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ptrjong
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Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 4:48 am

Dear forum,

Could somebody explain - in layman's terms if possible - why wing sweep recuces drag for fast-flying aircraft? What does it actually do?
I've read a few different explanations for this over the years, but none of them has satisfied me. And surprisingly, the basic subject does not seem to have been discussed here.

Thanks in advance,

Peter
The only difference between me and a madman is that I am not mad (Salvador Dali)
 
vikkyvik
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 5:11 am

Basically, you turn your airfoil shape at an angle to the oncoming airflow. The airflow therefore "sees" a thinner (or less cambered) airfoil. This reduces the acceleration of the air somewhat over and under the wing. So, for the same airspeed, with a swept wing, the air is accelerating less over (and under) the wing than it would be with a straight wing.

This reduced acceleration means that you can fly faster and still have subsonic airflow over the wings. The airplane Mach number at which any local airflow goes supersonic is called the critical Mach number. So, by sweeping the wings, you're increasing the critical Mach number.

Drag starts to rise when airflow goes supersonic, due to shockwaves, etc. So by sweeping the wings, you're delaying the onset of this drag till a higher Mach number.

So:

swept wings = slower local air velocity = higher critical Mach number = later onset of shockwaves = later onset of rise in drag

At least, that's the way I learned it.

Note that by turning the airfoil at an angle, you'll also need to go faster to generate the same amount of lift. But since that's what you were trying to achieve in the first place, it works out well.

Let me know if I missed anything.

~Vik
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ptrjong
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 5:30 am

Thanks a lot Vik,

That's a very good explanation - some bits I read are coming together now.

One of my books said that at higher supersonic speeds, when the airflow over a swept wing does get completely supersonic, it doesn't matter any more - and that that was why the F-104 Starfighter had straight wings. I don't believe that - if that were true, the Concorde could have had straight wings I suppose. I assume the 104 could just get away with its very thin wing in the transsonic region.
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Starlionblue
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 5:52 am

Quoting Ptrjong (Reply 2):
One of my books said that at higher supersonic speeds, when the airflow over a swept wing does get completely supersonic, it doesn't matter any more - and that that was why the F-104 Starfighter had straight wings. I don't believe that - if that were true, the Concorde could have had straight wings I suppose. I assume the 104 could just get away with its very thin wing in the transsonic region.

Indeed. Once you are supersonic it doesn't matter. But since it takes massive energy expenditure to get past the transonic region, a swept wing which decreases energy requirements, is a good idea.

Incidentally, the advantages of a swept wing were more or less stumbled upon by the designers of the Me-262 Schwalbe. Originally it was to have a straight wing, but weight and balance was way off due to the wing mounted engines. So they swept the wing and all of a sudden speeds were higher than anticipated. The proposed Schwalbe II and III had much more dramatic sweep.

[Edited 2006-07-21 22:54:13]
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aeroweanie
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 7:13 am

To quote the great aerodynamicist R.T. Jones (as related in Bill Sears autobiography "Stories From a 20th-Century Life"):

"Suppose a cylindrical wing (constant chord, incidence, etc.) is placed in an airstream at an angle of yaw - ie., it is swept back. Now, even if the local speed of the air on the upper surface of the wing becomes supersonic, a shock wave cannot form there because it would have to be a sweptback shock - swept at the same angle as the wing - ie., it would be an oblique shock. Such an oblique shock cannot form until the velocity component normal to it becomes supersonic."
 
SlamClick
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 8:58 am

Quoting Ptrjong (Reply 2):
the F-104 Starfighter had straight wings. I don't believe that - if that were true, the Concorde could have had straight wings I suppose.

To make the comparison more apple to apples take a design that was a contemporary of the F-104, the F-102 and F-106. The 102 was underpowered and had an inefficient intake so its performance was disappointing but the F-106 might be compared with the 104 better than Concorde could.

Each of these fighters represented a different solution to the problems of trans sonic and supersonic flight. The 106 achieved its goals with a delta wing rather similar to that on Concorde. Convair went with the delta wing similar to what had flown on the XF-92 and Lockheed used the diamond-section 'straight' but tapered wing similar to that seen on the Douglas X-3 Stiletto or the fins of the missiles of the time. Any of the usual problems associated with the straight wing were probably all but eliminated by the extremely low aspect ratio of the wing. It also had anhedral and very high loading. By contrast the F-106 wing was enormous for such a fast airplane.

I'll let others discuss benefits and penalties associated with these types but suffice to say they were just different solutions, different compromises.
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Starlionblue
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 10:03 am

Quoting SlamClick (Reply 5):

I'll let others discuss benefits and penalties associated with these types but suffice to say they were just different solutions, different compromises.

Indeed. The F-104 went fast in a straight line  Wink
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ptrjong
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RE: Wing Sweep - What Does It Actually Do?

Sat Jul 22, 2006 5:34 pm

The reason I mentioned Concorde is that it actually flew at Mach 2 most of the time, unlike any fighter. But Starlionblue has explained why a swept wing was still a good idea for Concorde.
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Buyantukhaa
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RE: Wing Sweep - What Does It Actually Do?

Mon Jul 24, 2006 8:42 pm

Quoting AeroWeanie (Reply 4):
Such an oblique shock cannot form until the velocity component normal to it becomes supersonic."

Would that component then be along the axis of the cylinder, or normal to both the axis and the airflow?
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vikkyvik
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RE: Wing Sweep - What Does It Actually Do?

Mon Jul 24, 2006 9:41 pm

Quoting BuyantUkhaa (Reply 8):
Would that component then be along the axis of the cylinder, or normal to both the axis and the airflow?

I believe that's the velocity normal to the shockwave itself. Also normal to the axis of the cylinder.

~Vik
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3DPlanes
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RE: Wing Sweep - What Does It Actually Do?

Tue Jul 25, 2006 12:14 pm

I'm going out of my expertise here, so any of you engineers can feel free to smack me down...

As I understand it, a big reason for using a delta wing (like on Concorde) is to keep all of the aircraft within the shock cone trailing back from the nose at the higher Mach numbers.

With the F-104 having the wings set well back from the nose, and being as stubby as they were, they didn't need to use a delta shape to keep the wings clear of the shock...
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aeroweanie
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RE: Wing Sweep - What Does It Actually Do?

Tue Jul 25, 2006 1:00 pm

Quoting BuyantUkhaa (Reply 8):
Would that component then be along the axis of the cylinder, or normal to both the axis and the airflow?

Along the axis of the cylinder...
 
BoeingOnFinal
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RE: Wing Sweep - What Does It Actually Do?

Tue Jul 25, 2006 6:58 pm

Quoting Starlionblue (Reply 3):
Indeed. Once you are supersonic it doesn't matter.

What doesn't matter? Could you explain that a bit further please  Smile
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Starlionblue
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RE: Wing Sweep - What Does It Actually Do?

Tue Jul 25, 2006 8:34 pm

Quoting BoeingOnFinal (Reply 12):
Quoting Starlionblue (Reply 3):
Indeed. Once you are supersonic it doesn't matter.

What doesn't matter? Could you explain that a bit further pleas

Let's start with what Ptrjong says.

Quoting Ptrjong (Reply 2):

One of my books said that at higher supersonic speeds, when the airflow over a swept wing does get completely supersonic, it doesn't matter any more - and that that was why the F-104 Starfighter had straight wings. I don't believe that - if that were true, the Concorde could have had straight wings I suppose. I assume the 104 could just get away with its very thin wing in the transsonic region.

I'll preface by saying I am a layman, so I may not be getting this right, but here goes.

Sweep delays shockwaves so that you can fly closer to the speed of sound with less energy. These shockwaves are caused by the compression of air with increased speeds. Once you get through the sound barrier, these shockwaves will be behind the aircraft so wing shape becomes less relevant to the shock wave phenomenon.
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vikkyvik
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RE: Wing Sweep - What Does It Actually Do?

Tue Jul 25, 2006 10:05 pm

Quoting AeroWeanie (Reply 11):
Along the axis of the cylinder...

Ahh. So I was incorrect here:

Quoting Vikkyvik (Reply 9):
I believe that's the velocity normal to the shockwave itself. Also normal to the axis of the cylinder.

Always good to know.

~Vik
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Jetlagged
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RE: Wing Sweep - What Does It Actually Do?

Tue Jul 25, 2006 10:57 pm

Quoting Starlionblue (Reply 13):
Once you get through the sound barrier, these shockwaves will be behind the aircraft so wing shape becomes less relevant to the shock wave phenomenon.

The shock waves won't be all behind the aircraft. They will all have to start from a point on the aircraft. There will be a conical shock from the nose, and leading and trailing edge wing shocks as well, at the very least. Swept wings do reduce wave drag, as I recall, even supersonically.

Concorde's wing was ogival shape partly to keep it out of the nose shock cone, as 3DPlanes suggested above. Also the slender delta provides a good compromise between supersonic drag and low speed lift, without the need for swing wings (as proposed on the Boeing SST) or flaps (so no need for a canard foreplane). The camber and leading edge profile were all carefully optimised, for aerodynamic reasons not just elegance, though they did make the Concorde more beautiful than the Tu-144.
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ptrjong
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RE: Wing Sweep - What Does It Actually Do?

Wed Jul 26, 2006 5:28 am

Quoting 3DPlanes (Reply 10):
to keep all of the aircraft within the shock cone trailing back from the nose at the higher Mach numbers.



Quoting Jetlagged (Reply 15):

Interesting. One of those books I didn't believe gave that as the only reason for sweeping the wings.

Will the airflow at the nose of a subsonic aircraft remain subsonic, ie, will there be no shock cone traliling back from the nose, and is this why airliners can have swept but long-span wings? Or is the cone simply wider due to the slower speed?

What about the X-29/forward sweep? Doesn't this put the wing within the cone? Or is wingspan short enough?
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benair
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RE: Wing Sweep - What Does It Actually Do?

Wed Jul 26, 2006 11:58 pm

One more word to the Concorde-wing

For supersonic flight it is really just two different solutions whether to use a swept wing or a straight but really thin one with a sharp nose.
However, for Concorde there was another design criteria besides supersonic cruise and that was take-off and landing.

A straight wing with thin and sharp airfoil like the F-104 has really bad characteristics for high lift. So take-off and landing speed is way to high for a civilian airliner. However, a delta wing (or a straight wing with 'strake', compare F-18) produces a considerable amount of extra lift at high angles of attack. That is due to vortices generated at the leading edge. The picture shows F-18, but the concept is the same for Concorde.

Big version: Width: 325 Height: 264 File size: 13kb


Only with this additional vortex lift was Concorde able to fly slow enough for take-off and landing.

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