2. an introduction to the helicopter

Active Aeroelasticity and Rotorcraft Lab.

2. An Introduction to the Helicopter

2020

Prof. SangJoon Shin

Active Aeroelasticity and Rotorcraft Lab., Seoul National University

I. Helicopter Configurations

II. Helicopter Control Methods

III. An Intro to the Helicopter

IV. Pilot Controls

V. Rotor Types

VI. Mechanics of Rotor Control

VII. Conventional Helicopter Design Features

VIII. Flight Characteristics of the Helicopter

Overview

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Helicopter Configurations

5 main types

1. Single rotor

• the most common type.

Simplicity of configuration

• “Gyrodyne” – anti-torque rotor

pulling the vehicle, in front of

the MR

• “Jet rotor” – no anti-torque

needed except bearing friction

simplicity. small storage space,

but high specific fuel

consumption


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5 main types (Contd.)

2. Coaxial

• Over-all dimension only by

the rotor and of a saving of

power over single MR-TR.

But, rotor hubs and controls

become more complex and

rotor weights tends to

increase.


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3. Side-by-side rotors

• laterally displaced rotors →

reduction in power

• similar to the aspect ratio

effect in fixed wing

• But, high fuselage parasite

drag, high structural weight

relatively complex gearing and

shafting

• “synchropter” – Sacrifices

some lifting efficiency gains


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4. Tandem rotors

• Pros: Clean fuselage, large

available CG range

• Cons: Transmission, shafting

weights. Loss in lifting efficiency in

forward flight (∵ one rotor is

working in the wake of the other)

but can be minimized by stagger,

i.e., by placing the rear rotor

above the front rotor

5. multi-rotors – simplification in

control system


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Helicopter Control Methods

Helicopter control methods

• 4 independent controls rather than the control for 6 independent

forces and moments

① Vertical Control

– by increasing or decreasing the pitch

② Directional Control (Fig. 2-7)


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Helicopter Control Methods

Helicopter control methods (Contd.)

• Lateral Control (Fig. 2-8)

- Tilt and side ward motion

• Longitudinal Control (Fig. 2-9)


Pilot’s Controls

① Control stick – Longitudinal and lateral control

② Pedals – directional control

③ Pitch lever

④ Throttle – Const. speed governor

(adjust rotor pitch / adjust throttle for a given pitch setting)

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Rotor Types

① Articulated

② Teetering, seesaw

③ Hingeless, Bearingless

• Offset of the hinges

• Flapping – stability and control

• Lead lag – vibration

• Number of blades – vibration

characteristics, weight, mechanical

complexity, storage space

▲ Fig 2-11 Three-bladed articulated rotor system

▲ Fig. 2-12 Two-bladed “see-saw” rotor system

▲ Fig. 2-13 Two-bladed rigid rotor system


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Mechanics of Rotor Control

① Producing moments about the rotor hub

② 2. Tilting the resultant rotor lift vector

③ 3. Combination of both


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1. Control by tilting the rotor hub – mechanically awkward

2. Control by cyclic pitch change - conventional way of achieving control in

both rigid and flapping rotors. usually accomplished by a linkage from

the blades to a “swash plate”.

- Rigid blades : swash plate tilt → moment about the rotor hub

- Flapping blades : swash plate tilt → tilt of the thrust vector, no moments.

If non-zero hinge offset

- Non-zero moment about the hub

- Thrust vector tilt


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2. Control by cyclic pitch change

(Contd.)

Alternative means

- Servo-tabs

- Servo-rotors

Prevent the feedback of forces from

the rotor into the control system.


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Conventional Helicopter Design Features

1. Rotor blades

– low pitching moment coefficient

Ex) NACA 00 series (0012, 0015) and NACA 230 series (23012, 23015)

- Airfoil thickness ratio 9~20%

- Planform, twist

Blade construction

- Steel spar, fabric covering

(most early rotor blades)

- Plywood-covered blades

- All-wood blade

- Metal blades (Sheet stock or extrusions)


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2. Rotor hubs

- Forged from steel or dural

3. Rotor control linkages

- Linkages are arranged to minimize 𝛿3(pitch-flap) and 𝛿4(pitch-lag) coupling

4. Control system

- Stick motion 1” → cyclic pitch change 1~2º

Side by side – swash plate may be free to tilt only in a fore and aft.

→ lateral control

Some for longitudinal control in tandem configuration



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5. Fuselage design – several factors (5)

6. L/G – shock absorber → softer action in landing, provide damping for

ground resonance, Loading angle be sufficient to permit high pitch-

up attitude

7. Transmission systems – engine and rotor 10:1, planetary gear train,

bevel gear, clutch, free-wheeling unit



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Flight characteristics of the helicopter

1. Performance

induced power: required to produce lift,

60~70% of total

profile-drag: required to drag the blades

through the air, 30~40% in

hover.

In forward flight, parasite power∝ 𝑉3

Power available - horizontal

▲ Fig. 2-18 Induced power decreases

with V, profile power increases slightly

▲ Fig. 2-19 Resultant power required minimum

power required at 40mph, mostly 40~60 mph


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1. Performance (Contd.)

Ground effect

- A helicopter is able to hover very near the ground even when it has

insufficient power to hover away from the ground.

Ground stops the rotor downwash, or induced velocity → decreasing the

induced power required.

- Overloaded helicopter : may take off in a wind or by making a run on the

ground to attain a small forward speed → marginal hovering performance

- Best climb : will occur at about the speed for minimum power in level

flight → the greatest excess power available



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1. Performance (Contd.)

Ground effect (Contd.)

- Top speed in level flight : reductions in fuselage drag by “cleaning up”

the fuselage are far more effective

- Blade stall : causes vibration of the helicopter and controls or

considerable increase in profile drag power

- Compressibility effect : advancing side → limit in forward speed 200mph

- Vertical descent : the rotor is about as effective as a parachute of the

same diameter



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2. Control forces

- Stick forces rather than stick displacement, but do not arise from straight

forward sources on an airplane

3. Vibration characteristics

- Even multiples of the No. of the blades

reduction of input forces → blade design

natural frequencies of the fuselage → avoiding natural freq

4. Ground resonance

- Coupling between blade lag motion in the rotating frame or

sideward or fore/aft motion of the shaft

→ destructive



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5. Stability characteristics

- high control sensitivity in roll → short-period, pilot-induced lateral

oscillation, due to low rotor damping

Sluggish in response to a sudden control deflection → due to the mass of

the helicopter.

Dynamically unstable in hovering & dynamically unstable in pitch in forward

flight, but mild

→ due to low rotor damping, angle-of-attack instability


2. an introduction to the helicopter

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