Why Does Wing Dihedral Make Planes Stable?

this episode of real engineering is brought to you by curiosity stream sign up today and get free access to my new logistics of d-day series on watch nebula comm 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 jet fighters are designed to be aerodynamically unstable and 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 wind dihedral angles which is simply the angle the wing makes with respect to a perfectly horizontal line 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 world war ii era fighters and everything in between like modern-day f-16s with low mounted straight wings or stranger still the gull wings of sea planes and inverted gull wing 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 winks meaning they point upwards in straight and level 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 the wing 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 role stability which we explored briefly in my white jet fighters can be to on stable 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 pilots an important safety feature this works as a result of a plane's tendency to sideslip when it rolls meaning the plane begins to move sideways and downwards in the direction of the bank this introduces a new airflow to the wing which has both a vertical and sideways component as a result of the dihedral angle this flow strikes the underside of the lower wing with a greater angle of attack than 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 winks where the wings pointed downwards here when site slip 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 is 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 realizing 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 inside slip with high mounted wings it will be deflected into the underside of the wing increasing the lift on the lower wing and thus providing a restoring moment while a low mounted 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 the 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 is 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 its sixth 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 f16 this made the f16 slightly unstable in role allowing for better maneuverability the f16 was in fact one of the first planes in existence 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 6 degrees like a Boeing 737 will lose just 0.55 percent of its vertical lift in exchange for a roll stability 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 wingtips late in the development stage to increase roll stability this is called a cranked dihedral one of my favorite planes the f4u Corsair features inverted gull wings where the wings initially protrude out at an an Hedra angle before sweeping back to dihedral this wasn't for any fancy aerodynamic reason but for a similar reason to our a n2 55 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 km/h this would require not only a large engine to achieve the power needed to overcome the immense drag at this speed but enlarged propeller to provide the 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 curving the wing downwards where it was located the gull wings of sea planes again used 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 shows a particular wing dihedral or anhedral angle like the yonkers ju 87 another inverted gull wing World War - IRRI plane flown extensively by the Germans I will be exploring many of these planes and other less examined factors that influence 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 explored the reasons Normandy was chosen as the final landing location factoring in everything from Allied fighter plane ranges to the geology of Normandy it's a fascinating topic that you can learn more about for just $2.99 by signing up to curiosity stream who 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Channel: Real Engineering

Views: 733,406

Rating: 4.947166 out of 5

Keywords: engineering, science, technology, education, history, real, plane, aviation, aeronautical, anhedral, dihedral, f4u

Id: I8iLR2xRNKY

Length: 9min 39sec (579 seconds)

Published: Sun Nov 10 2019