Chapter 8 - Wind Tunnels of the Future

The Demands of Near Future Aerospace Vehicles on Wind Tunnels


Advanced Commercial Aircraft

An Energy Saving Transport


[123] When a contemporary trans Atlantic transport rises from the runway, roughly 40 percent of its weight is fuel, compared to only 10 to 15 percent payload. The cost advantages in shifting some of the 40 percent fuel load to profitable payload are manifest. Even with many years of airplane improvements behind us, promising areas remain for further research and possible fuel savings.

It is the aircraft drag that consumes fuel wastefully. The frontier in drag reduction is drastically cutting skin friction. Today's airfoils operate with the boundary layer almost entirely in a turbulent state. If the layers of air close to the wing and fuselage would smoothly slide over one another without stirring up local eddies and vortices, skin friction drag would plummet. This smooth or laminar flow can be encouraged by smooth surfaces and carefully controlled pressure gradients, but large areas over the wings still break up into turbulence even with the best precautions. A better way to induce substantial regions of laminar flow is to suck small amounts of air from the boundary layer through thin slots in the aircraft structures. But boundary layer suction takes energy as well as additional equipment on the airplane. Better ways of promoting laminar flow must be found, for the payoff is high. Studies show that with everything considered, fuel consumption can be reduced up to 30 percent by actively creating large areas of laminar flow on the wings, fuselage, engine nacelles, and control surfaces. For a long range transoceanic flight, the payload fraction could almost double to 30 percent.

Uncharacteristically, it is the shortcomings of wind tunnels that partially deter the development of aircraft with laminar flow control. To date it has been impossible to create extensive areas of laminar flow on models in wind tunnels at Mach numbers and Reynolds numbers typical of full sized aircraft. The airflow in many existing wind tunnels has so much turbulence, noise, and other flow disturbances in it that it prematurely induces turbulent flow over the model. It is difficult to discover new ways of maintaining laminar flow over airfoils when the experimental equipment is a major culprit. NASA is now studying new ways to modify tunnels at Langley and Ames to remove this limitation on laminar flow research.


Spreading Out the Load


Commercial air transports are essentially winged box cars-a bit more streamlined perhaps, but still containers with appendages to provide lift and propulsion and control forces. However, if a cargo plane....


idealized transport

[124] An idealized transport configuration with laminar flow control (LFC) applied to the wings, struts, engine nacelles, arid fuselage. By reducing fuel consumption with LFC, payload fractions can be raised to about 30 percent, a large increase over current aircraft. [Original picture was in color, Chris Gamble, html editor] made large enough, with the wings growing proportionally, the wings will eventually become cavernous enough to hold cargo in addition to the fuel they customarily store today. This concept of "distributed load transports" becomes feasible at wing spans of 500 feet and gross weights of 3 million pounds (about four times the weight of contemporary wide body cargo planes). Aircraft of this size could begin competing with ocean going freighters on intercontinental runs.

It is more than a question of size. With cargo weight out in the wings, the lift forces are largely balanced where they are created. The bending moments on the wing roots are thus diminished, greatly reducing structural weight. Estimates put the payload fraction of a very large distributed load cargo plane at 40 to 50 percent, compared to 10 to 15 percent today, with direct operating costs per ton mile only a fraction of those of today's smaller air freighters.

Aerial behemoths of this size are not easily designed, built, and operated, especially with existing ground facilities. Considering only that portion of the challenge affecting wind tunnels, an aerodynamicist would certainly question the ability of existing tunnels to simulate the Reynolds numbers encountered with wings thick enough to walk into. (The Reynolds number increases directly with size.) Transonic tunnels now operating could attain Reynolds numbers of about 15 x 106 for the model of such an

aircraft. The real aircraft, however, would have a Reynolds number of roughly 125 x 106 based on the wing chord. The consequences of an order of magnitude lower test Reynolds number would be an overconservative design-due primarily to the wind tunnel predicting a lower cruise Mach number than actually would prevail on the full size plane. Faced with pessimistic wind tunnel data, the designer would likely elect to provide a thinner wing, with an attendant weight increase, to attain the desired speed. Cargo space would then be sacrificed needlessly.

Fortunately, a new NASA wind tunnel, designated the National Transonic Facility, will be in operation at Langley in 1982. This tunnel will be able to provide full scale tests of the various proposed configurations of the distributed load aerial freighters.


V/STOL Aircraft: Slow and Complex


A pithy saying among aerodynamicists is that a single helicopter blade faces more aerodynamic problems in one revolution than a fixed wing aircraft meets in its entire lifetime. Despite the low speeds of helicopters, the rotor tips whip around near the speed of sound. Blades are sensitive to both Reynolds number and Mach number effects. The same holds true for winged craft with tilting rotors. In fact, aerodynamic difficulties are multiplied in the potentially unstable transition region where vehicle lift is transferred from the rotors to the wings. Very large wind....


loader concept

[125] A swept wing span loader concept. The cargo space is inside the thick wings. (Courtesy The Boeing Company) [Original picture was in color, Chris Gamble, html editor]


...tunnels with air speeds of 200 to 300 mph are essential to the successful evolution of future rotorcraft.

After a hiatus of several years, interest in STOL aircraft has been renewed. The market for STOL craft transcends big city commuter traffic. Bush pilots, geologists, and others need STOL vehicles for operations in underdeveloped countries and in the widening search for new energy sources.

The extra lift needed for STOL comes from two sources: the downward vectoring of the jet thrust and additional wing lift from additional forced circulation of air around the wing. For example, a NASA quiet short haul research aircraft (QSRA) being studied at Ames employs upper wing surface blowing to more than double the lift over that of a conventional wing. Runways less than 1500 feet long are sufficient for planes of this type. Another promising STOL transport design has wing flaps that deflect the jet exhaust downward from underwing engines to increase lift at takeoff. Many other powered lift concepts are being investigated. They all must be tested in wind tunnels to check the designs before committing pilots and expensive equipment to flight tests.

The main problem in powered lift STOL is model size. The models must be large enough to adequately duplicate engine airflow and exhaust patterns and the complicated wing flap systems. If models are too small, the Reynolds numbers will be low, and flow separation over the flaps will be premature, leading to pessimistic results. Tunnel wall boundary effects are also important considerations that fade with increasing tunnel size. What this means is that V/ STOL...


XV-15 tilt rotor research aircraft

[126] The XV-15 tilt rotor research aircraft undergoing tests in the Ames 40 x 80-foot tunnel. The critical transition occurs when the rotors tilt forward for horizontal flight.

V/STOL fighter concept

A V/STOL fighter concept in the Ames 40 x 80-foot tunnel.

V/STOL tunnel

A model being tested in the Langley V/STOL tunnel employs upper surface blowing over the large flaps to turn the engine jets downward for powered lift. [Original picture was in color, Chris Gamble, html editor]


....wind tunnels using scale models should be very large. Full scale testing necessitates tunnels on the order of 100 feet in diameter at the test section. (A new leg being added to the Ames full scale tunnel will provide a suitable 80 x 120-foot test section).


Future Military Aircraft and Missiles


Military aerospace vehicles have more freedom relative to commercial aircraft in regard to economy, efficiency, and safety. It is not surprising therefore to find many aerodynamically radical military craft in [127] various stages of research and development. The demands that they will make on NASA or Air Force wind tunnels will be correspondingly more severe than those for civilian craft.

Even though the B-1 bomber may not enter mass production, large, manned, supersonic bombers are still being considered. Variable sweep wings, such as those on the F-111, are also in the running. In one design for on the deck, high speed flight, the wings sweep so far back that they disappear into the fuselage. Vehicle lift then comes from the fuselage alone. Finally, there is the almost grotesque pivot wing that seems intuitively unstable. Yet NACA wind tunnel tests demonstrated acceptable flying qualities for the pivot wing as early as 1946. It is still a candidate for future supersonic bombers as well as commercial transports.

Fighters must fly faster than bombers. Designers now talk of aerial battles at Mach 4.5 above an altitude of 100 000 feet. At these near hypersonic speeds, intense shock wave interaction has led to the consideration of scimitar shaped wings that seem to come right out of Buck Rogers and H. G. Wells. Even stranger are the swept forward wings that seem the antithesis of streamlining. Actually, it makes no difference in drag reduction at high speeds whether the...


bomber with pivoting wings

A bomber with pivoting wings rotated back almost flush with the fuselage for high speed flight. Body lift is sufficient for high speed flight at low altitudes. (Courtesy Rockwell International)

NASA pivot wing AD-1

A flight model of the experimental NASA pivot wing AD-1. The wing is shown canted in the high speed position. At low speeds it is perpendicular to the fuselage axis. [Original picture was in color, Chris Gamble, html editor]

Mach 4.5 interceptor

A model of a Mach 4.5 interceptor with scimitar wings undergoing tests in a wind tunnel at Arnold Engineering Development Center. (Photo, AEDC)


[128]....wings are swept forward or backward. But in terms of boundary layers and flow separation, the swept forward wing promises significantly higher lift to drag ratios in maneuvering flight and better low speed performance. Swept forward wings are still in the embryonic stage, but wind tunnels are already amassing the aerodynamic data needed for preliminary design.

Quasi hypersonic fighters would be impressive, but most future aerial battles would probably occur at transonic speeds. The key criterion to success here would be maneuverability near Mach 1. A NASA/Air Force program called HiMAT (Highly Maneuverable Aircraft Technology) has the goal of 8 g turns at Mach 0.9 at an altitude of 25 000 feet-a formidable technical challenge. Some configurations employ movable, two dimensional nozzles at the trailing edges of the wing/fuselage fed by jet engine exhaust to induce extra lift. Testing at the Langley 16 foot transonic tunnel helped prove this concept. To fill the gap between wind tunnel tests and costly flight testing, HiMAT uses a remotely controlled, reduced scale prototype released from a B 52 at 45 000 feet.

Although maneuverability may be the key to aerial dogfights, military missions such as air defense interception and reconnaissance want as much speed as possible. This means true hypersonic flight-Mach 5 and above. At present no American aircraft is capable of sustained hypersonic flight. In fact, the X-15 was the only real manned hypersonic craft ever built in the United States, but its rockets could provide power for only a few minutes because of their high fuel consumption rate. The key to sustained hypersonic flight is the Supersonic Combustion Ramjet or SCRAMJET, which uses high energy hydrogen as a fuel burning oxygen from the atmosphere.

The development of a SCRAMJET and its integration into a hypersonic aircraft are fraught with the kinds of difficulties that only wind tunnels can help. Local airflow fields change radically with the angle of attack and Mach number in a SCRAMJET, especially around the aircraft's forebody and engine inlet. In addition, vehicle surface temperatures may reach 2000° F at Mach 6 and an altitude of 100 000 feet. The liquid hydrogen fuel consumed by the SCRAMJET makes it an ideal coolant for these hot spots, but...


scale mockup of a fighter

Full scale mockup of a fighter with swept forward wings. (Photo, Rockwell International) [Original picture was in color, Chris Gamble, html editor]

Himat is released from its B 52

[129] Himat is released from its B 52 mother ship in this artist's conception. (HiMAT = Highly Maneuverable Aircraft Technology.) Maneuverability is achieved by canard control surfaces. [Original picture was in color, Chris Gamble, html editor]

HiMAT experimental aircraft

The HiMAT experimental aircraft on the ground.


[130] The SCRAMJET geometry is radically different from that of conventional jet engines. Wind tunnel tests are essential in defining the airflow patterns near the inlets and nacelles.


....wind tunnel experience will be essential in proving such a radical cooling system embedded in an equally radical aircraft.

At Langley, a 20-megawatt arc heated wind tunnel, called the SCRAMJET Facility, has been built to explore air flow around and through a small scale operational SCRAMJET. To move on to full scale testing, a much larger tunnel, such as the Langley 8 foot high temperature structures tunnel, would have to be modified to incorporate an oxygen replenished core to sustain combustion.

As manned military aircraft look more and more like unmanned missiles, the missiles themselves are becoming more sophisticated aerodynamically and look more like aircraft. Early missile design centered on brute force propulsion and accurate guidance and control, with aerodynamic performance being secondary. Now missile engineers are drawing heavily on the immense backlog of wind tunnel experience with manned supersonic aircraft and space vehicles of all sorts. Probably the most important of the new unmanned missiles is the air launched cruise missile. It is actually a small aircraft. A leading design incorporates variable sweep wings that retract completely into the fuselage for ease of stowage on the carrier aircraft. Once launched, the wings extend for long range cruise. Such craft have long been a familiar....


cruise missile

An air launched cruise missile. (Photo, The Boeing Company) [Original picture was in color, Chris Gamble, html editor]


....sight in wind tunnels. A large body of relevant experience is already available.


Beyond the Space Shuttle


During the early 1980s, the Space Shuttle will be the key to quick, easy, and economical access to outer space. It is only the first step in the development of a space transportation system that promises an order of magnitude reduction in payload costs through the recovery and reuse of the orbiting portion of the launch system. The 1981 Space Shuttle will carry about 65 000 pounds into low Earth orbit at an estimated cost of about $300 per pound. Larger shuttles based on the same technology will likely double the payload and cut the cost in half. To those visionaries who foresee vast space enterprises-orbital industrial manufacturing, manned scientific stations, staging platforms for ambitious missions to the other planets-even advanced shuttle craft could not carry the anticipated cargoes of men and materials. The proponents talk in terms of 100 000 tons per year into space at costs a factor of ten lower than today.

Several immense launch vehicles are under study. The Heavy Lift Launch Vehicle is a huge, recoverable rocket system launched vertically like the launch vehicles of Apollo days. Other concepts take advantage of lifting surfaces and air breathing engines to carry vehicles high into the sensible atmosphere where rockets can take over and insert large payloads into orbit. The proposed craft are truly gigantic: One winged supersonic launcher weighs 5 million pounds with a wing area of 50000 square feet-over ten times the wing area of the Concorde. Single stage to orbit launchers combine all the flight regimes of supersonic aircraft, vertical launch vehicles, and reentering spacecraft; their immense sizes outstrip the capacities of most wind tunnels.


heavy lift launch

[131] A possible configuration of a heavy lift launch vehicle. Taking off vertically,, like the Space Shuttle, this launcher could place up to 500 tons into low Earth orbit. Both stages are recoverable. (Photo, The Boeing Company)

future launch vehicle

Large future launch vehicles may take off horizontally like conventional aircraft. Air breathing engines would take the launcher to Mach 3.5 at 60 OOO feet, after which separation would take place, with rockets propelling the second stage into orbit.