Set of flashcards HE 14 (Page 1 of 2)

Flashcards	56
Language	Deutsch
Category	Technology
Level	University
Created / Updated	15.06.2015 / 07.06.2018
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How must the pilot react when he or she realizes that a ground resonance episode is developing?

Immediately reduce collective pitch.

Immediately switch on the stability augmentation system.

Immediately stop the rotor by starting the rotor brake.

Immediately interrupt the contact with the ground, if the rotor RPM are high enough, otherwise stop the rotor.

What are the consequences of a main gearbox failure on a conventional multiengine helicopter?

Emergency landing by autorotation must be performed.

Emergency landing by autorotation does not need to be performed

Emergency landing by autorotation must not be performed except in the early take off and late landing phases.

Emergency landing by autorotation must be performed only by Cat B and C helicopters.

What can you say about the power required by a fixed wing aircraft and the one required by a
conventional helicopter, when the two aircraft have the same payload and fly at the same height
and at a speed of approximately 150 kt?

The power required by the fixed wing aircraft is normally smaller.

The power required by the helicopter is normally smaller.

The power required by the two aircraft is normally very similar, as it depends on speed and mass only for both of them.

The power required by the two aircraft is normally very similar, as it is a function of height only for both of them.

Smaller rotors are aerodynamically more efficient at high speed. Of course they also generate a
higher induced velocity and hence require a higher induced power. Is this a significant problem?

No, it is an advantage: the bigger the induced velocity, the smaller the power required.

No, at high speed induced velocity is normally very small.

No, at high speed induced velocity is normally very large.

No, induced power is negligible at all speed on helicopters.

What can we say about the power required by the tail rotor of a conventional helicopter?

It is normally lower at low speed.

It is normally lower in hover.

It is normally lower at high speed.

It depends only on main rotor collective settings.

What are the causes of the limited max speed of conventional helicopters?

Retreating blade stall.

Compressibility effects on advancing blade.

Compressibility effects and stall of advancing blade.

Compressibility effects on advancing blade and retreating blade stall.

Which requirements distinguish performance class 3 operations form the other performance
classes?

The need to perform an emergency landing if one engine failure occurs.

The need to perform an emergency landing if one engine failure occurs in the early take off or late landing phases.

The need to perform an emergency landing if the helicopter is single engine and the engine fails.

The need to perform an emergency landing whenever the tail rotor fails.

What do performance class 1 operations involve?

The fact that a gearbox failure does never impose an emergency landing.

The fact that a tail rotor failure does never impose an emergency landing.

The fact that one engine failure does never impose an emergency landing.

The fact that one engine failure does never impose an emergency landing except in the early take off and late landing phases.

Do you always need a Cat. A helicopter for performance class 1 operations?

No.

Yes

No, it does not depend on the helicopter category.

No, also multi engine Cat. B may do that.

Singe engine helicopters are:

Either Cat. A or B depending on take off mass and power available

Always Cat. A.

Always Cat. B

Either Cat. A or B depending on type of rotor (co-axial rotors may be Cat. A).

What are category A and B helicopters?

Cat A helicopters, unlike cat B, have a certain stay-up capability in case of engine failure.

Cat A helicopters, unlike Cat. B, are never single engine.

Cat. B helicopters are always single engine.

Cat A helicopters, unlike cat. B, are always multi engine.

What is the ratio (approximately) of the rotation speed of the last turbine stage to the rotation
speed of the main rotor?

About 1000.

Up to 100.

20,000.

Lower than 10.

Are the consequences of a main gearbox failure different on a Class A and on a Class B
helicopter?

No, the failure is equally critical.

No, the failure is non-critical in both cases.

Yes, Class A helicopters can fly for a short time without a working gearbox.

Yes, on Class B helicopters it is more critical because of the lack of “stay-up” capability.

What should the pilot do in case of a failure of the tail rotor transmission?

Disconnect the main rotor from the engines.

Continue flying.

Disconnect the tail rotor from the engines.

Disconnect the main rotor from the engines and perform an emergency landing.

Why do helicopter gearboxes need intensive maintenance?

Because of the difficulty of lubricating moving mechanical parts.

Because of the long percentage of flight hours spent in hover flight.

Because of the large number of moving mechanical parts subject to wear and tear (D: wear and tear = Verschleiss).

Because of the large torque generated by the engine.

Could a turboshaft engine designed for fixed wing aircraft always be considered for installation
on a helicopter?

No, helicopter engines have centrifugal compressors, fixed wing engines only have axial compressors.

No, the requirements for helicopter engines are very different from those of fixed wing aircraft.

No, helicopter engines have two output shafts: one to the main rotor and one to the tail rotor.

No, helicopter engines have a higher number of turbine stages.

Why is it that on helicopters with coaxial rotors, while performing an autorotation, yaw control
inputs are reversed with respect to the normal working state of the rotor?

Because it is artificially reversed by a mechanical system to reduce the pilot’s workload.

Because in autorotation the energy flow is reversed: from the rotors to the gearbox.

Because in coaxial helicopters the energy flow is reversed in all flight conditions: from the rotors to the engines.

Because in autorotation the energy flow is reversed: from the gearbox to the rotors.

Is a separate anti-torque system installed on helicopters with co-axial rotors?

No, because helicopters with coaxial rotors are extremely slow.

No, because the rotors both rotate in the same direction.

No, because torque is controlled with lateral cyclic.

No, because the rotors are counter rotating

The Fenestron system is:

Scalable means that this solution can be easily increased or decreased in size to match smaller
or larger helicopters.)

Highly scalable.

Very safe for ground operations

Equipped with cyclic control.

Very noisy.

The NOTAR system is:
Scalable means that this solution can be easily increased or decreased in size to match smaller
or larger helicopters.)

Highly scalable.

Very efficient at high speed.

Very quiet.

Very noisy.

What are the main disadvantages of the conventional tail rotor with respect to solutions such
as Fenestron and NOTAR?

Higher weight and mechanical complexity.

Acoustic footprint (noise) and risk of accidents on the ground.

Acoustic footprint (noise).

Risk of accidents on the ground.

Is tail rotor effective in autorotation?

Yes, it is driven by the engine.

No, there are sources of power to drive it.

Yes, it is driven by an auxiliary power unit.

Yes, it is driven by the main rotor.

What is the tip-path-plane (TPP)?

The plane normal to the rotor mast.

The plane identified by the paths of all blades tips.

The plane identified by all blade hinges.

The plane identified by the position of the blade tips in hover.

What is a virtual flapping hinge?

It is an idealized hinge, made by a rigid blade-to-hub attachment.

It is a flapping hinge with a very small hinge offset.

It is an idealized hinge, made by a flexible blade-to-hub attachment.

It is a flapping hinge with lubrication.

What is the relation between the flapping hinge offset and the type of rotor head?

Hingeless and bearingless rotor heads are a better solution for large hinge offset designs, because the lack of mechanical hinges makes the blade stiffer.

Hingeless and bearingless rotor heads are a better solution for large hinge offset designs, because the lack of mechanical hinges makes the rotor head lighter and less draggy.

Articulated rotor heads are a better solution for large hinge offset designs, because the presence of mechanical hinges makes the rotor lighter and less draggy.

Articulated rotor heads are a better solution for large hinge offset designs, because mechanical hinges require less maintenance.

What is the relation between the so-called flapping hinge offset and the maximal control moments
around the pitch and roll helicopter axes available to the pilot?

The bigger the flapping hinge offset, the smaller the max control moments.

The bigger the flapping hinge offset, the bigger the max control moments.

None, control moments only depend on available power.

None, control moments only depend on cyclic pitch excursion (range).

What is the difference between hingeless and bearingless rotor heads?

In hingeless rotor heads the rotation around the feathering (or pitch) axis is accommodated by a mechanical bearing.

Hingeless and bearingless are synonyms (= the two words have the same meaning).

In bearingless rotor heads the rotation around the feathering (or pitch) axis is accommodated by a mechanical bearing.

Hingeless rotors are stiffer.

How are main rotor blades attached to the rotor head (also called rotor hub) on conventional
helicopters?

Blades are always rigidly attached to the rotor hub, this means that they cannot move or rotate.

Blades may be attached by means of mechanical hinges or flexible elements which behave as virtual hinges.

Blades are always attached by means of mechanical hinges.

Blades are always rigidly attached to the hub, between blade and hub special rubber elements are installed.

How is directional control on conventional helicopters achieved?

Tail rotor collective and lateral cyclic.

Tail rotor cyclic.

Main rotor collective.

Tail rotor collective.

How can dynamic stability of helicopters be further improved?

By means of mechanical systems such as SAS (Stability Augmentation System).

By means of computer-controlled systems such as FADEC (Full Authority Digital Engine Control).

By means of computer-controlled systems such as SAS (Stability Augmentation System).

By means of auto-stable airfoils.

Do helicopters exhibit (= have or show) a higher dynamic stability than isolated rotors?

Yes, this may happen in hover thanks to aerodynamic surfaces.

Yes, this may happen in forward flight thanks to aerodynamic surfaces.

Yes, this may happen in hover thanks to the higher mass.

Yes, this may happen in forward flight thanks to the higher mass.

Are helicopter rotors dynamically stable? (In this question, an isolated rotor is considered.)

No, neither in hover nor in forward flight.

Not in hover.

Not in forward flight.

No, neither in hover nor in forward flight at low advance ratio.

Are helicopter rotors statically stable? (In this question, an isolated rotor is considered.)

Yes, in forward flight.

Yes, in hover and forward flight at low advance ratio.

Yes, in hover and forward flight.

Yes, in hover.

Why does a rotor tend to flap back in forward flight?

Because the lift generated by the advancing blade is higher than the one generated by the retreating blade.

Because the lift generated by the retreating blade is higher than the one generated by the advancing blade.

Because of retreating blade stall.

Because of compressibility effects.

What is the main limiting factor for the position of the helicopter centre of gravity?

The limited excursion (range) of the tail rotor collective pitch control.

The limited excursion (range) of the lateral cyclic pitch control.

The limited excursion (range) of the longitudinal cyclic pitch control.

The limited excursion (range) of the collective pitch control.

Are tail rotors normally equipped with cyclic pitch?

No, only if the tail rotor is articulated

No.

No, only if the tail rotor is a see-saw.

No, only if the helicopter has a high max speed.

Are all bearingless rotors free from ground resonance also without dampers?

Yes, but only if the helicopters is equipped with skids.

No, not all of them, only the ones with very stiff blade roots, the so- called stiff-in-plane rotors.

No, only the ones with very soft blade roots, the so called soft-in-plane rotors.

Yes.

How are conventional helicopters protected against ground resonance?

With dampers around the flapping hinges of the blades and on the landing gear.

With dampers around the lead / lagging hinges of the blades and on the landing gear.

With the stability augmentation system or SAS.

With dampers around the lead / lagging and flapping hinges of the blades, as well as on the landing gear.

How could ground resonance be defined?

A kind of dynamic instability involving main rotor blades motion and the oscillations of the helicopter in the air.

A kind of dynamic instability involving main rotor blades flapping and the oscillations of the helicopter on the ground.

A kind of dynamic instability involving the helicopter oscillating on the ground when rotor max RPM is reached.

A kind of dynamic instability involving main rotor blades motion in plane (lead / lagging) and the oscillations of the helicopter on the ground.

What is the function of the so-called (delta 3)” angle on tail rotors?

(The delta 3 angle refers to the angle between blade axis and the pitch link and generates a mechanical
coupling between flapping and pitching).

To electronically maintain the blade flapping within acceptable limits in forward flight.

To mechanically maintain the blade flapping within acceptable limits in forward flight.

To increase tail rotor thrust.

To mechanically maintain the blade flapping within acceptable limits in hover.

HE 14

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