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Old 11th Oct 2012, 01:17
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Join Date: Jun 2006
Location: B.F.E.
Posts: 212
Slat drag on the DC-9 / MD-XX series


A quick question someone with a background in aerodynamics and experience on DC-9 airframes may be able to help with. In the MadDog, I fly regularly with a gentleman who insists that slats do not create any drag; he also insists that his copilots extend them any time we are below 235 knots "for safety reasons". Even if we are 20 miles out on an unrestricted visual approach. Even if the FMS-targeted CLEAN maneuvering speed is below 220 knots; even if the aircraft weighs 90,000 pounds on approach. Even if the "foot" is at 180. Even if we are 30 miles out and ATC asks us to slow to 230 on descent, at 90,000 pounds with a clean maneuvering speed of 218 knots.

Now, I'm not one to criticize someone for being extra conservative, but as this personal technique was not briefed at any point prior in the flight it resulted in a well-planned "idle to 1000 ft" visual approach getting completely screwed up and was a bit irritating. Even knowing the score on later legs, it remains a bit annoying. As the slats in the DC-9 series obviously create quite a bit of drag at 220-240 knots, we got way low way early and burned a TON of extra fuel. He did not accept the explanation that the slat drag botched the approach, which was planned to be in a clean configuration for as long as practical (per the typical approach profile that most folks here use).

Have of course come to a gentleman's agreement with him that if I can disprove him, he owes me drinks on our next trip. If he can disprove me I owe him. Have demonstrated the contrary position thusly: Set up a clean descent at flight idle at 220 knots; noted the descent rate (1200-1300 FPM). Had him extend the slats. Let the aircraft stabilize at the SAME airspeed. Noted increased descent rate (1700-2000 FPM).

No dice. Tried again on climbout my next leg. Left the slats extended for climbout and selected 230 knots climb speed. Note climb rate appx. 2000 FPM (forget the weight on that leg). Had him retract the slats. Stabilized in climb, CLEAN, still at climb lim and 230 knots, noted improved climb rate of 2700 FPM. Still no dice, insisted that there must be some other factor influencing the climb rate other than the slats. He plans to demonstrate a counterargumentative technique on a later flight, which I await in curiosity (weather intervened and he was unable to find conducive conditions to his demonstration on our last day). However, I believe that I have convincingly demonstrated that he is completely wrong. And I want my beer.

Somebody help me out here. Charts? Diagrams? Technical explanations? For any model DC-9, MD-XX, 717, jeez ANYTHING that might get through to this gentleman? Slats, as you well know, primarily serve the purpose of keeping boundary layer flow attatched at high angles of attack / low airspeed. Their primary function at HIGH airspeed is... nothing. They just stick out into the airstream and create... DRAG. They work GREAT for slowing down in the MD. Any additional lift they do create at high speed is of course balanced by induced... DRAG. How do you explain this to a brick wall? There are beers and Starbucks riding on this; it's important. Help a brother out. Thank you.

Last edited by hikoushi; 11th Oct 2012 at 01:19.
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