Re: Boeing 720?

From:         David Lednicer <dave@amiwest.com>
Date:         17 Jul 95 04:29:39 
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	I've been away for a couple of days and I came back to find this
interesting thread on the 720.

	First - Douglas and Boeing both developed transonic airfoils, for
the DC-8 and 367-80, respectively.  The difference was that Boeing got
lucky and theirs worked.  Douglas ran into an early drag rise problem
that necessitated a new leading edge being introduced after several
aircraft had left the assembly line.  (Drag rise is the sudden increase
in drag that happens as the Mach number is increased)  I have both the
707-100 wing and the early DC-8 wing and I would rate Douglas's airfoils
as more sophisticated, but alas, they didn't live up to expectations.
Dick Shevell has written about this and indicates that the initial wind
tunnel tests of the airfoils were conducted at too low of a Reynolds
number.

	For the record, the initial DC-8's wing airfoils were the
DSMA-128 inboard, DSMA-87 at midspan and the DSMA-88 at the tip.  After
the new leading edge was added, the DSMA-87 became the DSMA-277, and I am
not certain of the designations of the other airfoils.  The 367-80 wing
uses recambered NACA 63.5, 64.5 and 65 series airfoils.  I believe the
707 wing also has a leading edge mod that was added to the outboard wing.

	The 720 grew out of the threat from the Convair 990's high cruise
speed.  Bill Cook (Boeing Chief of Engineering at the time) says he knew
that Boeing had made the 707-100's inboard wing airfoil too thick.  When
challenged to get a higher cruise Mach number to equal the 990, he proposed
adding a leading edge glove to reduce the thickness to chord ratio.  Boeing
legend is that the glove is worth .02 in cruise Mach number.  If you look
closely, the 727 wing has a bit of a inboard leading edge glove on it
too.  The 707-320's inboard trailing edge extension (called a Yehudi,
because they were "fiddling around" in the wind tunnel when they
developed it), also lowers the root thickness to chord ratio, accomplishing
a similar improvement.

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David Lednicer             | "Applied Computational Fluid Dynamics"
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