A long time ago, when I'd just begun working in North Central Airline's flight training department, the chief pilot called me into his office one fine day.  He asked me to go over to the local Weather Bureau's office and borrow a wind anemometer.  I can still recall carrying that three cupped thing back to the airline's hangar offices, all the while wondering what in the world he intended to do with it when I got back!

Captain Hinke didn't leave me waiting in suspense for very long!  He told me that we had an airport on the system with an engine overhaul shop located next to the small lean-to serving as our passenger terminal.  Every time one of our DC-3's taxied away from this terminal (four times a day) and made a right turnout, they’d blow up dust and grit from the ramp.  Invariably, some of it entered the shop.  This small engine shop proprietor had rather heatedly discussed the matter with the local station manager and it was left up to us to do something - and - preferably sooner rather than later!

I guess you can see it coming, can't you?  I've won some bets in more recent years, but back in those days Art Hinke won most of them with me.  He asked me whether the wind velocity behind the airplane was greater with the flaps up or down.  I, of course, bit and opined that everyone knew that the good guy always lowered his flaps to prevent blasting the people behind the fence any more than necessary with the props.  Art just slyly grinned and asked me if I wanted to bet ten bucks (a goodly amount of money at the time) on it?  Well, certainly I would!  This was something that everyone knew, be really easy money!  Beware, a fool and his gold are soon parted - especially when dealing with some US Naval Aviation Reserve aviator!

So, we took the anemometer out to the ramp and positioned it about fifty feet or so behind the DC-3’s tail.  Cut me some slack on the exact distance, this was a long time ago but it was a representative distance.  A couple of us stood behind the airplane to hold the anemometer and record the results on a paper tablet.  Art climbed into the cockpit and fired up both Wright engines.  We faithfully recorded the results at (as best I recall) idling speed, 800 RPM and 1,000 RPM, with flaps up and flaps down.  I was amazed; you could watch the little aluminum cups on the anemometer spin faster as the flaps were lowered and slow as they were raised. 

Now, who's gonna be the first to answer why?  I guess we need to picture in our mind the venturi effect.  Lowering the flaps creates a venturi behind the engines at the wing’s trailing  edge, resulting in an increase in velocity of the airflow from the propeller.  Raising them increases the clearance under the wing through which the air must pass, lowering the velocity.  Simple, when you see it.  I wish I still had the bulletin written back then to all the line pilots, in which we described the results.

Now, what did we do at Worthington, you say?  We sent out a bulletin asking the pilots to taxi their DC-3 ahead a hundred feet or so before turning around on the ramp.  That way the prop blast wouldn't needlessly dust off the overhaul shop for Howard Libersky and his opened engines in the shop.  But Old Wives' Tales die hard, popular opinion on the part of the public still thinks that the considerate pilot always lowers his flaps in order to lower the prop wash - even though the exact opposite is actually true!  And it only cost me ten bucks to acquire this particular bit of knowledge.  Looks to me like another of those “danged if you do and danged if you don’t” deals - that simply can't be won!

R.L. Sohn©1999

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