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"Peaky" Foils Questions

Posted: Sun Jul 15, 2007 5:17 am
by Blackbird
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

RE: "Peaky" Foils Questions

Posted: Sun Jul 15, 2007 10:35 pm
by Jetlagged
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.

RE: "Peaky" Foils Questions

Posted: Sun Jul 15, 2007 11:24 pm
by F27Friendship


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

RE: "Peaky" Foils Questions

Posted: Mon Jul 16, 2007 2:30 am
by Blackbird
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

RE: "Peaky" Foils Questions

Posted: Mon Jul 16, 2007 3:27 am
by F27Friendship
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.

RE: "Peaky" Foils Questions

Posted: Mon Jul 16, 2007 10:42 am
by Blackbird
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

RE: "Peaky" Foils Questions

Posted: Mon Jul 16, 2007 6:35 pm
by F27Friendship
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

RE: "Peaky" Foils Questions

Posted: Tue Jul 17, 2007 1:04 am
by grandtheftaero
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.