IGE) | 20,260 lbs | |
| Max Gross Weight (OGE) | 18,700 lbs |