Mastropiero From Spain, joined Dec 2005, 121 posts, RR: 0 Posted (4 years 2 weeks 1 day 22 hours ago) and read 2915 times:
So, I´ve read in lots of threads about parts of the wing going supersonic and I really don´t get it. How is it possible that parts of a rather large piece of metal that flies sub-sonic can reach supersonic speed?
YWG From Canada, joined Feb 2001, 1140 posts, RR: 2 Reply 2, posted (4 years 2 weeks 1 day 22 hours ago) and read 2909 times:
Some turbine prop tips have been known to go supersonic too. The idea is the the tip is farther away from the hub and thus has a greater distance (radius) to travel in the same amount of time as the root of the wing. Thus the Tip has to travel faster.
Airbuske From United States of America, joined Jun 2007, 463 posts, RR: 0 Reply 4, posted (4 years 2 weeks 1 day 17 hours ago) and read 2808 times:
Quoting Pilotpip (Reply 1): As air goes over the top of a wing, it accelerates. If you're going .80 mach, it's possible that some of that air will be going faster than mach 1.
Two technical terms must be understood :
freestream mach number : mach number of the undistrubed airflow that lies far ahead of the body that is moving through the air.
local mach number : mach number of the disturbed airflow that lies close to the body that is moving through the air.
The Mach number displayed on the flight display in the cockpit or in a sales brochure is the freestream mach number. It is the mach number of the undisturbed airflow far ahead of the entire aircraft.
Logically, the freestream mach number of the entire aircraft is the free stream mach number of a particular segment of the aircraft, like say, the wing. As Pilotpip explained, due to conservation of mass, over parts of the upper surface of the wing, the air is moving faster than the "surrounding" freestream air. i.e. the local mach number is greater than the freestream mach number.
This is why even though the pilot reads that the airplane is flying at Mach 0.8, certain parts of the wing will most definately be travelling at speeds greater than Mach 0.8 (not necessarily supersonic).
For a simple to understand yet techinally correct explanation of why this is so, please read the following article written by my favourite professor.
Vikkyvik From United States of America, joined Jul 2003, 8203 posts, RR: 28 Reply 6, posted (4 years 2 weeks 1 day 17 hours ago) and read 2803 times:
AIRLINERS.NET CREW PHOTO SCREENER
Quoting Pilotpip (Reply 1): As air goes over the top of a wing, it accelerates. If you're going .80 mach, it's possible that some of that air will be going faster than mach 1.
I'm sure some of the engineering types around here can give a much more detailed explanation.
There's actually not much more detail to give
To understand this phenomenon, understand that a wing produces lift (without getting into the other ways of modeling it) by having a lower pressure on the upper surface, and a (relatively) higher pressure on the lower surface of the wing. A pressure differential multiplied by an area equals a force.
When moving air goes through a pressure decrease, it gains speed (and vice versa). So the air going over the top of the wing gains speed and loses pressure.
When an aircraft is moving fast enough, the increase in speed of the airflow over the top of the wing will be enough for the airflow to start going supersonic.
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