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Why Do Backwards Wings Exist?

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13:11   |   Jul 20, 2019

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  • This episode of Real Engineering is brought to you by CuriosityStream, watch over 2,400
  • documentaries for free for 31 days at curiositystream.com/realengineering.
  • On December 12th, 1984, the United States Air Force and NASA began testing an unusual
  • aircraft.
  • One that broke all aircraft design convention.
  • It’s wings pointed forward.
  • However this experimental aircraft dubbed the X-29, was not the first of its kind.
  • The German’s also experimented with the concept in the late stages of World War 2
  • with the Junker JU-287, and it’s prototype airframes would eventually end up in the hands
  • of the Soviets who took and developed the design into the okb-1 ef 131 and OKB-1-140.
  • [1]
  • All of these early iteration designs ran into the same problems.
  • This design was incredibly aerodynamically unstable.
  • When the wing deflects the force of the oncoming wind tends to make it deflect even more.[2]
  • This is a rather obvious design flaw.
  • Intuitively you just now looking at it that something doesn’t quite make sense.
  • So why did Germany, The Soviet Union and the United States all see the design worthy of
  • consideration?
  • To understand this, we first have to explore why wings are swept beyond a perfectly perpendicular
  • angle in the first place.
  • Looking at most aircraft developed during world war 2, you can see that nearly all of
  • them had straight wings.
  • The Spitfire, the mitzubishi A6M Zero and the P-51 Mustang.
  • It was only during the later stages of the war, as more powerful engines came to the
  • fore that other designs started to emerge, and in one case the straight wing became a
  • major design flaw that put the crew of the P-38 Lightning in serious danger.
  • These problems arose directly as a consequence of how wings generate lift.
  • An aerofoil is designed to make use of bernoulli’s principle, where a low pressure zone is created
  • on top of the wing as a result of airflow moving faster.
  • People like to say this false , but it’s just one of the ways a wing generates lift,
  • there is a lot more to the story.
  • Because this airflow actually speeds up as it crosses the wing, it can reach supersonic
  • speeds long before the plane itself reaches supersonic speeds.
  • [3] This causes problems because supersonic flow means shock waves form, which can disrupt
  • normal airflow over the wing.
  • On November 4th 1941, these problems resulted in the death of Ralph Virden, an expert test
  • pilot, during a high speed test dive of the P-38 lightning.
  • The causes of the crash were unclear at the time.
  • This version of the P-38 had been altered with superior servos for the control surfaces
  • to help the pilot overcome aerodynamic stiffening, where the force of the oncoming air at high
  • speeds makes it difficult to move the control surfaces, but the plane still entered an uncontrolled
  • dive regardless of these measures.
  • The engineers eventually discovered the airflow was separating from the surface of the wing
  • as a result of shockwave formation.
  • This reduced the lift the wing could generate, while increasing the lift on the tail wing
  • directly downstream of the flow separation.
  • This moved the centre of pressure and forced the plane to pitch downwards and gain even
  • more speed,[4] making it next to impossible to recover from.
  • To solve this issue they incorporated a dive flap on the lower surface of the wing, where
  • airflow was not reaching supersonic speeds, which could be deployed during high speed
  • dives to allow the wing to increase lift and recover from the dive.
  • As technology advanced however aeronautical engineers started to see that straight wings
  • were not suitable for transonic or supersonic speeds, and gradually started to adopt swept
  • wings.
  • The Germans confirmed the theory with high speed wind tunnel testing in 1939 by testing
  • two wings, a straight wing and a swept wing with a 45 degree sweep.
  • Proving that a swept wing could delay the onset of supersonic flow AND reduce drag.
  • [5] They recognised that the swept wing would allow a plane to fly faster before shock waves
  • formed, long before the technology that would enable them to fly faster was even invented.
  • They used this knowledge to develop the Messerschmitt P.1101, a jet powered plane that could actually
  • change it’s sweep angle before flight.
  • But the end of the war came before the German’s could finish it and test the aircraft
  • Air flow over a wing perpendicular to the freestream air has one component, the chordwise
  • flow, [6] which is air that flows over the chord of the aerofoil.
  • The chord is the imaginary line running from the leading edge to the trailing edge of an
  • aerofoil.
  • Chordwise flow does accelerate over the aerofoil, and thus contributes to lowering the speed
  • at which supersonic flow begins.
  • Called the critical mach number.
  • Now let’s look at the flow components over the Bell X-5s wing at its lowest sweep angle,
  • 20 degrees.
  • Here we can separate the airflow into two components.
  • [6] The chord wise flow, which is now offset at a 20 degree angle relative to the freestream,
  • and the new second component the spanwise flow, which flows along the length of the
  • wing and does not accelerate and thus does not lower the critical mach number.
  • At lower speeds, where supersonic airflow is not a worry, you want as much of that airflow
  • to be chordwise and thus generate the necessary lift to fly.
  • However as the speed of the plane begins to climb, we are generating more than enough
  • lift thanks to the increased air speed, and thus can afford to convert some of that airflow
  • into spanwise flow.
  • We do this by increasing the sweep angle, which the Bell X-5 could do in flight to a
  • maximum sweep angle of 60 degrees.
  • [7] As the sweep angle increases a larger portion of that airflow is converted to spanwise
  • flow, which is great for increasing the top speed of an aircraft, but can cause some troublesome
  • stall characteristics.
  • Because a large volume of air is now originating at the root of the wing and travelling down
  • to the tip of the wing, stall will begin at the tip of the wing and move towards the root.
  • This is a problem because our ailerons, the control surfaces that allow us to roll the
  • plane, are located on the outer wing.
  • If stall occurs on the outer wing, we will lose roll control.
  • [8] A major problem for say at fighter jet attempting a high angle of attack maneuver
  • while maintaining full control, and this is one of the problems forward swept wings were
  • trying to fix.
  • This reverses the direction of the chordwise flow, so it originates at the wing tips and
  • travels to the root of the wing.
  • [9] Meaning stall occurs at the root of the wing first, allowing us to maintain control
  • of the plane for much longer.
  • Not only that, but it reduced induced drag as a result of wing tip vortices.
  • Where high pressure air from the lower wing travels and mixes with low pressure air on
  • top of the wing at the wing tips.
  • The Ju 287 was designed this way not to benefit from the superior aerodynamics characteristics,
  • but to move the wing box rear wards, which allowed the bomb bay to move forward closer
  • to the centre of gravity of the aircraft, which in turn allowed the plane in-carry a
  • larger bomb, while not increasing the trim drag to keep the plane balanced.
  • [10] But ultimately the materials of the time could not facilitate it.
  • Under normal wing loading, the main force being exerted on the wing is upwards bending.
  • Where the force of lift pushes the wing upwards, while the weight of the fuselage pushes downwards.
  • To survive this we need to build an adequately strong and stiff wing.
  • This is achieved through a beam called a spar which runs the length of the wing.
  • With a forward swept wing an additional stress is introduced, where the force of oncoming
  • air is attempting to twist the wing.
  • We can imagine this with a free body diagram with springs representing the stiffness we
  • need to incorporate into the wing.
  • [11] Here the kb is the spring stiffness that will be needed to resist bending, and kt is
  • the spring stiffness needed to resist twisting.
  • Creating a structural member that can act like this torsion spring over the entire wing
  • however is no easy task and would require enough additional weight to negate any positive
  • attributes the forward swept wing would provide.
  • But that changed when advanced composite materials became available.
  • Allowing planes like the X-29 and the Russian equivalent the SU-47 to be made.
  • Both planes used carbon fiber reinforced plastics laid up so the fibres would resist that twisting
  • motion.
  • I will focus on the X-29 from here, as information on American technology is far more freely
  • available.
  • The X-29s primary structural member for resisting this twist was a closed box section, located
  • here, [12] which was constructed of crisscrossed composite tape that reached up to 156 layers
  • deep.
  • Essentially creating that spiral spring shape within the structural member, but with extremely
  • stiff and lightweight composites.
  • Taming that twisting problem, and allowing the plane to fly successfully.Wind tunnel
  • tests showed the forward swept design would provide a 20% gain in efficiency compared
  • to same plane with aft swept wings.
  • [9]
  • This along with a supercritical wing design [13], which flattens the top edge of the aerofoil,
  • to minimise the acceleration of the air over the top edge, while introducing a concave
  • curve to the lower surface to increase lift, allowed the X-29 to spend less fuel flying
  • at a higher mach number.
  • Another drag reducing benefit of the forward swept wing was the shifting of centre of pressure
  • rear wards.[14] Typically the lift generated by an aft swept wing needs to be counteracted
  • by a tail wing which generates downwards pressure to maintain pitch stability.
  • This downwards pressure is wasted energy that contributes to drag.
  • With a forward swept wing the centre of pressure is moved to the rear of the aircraft behind
  • the centre of gravity, and thus to maintain static pitch control these pitch control surfaces,
  • called canards, need to generate lift forward of the centre of gravity and thus contribute
  • to useful lift.
  • This would seem like an obvious feature to incorporate into every aircraft, but leads
  • to instability that requires the control surfaces to constantly adjust to maintain a stable
  • flight, and this was one of the massive challenges the designers faced.
  • The X-29 was incredibly unstable, especially in pitch, even when compared to modern jet
  • fighters.
  • This means the flight control computers had to be constantly adjusting the control surfaces
  • to maintain stable flight, about 40 times a second.
  • [15]
  • To do this the X-29 had three flight control computers, to provide redundancy if one failed.
  • As the plane would become essentially impossible to fly without the aid of a computer.
  • Which made it even more worrying when all three shut down while preparing to take off.
  • [16]
  • This caused the plane to be grounded.
  • Delayed testing, which was due to accelerate with the arrival of a second X-29 fitted with
  • a parachute system to allow high angle of attack maneuvers to be safely tested.
  • The spin parachute was installed to provide positive recovery from spins, as spin-tunnel
  • tests had indicated that the X-29A ailerons and rudder provided poor recovery from fully
  • developed upright spins.
  • Eventually the problems were solved and high angle of attack testing resumed and proved
  • the X-29s capabilities, but the program ultimately ended on December 8th 1988, almost four years
  • to the day of it’s first flight.
  • In between that first and last flight the X-29 completed 242 flights with 179 combined
  • flight hours.
  • Giving valuable scientific data and design experience in composite airframes and computer
  • aided flight.
  • Ultimately forwards swept wings weren’t incorporated into newer generations of planes,
  • as the benefits simply did not outweigh the cons.
  • From the additional structural requirements, the poor pitch stability and perhaps most
  • importantly it’s negative effects on stealth design [17], forward swept delta wings won
  • out in the end.
  • This is just one of many unusual plane designs that originated in world war 2 era Germany,
  • among other novel military vehicles.
  • You can learn more about these innovative machines with this documentary titled “Hitler’s
  • Miracle Machines” on curiosity stream.
  • You can watch them for free, by signing up to curiositystream using the code realengineering,
  • or using the link the description.
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  • As usual thanks for watching and thank you to all my Patreon supporters.
  • If you would like to see more from me, the links to my instagram, twitter, discord server
  • and subreddit are below.

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References:
[1] http://hugojunkers.bplaced.net/junkers-ef131.html
[2] https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-008-DFRC.html https://apps.dtic.mil/dtic/tr/fulltext/u2/a124715.pdf
[3] http://www.dept.aoe.vt.edu/~mason/Mason_f/ConfigAeroTransonics.pdf
[4] http://bit.ly/2Y17MM2
[5] https://apps.dtic.mil/dtic/tr/fulltext/u2/271130.pdf
[6] http://www.srmuniv.ac.in/sites/default/files/downloads/class5-2012.pdf
[7] https://www.nasa.gov/centers/armstrong/news/FactSheets/FS-081-DFRC.html
[8] https://repository.lib.ncsu.edu/bitstream/handle/1840.16/2141/etd.pdf?sequence=1
/watch?v=pg-Qi5TYzK0
[9] page 18 https://www.nasa.gov/sites/default/files/files/Sweeping_Forward.pdf
[10] /watch?v=LOmvrk3LPGc
[11] https://apps.dtic.mil/dtic/tr/fulltext/u2/a124715.pdf
[12] https://www.nasa.gov/centers/dryden/pdf/88172main_H-1574.pdf
[13] page 24 https://www.nasa.gov/sites/default/files/files/Sweeping_Forward.pdf
[14] https://www.nasa.gov/centers/dryden/pdf/120266main_FS-008-DFRC.pdf
[15] Page 124 https://www.nasa.gov/sites/default/files/files/Sweeping_Forward.pdf
/watch?v=LOmvrk3LPGc
[16] Page 127 https://www.nasa.gov/sites/default/files/files/Sweeping_Forward.pdf
[17] Page 208 https://www.nasa.gov/sites/default/files/files/Sweeping_Forward.pdf

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