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EXPERT INTERVIEW: Thursday May 7 th , 2015 TECHNISCHE UNIVERSITÄT WIEN INSTITUTE FOR ENGINEERING DESIGN AND L OGISTICS ENGINEERING REPORT IFAR V ERTICAL L IFT VARIABLE ROTOR SPEED

Thursday May 7th ExpErt IntErvIEw Ifar vErtIcaL LIft ......the average man’s flying machine to be used - Not right away but inevitably - much as the average man uses his automobile

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Page 1: Thursday May 7th ExpErt IntErvIEw Ifar vErtIcaL LIft ......the average man’s flying machine to be used - Not right away but inevitably - much as the average man uses his automobile

ExpErt IntErvIEw:Thursday May 7th, 2015

TECHNISCHE UNIVERSITÄT WIEN

InstItutE for EngInEErIng DEsIgn anD LogIstIcs EngInEErIng

R E P O R T

Ifar vErtIcaL LIft varIabLE rotor spEED

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Expert Interview 2015

PrefaceOver the decades Rotorcrafts have been established for vertical lift operations. Besides the classical helicopters, tiltrotor- and tiltwing aircraft have also been developed and entered service for special op-erations.

However in recent years, driven by new operational challenges, especially high speed helicopter concepts have been evolved and novel concepts for vertical lift aircraft were presented by all major manufacturers. The requirements for future rotorcraft were discussed on many occasions and scientific congresses. Besides new mission profiles and operational demands, the development is influenced by new regulations from the Authorities.

New concepts for vertical lift will require new solutions for propulsion. Apart from the totally new technolog-ical approaches for engines and electrical aircraft, all drive train components will have to be adapted for new rotorcraft concepts. Since 2008 TU Wien is doing active research in the field of drive train technology for aircraft and would like to focus with new projects on these challenges, together with multinational compa-nies and research institutes.

In addition to the Vertical Lift Initiative activities of TU Wien in IFAR, this international Expert Day would like to discuss new drive train concepts and challenges for future rotorcraft. Discussion and evaluation of new drive train concepts is envisaged to outline new tech-nological solutions in accordance with certification specifications and to build up international coopera-tion early enough to reach a suitable TRL level and to be able to support the concepts of rotorcraft OEMs.

The InTervIewThe first organised Expert Interview, held on May 7, 2015 is aimed at connecting international institutions that are engaged in rotorcraft for the purpose of ex-change of ideas and information and gaining new in-sights in this aviation sub area.

In recent years due to proceeding operational de-mands, especially for high speed rotor concepts, new ideas for vertical lift and new solutions for propulsion are required. Beside these main topics a prospect at gearbox design and transmission variable gear boxes is given.

ParTIcIPanTsProf. Vladimir Zhuravlev

Moscow Aviation Institute

Pavel Zhuravlev Moscow Aviation Institute

Roland Feil Munich University of Tech-nology

Martin Lawall Airbus Helicopters

Michael Pfeiffer Airbus Helicopters

Prof. Michael Weigand Vienna University of Tech-nology

Hanns Amri Vienna University of Tech-nology

Manfred Grafinger Vienna University of Tech-nology

Katharina Hartenthaler Vienna University of Tech-nology

abouT exPerT InTervIew

Michael Weigand

Dean of Machine Design Department

Disclosure :The report of the expert interview is created by Hanns Amri from TU Wien with support from Katharina Hartenthaler. All rights reserved!

© TU Wien 2015.

The ToPIcsAbout Expert Interview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1IFAR Vertical Lift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .2Russian Rotorcraft Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3Western Rotorcraft Trends . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7Russian high speed rotorcraft . . . . . . . . . . . . . . . . . . . . . . . . . 10Variable Rotor Speed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Difficulties and benefits . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Variable Transmission Drive Trains . . . . . . . . . . . . . . . . . . . . . 16Conclusion of the Expert Interview . . . . . . . . . . . . . . . . . . . . 19A few words to the end . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

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IFAR Vertical Lift

Speaker: M. Weigand

Ifar organIsaTIonIFAR, the International Forum for Aviation Research, is the world´s only aviation research establishment net-work. IFAR is established on a voluntary, non- binding basis. IFAR aims to connect research organisations worldwide, to enable the information exchange and communication on aviation research activities and to develop among its members a shared understanding on challenges faced by the global aviation research community. IFAR develops views and recommenda-tions, e.g. the IFAR Framework Document on future research strategies and facilitates opportunities for networking and partnerships.

IFAR develops a regularly updated IFAR Framework Document outlining global research objectives and technological opportunities for use by its members. The focus of IFAR is on non-competitive aviation re-search and development related to global technical challenges and steps to reduce the impact of aviation on climate and the environment.

Beyond the scientific and technical activities IFAR also focus on exchange, education and promotion of young scientists and engineers via development of an action plan and specific measures.

Ifar program prEsEntatIon

IFAR discusses global air transport with the goal of minimal travelling times. Vertical Lift (VTOL) aircraft can be one important part of a global air transporta-tion system, which attains this goal. Currently OEMs of VTOL aircraft are working on new products based on individual motivation and scenarios and completely new concepts come up.

The basic question is: “Are Rotorcraft a part of future global transportation system?!” The topic is about the requirements for new rotorcraft and to get an idea how a new rotorcraft concept should look like. The focus is on speed requirements, efficiency and noise. Also already existing concepts should be presented and discussed.

Figure 1 Participants LTR: Martin Lawall, Michael Pfeiffer, Roland Feil, Prof. Michael Weigand, Prof. Vladimir Zhuravlev, Hanns

Amri, Katharina Hartenthaler, Pavel Zhuravlev, Manfred Grafinger

Ifar verTIcal lIfT

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Expert Interview 2015

Speakers: V. Zhuravlev, P. Zhuravlev

sPecIfIc feaTures of avIaTIon In-dusTryIt is necessary to conduct a lot of scientific and engi-neering work to develop new aircraft and their sub-systems. This work can take up many years and even decades. Its costs can amount up to billions and doz-ens of billions of US dollars (Euros).

The success of the development programs for new aircraft types depends on the usage degree of the advanced scientific development results in these new aircraft.

New aircraft design, which are intended for serial pro-duction, can be considered as a process of harmoni-zation of the requirements, which are created by the potential customer, and capabilities of the creator of these aircraft. At this part it is necessary to take the existing and future trends of the aircraft market into account as well as the scientific-technical and manu-facturing potential of the companies, which develop and manufacture the aircraft.

Rotorcraft are super-complex technical systems. This is especially true for conceptually new rotorcraft. The success of their creation is defined by the accumulat-ed scientific and engineering potential of the compa-nies and research centers, which carry out these de-velopments.

cIvIl helIcoPTers

People dream of big things...“... After the war is over... the helicopter may well become the average man’s flying machine to be used - Not right away but inevitably - much as the average man uses his automobile. That is a very big statement. But it is hard to escape.” (I. Sikorsky - Excerpt from LIFE Magazine June 21, 1943)

...but those things do not always come true!The civil market disappoints. Even 70 years after the vision, it still didn’t come true! The helicopter has failed, so far, to live up to the early vision in two areas. First in the personal transportation and second in the intercity airline operation.

Why is that?The reasons, why personal helicopters aren’t in wide-spread use are:

• Too expensive to buy and maintain• Reliability inadequate• Few suitable engines• Too hard to fly• Safety concerns• Too noisy - inside and outside• City and town ordinances• Too easy to get lost • Air-to- Air collision concerns

The reasons, why there aren’t more helicopter airlines are:

Inadequate:• Economic viability • Infrastructure (Vertiports, ATC)• Passenger acceptance• Community acceptance

russIan roTorcrafT Trends

Figure 2 Live Magazine

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IFAR Vertical Lift

Community Acceptance Problems:• Excessive NOISE (big problem)• Perceived safety problem• “Not in my back yard”

Is there a way out?The technology is mostly in place. A demonstration program is needed for a quiet, high- efficient heli-copter. A successful demonstration will help to open doors to new markets.

Main factors for improving competitiveness of civil helicopters are:

• Increase of flight safety and high level of control automation

• Introduction of strict limitations on noise, emis-sions and navigation

• Increase of flight speed and hauling capacity• Reduction of transportation costs and fuel con-

sumption per unit of transportation work• Increase of comfort (reduction of vibration, oscil-

lation, noise)• Increase of reliability and decrease of mainte-

nance costs• Reduction of time required for new helicopter

developments• Decrease of serial production costs• Decrease of helicopter price

Combining developer abilities, design processes and main customer requirements, the resulting criteria for the continuity of civil helicopters are safety, noise, reliability, maintenance, dependence on fuel and of course costs. Compared to these important topics high speed is not that important.

Aviation is a very complex industry so developing new concepts and achieving acceptable results takes a long time. Reducing noise will be the main goal for future developments.

What are the reasons for expanding the fields of helicopter usage?

• Aggravating problems of supplying the world economy with petroleum and wide usage of hel-icopters for reconnaissance and operation of oil and gas basins (especially offshore ones).

• Development of new weapon systems and new tactics of usage of helicopters as main combat aircraft and not just as support tools.

• The increasing demand for expansion of helicop-

ter usage in various branches of economy (trans-portation works, building, forest protection, search-and-rescue operations etc.).

• Improvement of helicopter structure and per-formance, which would provide a considerable decrease of direct operating costs for new heli-copters.

There is a dynamic change of helicopter fleet. First helicopter operations started during WW II. From the middle of 1970s the number of helicopters has sur-passed the number of airplanes in military forces of many countries. From the end of 1990s the number of the civil helicopters in the world fleet has exceeded that of military helicopters.

helIcoPTer MIssIons In russIaThe average distances of transportation by helicop-ters in Russia are around 160 km. Main transportation activities that are carried out in fuel-and-energy com-plex and geology are pre-planned and thus do not re-quire a considerable reduction of flight time.

Russian helicopters are mainly used for oil and gas ex-ploration. Also special transportation operations and search-and-rescue operations are important scopes of application. For every design effort it is necessary to ensure the financing of new inventions and thus of new helicopters. Therefore it is necessary to analyse and take into account the requirements of the main customers. Since the focus of this research is to find out the prospective areas of application of high-speed helicopters, the following analysis is focused on re-quirements of the customers that the authors find the most promising for such new rotorcraft.

Figure 3 Oil platform – clearly a place, where usage of heli-

copters would be extremely beneficial!

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Expert Interview 2015

oIl and gas IndusTryThe oil and gas industry is the most important costum-er for helicopters. They need helicopters to inspect oil and gas pipelines, to reach stations on- and off-shore and to transport people and goods. Oil and gas com-panies They also obviously have enough money to buy new helicopters.

Due to the variety of missions in the oil and gas in-dustry, various helicopter configurations are need-ed. Some missions require fast helicopters and other missions require helicopters with a given payload and range.

Off-shore rig Obvious requirements for such missions are compact dimensions, safe continuation of operation ( safe land-ing) in case of engine failure (provided by e.g. an extra third engine or boat shaped fuselage and ballonets to stay afloat in case of an emergency landing on water). There is also a possible need for high-speed flight and hover capability in case of rescue and/or urgent fail-ure/ repair mission on an off-shore rig (similar to the situation that occurred in Gulf of Mexico).

PaTrol, search and rescue MIssIonsCoast guard is another potentially important customer for high-speed rotorcraft. For this customer the main missions are patrol and search-and-rescue operations. Thus they are considered in details below.

The probability that the violator ship would not be ar-rested Pn/a is the objective function, which is used for evaluation of effectiveness of patrol operation:

K2 = Pn/a → min.

The process of evaluation of effectiveness of a patrol

operation can be divided into two stages

1st stage: Determination of• the necessary quantity of patrol, search and res-

cue vehicle bases (PSRVB) • the total quantity of patrol, search and rescue ve-

hicles • the distribution of patrol, search and -rescue ve-

hicle bases in the region.

2nd stage: Determination of values of the objective function used for definition of effectiveness of opera-tion for all vehicle types.

The critical values of the following weather parame-ters (conditions) and the appropriate ranges of vari-ation should be taken into account during the calcu-lations:

• The light conditions: day or night.• The amount of cloudiness: clear or cloudy weath-

er.• The wind velocity • The wave height• The water temperature• The air temperature

The probability of carrying out a successful rescue operation over the whole zone and during the whole year is the objective function, which can be used to evaluate the effectiveness of application of a certain type of PSRV during a rescue operation .

The probability of the successful rescue depends on the distance from base and the speed of the vehicle.

As can be seen from above listed requirements for the operations of the search-and-rescue and/or pa-trol-and-interception helicopter are the following:

Figure 5 Application of Mi-8 for evacuation of people in the

mountains

Figure 4 Example of a mission on an off-shore oil rig

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IFAR Vertical Lift

• The helicopter should be capable of high-speed flight during the search phase of the mission and/or time of flight from the base to the detect-ed people in distress/intruder vessel.

• Also hover capability with acceptable downwash is required for taking the rescued people aboard or boarding the intruder vehicle.

High-speed helicopters or helicopters with variable transmission speed are definitely very useful for SAR operations!

MIlITary MIssIonIt is unclear whether there is a requirement for higher flight speeds of future military helicopters. On the one hand the anti-aircraft systems work well even against airplanes flying at very high speeds (compared to ro-torcraft) and the rotorcraft can not fly faster in nap-of-the-earth area. On the other hand – there might be some future “classified” applications, we don’t know about, like quick flights on own territory along the frontline with some special missions.

new helIcoPTers In russIaThere are three main trends in civil helicopter design in Russia. One is the improvement of existing helicop-ters. The second is a new design approach for new hel-icopters without additional emphasis on the increase of flight speed and the third is a new design approach for new helicopters with additional emphasis on the increase of flight speed (high-speed rotorcraft pro-gram). The first and third trends are analysed below.

IMProveMenT of exIsTIng helIcoPTersThis is an important part of helicopter design because the customer already knows the product. The de-velopment and certification costs for a redesign are

relatively small. And there is still a reserve in the old designs due to the conservative design approaches of the past. The insufficient precision of analysis and synthesis at the early stages of design of many exist-ing “old” helicopters resulted in the emergence of the so-called “unplanned” reserves in the design. Such reserves allows to carry out a modernisation and en-hancement of capabilities of the existing helicopters. For example, the new modification of Mi-8 – Mi-171A2 is pushing hard for an increase of flight speed and de-crease of fuel consumption.

hIgh-sPeed roTorcrafT PrograMNew design approach for new helicopters with ad-ditional emphasis on the increase of flight speedDue to the increase of precision of calculations and predictions at the early design stages the more mod-ern helicopters, which were designed during the re-cent decades, do no incorporate “unplanned” reserves in their design. Projects or helicopters which were designed only a few years ago, were created at the “almost maximum use of capabilities with no reserve (room for enhancement)”. So an enhancing redesign won’t be possible in the future without substantial investments and changes in the initial project, which would essentially lead to creation of a new helicopter. To use new materials is not the solution, because then there is still a need for a design of new structure The increased precision of analysis at early design stages allows to introduce planned reserves for future devel-opment into the new helicopter projects. This leads to the idea of introducing a family concept in the heli-copter industry. Such concept is already actively re-searched and explored by Agusta Westland.

Thus overall it might be beneficial to research, which reserves should be introduced into the basic design of new helicopters for taking into the account the possi-ble future increase of their flight speed.

What are the possible areas of usage of high- speed helicopters in Russia?Fuel and energy complex and geological exploration are the promising areas of the possible high-speed helicopter usage. Duty (flying) time of helicopters for fuel-and-energy complex and geology previously and nowadays makes up to more than 60% of the total fly-ing time of the Russian helicopter fleet.

However, it is necessary to mention that the helicop-ters would be used in fuel-and-energy complex at the distances of no more than 600 km (longest range val-

Figure 6 Military Mission for a Helicopter NOE: Nap -of-Earth

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Expert Interview 2015

ue). These are the expected distances of flight from the main shore bases to the planned regions of drilling op-erations at the most remote field – Shtokmannovskoe gas condensate field in the east part of Barentz sea. Also modern Russian fuel-and-energy complex is charac-terized by a gradual and rather fast process of building a network of land roads in the places of development of large fields. This leads to a substantial reduction of

the helicopter flight ranges. For example, the building of the new railroad line “Obskaya – Bovanenkovo” with the distance of 525 km onto the Yamal peninsula.

Overall it is obvious that patrol, search-and-rescue and emergency sanitary flights (including off-shore rig op-eration) are those that require substantial reduction of flight time.

wesTern roTorcrafT Trends

Figure 7 US and European programs for developing new ro-

torcraft technology

Figure 8 Agusta Westland - NextGenCTR

Figure 9 Airbus Helicopters - LiveRCraft

Speaker: Hanns Amri

At the moment there are 5 large research programs in the field of new helicopters in the US and in Europe.

clean sky 2Clean Sky 2 will deliver break-through technologies for incorporation into the next generations of aircraft from 2025 onwards. By spearheading European aer-onautics research culminating in demonstrations of game-changing new vehicle configurations, Clean Sky 2 will enable the aeronautics industry to introduce innovations in time scales that otherwise would be unachievable. By doing so, it will drive environmen-tal improvements, increase transport efficiency, and create jobs and growth for Europe. The major goals of Clean Sky 2 are:

• Accelerate the development of aircraft which are operated worldwide

• Achieve sustainable growth, wealth creation and stable employment in fields of high technology

• Win global leadership for European aeronautics with a competitive supply chain

• Demonstrate several aircraft systems at the air-craft platform level

NextGenCTR- AW609Agusta Westland is working on a tilt rotor program, the AW609. This rotorcraft is able to fly 333 kt (171 m/s) with a take-off weight of 7.6 tons and a useful load of 2.5 tons. The travelling distance is about 1300 km with 9 passengers. In the“Clean Sky 2” project Agusta Westland plans a larger version of the AW609 at least as a prototype. The size should be comparable to an Airbus A320.

LiveRCraft- X³Airbus Helicopters was working on its X3 experimental project. It is a compound helicopter with one main ro-tor and two propellers mounted on short wings. The X3 set up a new speed record of 255 kt (131m/s) on the 7th of June 2013. In “Clean Sky 2” Airbus Helicopters is planning an advanced version of the X3.

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IFAR Vertical Lift

elecTrIc vTolThere are also some investigations in electric rotor-craft. The goal is to develop and test new electric tech-nologies in rotorcraft. Electric drivetrains allow highly innovative configurations. The advantages of electric technology are a high flexibility of electric motors, a reduction of CO2- gases and a power independence form the height.

Project ZeroAgusta Westland designed an electric driven VTOL, called Project Zero. The rotorcraft is equipped wiht a direct rotor drive with no reduction gearbox. There is an individual blade control system for the swashplate, which leads to less primary flight control. EMA (Elec-tro-Mechanical Actuators) are used for hydraulics-free Fly-by-Wire system. Rechargeable Lithium batteries are the energy source, with flexibility to hybridize with other energy sources (e.g. Diesel engine). Stabil-ity Augmentation System (SAS) and autopilot control laws are implemented for a twin rotor-in-wing aircraft configuration, including collective and cyclic blade pitch and rotor tilting control.

Joby S2An other, more futuristic design concept is Joby S2. This program wants rotorcraft to become an individ-ual transportation system, with electric power. The goal of the design study was, the achievement at the best civil transport VTOL aircraft performance with the newly available electric propulsion technologies. The S2 has fixed pitch propellers. All propellers are de-signed for hover except those on the wing tips. The lift

propellers are stowed and folded during cruise flight. The designer expect a very high lift to drag coefficient of 20 during cruise. In hover the need much more power. If this design concept is realistic, that remains to be seen in the future.

nasa- fuTure verTIcal lIfTNASA started a VTOL program with the goal to transport 90 passengers over 1852 km (1000 nm) with a speed of 155 m/s (300 knots). They investigated different ro-torcraft configurations and found out, that a tiltrotor concept would fit best to their requirements. To ena-ble an efficient flight, NASA calculated a need for vari-able rotor speed up to 50 % from the maximum value. To reach this capability different concepts of varying rotor speed were investigated. They could use either variable speed turbine technology or a gearbox tech-nology with two speeds or continuously variable.

daPra- xPlaneDefence Advanced Research Projects Agency started the xPlane program. In the framework is an aircraft that could have the following properties:

• fly at 300 – 400 kt (555 – 740 km/hr) • raise hover efficiency from 60% to at least

75 %; • increase cruise lift to drag ratio from 5-6 to at

least 10• maintain the ability to carry a useful load of at

least 40 % of the vehicle’s projected gross weight of 10,000 – 12,000 lb (4.5 – 5.4)

Figure 10 Agusta Westland - Project Zero

Figure 11 The Joby S2 VTOL Concept

Figure 12 NASA - Large Civil Tiltrotor

Figure 13 DARPA- xPlane

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Expert Interview 2015

JMr-TdJoint Multi Role Technology Demonstration (JMR- TD) is a program of US- Army’s Aviation Applied Technol-ogy Directorate (AATD). JMR-TD should expand the VTOL speed spectrum and define requirements for next generation vertical flight. The goal is to create an demonstrator with the following performance param-eters:

• cruise flight with minimum speed of120 m/s (230 knots)

• at an altitude of 1830 m with a temperature of 35 °C (6000 Ft /95 °F)

• design gross weight of 13600 kg (30.000 lb)

RaiderThe Sikorsky Boeing team is designing a coaxial com-pound helicopter based on the Sikorsky X2. It uses a RPM drive system to control main rotor blade tip speed for high speed cruise. The SB>1 Defiant has a coaxial rotor and a pusher propeller.It will carry 12 troops with a design weight of 13.6 t and a cruise speed of 230 ks (118 m/s).

AVX- RotorcraftAVX company designed a coaxial compound hel-icopter with two fans on each side. It is designed to transport 14 troops with a maximum flight speed of 236 kt (121 m/s). It has a design weight of 12.7 t and a useful payload of 4.4 t. The maximum cruse range is 3900 km. It has a rear door and two side doors.

Valor V280Bell Helicopters has invented the V-280 Valor. It is a til-trotor of the “third generation”. The engines are fixed. The vehicle can carry 11 troops or 5.4 t useful payload with a design cruise speed of 280 kt (145 m/s). The Val-or has a range of 3900 km.

Karem TR75 JHLKarem Aircraft developed also a tiltrotor concept. It is called “optimum-speed tiltrotor (OSTR)”. The TR75 JHL should have a maximum speed of 360 kt (185 m/s). Such speeds should be attainable with the OSTR tech-nology combined with a multi- speed gearbox.

conclusIon• There is a need for higher speeds. This should be

realized with new rotorcraft configurations.• Electricity could enable new rotorcraft architec-

tures in the far future.• New drive-train architecture is required to make

full use of the potential of the new technologies

Electrification leads to a wide range of new opportu-nities. This results in a higher flexibility of electric en-gines and propulsion, Fly-by-wire systems, hybridisa-tion with other energy sources like diesel engines or new battery inventions. On the other hand electrifica-tion comes along with several problems. The engines should supply enough power at while having small weight (do not decrease payload) and dimensions. (to take up less available space). In case of failure the sup-ply of the rotors, auxiliary units, autorotation etc. have to be ensured. These requirements have to be regulat-ed in the certification documents. Figure 14 AVX- Rotorcraft

Figure 15 Sikorsky Boeing - Raider

Figure 16 Bell Helicopters - Valor V280

Figure 17 Karem - TR75 JHL

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IFAR Vertical Lift

Speakers: V. Zhuravlev, P. Zhuravlev

hIsTorIcal ProJecTsOne of the first Russian high speed helicopters was the Kamov Ka- 22. It was a side-by-side rotor helicopter with two additional propulsive propellers to increase flight speed. The maximum horizontal flight speed was 370 kph. First flight took place in 1959 and four prototype helicopters were produced. Development stopped after two helicopters catastrophes.

Another historical Russian concept was the MIL Mi-30. It was a tiltrotor concept. The project was stopped before the creation of a prototype. The research was done in the years 1972-1991.

russIan helIcoPTers hIgh sPeed PrograM 2010Russian Helicopters Cooperation started a high speed rotorcraft development program in 2010. The first goal of the program is, to increase forward flight speeds up

to 350 km/h. The final goal of the program is to create a helicopter with a forward flight speed of up to 500 km/h. The first concepts were the Mil Mi-X1 and the Kamov Ka-92. Due to the design bureau philosophies Mil design had a single main rotor and Kamov design had a coaxial main rotor concept. Both concepts are using pusher propellers for additional forward thrust. Ka-92 was researched as a 16 t helicopter with a max-imum flight speed of 460 km/h and a cruise flight speed of 420 km/h. The flight range is about 1400 km with a capacity of 30 passengers. The helicopter is driven by two 3200hp BK-3200 engines.

Based on these two design ideas, the design bureaus started to develop concrete concepts.

The MIl hIgh-sPeed helIcoPTer Pro-graM

Mil Company foresees the following main potential threats for the helicopters of the new generation in case the advanced technologies of high-speed heli-copter development are not implemented in time in Russia:

• The loss of competitive abilities of the civil heli-copters on the world market

• The threat of falling behind considerably in terms of the operational effectiveness of combat heli-copters

Limitations of the traditional single-rotor configura-tion force the Mil company to search for ways of im-proving the existing helicopters to achieve higher speeds rather than designing brand new machines. The MIil philosophy:

russIan hIgh sPeed roTorcrafT

Figure 18 Ka-22 Side-by-side Rotor Helicopter

Figure 19 Concept drawing of MIL Mi-30

Figure 20 MIL Mi-X1 and Kamov Ka-92

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«The helicopter with a standard configuration still holds huge opportunities for the increase of maximum flight speed, which have not been used up till now. The max-imum (and frequently cruise flight) speed at altitude of the most modern helicopters are limited not by the pow-er of the engines, but by the detachment of the flow on the part of the blade, which moves along the flow where the high angles of attack occur. A number of devices, such as boundary layer suction and blowing, high-lift de-vices of the blade profile and others are still waiting to be used for a considerable increase of the helicopter flight speed.» (a quote from M.L. Mil)

MIL has a four stage plan to increase the forward flight speed of a single main rotor helicopter. According to this plan the forward flight speed should grow as fol-lows (numbers in the list denote program stages):

1.) 380 km/h2.) 430 km/h3.) 450 km/h4.) 680 km/h

Initially a traditional single rotor helicopter is used. In future a Jet engine will be the most effective and the only possible way for implementation of the fourth stage.

Stage 1: Further aims• High load factors• Low fuel consumption• High safety• Long life (long service terms)• Low operational costs• Acceptable price for the customer

Stage 1: Problems of rotor• Necessity of increase of lift-to-drag ratio• Large control loads

• High vibrations• Growth of required power with increasing speed• Decrease of payload-to-take-off-weight ratio • Problems with rotorcraft dynamics

Stage 1: Ideas for implementation• Airframe with low drag coefficient• Research for selection of helicopter parameters,

which would allow to solve problems with heli-copter dynamics at high speeds of flight

• Absence of configurational changes due to less design effort

• The higher the flight speed the harder it is to in-crease the pitch. Pitch link force increases very fast over 300 km/h.

The main idea consists in modifying the existing Mi-24 helicopter to achieve the first stage goals of the high-speed helicopter program. The PSV flying laboratory is created on the basis of a series-produced Mi-24K helicopter.

Stage 1: Adaption of Mil Mi-24• New aerodynamic configuration of rotor blades• Redesigned nose / fuselage• New engines• Modification of exhaust pipes• Vibration absorber added• Stall regulation system (sensors)• Removal of military equipment• Three configurations:

ħ Without wing ħ Redesigned serially produced wing ħ New wing

For every adaption, redesign or invention new ideas have to be collected and analysed. This is necessary for generating new projects and achieving good re-

Figure 21 Four stage plan from Mil for high speed helicoptersFigure 22 Early Sketch PSV 1 Flying Laboratory

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IFAR Vertical Lift

sults. Furthermore important final conclusions about every aircraft have to be drawn to avoid mistakes. If new blade configuration works it will be applied for helicopters in serial production.

Stage 3 and 4 additional investigations:• Rigid main rotor• New turbines• Vibration reduction• Additional pusher propulsors (pusher propeller,

jet engine etc.)• Aerodynamic configuration• New blade structures and materials

Stage 4 is more hypothetical ambition compared to the current state of scientific knowledge. More re-search activities have to be carried out to achieve these high speeds. Certain helicopter configurations are better suited for higher speeds or higher altitudes than others.

kaMov- hIgh sPeed PrograMOver time Kamov changed the type of design of their high speed helicopters. Instead of designing a pas-senger helicopter, the company decided to design an UAV. In this case Kamov keeps in mind one of the main advantages of a UAV - much less risk for the pilot in case of a malfunction. (which is likely to happen dur-ing the adaptation of a new advanced technology)

Coaxial helicopter configurations offer a better op-portunity for high-speed flight (due to compensation of rolling moment of main rotor). Therefore addition-al propulsive units like a pusher propeller can be in-stalled without additional design changes. A high speed Kamov UAV with artificial intelligence shall be embedded into existing aviation systems and shall be stationed on ships. Introduction of an UAV with a high level of artificial intelligence, unified on-board systems and open architecture.

Kamov has the following ideas to reach the goal of high flight speeds:

• Usage of new propulsive units• Rigid main rotor leads to higher lift-to-drag ratio• Increase of flight speed, range and time by ac-

cepting stronger vibrations

Figure 23 MASK 2015 Mil high- speed demanstrator

Figure 24 Possible design sketch of a stage 4 Mil high-speed

helicopter program

Figure 25 Kamov- high speed UAVs

Figure 26 Multi- Mission Capability of high speed UAVs

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Expert Interview 2015

TsagI InvenTIonsFundamental research to support high-speed helicop-ter developments. TsAGI r conducts the following re-search on the aerodynamics of high-speed helicopter main rotors.

• New shape of rotor blades• Piezoelectric actuators firmly integrated in the

trailing edge of airfoil, built into the blade for stall reduction

• Negative offset of flapping hinge Figure 27 Prototype negative offset of flapping hinge

varIable roTor sPeed

State of the art helicopters produce lift and propulsion with the main rotor. Main rotor and tail rotor are driv-en by a turbine with constant speed and the power transmission is done by a constant speed transmission gear box.

The usage constant speeds simplifies the structure of blades and the whole rotor system. The reason is, that there is only one “design speed” for calculation and design of the rotor system. The eigen frequencies of the rotor are adjusted to the rotor harmonics ,so that they are not in the same frequency region. Therefore there is only a “drive through” these frequencies with-out loads during start or shut down of the engine. The best lift-to-drag-ratio of the rotor is used in the design operational state of the rotorcraft.

However, most of the time a rotorcraft works not in its design operational state. Therefore the rotorcraft cannot take advantage of the most efficient angle of attack of the rotor. Enabling the rotorcraft operation in the efficient mode (design operational sate) over a wide range of the flight envelope could increase the efficiency of the rotorcraft. This may lead to a reduced

power demand and so to less fuel consumption. This might also be interesting from the ecological point of view for future rotorcraft.

If we consider a a rotor with variable rotation, there are two basic facts we have to think about. Firstly, “how can we vary the rotor speed?” To answer this question one needs to have deeper understanding of the rotor-craft drivetrain.

For example if you consider a gear shift during flight, it is clearly a difficult task, compared with the gear shift mechanism of an automotive ground vehicle. It is necessary to ensure, that the power transfer should not be interrupted during shift and the change of the rotor speed should not be abrupt. The turbine works with a nearly constant speed, so the adaptation of the rotor speed must be done by the gearbox.

Second question is “What would be the dynamic be-haviour of the rotor, when the speed is varied?” This question requires a deeper understanding of the rotor itself.

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Variable Rotor Speed

Speaker: Roland Feil

Today rotational speed of helicopter main rotor is pre-defined at a specific single operational point. Future research will adapt the rotational speed to actual re-quirements from the rotor system according to chang-ing flight conditions.

benefITs froM varIable roTor sPeedFirst let’s take a look at the efficiency of a helicopter with variable rotor speed compared to that with a fixed rotor speed. The example shows results of us-age of Model intended for comprehensive analysis of a coaxial helicopter in forward flight. State of the art technology is one rotational speed that is prede-fined for a single operation point. With the variable rotational speed technology it is possible to adapt the rotor speed to the actual requirements from the ro-tor system. This leads to a significant reduction of re-quired power. Variable rotor speeds may also enhance manoeuvrability at higher flight speeds.

In Figure 28 you can see the nominal rotor speed and the optimized rotor speed over the forward flight speed. The colour in the background indicates the re-quired power. In hover there is a possible reduction of 25 %. At 40 kts flight speed there is a possible reduc-tion of 31% and at 80 kts there is a possible reduction of 11%. This indicates, that a variable speed rotor can increase the efficiency of a helicopter.

Assuming that we have a helicopter with a collective pitch of 7.5° by maximum forward speed of 100%. A

collective pitch of 10° increases the rotor thrust but also the required power, if the rotor is driven with the same rotational speed. If we can decrease the rotor speed to have the same thrust at 10° collective pitch that is necessary for a flight at 100%, we can reduce the required power. So we have some power margin left to incerse the flight speed.

Thrust and rotor speed have a linear correla-tion (upper graph in Figure 30) but rotor speed and power have a polynomial (cubic) correlation (lower graph in Figure 30). In the example giv-en here the rotor rotational speed is decreased to 82 % of the nominal speed and power is reduced by 40 % for the same output thrust.

Extending the flight envelope to higher ceiling re-quires increased tail rotor thrust. Hence, without af-fecting the rotor geometry, one can simply increase the rotational speed of the tail rotor.

dIffIculTIes and benefITs

Figure 28 Rotor Speed Optimisation

Figure 30 Thrust and power of the main rotor

Figure 29 Thrust and power of the tail rotor

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Expert Interview 2015

• Vibrations ħ Vibratory loads may affect the systems ħ Lifetime ħ Air resonance ħ Interactions between rotor and structure

• Increasing weight of gearbox• New failure scenarios

The dynamic behaviour of a rotor characterizes the vibratory loads of the whole helicopter. Rotor blades are, as a rule, designed so that their flap, lag, and tor-sional eigen frequencies do not match with the rotor harmonics at nominal rotor speed.

The use of variable speed rotor systems offers the op-portunity to operate the rotor with an arbitrary rota-tional speed. As a consequence, the rotor system must be able to be operated with loadings equal or close to a rotor harmonic. High vibratory loads must be tol-erated. Hence blade design concerning stability and stiffness is essential.

One side effect of application of variable speed tech-nologies is the weight increase. A helicopter’s total weight balance must account for additional weight from a complex drivetrain and rotor.

requIreMenTs

A variable speed rotor requires a system that allows to vary its speed. The following solutions are possible:

• A speed variable turbine• A variable gearbox

Figure 29 shows the tail rotor thrust and power for various altitudes and three rotor speeds. Thrust re-duction due to lower density at higher altitudes can be controlled by varying the rotational speed. This results in higher ceiling levels for the rotorcraft range of operations. A sufficiently powerful engine supply is assumed since the power grows as a cubic function while thrust increases linearly. The conclusion is that the tail rotor can be optimised in terms of power ac-cordance with the individual flight state, just like the main rotor.

dIffIculTIesThe variation of rotor speed may be accomplished within the framework of feasibility, certification spec-ifications and compliance with general limitations of the flight envelope (see Figure 31). A rotational speed that is too low may not comply with the autorotation requirements.

A rotational speed that is very high affects noise lim-its. Compressibility effects may occur in high speed flight conditions. In order to counteract a retreating blade stall, the rotational speed of a rotor must also be above a certain minimum in high speed flight. The aforementioned limits are based on physical require-ments as well as on safety and certification standards.

The difficulties of a variable rotor system are:• Rising complexity of rotor and drive train

system• Additional requirements for rotor design:

ħ New degree of freedom ħ Stability of blades must consider all possible

rotor speeds• Stability and Control

Figure 31 Limitations of the flight envelopeFigure 32 Frequency diagram overlapping of rotor frequen-

cies with harmonics are marked in blue

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Variable Rotor Speed

A variable gearbox can be designed as a discontinu-ous system. This means that there would be a defined number of gears within such gearbox. The gears are connected via a clutch which leads to load relieving during shifting. It is an existing technology (A160 Hummingbird, etc.).

But a gearbox can also be designed as a continuous system. Then the variable speed can be adapted ac-cording to the flight states. A change of speeds with applied loads is possible. But it needs to be investigat-ed additionally.

For each mentioned above gearbox system there are new requirements from a variable speed rotor on its gearbox, which is driven by it.

conclusIonVariable rotor speed is only usable on completely new systems. The whole system has to be taken into account during the development. The reason for this is that the fuselage and structural components influ-ence the stability of the rotor.

• You need a whole dynamic model of the helicop-ter to figure out the vibrations and noise.

• There is a high potential for application of varia-ble rotor speed technologies.

• Future requirements demand innovative solu-tions that support the expansion of a rotor- craft’s flight envelope

• Speed variation technologies are applicable for various rotary wing architectures

• It is still necessary to conduct a lot of research and development activities to mature the appro-priate technologies.

Speaker: Hanns Amri

The concept of using a gearbox with varia-ble ratio has been employed since the 1980s by R. Moore. Shifting gears during a rotorcraft flight, is a complex and difficult task, because the transfer of power should be secured while shifting and a speed change of the rotor should not be abrupt. Hence, con-tinuous solutions may be favoured to ensure not only a smooth speed variation but also operation of the turbine at its intended design point.

exIsTIng TechnologIes

helIcoPTer TransMIssIon sysTeMMoore was one of the first researchers to identify the advantages of a variable rotor speed, which was changed by using a variable transmission gearbox. In 1986, he invented a gearbox with two gear ratios, which was suitable for helicopters.

The system basically consists of a planetary gear, an overrunning clutch and a break system. It enables two speeds. The speed is changed by engaging or disen-gaging the break system.

Two sPeed TransMIssIon wITh sMooTh Power shIfT

In 2006, Ai invented a system for smooth shifting with high efficiency. The system consists of two planetary gears, which are connected via a common planet car-rier and two electric motors. One ring gear is always locked during operation. When there is a request for changing the gear ratio, the locked ring gets un-locked. So both rings are able to rotate. One electric machine decelerates one ring gear. With the energy of deceleration the second electric unit accelerates the second ring gear. When the revolution of the first ring gear is almost zero the ring becomes locked and the other transmission is then used.

varIable TransMIssIon drIve TraIns

Figure 33 Sectional view of a model based on Moore’s inven-

tion

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Expert Interview 2015

PlaneTary dIfferenTIal drIveNASA invented this type of speed variation in its pro-ject titled “Concepts for Variable/Multi-Speed Rotor-craft Systems”. The concept consists of a planetary gear and a controller or variator. The input shaft is con-nected to the sun gear; the output shaft is connected to the planet carrier; and the controller is connected to the ring gear.

By changing the speed of the controller, the speed of the output shaft is also changed without altering the speed of the input shaft.

This system can also function with a variator. In this case, the system would include two planetary gears. The input is connected to the sun gear of the first planetary gear; the planet carrier of the first planet gear is connected to the sun gear of the second planet gear; and the planet carrier of the second planet gear is connected to the output shaft. The ring gear of the first planet gear powers the variator, which changes the speed of the second ring gear and therefore de-fines the speed of the output shaft.

Power TransMIssIon aPParaTus for hel-IcoPTers

In 1999 the Advanced Technology Institute of Com-muter-Helicopter (ATIC) invented a continuous var-iable transmission system (CVT) that promises to be suitable for main and tail rotors of helicopters. The CVT system is a roller toroidal transmission combined with planet gears.

Roller toroidal transmissions are friction gears that consist of a driving disc on the input side, a driven disc on the output side and some rolls. The rolls are placed along a concentric circle between the discs and are able to rotate around their axis. The axis of the rolls can be tilted. Tilting the axis changes the radius be-tween the rolls, the driving disc and the driven disc. Due to this change of the radius, the transmission ra-tio is altered. This system is frictional and does not pro-vide a positive traction.

shIfTIng uP a gearA flight with a discontinuous variable transmission could look like the following:

1. First gear is optimized for take off and hover - high efficiency during take off.

2. Acceleration in the first gear – decrease of effi-ciency

3. Efficiency of first gear is less or equal to that of the second gear – shifting region.

4. Acceleration in the second gear- increase of the efficiency

5. Second gear is optimized for cruse flight- high efficiency during cruise flight.

Figure 34 Sectional view of a model based on Ai’s invention

Figure 35 Planetary differential drive

Figure 36 General schematic of a roller toroidal transmission

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Variable Rotor Speed

With a continuous variable gearbox, the flight would look different.

1. Take off and hover with the optimum rotor speed

2.-4. Adapting the rotor speed during acceleration 5. Cruise flight with high efficiency

Technology consIderaTIon

Benefits of variable gearboxes:• Wide range of speed variability• Auxiliary power independent from rotor speed• Different rotors can be driven with different

speeds• One technology for different rotorcraft configu-

rations• Independent from the power source

• Turbine or motor is always working in optimum mode (range)

Advantages of shifting gears:• Simple mechanical (or electrical)

implementation • Rotor is designed for specified operation points• Existing Technology (e.g. A160 Hummingbird)

Disadvantages of shifting gears:• Limited number of design points reduces poten-

tial rotor efficiency• Increasing gearbox weight• Need for a clutch system to shift to another gear

Advantages of continuous variable gears:• Continuously variable rotor rotational speed• Speed optimized rotor design• High level of ecological efficiency• Noise reduction• Reduction of fuel consumption• Flight envelope extension

Disadvantages of continuous variable gears:• High complexity • Even bigger increase of gearbox weight (as com-

pared to shifting gears)• The need for a rotor design on any point of the

speed range• Completely new technology required

faIlure scenarIosTorque and speed change abruptly during gear shift. - The appropriate shocks and vibratory loads affect the whole drivetrain. Thus there is a danger of interrup-tion of power transmission during shifting.

For example slipping can occur in frictional systems. Overload during gear shifting can result in a reduction or even a loss of frictional transmission. This in turn could lead to a total loss (destruction) of the transmis-sion system.

In case of usage of fluid systems their efficiency can deteriorate due to the increase of fluid temperature. The temperature of the fluid will increases during ap-plication of high loads, which leads to a decrease of the transmission efficiency. This efficiency decrease in turn results in a further increase of the fluid temper-ature. The negative loop can lead to a total loss (de-struction) of the transmission system. Leakages also leads to a loss of the transmission system.

Figure 37 Gear efficiency over flight speed for a discontinuous

variable gearbox

Figure 38 Gear efficiency over flight speed for a continuous

variable gearbox

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Expert Interview 2015

Helicopters are used for a variety of missions. Among these high-speed helicopters with variable rotor speed have the biggest potential for application in off-shore missions for oil-and-gas industry as well as, search-and-rescue and special military operations (the latter requires additional special data for verification). Noise, emissions and fuel consumption are not that important during the off-shore and search-and-rescue operations. Especially For search-and-rescue missions it is essential that the vibrations in hover and noise are decreased while on the other hand high speeds are required for increasing the chances of victim survival.

High speed helicopters also have in long range trans-port missions or in VIP or passenger transport mission a possible field of usage.

In general variable rotor speeds can potentially ex-tend flight time and range by decreasing noise, emis-sions and fuel consumption and thus operation costs.

If the noise is decreased the acceptance of the com-munity will be improved. Then the market for civil helicopters will extend. The advantage of low-noise high-speed helicopters with variable rotor speed for the passenger is that they are travelling faster from A

to B with vertical take off and landing directly in the city centre. Time-consuming travels to the airport are eliminated. At the moment the economic viability and the missing acceptance due to noise and safety prob-lems prevail the desire for helicopter airlines.

High flight speed and variable rotor speeds both have high potential. Still a lot of more additional research and development activity is required because of the increasing complexity of the rotor and the whole helicopter structure including aerodynamic, propul-sion, transmission and rotor blade considerations. Everything is important and affects every part of the helicopter.

fundaMenTal requIreMenTsBased on previous considerations, the following fun-damental requirements can be summarized for usage during design a transmission variable transmission gearbox.

• Weight increase should be minimized• Transmission efficiency should be high• Continuous variable transmission should be cho-

sen over other transmission types• The transmission should provide independent

drive of auxiliary units• The transmission should provide independent

rotational speed of every rotor (e.g. main and tail rotors)

• The transmission system should be safe and re-liable

conclusIon of The exPerT InTervIew

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Conclusion

This was the first Expert Interview, which took place at the Technische Universität Wien and was organized by the Institute for Engineering Design and Logistics Engineering.

First of all I want to say thank you to all partici-pants for coming and for

the great discussions. A special thanks goes to the speakers for presenting us their knowledge and pre-paring the presentations.

I hope you all enjoyed the day, you could take some-thing with you and that you might have met new peo-ple or that you were able to deepen your acquaintanc-es.

Hanns Amri

a few words To The end

Published by: TU Wien, Karlsplatz 13, 1040 Vienna, AUSTRIA© TU Wien 2015.

General CONTACT:TU Wien - Institut für KonstruktionswissenschaftenGetreidemarkt 9/307, A-1060 Wien

[email protected] http://www.ikl.tuwien.ac.at Tel: +43 1 58801 30601

CONTACT to the Organisator:Hanns AmriGetreidemakt 9 / E307-31060 ViennaAustriaTel.: +43 1 58801 [email protected]

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InstItutE for EngInEErIng DEsIgn anD LogIstIcs EngInEErIng

Published by: TU Wien, Karlsplatz 13, 1040 Vienna, AUSTRIAGeneral CONTACT:TU Wien - Institut für KonstruktionswissenschaftenGetreidemarkt 9/307, A-1060 Wien

[email protected] http://www.ikl.tuwien.ac.at Tel: +43 1 58801 30601