[kach22i]

Pilotes Anciens: June 2015

A 1:5 model of the 917 as tested in various configurations in the SERA wind tunnel in
February, 1969, two months before the 917 was homologated by the FIA.......

The key to the 917's aero stability was at the rear, and was unlocked with the Horsman K tail (at the Zeltweg test) and a large wing across the full width of the LH tail (in the spring of 1970).

Monday, July 2, 2012
A Long Tale of Many Tails
http://pilotesanciens.blogspot.com/2012/07/long-tale-of-many-tails.html

So here is the story of the 917's tail, based on my own reading and picture research. ...........

In testing it quickly became clear that the 917 was unstable at racing speeds. But the source was not conclusively identified as aerodynamic lift until after the 1969 season was over. Both the short and long tailed cars suffered. It was capable of 190-220 m.p.h., 20% (or more) faster than a 908. The Ford GT-40, the first racing sports car to run 400+ horsepower, had suffered from aero instability too. Ford solved the problem with a rear spoiler and a blunt nose. With power to spare, higher drag was a price Ford was prepared to pay. But the Porsche Way, going back to the 356 road car--the first Porsche ever--was low drag. A "barn door" solution was not going to come from Stuttgart.

Instead, he says, he noticed that while the fronts and windshields of the test coupes were covered with dead gnats, there were none on the rear spoilers. He concluded that air was not flowing to or over them. Wyer felt so strongly that Horsman had been denied proper credit for the breakthrough that he said in his memoir that the revision should be called "the Horsman Tail,"not named after him and his team. .................

Horsman says in his memoir that he was afraid that his idea would be stopped cold by Porsche's racing management because it was high drag and "Not Invented Here." So, toward the end of the test day, he took pains to casually ask if his mechanics might borrow a coupe and some sheet aluminum. This was the modification Horsman came up with: