Principles of Flight for IFR Helicopters
IFR helicopter flight adds performance and stability considerations to basic rotary-wing aerodynamics. Candidates must understand how speed, density altitude, mass and bank angle interact, and why IFR flight in a helicopter demands a wider performance margin.
Concepts to master
- The power required versus power available curve, translational lift, and the region of reverse command behind the power curve.
- Density altitude: the effect on power available and on the hover ceiling.
- Retreating blade stall and the conditions that provoke it: high speed, high mass, high altitude and high load factor.
- Settling with power (vortex ring state): low speed, high rate of descent, power applied; the recovery is forward cyclic and reducing collective.
- Stability: the helicopter is less stable than a fixed-wing aircraft, so IFR flight relies on the attitude indicator and often on the autopilot.
- Bank angle and load factor: in a level turn, load factor rises with bank angle, increasing the power required.
Common traps
Questions ask what happens to the power required as you slow behind the power curve (it increases). Another trap is confusing settling with power with retreating blade stall.
Rule of thumb
At 60 degrees of bank the load factor is 2; the stall speed rises by the square root of the load factor, and the power required rises with it.
Worked example
At 30 degrees of bank the load factor is about 1.15. In a level turn at that bank, the power required increases by roughly 15 percent over straight flight, which matters at a high density altitude.