ALexeu
Topic Author
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Supersonic A/C Airfoil

I read somewhere that supersonic A/C have symmetrical airfoil. This didn't make any sense and my
aerodynamics professor told me that i was right (they use asymmetrical). So my question is which airfoils do supersonic A/C have (i assume asymmetrical, but which type of asymmetrical are most widely used, e.g. Concorde)? I am talking about wing airfoil, not tail unit airfoil (which is symmetrical for both subsonic and supersonic).

Cheers

mNeo
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RE: Supersonic A/C Airfoil

From what i have in my aerodynamics textbook, a true supersonic airfoil is indeed symmetrical. But that is not practical because a symmetrical airfoil does not create any lift at AOA of 0 degrees. From what i remember from lecture there was one production military fighter that was with a fully symmetrical wings, (maybe someone could remember what it was F-104??)

And as for what airfoil different planes use, that is a very complicated answer. I know that my Piper Arrow uses an NACA airfoil, but im sure that every plane has a custom airfoil. All i remember about the concorde is that when that plane slowed down, the induced drag went off the chart.

ALexeu
Topic Author
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RE: Supersonic A/C Airfoil

I heard that there is also triangular assymetrical airfoil for supersonic A/C.
Are there any pics of symmetrical airfoil (on wing)?

If we are using Bernoulli's principe then symmetrical airfoil would not produce any lift...

Cheers, Alex

joness0154
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RE: Supersonic A/C Airfoil

 Quoting AlexEU (Reply 2): If we are using Bernoulli's principe then symmetrical airfoil would not produce any lift...

I don't have much experience with airfoils, etc.

But wouldn't a symmetrical airfoil produce lift at any AOA other than 0? The air going over the wing would have a longer distance to travel than the air under it, which produces a relative low pressure, etc...

Thats what I would think anyways....
I don't have an attitude problem. You have a perception problem

jetmech
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RE: Supersonic A/C Airfoil

 Quoting AlexEU (Reply 2):If we are using Bernoulli's principe then symmetrical airfoil would not produce any lift...

IIRC, many supersonic airfoils are a very thin symmetrical diamond shape in cross sectional. These air foils produce lift at angles of attack other than zero by a combination of shock waves over the top and bottom surfaces.

 Quoting Mneo (Reply 1):All i remember about the concorde is that when that plane slowed down, the induced drag went off the chart.

The Concorde has a delta wing, and lift was produced at low speeds by having a very high AOA. This produced massive vortex structures from the leading edge over the top surface of the wing. Because the vortex structures spun so fast the pressure inside them was reduced, which gave a net lift force. Unfortunately, this method of producing lift is not really too efficient, so you also get lots of drag.

Regards, JetMech
JetMech split the back of his pants. He can feel the wind in his hair .

lehpron
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RE: Supersonic A/C Airfoil

A diamond shaped triangular wedge foil produces the lowest wave drag superonically, but subsonic flow doesn't like sharp corners and adds turbulence drag subsonically. So most supersonic airfoils, whether fins or wings, are bi-convexal with a small angle of attack to generate lift for wings. They are symetical with an angle, don't need much maybe 1-2 dgerees is more than enough above Mach 1.5.

There is no standard for supersonic foils, every wing is tailored for a specific flight category. Generally, thinner foils are best to reduce drag (and thus thrust and fuel consumption) but lift drops so a larger surface is needed. But thinner foils aren't subsonic freindly, and foils have to be changed in some way so that the plane can land and takeoff.
The meaning of life is curiosity; we were put on this planet to explore opportunities.

boeingfixer
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RE: Supersonic A/C Airfoil

 Quoting AlexEU (Thread starter):I am talking about wing airfoil, not tail unit airfoil (which is symmetrical for both subsonic and supersonic).

You may want to refresh your understanding of horizontal stabilizers and what they do before you make a statement like that. On fixed wing aircraft, the centre of lift is behind the centre of gravity which requires a down force from the H-Stab. That down force requires that the H-Stab is either at a negative angle of incidence or an inverted airfoil in the case of larger aircraft.

Cheers,

John
Cheers, John YYC

aeroweanie
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Joined: Fri Dec 03, 2004 9:33 pm

RE: Supersonic A/C Airfoil

Some supersonic aircraft do have symmetrical biconvex airfoils (F-104 for example), others don't. Judge for yourself from this list: http://www.ae.uiuc.edu/m-selig/ads/aircraft.html

Blackbird
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RE: Supersonic A/C Airfoil

AlexEU,

That's not entirely true!

If you had a straight-wing for supersonic use, or possibly if the shockwave swept back beyond the sweep-angle of the wing you'd use a symmetrical foil.

The reason is that the some of the principles behind a regular airfoil -- the curved portion of the top section of the foil producing a low-pressure zone on the top seems to go largely in reverse at supersonic speeds. Convex curvature increases pressure and slows the airflow down which reduces lift. AoA will have to be increased to negate this, and more to produce positive lift.

While inverse-camber would be better for supersonic flight as pressure would be increased on the bottom, it often isn't exactly the best choice as the design would produce negative lift at subsonic speed requiring higher AoA's to get positive lift while subsonic. Since supersonic foils are kind of thin, this could make takeoff speeds very high, and critical AoA's quite low. So instead symmetrical are typically used since it's the next best thing. Sure at 0-degrees AoA it produces no lift, but once you get the AoA up you start producing lift. It produces better performance at supersonic speed than a standard subsonic airfoil, without substantial losses at subsonic speed (although there are some losses). I should note that even with this said, you will still require a higher AoA at supersonic speed as the L/D ratio falls off due to the thicker boundary layer, increased turbulence, and a lower pressure differential (the low-pressure zone up top isn't as low as at subsonic speed), but it would be substantially better than a subsonic airfoil.

These particular airfoils only have some limited applications. Straight wings experience a massive shift in the center of pressure (on a straight wing once the upwash up front goes away, the center of pressure shifts back to the 50% mark). This produces a strong nose-down tendency in supersonic flight requiring considerable longitudinal trim deflections which produce excessive amounts of drag if you're looking for endurance. Not to mention, the thickest point on the wing is often half way down the wing (due to the center of pressure being at 50% instead of 25% at subsonic flight) which does degrade low-speed performance (Still better than an inverse camber supersonic-foil). The airfoil-thinness can pose a problem as fuel might not be able to be carried in the design (the F-104 could not carry any fuel in it's wing and needed the fuselage to do the whole job). The serious advantage they do have is that they have very low drag when flow over them is fully supersonic compared to other airfoil types. They call this supersonic-drag. In fact even if shockwaves produced on the other parts of the plane sweep beyond the wing's leading edge, it's not that big a deal. Tapered, or trapezoidal wings (low-sweep) a'la the F-104 type will feature a shockwave that will form on the root and sweep back... even if they sweep back well beyond the wing's leading edge, it's not a significant problem. In fact, the F-104 aircraft's geometry (if airframe and temperature restrictions weren't an issue -- and they are) could easily allow for speeds in excess of Mach 3 maybe 4 (I've been told it's shape could allow for twice the speed if engine temps and structual limitations didn't matter)

There are other types of airfoils though used for supersonic flight though and a large number of them do not have fully-supersonic flow over them.

Now there are other wing-designs that exists, but I'm rather tired right now and feel frankly like getting a good nap.

Andrea Kent

ALexeu
Topic Author
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RE: Supersonic A/C Airfoil

Thanks a lot for replies    you answered my questions.

Jetlagged
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RE: Supersonic A/C Airfoil

 Quoting AlexEU (Reply 2):If we are using Bernoulli's principe then symmetrical airfoil would not produce any lift...

This is a mis-use of Bernoulli's equation, combined with the erroneous "equal time" theory of lift. You cannot predict the pressure distribution around an aerofoil merely by comparing the length of the upper surface to the lower surface. In this way, a flat plate would produce no lift. The heavily cambered thin aerofoils, as used in the early days of aviation, would not produce more lift than an uncambered aerofoil of the same thickness.

Symmetrical aerofoils produce lift at an AOA, and some subsonic aircraft used them too. Not producing lift at zero AOA is not a disadvantage. All wings have a zero lift AOA, whether symmetrical or asymmetrical section.

An aircraft designed only for supersonic flight would probably use rhomboid section symmetrical aerofoils. Many air launched missiles do have fins with such sections. However most supersonic aircraft spend most of their time flying subsonic. So a compromise has to be reached between subsonic efficiency and minimal wave drag when supersonic.
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ALexeu
Topic Author
Posts: 1447
Joined: Tue Oct 16, 2007 9:01 am

RE: Supersonic A/C Airfoil

So, my aerodynamics proffesor was either confused or wrong... i will ask again. However she did mention rhomboid airfoil.

 Quoting Boeingfixer (Reply 6):That down force requires that the H-Stab is either at a negative angle of incidence or an inverted airfoil in the case of larger aircraft

AFAIK, supersonic a/c do not have elevators, they move the hole h-stab, but i understand what are you saying.

Cheers

Blackbird
Posts: 3384
Joined: Wed Oct 06, 1999 10:48 am

RE: Supersonic A/C Airfoil

AlexEU,

Not all supersonic aircraft move the whole stab for pitch control. The HSCT concept, which to my knowledge was plenty stable featuerd a large elevator for pitch control and a horizontal stabilizer for trim.

Fighters often move the whole stabilizer for pitch control for the purpose of improved pitch control for pulling high-G forces...

Andrea Kent

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