Why Do Backwards Wings Exist?

Why Do Backwards Wings Exist?

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. This will give you a month of completely free
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    Ron Pearson

    Russia is still using vacuum tubes in their fighters so they must have figured out a way to fly these fighters stabel without computers.

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    Faster Than You

    Love the look… been intetested since the 80's in reverse wing aircraft
    But I love all variants since Biplanes lol 😎

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    Funny how he doesn't mention the Spitfire had a 24 degree sweep while the Me 262 only had a 14 to 18 degree sweep and that the Spit has the highest recorded dive speed of any jet or piston aircraft from the war era. The truth is that Germany was not as advanced as the Brits during this period or the US in some ways.

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    Syed Imran SHAH

    Its not the first time that sensors have been installed on forward wings. Our people's brains always stop at these things. They will ignore all other things. Thats why General Zia got the nationality of Israel and started praying five times a day. He knew what Pakistanis want to see. They are not interested in nationality or any other thing.

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    Dottor B

    Saw one at Long Island, MacArthur Airport just on the other side of the security fence. Not sure what it was doing there. Looked like they were using it for something. Very odd with the NASA logo on the tail.

    Remember being amazed by this in the 80s, my friend's dad worked at Grumman, back then said it was so unstable it would be impossible to fly without computers. Seemed nuts to me then, still seems nuts.

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    polymer engineer here: 9:00 the wings could withstand the bending-twisting because the used layup negates it by design. Just check out the ABD Matrix and used layup to see for yourself.

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    B Rumm

    1:59 "people like to say this is false" – well, it's not false in principle, but your wing animation repeats the common misconception. You say "the wing generates lift" but your wing does not generate any lift.

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    There may already be a bunch of comments on this, but I have to mention that Bernouli's principle (the equation you displayed) is only applicable when the air is flowing along a streamline. Because the wing is in open air, the air is not forced to flow from the leading edge to the trailing edge of the wing. Testing shows the air over the top of the wing actually meets at a point a ways behind the trailing edge of the wing while the air below meets more or less at the trailing edge, thus not on a streamline. (air is also compressible and viscus but these are not as important at low speeds)

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    Germany has its inventions and minds stolen by the Soviets and the US, along with their culture and identity destroyed. And for what? All so that the Jews could gain sympathy to gain Palestine. Was it worth it? We fought the wrong enemy.

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    The Westland Lysander also had swept forward wings, and before WW2. This was done for very different reasons but it did give the aircraft more manouevrability than it might otherwise have had, enough to make this army observation aircraft suitable to be drafted in as a fighter.

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    Mikee Maus

    Out of the Three Russia, China and the US. Only the Russians successfully solved the wing flutter problem…and they did it very simply. Instead of covering the wings in Sheet aluminum, which is light and can allow a great amount of flex, which interferes with stability and control, they covered the wings in thin sheets of cold rolled Steel….

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    Joseph Hinton

    All three nations experimented with the forward wind swept design because they all absorbed Nazi German engineers and scientists into their ranks after WWII. Anyone ever heard of the U.S.'s "Operation Paper Clip?" Or rocket scientist Warner Van Braun?

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    They were an attempt to make the plane more unstable which should mean, as the designers thought, that it would turn faster and therefore be better in a dogfight. But what they failed to see is that the large surface area at the rear prevents the craft from whipping around as they'd hoped. So what they ought have done was to connect that giant flap to the fuselage by means of an axis pole so that it becomes rotatable. Then they'd be able to whip around like nothing anybody's ever seen.
    Ya see how the front canards twist? Yeah, that's supposed to be how the main wing works.

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    Dennis Møller

    Why not create the wing completely round?
    Oops, I just created what many people called a UFO. A very high technology of flying crafts.

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    Ralf Hartung

    Backward wings? I don't see such wings here. I only see forward wings. They are called forward wings because they are swept forward. Alternatively they can be called negative v-wing. Backward wings are swept backwards. Hence the name.

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    Anyone calling themselves "Real Engineering" should know that there are no "backward wings." And if there were, the examples shown wouldn't be it. So take a note: any airfoil that causes lift is a "wing" regardless of its particular orientation. By contrast, a fictitious "backward wing" would be an airfoil that caused lift when the plane is in reverse (for lack of better term), again, regardless of its orientation.

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    Paul Gardner

    There's simply no way you read all these comments, but I just have to say how impressed I am with the amount of research and knowledge you manage to cram into a video. This is truly the best channel on YouTube. I am not exaggerating.

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    robert juker

    if everyone didnt beleive those lies about germany and they hadnt lost the war. airplanes would be even more advanced and fast today ,they were genuises

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    Michael McClain

    I stopped watching when you try to explain lift only using Bernoulli's principle, which is not the major factor in how wings generate lift. There are aircraft with symmetric foils! Most of an aircraft's lift is not from Bernoulli's principle, just from deflecting air downward. The foil shape helps keep the air flow clean and efficient.

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    Kazuki Kashiwazaki

    But in a nutshell the forward swept wing design makes the aircraft much more stable, but puts the outward halves of the wings under immense air resistance stress anytime the aircraft goes up, down, even when executing a simple barrel roll.

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    Teabag McPick

    Why do you still persist with Bernoulli as the reason wings create lift? It's nonsense. A flat wing will create lift. Angle of Attack/Incidence is why wings work. Bernoulli is why they can be made more efficient.

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    7:37 "we need to build an adequately strong and stiff wing" at the precise moment that the jetliner is using sheer engine power, not aerodynamic lift, to gain altitude.

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    Can you make a video about calculations of C.G. of aircraft? Please include like the most forward and most aft cg calculation. Btw are there different calculations for most forward and most aft cg calculation for different aircraft? like an engine mounted on front of the aircraft like cessna 172 and engine mounted on the vertical stabilizer like LISA Akoya.

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    Daryl Lackey

    NASA: "We want you to test fly this jet. We know under certain conditions you will not be able to recover so we've included a parachute."
    PILOT: "Um… ok"

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    Rich Purslow

    well executed documentary. Interesting concept with the forward sweeping wing. do you reckon computers today would be better for this concept.

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    Do you really need that annoying music. I want to listen to what you have to say without distraction, is that tooo much to ask?

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    X Critic

    Ok but why do slower aircraft like biplanes and seaplanes have perfectly straight wings? Is it better for gliding at lower speeds and altitudes?

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