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8/12/2019 Rotorcraft Technology http://slidepdf.com/reader/full/rotorcraft-technology 1/51 1 ROTORCRAFTS TECHNOLOGY About the title: Whereas title refers to “Rotorcrafts”, mainly helicopters are treated through this course.

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ROTORCRAFTS TECHNOLOGY

About the title: Whereas title refers to “Rotorcrafts”, mainly helicopters are

treated through this course.

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Helicopters history

The word “helicopter” comes from the association of two Greek roots: “Helix”

and “Pteron”, literally: “wing in spiral”.The locution “wing in spiral” helps to remind the major specificity of thehelicopter aeromechanic.

This specificity is also reminded by English locution “Rotary Wing”, referring tothe family comprising the helicopter concept (Rotary Wing = “VoilureTournante” in French)

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First attempts and first “flights”

At the time, engineers were puzzled by technical problems :

-Insufficient available power,

-Poor mechanical properties of available materials,

-Control and stability issues,

-The first forward flights raised the limiting problem of “lift asymmetry”.

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Through autogiro…

End of twenties, Spanish engineer Juan DeLa Cierva played a key role in rotary

wing aircraft history, working on the development of the autogiro.He developed these rotary wing machines from fixed wing “biplane” airplanes,first replacing the upper wing by a rotor mounted free on its vertical shaft. Then,taking advantage of flight testing, he progressively removed the lower wing.

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… And it’s application

During world was II, the Focke Achgelis FA330 was designed to be carried

onboard of German “U boots”.The purpose was to make it glide, pulled behind the submarine, in order to use itas an aerial observation platform, searching the horizon for enemy boats.

Due to the low towing velocity, it would have been difficult to use a fixed wingultra light aircraft for this application, unless the fixed wing aircraft has a largewing surface.

Additionally, this ultra light autogiro aircraft is foldable and compact, whichmakes it better suited to the application than a fixed wing ultra light aircraft.

More info: http://robroy.dyndns.info/targetkites/Fa-330/intro.html

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… Toward more mature machines

At this time, increasingly powerful engines become available at an affordable

weight, which enables more remarkable flights.But the success and development pursuit of rotorcrafts certainly comes from amajor feature of these machines already demonstrated at this time;

Their ability to perform safe (power-off) glided landing, in the frame of amaneuver called AUTOROTATION.

Without the key ability to perform AUTOROTATION, (at the manner of anautogiro), the helicopter development would have certainly led to a dead end.

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Summary of the first 30 years

Sikorsky VS300 is sometime referred to as « the aircraft that created an

industry ».Between 1939 and 1941, Igor S.Sikorsky and his team (located in Stratford,Connecticut) worked on several configurations of this aircraft, to achieve asuccessful final configuration.

Although the first architecture of the VS300 was already including a collectiveand cyclic pitch control for the main rotor, as well as a collective pitch controlledtail rotor, it’s only after reverting to less ambitious configurations that Sikorskycould achieve this first (and final) configuration to work properly.

In order to distinguish the technical issues coming from the main rotor cyclic and

collective controls, he first removed all the cyclic control from the main rotor,(leaving only collective control), and ensured the forward and lateral flights bythe means of vertical axis auxiliary propellers, located at the back and on thesides of the aircraft.

It’s only after several main rotor head improvements that he could successfullyget a swashplate system to work, finally removing the vertical axis auxiliarypropellers.

Even still today, the VS300 architecture can be considered as the dominantWORLDWIDE ADOPTED HELICOPTER ARCHITECTURE, and this is whyit’s introduced here as the first major breakthrough in helicopter history.

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Toward the first industrial machines in the US… (1/2)

Following the VS300 success, US Army used Igor S.Sikorsky to develop several

helicopters.At the time, key applications were military, as well as in the field of rescuemissions.

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Toward the first industrial machines in the US… (2/2)

France bought S55 from the US. At the time, an existing plant located in

Marignane, beside the “Etang de Berre” was used to ensure maintenance of theseaircraft.

This maintenance license has helped France to re-started its helicopter businessfollowing world war II.

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…And in France (1/2)

The re-start of helicopter business in France following world war II:

SE3101 aircraft (piloted by French test pilot Jean Boulet) was using one of thetwo Focke-Achgelis FA61 main rotor. In its book “L’histoire de l’hélicoptère”,Jean Boulet explains that taking this existing main rotor is the origin of whyFrench rotors rotate clockwise (seen from above) whereas US manufacturersmain rotors rotate counterclockwise.

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…And in France (2/2)

The advent of the turbo shaft engine on helicopters:

Charles Marchetti, head of SNCASE design office, thought that piston enginesdrawbacks (weight to power ratio and size) could justify the attempt to use turbo-shaft engines, at that time already available on bigger aircrafts.

With the engineer René Mouille, along with the help of Joseph Szydlowski, (whofounded Turboméca company in 1938), they adapted Artouste engine (initiallydesigned as an airplane Auxiliary Power Unit) on the Alouette II.

Since this success, which has consolidated the creation of the helicopter industryin France, all helicopter manufacturers worldwide have adopted the idea, and thisis why it’s introduced here as the second major breakthrough in helicopter

history.A side benefit is that the vibration levels are reduced, since the torque pulsation islower on a turbo-shaft, than on a piston engine.

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Has anybody ever tried to ?...

The V/STOL wheel proposed on http://www.vstol.org/ website is an inventory

work that proposes an overview of “ever tried” Vertical and/or Short Take-Offand Landing aircrafts.

A prototype of each kind is detailed (in an explanation sheet), representing eachfamily.

This wheel illustrates the human creativity and perseverance, and also remindsthat, a bit like living species, technical concepts carry “genes”.

Depending on several intrinsic or extrinsic influences, these genes can be either“dominant” (in which case the species or the feature develops) of “recessive” (inwhich case the species or the feature extinguishes).

-Intrinsic influences: Technical simplicity or complexity, costs, safety, efficiency,etc…

-Extrinsic influences: End-user needs, politics, availability of technologic brick,etc…

It’s important to remind that:

-If not serialized, a concept has little chance to prove it can be dominant,

-Nowadays dominant concepts and products are the results of all this “geneticmixing”,

-Today’s dominant concepts are not necessarily tomorrow’s dominant concepts.

Note that acronym “STOVL” (Short Take-Off Vertical Landing) is also found,

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although there is no real typological difference between a V/STOL and a STOVL.

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Dominant concepts

Today, the four rotary wing dominant concepts are:

- The Helicopter (also comprising “tandem helicopter” sub-concept)

- The Compound (also sometimes referred to as “gyrodyne”)

- The Tilt Rotor

- The Autogiro

What about tomorrow ?

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Qualitative Summary

The following is a qualitative comparison, enhancing the major differencesbetween the main rotary wing concepts:

- The Helicopter is the best solution for missions requiring an important part ofhovering flight. It’s well adapted to missions that can deal with low speeds,and short to medium ranges.

It’s essentially limited by its maximum airspeed, as well as its operational cost.

- The Compound enables an increase in maneuverability at high speeds, but onlybrings a limited increase of maximum speed (+50%). It’ s limited by itsoperational cost and weight convergence.

- The Tilt Rotor overcomes natural helicopter limits for medium and long rangemissions. However, hovering capacities are affected, although still remainingreasonable.

This concept has entered serial/operation phases in the US with military programV22, and is still in development/certification phases in Italy/US with A609program.

- Concerning the Autogiro: Although unique (no hovering capacity, but low cost,

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low range and low speed), its flight envelope has not presented enough advantages so thatthis concept has other application today than ultra light / leisure applications.Nevertheless, as previously mentioned, the autogiro role was key is rotary winghistory.

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Emergency Medical Service (EMS) (Mostly a para-public use, but can be also acivilian or military use)

Purpose of the mission can be:-Extraction of injured people from the accident scene to hospitals

-Taxi of medicalized people between hospitals (same function as ambulance)

Typical equipment can comprise: Medicalized interior with life sustaininginstallation, wire strike protection system

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Water Bombing Mission (1/2) (Mostly a para-public use)

Mission features :

-Following fire alert, (given by ground or aerial surveillance entities), go as fastas practicable to the fire scene

-Water the fire scene, performing short rotation between water refill point andfire scene

-Water quantities: (depending on carrier) 900 liters (AS350) to 8000 liters (S64Aircrane)

Typical equipment can comprise: Water tank (external or internal), door system,refill water pump, loudspeaker.

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Oil and Gas Mission (Civilian use)

Purpose of the mission is mostly to make oil industry workers commute between

shore and oil rig platforms. Most of the time of this demanding mission is spentabove water, sometimes in bad weather conditions.

Other uses in oil industry comprise:

-Oil and gas exploration

-Transport of material, when commuting by boat takes too much time

Typical equipment can comprise: Interior with passengers seating, emergencyfloatation gear and life raft

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Port Piloting (Civilian use)

In the world of portal activities, a port pilot is an employee of the port, who’s role

is to guide boats that enter the port. This personnel has to be physically on theincoming boat.

Most of the time, port pilots go to the incoming boats by means of small fastboats. In some huge ports where cargo traffic is very intense, such as in “LeHavre” (France), driving port pilots to incoming boats can be done by helicopter,hoisting the pilot on the deck on the boat.

Typical equipment can comprise: Emergency floatation gear, life raft, hoistsystem

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Police Mission (Para-public / military use)

General purpose is law enforcement, including missions that require tracking and

interception of criminals, such as drug traficants for example.Repelling and roping can be part of the mission, when the velocity of egress isdeterminant.

Typical equipment can comprise : Searchlights and FLIR (Forward Looking InfraRed) for night missions, loudspeakers, wire strike protection system, personnelweapons (para-public) or heavier mounted weapons (military)

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Search and Rescue (SAR) (Para-public / military use)

General purpose is the search for and the aid to people who are in distress or

imminent danger.Two main fields of search :

-Mountain rescue

-Sea rescue

Typical equipment can comprise : Hoist(s), Searchlights and FLIR (ForwardLooking Infra Red) for night missions, loudspeakers, wire strike protectionsystem, as well as emergency floatation system and life raft for sea rescue

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Aerial work (Civilian use)

The aim of the mission is to lift loads in difficult cases, when access of the

working site is remote, or impossible by other means.Typical application can be: Post installation in mountain area, concrete hoppertransport in mountain area, cableway installation for ski facility

Typical equipment comprise: A belly mounted cargo hook, cargo mirrors, wirestrike protection system

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VIP Transport (Civilian use)

The aim of the mission is ensure fast and safe transport

Generally but not exclusively in urban areas

Typical equipment comprise: A comfort interior with galley, a floatation systemfor sea areas

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Anti Submarine Warfare (ASW) & Anti Surface Warfare (ASuW) (Military use)

Based on ships or on-shore, the purpose of the mission is to search for, locate and

sometimes engage submarines (ASW) or boats (ASuW),These missions are among the most complex, in term of number of functions tobe ensured, complexity of each function, payload.

Typical equipment comprises: Foldable capability for main rotor and tail boom(when h/c is stored inside boat hangar), harpoon system for ship-deck landing,naval radar, FLIR, sonar, defense suite, torpedo (ASW) or missile (ASuW), etc…

Note: A good course of submarine warfare is accessible through 1990 the movie“ The Hunt for Red October ” (French title: A la poursuite d’Octobre Rouge)from director John McTiernan.

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Aerodynamics Background (1/3)

For a certain rotor span, (for example @ 50% of rotor radius) a blade cross

section is looked at.Profile is characterized by its geometric contour, its geometric chord, its leadingand trailing edges.

“Théta” is the PITCH ANGLE: It’s defined as the angle comprised between aplane that is normal to the main rotor shaft, and the geometric chord.

“i” is the INCIDENCE ANGLE: It’s defined as the angle comprised betweenrelative wind and geometric chord.

Exactly like in the case of an airplane wing, under the action of relative wind“VR”, profile reacts elemental effort “dFN”.

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Aerodynamics Background (2/3)

Summing the elemental contribution of every “dFz” of each cross section along

each blade, and then summing the contribution of each blade, the overall rotordisc lift “Fz” is found.

In hover flight, relative wind “VR” is obtained only by the rotation of the mainrotor (peripheral speed “U”). In practice, it’s useful to remember that peripheralspeed U worth approximately 200 m/s (0 / +10%) for all helicopters:

- The minimum speed is given by the need to be aerodynamically efficient

- The maximum is given:

* In hover flight by noise considerations

* In forward flight by issues encountered on advancing andretreating blades

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Aerodynamics Background (3/3)

Helicopter control is simply provided by main rotor disc tilt, in the direction

where displacement is expected.A practical solution to achieve this tilt would be to tilt the main rotor shaft,though driving the tilt of the entire rotor disc.

Although this is achievable for small rotor discs (7 to 8m diameter), with smallinertia and small load to surface ratios, (this solution is still used on ultra-lightautogiros) this solution is not practical for larger rotors, due to the mechanicalefforts that would result.

As a consequence, another solution had to be found !

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NOTE: For the following chapters, the word “hinge” will refer to: Either anarticulation with a physical “swiveling” pivot/axis/point, or a soft member,

playing a similar role to the pivot/axis/point above.The locution “rigid rotor” is sometimes used to designate rotors that belong to thesecond above category. In reality, these rotors have blade travels that are similarto those of the first category above. Author proposes that “rigid” rotor locutionshould be reserved to rotors that have “little or no blade travel”, like the case ofan airplane propeller, for example. The second category above is then betterdesignated by: “Bearing less Main Rotor” (BMR).

Origin and role of rotor hinges (1/6)

The most usual solution to achieve main rotor disc tilt is the cyclic pitch anglevariation. The idea is to decrease lift in half of the rotor disc, and to increase liftin the opposite half. For example, if the intent is to tilt the rotor disc forward toachieve a forward flight, lift is decreased in the forward area of the rotor disc, andincreased in the backward area.

The most usual way to achieve this control is a system called swashplate, thatcontrols the pitch of each blade through its revolution.

If we introduce “Psi” as the azimuth angle (Psi=0° / blade in rear location,Psi=90° / blade in advancing location, Psi=180° / blade in front location,Psi=270° / blade in retreating location), then: Théta = Sin(Psi+Fi).

Note that due to the gyroscopic precession effect, the cyclic pitch has to be made

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maximum (respectively minimum) 90° before the azimuth where the lift increase(respectively lift decrease) is expected. In the frame of our previous forward flightexample: In order to decrease lift in the forward area of the rotor disc, and to increase it inthe backward area, cyclic pitch has to be made minimum for Psi=90° and maximum forPsi=270°.

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Origin and role of rotor hinges (2/6)

When forward flight is achieved, the following relevant phenomenon has to be

considered:If U is the peripheral speed of the blade and V is the helicopter velocity along theflight path with respect to the wind, then the advancing blade (Psi=90°) has itsrelative speed equal to U+V, whereas the retreating blade (Psi=270°) has itsrelative speed equal to –U+V.

As a consequence, advancing and retreating blades have uneven speeds. The fieldof airspeeds vectors on the disc can be easily computed.

If one is looking at the sign of the projected airspeed component along the flightpath, it’s interesting to notice that an area called the “circle of inversion” appears.

This area is the area where the projected airspeed component along the flight pathis negative. In other words, in this area, the blade attacks the air through itstrailing edge.

Circle of inversion is responsible for what is also referred to as “LiftAsymmetry”.

If not treated, lift asymmetry can generate uneven efforts (FA and FR) and thenalternating loads that are source of fatigue.

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Origin and role of rotor hinges (3/6)

Assuming that each blade is embedded into the rotor hub, the moment

distribution is maximum at the blade root, and decreases toward the blade tip.For a long time, such moments have been detrimental to rotor integrity, asalternating high loads are source of fatigue.

The easiest way to alleviate the effects of lift asymmetry is to free the blade inflapping by means of a flapping hinge.

Flapping hinge brings two benefits:

1- Bending moment nulls at the blade root, which greatly decreases bendingmoment, and blade and rotor material resistance issues.

2- Allowing a vertical motion of the blade allows a vertical speed of the blade.This vertical speed sums to relative wind speed and decreases the incidence ofthe advancing blade and increases the incidence of the retreating blade, thoughdecreasing lift of the (fastest) advancing blade and increasing lift of the (slowest)retreating blade. As consequence, lift asymmetry is lowered.

NOTE: Flapping hinges can be divided in 2 major families:

1- Articulated rotors: Flapping hinge is a clear articulation with a unique axis,done by means of bearings, or elastomeric components.

2- Rigid rotors: Flapping hinge is an area where the deformation is going to

occur, such as a beam, made out of titanium or fiberglass. The “Rigid rotor”

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locution is a trap, as very few rotors are totally and completely “rigid”.

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Origin and role of rotor hinges (5/6)Under aerodynamic forces influence, blades swivel and lift around their

respective flapping hinge.- In hover flight, or under the effect of the collective pitch, the blades describe acone, whose axis is the same than the rotor shaft. The static conicity position isreached. Flapping angle is usually denominated as “Béta 0”, or the “angle ofstatic conicity”.- In forward flight, or under the effect of any cyclic pitch, the blades describe acone, whose axis (sometimes called the “virtual axis”) is not the same than therotor shaft. This cone can be pictured as the same cone than the previous one, butinclined by the effect of the cyclic pitch. This second flapping angle is usuallydenominated as “Béta c”, or the “angle of cyclic conicity”.

As the rotor is driven by a shaft going through an axis that is not the same that theaxis of the cone formed by the blades, then arcs A1A2 , A2A3 , A3A4 et A4A1that correspond to even time intervals are uneven in length. This results in acontinuous blade acceleration and deceleration in the blade plane. In the lack ofhinge, in the same way than for flapping, alternate bending and fatigue wouldresult.

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Origin and role of rotor hinges (6/6)

As a summary, the 3 blade hinges are:

- The pitch hinge

- The flapping hinge

- The dragging hinge

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Flight regimes of a helicopter and autorotation (1/2)

Two main flight regimes can be distinguished:

-“Power-On” flight regime:

This flight regime is the “regular” helicopter flight regime. As previouslyexplained, control is achieved by tilting the rotor disc toward the direction whereflight is expected. As such, it shall be noticed that relative wind crosses the rotordisc from top to bottom.

-“Power-Off” flight regime:

This flight regime is also called “Autorotation”. During this phase, it shall benoticed that relative wind crosses the rotor disc from bottom to top. During thedescent phase of autorotation, rotor collective pitch is adapted so that relative

wind keeps on rotating the rotor disc, therefore maintaining the lift. For the bestlift to drag ratio, pilot is to maintain horizontal speed around Vy. Approaching theground, flare maneuver is initiated (pulling the cyclic control backward), in orderto slow down vertical and horizontal speeds, until the helicopter safely reachesthe ground.

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Flight regimes of a helicopter and autorotation (2/2)

The autorotation maneuver can be initiated anywhere outside of the height /

speed domain (yellow/hatched area).As a matter of fact, the helicopters needs :

- Either height (above 150m or so),

- Or speed (above 50kts or so),

- Or a combination of both (height and speed)

In order to be able to perform a safe autorotation maneuver.

In the case of a twin engine helicopter, height / speed diagram is more favorable,as the remaining functioning engine still helps the pilot to perform a safe landing.

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NOTE: The following part of the course is a description of:

- Some elemental technological bricks,

- Some bricks arrangements.

If an arrangement is not treated, it doesn’t necessarily mean that it’s impossible.However, it should mean said arrangement present some “recessive feature”compared to some generally “universally adopted” multi-purpose arrangement.

Reader should keep in mind that helicopter architecture is a matter ofcompromise, certainly more than in the case of some fixed wing airplane thatcertainly have more specialized missions.

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Helicopters typology (1/2)

Gross weight classes for helicopters:

Certification specifications CS27/FAR27 and CS29/FAR29 divide the helicopterworld in (only) 2 gross weight categories:

- CS27 / FAR27: Up to 3175kg (7000lbs) & 9 or less passenger seats

- CS29 / FAR 29: “Transport” category: Above 3715kg (7000lbs)

However, some US forestry agencies, as well as manufacturer Sikorsky proposean (unofficial) 4 categories breakdown :

- Light: Up to 6000 lbs (2722kg)- Intermediate:From 6000 lbs (2722kg) to 14000 lbs (6350kg)- Medium: From 14000 lbs (6350kg) to 32000 lbs (14515kg)- Heavy: Above 32000 lbs (14515kg)

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Helicopters typology (2/2)

Multi-purpose or dedicated helicopters:

The author proposes to distinguish 2 categories of helicopters:

- Multi-purpose helicopters,

- Dedicated helicopters.

The first category offers a possibility for a fast (up to a few hours) missionchange. Reconfiguration might be done at end-customer level. A given type ofaircraft can ensure several types of missions. As a consequence, its design isoptimized with respect to a group of several missions. The more missions thehelicopter type ensures, the more compromises have to be done duringdevelopment phase.The second category encompasses more specialized tools. It includes aircraftdesigned only for one or a few missions. Although some helicopters of thiscategory might offer the possibility to be reconfigured, it will require a long time(a few days) to do so. Reconfiguration will often require OEM participation, or askilled/qualified end-customer. Design optimization for this kind of helicopter isusually more visible than for the first category.

Note that the multi-purpose / dedicated character of a helicopter is mostlyattached to the possibilities offered by the aircraft cabin and its ingress/egress.

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General architecture of a machine

The following is considerations applicable to conventional helicopters

architectures. It applies to most of helicopters belonging to the multi-purpose /medium gross weight type.

Some of these considerations might also apply to other types of helicopters.

More than a century of “genetic selection” has built a relatively clear line (called“upper deck” or “mechanical deck”) between dynamic assemblies (located ontop) and the remainder of the helicopter airframe (located at the bottom). This isdue to several reasons:

-Basic stability consideration : With this architecture, rotor lift (applied at thecenter of the rotor) is applied above the CoG (located more or less at the center of

the cabin), which ensures natural stability.-Weight optimization reason : Mechanical parts are dense and heavy. It’ morerational to group them together, so that to minimize interconnecting shafts innumber and length. Shafts are heavier where high torques are required. As such,it’s logical to install the rotor hub and blade assembly as close as possible to themain gear box. Engines are also generally located beside the main gear box, atthe front or at the rear.

-Safety reasons : Occupants are to be located (as far as practicable) away fromrotating main and rear rotors, so that they are not endangered by potentialcontact. As such, “under the rotor, close to the center” is a privileged place for

occupants. Ground personnel shall not be forgotten, as their intervention on

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ground is relevant to the h/c operation. As such, medium helicopters rear rotors are usuallylocated more than 6ft above the ground. In the case of light helicopters, for which thisdesign practice is not achievable, the shrouded tail rotor is of great interest for groundpersonnel safety as the shroud materializes the rotor.