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Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

### "Peaky" Foils Questions

First of all, what about the pressure distribution caused them to be called "peaky" foils?

Second of all, how exactly did they work differently than standard foils? And what aircraft used them?

Andrea K

Jetlagged
Posts: 2564
Joined: Sun Jan 23, 2005 3:00 pm

### RE: "Peaky" Foils Questions

 Quoting Blackbird (Thread starter):First of all, what about the pressure distribution caused them to be called "peaky" foils?

The shape of the pressure distribution shows strong peaks, especially near the upper surface leading edge.

 Quoting Blackbird (Thread starter):Second of all, how exactly did they work differently than standard foils? And what aircraft used them?

They work like standard aerofoils. No advantage to a peaky pressure distribution, plenty of disadvantages.
The glass isn't half empty, or half full, it's twice as big as it needs to be.

F27Friendship
Posts: 1099
Joined: Sat Jul 14, 2007 11:45 pm

### RE: "Peaky" Foils Questions

On the bottom left you can see a pressure distribution of a "conventional" airfoil. There are better examples, but you can see there is a disctinct peak. On the bottom right, you see the pressure distribution of a supercritical airfoil, which uses isentropic recompression to have somewhat constant maximum pressure over the larger part of the airfoil

Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

### RE: "Peaky" Foils Questions

What does isentropic mean? Also, what does the inverse camber on the underside too?

I know the curve in the back (the positive camber) is to produce lift in the back area...

Andrea K

F27Friendship
Posts: 1099
Joined: Sat Jul 14, 2007 11:45 pm

### RE: "Peaky" Foils Questions

This is just an example, but the the inverse camber at the back at the underside is probably there to generate rear loading, something usually done more at the outboard of the wing.

Isentropic means that the airflow has a constant entropy, but then we're getting at thermodynamic theory.
The main thing is, is that it allows for supersonic flow over the wing (above critical c-p, hence supercritical airfoil). There is a first shock, but behind it there are expansion waves, keeping the flow attached, followed by a weaker shock at the end (the steep line).

First Airbus (A300) used a so called sonic rooftop airfoil. It also had a plateau-like c-p diagram, but it was limited just under c-p critical.

Blackbird
Topic Author
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Joined: Wed Oct 06, 1999 10:48 am

### RE: "Peaky" Foils Questions

I thought the airflow accelerated to supersonic speed without a shock, then slowed down and formed a shock?

So it forms a shock, then expansion waves form and then it goes back down to subsonic speed forming a weak shock at the back?

Andrea K

F27Friendship
Posts: 1099
Joined: Sat Jul 14, 2007 11:45 pm

### RE: "Peaky" Foils Questions

Whoops, almost (I explained it wrongly at first), you are right

The flow reaches supersonic flows and through expansion waves recompresses, eventually followed by a weak shock

grandtheftaero
Posts: 247
Joined: Sat Nov 29, 2003 1:05 pm

### RE: "Peaky" Foils Questions

 Quoting Jetlagged (Reply 1):They work like standard aerofoils. No advantage to a peaky pressure distribution, plenty of disadvantages.

There is at least one advantage. As you know, wings are swept in modern transonic aircraft to increase Mcritical. However due to interference effects at the wing-to-body join, the spanwise lines of constant pressure (isobars) along the wing become unswept, even though the physical geometry is swept. Since for subsonic speeds, the freestream interacts with these isobars, the freestream "sees" an unswept wing near the wing-to-body join.

In military aircraft, this can be solved by aggresive sweeping of the wing near the fuselage as seen on the B-1 or F-16. In commercial airliners, the isobars can be tailored by using so-called "peaky" airfoils as described above.

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