NASA's Weirdest Experimental Plane

this episode of real engineering is brought to you by brilliant the problem-solving website that teaches you to think like an engineer since its inception in 1958 nasa has been involved in countless strange and bizarre experiments in the name of advancing the united states understanding of aeronautics after all nasa stands for the national aeronautics and space association their mission is to pioneer the future in space exploration scientific discovery and aeronautic research to do this they have built and tested some of the weirdest planes the world has ever seen each designed to push the limits of our understanding and evaluate the value of new innovative designs the x5 featured variable sweep wings that allowed the plane to alter its sweep angle during flight the x-15 a sleek rocket-powered plane designed to push for the record of the fastest plane in existence smashing into the hypersonic regime at mach 6.7 and the x-29 with its odd forward-swept wings gave us valuable data on the flight characteristics of this interesting wing configuration these are all weird and wonderful planes but one stands out to me as the weirdest a plane that looks like the weird lovechild of the x5 and the x29 a variable sweep wing but with a single wing pivoting about a single point resulting in the plane having this bizarre asymmetric wing configuration with one wing pointing forward and one pointing backwards what could the designers of this bizarre plane have been thinking what advantages did they possibly foresee the primary advantage of variable sweep wings was well established before the ad1 with the x5 experimental aircraft which first flew in 1951 when nasa was called naka when the organization was focused purely on aeronautics variable sweep wings have a couple of advantages during takeoff and landing you want to maximize lift so the plane can minimize its speed making it easier and safer to take off and land particularly on short runways like aircraft carriers so a variable sweep wing can take off with its wings set to its lowest sweep angle maximizing lift then as it speeds up and generates more lift it can increase its sweep angle which lowers the dry the wing generates as a result of the lower frontal area increased sweep angle also increases the critical mach number of the plane the critical mach number is the speed at which supersonic flow first appears at some point along the plane's surface this happens before the plane itself reaches supersonic speeds because air speeds up as it travels across the wing's upper surface this can cause some funky aerodynamics as flow separates from the wing's surface when supersonic speeds are reached which can interfere with controlled surfaces and drastically increases drag this prevents some planes from flying faster for example airliners cruise just below critical mach number so increasing the critical mach number is advantageous to understand this let's look at the flow components over the bell x5's wing at its lowest sweep angle 20 degrees here we can separate the airflow into two components spanwise flow which flows along the wing and cordwise flow which flows along the cord of the wing the spanwise flow runs along the length of the wing and does not accelerate and thus does not lower the critical mach number at lower speeds when supersonic airflow is not a worry you want as much of that airflow to be cord wise which is the flow that generates lift however as the speed of the plane begins to rise we are generating more than enough lift thanks to the increased airspeed and thus can afford to convert some of that airflow into spanwise flow we do this by increasing the sweep angle which the bell x5 could do in flight to a maximum sweep angle of 60 degrees as the sweep angle increases a larger portion of that airflow is converted to span wise flow which increases the critical mach number however it can cause some troublesome stall characteristics because a large volume of air is now originating at the root of the wing and traveling to the tip of the wing a stall will begin at the tip of the wing and move towards the roof this is the problem because our ailerons the control surfaces that allow us to roll the plane are located on the outer wing if a stall occurs on the outer wing we lose roll control a major problem for say a fighter jet attempting a high angle of attack maneuver while maintaining full control and this is one of the problems the forward swept wing was trying to fix like the x-29s which we have explored in a previous video this reverses the direction of the span wise flow so it originates at the wing tips and travels to the root of the wing 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 reduces induced drag as a result of the wingtip vortices where high pressure from the lower wing travels and mixes with the low pressure air on top of the wing at the wingtips so the advantages of variable sweep and forward swept wings are established but this plane is some weird indecisive frankenstein's monster blend of both we have one wing forward swept and one wing backward as you have probably guessed this bizarre asymmetry results in some strange control issues side force on the plane the force that attempts to push it to the side increases with the sweep angle as more of that span wise flow is redirected down each wing in a single direction to compensate for this the plane has to increase its bank and side slip angle flight at the maximum sweep angle of 60 degrees at 260 kilometers per hour required one degree of nose right side slip and seven degrees of right side down bank to remain in straight and level flight as we saw the sweep direction impacts where the stall occurs the forward pointing wing will first stall at the root while the rear facing wing will first stall on the wingtip on top of this the rear-facing wing stalled earlier than the forward facing wing meaning we lose lift on one wing first causing the plane to roll and yaw to the left while also having decreased ability to control roll as we lose aileron control on that wing if not recovered this would result in a deep spinning dive there are more stability and control issues with this plane that i can explain in this video but i have linked an excellent nasa report in the references for the aviation nerds that want to really dive deep into this complicated subject the weird aerodynamics were obviously predictable so what were the anticipated advantages of this bizarre configuration the first and most obvious is the simplification of the sweep mechanism a single pivot point for a sweep mechanism is simpler in construction than a twin pivot but it also drastically reduces the stress being placed on the mechanism to understand this let's first see how the forces on the mechanism changes as the sweep angle increases starting with the oblique wing let's imagine this as a simple beam that is being pushed upwards at its end point at zero degree sweep the beam is simply under symmetric bending which means the center pivot point is only being pulled upwards creating tension in the linkage attaching it to the plane as the sweep increases in level flight the forces should remain the same meaning that linkage is only ever in tension in level flight now let's imagine a single wing as it rotates around this pivot point once again we will imagine it as a beam being lifted upwards here there is no equal lift on the other side of the pivot point so this manifests in the mechanism as bending not tension and as the sweep increases we begin to incorporate a torque around the y-axis of the plane as the force lifting this beam gets further from the center of mass these more complicated forces result in a pivot that needs to be stronger and we need two of them so a single pivot point reduces the complexity and structural weight needed to facilitate a variable sweep wing next we reduce wave drag as a result of conforming more closely to the area rule this simple rule tells us the distribution of the cross-sectional area along the length of a transonic or supersonic plane should look something like this that includes the wings engines everything this gradual increase and decrease of cross-sectional area results in a reduction in wave drag which is the drag that arises from shock waves forming around the plane as it approaches the speed of sound so let's take a cross-sectional scan of the early version of the f-102 delta dagger which was designed with a straight fuselage it looked like this we can see sudden bumps where the engine the cells protrude from the fuselage and a steep rise in cross-sectional area as the large delta wing increases in width before a sudden drop along the straight trailing edge of the wing this plane would have had quite a lot of wave drag because it deviates from the ideal distribution so much so the engineers later redesigned the f-102 so the fuselage tapered in along the length of the wing thus decreasing the rapid rise in cross-sectional area as the wing widened this decreased fuel consumption and increased the plane's top speed now you probably see where this is going by having one wing pointing forward and one pointing backwards it naturally distributes the cross-sectional area of the wing over the entire length of the plane allowing it to conform more closely to the area rule and thus decreases the effects of wave drag finally traditional variable sweep wings have one last disadvantage as the sweep of the wing increases the center of lift of the plane shifts backwards which if not compensated for with pitch controls will cause the plane to nose down using the control surfaces in this way will increase drag to keep the plane in straight and level flight by having one wing shift forward and one wing shift backward the center of lift remains relatively static and pitch trim does not need to compensate decreasing the fuel lost to drag the designers of this plane envisioned it as a plane that could take off from short runways and be capable of loitering in the air for extended periods of time making it suitable as an air-to-air refueling tanker or submarine hunter in the end any advantages the designer sought using this flight configuration did not overcome the extreme disadvantages it experienced in stability and control it incorporated all the structural issues associated with ford swept wings which we have explored in a previous video it required extreme trim alterations in straight and level flight and became near impossible for a human to control while performing extreme maneuvers even its more traditional brother in symmetric variable wings fell out of favor in the 70s due to its own disadvantages in heavy mechanisms and trim drag and advances in flight controls and materials made delta wings the new norm but that's a video for another day nasa knows the best way to learn is by applying 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Channel: Real Engineering

Views: 981,723

Keywords: engineering, science, technology, education, history, real

Id: SpiMXHgItHg

Length: 13min 14sec (794 seconds)

Published: Sat Jan 23 2021