On May 21, 1999, about 1412 Eastern Daylight Time, a Piper PA-11, N78659, was substantially damaged during a landing at Leesburg Municipal/Godfrey Field Airport (JYO), Leesburg, Virginia. The certificated flight instructor and commercial pilot were not injured. Visual meteorological condition prevailed for the flight that originated from York Airport, York, Pennsylvania. No flight plan was filed for the instructional flight conducted under 14 CFR Part 91. The instructor was introducing the commercial pilot to tailwheel operations, and the commercial pilot had accomplished approximately six landings in the airplane. The instructor stated that during the next landing, they experienced a sudden crosswind, followed by a tailwind. The airplane stalled, bounced, and traveled off the left side of the runway. The instructor further stated that he took control of the airplane, prevented a ground loop, but was unable to keep the airplane on the runway. The airplane struck a runway sign, causing substantial damage to the empennage. The commercial pilot stated "...upon touchdown, the plane veered sharply to the left, going onto the grass and hitting a sign with the horizontal stabilizer. There was a slight crosswind on final, touchdown was normal, then the wind appeared to shift to a tailwind." Examination of the wreckage by a Federal Aviation Administration Inspector did not reveal any pre-impact mechanical malfunctions, nor did the pilots report any. The approach was flown to Runway 35, a 5,500 foot long, 100 foot wide, asphalt runway. The reported winds at JYO, at 1400, were from 270 degrees at 5 knots.