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30-584: In 1956, Stroukoff had already gained much experience working on the C-123 Provider, having completed two contracts based on that airframe. Its YC-123D had introduced a Boundary Layer Control (BLC) system to the C-123B. This system forced pressurized air over the upper wing surfaces of the airplane, making the wing work as if it were flying at much greater airspeed. This greatly improved landing and take-off performance, gross weight capability, and lowered

60-617: A splitter plate . Much research was conducted to study the lift performance enhancement due to suction for aerofoils in the 1920s and 1930s at the Aerodynamische Versuchsanstalt in Göttingen . An example of an aircraft with active boundary layer control is the Japanese sea plane ShinMaywa US-1 . This large, four-engined aircraft was used for anti-submarine warfare (ASW) and search and rescue (SAR). It

90-434: A wake . A boundary layer exists whenever there is relative movement between a fluid and a solid surface with viscous forces present in the layer of fluid close to the surface. The flow can be externally, around a body, or internally, in an enclosed passage. Boundary layers can be either laminar or turbulent . A reasonable assessment of whether the boundary layer will be laminar or turbulent can be made by calculating

120-417: A curved path. The spin causes boundary layer separation to be biased to one side which produces a side force. BL control (roughening) was applied to golf balls in the 19th century. The stitching on cricket balls and baseballs acts as a boundary layer control structure. In the case of a freestream flow past a cylinder, three methods may be employed to control the boundary layer separation that occurs due to

150-466: A laminar boundary layer to reduce skin friction have not been demonstrated for dolphins. This became known as Gray's Paradox . The wings of birds have a leading edge feature called the Alula which delays wing stalling at low speeds in a similar manner to the leading edge slat on an aircraft wing. Thin membrane wings found on bats and insects have features which appear to cause favourable roughening at

180-417: A rapidly expanding duct of pipe. Separation occurs due to an adverse pressure gradient encountered as the flow expands, causing an extended region of separated flow. The part of the flow that separates the recirculating flow and the flow through the central region of the duct is called the dividing streamline. The point where the dividing streamline attaches to the wall again is called the reattachment point. As

210-405: Is applied only to the leading edge region of a swept wing and NLF aft of that. NASA-sponsored activities include NLF on engine nacelles and HLFC on wing upper surfaces and tail horizontal and vertical surfaces. In aeronautical engineering, boundary layer control may be used to reduce parasitic drag and increase usable angle of attack . Fuselage-mounted engine intakes are sometimes equipped with

240-443: Is approximately stated as where s , y {\displaystyle s,y} are streamwise and normal coordinates. An adverse pressure gradient is when d p / d s > 0 {\displaystyle dp/ds>0} , which then can be seen to cause the velocity u {\displaystyle u} to decrease along s {\displaystyle s} and possibly go to zero if

270-555: Is called Natural laminar flow (NLF) and has been achieved by sailplane designers with great success. On swept wings a favorable pressure gradient becomes destabilizing due to cross flow and suction is necessary to control cross flow. Supplementing the effect of airfoil shaping with boundary layer suction is known as laminar flow control (LFC) The particular control method required for laminar control depends on Reynolds-number and wing leading edge sweep. Hybrid laminar flow control (HLFC) refers to swept wing technology in which LFC

300-450: Is generally undesirable in aircraft high lift coefficient systems and jet engine intakes. Laminar flow produces less skin friction than turbulent but a turbulent boundary layer transfers heat better. Turbulent boundary layers are more resistant to separation. The energy in a boundary layer may need to be increased to keep it attached to its surface. Fresh air can be introduced through slots or mixed in from above. The low momentum layer at

330-547: Is the Reynolds number . For a given adverse d u o / d s {\displaystyle du_{o}/ds} distribution, the separation resistance of a turbulent boundary layer increases slightly with increasing Reynolds number. In contrast, the separation resistance of a laminar boundary layer is independent of Reynolds number — a somewhat counterintuitive fact. Boundary layer separation can occur for internal flows. It can result from such causes such as

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360-429: The Reynolds number of the local flow conditions. Separation occurs in flow that is slowing down, with pressure increasing, after passing the thickest part of a streamline body or passing through a widening passage, for example. Flowing against an increasing pressure is known as flowing in an adverse pressure gradient . The boundary layer separates when it has travelled far enough in an adverse pressure gradient that

390-453: The pressure differential between the front and rear surfaces of the object. It causes buffeting of aircraft structures and control surfaces. In internal passages separation causes stalling and vibrations in machinery blading and increased losses (lower efficiency) in inlets and compressors. Much effort and research has gone into the design of aerodynamic and hydrodynamic surface contours and added features which delay flow separation and keep

420-564: The C-123's stall speed . The YC-123E had been another experiment in improving the C-123's ability to operate wherever it might need to, introducing Stroukoff's own Pantobase system: two high-stress skis fitted to the lower fuselage, wingtip mounted floats, along with sealing the fuselage itself. This gave the YC-123E the ability to operate on water, as well as ice and snow, and with the BLC from

450-483: The C-123B from 31,058 lb (14,088 kg) to 37,965 lb (17,221 kg), and a maximum loaded weight increase from 60,000 lb (27,000 kg) to 74,700 lb (33,900 kg). The aircraft's cruising speed was 219 mph (352 km/h), compared to the C-123B's 190 mph (310 km/h), and the YC-134 had a 1,600-mile (2,600 km) range with a 24,000 lb (11,000 kg) payload. The BLC allowed

480-505: The P-51 airfoil done in the high speed DVL wind tunnel in Berlin showed the laminar flow effect completely disappeared at real flight Reynolds numbers . Implementing laminar flow in high-Reynolds-number applications generally requires very smooth, wave-free surfaces, which can be difficult to produce and maintain. Maintaining laminar flow by controlling the pressure distribution on an airfoil

510-517: The Reynolds numbers involved, thereby enabling these creatures to fly better than would otherwise be the case. Balls may be given features which roughen the surface and extend the hit or throw distance. Roughening causes the boundary layer to become turbulent and remain attached farther round the back before breaking away with a smaller wake than would otherwise be the case. Balls may be struck in different ways to give them spin which makes them follow

540-476: The World's Aircraft 1958–59 General characteristics Performance Related development Related lists Boundary layer control Boundary layer control refers to methods of controlling the behaviour of fluid flow boundary layers . It may be desirable to reduce flow separation on fast vehicles to reduce the size of the wake (streamlining), which may reduce drag. Boundary layer separation

570-536: The YC-134's take-off distance to decrease from 1,850 feet (560 m) to 750 feet (230 m), very similar to that of the YC-123D. The U.S. Air Force, however, deemed that the YC-134 did not offer substantial improvement over the C-123, nor did it have a requirement for a piston-engined amphibious assault transport, and decided to purchase the Lockheed C-130 Hercules . Data from Jane's All

600-399: The adverse pressure gradient is strong enough. The tendency of a boundary layer to separate primarily depends on the distribution of the adverse or negative edge velocity gradient d u o / d s ( s ) < 0 {\displaystyle du_{o}/ds(s)<0} along the surface, which in turn is directly related to the pressure and its gradient by

630-444: The adverse pressure gradient. Rotation of the cylinder can reduce or eliminate the boundary layer that is formed on the side which is moving in the same direction as the freestream. The side moving against the flow also exhibits only partial separation of the boundary layer. Suction applied through a slit in the cylinder near a separation point can also delay the onset of separation by removing fluid particles that have been slowed in

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660-541: The boundary layer. Alternatively, fluid can be blown from a faired slit such that the slowed fluid is accelerated and thus the point of separation is delayed. Laminar flow airfoils were developed in the 1930s by shaping to maintain a favourable pressure gradient to prevent them becoming turbulent. Their low-drag wind tunnel results led to them being used on aircraft such as the P-51 and B-24 but maintaining laminar flow required low levels of surface roughness and waviness not routinely found in service. Krag states that tests on

690-496: The differential form of the Bernoulli relation , which is the same as the momentum equation for the outer inviscid flow. But the general magnitudes of d u o / d s {\displaystyle du_{o}/ds} required for separation are much greater for turbulent than for laminar flow, the former being able to tolerate nearly an order of magnitude stronger flow deceleration. A secondary influence

720-412: The flow attached for as long as possible. Examples include the fur on a tennis ball, dimples on a golf ball, turbulators on a glider, which induce an early transition to turbulent flow; vortex generators on aircraft. The flow reversal is primarily caused by adverse pressure gradient imposed on the boundary layer by the outer potential flow . The streamwise momentum equation inside the boundary layer

750-567: The flow goes farther downstream it eventually achieves an equilibrium state and has no reverse flow. When the boundary layer separates, its remnants form a shear layer and the presence of a separated flow region between the shear layer and surface modifies the outside potential flow and pressure field. In the case of airfoils, the pressure field modification results in an increase in pressure drag , and if severe enough will also result in stall and loss of lift, all of which are undesirable. For internal flows, flow separation produces an increase in

780-401: The flow losses, and stall-type phenomena such as compressor surge , both undesirable phenomena. Another effect of boundary layer separation is regular shedding vortices, known as a Kármán vortex street . Vortices shed from the bluff downstream surface of a structure at a frequency depending on the speed of the flow. Vortex shedding produces an alternating force which can lead to vibrations in

810-491: The previous YC-123D, the new aircraft could effectively be operated from almost any runway surface available, and airstrips of shorter length. The product of a US Air Force contract in 1956, a single C-123B from the -CN production block (serial 52-1627) was modified by Stroukoff Aircraft to become the YC-134 . This aircraft was heavily modified with the following new features: These features gave an empty weight increase over

840-432: The speed of the boundary layer relative to the surface has stopped and reversed direction. The flow becomes detached from the surface, and instead takes the forms of eddies and vortices . The fluid exerts a constant pressure on the surface once it has separated instead of a continually increasing pressure if still attached. In aerodynamics , flow separation results in reduced lift and increased pressure drag , caused by

870-470: The surface can be sucked away through a perforated surface or bled away when it is in a high pressure duct. It can be scooped off completely by a diverter or internal bleed ducting. Its energy can be increased above that of the free stream by introducing high velocity air. British zoologist Sir James Gray stated that dolphins appeared to have a turbulent boundary layer to reduce the likelihood of separation and minimize drag, and that mechanisms for maintaining

900-571: Was capable of STOL operation and very low air speeds. Its replacement in the SAR role, the ShinMaywa US-2 , uses a similar system for its capability to fly at 50 knots. This feature is also used in Boeing's 787-9 Dreamliner aircraft. Boundary layer separation In fluid dynamics , flow separation or boundary layer separation is the detachment of a boundary layer from a surface into

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