Pilot1113
Topic Author
Posts: 2276
Joined: Thu Aug 05, 1999 1:42 pm

Super Critical Wing

Mon Jul 10, 2000 2:51 am

Is the Super Critical Wing in use on modern airliners?

I believe I've heard it was on 717, but I can't be sure. I don't see why more airplanes don't have it, as it does have a lot of advantages.

One of the biggest is that you can hover close to the speed of sound because when the air finally does slow to 759 from 760 MPH, the sonic boom is created at the end of the wing as opposed to the middle where it can create problems. Coffin corner, anyone?

- Neil Harrison
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

RE: Super Critical Wing

Mon Jul 10, 2000 7:19 am

the 777 was the first american airliner to use a true supercritical wing.

However the 757 has a hybrid (half and half) sort of supercritical design. The same goes for the A340. Cessna's Citation X is also a supercritical design.

I assume that you know how a supercritical wing is designed since you asked about it, but you should realize that they don't work very well at low speeds...
 
Buzz
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RE: Super Critical Wing, Part 3

Mon Jul 10, 2000 2:33 pm

Hi Aaron, and Pilot 1113, Buzz here. If you've changed wingtip lights on a 757 you can see the shape. It sure seems supercritical to me.
It looks like the A320 also has some supercritical shape to it.
I'd have to look in the ARG to see if the wing is something special. I wouldn't be suprised, if you can save a percent or two in drag it adds up over the life of the airplane.
g'day
Buzz Fuselsausage; Line Mechanic by night, DC-3 Crew Chief by choice.
 
richie
Posts: 139
Joined: Wed Dec 22, 1999 10:28 pm

RE: Super Critical Wing

Tue Jul 11, 2000 3:06 am

Well, the 717s wing is certianlz not supercritical, as it is nothing but an pbeefed DC9-30 wing. Remember, the 717 (formerlz known as MD95) is a modernised DC9-30, nothing less, and a verz nice machine, also.

The problem with supercritical wings is, that thez are very likelz to used their efficiencz is contamination (rain, insects etc.) so that the ones used by the manufacturers are still a kind of compromise.
 
aaron atp
Posts: 517
Joined: Sun Mar 19, 2000 1:17 pm

RE: Super Critical Wing

Tue Jul 11, 2000 3:53 am

Buzz, take another look at that 757 wing, and tell me which part of the wing appears to be of a supercritical design, root/mid-span/tip...


aaron
 
Pilot1113
Topic Author
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RE: Super Critical Wing

Tue Jul 11, 2000 3:54 am

Thanks all, for your input!

I was under the impression that this was a relatively new technology and wasn't widely in use yet. In addition, I thought that it was the "Holy Grail" of wings. In that it was supposed to work at both high and low airspeeds while shoving the sonic boom off the wing.

- Neil Harrison
 
aaron atp
Posts: 517
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RE: Super Critical Wing

Tue Jul 11, 2000 10:40 am

if you look at something like the original whitcomb supercritical airfoil, you'll be able to see a few of the disadvantages... the first: lift is centered at the rear of the wing because of the aft camber (imagine a flap at the end of the wing) which creates a bad pitching moment and a need for efficiency robbing taildown force. Secondly, it creates difficulty in providing strength and makes control surface/flap design very complex.

Whitcomb (the area rule guy) created his famous airfoil in the mid sixties, so they have indeed been around for a long time; but without the capability to predict supersonic airflow with precision, it took a long time to create feasible designs.

I think a lot of new ideas were discovered when designing the 777 wing because of the advances in analyzing the results and the greater ability to predict supersonic flow and compressibility properties. You'll certainly see even more advanced supercritical designs in the new airbus.

For people that don't have a clue about this discussion, supercritical airfoils work by creating a high pressure area underneath the airfoil, rather than low pressure above it. Much like using an aerating chopper-prop on a high speed racing boat to circumvent cavitation and prop "blow-out".



aaron
 
Pilot1113
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RE: Super Critical Wing

Tue Jul 11, 2000 10:55 am

Aaron atp wrote:
-------------------------------
For people that don't have a clue about this discussion, supercritical airfoils work by creating a high pressure area underneath the airfoil, rather than low pressure above it. Much like using an aerating chopper-prop on a high speed racing boat to circumvent cavitation and prop "blow-out".
------------------------------

To me the profile of the wing looked like it had a flat top and a flat bottom with a little indent towards the end of the wing. Correct me if I'm wrong, it's been awhile since I've seen the sketch.

- Neil Harrison
 
aaron atp
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Neil

Tue Jul 11, 2000 11:45 am

yep, that's it... the little flap at the end creates high pressure under the wing which creates positive lift

whitcomb supercritical airfoil
modern supercritical airfoils aren't designed with such a sharp trailing edge


it has to operate at a higher AOA at low speeds to develop the same amount of lift when compared to conventional wings on the same aircraft, but at high speeds, it is much more efficient. I'm going to hold off delving into the reasons for the sake of brevity.

When examining this airfoil, remember that the NACA 2412 on your cessna doesn't account for effects outside the realm of an ideal gas. Fluids act differently in the transonic realm of compressability. Normal thinking does not apply to the shape of supercritical airfoils, even if you think you have a perfect grasp of the writings of newton and bernoulli. You may think it looks a lot like a symmetric airfoil or an asym that develops negative lift from its camber; but it certainly doesn't act like either at high speeds


aaron
 
fr8tdog
Posts: 120
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RE: Super Critical Wing

Wed Jul 12, 2000 1:24 pm

aaron negative lift from a asyemtric airfoil ? never heard of it nor do my aerodynamic books reflect that .....
The only negative lift would be from a horizontal stab.....
 
aaron atp
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What?

Wed Jul 12, 2000 2:52 pm

umm.. I hope you misunderstood me. Of course an efficient aircraft design would never use a main wing that creates negative lift at neutral angles of attack, but I was only describing the appearance of some supercritical airfoils.

This statement gets me: negative lift from a asyemtric airfoil? never heard of it nor do my aerodynamic books reflect that. You seem to challenge the existense of asymmetric airfoils that create negative lift. More on that later...


The only negative lift would be from a horizontal stab..... This doesn't make sense when coupled with the first statement. Do you misunderstand the meaning of "airfoil" to mean "the main wing of any aircraft?"


an airfoil doesn't need a negative AOA or incidence to create negative lift if its mean camber is negative. What happens if you mount a NACA 2412 upside down as the horizontal stab? Negative lift from camber!

you can design any airfoil to create positive or negative lift from camber at zero AOA... besides, even the wings of your cessna 152 will create negative lift at negative angles of attack greater than about -3 degrees... How else would negative load factors be created?


in the context above, look at the cross-section... see that it has more lower camber from horizontal station .1 to .7 underneath the x-axis than upper camber. We'll call this negative mean camber for that portion of the wing. However, when you look at the wing as whole, you can see that the "flap" that occurs after station .7 gives the wing a positive CLo anyway.


Remember what the coefficient of lift is?
CL = (CLo +2pi(a)) / (1+(2/AR))

where:
CLo is the lift from camber
2pi(a) is lift from AOA "a" ("a" is AOA radians)


if the design of the wing creates low pressure below and high pressure above it at zero degrees AOA, CLo is negative. All you have to do is flip a "normal" asymmetric airfoil over and you now have one that creates negative lift from camber at zero degrees AOA... Of course, the wing can generate negative or positive lift; depending on whether the AOA is positive or negative, and regardless of the sign of CLo.

for the same reason a symmetric airfoil can generate negative or positive lift. CLo just happens to be zero with a symmetric design


Just realize that I was stating that it may look like it creates negative or no lift from camber in my post; but yes, the whitcomb supercritical does indeed have positive CLo. However, you seemed to challenge the fact that CLo could ever be negative...


aaron





 
chris28_17
Posts: 1372
Joined: Sun Jul 09, 2000 4:26 am

RE: Super Critical Wing

Wed Jul 12, 2000 3:08 pm

My head hurts after reading all that! and I thought I was smart... you all have greatly humbled me !!!
Just thought you should know!!!!

--CHRIS--
 
Pilot1113
Topic Author
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Aaron ATP

Thu Jul 13, 2000 7:41 am

Wow!!!

Damn... I'm going to have to print that one out and translate it using my "Aircraft Basic Science" textbook!  

I've always wondered... what does that ATP stand for in your name? Is it Airline Transport Pilot?

- Neil Harrison
 
chris28_17
Posts: 1372
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RE: Aaron ATP

Thu Jul 13, 2000 2:43 pm

I'd like to think its related to our old UAX planes the BAe ATP's (advanced turbo prop)
they were horrible but i miss them.
i certainly doubt it but im curious too aaron!!


CHRIS
 
fr8tdog
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RE: Super Critical Wing

Sat Jul 15, 2000 3:46 pm

Aaron , hmmmm tounge in cheek .... sorry I just reread your post and I misread what your meaning was intended for.... I truly thought that you where indicating the main airfoil.
Now as for negative lift ,the referance to that is, the horizontal stab for stabiliztion purposes between the Cp and Cg of the main lifting wing. Yes if you have a neg aoa there will be negative lift momentarily (unless the wing is in a continuous arc) however its primary design is to provide positive lift even if you roll inverted its still positive lift in climbing, level, or descending flight (provided you dont have neg G's)
I dont have my aerodynamic books with me and just did some quick research on the net. I was looking for pressure distribution and lift vectors on the SCW. you might want to look at RPI.edu under flow dynamics.
to me (which I have limited time) the Cl is at two points on around the leading edge and 2nd around the begining of the aft apex of the upper chamber. they did not do any testing at CLo (if I remember correctly, lift coefficient section at M where M=0) its been a while so dont chastize me to badly (wink)
Paul
 
aaron atp
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Sounds Good To Me

Sun Jul 16, 2000 3:45 am

I think we all know what it looks like in a more "normal" subsonic situation, so here's our DTE airfoil so everyone can get a visual picture of what's going on around a transonic airfoil:
pressure contours

I think we're getting past the scope of this forum now... If I have to explain what a bound vortex is, or prove that Bernoulli's priniple is just a derivitive of the first law of thermodynamics; I'll be pissed. The same goes for DTE airfoils.


I don't think there is anything to argue, so I'll shut up now

 
User avatar
RayChuang
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RE: Super Critical Wing

Sat Jul 22, 2000 9:57 pm

I think the first airliner that used a wing that was strongly influenced by Whitcomb's work at NASA was the Airbus A300B2/B4 series.

While it was not a true Whitcomb shape, it was actually fairly close to the Whitcomb ideal.
 
JETPILOT
Posts: 3094
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RE: Super Critical Wing

Sun Jul 23, 2000 2:51 am

The A300 planes do have a hybrid supercritical airfoil.

The Airbus A300 has to be de-iced anytime the the temperature and dewpoint are within 3 degrees of each other, and the temperature is below 35 degrees F.

They do not take well to any wing contamination.
 
Pilot1113
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Jetpilot

Sun Jul 23, 2000 8:50 am

I agree that they don't take well to contamination. Wasn't it you who said, "When you're flying a contaminated wing, you're basically flying an untested wing..."

- Neil Harrison

P.S. Thanks to all for the responses. Especially Aaron ATP with his pictures and rather indepth discusstion.

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