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aslan1982
2nd Mar 2010, 15:59
Are the modern boeings and airbus's fitted with super critical wings?

Is this why they can cruise comfortably above their critical mach number because the super critical wing has a much lower wave drag effect after passing Mcrit.

Ive read alot of threads regarding Mcrit and paid special attention to a poster called Old Smokey who seems to know his stuff

He advocates that Mcrit is about .02/.03 below lrc
LRC on 737 800 = .785 so Mcrit would be around .75 ish

Mach trimmer comes in at .615 and the limit on operating without it is .74

all relating to the 737

Above MMO .82 + .02 is where wave drag starts to cause problems and thats why we are limited to a speed just below that ie .82 MMO

Its all kinda confusing depending on what you read and how you interpret it and if im talking absolute nonsense please let me know.

All i have written above has come from posts on this forum and from the 737 technical site

XPMorten
3rd Mar 2010, 16:51
Yes, modern airliners have supercritical airfoils (wing).

Supercritical airfoils DELAY the point of Mach Critical Drag Rise MCDR.
So, in other words you can cruise faster before reaching MCDR.

Acf with supercritical airfoils are designed to cruise just BELOW MCDR while older more
conventional wings like the classic B747 will cruise ABOVE MCDR.

More sweep and a thinner supercritical airfoil will also increase MCDR

Recommend THIS (http://www.xplanefreeware.net/morten/DOCS/supercritical.pdf) document for more details.

XPM

aslan1982
3rd Mar 2010, 17:25
Thanks for your reply

Is the mach critical drag rise higher than the Mcrit number

In your article it says something about the MCDR being 20 % more than Mcrit if I am to understand this correctly. 1st paragraph 14 lines down

regards

aslan

XPMorten
3rd Mar 2010, 19:22
yes, Mcdr > Mcrit

Mcrit is where the local airflow on the upper wing surface starts to
exceed M1.0.
Mcdr is where the DRAG is 20% above the one at Mcrit.

How far apart Mcrit and Mcdr is in speed I guess depends on the airfoil.

http://www.xplanefreeware.net/morten/DOCS/MCDR.jpg

aslan1982
3rd Mar 2010, 21:58
thankyou very much for your help.

this explains alot to me

:D

Mad (Flt) Scientist
3rd Mar 2010, 23:23
Are the modern boeings and airbus's fitted with super critical wings?

Perhaps not. The term supercritical is associated with a patented design, and certainly not all high-speed aircraft use these specific airfoils, and there have been patent cases on that very topic.

Wouldn't surprise me if Airbus uses airfoils with the same design intent, but which are not strictly, legally, "supercritical".

Pugilistic Animus
5th Mar 2010, 04:31
MFS you'd better watch it you might get bad MOJO giving away the secrets like that:}

PA

john_tullamarine
5th Mar 2010, 11:43
supercritical ?

... is that not what one's wife generally is in respect of one generally .. and one's itinerantly errant behaviour specifically ?

galaxy flyer
5th Mar 2010, 13:29
J_T

Strangely, we seem to have married sisters--10,000 miles apart. :}

GF

FE Hoppy
7th Mar 2010, 00:24
Mine was Hypersonic which when combined with Supercritical was really SHOCKING!!

john_tullamarine
7th Mar 2010, 10:55
Then I think that I'll stick to the wife I know and adore ... she's somewhere between lovely and perfect.

rak64
7th Mar 2010, 11:18
Thank you XPMorten for your explanations.

In aerodynamics are 4 phenomena related to supersonic well known.

a) supercritical wing

b) area rule esp. for fuselages

c) supersonic acceleration esp. in rocket motors

d) supersonic diffusers esp. used in ramjets or MIG-21

Every single phenomena is well discovered and explained, but the explanations are unequally.

Lets start with d) supersonic diffusers using chocks oblique and normal ones. The difference is the flow behind a chock, if supersonic it will be a oblique, if subsonic it will be a normal, a straight shock. Short supersonic supersonic means oblique; supersonic subsonic means normal shock.

In c) airstream is accelerated subsonic, reach Mach 1 at the throat, than get accelerated supersonic further in the nozzle. Conclusion in subsonic smaller diameter cause acceleration, in supersonic bigger diameter cause acceleration.

Now we look to b) Area rule say reduce fuselage diameter in the mid section to make the aircraft fit for supersonic. No deep explanations why.

The same with the supercritical wing. I think I can explain why, hopefully i'm right.

Air get accelerated under subsonic conditions if the diameter is reduced (you can see the the first part of a wing in that way). Than it reaching locally a speed which case a shock, that is usually around Mach 1, if flying in high supersonic speeds. The question is: What is the speed behind the shock? If assuming it stay supersonic (what seems strange in the beginning), than look at the diameters and what rule is to apply. If the diameter is widening than a supersonic steam get accelerated! That means if you see the shock as supersonic, than it keeps supersonic if diameter widening. That happens for supercritical wings and for the earea rule!
So its supersonic supersonic means a oblique shock.

If (like in conventional profiles) the diameter following a shock is reducing than the supersonic stream will get slowed down further to subsonic speed. What imply a greater change of speed and a greater energy loss. So the reason for supercritical wing and area rule are the same, keep the speed supersonic to minimise the losses of energy.

Sorry I developed the idea (shockwaves are supersonic in the beginning) while writing, even I did not think through this matter in english before, hopefully you can agree with me.