Re: DC-10 Engine pod Fins

From:         David Lednicer <dave@amiwest.com>
Date:         22 Mar 94 09:52:51 PST
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	To quote from Dick Shevell's AIAA paper "Aerodynamic Bugs: Can 
CFD Spray Them Away":

	"DC-10 wind tunnel tests showed a significant loss in maximum lift 
coefficient in the flap deflected configurations, with landing slat 
extension, compared to predictions.  This resulted in a stall speed 
increase of about 5 knots in the approach configuration.  The initial 
wing stall occured behind the nacelles and forward of the inboard 
ailerons.  The problem was traced by flow visualization techniques to the 
effects of the nacelle wake at high angles of attack and the absence of 
the slat in the vicinity of the nacelle pylons.  The solution was 
developed in the NASA Ames Research Center 12 ft. pressurized tunnel and 
turned out to be a pair of strakes mounted forward on each side of the 
nacelles in planes about 45 degrees above the horizontal.  The final 
strake shape was optimized in flight tests.  The strakes are simply 
large vortex generators.  The vortices mix the nacelle boundary layer air 
with the free stream and reduce the momentum loss in the wake.  The 
vortices then pass just over the upper surface of the wing, continuing 
this mixing process.  The counterrotating vortices also create a downwash 
over the wing region unprotected by the slat, further reducing the 
premature stall.  The effect of the strakes is to reduce the required 
takeoff and landing field lengths by about 6%, a very large effect."

	Rumor has it that Douglas has the patent on nacelle strakes, but 
the patent calls for pairs of strakes (one on each side).  Hence, Boeing 
aircraft only use single nacelle strakes (737-300, -400, -500 and 767-200 
and -300).  Boeing calls them "nacelle chines".

	John McMasters relates that Boeing had a similar problem on the 
707-700 (the last civil production 707, which had CFM-56s) and used nacelle 
strakes to fix the stall.

ps - as to why there are now more than twice as many DC-8s in service 
than 707s - the US government has been a major buyer of airline surplus 
707s.  The last I heard they had bought over 200 old 707s, of which 
nearly 100 are at Davis-Monthan, stripped of parts.  It must be nice to 
have the US government supporting your hull value!



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David Lednicer             | "Applied Computational Fluid Dynamics"
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