[American Institute of Aeronautics and Astronautics Aircraft Design and Operations Meeting - Seattle,WA,U.S.A. (31 July 1989 - 02 August 1989)] Aircraft Design and Operations Meeting - The design challenge of applying tiltrotor technology to the civil mission

. , , , . . . . . . .. ~. . 370 L’Enfant Promenade, S.W., Washington, D.C. 20024 AlAA-89.2067 THE DESIGN CHALLENGE OF APPLYING TILTROTOR TECHNOLOGY TO THE CIVIL MISSION i/ John F. Ward" Ward Associates Easton. Maryland Abstract The paper addresses the challenges the designer must face in developing a viable civil tiltrotor that is capable of capturing the full market potential. The issue is one of efficiently applying available technology to create an aircraft capable of safe, reliable, all-weather, scheduled commercial operations at urban vertiports and at high density airports. This challenge to increase air transportation capacity, while minimizing the impact on the surrounding urban community, will require the design of sa€e tiltrotor aircraft with true vertical takeoff and landing (VTOL) capability. The key design requirements are discussed with an emphasis on aircraft propulsion, flight path management, guidance systems, and terminal area procedures. Introduction The United States currently enjoys a favorable position in tiltrotor research, technology and military development. The Department of Defense (DOD) V-22 Osprey Program represents a major investment in military development and application. The technology readiness which permitted the launching of this program was the result of Over 30 years of sustained research and technology validation effort by the DOD, the National Aeronautics and Space Administration (NASA), and industry. While vertical takeoff and landing capability represents a primary asset of the tiltrotor concept, a number of €actors have combined to drive the initial civil tiltrotor scenario toward a more conventional short takeoff and landing (STOL) operational mode. These factors include: (1) a tendency to evolve preliminacy tiltrotor operating procedures from fixed-wing experience, (2) utilization of current microwave landing system (MLS) capability a s an approach and landing aid, limiting glide slope angles to 10 degees or less, ( 3 ) from a revenue perspective, a desire to be able to operate the tiltrotor in the STOL mode to achieve maximum payload capability, and ( 4 ) a recognition that the current V-22 military design requirement does not translate directly t o a one- engine-inoperative (OEI) VTOL emergency President, Ward Associates Member AIAA, Fellow AHS President, Ward Associates Member AIAA, Fellow AHS operating capability for a civil transport aircraft mission, especially with a full passenger load. Potential operators of urban landing sites, called "vertiports," are demanding true vertical takeoff and landing capability. 1 They see the tiltrotor's potential to help provide relief from transportation gridlock and airport capacity limits. They cannot always produce sufficient real estate acreage and unobstructed terminal airspace to accommodate a STOL-type operation. Therefore, they require the tiltrotor's VTOL capability in order to build and operate vertiports where they may be needed most. The utilization of the tiltrotor to offload fixed-wing runways at high density airports will also require true VTOL operation from colocated vertiports. 2 In order to achieve airport capacity enhancement, the tiltrotor, and helicopter, VTOL ground and air operations must not interfere with the full utilization of runway slots by medium and long haul fixed-wing transport aircraft. In order to meet the requirements for true VTOL commercial operations at the close-in urban setting and at vertiports colocated at maior airoorts. an auqmented i/ aircrift/vertiport operating infrastructure will be required. The following discussion addresses some key highlights of civil tiltrotor design studies, looks at the current activity from the perspective of earlier STOL program results, and offers a view of the design reauirements and ChallenQes as miaht be seen by the potential commercial operator. This leads to the definition cf an augmented, all-weather, VTOL operating capability requirement which challenges the designers O F commercial tiltrotor aircraft and supporting systems to apply available technology in the most effective manner possible. Civil Tiltrotor Initiative The opportunity now exists t o initiate a program for the timely integration of civil tiltrotor operations into the national air transportation system. In response to bipartisan Congressional interest and direction, a National Civil Tiltrotor Implementation Plan is being prepared under the leadership of the Federal Aviation Administration (FAA). This plan will define the necessary effort and investment v Copyright @ American lnslnule or Aeronautic* and A ~ i r ~ n a u f i c ~ , IIIC.. 1989. All rights reserved. 1 required to achieve a state of readiness for the operational evaluation and commercial introduction of tiltrotor operations in the 1992 to 1995 timeframe.3.4 The planning effort involves the FAA, the Department o f Transportation, DOD, NASA, v industry and the Department of Commerce. In addition to airframe and crew certification. the uroqram ulannina focuses on the near term inkrastructure ~ ~~ requirements for civil tiltrotors including: air traffic control, terminal area operations, facilities and equipment, and tiltKOtOK landinq sites, which are similar to heliport facilities. 5 In addressing the initial introduction of the civil tiltrotor into the national air transportation system, the planning effort identifies the use of the V-22 aircraft with minimum modification as a practical means of conducting a limited civil demonstration to gather operational data. In a related effort, a vertiport Working Group is drafting preliminary vertiport design standards for initial publication by the end of 1989. This effort is a follow on to the r cent Advisory Circular on heliport design. 8 The initial planning also envisions the early use of the Microwave Landing System at the vertiport sites. The impetus for the current planning effort was the successful technology validation achieved with the NASA/Army XV-15 Tilt- Rotor Research Aircraft and the subsequent launching of the military V-22 Osprey Program by the Department of Defense. A joint FAA/NASA/DOD Civil Tiltrotor Steering Group issued findings recommendations in early 1988. ?':he Steering Group recommended that the government pursue supporting activities to provide f o r the timely transfer of tiltrotor technology to the civil Sector for the public benefit. This recommendation was based on an assessment of the results of a research study of civil tiltrotor missions and applications. 8 The contracted research study addressed potential markets, technical design studies of v-22 derivative aircraft, (Fig. 1) and new civil tiltrotor designs (Fig. 2 ) with passenger capacity from 8 to 75 seats. In addition, the study analyzed potential risk areas, national issues, vertiport facilities, and economics. In a parallel effort, a study for the Port Authority of New York and New Jersey, which drew upon the ongoing technical design studies, evaluated the feasibility of VTOL intercity air transportation using the tiltrotor. 9 The study was based On a vehicle cruise speed of 300 knots-and 1 hour trip time. This mission capability could provide a tiltrotor short haul air transportation network centered in the New York metroPolitan area and reachina 'V CTR-22C Figure 1. - civil derivative candidate designs based on V-22 military design. (From reference 8 . ) Figure 2. - CTR-~SOO: a 75-passenger civil tiltrotor design candidate, wing span = 63.0 feet, rotor diameter = 4 6 . 0 feet. (From reference 8 . ) throughout- the northeastern United-States --' and into Canada (Fig. 3 . ) 2 F i g u r e 3 . - N o r t h e a s t e r n U.S. r e g i o n a l o p e r a t i o n s ' c a p a b i l i t y f o r c i v i l ' t i 1 t r o t o . r f l i g h t s o r i g i n a t i n g i n t h e ' m e t r o p o l i t a n New ~ o r k / N e w J e r s e y a r e a . T e c h n i c a l d e s i g n s t u d i e s a r e c o n t i n u i n g and t h e F A A i s f u n d i n g a d d i t i o n a l o p e r a t i o n a l f e a s i b i l i t y s t u d i e s a: a, result o f , g r a n t a p p l i c a t i o n s . from numccous, ,st ,a. te and l o c a l governments . . O n a n . a t i o n a 1 b ,as is , t h e u l t i m a t e s c e n a r i o might be a s e r i e s o f r e g i o n a l t i l t r o t o r ne tworks t h a t s e r v e t h e m a j o r m e t r o p o l i t a n a r e a s , h u b a i r p o r t s , r e g i o n a l b u s i n e s s c e n t e r s and communi t i e s S C K O S S t h e c o u n t r y ( F i g . 4 . ) . . I. . , . 300 NM RADIUS &.-/ ,. . F i g u r e 4 . - P o t e n t i a l c i v i l t i l t r o t o r o p e r a t i o n a l r e g i o n s c e n t e r e d on , : . _ , . . , . .. , " i ..,. I .' .-.: :: # \ , , I n t e g r a t i n g t h e t i l t r o t ' o r ' s nominal m i s s i o n r a d i u s o f 3 0 0 n a u t i c a l m i l e s ' ~ ( n m ) w i t h h e l i c o p t e r f e e d e r s e r v i c e a t 1 2 5 nm o r l e s s , and c o n n e c t i n g t o new v e r t i p o r t s and e x i s t i n g a i r p o r t s would p r o v i d e a modern r e g i o n a l a i r t r a n s p o r t a t i o n s y s t e m t h a t c o u l d s i g n i f i c a n t l y enhance c i v i l a v i a t i o n c a p a c i t y . The t i l t r o t o r ' s p romise l i e s i n t h e o p p o r t u n i t y t o p r o v i d e r e l i e f t o t h e growing problem of tr 'xnsgo'ortation g r i d l o c k and a i r p o r t capkccity s a ' t u r a t i o n ' . However, t h i s ' . r e q u i r e s ' t h a t t h e tilt rotor^ b e d e s i g n e d t o o p e r a t e s a f e l y ' a n d r o u t i n e l y a i r p o r t s o n a n o n i n t e r f e r e n c e b a s i s . While i t a p p e a r s t h a t t h e e n a b l i n g t e c h n o l o g y i s a v a i l a b l e . t h i s w i l l be a maior c h a l l e n q e i n t o t h e u r b a n environWe'fit"an8 i n t o busy ./ i n a v i a t i o n system- d e s i g n ; : ~ . . . . , ~ 2 , .~' , j . ,., C i v i l V/STOL P F Y s @ ~ c . t , i v e i n ' No'rth America Th'ere i s a i u r k i n g s e n s e o f . ' d a j a vu . . ., . , ... .,., . :.,.l , . . . . . .. a s s o c i a t e d w i t h th,e. c u r r e n t a c t i v i t y i n c i v i l t i l t r o t o r t r a n s p o r t a t i o n a s s e s s m e n t and, f o r e c a s t i n g . The. September ,1968 i s s u e of - t h e , A I A A month ly p , u b l i c a t i o n "Ae,r onau t i c s and AS t r Onau t i,cs" ( A L A ) f e a t u r e d ' t h e t o p i c "V/STOL - I t s Day Has Comg." Twenty-one y e a r s a g o t h e f e a t u r e a r t i c l e s . , c a l l e d a . t t e n t i . o n t o s t u d i e s c o n c e r n i n g economic , o p e r a t i o n a l , and f i n a n c i a l a s p e c t s of i n t r o d u c i n g V/STOL a i r c g a f t , i n t o t h e t r a n s p o K t a t i o n s y s t e m . These a r t i c l e ? complemented e a r l i e r a r t i c l e s , i n t h e September 1965 i s s u e o f A L A , c under t h e h e a d i n g "V/STOL - T u r n i n g P romise t o R e a l i t y . " - , . / , , . , . . . j l I t a i r p o r t c o n g e s t i o n r e l i e f , r e l i e f of c l o g g e d g r o u n d - t r a n s p o r t a t i o n l i n k s , t h e N o r t h e a s t C o r r i d o r , STOLports and v e r t i p o r t s , and t h e need f o r d e m o n s t r a t i o n programs. I n 1 9 6 8 t h e s t a t e d p romise o f V / S T O L , ~ ~ $ a c a r b o ? copy o f t h e p romise o f t i l t r o t o r two d e c a d e s l a t e r . I t i s i n t e r e s t i n g t o n o t e t h a t i t was r e a l i z e d a t L/ t h a t t i m e t h a t f u l l e x p l o i t a t i o n o f t h e a i r f o r s h o r t h ? " l L o p e r a t i o n s m o s t a w a i t t h e deve lopment o f s u i t a b l e h iqh - speed a i r c r a f t such a s t h e t i l t w i n t i l t r o t o r , and s t o w e d - r o t o r VTOLs.Ia'The w a i t f o r t i l .., a i r c r a f t The were r e c o g n i z e d . VTOL a i r c r a f t were s e e n t o t a k e l e s s a i r and g round maneuver ing t i m e t h a n c o n v e n t i o n a l STOL a i r c r a f t . The VTOL c o u l d approach and d e p a r t from any d i r e c t i o n r a t h e r t h a n t h r o u g h a runway and approach " t u n n e l . " I n a d d i t i o n , t h e VTOL would have g r e a t e r p o t e n t i a l f o r a l l - wea the r l a n d i n g s b e c a u s e of t h e a b i l i t y t o i n d e p e n d e n t l y ' c o n t r o l v e r t i c a l and h o r i z o n t a l v e l o c i t y i n t h e f i n a l a p p r o a c h . I n o t h e r words , t h e a b i l i t y t o " s t o p and l a n d i n s t e a d o f l a n d , and s t o p . " F i n a l l y , a VTOL t e r m i n a l would t a k e l e s s h i g h - p r i c e d dqwntown. l a n d . , :,.jP ,:j:, 0 r e a d t h e d e s c r i p t i o n s o f .'.iliii. 3,;: ii: Model 188 (Breguet 'b .4 lS) . STOL t r a n s p o r t i n the New York metropolitan area. It was reported that repeated operations with approaches down to 200-ft altitude over the Hudson River piers did not cause any noise complaints or apparent awareness of the aircraft's presence.11 The promise of this complexities of promoting a STOLport in New york.12 Valuable lessons were learned in the 1960/1970 STOL programs and many of them are discussed in the December 1970 issue of A&A under the theme "Toward City- Center Air Service" As seen in 1970, the key lessons learned in the STOL program included the need to address the community concerns regarding noise and safety, a need for an early and factual public-education program, and the need for strong and sustained leadership. In a guest editorial in the December issue of A&A on the question of leadership in V/STOL aviation, Dr. Raymond Bisplinghoff, then the Deputy Director of the National science Foundation, observed that 'I .... in most of society's problems, including short haul trasportation, we are handicapped not so much by a shortage of science and technology, as by a shortage of entrepreneurial ability to plan and execute new programs." 13 while the U.S. STOL program foundered, the Canadian government moved ahead to accomplish a Montreal-Ottawa STOL demonsration during 1 9 7 4 to 1976. The results of this operational evaluation were published in 1 9 7 8 giving details on financial and operational statistics, STOL passenger acceptance, the competitive environment, economic viability, and local community acceptance .I4 Tiltrotor's Civil Application Potential - observation was shattered by the L program funding, air service activities, As a result of the NASA/Army and Bell Helicopter TeXtrOn flight research and mission suitability demonstrations using the XV-15 Tilt-Rotor Research Aircraft, and the Navy/Marine V-22 Osprey development, the tiltrotor is now a fully validated new aircraft concept. The technology has been researched, developed, validated, and demonstrated for both military and civil missions.l5 This technology is now being applied to fulfill the Marine military assault transport requirement as well as other Navy and Air Force mission requirements, including search and rescue. The stage is now set to begin the commercialization of the fruits of this national investment in rotorcraft technology for the public benefit. The 1988 White House Office of Science and Technology Policy (OSTP) report on National Aeronautical R S D Goals - Agenda for Achievement includes the following recommendation as part of an action plan to strengthen U.S. competitiveness in the world marketplace: W "Enhance the safety and capacity of the National Airspace System through advanced automation and electronics technoloov and - ,. - - - - new concepts including vertical nd short takeoff and landing aircraft." 1: An impressive array of proven technology is now available for application in an enhanced public aviation transportation system. The integration of modern helicopter and tiltrotor operations with commercial fixed-wing operations offers the potential for a number of public benefits, including: - Air transport capacity enhancement without major new airport real estate acquisition. 17 with quick-response Emergency Medical Service (EMS.) Every person in the United States could be within the critical "Golden H O ~ C ~ of a regional trauma center. 18 Tiltrotor makes it possible to extend this coverage to all regicns of the country. This can be achieved by integrating tiltrotor and helicopter EMS operations utilizing autonomous instrument landing approach methods. - Virtually 100% coverage of the nation - A fleet of civil aircraft that could be temporarily diverted to use in disaster relief, search and rescue and emergency utilization where the unique capabilities of vertical takeoff and landing are required. helicopters and tiltrotor aircraft in the Civil Reserve Aircraft Fleet (CRAF) where government investment in augmented aircraft design features provides the capability to utilize these aircraft in national emergencies. The CRAF program also provides a pool of trained and proficient crews and a rapid accumulation of operational experience, which can benefit military operational readiness. - A network of air transportation links between urban and rural regional enterprize zones for the rapid movement of materials, personnel and high-value finished products between heliport, vertiport and airport sites. - Inclusion of modern commercial civil Tiltrotor Design Studies Conceptual design studies for civil helicopter and tiltrotor configurations have been carried out by industry and government over the past 20 years. The engineering design studies done in 1975 concluded there was no fundamental technological constraint which limits vehicle size up to 1 0 0 assengers - the largest size studied. New studies are now under way to reassess the earlier studies of a very large civil transport (VLCT) single rotor helicopter design. 21 4 1 n t h e r e c e n t s t u d y o f f i v e c a n d i d a t e c i v i l t i l t r o t o r d e s i g n s , s p o n s o r e d by t h e NASA/DOD/FAA, s i z e s r a n g i n g from 8 - t o 75- p a s s e n g e r v e h i c l e s were examined i n d e t a i l ? ' The s t u d y r e s u l t s , r e p o r t e d a t t h e 1988 N a t i o n a l Forum o f t h e American H e l i c o p t e r s o c i e t y , i n c l u d e d a d i s c u s s i o n O f t h e m a r k e t s e g m e n t s , VTOL i n t e r c i t y t r a n s p o r t s y s t e m , a n d v e r t i p o r t concepts. Miss ion p r o f i l e s and d e s i g n ground r u l e s were e s t a b l i s h e d b a s e d on e x i s t i n g F e d e r a l viat ti on R e g u l a t i o n s , i n c l u d i n g t h e u n p r e c e d e n t e d a b i l i t y t o hove r O u t o f g round e f f e c t ( H O G E ) a t d e s i g n g r o s s we igh t w i t h one e n g i n e i n o p e r a t i v e (OEI ) . The d e s i g n g u i d e l i n e s f o r t h e s t u d y were d e v e l o p e d b a s e d on a v a i l a b l e t i l t r o t o r e x p e r i e n c e (XV-15 and V - 2 2 ) augmented by p r e l i m i n a r y i n p u t r e c e i v e d from p o t e n t i a l o p e r a t o r s . Some o f t h e key t e c h n i c a l d e s i g n g u i d e l i n e s e x c e r p t e d f rom t h e f u l l l i s t i n c l u d e : M i s s i o n C r i t e r i a : - 600-nm r a n g e f o r VTOL w i t h OEI, H O G E , s t a n d a r d d a y , s e a l e v e l . - 80n-nrn r a n g e f o r 7 5 0 - f t STOL c v e r a 35- E t o b s t a c l e , O E I , s t a n d a r d d a y , s e a l e v e l . A i r c r a f t S t r u c t u r a l Des ign C r i t e r i a : - L i m i t l o a d f a c t o r of 3 . 0 a t d e s i g n g r o s s we igh t ( D G W ) . - A l l c o m p o s i t e p r i m a r y s t r u c t u r e . - P r e s s u r i z e d c a b i n f o r o p e r a t i o n s above 1 0 , 0 0 0 f e e t ; c a b i n a l t i t u d e n o t t o e x c e e d 8 , 0 0 0 f e e t . P r o p u l s i o n s y s t e m S i z i n g C r i t e r i a : - 30-second OEI emergency e n g i n e r a t i n g Of 1 2 5 % o f t h e e n g i n e t a k e o f f r a t i n g assumed t o s a t i s f y t h e H O G E , O E I , a t DGW. - F u e l System s i z e and w e i g h t assumed a wet wing and t a n k s f o r f u l l p a s s e n g e r l o a d o v e r a 600-nm r a n g e , w i t h f u l l IFR r e s e r v e s . O p e r a t i o n a l C r i t e r i a : - R o t o r , wing , e n g i n e i n l e t , and w i n d s h i e l d d e i c i n g p r o v i d e d . Another ma jo r d e s i g n s t u d y i s underway in Europe . I n 1986. a j o i n t a c t i v i t y among E u r o p e ' s h e l i c o p t e r and f i x e d - wing m a n u f a c t u r e r s was i n i t i a t e d t o i n v e s t i g a t e t h e f e a s i b i l i t y o f a European t i l t r o t o r a i r c r a f t f o r t h e y e a r 2 0 0 0 . 23 Fol lowing t h e go-ahead d e c i s i o n i n September 1987 , t h e p a r t n e r compan ies a r e now working o n a 3-year p h a s e t o s t u d y S p e c i f i c t i l t r o t o r component t e c h n o l o g i e s , i n v e s t i g a t e c e r t i f i c a t i o n and i n f r a s t r u c t u r e , a i r t r a f f i c c o n t r o l 5 p rob lems and t o c o n d u c t m a r k e t s u r v e y s f o r a commerc ia l p r o d u c t . T y p i c a l m i l i t a r y t i l t r o t o r p o t e n t i a l m i s s i o n s have a l s o been e s t a b l i s h e d and t r a d e - o f f s t u d i e s o f m i l i t a r i z e d v e r s i o n s have been i n i t i a t e d . The g e n e r a l d e s i g n r e q u i r e m e n t s have been d e f i n e d f o r a b a s e l i n e c o n f i g u r a t i o n , which is 30-passenge r commerc ia l v e h i c l e . ( F i g . 5 ) These p r e l i m i n a r y d e s i g n r e q u i r e m e n t s a r e s i m i l a r t o t h o s e e s t a b l i s h e d f o r t h e NASA/FAA/DOD c i v i l t i l t r o t o r s t u d i e s , e . g . , 6 0 0 nm r a n g e , 300 k n o t c r u i s e s p e e d , C a t e g o r y A c a p a b i l i t y t o f l y away from a l a n d i n g emergency w i t h O E I , and a p r e s s u r i z e d f u s e l a g e . However, some n o t a b l e a d d i t i o n a l r e q u i r e m e n t s have been l i s t e d . These i n c l u d e : c r u i s e a l t i t u d e of 7500 m (24600 E t ) , low e x t e r i o r n o i s e l e v e l , l i g h t n i n g p r o t e c t i o n , a u t o r o t a t i o n c a p a b i l i t y , and emergency l a n d i n g c a p a b i l i t y i n t h e f ixed -wing mode. The u n d e r l y i n g i n t e n t of t h e l a s t two r e q u i r e m e n t s i s t h a t i t s h o u l d h e p o s s i b l e t o r e t u r n t o t h e ground s a f e l y f o l l o w i n g t o t a l power f a i l u r e . w F i g u r e 5.- 30-passenge r b a s e l i n e E U R O F R R t i l t r o t o r d e s i g n from r e f e r e n c e 2 4 . Wing s p a n = 4 8 . 2 f e e t , r o t o r d i a m e t e r = 36.8 f e e t . Another s t a t e d i n t e n t of t h e EUROFAR d e s i g n i s t o p e r m i t a l l - w e a t h e r f l i g h t and a p p r o a c h e s i n a n u r b a n env i ronmen t under I n s t r u m e n t M e t e o r o l o g i c a l C o n d i t i o n s (IMC). The e x p r e s s e d d e s i r e i s t o p r o v i d e a c a p a b i l i t y f o r a hands -o f f t r a n s i t i o n and approach t o a low hover i n z e r o - z e r o v i s i b i l i t y a t a r e s t r i c t e d s i t e . T h i s i s c l e a r l y r e c o g n i z e d by t h e EUROFAR C e r t i f i c a t i o n and I n f r a s t r u c t u r e Group a s a f o r m i d b l e d e s i g n c h a l l e n g e i n v o l v i n g t h e c a r e f u l i n t e g r a t i o n o f a v a i l a b l e a i r c r a f t and i n f r a s t r u c t u r e t e c h n o l o g i e s . 2 5 What Might t h e O p e r a t o r s Want? s i n c e new commerc ia l t i l t r o t o r s a r e n o t l i k e l y t o be b u i l t on s p e c u l a t i o n , l a u n c h i n g c u s t o m e r s w i l l have a n i m p o r t a n t i n f l u e n c e i n t a i l o r i n g t h e f i n a l d e s i g n r e q u i r e m e n t s . The o p e r a t o r s ' r e q u i r e m e n t s s h o u l d s p e c i f y what i s r e q u i r e d a s an end r e s u l t and n o t be t o o s p e c i f i c on t h e i/ W' details of how to accomplish it. However, the prudent customer must practice the art of the possible by assessing the real benefits of applying new technology vs. th economic viability of the resulting design. - Technology €OK its own sake is not a viable 56 commercial option. For Example, during the 1982 National Specialists Meeting On Commercial Helicopter Design, sponsored by the American Helicopter Society (AHS) and the Helicopter Association International (HAI), operators expressed caution in embracing new technology. However, they recognized that when properly applied, with credible guarantees from the manufacturer, new capabilities are welcome. Some of the operators' observations and expressed needs discussed at the 1982 Specialists Meeting have relevance to similar issues and requirements that are likely to develop for the commercial tiltrotor. A number of the key points were: - Civil derivatives of military designs must be looked at to assess the trade- of€ of lower first cost vs. non-optimum lifetime cost issues. (More recently, i t has been observed that in the case of the helicopter the commonality of civil and military designs is eroding and technology divergence is at hand.) 27 requirements included: 28 - The operators' guidance for new design Increased instrument flight capability v High accuracy approach guidance sy s temS Reduced noise Improved OEI performance Improved maintainability Reduced direct operating costs Pressurized cabins to fly above the weather Improved crashworthiness, including energy absorbing seats and the use of crash-resistant fuel cells based on proven military systems Reduced clear area requirement for VTOL operations Today,it appears that, to varying degrees, all of these same topics are being addressed in the development of preliminary design requirements for civil tiltrotors. The last topic in the listing above, regarding "clear area," has important implications for the successful civil application of the tiltrotor. The issue in 1982 centered on the requirement to have a 2500 foot clear area for twin engine helicopters in commercial passenger operations in order to allow for emergency reland o r safe cpntinued climbout after an A consensus developed among the operators at the Specialists Meeting that the new ueneration of aircraft must be cavable of engine failure. 9 " t d k i n g off a,;? proceeding on their way vertical!y." The helicopter, mrftinq its full potential, was seen as a vertical machine and, if properly designed, did not need stop distances of 400 to 700 feet. It was also suggested that for scheduled service some level of Category-111-type operating system would be required to provide all-weather landing capability. The civil operators identified the requirements that they felt should be incorporated in the specifications for any new generation of vertical lift aircraft. These are especially pertinent as the new civil tiltrotor design requirements are being drafted. Of particular importance is the requirement to have true VTOL, all- weather capability for viable scheduled service in operating environments having limited ground space and minimum terminal air space. Future Vertipor t Scenarios In order to significantly impact civil aviation capacity limits and fully capture the potential market, the tiltrotor must be capable of serving vertiport sites at close-in urban locations and at major airports.30~31 To be most effective, these facilities must exploit the VTOL mode of operation in order to minimize the real estate required to four acres or less. These facilities may be at ground level or on the top of suitable structures, thus achieving increased obstacle clearance. The true usefulness and resulting benefit of a central business district VTOL service will be critically dependent on its convienience, trip time saved, environmental impact, and reliability of service. In the congested airport scenario, the introduction of scheduled tiltrotor VTOL service from colocated, autonomous, vertiports could off-load fixed-wing commuter demand for runway s l o t s . These aircraft slots could then be allocated to larger fixed-wing transport aircraft to achieve an overall increase in system capacity without prohibative infrastucture investment. j2 Again, the demand for schedule reliability, minimum real estate requirement, and minimum environmental impact will be the key to success. The potential supply and demand trade-offs. made possible by the introduction of the tiltrotor, would be among airport owners, air carriers, and commuter operators. It would likely foster new investment stategies, modified fleet mix, and increased operational systems integration at the congested airports. I 5 The C r i t i c a l I s s u e A major d e s i g n i n t e g r a t i o n c h a l l e n g e e x i s t s i n m e e t i n g t h e r e q u i r e m e n t f o r t r u e VTOL, s c h e d u l e d commerc ia l o p e r a t i o n s from u r b a n c e n t e r v e r t i p o r t s and autonomous v e r t i p o r t s c o l o c a t e a t c o n g e s t e d a i r p o r t s . Assuming t h a t t h e d e s i r e d l a n d i n g s i t e s a r e made a v a i l a b l e and t h a t t h e c i v i l t i l t r o t o r a i r c r a f t d e s i g n s a r e p r o d u c e d , i t a p p e a r s t h a t a new t e r m i n a l a r e a p r e c i s i o n o p e r a t i n g s y s t e m w i l l b e r e q u i r e d t o f u l l y e x p l o i t t h e t i l t r o t o r p o t e n t i a l . P o t e n t i a l v e r t i p o r t owners and o p e r a t o r s a r e b e g i n n i n g t o l o o k a t s i t e f e a s i b i l i t y w i t h a n e y e toward u s i n g s m a l l , b u t c o s t l y , l a n d a r e a s a v a i l a b l e i n c l o s e p r o x i m i t y t o h i g h d e n s i t y c e n t r a l b u s i n e s s d i s t r i c t s and a t e x i s t i n c l m e t r o o o l i t a n a i r o o r t s . The ~. ~ ~ p r e l i m i n a r y c i v i l - t i l t r o t o r i e s i g n s t u d i e s h a v e c o n f i r m e d t h e a b i l i t y t o p r o v i d e t r u e VTOL c a p a b i l i t y , i n c l u d i n g O E I , HOGE, emergency o p e r a t i n g c a p a b i l i t y w i t h t h e u s e o f p r o p e r emergency e n g i n e r a t i n g s . The c u r r e n t e f f o r t t o d e v e l o p new v e r t i p o r t and l a r g e h e l i p o r t d e s i g n g u i d a n c e is , by p r a c t i c a l n e c e s s i t y , i n c o r p o r a t i n g t h e C u r r e n t g e n e r a t i o n Microwave Landing Sys tem (MLS) a s a n i n t e g r a l p a r t o f n e a r t e r m v e r t i p o r t d e s i g n o p t i o n s . 3 3 ( F i g . 6 ) C o n s i d e r a b l e p r o g r e s s h a s been a c h i e v e d i n a d a p t i n g t h e c u r r e n t MLS c a p a b i l i t y t o t h e t i l t r o t o r / v e r t i p o r t r e q u i r e m e n t . I n n o v a t i v e and p r e c e d e n t s e t t i n g d i s c u s s i o n s and a p p r o a c h e s a r e b e i n g p u r s u e d t o p r o v i d e s t e e p a p p r o a c h a l l - w e a t h e r l a n d i n g g u i d a n c e w i t h t h e MLS equ ipmen t . The g o a l i s t o p r o v i d e t h e h i g h e s t g l i d e p a t h a n g l e f e a s i b l e w i t h i n t h e e l e v a t i o n s c a n a n g l e l imi t s o f t h e MLS (0.9 t o 1 5 d e g r e e s . ) E a r l y d i c u s s i o n s h a v e d e a l t w i t h t h e f e a s i b i l i t y o f g l i d e s l o p e a n g l e s o f 6 t o 1 0 d e g r e e s . I n some i m p o r t a n t v e r t i p o r t s i t i n g , e v e n t h i s r e l a t i v e l y s teep g l i d e s l o p e c a p a b i l i t y o f t h e p r e s e n t MLS may n o t b e s u f f i c i e n t . T h e r e w i l l b e a need t o l o c a t e v e r t i p o r t s , c a p a b l e o f s a f e and r e l i a b l e s c h e d u l e d o p e r a t i o n s , i n a r e a s domina ted by t a l l s t r u c t u r e s and a need t o o p e r a t e i n d e s c r e t e , n o n - c o n f l i c t i n g t e r m i n a l a i r s p a c e a t a i r p o r t s . W i t h o u t e x t e n s i v e a d a p t a t i o n , t h e c o n v e n t i o n a l MLS i s l i k e l y t o be i n c o m o a t i b l e w i t h Some d e s i r e d v e r t i n o r t .. .. ~... - s i t e s . AS a n example , MLS s i t i n g a t t h e Wall S t r e e t h e l i p o r t h a s n o t been f e a s i b l e b e c a u s e o f t a l l s t r u c t u r e s i n c l o s e p r o x i m i t y t o h e l i p o r t s i t e and t h e p o t e n t i a l h e i g h t s o f t h e mar ine t r a f f i c w i t h i n t h e o t h e r w i s e c l e a r a r e a on t h e E a s t R i v e r . T h i s s i t u a t i o n , i n c o m b i n a t i o n w i t h e x i s t i n g t e r m i n a l i n s t r u m e n t p r o c e d u r e s (TERPS) r e q u i r e m e n t s , r e s u l t e d i n t h e p l a n t o l o c a t e t h e MLS i n s t a l l a t i o n a t t h e " F i s h p o r t " s i t e a c r o s s t h e r i v e r . A proposed i n s t r u m e n t approach would p r o v i d e a 3 . 7 d e g r e e g l i d e s l o p e w i t h a d e c i s i o n h e i g h t ( D H ) o f 4 7 3 f e e t above mean sea l e v e l ( A M S L ) and one S t a t u t e m i l e v i s i b i l i t y minima. A f t e r a r r i v i n g a t t h e D H , t h e a i r c r a f t would p r o c e e d v i s u a l l y a l o n g t h e ~ r o o k l y n s h o r e l i n e f o l ~ o w i n g c o u r s e l i g h t s , t h e n t u r n t o c r o s s t h e E a s t R i v e r t o t h e Wall S t r e e t h e l i p o r t t o l a n d . _- The s c o p e of t h e ongo ing d i c u s s i o n s by t h e V e r t i p o r t Working Group g o ' w e l l beyond t h a t o f t h e c u r r e n t Wal l S t r e e t h e l i p o r t s t u d y . A S a r e s u l t , t h e a p p l i c a t i o n of c u r r e n t MLS equipment c a p a b i l i t y f o r p r e c i s i o n g u i d a n c e i n t h e v e r t i p o r t t e r m i n a l approach i s showing c o n s i d e r a b l e p romise f o r t h e e a r l y i m p l e m e n t a t i o n o f modern t i l t r o t o r and h e l i c o p t e r I M C o p e r a t i o n s . The u n d e r l y i n g p r e m i s e o f t h i s p a p e r i s t h e b e l i e f t h a t , u l t i m a t e l y , t o f u l l y r e a l i z e t h e p o t e n t i a l m a r k e t demand f o r t h e t h e c i v i l t i l t r o t o r c a p a b i l i t y i t w i l l be e s s e n t i a l t o p r o c e e d w i t h t h e d e s i g n , deve lopmen t , and c e r t i f i c a t i o n o f a t r u e VTOL p r e c i s i o n t e r m i n a l a r e a o p e r a t i n g Sys tem, which p r o v i d e s 9 0 d e g r e e , v e r t i c a l approach and d e p a r t u r e g u i d a n c e . Only by f u l l y e x p l o i t i n g t h e VTOL c a p a b i l i t y i n t h i s way f o r t h e p u b l i c ' s use w i l l t h e maximum r e t u r n on i n v e s t m e n t i n t h e c i v i l t i l t r o t o r and i t s i n f r a s t r u c t u r e b e a c h i e v e d . ALTITUDE. F T 4 r1000 3 I TRANSITION CRU,SE VERTIPORT ~_*___.~~ ~~ ri000 I . _ _ ~ L -.A 0 1 2 3 1 5 6 7 8 9 DISTANCE, FT F i g u r e 6 . - P o s s i b l e VTOL o p e r a t i n g s c e n a r i o u t i l i z i n g t h e Microwave Landing System (MLS) approach geomet ry . W Meet ing t h e System Des ign C h a l l e n g e The c h a l l e n g e i s t o d e v e l o p a s a f e , r e l i a b l e , n o n - i n t r u s i v e VTOL p r e c i s i o n t e r m i n a l a r e a o p e r a t i n g s y s t e m f o r u s e i n t h e u rban and c o n g e s t e d a i r p o r t env i ronmen t . I n t h e u r b a n e n v i r o n m e n t t h e s y s t e m must a c h i e v e low p e r c e i v e d n o i s e l e v e l s and r e s u l t i n minimum i n t r u s i o n upon t h e s u r r o u n d i n g community. I n t h e a i r p o r t a p p l i c a t i o n , t h e same a t t r i b u t e s a r e r e q u i r e d i n a d d i t i o n t o t h e a b i l i t y t o o p e r a t e w i t h m i n i m u m impact o n t h e e x i s t i n g f ixed -wing a i r t r a f f i c control IATC) s y s t e m and t e r m i n a l a r e a o p e r a t i o n s . F i g u r e 7 d e p i c t s a; c h n d i d a t e cor icept t o a c h i e v e t h e r e q u i r e d o p 6 r ~ t i o ~ a l " c a p a b i l i t y , . p e , y T P L ~ p r e c i s i o n i n s t r u m e n t t e rmin5'1:. op& r a t I n g s y s f o l lowing e leme n ts'i a': c'iV'i.1 t ilrt roto'r ai rcrnf t c i i G b l 6 '0f"sBZ'S n'&l2rz 'eP8/zero m i s ' m a d e up ' o f t h e __._._.. . ~~ ins t rument .VTOL a r r i v a l s an8:dkpa .~ ALTITUDE. FT 3 t 3 2 1 0 1 2 3 2 4 6 DISTANCE FT F~i9Ur.e . , . ., 7.' ,. , :.o-p : : 6 i i i i , ~ Ach iev ing a c a p a b i l i t y s i m i l a r to . t h e VTOL pr e c is i o n i n s t r u'me n t 1 and i ng' s y s t e m d e s c r i b e d above r e q u i r e s t h e s o p h i s t i c a t e d d e s i g n integration^ a n d e n g i n e e r i n g . ,. developm'ent o f e a c h eIemB'nt o f t h e t o t sys t em. The e f f e c t i v e a p p l ' i c a t i o n of a v a i l a b l e t e c h n o l o g y i s the"key . . to ' t h e ~ s u c c e s s oE s u c h a c h a l l e n g ' i n g s y s t e m s ' ' d e s i g n e f f o r t . The e n a b l i n g t e c h n o l o g y e x i s t s t o d a y t o d e s i g n . a VTOL 'tet ' ' oper s y s t e m o f f ' h i s t y p e . The of d e s i g n i n g t b e i n t e g r c a n dra* dpOX 8 n " e x t e n s i v e t e c h n o l o g y b a s e now a v a i l a b l e f r 0 m " p r i o r i n v e s t m e n t s i n : ' governme'rlt and i n d u s t r y rc;se.eich and ' ' t e c h n o l o g y ; NOW, t h e d e s i g n t a s k i s t o i n E e g i a f e t h i K f e c h n o l o r J y ' , v a l i d a t e a n mission. ' U B @O'L .;.Vlibiion:sL. St??, ~ A i r c r a f t Des ign Augmenta t ion . The a i r c r a f t d e s i g n would need t o b e enhanced t o g i v e rgency". e n g i ne 8 System (PNVS); laser rangefinder; IR and TV tiltrotor's inherent operational advantage systems with multiple fields of view; is eroded. 52 While its STOL overload digital map displays: automatic approach, performance is a significant asset, hover and climb out: and pre-programmed inordinate utilization rates would suggest input for navigation and comm~nications.39-~9 that a more conventional STOL aircraft might be used more cost effectively. Suffice it to say that the technology available in this category far exceeds that required for safe automated VTOL operations. In fact, the basic VTOL terminal area automatic landing technology was validated an demonstrated by NASA in the early 1970s.fo GroJnd Based Guidance System Design hu;ncntation. The cirrent MLS eqilpmcnt, w i c h ,l m3ximum glide slope canabilltv of 1 5 degees, is not suitable f o r VTOL preGision guidance in confined terminal area environments. It may be appropriate to assess the feasibility of adapting the MLS design to provide 90 degree VTOL guidance, or MVLS, capability. It is interesting to note that in June 1963 the FAA first evaluated a VTO Approach and Landing System (VAPS). A glide path could be set at any angle from -5 to 90 degrees. Vertiport Design Augmentation. From the VTOL vertiport point of view, the design task may be less demanding. The automated vertical approach and departure would require the colocation of the VTOL landing guidance system. The concerns €OK normal MLS obstacle limitation surfaces and final approach areas associated with low angle glide slopes would be considerably relaxed O K eliminated. All the required precision approach procedures to bring the aircraft to a hover over the landing site would be accomplished at altitude. Timing and Economic Issues Recognizing that V/STOL history is consistently marked by overoptimism, some feel for possible timing is offered. At best, a civil VTOL terminal area operating system of the type envisioned in this paper is in the category of a second generation or special-class vertiport. While all of the basic technology appears to be in place, it is estimated that it would take approximately 3 to 5 years from a go-ahead decision to adequately demonstrate the operational feasibility of such an integrated system using a modern helicopter. Full development and certification for operational use could occur in the 2000 - 2005 time frame. Early estimates of tiltrotor costs are the subject of much debate. In the absence of production and operating data, preliminary estimates of trip costs range from one-and- one-half to four times the prices on conventional fixed-wing aircraft. when the cost of portal-to-portal trips are totaled, including the cost and time of ground transport that must be added to the airport-to-airport costs, the tiltrotor becomes competitive and seemingly within the grasp of business travelers. In general, without effective utilization of its VTOL capability, the civil ,d The augmentation of the civil tiltrotor with automated VTOL capability will likely increase the initial investment costs and operating costs to some degree. However, this should he offset by the ability to provide reliable, all-weather, scheduled commercial operations between urban business centers, major regional airports and regional vertiport/heliport networks. Concluding Remarks Over the past 20 years there has been a small, but growing recognition that the commercial helicopter, and now tiltrotor, mission capability should include true VTOL terminal area, all-weather operations. This requirement was identified in 1969, at the time of the STOL progam, again in 1982 by helicopter operators, and most recently by potential vertiport operators. This presents a sophisticated system design challenge. It appears that the enabling technology base is readily at hand to design an instrument terminal area VTOL operating system which utilizes adequate emergency engine ratings, automated flight path management, vertical landing guidance, and modern vertiport facilities. This capability should make it possible to extend the civil tiltrotor infrastructure to close-in urban vertiports and to W colocated vertiports at high capacity airport f ac il it ies . The near-zero/zero VTOL operations of the type described, if demonstrated to be Operationally safe and economically viable, would provide enhanced service with minimum impact on the surrounding community, i.e., noise, real estate required, and intrusion. These same attributes would also apply for airport operations; and, in addition, the autonomous VTOL operations would minimize the impact on normal fixed-wing operations, while enhancing total capacity. Acknowledgement The author acknowledges, with appreciation, the willingness of numerous individuals to discuss the general background and technical information relating to various aspects of the material included in this paper. The author's interpretation of this information and the information contained in the referenced documents does not necessarilv represent the position of anv other gove;nment o r private individual 0; organization. References 1. 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