Aerodynamics in Flight

34 Questions

Which of the following is true about hovering flight?

The helicopter maintains a constant position above the ground

What happens if the amount of thrust is greater than the actual weight during hovering flight?

The helicopter gains altitude

What are the main aerodynamic forces acting on a helicopter in powered flight?

Lift and thrust

Which direction does the helicopter fuselage turn when the engine turns the main rotor system counterclockwise?

Clockwise

What is the purpose of an antitorque rotor or tail rotor in most helicopter designs?

To counteract torque-induced turning tendency

What is translating tendency in hovering flight?

The tendency of the helicopter to drift in the same direction as tail rotor thrust

What causes rotor blade coning in a helicopter?

The centrifugal force generated by the rotation of the rotor system

Which part of the blade is known as the driven region?

Part A

What is the size of the driven region dependent on?

All of the above

Where does the driving region of the blade normally lie?

Between 25 to 70 percent of the blade radius

During aerodynamic flapping of the rotor blades, the advancing blade achieves maximum upflapping displacement over the nose and maximum downflapping displacement over the tail. This causes the tip-path plane to tilt to the rear and is referred to as

Blowback

In sideward flight, the tip-path plane is tilted in the direction that flight is desired. This tilts the total lift-thrust vector

Sideward

In rearward flight, the tip-path plane is tilted

Rearward

During autorotation, the main rotor system is being turned by

Relative wind

Which of the following is true about the movement of the cyclic pitch control in a rotor system using three or more blades?

It changes the angle of attack of each blade by the same amount

What happens when the angle of attack of the rotor blades is increased while their velocity remains constant?

Additional vertical lift and thrust is generated and the helicopter ascends

What happens if lift and thrust are greater than weight and drag in a no wind condition?

The helicopter ascends vertically

What happens to the rotor system efficiency as the helicopter accelerates in forward flight?

It becomes more efficient due to increased airflow

What is the phenomenon known as ground effect?

The change in air circulation patterns when hovering near the ground

What is gyroscopic precession?

The resultant action of a spinning object when a force is applied

How does gyroscopic precession affect the movement of the tip-path plane in a two-bladed rotor system?

It causes the tip-path plane to tip forward

Why are two-bladed rotor systems normally subject to Coriolis Effect to a much lesser degree than articulated rotor systems?

Because the blades are underslung with respect to the rotor hub

Which one of these best describes the transverse flow effect?

The angle of attack at the front disc area increases causing the rotor blade to flap up, and the angle of attack at the aft disc area decreases causing the rotor blade to flap down.

Which one of these best describes dissymmetry of lift?

The relative wind encountered by the advancing blade is increased by the forward speed of the helicopter, while the relative wind speed acting on the retreating blade is reduced by the helicopter’s forward airspeed.

Which one of these best describes the purpose of blade flapping in a rotor system?

To equalize lift across the rotor disc by automatically adjusting the angle of attack of each rotor blade.

Which one of these best describes retreating blade stall?

The retreating blade stalls because of a high angle of attack and slow relative wind speed, causing a nose pitch up, vibration, and a rolling tendency—usually to the left in helicopters with counterclockwise blade rotation.

The driven region of the rotor blade produces lift and drag simultaneously.

False

The size of the driven region is solely dependent on the rate of descent.

False

There are two points of equilibrium on the rotor blade where total aerodynamic force is aligned with the axis of rotation.

True

The driven region, also called the ______ region, is nearest the blade tips.

propeller

The overall result is a ______ in the rotation of the blade.

deceleration

There are two points of equilibrium on the blade—one between the driven region and the driving region, and one between the driving region and the ______ region.

stall

The driving region, or ______ region, normally lies between 25 to 70 percent of the blade radius.

autorotative

The size of the driven region varies with the blade pitch, rate of descent, and rotor ______.

r.p.m.

Study Notes

Hovering Flight

If the amount of thrust is greater than the actual weight during hovering flight, the helicopter will climb.
Translating tendency in hovering flight refers to the sideways movement of the helicopter's center of gravity.

Aerodynamic Forces

The main aerodynamic forces acting on a helicopter in powered flight are lift, weight, thrust, and drag.

Rotor System

The direction of rotation of the main rotor system determines the direction of the helicopter's fuselage turn; counterclockwise rotation means the fuselage turns to the right.
The antitorque rotor or tail rotor counteracts the torque created by the main rotor, preventing the helicopter from spinning.
The driven region of the rotor blade produces lift and drag simultaneously, and its size is dependent on the blade pitch, rate of descent, and rotor RPM.

Rotor Blade Movement

Rotor blade coning occurs due to the change in angle of attack and velocity of air particles along the blade span.
The driven region of the blade is near the tip, while the driving region normally lies between 25 to 70 percent of the blade radius.
During aerodynamic flapping, the advancing blade achieves maximum upflapping displacement over the nose and maximum downflapping displacement over the tail, tilting the tip-path plane.

Tip-Path Plane Tilt

In sideward flight, the tip-path plane is tilted in the direction of desired flight, tilting the total lift-thrust vector.
In rearward flight, the tip-path plane is tilted forward.
During autorotation, the main rotor system is turned by the upward flow of air through the rotor disk.

Rotor Control

The cyclic pitch control moves in the same direction as the rotor disk rotation in a three or more bladed rotor system.
Increasing the angle of attack while maintaining velocity increases lift and drag.

Helicopter Performance

If lift and thrust are greater than weight and drag in a no-wind condition, the helicopter will climb.
Rotor system efficiency decreases as the helicopter accelerates in forward flight.
Ground effect is the phenomenon where the rotor system efficiency increases due to the proximity of the ground.
Gyroscopic precession causes the tip-path plane to tilt, affecting the movement of the rotor system.

Rotor System Design

Two-bladed rotor systems are less affected by the Coriolis Effect than articulated rotor systems.
The transverse flow effect refers to the uneven airflow around the rotor blades due to the angle of attack.
Dissymmetry of lift occurs when the advancing blade produces more lift than the retreating blade.
Blade flapping helps to equalize lift across the rotor disk.
Retreating blade stall occurs when the retreating blade reaches a critical angle of attack.

Test your knowledge on aerodynamic flapping of rotor blades and the concept of blowback. Learn about the displacement of the advancing and retreating blades and how it affects the tilt of the tip-path plane.

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