## Helicopter Aerodynamics

speaker notes for Philip Greenspun; revised February 2009

Site Home : Teaching : Short Talks : One Element

### What You'll Learn Today

• What is interesting about helicopters
• How helicopters fly
• Helicopter Maneuvers
• Autorotations

### What is Interesting About Helicopters?

• fly low and slow with great visibility (no requirement to stay 500' and 1000' above obstacles)
• impress friends; take off and land at an airport and they think you're a hero
• cheat gravity; fly in arbitrary directions
• land off-airport without ending up on the NTSB Web site

### How Helicopters Fly

• a wing is a wing is a wing; a stalling angle of attack is a stall (figure 2-3 and figure 4-2 from Airplane Flying Handbook)
• airspeed + angle of attack leads to lift; easiest way to understand Bernoulli's Principle is conservation of energy; kinetic energy increases as velocity increases, so static pressure must decrease (figure 3-5 from Pilot's Handbook of Aeronautical Knowledge)
• helicopter in a hover relies on engine-driven rotation to generate airspeed
• pull up on the collective to add angle of attack and the helicopter goes up (explain the swashplate and pitch links here), figure 1-7 shows the controls
• move the cyclic and the helicopter translates forward, backwards, or sideways
• the anti-torque rotor

### Helicopter Maneuvers

• pickup
• hover
• takeoff (why we don't simply lift straight up), figure 11-2, figure 9-8
• landing (why it is nice to have a runway)

### Autorotation

• three sources of energy: potential, kinetic, blade inertia (assume 500', 70 knots, 400 rpm); figure 3-20
• importance of maintaining airspeed to treetop level; figure 11-1
• flaring from 70 knots to 0 arrests vertical descent
• blade inertia cushions drop from 5', not from 500'