Full scale subsonic wind tunnel requirements and design studies

section, and the addition of a new 80- by 120-foot low-

con- 40- by 80-foot wind tunnel. The modifications being sidered are repowering the wind tunnel to provide a test speed of 300 knots in the existing 40- by 80-foot test section, and the addition of a new 80- by 120-foot low- speed test section.

FULL-SCALE WIND TUNNEL DESIGN STUDIES Aircraft Size and Speed Trends Relating to Facility Requirements increasing Conventional aircraft.- Over the years the demands of mission requirements and economics have dictated a long term trend toward larger aircraft. A result of this is that the existing full-scale wind tunnels are no longer As shown by capable of testing most operational aircraft.

figure 14, even modern fighter aircraft tax the capability of these facilities. As a result of this, current research and development tests for the F-14 and F-15 fighter aircraft are being performed with 3/4-scale models rather than full- get scale test vehicles; and the use of such models will not structures, propulsion, at the important interface between and aerodynamics.

Rotorcraft.- Figure 15 shows the variation of rotor and payload for diameter or aircraft span with gross weight tilt rotor aircraft.

single rotor compound helicopters and on These variations result from fairly well-defined limits rotor disk loading and rotor weight. Therefore, the trends shown on figure 15 can be expected to be valid into the be foreseeable future. The largest diameter rotor which can tested in the 40- by 80-foot wind tunnel is about 60 ft., and, for this size rotor, the tests are limited by wind of the flow to low wake angle con- tunnel wall constraint ditions (i.e., high speeds and low lift coefficients).

with this limitation on aircraft span, Figure 15 shows that, far too small to test full- the 40- by 80-foot wind tunnel is scale transport rotorcraft, and is capable of testing only the smaller utility and tactical aircraft.

Figure 16 shows the increase in the rotorcraft speed records with time, and demonstrates the feasibility of op- erating advanced rotorcraft up to speeds of the order of 300 knots. However, it should be recognized that the more recent high speed records have been achieved only with small experimental rotorcraft having extremely limited flight envelopes. There is as yet no operational rotary wing air- craft capable of flight speeds over 200 knots. In contrast, the maximum speed of the 40- by 80-foot wind tunnel is only 200 knots. This deficiency in facility speed capability is analogous to that which existed for transonic facilities in the forties which prompted the construction of the X series of high-speed research aircraft.

The economics of transport missions dictates higher flight speeds than are available from current helicopters.

Even for relatively modest stage lengths, speeds of 250 to 350 knots are required for economic operation. As discussed previously the most serious technical risks for high-speed rotorcraft are rotor dynamic stability and vibratory loads in high speed flight. The magnitude of these problems can be expected to increase at least with the flight velocity squared. Therefore, the increase in rotorcraft speeds from the current 200-knot level to the 300-knot level can be ex- pected to more than double the rotor dynamic stability and vibration problem.

In summary, the long-term trends in size and speed requirements for advanced rotorcraft indicate a need for vehicles having rotor diameters or spans of up to 100 ft., and capable of flight speeds of 300 to 350 knots. A full- scale wind tunnel capable of testing these rotor systems would require a test section size of at least 60- by 120- feet and a speed of at least 300 knots.

High-disk loading V/STOL aircraft.- Figure 17 (from reference 13) shows the typical variation of aircraft span with gross weight and payload for a variety of high-disk loading V/STOL and STOL transport aircraft concepts. In general, STOL aircraft concepts for which the wing carries a major share of the weight (such as the externally blown flap and the augmentor wing) are near the upper bound of the shaded area on figure 17, while concepts for which the propulsion system carries the major share of the weight (such as lift fan aircraft) are near the lower bound of the shaded area. While the size trends shown on figure 17 for high disk loading V/STOL aircraft are not as well defined as those shown on figure 15 for rotorcraft, they nevertheless indicate that aircraft having wing spans from 60 to 100 ft. will be required to perform the transport missions envisioned for these aircraft.

The primary technical problems for these aircraft are in the low-speed flight range (up to about 150 knots) where there is strong interference between the flow through the propulsion system and that over the airframe. Under these conditions the aircraft wake is deflected through a large angle and the flow constraint effects of the wind tunnel walls become the limit- ing factor in determining the wind tunnel size requirements.

On the other hand, in high speed flight these aircraft are more or less conventional in their operation, and require no special test requirements other than those used for cruise flight of conventional aircraft.

Therefore, the test require- ments for most high-disk loading V/STOL aircraft can be met in a wind tunnel with a maximum speed capability of 150 knots.

However, the effects of the constraint of the flow become increasingly serious as the speed is reduced (and the wake angle is correspondingly increased).

Therefore, these air- craft dictate the size requirements of the proposed facility.

Facility Size and Speed Requirements The aircraft size and speed trends discussed in the pre- ceding paragraphs are summarized on figure 18 along with the approximate test section widths required to accommodate tests of these aircraft.

At the lower speeds, corresponding to the transition flight regime of V/STOL aircraft, the size of the test section increases rapidly as the flight speed decreases.

This increase in size is required to alleviate the growth of wind-tunnel wall constraint effects with increasing wake angle as the speed is reduced. These test-section width require- ments are shown as approximate areas rather than definitive lines since they are dependent on a number of factors. How- ever to conduct tests of large V/STOL aircraft having wing spans of 100 ft. at speeds of 50 knots and less, a test

Next: section width →

section width

section width of 200 feet is indicated. On the other hand, at the high speeds and low lifts corresponding to the regime of prime interest for advanced rotorcraft, a test-section width of at least 120 ft. would be required.

Wind Tunnel Configuration Studies A number of wind tunnel configurations have been studied to determine the best compromise between the conflicting speed and size requirements discussed in the preceding section.

Some of these designs are shown on figure 19.

Broadly speak- ing, they included a simple closed-return design similar to the 40- by 80-foot wind tunnel but scaled up to a test section size of 75- by 150-foot and a speed of 300 knots, an open- return wind tunnel having the same size and speed character- istics, and a number of two test-section designs. These studies showed that the cost of the structure in a wind tunnel of this size was the most important element of the cost, and this fact focused attention on the open-return wind tunnel designs which minimize the amount of structure.

The main disadvantage of an open-return wind tunnel is that the flow in the test section is not isolated from the effects of external winds. Thus, the flow quality in the test section may vary from day to day unless special atten- tion is paid to the design of the inlet and exhaust sections of the wind tunnel. A number of experimental investigations have been conducted to develop inlet and exhaust sections which will ensure satisfactory flow quality in an open return wind tunnel. The investigations conducted at the Ames Research Center are reported in reference 15, wherein it is concluded that satisfactory flow quality can be achieved in the test section of an open return wind tunnel under nearly all weather conditions prevailing in a moderate environment such as exists at the Ames Research Center.

The main advantages of the open-return-wind tunnel are: (1) it provides the minimum structural cost, and (2) it does not require a purging system to eliminate engine exhaust gases from the wind tunnel air flow. This latter point is particularly important for the full-scale wind tunnel since it is intended to operate V/STOL aircraft propulsion systems in it. The power required for an open-return wind tunnel is about the same as that of a closed return wind tunnel, since the kinetic energy lost at the exit of the open-return wind tunnel is about equivalent to the energy lost in the corners of a closed return wind tunnel.

In summary, the cost of an open-return wind tunnel will be less than that of a closed return wind tunnel provided that the cost of the inlet and exit treatment required for satisfactory flow quality in the open-return design is less than the cost of the return circuit, the heat exchanger, and the exhaust gas purging system in the closed-return wind tunnel. The design studies and wind tunnel experiments con- ducted to date show that satisfactory flow quality can be achieved in an open-return wind tunnel with relatively economical treatment of the inlet and exit sections of the facility.

The concept selected for the proposed full-scale sub- sonic wind tunnel is shown on figure 20. This is an open- return wind tunnel having two test sections driven by a common power section. The particular combinations of test section size and speed were selected to give the best compromise between the conflicting test requirements for high-disk loading and low-disk loading V/STOL aircraft discussed in the preceding section of this paper. This design provides the maximum test capability for the minimum cost. It also provides for high utilization of the facility since tests can be conducted in one test section while the other test section is being prepared. Finally, it provides an advantage in budgeting for the facility in that the con- struction can be phased if necessary, with one test section brought into operation initially and the second test section added at some later date.

ECONOMIC CONSIDERATIONS The overall judgement that must be made in considering the proposed facility is whether the research and develop- ment value of the work performed in this facility will justify its cost. Since this facility should have an oper- ational life of at least 25 years, it is obviously impossible to make a detailed and specific cost-benefit analysis. How- ever, it is possible to arrive at a reasonable perspective on the value of the proposed facility by considering two factors: (a) the estimated cost of future aircraft programs and the possible contributions of the full-scale wind tunnel in reducing these costs; and (b) the experience of the existing full-scale wind tunnels.

Costs of Typical Aircraft Programs The prime justification for the proposed facility is that the cost of the facility ismreasonable in terms of the cost of the aircraft programs which it would support, and in terms of the savings that can be realized for these programs over the costs that would be incurred in the ab- sence of the facility. This assessment is difficult to make. However, some perspective on this can be realized by considering the costs of typical aircraft programs such as are described in reference 16.

Figure 21 compares the funding schedule for a typical production aircraft program (from reference 16) with the cost of the proposed facility. Figure 21 shows that the first major input from tests in the facility occurs prior to flight test when both the level of funding and the rate of increase of funding are low. These tests typically would involve full-scale demonstrator hardware of the high-risk items (e.g. rotors, fans, engines) installed in inexpensive "boiler plate" airframe mockups. Data obtained at this early stage of the program has extremely high leverage on program costs, since the funds committed are still low.

The second period of major influence of the facility shown on figure 21 corresponds to the early flight test stage.

If unanticipated problems are encountered in flight, the aircraft can be returned to the full-scale wind tunnel for rapid and safe exploration of these problems. The alter- native to this is continued flight tests with the accompany- ing risks to the aircraft and pilot. By the time flight tests can resolve major discrepancies the cost of the pro- gram exceeds that of the facility. In addition, by that time commitments have been made such that the cost incurred by changing the design may be as much as the funds expended.

Thus, the cost of the proposed facility should be viewed as an insurance premium which reduces the risk of potential losses in advance technology programs to acceptable levels.

It is quite likely that, without the assurance provided by early tests of critical components in the proposed facility, many advanced aircraft programs (e.g., high-speed rotorcraft and V/STOL aircraft) will not be initiated due to excessive technical and financial risk.

Experience with Existing Full-Scale Wind Tunnels NASA has operated full-scale wind tunnels since the late 1920's, and there is considerable historical evidence of the value of this type of facility. The first NASA full-scale wind tunnel (the 20-foot Propeller Research Wind Tunnel) showed the absolute necessity of using cowls for radial engines and variable pitch propellers for high performance aircraft. The incorporation of these features provided the first high performance transports which launched the era of practical commercial air transportation. Similarly, the application of this research to fighter aircraft culminated in the high-performance radial engine fighters of World War II fame.

While the prime justification for both the 30- by 60- and the 40- by 80-foot wind tunnels was for drag reduction studies of military aircraft, the major contributions of these facilities have been in technological areas which were not anticipated at the time the facilities were planned.

The contributions of these facilities to the research and development of V/STOL aircraft is an example of this. The itself by 40- by 80-foot wind tunnel has more than paid for preventing failures of experimental V/STOL aircraft in flight. These aircraft encountered failures during tests in the 40- by 80-foot wind tunnel which could have been catastrophic in flight. All of these failures involved the complicated interface between aerodynamics, dynamics, and structures. Therefore, tests of the full-scale hardware were the only way that these problems could have been dis- covered. For example, the XV-1 compound helicopter encoun- tests tered a rotor speed instability during the wind tunnel which required changes to the rotor control system. Tests of the XV-3 in the 40- by 80-foot wind tunnel were requested after a catastrophic rotor-pylon whirl instability was en- countered in flight, resulting in loss of the aircraft and serious injury to the pilot. After two tests in the 40- by 80-foot wind tunnel, separated by a one-year analysis effort, this stability problem was alleviated so that a highly successful flight research program could be completed. As a result, the tilt rotor aircraft is considered today to be one of the more promising high performance rotary wing air- craft concepts. The first wind tunnel test of the XH-51 rigid-rotor helicopter ended in a break-up of the rotor due to a bonding failure. The rotor'blade was redesigned, a successful wind tunnel test was completed, and the XH-51 went on to a highly successful flight research program which culminated in a rotorcraft speed record. During wind tunnel tests of the XV-5A lift-fan airplane, structural failure of the fan inlet guide vanes was encountered. If these had failed in flight and entered the fan rotor the aircraft would have been lost. Also, excessive deflection of the fan exit louver control mechanism was encountered during these wind tunnel tests. This would have severely limited the fan- supported flight envelope of the XV-5A. Both of these problems were remedied following the wind tunnel tests, and the XV-5A airplane has completed a series of successful The lift-fan propulsion system is flight research programs.

to be one of the most promising con- currently considered cepts for a high performance V/STOL airplane.

The total cost of the aircraft programs which have been saved by tests of the full-scale aircraft in the 40- by 80-foot wind tunnel has more than offset the total cost of construction and operation of this facility for 25 years.

these savings could be added the savings due to the To aircraft which had flight test programs of cancellation of fundamen- by full-scale wind tunnel tests to have been shown aircraft is the A partial list of such tal deficiencies.

low- wing, the Avrocar, and the Vanguard Kaman K-16 tilt fan-in-wing airplane.

disk loading the full- Actually, the most important contributions of it is tunnels have been in research areas where scale wind on the value impossible to put a firm dollar magnitude nearly full-scale contribution. The contributions of the of the the externally-blown flap wind tunnels to the development of STOL aircraft are almost and the augmentor wing turbofan fact that these concepts are today solely responsible for the types for application to considered to be the most promising transport aircraft. The large commercial and military STOL tunnels was also instrumental in use of the full-scale wind conventional landings of establishing the feasibility of the This included tests of all of lifting body spacecraft.

wind tunnel vehicles in the 40- by 80-foot full-scale flight in the 30- by 60-foot plus studies of free-flight models studies added immeasurably to the confi- wind tunnel. These initiated flight tests could be dence level required before aircraft. This is these radically new and different with the full-scale best example of an application of perhaps the have been to fill a need which could not wind tunnels justified.

the facilities were anticipated when full-scale wind tunnel the cost of the proposed Since cost of current compared with the typical has already been the cost of it is of interest to compare aircraft programs, tunnels with representative air- the existing full-scale wind were justified.

at the time these facilities craft current 22 which shows the presented on figure This comparison is tunnel cost with time.

in airplane cost and wind variation full-scale that the cost of the proposed This figure shows aircraft is, if to the cost of current wind tunnel relative 60- and 40- by than the cost of the 30- by anything, less the time to aircraft costs at 80-foot wind tunnels relative and the In addition, the need these facilities were built.

transportation is for-both military and civil air potential existing it was at the time the much more apparent now than retrospect our justified. In full-scale facilities were and courage a high degree of foresight predecessors showed during the great depression in building these facilities (the 30- by 60-foot wind tunnel) and at the outset of World War II (the 40- by 80-foot wind tunnel).

CONCLUSIONS The following conclusions were drawn from the studies discussed in this paper: 1. Basic factors such as growth of the population and economy will create a growing demand for transportation in the United States, and the major portion of this transportation demand will be for trips between 50 and 500 miles.

2. The problems of aircraft noise and air traffic congestion must be alleviated if this demand for air transportation is to be met. The use of V/STOL aircraft will provide significant reductions in air traffic congestion by shifting most of the short-haul traffic away from the major airports to small V/STOL ports. In addition, the use of V/STOL aircraft will subject less land area to high noise levels than will surface transportation systems.

3. There are a number of military missions which require the development of high-performance V/STOL aircraft.

4. The major problem associated with the development of advanced V/STOL aircraft is the technical and financial risk of the aircraft development pro- gram. This risk can be significantly reduced by conducting full-scale tests of the critical components of the aircraft (e.g., V/STOL propul- sion systems and rotors) prior to the go-ahead for the complete aircraft system. To conduct these tests, a subsonic wind tunnel capable of testing vehicles with spans up to 100 feet and at speeds of about 300 knots is required.

5. While the primary justification for the new full-scale wind tunnel is for the development of V/STOL aircraft, experience with the existing full-scale wind tunnels shows that this facility will be very useful in the development of conventional aircraft as well.

6. A two-test section, open-return wind tunnel provides the maximum test.capability for the minimum cost.

REFERENCES 1. Anon.: Report of the Joint DOT-NASA Civil Aviation Development Policy Study, Vol. II, NASA SP-266, 1971.

2. Galbreath, A.; and Warfield, R. M.: Terminal Area Airline Delay Data, 1964-1969, Federal Aviation Administration, Washington, D. C., September 1970.

3. Wimpress, J. K.: Aerodynamic Technology Applied to Takeoff and Landing. Annals of the New York Academy of Sciences, Vol. 154, Art.

2, Pages 962 to 981, November 1968.

4. Deckert, W. H.; and Hickey, D. H.: Summary and Analysis of Feasibility-Study Designs of V/STOL Transport Aircraft, J. Aircraft, Vol. 7, No. 1, January-February, 1970.

5. The Boeing Company: Study of Aircraft in Short-Haul Transportation Systems, NASA CR-986, 1967.

6. Fry, B. L.; and Zabinsky, J. M.: Feasibility of V/STOL Concepts for Short-Haul Transport Aircraft, NASA CR-743, 1967.

7. Marsh, K. R.: Study of the Feasibility of V/STOL Concepts for Short-Haul Transport Aircraft, NASA CR-670, 1967.

8. The Lockheed-California Company: Study on the Feasibility of V/STOL Concepts for Short-Haul Transport Aircraft, NASA CR-902, 1967.

9. Anon.: Technical and Economic Evaluation of Aircraft for Intercity Short-Haul Transportation, Federal Aviation Agency-Air Defense System-74, Vols. I, II and III, April 1966.

10. Kuhn, R. E.; Kelly, M. W.; and Holzhauser, C. A.: Bringing V/STOL's Downtown, Astro. Aero., September 1965.

11. Simpson, R. W.: Summary and Recommendations for the NASA/MIT Workshop on Short Haul Air Transport, Flight Transportation Laboratory Report 71-4, October 1971.

12. Tapscott, R. J.: Rotorcraft Applications and Tech- nology, Paper No. 20 in Vehicle Technology for Civil Aviation, The Seventies and Beyond.

NASA SP-292, November 1971.

13. Kelly, M. W.; McKinney, M. 0.; and Luidens, R. W.: The Requirements and Justification for a New Full- Scale Subsonic Wind Tunnel, NASA TM X-62,106, February 1972.

14. Mitchel, James G.: The Test Facilities Role in the Effective Development of Aerospace Systems, AFSC-TR-71-01, September 1971.

15. Mort, K. W.; Eckert, W. T.; and Kelly, M. W.: The Steady-State Flow Quality in a Model of a Non- Return Wind Tunnel, NASA TM X-62,170, September 1972.

16. Harris, N. D.: Forecasting Military Aircraft, Space/Aeronautics, 1968.

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