I have heard of transonic speed and it is between supersonic and not supersonic. What is it and why does it happen to aircraft and how does it produce a lot of drag. How do you prevent it from happening?
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asked Mar 5, 2019 at 21:43
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$\begingroup$ Related: How does a weak shock-wave boundary-layer interaction create wave drag (other than through direct shock losses)? $\endgroup$– user14897Commented Mar 5, 2019 at 22:27
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2 Answers
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Airflow around aircraft and especially over wing is not uniform. The lowest transsonic speed is when the airflow at some point of aircraft (usually over wing) goes supersonic. Depending on aircraft, this happens around M0.8. When the airflow around entire aircraft is supersonic the transsonic region ends.
The excessive drag is due to the shockwave(s) that forms in the airflow at the location where airflow has accelerated to speed of sound. As airflow ahead of it hits the shockwave the energy of flow dissipates and there is sudden drop in air pressure across the shockwave. As there is less pressure behind the shockwave than there is in front of it additional drag is induced in the process. This called wave drag.
This effect can be counteracted by designing an airfoil that forms the shockwave initially as close as trailing edge as possible and then the entire airfoil goes supersonic as quickly as possible as speed increases. See https://en.wikipedia.org/wiki/Supercritical_airfoil
The formation of shockwave is the reason why supersonic aircraft has pointed noses as air is not pushed ahead of an aircraft. The pointed nose creates an oblique shockwave that has creates less drag along the aircraft flight path.
Pointed cones can also be seen in engine intakes of supersonic aircraft (see SR-71 and Mig-21). The additional oblique shockwave formed by the cone ahead of the engine is used to slow air entering the engine to subsonic speeds. The cone is moved back and forth as mach number changes to keep the shockwave away from the actual engine. Concorde used adjustable ramps within engine intake for same effect.
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1$\begingroup$ That was a helpful answer, but how exactly is this drag force transmitted onto the surface of the wing? To me, this point does not seem as intuitive as for example "form drag", where the force is clearly transmitted onto the surface due to surface friction. Can wave drag exist with zero viscosity effects? $\endgroup$ Commented Dec 9, 2022 at 2:51
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1$\begingroup$ @AdityaSharma It cannot. Wave drag is induced by miniscule imperfections on the wing surface. Layer of air next to the wing that becomes stagnate does not propagate pressure wave upstream as the stream has reached supersonic speed - thus again stopping abruptly next layer as it hits stagnate air. So it is eventually the wing surface that stops ever increasing amount of air, causing the drag. The drag can be so substantial that it was called “sound barrier” as engines of the time could not overcome it. $\endgroup$ Commented Jan 3, 2023 at 13:38
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$\begingroup$ @busdriver Makes sense, thank you! $\endgroup$ Commented Jan 3, 2023 at 21:12
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As I understand it, total pressure (i.e. static + dynamic in incompressible region) decreases across a shock. Wave drag is due to the difference in total pressure across a shock (larger upstream than down stream).
On the other hand, static pressure increases across a shock wave causing an adverse pressure gradient downstream leading to flow separation and a further increase in drag.
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$\begingroup$ Welcome to aviation.se. Do you have any sources or references? $\endgroup$– FedericoCommented Dec 8, 2022 at 6:04