HELI 18

HELI ZHAW AV15 6. Semester

139

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L. H.

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L. H.

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This flashcard set delves into the intricate mechanics and dynamics of helicopters, focusing on university-level aerodynamics, rotor systems, and flight control mechanisms. It covers key aspects such as rotor blades, ground effect, flight dynamics, and the role of the pilot. This set is invaluable for aerospace engineering students and professionals seeking to understand the complexities of helicopter design and operation.

Cartes-fiches

139

Utilisateurs

Langue

Anglais

Catégorie

Technique

Niveau

Université

Créé / Mis à jour

26.04.2018 / 18.06.2021

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Big helicopters, over 10 tonnes of mass, never have skid landing gears. Why?

Passengers comfort would be disappointing,

Ground resonance could not be avoided,

Hangaring and taxiing would be very difficult,

Aerodynamic drag would be too high,

Skid landing gears may also not have dampers. How is energy dissipated if the helicopter oscillates on the ground?

By the main rotor,

By the pilot,

By an electronic control system called FADEC,

Friction between skid and ground,

What is ground resonance?

An instability mode involving the oscillations of the helicopter on the ground and the main rotor,

An instability mode involving the main rotor on the ground,

An instability mode involving the fuselage and the landing gear,

An instability mode of the landing gear,

How is ground resonance prevented, normally?

With a control system called FADEC,

With dampers on the landing gear,

With dampers on the rotor and the landing gear,

With flapping hinges,

What are the main advantages of a Fenestron-like tail rotor?

Low vibration level,

Low noise, safety for ground personnel,

Aerodynamic efficiency,

Low mass,

What are the advantages of a NOTAR-like anti-torque system?

Low vibration level,

Aerodynamic efficiency,

Low noise, safety for ground personnel,

Low mass,

What are the advantages of a conventional tail rotor system (i.e. a “normal” tail rotor)?

Good aerodynamic efficiency in most of the flight envelope,

Low noise, safety for ground personnel,

Low vibration level,

Low vulnerability (that is to say that the helicopter is difficult to damage in, say, a combat scenario),

What is a category A helicopter?

A helicopter with stay-up capability in case of power unit failure,

A helicopter without stay-up capability in case of power unit failure,

A helicopter with stay-up capability in case of tail rotor failure,

There is no such thing as a category A helicopter,

What is a category 1 helicopter?

A helicopter without stay-up capability in case of power unit failure,

A helicopter with stay-up capability in case of power unit failure,

There is no such thing as a category 1 helicopter,

A helicopter with stay-up capability in case of tail rotor failure,

Can a single-engined helicopter be category A?

Yes,

No,

Only in performance class 1 flights,

Only if the helicopter has no tail rotor,

What is the relation between manoeuvrability and rotor head geometry?

The larger the flapping hinge offset, the smaller the lift force generated by the rotor,

The larger the flapping hinge offset, the smaller are the control moments,

The larger the flapping hinge offset, the larger is the drag force generated by the rotor,

The larger the flapping hinge offset, the larger are the control moments,

What are articulated rotors?

Rotors with articulated hinges,

Rotors with no hinges,

Rotors with rigid blade attachments,

Rotors with flapping, lead-lagging and feathering hinges,

Which is the best definition for hingeless rotors?

Rotors with no hinges,

Rotors with (bendingwise) flexible blade attachments and a feathering hinge,

Rotors with flexible blade attachments,

Rotors with (bending- and torsionwise) flexible blade attachments (and no feathering hinge),

Which is the best definition for bearingless rotors?

Rotors with (bendingwise) flexible blade attachments and a feathering hinge,

Rotors with (bending- and torsionwise) flexible blade attachments (and no feathering hinge),

Rotors with no hinges,

Rotors with flexible blade attachments,

What can be the advantage of a larger hinge offset?

Lower drag force,

No ground resonance,

Larger forces can be generated,

Larger moments can be generated around the helicopter axes,

What can be a disadvantage of a larger hinge offset?

Ground resonance,

Higher drag forces,

Unsteady airloads (e.g. turbulences) are felt more heavily by the passengers,

Smaller forces can be generated by the rotor,

See-saw rotors have zero hinge offset. In which situation is the helicopter more difficult to control?

Manoeuvres with small or zero load factor,

Manoeuvres with a large positive load factor,

Manoeuvres which require a large lift force,

Manoeuvres with increase of drag force,

Are see-saw rotors suitable for helicopters which need to be very manoeuvrable?

No, because control moments would be too small,

No, because control moments would be too large,

Yes, because control moments are very large,

Yes, because control moments are very small,

Hingeless and bearingless rotors may have large hinge offset. In this case, are they suitable for helicopters which need to be very manoeuvrable?

Yes, because control moments are small,

Yes, because control moments are large,

No, because control moments are large,

No, because control moments are small,

How can the pilot control the amount of rotor thrust generated by the rotor on a conventional, modern helicopter?

By controlling the power available,

By acting on the pedals,

By acting on the collective pitch lever,

By acting on the cyclic pitch lever (stick),

How can the pilot change the orientation of the trust force vector generated by the rotor?

By acting on the collective pitch lever,

By acting on the cyclic pitch lever (stick),

By acting on the pedals,

By controlling the power available,

What is the consequence of the pilot acting on the pedals?

Main rotor thrust is modified by a change in main rotor cyclic pitch,

Main rotor thrust is modified by a change in main rotor collective pitch,

Tail rotor thrust is modified by a change in tail rotor collective pitch,

Tail rotor thrust is modified by a change in tail rotor cyclic pitch,

What can be said of the relative velocity on the rotor blades in forward flight,

The relative velocity is uniform across the rotor disc,

The advancing and the retreating blade are immersed in air moving with the same velocity,

The retreating blade is immersed in higher velocity air, the advancing blade is immersed in lower velocity air,

The advancing blade is immersed in higher velocity air, the retreating blade is immersed in lower velocity air,

What is the origin of the “lift dissymmetry” in forward flight on rotors without flapping hinges?

The identical relative air velocity on the advancing and the retreating blades,

The different relative air velocity on the advancing and the retreating blades,

Blade flexibility,

Blade stiffness,

What is the reaction of an articulated rotor to a sudden horizontal wind component if not corrected by the pilot?

The rotor does not react,

The rotor tilts in the same direction of the wind (this is called “flapback” when referred to wind from front),

The rotor tilts in the opposite direction, thus generating a thrust component opposite to wind,

The rotor becomes unstable,

Consider a blade section. What is the “pitch” angle?

It is the angle between a reference line on the blade section and freestream velocity,

It is the angle between a reference line on the blade section and the plane of rotation (it is a purely geometric quantity),

It is the angle angle between a reference line on the blade section and the relative velocity (freestream + rotation),

It is the angle angle between a reference line on the blade section and the induced velocity,

Consider a blade section. Is pitch angle identical to angle of attack?

No, angle of attack also depends on the direction of the relative velocity,

Yes,

No, angle of attack depends only on induced velocity,

No, angle of attack is not defined for helicopters,

How would you describe the aerodynamics of the helicopter?

It is dominated by the rotor downwash, which generates large vortices and makes aero- dynamic forces difficult to predict,

It is very similar to the one of fixed wing aircraft, with the rotor behaving exactly like a wing, and generating very small induced velocity,

It is dominated by the rotor downwash, which makes the rotor thrust force irregular and un- steady,

It is very similar to the one of fixed wing aircraft, with the rotor behaving exactly like a propeller generating very large but concentrated induced velocity,

Are helicopters fast, compared to fixed wing aircraft (with similar payload)?

Yes,

The flight envelope of both vehicles cover an almost identical speed range,

No, they are normally slower,

It depends on the power available,

What are the physical phenomena which limit the helicopter maximum forward speed?

High Mach number of the flow around the retreating blade,

High Mach number of the flow around the advancing blade,

Compressibility effects on retreating blade and stall on advancing blade,

Compressibility effects on advancing blade and stall on retreating blade,

Why is there an “avoid area” at low forward speed and low height from the ground?

To limit air pollution in proximity of the heliport,

Because it is difficult to initiate autorotation and complete an emergency landing in these conditions,

Because the pilot has little visibility on the heliport,

To avoid interference with air traffic,

Why is there an “avoid area” at high forward speed while flying in proximity of the ground?

To limit air pollution in proximity of the heliport,

Because it is difficult to initiate autorotation and complete an emergency landing in these conditions,

Because the pilot has little visibility on the heliport,

To avoid interference with air traffic,

Some commercial helicopters are designed to be fast. What could the maximum speed be?

About 250 kt,

About 150 kt,

Almost Mach 1,

About 50 kt,

What is the function of the tail rotor?

Provide torque balance,

Provide additional forward thrust,

Provide lateral and directional dynamic stability,

Provide lateral and directional stability,

Some helicopters have multiple and large vertical tailplanes. What is their function?

Provide directional control,

Provide directional stability,

Provide a support for the tail rotor,

Provide lower drag force,

How can autorotation be defined?

It is a working state of the rotor, whereby thrust is generated without the need of engine power,

It is a working state of the rotor, whereby the helicopter can perform descent flight,

It is a descent flight,

It is a descent flight at zero collective pitch angle,

How is an emergency landing from horizontal flight performed?

Lower collective until the autorotative state is reached, descend in autorotation,

Raise collective until the autorotative state is reached, descend in autorotation,

Lower collective until the autorotative state is reached, descend in autorotation, perform flare manoeuvre,

Raise collective until the autorotative state is reached, descend in autorotation, perform flare manoeuvre,

At which horizontal speed should the pilot initiate autorotation, if he/she is in the position to control horizontal speed?

Hover, in ground effect,

Hover,

At V-max,

At V-best range or V-best endurance,

What could be the advantage of performing autorotation at V-best range?

The pilot can perform a more elegant flare,

The pilot can delay the impact by a few seconds,

The pilot has a wider choice for a landing spot,

The pilot can save fuel,

What can be said of isolated rotors’ stability with respect to translations and rotations?

They are statically unstable but dynamically stable,

They are statically stable but dynamically unstable,

They are statically and dynamically stable,

They are statically and dynamically unstable,

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