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Supersonic airflow above wing

Posted: Sat Jan 02, 2021 1:01 pm
by Sokes
Sorry, I didn't want to start a new topic. We discussed this recently, but I can't remember the topic name. If you remember, please post the link so that we can ask Mods to merge the topics.

I was wondering from which speed one avoids supersonic flow above the wing.

Also:
How does the A330 Neo wing compare with B787?
Assuming same mission, does the A330 Neo fly slower than A330?

Re: Supersonic airflow above wing

Posted: Sat Jan 02, 2021 2:09 pm
by Flow2706
The exact speed (which is actually a Mach number, called critical Mach number, Mcrit) to avoid supersonic flow on the top surface of the wing is depending on the design of the wing, so it's quite variable. Anyhow, modern jet transport aircraft are using supercritical profiles on the wings. These are designed to delay/reduce the onset of wave drag even if a part of the airflow is supersonic. The more important number in aircraft design in the Mdd, Mach drag divergence, i.e. the mach number at which wave drags becomes significant. For supercritical profiles Mdd is higher than Mcrit. Operationally neither of those two speeds are significant, the number relevant for operations is Mmo, which is the maximum operation mach number.

Re: Supersonic airflow above wing

Posted: Sat Jan 02, 2021 4:21 pm
by zeke
Sokes wrote:
Assuming same mission, does the A330 Neo fly slower than A330?


I would assume the neo would fly slightly higher the the ceo at the same weight. The Mach number I would think would be the same, however the aircraft flying lower may have a slightly higher TAS.

Re: Supersonic airflow above wing

Posted: Sat Jan 02, 2021 5:32 pm
by Sokes
TAS meaning?

Re: Supersonic airflow above wing

Posted: Sat Jan 02, 2021 5:59 pm
by LyleLanley
True Air Speed. It's the velocity of the aircraft relative to the air it's flying in.

Re: Supersonic airflow above wing

Posted: Sun Jan 03, 2021 1:04 am
by Starlionblue
LyleLanley wrote:
True Air Speed. It's the velocity of the aircraft relative to the air it's flying in.


To expand on that, TAS takes Indicated Airspeed (what the pitot tube is reading) and corrects for instrument error, position error, compressibility, and density.

Re: Supersonic airflow above wing

Posted: Mon Jan 11, 2021 7:26 am
by T54A
To answer the first question in simple terms: Each aircraft will have a different limiting speed to avoid the onset of supersonic flow above the wing. For a B744/A350 it will be just above Mach 0.9 (maybe 0.92. For a slower type like the A330/340 or B777 it will be in the high M08’s. This is a very simple explanation for a very technical question.

Re: Supersonic airflow above wing

Posted: Mon Jan 11, 2021 8:50 am
by Starlionblue
T54A wrote:
To answer the first question in simple terms: Each aircraft will have a different limiting speed to avoid the onset of supersonic flow above the wing. For a B744/A350 it will be just above Mach 0.9 (maybe 0.92. For a slower type like the A330/340 or B777 it will be in the high M08’s. This is a very simple explanation for a very technical question.


Supersonic airflow is not avoided. It is there as soon as the aircraft accelerates into the transonic region, which happens well before cruise speed.

Re: Supersonic airflow above wing

Posted: Mon Jan 11, 2021 3:00 pm
by IADFCO
Starlionblue wrote:
T54A wrote:
To answer the first question in simple terms: Each aircraft will have a different limiting speed to avoid the onset of supersonic flow above the wing. For a B744/A350 it will be just above Mach 0.9 (maybe 0.92. For a slower type like the A330/340 or B777 it will be in the high M08’s. This is a very simple explanation for a very technical question.


Supersonic airflow is not avoided. It is there as soon as the aircraft accelerates into the transonic region, which happens well before cruise speed.


In fact, if the lighting is just right (sun overhead at just the right angle) one can see the "shadow" of the shock over the wing from a window seat. It looks like a thin grey line on the upper surface of the wing.

For a general explanation of the physics involved: https://en.wikipedia.org/wiki/Shadowgraph.