helicopters stabilityandcontrol

8/17/2019 Helicopters StabilityAndControl

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Helicopters

Basic Theory

Miguel Silvestre Aerospace Sciences DepartmentUniversity of Beira Interior

April 2016

1.4 Stability and control


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1.4 Stability and control –

Introduction• The Stability and Control analisys in helicopter is in

general similar to fixed wing aircraft.

• The following items are the diferences of the

helicopter relative to the fixed wing aircraft.


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1.4 Stability and control –Main Rotor Moments About the CG

pitch

or rol.

- the No Feathering Plane (NFP) and the Rotor

Flapping Angle (RFA) define the rotor plane (TPP);

- the thrust is perpendicular to the TPP;

- any missalignement of the thrust direction to

the center of gravity creates moments

- these can be in

- This is the principle for the pilot to control the

aircraft:

- he changes the no feathering plane to tilt therotor and obtain a control moment


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1.4 Stability and control - trim

After CG

Forward CG

Cruise Trim - CG position influence

note:the same RTF, TPP and NFP.

The difference is the fuselage attitude


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1.4 Stability and control - trimCruise trim- Horizontal tail influence

After CG

Forward CG

LH

note:the same RTF, TPP and NFP.

Main rotor pitch moment must balance

horizontal tail moment depending on

the fuselagem attitude that depends on

the CG position


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1.4 Stability and control - trimFlapping hinge offset - direct head moment

Main rotor pitch moment,horizontal tail moment

depending on the fuselagem attitude and

flapping hinge offset moment must balance


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• RFA is proportional to speed and

• is proportional to thrust at a given speed. So,

– as thrust is proportional to the rotor’s angle of attack,

• Any RFA disturbance may tilt the rotor further back,

increasing the rotor’s angle of attack. Furthermore, itproduces nose up pitching moment that increases thrust

and the airaft AoA even further.

Important notes about rotor flapping


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Static stabilitynose up divergence problem:

In case of an angle of attack increase disturbance,

•thrust increases;

•rotor tilt angle increases;

•nose up moment is created;angle of attack is increased even further!

An Horizontal tail contributes with positive longitudinal static stability

The rotor alone, above the cg, is pitch unstable!


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HoveringHovering dynamic instability problem:

In case of an horizontal velocity disturbance,

•flapping angle appears;•rotor and thrust are tilted;

•horizontal acceleration is installed;

•horizontal velocity builds up untill rotor flapps in the opposite direction;

•the process is reapeated in the opposite direction with inceasing

amplitude!

For large helicopters, the hovering oscilation period is usually long enough for safe

reaction of the pilot. For small RC helicopters, a stabilization device is needed.

The rotor alone, above the cg, is also dynamically unstable in hover !


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Hovering

Lackner HZ-1 Aerocycle

(from:http://en.wikipedia.org/wiki/De_Lackner_HZ-1_Aerocycle)

One hovering dynamic stability solution concept: rotor below CG

http://en.wikipedia.org/wiki/De_Lackner_HZ-1_Aerocyclehttp://en.wikipedia.org/wiki/De_Lackner_HZ-1_Aerocycle


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Hoveringother hovering dynamic instability solutions:

Hiller control system

Bell control system

Bell-Hiller mixing device


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Bell control system

A flybar acts as a gyroscope maintaing its own plane of rotation. The

flybar acts as an auxiliary swashplane. The NFP is a mixture (or

average) of the swashplate and the flybar planes. As the rotor plane

goes away from the flybar plane, a cyclic pitch input is imposed that

tends to bring the rotor plane back to the flybar plane. The heavier the

flybar weights the more stable in hover the helicopter gets.


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Bell control system

Bell UH-1(http://es.wikipedia.org/wiki/Bell_UH-1_Iroquois )

http://es.wikipedia.org/wiki/Bell_UH-1_Iroquoishttp://es.wikipedia.org/wiki/Bell_UH-1_Iroquois


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Bell control system

Bell UH-1(http://www.aircav.com/huey/stabiliz.html )


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Bell control system

Bell UH-1(http://www.b-domke.de/AviationImages/Huey/Images/0419.jpg)


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watch: https://www.youtube.com/watch?v=8HcWmhEkXpg

https://www.youtube.com/watch?v=N99wK1uACUM

Less control force required (as in the Hiller system)

More stable than a Bell system (as in the Hiller system)

Faster cyclic response than the Hiller system due to some direct swashplate input

Slightly slower cyclic response than a pure Bell system

More complexity than either a Bell or Hiller system

https://www.youtube.com/watch?v=8HcWmhEkXpghttps://www.youtube.com/watch?v=N99wK1uACUMhttps://www.youtube.com/watch?v=N99wK1uACUMhttps://www.youtube.com/watch?v=8HcWmhEkXpg


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http://www.rchelicopterfun.com/images/HillerHead500pics.gif

• Bell-Hiller Control System

D i S bili Pi h


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Dynamic Stability - Pitch


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Bell 212

not an

antenna!


Dynamic Stability

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