Subtitles section Play video Print subtitles This episode of Real Engineering is brought to you by Curiosity Steam. Sign up today and get free access to my new Logistics of D-Day series on watchnebula.com. Plane wings come in a huge variety of shapes, sizes and configurations. We have explored how the aft swept wings of a traditional airliner and the forward swept wings of experimental planes like the x-29 affect the aerodynamic properties of the plane. We explained why modern day jet fighters are designed to be aerodynamically unstable, and in one of my very first videos we investigated why airliner manufacturers like Boeing and Airbus are installing increasingly complicated winglets at the tips of their wings. However we have never explored in detail the theory and practice behind wing dihedral angles. Which is simply the angle the wings make with respect a perfectly horizontal lie through the base of the wing, and planes throughout history have had a huge variety of variations with this feature. From the high mounted anhedral wing of the AV-8B Harrier, where the wing angles downwards, to the low mounted dihedral wings of airliners and WW2 era fighters and everything in between like modern day F-16s with low mounted straight wings or stranger still the gullwings of seaplanes and inverted gullwing of the F4U Corsair. So what's the purpose behind all these designs? To understand this, we first have to understand the dynamic lift generated by a wing as it rolls. Let's examine a plane with dihedral wings, meaning they point upwards. In straight and left flight the lift each wing generates does not point directly up. As the lift a wing generates is perpendicular to the aerofoil of the wing. This means the lift of wings points inwards at an angle towards the fuselage.  This seems like a strange design choice, as only the vertical component of lift contributes to getting the plane off the ground, and thus the horizontal component is wasted energy which results in additional fuel being wasted. So why do they do it? By tilting the wings upwards we gain roll stability, which we explored briefly in my “Why Jet Fighters Can Be Too Unstable” video. This means that when the plane rolls it will naturally correct itself and return to straight and level flight with no input from the pilot. An important safety feature This works as a result of a planes tendency to side slip when it rolls. Meaning the plane begins to move sideways and downwards in the direction of the bank. This introduces a new air flow to the wing which has both a vertical and sidewise component. As a result of the dihedral angle, this flow strikes the underside of the lower wing with a great angle of attack that the higher wing, and thus the lower wing now generates more lift than the upper wing, providing a restoring force to return the plane to straight and level flight. The exact opposite occurs with anhedral wings, where the wings point downwards. Here when sideslip occurs the upper wing achieves more lift due to a greater angle of attack, causing the plane to roll more. High mounted wings tend to be anhedral. Some sources will tell you that this due to the pendulum effect where the weight of the plane itself acts as a restoring force, like how a pendulum will naturally return to its original position when disturbed, but this is incorrect and I nearly uploaded this explanation on Saturday before thinking about it a little harder and realising that that didn't make any sense. The actual reason high mounted wings are roll stable is a result of how the airflow flows around the fuselage in sideslip. With high mounted wings it will be deflected into the underside of the wing, increasing lift on the lower wing and thus providing a restoring moment, while a low mount wing will have the airflow deflected into the top of the wing providing a moment which will increase the roll. So plane designers will use dihedral wings on low mounted wings to counteract the instability of low mounted wings and use anhedral wings to offset the stability of high mounted wings. We can see examples of anhedral wings in planes like AV-8B Harrier, where the designers needed to use a high mounted wing in order to utilize the direct thrust nozzles that would otherwise strike the top of the wing in a low mounted wing. Perhaps the most impressive example of high mounted anhedral wings is with the Antonov AN-225. The largest plane in the world. I could not find a reference to back this up, but I assume they chose this wing configuration to increase ground clearance for it's 6 turbofan engines, and thus allow the plane to have extremely short landing gear. Which would reduce the material needed for them, and make loading procedures much easier. We also see examples of low mounted wings with no dihedral angle at all, like the F-16. This made the F-16 slightly unstable in roll, allowing for better maneuverability. The F-16 was infact one of the first planes in existance to deliberately introduce instability for the benefit of energy efficient maneuvering. A technology made possible by fly by wire computers. Angling the wings like this does come with disadvantages. One of them is its effect on fuel economy. As stated earlier, only the vertical component of lift contributes to getting the plane off the ground, and thus the horizontal component is wasted energy which results in additional fuel being used, but the effect is tiny if we keep the dihedral angle relatively small. The loss in vertical lift is proportional to the cosine of the dihedral angle. A wing with a dihedral angle of six degrees, like a Boeing 737, will lose just 0.55 percent of it's vertical lift in exchange for roll stability. (Cos (0) - Cos (6) = 0.005478)  The configurations mentioned above are the most common, but there are configurations that appear in special circumstances. The F4 Phantom, a fighter created before fly by wire technology was mainstream, was found to be unstable during the testing phase and thus they retrofitted a dihedral angle to the wing tips late in the development stage to increase roll stability. This is called cranked dihedral. One of my favourite planes. The F4U Corsair features inverted gullwings where the wings initially protrude out with an anhedral angle before sweeping back to dihedral. This wasn't for any fancy aerodynamic reason, but for a similar reason to our AN-255. When the F4U Corsair was being designed the engineers had some design criteria they had to hit, namely it was required to be capable of achieving a top speed of 640 kilometres per hour.  This would require not only a large engine to achieve the power needed to overcome the immense drag at this speed, but a large propeller to provide thrust. This created ground clearance issues for the plane that would require longer landing gear, but a longer landing gear would need to be stronger, heavier and would take up more space. Instead the designers decided to bring the landing gear closer to the ground by curve the wing downwards where it was located. The gullwings of seaplanes again use them for clearance reasons, as they try to lift the wing mounted engines clear of damaging sea water spray. These are just some examples of how dihedral angles change for different design criteria, but from here you should be able to look at any plane and understand why it chose a particular wing dihedral or anhedral angle. Like the Junkers JU-87 another inverted gull-wing world war 2 era plane flown extensively by the German's. I will be exploring many of these planes and other less examined factors that influenced the outcome of World War 2, in my new Logistics of D-Day series available exclusively on Nebula. The streaming service, my creator friends and I made to provide a new place to upload our content free from the shackles of the YouTube algorithm. In my debut episode I explore the reasons Normandy was chosen as the final landing location, factoring in everything from Allied fighter plane ranges to the geology of Normandy. 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