Tuesday, January 5, 2016

SERA And The Porsche 917, Part Two

Note: Almost all factual information in this post comes from Porsche: Excellence Was Expected (pp. 675-678 and 685-690), by Karl Ludvigsen, and Porsche 917: Archive And Works Catalog, by Walter Naher.

It was news to me that Porsche consulted with SERA on 917 aero before the "Pink Pig" that ran at LeMans in 1971.  (Search this blog on "SERA" if you're not familiar with the Pink Pig.)  SERA and Porsche collaborated on significant aero development for the 917 Langheck (LH) before, during, and after LeMans in 1970--and finally "fixed" it for LeMans 1971.

Before we get into development, some background:

The body shapes of the 907, 908, and the first 917 LH models were essentially the same.  The 907 LH was developed in wind tunnel tests.  It had the lowest drag coefficient of any racing Porsche, then or later.  This was why, with only 2.2 liters, it became another Porsche "giant killer."  The 908 LH was a slightly upsized copy of the 907.  It was not wind-tunnel tested until after its configuration was finalized.  At 3.0 liters, it suffered no stability problems at speeds approaching 200 m.p.h.  The 917 LH was again a slightly upsized copy of the 908, and it too was not wind-tunnel tested until after configuration was finalized.

The original 917 LH at the LeMans Test Days, 1969: essentially a 907 or 908 LH, but unstable.  The bell-crank operated
winglets didn't help, and were illegal under FIA regulations after LeMans anyway.  Back to the drawing board.  SERA
was not involved in shaping this body.

After the 917 was introduced in the spring of 1969 it quickly became clear that it was unstable at racing speeds.  But the problem was not conclusively established to be aerodynamics until October.  Porsche fiddled with spring rates and suspension geometry.  There was no time to change the LH body before LeMans, but soon after the race it went into Porsche's wind tunnel, and SERA was asked for a second opinion (looking ahead to LeMans 1970).  SERA was an aerodynamic consulting firm run by Charles Deutsch, who was noted for his low drag bodies on the DB Panhards raced at LeMans in the early 1960's.  While the 917 K's aero stability was sorted with the iconic Horsman Tail in the winter of 1969-70,  Ferdinand Piech never gave up on a low-drag "solution" for LeMans.

SERA made detail changes throughout the car before the spring of 1970.  Only the windshield, doors, and roof remained unchanged.  The rear window remained, but without louvers.  A transverse slot provided for the engine's air intake and cooling fan.  The tail was more "lush," with curved reliefs around the now partially-shrouded wheel openings, and upswept at the very rear to a small "spoilerette."  It was homologated with or without vertical fins.  The contours of the front fenders were similar to the finalized (more square-nosed) K, but with smaller inlets for the oil cooler and brakes, similar to the 1969 car.  Kurt Ahrens crashed this configuration in March, 1970, when it aquaplaned before the LeMans Test Days.  Willy Kauhsen crashed one again after Test Days but before the race.

This could be unloading for either of the two Langheck tests at the Volkswagen proving grounds, or for the LeMans
Test Days in April.  In any event, the car as seen here is the "first cut" at a 1970 LH.  Note the smaller rear window
without louvers.  For reasons unrelated to LH aero, 1969's side exhausts for the front cylinders have been lost in
favor of conventional 6-into-2-into-1 exhausts exiting at the rear.  SERA was heavily involved in this shape.

At the LeMans Test Days in April, Ahrens and Vic Elford reported nose lift above 150 m.p.h.  The LH reached 226 m.p.h. on the Mulsanne Straight, but the front jumped around.  But if the front was "pinned," the rear became loose. The car was unstable at speeds above 190 m.p.h., especially when coming off-throttle.  The Test Days configuration was deemed "not acceptable."

After further wind tunnel tests and for the race:
1. The lower lip of the oil cooler inlet was cut back to the cooler itself.
2. The brake cooling inlets were enlarged.
3. The front deck was changed to a concave shape.
4. The rear deck was made identical to the K's arrangement, with a small vertical window
     recessed into sail panels.
5. Scoops were added to cool the transaxle and rear brakes.
6. Most importantly, a rear wing was hung between the vertical fins. This configuration (when tested
    after the race) yielded drag 9% higher than the 1969 car.

Vic Elford said the LH as raced in 1970 was driveable "but still not dead steady" compared to the K.  It was only 1-1.5 seconds faster than the K's over a 3:20-ish lap, and in the race itself Wyer's K's sometimes lapped faster.  Elford and Ahrens led a contested and often wet race for 17 hours, with an assist from the 4.9 liter engine, until a broken valve spring put them out.  Porsche's first overall win at LeMans went to Richard Attwood and Hans Herrmann in a "box stock, customer spec." 4.5.  The only LH ever to finish LeMans, the "hippie car" of Gerard Larrousse/Willy Kauhsen, finished second, after several mishaps, 5 laps behind Attwood/Herrmann.

The 917 LH as raced at LeMans in 1970.  Vic Elford / Kurt Ahrens led off-and-on and never unchallenged, for 17 hours,
until a broken valve spring retired them.  This view shows the cut-back lower edge of the oil cooler opening, the
concave nose panel, the rear scoop for cooling the transaxle and rear brakes, and the wing.

For 1971, Porsche and SERA addressed the LH's issues immediately after LeMans.  Development continued in the summer and fall of 1970.  One goal was improved stability; Gerard Larrousse also felt that cornering grip could be improved.  Rear wheel widths were taken from 15 inches to 17, the same as the K's.  This had not been done previously because it increased the LH's frontal area.  A wider, flatter, tail that shrouded the upper half of the wheels was adopted.  A sickle shaped opening was cut in it to cool the tires.  Decreased suspension travel at both ends of the car and lowered wheel arches were adopted.  This "got back" some of the increased frontal area and was possible because LeMans was flat and smooth compared to other circuits.  NACA ducts replaced scoops to cool the transmission and rear brakes.

Porsche technicians were also looking for ways to increase the load on the front tires.  A lower, wider, oil cooler allowed for a bigger concave front deck panel.  The cooler was also placed closer to the road to lessen the amount of air passing under the car.  The headlights were moved rearward and placed more horizontally to allow for much larger front brake ducts.  The profile of the nose was flattened in plan view (even more so than the already-flattened K).  The trailing edges of the wheel arches were relieved and radiused to prevent air from "packing" in the wheel wells at speeds over 150 m.p.h.  SERA suggested and/or tested many of the changes described above, but not those described below.

Spring rates were stiffened to minimize changes in the car's aerodynamic angle of attack.  (Interestingly, John Wyer had always considered the 917's angle of attack critical to stability, particularly under braking, and his Gulf cars were set up to maintain a positive angle of attack as much as possible.  But even the Wyer K's had squirmy front ends under heavy braking.)

The new body was made of a special foam core material (used also for the 908/3's) that was thin enough to be translucent before it was painted.  This got the LH's down to an average weight of 1855 lbs. without fuel.  All the changes described here were tested by Jo Siffert at Hockenheim in November and became the baseline LH for 1971.

This view of the new 917 LH "body-in-white" at the LeMans Test Days in 1971 illustrates the changes described in the
text.  The car was so fast, and stable, that John Wyer reversed his decision of a year previous and agreed to run two
 LH's in his three-car entry for the race.  Martini & Rossi ran one too, in a livery that became iconic.

At the LeMans Test Days in April, 1971, a Wyer-Gulf 917 K was used to set a "bogie" time of 3:25.  The LH was clearly faster at 3:17, the magnesium frame K (which eventually won the race) was only 2 seconds slower, and the Pink Pig was well off the pace at 3:25.  Four different drivers could detect no difference in handling between the LH and the K.  Jackie Oliver got the LH down to 3:14, about 6 seconds faster than the best K time, and clocked 239 m.p.h. on the Mulsanne Straight.  He reported the car as "dead stable."  The only alterations for the race were to open the underside of the nose ahead of the front wheels and to move the louvers forward.  Success, in the third year of trying.

No cigar, but a great-looking commercial livery.

The LH's led the first 4 hours of the race easily, challenged only by the Penske Ferrari 512 M.  The Elford/Larrousse LH was out at 5 hours when the cooling fan flew off, cooking the engine.  Both Wyer cars suffered failed left rear wheel bearings.  This put the Siffert/Bell car out of contention.  It eventually retired with engine failure.  Rodriguez/Oliver got their LH back up to 2nd when an oil line broke at 18 hours; it too retired with engine failure.  The race was run in completely dry weather, unlike the rain-soaked 1970 event.  The blistering dry-weather pace set by the LH's probably had something to do with their retirement.  But, to this day (2006), John Horsman, Wyer's race engineer, believes that the LH was insufficiently developed to be confidently expected to last 24 hours.

The LH fascinates me because it struggled with lift at the dawn of the aero era.  Jim Hall was exploring downforce with wings on his "unlimited" Chaparrals, but nobody else was, in a rigorously experimental way.  The Ford GT 40 had solved its lift problem with high drag.  The "Porsche Way" was low drag, the Mulsanne Straight was the key to a fast lap at LeMans, and LeMans was always the most important race on Porsche's calendar.  Even though he (grudgingly) accepted the high drag Wyer/Horsman K solution aside from LeMans, Piech was not about to give up on the LH.  (When Porsche entered the Can Am's short sprint races on twisty circuits with the massively powerful 917/10 and /30, Piech quickly became "all about" downforce.)

Even after the rear wing partially sorted the LH's instability, Piech had SERA pursuing a non-wing solution: the Pink Pig.  Today, of course, airflow is managed over, under, around, and through racing sports car bodies.  And engineers have decades of experience in doing this.  The Porsche 956/962 was successful in part because of its high downforce/low drag body.  For that matter, the LH's reason for existence itself no longer exists: LeMans is now squiggly in the places it used to be straight.

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