V/STOL RESEARCH NEEDS
This document contains a draft list of V/STOL research needs as seen by the AIAA’s Aircraft Systems V/STOL Technical Committee. It serves to motivate interest and possible areas of participation in the development of Centers of Excellence for V/STOL aircraft.
The use of the helicopter over the past 50 years has shown the value of aircraft that do not need long runways. The success of the AV-8A and AV-8B Harriers attests to the need and acceptance of higher disc loading types of Vertical/Short Take Off and Landing (V/STOL) aircraft. The decision to built a STOVL variant of the JSF also attests to that need. On the other hand there are challenges to meeting that need. The switch from Externally Blown Flaps to more conventional slotted flaps on the C-17, with the attendant reduction in short take off and landing (STOL) performance, illustrates the conflict of varying mission objectives and budget constraints on obtaining that performance. The development of a vehicle system is mixed with a complex blend of technical, as well as political and economic issues. The present discussion focuses on the technical issues important for V/STOL aircraft research.
Recent experience with the C-17, F-22 and F-18 E/F aircraft demonstrates that development of modern/next generation aircraft is a challenging technical undertaking. The addition of V/STOL capability requirements increases the range of difficulty for development of a successful vehicle. While the general problems, flow fields and operating constraints of most V/STOL types are understood, there are areas where the data base and design procedures are inadequate or poorly defined.
Traditionally NACA/NASA and the military services have played a large role in fundamental studies relative to V/STOL aircraft. Unfortunately today, while wind tunnel and simulator facilities are available from NASA, fundamental research in the areas of powered lift aerodynamics, propulsion, structures, test techniques, etc. have been dropped. Neither the Government nor University communities are structured to adequately undertake the needed fundamental research. Currently the responsibility has devolved to the aircraft developer to solve the problems unique to his particular aircraft by ad-hoc means.
The engineering technical community recognizes the need for a more systematic approach to the solution of the fundamental research needs. To deal with these needs, the AIAA V/STOL Technical Committee is attempting to establish at least one and possibly several university 'Centers of Excellence' (CoE). These CoE's will systematically undertake the more fundamental research topics and to provide graduating engineers with the needed skills to advance the state of the art in designing, developing, and maintaining V/STOL type vehicles.
The following sections identify the areas in need of fundamental research and highlight the work that needs to be done. V/STOL design issues can be addressed with computational fluid dynamics (CFD), empirical, analytical, and in particular experimental tools with the inherent advantages and disadvantages of each. Many of the sections listed below will include both experimental and computational activity.
The jets issuing from the aircraft induce suction pressures on the lower surface of the aircraft that subtract from the total lift being generated by the jets. These jet-induced effects depend on the number and arrangement of the jets, the jet geometry, the jet nozzle pressure ratio, the jet efflux characteristics, the disposition of the surface area surrounding the jets, and the operating mode of the aircraft (i.e. hover, transition, STO, VL, etc.).
When hovering in ground effect the flow from the impinging jets form a wall jet on the ground that spreads radially from the impingement point. The entrainment action of this wall jet, in addition to the jet flow prior to impingement, lowers the pressure under the aircraft; thus inducing a suckdown or lift loss. For multiple jets, an upward flow is created at the point where the wall jets meet, forming a fountain flow, which tends to impinge on the aircraft lower surface and reduce suckdown.
In transition out of ground effect, the jets are deflected aft by the interaction with the free stream and roll up onto vortex pairs. These vortex pairs, along with the entrainment and blockage effect of the jets induce suction pressures beside and aft of the jets. As in hover, these effects are highly configuration dependent. There are also effects of jet nozzle pressure, jet shape, and jet flow quality that are not adequately understood.
In STOL operation both the hover and transition effects are present but modified by the effect of crossflow and ground proximity. A ground vortex is formed ahead of the jet arrangement by the effect of the free stream in rolling the forward flowing wall jet back on itself. In addition there is an effect of the crossflow on the fountain type flow. There are, at present, limited data on these effects.
Most of the data on the above effects are for sonic or subsonic jets. There is some data that shows significant effects of the shocks formed in higher pressure ratio jets but these are poorly understood.
Additionally most of the data to date have been obtained with relatively uniform laboratory sonic or subsonic jets. Little is known about the effects of distortion and swirl in the jet. There is also little data on the jet efflux characteristics from full-scale engines and nozzles and on what jet characteristics must be modeled.
Flying qualities for a V/STOL aircraft need to be examined in both the low speed propulsive lift flight regime and in the conventional up- and away-flight regime. A V/STOL research program will involve the definition of an integrated flight/propulsion control (IFPC) system for low speed propulsive lift flight. The control system might employ a command mode for attitude, flight path angle, and flight path acceleration during transition, and translational velocity commands for hover and vertical landing.
Piloted flight simulation of these integrated control designs is conducted using a ground based simulator, such as the NASA Ames Research Center's Vertical Motion simulator. Flying qualities are assessed over the low speed flight envelope to document control power usage and system dynamics characteristics. These usage and characteristics along with V/STOL operational requirements are used as a basis for defining criteria for the V/STOL controls. Inherent to these studies is the definition of cockpit layout, inceptors and displays, response types, and control effectors. Not only do all these affect the pilot/machine interface, but also need to be considered for autonomous or ground based flight.
On selected configurations, Level 1 flying qualities have been achieved using translational velocity command control system concepts for low-speed operations over a wide range of wind, atmospheric turbulence, visibility, and sea conditions. Maximum control usage and thrust response may be established for each of the operational conditions. Control power criteria may be defined and compared with existing specifications where they are available. Other factors include: thrust margin in ground effect with hot gas ingestion; thrust transfer rates for attitude control; dynamic thrust response; and flight path control envelope during transition. Successful control system development is currently more an art than a predictable science. Each aircraft requires development of an accurate aerodynamic and propulsion database, which is then used to integrate a satisfactory control system.
Flight and ground-based research on jet powered V/STOL
aircraft has shown that engine inlet ingestion of exhaust gases, or air heated
by exhaust gasses (hot gas ingestion - HGI), can occur during both vertical
takeoff and vertical landing, the latter in particular. HGI can also occur in STOL operation at low
speeds and in ground effect. This
ingestion may cause increased inlet gas temperatures and/or flow
distortion. Assuming an engine is
operating at its maximum rated temperature, an inlet temperature rise of 40°F
will cause about a 15 percent thrust loss.
In addition, spatial or temporal temperature distortion can cause compressor
stall, which would cause abrupt and catastrophic thrust loss during the
critical vertical flight phase.
Considerable research was done in the 1960's and 70's on many V/STOL
aircraft configurations and recently on the several Joint Strike Fighter configurations. The results of these investigations have provided only a general
understanding of the causes and some techniques for alleviation.
The Harrier STOVL aircraft flowfields may cause hot gas ingestion (HGI).
Flows from the multiple lift-jets create the opportunity for hot gas ingestion. As shown in Figure 5-1, these flows are usually considered as the source of near-field, intermediate-field, or far-field recirculation of the exhaust gases. In ground effect, the exhaust efflux impinges on the ground and is deflected to form wall jets. When two or more of these wall jets flow into each other they form a fountain which flows upward. Often a part of the fountain flow impinges on the lower surface of the fuselage. The impinging gases may flow along the surface and, often, into an engine inlet in addition to heating of the aircraft structure. Figure 5-1 shows an example of near field hot gas ingestion which is usually severe enough to create both an increased mean temperature and a large inlet flow temperature distortion.
The portion of the wall jet that flows away from the aircraft (Figure 5-1) is usually a smaller concern because these gases will cool rapidly. However, these flows rise due to buoyancy and head or cross winds can deflect them back to the engine inlets at hot enough temperatures to cause problems as an intermediate-field flow. Also in STOL operation, the forward flowing wall jet is rolled back on itself by the free stream to form a ground vortex. If the speed is high enough, this ground vortex is pushed aft of the inlet and HGI is avoided. However, at lower speeds the ground vortex can serve to carry heated air back to the vicinity of the inlet.
It has been found that these effects are very configuration dependent and strongly influenced by the location of the engine inlets and exhaust nozzles, nozzle deflection pitch and splay angles as well as the location of wing and tail surfaces. A useful technique for reduction of HGI is the use of lift improvement devices (LID). The LID is a group of strakes, either physical material or jet flow, on the bottom of the fuselage which trap and redirect the impinging fountain flow; this can both increase lift and protect the engine inlets from the hot gases. The LID may also be used to protect the stores from elevated temperatures. Research into new LID configurations and other methods to alleviate HGI are areas for continued research.
The Joint Strike Fighter (JSF) airplane uses the F119 engine, which is capable of very high nozzle pressure ratios (NPR). The use of convergent only, or convergent/divergent nozzles will result in operational conditions where there are complex supersonic shock/expansion flows. Many experimental investigations have shown that these flow conditions generate high noise and the potential for structural interaction with large acoustic fatigue loads in up- and away flight. During vertical landing there is an additional interaction between the jet plume, aircraft structure, and the ground. This interaction involves high amplitude peaks that are related to the jet screech phenomenon. Surface temperature distribution data show the presence of elevated temperatures in regions of high turbulence intensity. At selected combinations of NPR and ground height, resonant conditions have been found which produce amplified, non-linear loads. These loads may damage either the aircraft structure and/or the stores. Research is needed to understand the jet induced noise environment.
Selected mission scenarios are used to identify aircraft requirements. This process is iterative and is very dependent on the problem characterization. In addition to examining aircraft and operational sensitivities, the effect of varying scenario events must be examined. These iterations use computer codes such as the NASA developed ACSYNT (AirCraft SYNthesis) code to examine the interactions between potential missions and the aircraft design trades, including performance. Judgment is needed to identify practical aircraft configurations and mission constraints.
Typically, the aircraft type and the selected propulsion concept drive the V/STOL aircraft shape. The aircraft configuration is determined primarily by its design missions. Specific aircraft missions and the resultant operational parameters combine to determine configuration sensitivity to design trade-offs. Some of the design factors include: weight, cost, range, acceleration, maximum speed, payload, maneuverability, signature, integrated aircraft/propulsion controls, and configuration layout. Improving system trades and performance analysis early in the design process should lead to improved and less costly designs.
Current design practices place an even greater emphasis on sensitivity analysis. The Joint Strike Fighter Program is one example in which cost is an independent variable. This means mission performance is now a goal instead of a fixed requirement which can now be traded off versus life-cycle or unit fly-away cost. Rapid & accurate sensitivity analysis is necessary to develop aircraft in this new design environment.
The crucial trade between cost/weight and mission performance determines the suitability of an aircraft concept for a particular V/STOL application. Inherent in this trade is the integration of geometry for both up- and away-performance and for V/STOL operations. To achieve the most suitable configuration, the designer works with many design features such as engine location/size, jet arrangement, wing planform, horizontal control surface location, planform, and area. Integrating the results from various V/STOL component tests into one coherent database for design trades is another area requiring investigation.
Future V/STOL combat aircraft will likely use very high pressure, high temperature jets directed downward onto the ground. The impingement of this exhaust on the takeoff and landing surface can cause serious damage to some landing area materials. Tests that identify how much temperature and pressure affect the various types of surface materials can withstand are needed. These data should define the surface failure mechanism and the debris that is blown off.
In over-water operation (as in approaching a shipboard landing) altitude must be maintained to stay above the spray. And how high is the spray? What altitude must be maintained to stay above the spray? Studies are needed to determine the conditions where spray is generated. Techniques that can be used to minimize damage to the landing surface and/or protect the aircraft and ground personnel and equipment from spray and/or debris should be studied.
The integration of the propulsion system is the key to powered lift systems. In recent years advances have been made in engine technology, which could contribute to improve V/STOL vehicles. In particular, considerable progress has been made in increasing the thrust to weight ratio of power plants and this effort should be continued. Control of V/STOL aircraft usually involves modulating the thrust, changing nozzle area and extracting bleed air. Thus control of the engine becomes an integral part of the aircraft control system and the aircraft and engine control systems must be integrated. This coupling requires coordination between the propulsion and flying qualities specialists.
Inlets designed for high speed flight will experience internal separation in hover or at very low speeds with attendant high thrust losses. Blow in doors, sliding inlets to open a blowing slot, etc. help reduce the thrust loss. In some cases, a secondary auxiliary inlet is needed at low speeds to capture enough air for optimum engine performance. These problems are often configuration dependent but some generic work is needed to develop a database on the effects of various approaches.
Vectoring nozzles can be used to turn the flow from horizontal for cruise to vertical or near vertical for low speed flight or hover. Work is needed to minimize the adverse effects of internal flow disturbances on the nozzle coefficients and to minimize the weight and complexity of these nozzles.
Some configurations require ducting of hot, high pressure gas (compressor bleed or engine exhaust) from the engine to remote nozzles for trim or control. The local flow separation and the losses in long duct runs, corners, nozzles, etc. must be minimized.
In some concepts an extra turbine stage is added to the engine so that shaft power can be extracted to drive a remote fan. This approach also requires improved high speed shaft technology such as a gearbox to match the required speeds, and a clutch to engage and disengage the fan from the engine. Additionally, inlet flows and associated ram drag from these vertically, or near vertically, mounted remote fans needs to be addressed during the transition phase between jet- and wing-borne flight. Continued work is required on all of these elements of the system to maximize performance, minimize weight, and improve reliability.
For single engine V/STOL aircraft, engine reliability is crucial. Research that emphasizes engine reliability is directly applicable to vehicle viability. Prognostic and health management techniques are also being developed to improve propulsion system reliability and maintainability. Multi-engine V/STOL aircraft are usually designed with the intention of being able to maintain flight in case of the failure of any one engine. An emergency rating for these engines is proposed as a way to minimize the propulsion system weight. This approach raises questions such as:
- How much overload should be designed for?
- Must the engines be removed and inspected after every use of the emergency rating?
- What damage to the engine (if any) can be permitted?
- How many times can the overload rating be used?
V/STOL research makes use of small-scale model tests to more economically obtain experimental information. There are many test techniques used to obtain experimental information. There are many details associated with the test techniques used to obtain valid data that accurately represent the full-scale airplane. Research issues include airframe modeling, jet simulation, hover and wind tunnel test environments.
In the 1990 AIAA Aerospace Engineering Conference there were two sessions which presented experiences from several aircraft development programs. The conclusions from these papers led to a fairly consistent consensus:
• Small-scale testing is less expensive, enables rapid model changes, very high run rates, and generally provides results that can be interpreted with confidence. These tests are especially effective for most configuration development activities.
• V/STOL, rotors, and model tunnels appear to have no significant scale effects between 1/5 scale and full scale except possibly for V/STOL inlet effects and hot gas ingestion. The conclusion for 1/10 to 1/5-scale models is less certain.
• Full-scale tests provide the needed verification of selected small-scale results as well as confidence in the results and good public relations photographs.
Recent test experiences, however, have shown that research is needed to identify which engine jet characteristics are required for adequate simulation in a model. Critical jet features include model plenum contraction, internal flow separation, flow quality upstream of the jet nozzle, exit pressure profile, efflux turbulence, pressure and temperature decay. Both transition and hover lift loss out of ground effect are especially sensitive to jet decay. Even full-scale testing can be misleading as the jet efflux for the operational jet is not known or fully developed, accurate calibration of the propulsion system can be difficult, model support effects are not necessarily known, etc. A more complete understanding of the influence of all of these and perhaps other factors is needed to support future aircraft development programs.
For short takeoff and landing the relative motion between the aircraft and the ground must be properly represented in the wind tunnel. Test results obtained using moving models are not always consistent with those obtained in wind tunnels using a fixed model and a moving belt ground plane. There are data available which illustrate this problem but no clear solutions are currently identified.
There are no facilities available in this country suitable for investigating the lift loss, HGI, and ground vortex flow fields generated in STOL operation by the high pressure ratio, high temperature jets of advanced V/STOL aircraft. The available facilities use fabric belts that cannot withstand the temperature involved and distort under the impingement of these high pressure jets. A new facility is needed.
The effects of wind tunnel or hover test cell size relative to the model size have received significant research effort. Some correction techniques have been developed but there is not a consistent use or acceptance of either the methods or of the importance of them. This should be resolved and industry standards should be established. Included in this issue are the effects of model support systems, the use of metric or non-metric propulsion systems, propulsion inlet effects, and other modeling technique issues.
The take-off and landing environment is harsh on a V/STOL aircraft. The propulsion system will generate a lot of heat and noise. This heat and noise can affect areas of the airframe not usually exposed to an adverse environment during traditional up-and-away flight. New or improved airframe materials may be needed in critical undersurface areas of the V/STOL aircraft that can accommodate the higher temperature and acoustics, without greatly increasing the weight or performance characteristics of the airplane.
The special logistical requirements imposed by V/STOL aircraft when operating from a conventional base, from austere sites, or from on board ships need to be defined. The logistic needs must be minimized in the design of the aircraft and its support equipment. Supply problems become most severe for operations from austere sites or from small ships. How fuel, armaments, and stores are brought to a remote site or small ship and then loaded onto the aircraft deserve study. Safety (fire) and security precautions must be accounted for. Maintenance done on the small ship or remote site should be minimized or deferred until the aircraft can get back to a main base. Special features may need to be designed into the aircraft to accommodate these issues.
A unique application of V/STOL concepts is the use of the ground loiter concept for support of forward troops. The practical operation support issues are a fertile study area. These issues include identification of forward area ground support, refueling, security and aircraft operational procedures.
There are V/STOL unique effects on personnel during ground or hover operations, such as noise and jet flow temperature and velocity. These effects need to be addressed for high performance V/STOL aircraft and associated propulsion systems.
Unmanned Aerial Vehicles (UAV) are being used or proposed for many missions including some using V/STOL capability. Successful operations will depend upon not only all the normal design and operational issues of a manned V/STOL aircraft, but also upon the systems that provide for ground based or autonomous operations.
When the pilot is taken out of the loop, non-conventional configurations become more feasible, such as Vertical Attitude Take Off and Landing (VATOL) concepts. Issues involving high angle of attack low-speed flight, inlet distortion, and landing gear configurations are some of the issues that will need investigating for this concept.
For autonomous operations, much research will be needed in order to make up for the lack of a pilot in the mission environment. Smart and adaptive control algorithms will be needed for rapid responses to threats and/or loss of signal. Sensing and computing requirements will remain a high priority, as will the rapid transfer of data between the vehicle, other aircraft, and the ground control station.
The above is written from the background of operations of the Harrier and the development of the V/STOL versions of the Joint Strike Fighter. It therefore applies to fighter/ground-support type aircraft. It is anticipated that V/STOL or SSTOL (super STOL) transports, probably using Externally Blown Flap (USB) or Upper Surface Blowing (USB) concepts, will be needed in the future. Also other uses, such as V/STOL aircraft for business use (many companies use both helicopters and executive jets) will probably develop. Much of the research done in response to the above write-ups will apply, but there will be special needs that must be addressed as the need for other types become apparent.
The research and development done on V/STOL aircraft over the last 40 years has provided a good general understanding of the flow fields, operational techniques, etc., of most V/STOL concepts. However many of the problem areas are configuration sensitive and the database and analytical techniques needed for orderly design are not fully in hand. Many problems encountered in development are solved by ad hoc methods or are avoided all together during operations. Both of these techniques offer higher risk to the aircraft development that can be translated directly into lost performance, time, or money.
Analytical tools such as CFD can be powerful design aids in the development process, but they must have an adequate database behind them to verify their validity. Experiments are becoming more expensive, and difficult to schedule, so improved experimental planning, development and techniques are also becoming important. Information Technology is also developing into a useful tool, but it will only provide limited help if the information is not available in the first place.
The above sections identified are areas where systematic work is needed to provide a solid footing for improved V/STOL design procedures. There is plenty of work to do.
If you have any comments about the CoE, please contact:
Doug Wardwell
NASA Ames Research Center
MS 237-2
Moffett Field, CA
94035-1000
Tel: (650) 604-6566
Fax: (650) 604-6990
dwardwell@arc.nasa.gov