Visualizing Transsonic Aerodynamics
General Basic Aerodynamics Theory (Without Math Equations)
– Total lift is the vector sum of lift due to Bernoulli and deflection forces (Newton’s Laws)
– Total drag is the vector sum of drag due to induced and parasitic drag
– Mach 1.0 is the speed that shockwaves form in a region of space at a certain temperature.
General Advanced Aerodynamics Theory (Without Math Equations)
– we do not have a perfect description of all the causes and effects that define flight over time, because flight is a complex, 3D, dynamic motion with both near and far airflow effects
– a combination of Navier-Stokes and other equations, plus classical physics can be used to model flight with some utility, but wind tunnel and test flights are still needed to see what really happens
– some examples of observable but mathematically unexplained effects are vortices and flow separation. Also, shockwaves are high frequency events that dance, not always standing waves
– to compensate for the difficulty of modelling flight, designers compensate with large, powered controls and test the flight envelope
– air is not compressible at slow speeds, so still acts as a viscous fluid (fairly easy to model mathematically)
– aircraft propellers cannot be supersonic for various practical reasons (except in NASA X-plane tests)
– flight control surfaces are relatively small and designed and positioned for subsonic flight
– induced drag is significant but increasing engine power is a reasonable solution
– easy to test in a wind tunnel, first invented by the Wright Brothers
– to model airflow, think in terms of flowing water from high pressure to low pressure
– to visualize, stick your hand out of a moving car and change the angle of your palm.
– generally considered to start above airspeeds of Mach 0.7 thru 1.3
– airflow speed over some parts of the airplane are supersonic and compressible
– shock waves form and move aft, causing flow separation, blanking out controls designed for subsonic flight
– flying tail is needed
– can be difficult to test in small wind tunnels due to reflections from the wall
– critical Mach number is highest possible air speed that no part of the plane is supersonic (no shock wave). Laminar flow has achieved Critical Mach 0.92 and is the practical limit on airliner efficiency
– the Bell X-1 used a rocket engine to exceed Mach 1.0 for the first time. The British did build an aircraft capable of exceeding Mach 1.0, the Miles M.52, but never flew it (and rightfully so) for safety reasons. Note the X-1 structure was designed for 18 Gs and Chuck Yeager was uniquely qualified both a mechanic and a test pilot.
– it’s generally believed that no production piston-powered aircraft has actually exceeded Mach 1.0 because of the thickness of wings causing a large bow wave, and control reversal designed for subsonic airplanes
– jet engine intakes must be positioned to avoid the boundary layer with the fuselage to reduce inlet turbulence
– area rule – the cross-sectional (flat plate) area of an aircraft as seen by the relative airflow should be constant. ie. Coke bottle with wings. Good example is the F-5.
– swept wing is not necessary for transsonic or supersonic flight, but does help with area ruling
– wings are most commonly aft-swept, but can be made forward-swept with stiff-enough materials. See the X-29, an F-5 with modified forward-swept wings and a front canard
– to model airflow, think in terms of “3d pipes” and “bubbles”
– the sound barrier is a physical limit for airplanes that are not designed for incompressibility effects (thick wings and no flying tail)
– to visualize, stick your hand out of a moving car with your palm forward (perpendicular to the ground)
– airflow is supersonic around the entire aircraft
– very thin wings and area ruling are needed to reduce wave drag
– significant skin heating effects. The SR-71 skin temperature exceeds 800 F.
– super-cruise (sustained flight above Mach 1.0 without afterburner) takes considerable design effort and metallurgy. The F-22 can supercruise at Mach 1.8 and the Eurofighter at Mach 1.5.
– need supersonic wind tunnel for testing
– control surfaces need to be fairly large and tested for various flight regimes by test pilots before end-users. Often stalls and spins are dangerous.
– X-15 was used for most of USA’s supersonic flight science, mostly flown by Scott Crossfield
– very noisy acoustically as the airflow is “torn” away from the plane instead of flowing around it
– to visualize, do a belly flop from a low or medium height diving board.
Airspeed and Aerial Combat
– head-on passes don’t favor higher air speeds for either airplane, as the closing speed is the same for both airplanes
– maneuvers lose airspeed, so dogfights end up at subsonic speeds. thrust vectoring is very helpful for changing the direction, especially the nose for firing
– spins are not helpful for dogfights, leaving the airplane a sitting duck (slow and out of control)
– rate of climb is always important.