Document
NASA Technical Memorandum 81281
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NS-120066
' Ill tllltlllltllttll till I_t
Recent Progress In V/STOL
Aircraft Technology
L. Roberts
W. Deckert
D. Hickey, Ames Research Center, Moffett Field, California
I / SA
National Aeronautics and
Space Administration
Ames Research Center
Moffett Field, California 94035
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I0-i RECENT PROGRESS IN V/STOL AIRCRAFT TECHNOLOGY L. Roberts, W. Deckert, and D. Hickey Ames Research Center, NASA Moffett Field, Salifornia 94035, d.S.A.
Sb_MARY Recent results from wind-tunnel and flight-tests investigations for V/STOL aircraft are reviewed.
Primary emphasis is given to tecnnical results relating to threetypes of subsonic aircraft: a quiet STOL aircraft; a tilt rotor aircraft, and a turbofan V/STO, aircraft. Comparison and correlation between theo- retical and experimental results, and between wind-tunnel and flight-test results, is made. The Quiet STOL aircraft technology results are primarily those derived from'the NASA/Boeing Quiet Sho_: Haul Air- craft (QSRA) program. The QSRA aircraft uses an upper surface blown flap and develops a usable engine- out landing approach lift coefficient of 5.5 and landing distances less than 1,000 ft. The tilt rotor aircraft technology results are those obtained from t_e NASA/Army/Navy/Bell (XV-15-TRRA) aircraft flight investigations. The TRRA is a twin rotor research aircraft capable of vertical takeoff and landing and cruise speeds of 300 knots. The turbofan V/STOL aircraft tecnnology results are from static ground facil- ity and wind-tunnel investigations of a NASA/Navy/Grumman full-scale lift/cruise fan aircraft model, which features two tilting nacelles with TF-34 engines.
i. INTRODUCTION The past several years have seen steady progress in the research and technology deveIooment of s_ort takeoff and landing (STOL) and vertical takeoff anO landing (VTOL) aircraft configurations. There now appears to be emerging a growing recognition that this technology can play an important role in providing a military logistics capability that will offset the growing vulnerability of large forward air bases and of large ships at sea. When the technology and the mission needs are made compatible with each other, we can expect to see a new generation of military logistics aircraft that will include long-range and short- range aircraft designed to operate with great versatility from a variety of bases.
Tnis paper reviews some of the recent STOL and VTOL technology advances in the United States that have derived from collaborative programs between NASA, the Army, the Navy, and the aircraft industry. The pri- mary emphasis here is on three kinds of aircraft that can potentially fill the current "versatility gap' between conventional military transport aircraft and conventional helicopters. The results from several representative investigations are di;cussed to demonstrate the improvements taat have been made through wind-tunnel, simulator, and flight programs. In addition, an attempt is made to summarize applicable tech- nology with respect to the readiness of these types of aircraft for future military applications.
_2. MISSIONS AND VEHICLES Currently, only two means of air-logistics are avai!ahle to support military and naval operations: I. Large conventional takeoff and landing (CTOL) aircraft capable of long-range missions but limited tc arrival points that have long runways 2. Helicopters capable of taking off and landing at a wide variety of locations, including unprepareQ sites, b_t with limited payload and range capabilities.
As the need for greater basing flexibility and faster response continues to grow - in Europe, in the Middle East, and e?sewhere - the limitaticns of large aircraft serving fixed, concentrated supply bases that are subject to r_nway denial by eneEy attack, are now recognized. Similarly, the concentration of naval resources in a relatively few large ships places severe limitations on the flexibility of the deploy- ment of naval forces; as a result, greater attention is being directed to the use of a distributed sea- going force having appropriate air logistics. Although the helicopter has played a vital role in very- short-range logistics its inherently limited range and speed prevent it from fulfilling many missions in which material must be rapidly transported over intermediate distances.
The full exploitation of air'logistics requires additional kinds of aircraft - vehicles capable of bridging the gap between conventional aircraft and helicopters; that is, aircraft that can operate effi- ciently over intermediate ranges a_d also be capable of operating into short-to-intermediate runways, including ships at sea. Figure I is a simple representation of the air logistics gap described in terms of two parameters: aircraft range and field length. The helicopter is at one extreme and the CTOL trans- port is at the other; between the two there has been no other successfully developed form of air transportation.
The design of a single aircraft, capable of efficient cruise over long range and of efficient hover for short range, must be considered too difficult a task for the near. ter_. It is not surprising, there- fore, that a number of concepts have been proposed that partially fill the void suggested in Fig. i. Three of these concepts - the fixed wing STOL transport, the tilt-rotor V/STOL transport, and the lift-cruise fan transport - attempt to perfor_ in the region between the CTOL and the helicopter, Because of recent tech- nical progress, it is worthwhile to review the status of these concepts,:and to assess the degree to which each may be ready for development.
When these three concepts are characterized in terms of range and field length (Fig. 2), it is clear that the fixed-wing STOL transport, which derived from the CTOL, sacrifices some range capability in order to achieve short-field operation capability. Similarly, the tilt-rotor transport, which derives froE the helicopter, sacrifices some hover performance to achieve greater range. Because of the derivative nature of these two concepts, their tecnnical development has progressed more rapidly than that of the third
10-2
,_GENERAT,ON_--_>_ CTOL _:_
10,000 10,000 I- _ " _ _
LONG RANGE __i LONG FIELD LENGTH ....... ! TILT ROTOR / /
/
VERSATILITY INTERMEDIATE RANGE .J SHORT FIELD CAPABILITY
e NEEDED
w
J
SHORT RANGE SHORT FIELD LENGTH HELICOPTER i I !
i 10 100 1000 10,000 10 100 1000 10,0100 RANGE, miles RANGE, rmlel Figure 1. Aircraft range and field length Figure 2. New technolog, y aircraft: powered-lift capabilities.
STOL, tilt-rotor, turbofan VTOL.
concept, the lift-cruise fan aircraft, which is a more radical departure from current aircraft design.
However, this third concept is potentially capable of vertical takeoff, nigh cruise Speed, and long range.
In the remainder of this paper the technical progress recently made with each of these three V/STOL air- craft concepts is described.
3. RECENT DEVELOPMENTS IN V/STOL TECHNOLOGY A number of V/STOL concepts are being actively pursued, including the ABC rotorcraft, the X-wing _stoppable rotor) concept, and various approaches to powered-lift STOL and VTOL aircraft. This paper, however, is restricted to a Discussion of some recent work on three aircraft: (I) a STOL powered-lift aircraft (the QSRA), (2) a compound rotorcraft (the TRRA), and (3) a turbofan V/STOL aircraft. These three programs are thought to represent the forefront o_ technology-readiness for logistics/utility venicles designed to fill tne capabilities gap between helicopters and CTOL aircraft.
3.1 STOL Transport Technology There has been significant progress in powered-lift technology for STOL transport aircraft during the last 10 years. The primary effort within NASA, undertaken in conjunctlon with the Boeing Company, has been the flight demonstration of an advanced upper-surOace-blowing (USa) powered-lift concept. The air- craft used in this program, the Quiet Short-Haul Research Aircraft (QSRA), has provided proof-of-concept verification of the USB apprcach and has been used extensively to investigate terminal-area operations for STOL aircraft.
The QSRA (Fig. 3) utilizes a deHaviTIand C-8 Buffalo fuselage and empennage, and a new swept wing with four Lycoming YF-IO2 turbofan engines mounted above the wing. As i]lustrated in Fig. 4, the USB concept "D"-NOZZLE / USB FLAP !COANDA SURFACE) LANDING FLAP SETTING = 50 deg Figure 3. QSRA in landing configuration. Figure 4. Upper-surface-blowing wing-engine schematic.
is used for propulsive-lift. The wing, nacelles, and propulsior system were designed to be representative of a configuration capable of efficient cruising flight at Mach 0.74. To pursue the flight research objec- rives at minimum cost, the configuration uses fixed landing gear, fixed leading-edge f]aps, and other con- figurational compromises that limit maximum airspeed to 190 knots. Some of the QSRA characteristics are presented in Table I.
The airframe modlfication program was completed by the Boeing Com_ercial Airplane Company in July 1978.
In 1979-1980, QSRA research activities i_cIuded the areas of configurational aerodynamics and acoustics, as well as flight control systems research to expand the research flight envelope; a cooperative program with the U.S. Navy that featured unarrested landings and unassisted takeoffs aboard the U.S.S. Kitty Hawk; and 10-3
a guest evaluation program in which pilotsfrom other
NASA centers, themilitaryservices, airlineorgani-
zations, theFAA, and industry participated. Pres-
entlytheQSRA is being flown at reduced ergine set-
tingsto determine thelow-speed characteristics CONTROL MARGIN LIMIT
aoplicabie to theclasso#propulsive-lift aircraft LANDING FIELD LENGTH: 1530ft
thatfeature lowto medium thrust-to-weight ratios.
TURN RADIUS: 380 It
Asoc February 1981, theQSRA _a_ accumulated
APPROACH SPEED: 280 _esearch _lighttours.
The _SRA flight research envelope is shown in
Fig.5. The upper boundary is limited bytheusaole
_4
approach lift-coef#icient when one engine is inoper-
MIN,MUM I "
ativeand wh_Ie meeting theusual commercial safety
WEIGHT _ RESEA CH : / _argins, sbch as speed, angle of attack, and normal
_3
-- --L_ I ENVELOPE !
acceleration. This approac _ lift-coefficient is 5.5 with USB flaps and double-slotted flaps (located out- board o_ t_e USB region) deflected 50 ° and 59 ° ,
_2
respectively. By design choice, the left-hand bound- ary is for a wing loading of 65 Ib/ft 2 and the right- LANDING FIELD LENGTH: 4490lt I _a_d boundary is for a wing loading of 100 Ib/ft 2.
TURN RADIUS: 1275 ft As shown in Fig. 5, at 7ower wing Ioadings the upper APPROACH SPEED: 120 knots part of.the researcn envelope is bounded by aircraft I I I control a_d safety margin limits at an approach air- 80 100 110 40 60 speed of 65 knots. Other powered-lift aircraft have WING LOADING, Ib/_l ft also experienced limitations in approach-lift coeffi- cient because of inadequate control (rather than Figure 5. QSRA flignt envelope: lift coefficie,t because of inadequate propulsive-lift performance), vs wing loading.
particularly for the required engine-out case.
The lower boundary for the QSRA flight- research envelope is somewhat arbitrarily defined as the CTOL configuration that features no deflec- tion of the USB flaps (with double-slotted flap deflection of 59°). With an engine out and oper- ating in compliance with commercial margins, the QSRA has a minimum turn radius of 380 ft, a typical fl_ghtpath approach angle of 7.5 ° , and a required OSRA FLIGHT TEST RESULTS field length of 1,530 ft. With a 30-knot wind over the deck of the U.S.S. Kitty Ha_k, the QSRA _anding ground-roll was 200 ft (approximately twice its fuselage length), without using arrest- ing gear and without reverse zhrust.
Trimmed lift coefficients as functions of angle of attack for the QSRA and for a typical CTOL short- ha_l turbofan transport are presented in Fig. 6; data in the figure are for all engines operatimg.
Z The approach coefficients of the QSRA are several times that of the CTOL transport. With the QSRA, u.
.the percentage of lift-loss wit_ one e_gine inop- u. 4 erative is low, which permits an operational lift- coefficient close to maximum lift coefficient.
F- Note from the previous discussion and from Fig. 5 ..¢ that the QSRA approach lift-coefficient is a maxi- mum of 5.5 with all engines operating or with one engine out. Using a typical flightpath approach a_gle of 7.5 ° , the QSRA fuselage attitude is essen- tially horizontal during approach. This character- istic simpllfies landing flare _equirements and implies 3assenger comfort. At maximum engine thrust levels _not shown on Fig. 6) the QSRA has flown at maximum lift-coefficients above 10.
I I I I I 0 5 10 I_ 20 25 TABLE I QSRA CHARACTERISTICS ANGLE OF AT.rACK, (leg 50,000 lb Design takeoff gross weight 60,000 lb Figure 6. QSRA lift capability: lift coefficient Maximum takeoff gross weight Demonstrated maximum takeoff gross vs angle of attack.
57,000 Ib weight 83 l.P/ft2 Wing loaaing at design gross weight Thrust-to-weight at design gross O. 50 weight, measured Maximum sink rate at design gross 12 ft/sec weight 10,670 lb Wing fuel capacity 5,409 Ib Fuselage fuel capacity 387 n.mi.
Range'with 45-min reserve (2,800 Ib) 2-I/4 hr Typical test mission duration 170 KTAS Long-range cruise speed at 10,000 ft 15,000 ft Design ceiling 10-4 Figure 7 is a plot of Federal Air Regulation (FAR) takeoff field length (based on FAR rules for transport aircraft) as a function of installed thrust- to-weight ratio for a STOL transport based on QSRA tecnnology and for a CTOL short-haul turbofan air- craft with comparable wing loadings. At high thrust- to-weight ratios the takeoff field length for the STOL Z transport is about one-half that of the CTOL aircraft.
-I.LI For example, for a wing loading of 110 Ib/ft 2 and a ,.I W/S. Ib/sq ft thrust-to-weight ratio of 0.50, the STOL field length ,--I is 1,750 ft; that of the CTOL is 3,350 ft. Note that _u at low thrust-to-weight ratios, typical of values used u. 4000 on CTOL transports, field length differences remain significant. For example, for a wing loading of 110 Ib/ft 2 and a thrust-to-weight ratio of 0.30, the STOL and CTOL fie]d lengths are 3,200 and 5,300 ft, respectively.
20oo Also notice the implications of Fig. 7 if STOL and CTOL designs are compared at a fixed field length.
For example, for a wing loading of 110 Ib/ft 2 and a takeoff distance of 4,000 ft, the STOL and CTOL required thrust-to-weight ratios are about 0.28 and ,i -I I I i i i .24 .28 .32 36 40 _44 .48 .52 0.40, respectively. These differences in required INSTALLED THRUST TO WEIGHT RATIO thrust-to-weight ratios at equal field distances nave significant implications in aircraft design. Using the safe engines, whether existing engines or advanced Figure 7. STOL and CTOL field length vs engines, the STOL gross weight could be higher than thrust/weight.
that of the CTOL, with an accompanying larger useful load or payload for the STOL aircraft. At equal field lengths, the STOL aircraft would feature a much larger useful load, relatively small penalties in cruise airspeed and empty weight, and therefore a higher productivity. :t should be noted that the state of the art of STDL technology makes this an opportune time for in-depth aircraft design studies in which powered lift would be viewed from mar> aspects, not just as an approach to short fie]d lengths.
Sunm_ary conclusions from the results of the QSRA flight research to date include the following: 1. A four-engine USB configuration can have very high performance and at low speeds can be a rela- tively easy aircraft to fly.
2. At any-design thrust-to-weight ratio, including the low ratios typical of CTOL transports, a propulsive-lift design can operate from significantly smaller fields than its CTOL counterpart.
3. Even when short field length is not a mission constraint, a propulsive-lift aircraft may compare favorably with its CTOL counterpart in terms of commonly used figures of merit, such as useful load and overall productivity.
Propulsive-lift aircraft are often discussed only in tme context of high thrust-to-weight ratios and very short field lengths; corsequently, other potential attributes of _ropulsive-lift aircraft, such as conclusions (2) and (3_ above, are less well understood.
3.2 Tilt-Rotor Aircraft Technology The technology associated with tilting prop-rotor aircraft has been focused in recent years on the development of a research aircraft, the XV-15 Tilt Rotor Research Aircraft (TRRA), following extensive ground-based technical programs. The TRRA has been developed by a team comprising Bell Helicopter, NASA, and the U.S. Army and Navy; this team has successfully brought two aircraft to flight status. Additional advanced technology on advanced tilt _otors and control systems is currently being undertaken by NASA, Boeing-Vertol, and Bell. It is anticipated that the ongoing flight programs will provide the technological confidence necessary to the construction of larger aircraft and to the definition of the various operating modes that fall within the broad flight envelope of this type of aircraft.
The XV-15-TRRA _Fig. 8) is powered by two Lycoming T-53 turboshaft engines that drive three-bladed metal rotors that are 25 ft in diameter, have a blade twist of 45 °, and are gimbal-nw)unted to the hub with ......... AIRPLANE MODE .... TILT ROTOR MODE ........ ; HEUCOPTER MOOE ___; Figure 8. XV-15 tilt-rotor aircraft in airplane, tilt-rotor, and hel.icopter configurations.
10-5 an elastomeric spring for flapping restraint. At the VTOL design gross weight of 13,000 Ib, wing loading is 77 Ib/ft 2 and disk loading is 13 Ib/ft 2. The two aircraft have accumulated approximately IC9 flig-t hours. Following proof-of-concept, the program will move into research directed toward establishing the suitability of this kind of aircraft to various applications, The XV-15 research flight envelope is presented in Fig. g. The envelope, presented in terms of density-altitude as a function of true airspeed, is for level flight, with the predicted envelope as s_own and with flight conditions superimposed on the pre- dicted envelope. Based on flight results to date, o HELICOPTER MODE the demonstrated flight envelope is expected to z_ TILT ROTOR MODE NORMAL RATED coincide with the predicted envelope. Flight hover- o AIRPLANE MODE ing performance agrees well with predicted perfor- mance, and sideward flights and rearward flights to
POWER L'M'T
25 krots _ave been conducted with favorable results.
As originally expected, one-engine-cut hovering PREDICTED flight is possible at lowe_ gross weights while in AEROE LASTIC STABILITY ground effect. As show_ in Fig. 9, a maximum .20 _ BOUNDARY cruise airspeed of 300 knots has been demonstrated at aaout 15,500 #t density-altitude. The XV-15 flight envelope is large; for example, it encom- passes the combined envelopes of the UH-1H Iriquois helicopte r and the OV-ID Mohawk fixed-wing aircraft.
_o
Historically there has been a concern regard- _ _ _;z_ c_" /
/
ing the tilt-rotor aircraft concept with respect to aeroelastic rotor stability during high-speed cruise flight. Figure 9 shows the location of the 100 200 300 4170 predicted limiting rotor stability boundary for TRUE AIRSPEED. knott the XV-15. Flight results up to the maximum level- flight airspeed show that damping ratios are equal Figure 9. XV-15 flight envelope: altitude tO Or higher than predicted levels. Stability vs airspeed.
measurements have been made in windmilling descents and over the cruise rpm range at normal power. No significant changes in stability levels have been observed. Further exploratior and documentation of aero- elastic characteristics, putting the aircraft into a shallow dive to attain higher airspeeds, are planned.
The cruise airspeed of the tilt-rotor aircraft concept is not limited by the stability boundary shown 'n Fig. 9. That boundary is for the XV-15 specifically with its disk loading of 13 Ib/ft 2 in its particular configuration (i.e., wing stiffness, rotor type, etc.). Higher disk loading or improved rotor tecnnology or both will move the stability boundary to higher airspeeds.
Rotor shaft horsepower as a function of calibrated airspeed is presented in Fig. 10 for C1) the heli- copter mode (nacelle angle of 90°), (2} two tilt-rotor modes (nacelle angles of 30 ° and 60°), and (3) zhe airplane mode (nacelle angle of 0=). Also shown is the single-engine-maximum power line for t_is research aircraft. The speed-power curve for the helicopter mode is typical of helicopters, and the curve for the airplane mode is typical for fixed-_ing t_rboprop aircraft. The buckets of the helicopter mode speed- power curve and the airplane-mode curve (and all tilt-rotor mode curves) occur at about the same power, that is, at about 920 rotor s_aft _orsepower per rotor. "_is phenomenom leads to an XV-15 total config- urational minimum power envelope that is essentially flat from 50 to 170 knots calibrated airspeed. Thus there exists about a 120-knot range in airspeed that can be flown at minimum power by using the nacelle angle appropriate for a desired airspeed.
O HELICOPTER MODE VTRU E = A TILT ROTOR MODE 300 knotz AIRPLANE MODE @ 15.500 ft NACELLE ANGLE = 90 deg 60 30 . _'_ 0 2OOO n- wJ _ _ /_/ l-1 SINGLE I-1 ENGINE MAXIMUM -I- 0 r] POWER
12oo
,.r
-..<
o 8OO MINIMUM POWER ENVELOPE i-- 4OO I I I I ,I f 40 S0 120 160 200 240 CALIBRATED AIRSPEED, knots Figure }0. XV-15 power envelope: rotor shaft horsepower vs airspeed.
10-6 Figure i0 has several implications. Flight at minimum fuel flow (about 800 lb/hr for the XV-15 research aircraft) occurs anywhere within the 120-knot-wide minimum power envelope. This characteristic is of interest for a wide variety of flight conditions; for example, it would provide flexibility and endurance for search and rescue missions and efficient near-termlnal operations. Othe_ favorable implica- tions pertai_ to such characteristics as noise levels, STOL performance, life of component parts, and single-engine conversion and reconversion in level flight. It may be of interest for some military mis- sions to note that the cruise speed of the tilt-rotor aircraft with one-engine inoperative is much higher than that of the typical helicopter under full power.
A closer look at the XV-15 _light modes is presented in Fig. 11. Nacelle angle may be variec from 95 ° (used for autorotation) to 0°, For research purposes two rates of conversion can be used - either 7.5 = or 1.7 ° of nacelle angle per second. Flight within the conversion corridor (tilt-rotor mode) is straight- forward. BecaJse the aircraft accelerates and decelerates well within the corridor, the pilot does not have to concern himself with a precise conversion sched_le. At present, the conversion corridor has a width of from 50 to 70 knots, and further expansion is possible.
q HELICOPTER = J4 AIRPLANE IL MODE I MODE [[_ TILT ROTOR _J MODE - I lOO _////////////////////.
\%. i,, 8o
eo
Z •-_ 40 _///A J VMIN NN z
v////_ IN DESIGN
_ ENVELOPE
, _///////////////////._
0 40 80 120 160 200 240 CALIBRATED AIRSPEED, knotl Figure 11. XV-15 conversion corridor: nacelle angle vs airspeed.
Testing has been performed in steady level flight to a line approximating an angle of attack of 6".
Flights nave been conducted beyond this line, as shown by the cross-hatched area in Fig. 11, to investl- gate the low-speea boundary of the conversion corridor. High power climbs have been performed at low speed, at nacelle angles between 50 ° and 65 °. Wing stall boundaries nave been investigated at nacelle angles of 0 ° and 30°; stall speeds were determined to be 104 knots at 0 ° and 100 knots at 3D'. Stall char- acteristics were docile. It is expected that the corridor will De extended through the shaded region shown in Fig. 11. The anticipated high-speed boundary of the corridor is expected to be defined by power or infinite blade-life fatigue loads. The flap placard speed of 170 knots dictates the boundary at low nacelle angles.
Tilt-rotor aircraft technology is ready, at a reasonable risk, for application to a wide variety of potential military and civil aircraft. The technology is ready for application to aircraft design gross weights up to at least 30,000 lb (i.e., twice the XV-15 STOL gross weight of 15,000 lb) and possibly up tc 40,000 ]o.
It must be recognized, however, that the design of tilt-rotor aircraft requires close attention to vehicle dynamics. One fundamental 'potential problem is that as design gross weight increases, the fre- quencies of various pertinent rotor/nacelle/wing modes decrease and tend to merge together leaving less "design soace" between modes. For a given ]ow-disk loading, for example, there probably is a point at which increasing design gross weight will be difficult. Design studies, wind-tunnel experimental efforts, and piloted simulation investigations are needed to address the scaling of tilt-rotor aircraft to higher gross weights or to higher _isk loadings or both.
3.3 Turbofan V/STOL Technology The technology for turbofan V/STOL aircraft has also received considerable attention in recent years.
Several multiengine configurations were considered in conceptual design studies and extensive large-scale wind-tunnel investigations were conducted in the 196Ds and 1970s.
As a result of these efforts a more direct and simpler approach to turbofan V/STOL aesign is emerginG.
This approach attempts to minimize configuration complexity by reducing the numbers of engines and rotat- ing machinery, and by integrating the propulsion and control system into a separate module that can be adapted to different mission-specific aircraft. One design that incorporates these features is shown in Fig. 12. It was developed by Grumman and tested extensively at large-scale by NASA. The design I 10-7 HIGH BYPASS incorporates two tilting turbofan engines with con- trollable inlet guide vanes and a system of con- TURBOFAN trollable vanes in the engine exhaust flow.
Although this approach to the design of a turbofan V/STOL aircraft looks promising, a number of technical questions must be addressed. One important concern with any design incorporatin_ tilting nacelles is the possibility that premature inlet separation could impair performance during deceleration and descent. The induced aerodynamic effect of the large-diameter, high-velocity flow at various angles relative to the wing is also of concern. The effectiveness of the vanes in the exhaust in producing the necessary trim and maneuvering moments during transition and in hover also needs to be better understood. Finally (as I// _I/ / _ _" FAN EXHAUST is the case for all V/STOL aircraft) it is neces- sary that ground effects, the interaction between the airframe aerodynamics and the engine exhaust "flow, and exhaust reingestion be properly understood.
In view of the potential for this configuration Figure 12. V/STOL turbofan aircraft schematic.
to provide a simpler design for turbofan V/STOL air- craft of the future, a comprehensive investigation of its aerodynamic and propulsive characteristics has been undertaken as a joint effort between Grumman, _ASA, and the U.S. havy. The investigation utilized a full-scale model with TF-34 engines in NASA's static- test and wind-tunnel facilities, for the purpose of resolving the questions relating to possible aerody- namic and propulsive interactions. Figure 13 shows photographs of the model in the Ames 40- by 80-Foot Wind Tunnel and on the VTOL Aircraft Hover Research Facility.
-i !
40- bySO-f t WIND._I'VNNEL ....... HOVER FACILITY .............
Figure 13. V/STOL turbofan model in _O- by 80-Foot Wind Tunnel and hover facility.
Turning now to the specific technical concerns enumerated previously, the inlet airflow separation was first investigated and a flight boundary, in terms of nacelle angle versus airspeed, established from wind- tunnel data. Figure 14 shows the combination of nacelle angle and airspeed required to provide for a con- stant climb angle. Also shown is the inlet separation boundary giving an upper limit to the permitted nacelle angle as a function of airspeed; at 60 to 80 knots airspeed the maximum descent angle is y = -20 °.
The inlet separation boundary is quite conservative because the engine could undoubtedly have withstood much greater flow separation.
Forces induced on the model by the engine airflow can be inferred from the data in Fig. 15. A com- parison of the results for engine idling and a nacelle ang|e of 5 °, with the results at high-thrust coeffi- cient (C u - 3) and a nacelle angle of 40 °, snows that engine operation increased aerodynamic lift, had little effect on drag, but alteredpitching mount significantly. The lift on the nacelles when at large angles on the flow can be substantial. Apparently, the nacelle lift more than offsets any adverse effect of the jet on the flow over the wir.g, at least for nacelle angles up to 60 °.
The effectiveness of a control vane immersed in a turbofan airstream has been investigated, using a full-scale independent nacelle. The results from this study, with a nacelle on a complete airframe, are summarized in Fig. 16, which shows control power of the vanes in pitch at a nacelle angle Of 50 ©. The vane pitching moment contribution is linear until the vane is near maximum lift (at a deflection of 20°).
For the particular condition shown (_ = 4:, V® = 80 knots) approximately 5° of vane deflection was required for trim and ample control power remains for maneuvering. This was true of all transition flight condi- tions studied. Control power in hover out of ground effect (L/D = 5.07) and control power in ground effect (L/D = 1.15, almost wheel height) nave been determined and show that no degradation of control power occurs due to ground effect.
10-8 NACELLE INLET SEPARATION _ 80BOUNDARY, -_ = -10 deg// ,d_/////////////.
{ 4o
.
Z20 - 0 "v = +20 deg I I I I i O0 60 80 100 120 140 AIRSPEED, knot= _igure ]4. V/STOL transition corridor: nacelle angle vs airspeed.
C# C_ .,Q 2.4 r 2.0 " k-
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o 1.2- :/2 o ,0 3.,, ' _ A 20 1 49 / _ 5 0.77 _ .4 I" < / t I I I t I
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I & -,4 L _ 112 J ll.2 _4 J 0 4 8 16 20 0 ,4 .8 1.6 .4 0 -. -.8 ANGLE OF ATTACK, deg DRAG PITCHING COEFFICIENT MOMENT COEFFICIENT Figure 15, Lift, drag, and moment characteristics.
Ground effect does have an influence on lift during hover, however, and the magnitude of this effect will depend on the details of the configuration, and particularly on the placement of fuselage strakes.
Figure 17 shows the lift-to-thrust ratio as a function of neigh: above ground for two fuselage strake angles (15 ° and 45"). Ground effect is positive (lift/thrust > 1) for both strake angles and increases as the strake becomes more vertical. The spread in the data represents the variation in lift/thrust experienced over a 2-nin test period (due to the meandering of the fountain formed by the exhaust flow). As the strakes become more vertical the data spread caused by the meandering _low diminishes. These data are representa- tive of wind conditions up to 20 knots, Re-ingestion of high-temperature exhaust gas as a result of the short flow path to the inlet was minimal; this finding was confirmed by smoke-visualization studies.
In summary, as the result of an experimental program using a full-scale model of a turbofan V/STOL aircraft, a number of concerns regarding aerodynamic performance and control have been alleviated. For example: (1) evidently there exists an adequate transition corridor within which inlet flow separation can be avoided; (2) aerodynamic lift due to flow around the nacelle tends to offset the loss of lift that the nacelle produces on the wing; (3) control power produced by the exhaust vanes appears to be quite linear with va_e deflection and provides adequate control moment for maneuvering; (4) ground effect does produce a meandering fountain effect, but the lift/thrust ratio is not degraded in this condition; and (5) there is no evidence of high-temperature gas reingestion. All of the factors that inf]uence the performance and t I 10-9
-'IDt"-
2.0 1.16 / STRA_ 1.12
,1
CONTROL VANES IN FAN EXHAUST 1.08 i.- 1.04 1.00 15deg STRAKE L I I A I -1.2 .96 1 2 3 4 5 -30 -20 -10 0 10 20 30 CONTROL VANE ANGLE, de9 H/D Figure 16. Longitudina_ control effectiveness. Figure 17. Ground effect: lift/thrust vs height above ground.
stability characteristics of this type of aircraft have not yet been analyzed and the work is contln_ing.
Tests are planned for the 80- by _20-foot test section of the Ames Research Center's tunnel when that sec- tion becomes available in 1982.
4. CONCLUDING RZ_iARKS During the past lO years several technology development programs have been undertaken to demonstrate that various alternatives are available to fill the "versatility gap" that exists between the capabilities of CTCL transport aircraft and helicopters. There is little doubt that the technology is sufficiently developed for at least two new aircraft configurations - t_e upper-surface-blown STOL aircraft and the tilt-rotor VTOL aircraft - to permit their introduction in a military logistics role during the 1980s. A third-concept, the lift-cruise fan VTGL aircraft requires further technical development and demonstration; nevertheless, that development can probably be achieved by the early 1990s.
The logistics aircraft and missions that can become possible with new technology include the following: 1. Large powered-lift STOL transport aircraft capable of resupply over ranges in excess of 3,000 miles to landing zones having runways as short as 3,000 ft in length.
2. Medium cargo V/STOL rctorcraft capable of speeds o_ 350 mph and ranges up to 600 miles.
3. Small fixed-wing utility VTOL aircraft utilizing lift-cruise engines and capable of speeds of 500 mph and ranges up to 1,000 miles.
Perhaps the most significant step forward that must be made in the years ahead is the matching of new air-vehicle capaoi_ities to the evolving mission needs. It will not be sufficient to argue that STOL and VTOL aircraft cannot compete because they have reduced the range/payload efficiency. A thorough reassess- ment of the air-logistics needs of land and sea forces, in light of tne growing vulnerability of fixed military assets and in light of t_e growing confidence that the technology can provide a new generation of aircraft that will be both versatile and efficient, is necessary.
BIBLIOGRAPHY STOL Transport Technology Clay, C. W., "Technology for AII-Ne'_ Specialized Short-Haul Aircraft." Boeing Con_ercial Airplane Company, 60th Annual Meeting of Nationa_ Transportation Research Board, _ashington, D.C., Jan. 1981.
Cochrane, John A., "Conceptual Studies of a Long-Range Transport with an Upper Surface Blowing Propulsive Lift System." NASA TM-81196, May 1960.
Cochrane, J. A. and Boissevain, A. G., "Quiet Short-Haul Research Aircraft - Current Status and Future Plans." AIAA Paper 78-1468, Los Angeles, Calif., Aug. 1978.
10-10 McCracken, Robert C., "Quiet Short-Haul Research Aircraft Familiarization DocJment." NASA TM-81149, Nov. 1979.
qJeen S. and Cochrane, John A., "QSRA Joint Navy/NASA Sea Trials." AIAA Paper 81-0152, St. Louis, Miss., Jan. 1981.
Youth, Stan et al., "USB Propulsive-Lift Aircraft Design and Performance Evaluation Study." Report D340-I0108, The Boeing Commercial Airplane Company, Oct. 1980.
Tilt-Rotor Aircraft Technology Dugan, D: C., Ernart, R. G., and Schroers, L. G., "The XV-15 Tilt Rotor Research Aircraft." NASA TMo81244 (AVRADCOM TR 80-A-15), Sept. 1980.
Lawrence, W. S. and DuFresne, D. A., "The XV-15 - An Initial Navy Look." AIAA Paper 81-0155, Jan. 1981.
Schroers, L. G., "Initial Flight Test Results Compared to Design Predictions." AIAA Paper 80-0235, Jan. 1980.
Sc_roers, L. G., Dugan, D. C., Mart, R. L., and Erhart, R. G., "Operating Flexibility Exhibited in Flight Test of the XV-15 Tilt Rotor Research Aircraft." 36th Annual Forum of the American Helicopter Society, May 1980.
Wernicke, K. and Magee, J., "XV-15 F)ight Test Results Compared with Design Goals." AIAA Paper 79-1839, Aug. 1979.
Turbofan V/STOL Technology Betzina, Mark D. and Falarski, Michael D., "Aerodynamics of a Tilt-Nacelle V/STOL Propulsion System; V/STOL Aircraft Aerodynamics Workshop." Navy Workshop in Monterrey Navy Postgraduate School, May 1979.
Betzina, M. D. and Veta, R., "Aerodynamic Effects of an Attitude Control Vane on a Tilt-Nacelle V/STOL Propulsion System." AIAA Paper 79-1855, Aug. 1979.
Falarski, M. D., Dudley, M. R., Buchman, W., and Pisano, A., "Aerodynamic Characteristics of a Large-Scale Twin Tilt-Nacelle V/STOL Model." AIAA Paper 81-0150, Jan. 1981.
Grumman Aerospace Corp., "Large Scale Static Tests of a Tilt-Nacelle V/STOL Propulsion/Attitude Control System." NASA CR-152181, July 1978.
Kress, R. W., "Surface Combatant Fleet Offensive/Defensive Enhancement by High Performance Turbofan VTOL Aircraft." AIAA Paper 80-1881, Aug. 1980.
1 Ret:x3rt No 2. Government Acce_ion No. 3. Recipient's Catalog No.
NASA TM-81281
4, Tizle and Subtitle S. Report Date
RECENT PROGRESS IN V/STOL
6. Performing Organ,zation CoOe
AIRCRAFT TECHNOLOGY
7. Author{s) 8. Performing Or_nization Report No.
A-8512
L. Roberts, W. Deckert, and D. Hickey
10. Work Unit No.
9 Performing Organizat,on Name and Addre=
010-01-01
I1. Contract or Grant No.
Ames Research Center
Moffett Field, Calif. 94035
13. Type of Report and Period Covered 12. Spons_ing A_ncy Name and Addr_s
Technical Memorandum
National Aeronautics and Space Administration 14. Sponsoring Agency Code
Washington, D.C. 20546
5, S,Jpplementary Noles 18 AbstraCt
Recent results from wind-tunnel and flight-tests investigations for
V/STOL aircraft are reviewed. Primary emphasis is given to technical
results relating to three types of subsonic aircraft: a quiet STOL air-
craft; a tilt rotor aircraft, and a turbofan V/STOL aircraft. Comparison
and correlation between theoretical and experimental results, and between
wind-tunnel and flight-test results, is made. The quiet STOL aircraft tech-
nology results are primarily those derived from the NASA/Boeing Quiet Short
Haul Technology (QSRA) program. The QSRA aircraft uses an upper surface
blown flap and develops a usable engine-out landing approach lift coeffi-
cient of 5.5 and landing distances less than 1,000 ft. The tilt rotor air-
craft technology results are those obtained from the NASA/Army/Navy/Bell
(XV-15-TRRA) aircraft flight investigations. The TRRA is a twin rotor
research aircraft capable of vertical takeoff and landing and cruise speeds
of 300 knots. The turbofan V/STOL aircraft technology results are from
static ground facility and wind-tunnel investigations of a NASA/NAVY/Grumman
full-scale lift/cruise fan aircraft model, which features two tilting
nacelles with TF-34 engines.
18. Distribut|on Statement 17 Key Words {Suggested by Author(sl)
Unlimited
Quiet STOL aircraft
Tilt rotor aircraft
Turbofan V/STOL aircraft
STAR Category - 05
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Unclassified Unclassified _1 NO. Of Pa_s I 22 Dice"
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