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Questions and Answers
Why are vibrations more prominent during approach than during takeoff?
What happens to the parasite drag of the fuselage as the helicopter's forward speed increases?
What must happen to the total rotor thrust when the helicopter moves forward from a hover state?
Why does the collective pitch lever not need to be progressively raised for speeds up to minimum power speed?
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What is the term used to describe the gain in rotor efficiency when moving forward from a hover state?
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What is the primary reason for the increase in total rotor thrust required as the helicopter moves forward from a hover state?
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At what speed range does the collective pitch lever need to be progressively raised to maintain level flight?
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What happens to the vertical component of total rotor thrust as the helicopter moves forward from a hover state?
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What is the term used to describe the rotor thrust required to counteract the weight of the helicopter?
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What is the effect of wind conditions on a stationary hovering helicopter?
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Study Notes
Centre of Gravity and Fuselage Attitude
- The centre of gravity controls the position of the fuselage in relation to the disc (fuselage attitude) during hover and forward flight.
- This relationship can be affected in forward flight by the aerodynamic shape of the fuselage and loading on the stabiliser.
Load on Stabiliser
- The load on the stabiliser is equal to the amount of downwash on it at any given point in time.
- More downwash results in a greater downward force on the stabiliser, causing a pitch up.
- Raising the collective pitch increases downwash, causing a pitch up, while lowering it decreases downwash, causing a pitch down.
Transition
- Transition occurs when the helicopter changes from a hover to forward flight or vice versa.
- In hover, the total rotor thrust equals the weight, with some fuselage parasite drag included.
- In forward flight, the total rotor thrust must provide both a vertical force to balance the weight and a horizontal force in the direction of movement.
Effects of Forward Speed
- As forward speed increases, the axis about which the disc tilts will vary due to inflow roll and flapback.
- In Chetak/Cheetah, the cyclic must be moved forward and to the right to counter flapback and inflow roll.
Hover
- Hover is defined as a condition of flight with no horizontal or vertical motion, maintaining a constant direction.
- To lift the helicopter off the ground, a lifting force equal and opposite to the weight must be produced.
Pick Up and Climb to a Free Air Hover
- Raising the collective pitch lever increases rotor thrust, eventually equaling the weight, and then exceeding it to accelerate the helicopter vertically upwards.
- During this phase, the helicopter may experience ground resonance, which is avoided by passing through it quickly.
Vibrations During Transition
- Vibrations occur during transition, more prominently during approach than takeoff.
- These vibrations are due to the helicopter staying in its own wake for a longer time during approach.
Total Rotor Thrust and Forward Speed
- As the helicopter moves forward, the total rotor thrust must increase to provide the required horizontal force.
- The faster the forward speed, the greater the total rotor thrust must be to balance the weight.
Translational Lift
- Translational lift occurs when the total rotor thrust increases as the helicopter moves forward, allowing for a reduction in collective pitch and power up to a minimum power speed.
- This gain in rotor efficiency occurs when moving forward or hovering stationary in wind conditions.
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Description
This quiz covers the relationship between the centre of gravity, fuselage attitude, and aerodynamic forces in helicopters, including the impact of downwash on stabiliser loading.
