critical wing

[fly33]

What is the meaning of "critical wing " ?

Is this just another way of saying : " the moment Critical Mach Number occurs ???"

Or what....

[Ref + 10]

Howdy Fly 33,

I have heard this term thrown around as well but you made me check my books. The other better informed than me may well correct me but I agree with you in that it is a speed.

The way I understand is that it is the speed where acceleration beyond that speed will result in a marked increase in drag. It also relates to the formation of a shockwave where the airflow decelerates below the speed of sound on the area aft of the max camber.

Below Mcrit (from my undesrstanding) there is no shock wave along the length of the airfoil. I have also read that the Citation X has a permanent shockwave over the wing that can be seen by the passengers and it looks like a lenticular cloud.

I'm not exactly sure of it's significance in this forum but would love to hear from anyone who wants to enlighten me.

Hope I've helped and not caused more confusion.

Ref

[Keith.Williams.]

If you mean a "supercitical wing" it is one which has been designed to give an increased critical mach number.

Critical mach number (MCrit) is the lowest aircraft (freestream) mach number at which the airflow at any point on an aircraft is equal to the local speed of sound (Mach 1). If we confine our considerations to the wings, then MCrit is the lowest mach number at which the airflow over the wings reaches Mach 1.

Air flowing over the wings is accelerated, so its velocity becomes greater than the freestream TAS of the aircraft. When this acceleration plus the freestream TAS equal Mach 1, the aircraft is at its critical mach number. So if we express the acceleration over the wing in terms of mach number, we can say that MCrit is 1 minus the acceleration over the wing. If for example the air accelerates by Mach 0.2 over the wing, then MCrit will be 0.8

In order to increase the speed at which a subsonic aircaft can cruise, the value of MCrit must be increased. To do this it is necessary to reduce the rate of acceleration of the airflow over the wings. This is done by flattening the upper surface to reduce its camber. But this also reduces the lift generated, so supercritical wings also use a reflex camber or concave surface under the rear area of the lower surface.

The reduced camber also permits either a higher MCrit or a thicker wing which makes it easier to support the flight loads. In practice most supercritical wings make a trade off between thicker wings and higher MCrit. So overall supercritical wings tend to be flatter and fatter (thicker) than conventional wings and also have a reflex camber on the lower surface.

[fly33]

Hello Ref+10 and Keith.Williams, thanks for your reply . Appreciate it ! And yes , that's what I thought as well. And I do agree .

[Celtic Frog]

Guys, I've also come across this term with a few aircraft types, the one that comes to mind is the Dornier 228 turboprop..nothing to do with mach numbers .
It's described as having a "critical wing" because of it's streamlined shape making it more efficient / economic though nowhere near any critical mach number.
An aerodynamicist could likely tell you a lot more than me, but as I understand it, it's referred to a "critical" because it absolutely hates ice build up.
A big fat chunky wing of ..let's say an HS 748, has a well pronounced camber on its top surface, and so can carry quite a lot of ice build up before that camber shape changes significantly enough to seriously reduce th eamount of lift it produces.
A smaller sleeker wing with less camber, like that of the Do228, can only cope with a small amount of ice build up before its shape and therefore lift-producing capability starts to diminish.
But I have no idea how the designers calculate at which point such a wing can be described as "critical"

[FlapsOne]

'The unique design of a Supercritical Wing reduces the effect of shock waves on the upper surface near Mach 1, which in turn reduces drag.'

That's the NASA view.