An In-Depth Analysis Of The Porsche 908LH’s Suspension Activated Ailerons

One of the highlights of Rennsport Reunion V, for me, was being able to see one of Porsche’s most interesting aerodynamic concepts in action, the Suspension Activated Ailerons found on the rear bodywork of the Revs Institute’s 908 LangHeck. I’ve had a keen interest in the advancement of aerodynamic study in the late 1960s and early 1970s, because of all the progress made in that time. Prior to the 906, Porsche didn’t really pay much attention to aero, and didn’t really get a good grip on the subject until well into the 917 program. The race car featured in this story could be considered the beginning of Porsche’s aero obsession.

Active aero had been around for a while, most notably in Mercedes’ “Air Brake” 300SLs at Le Mans in 1955, and a few stilt-wing cars in Formula One and Can-Am in the early to late 60s. Those earlier versions were developed with a large single plane wing or air brake, but Porsche went one further, increasing stability in corners by experimenting with a number of different side-dependent articulated ailerons. The first Porsche recorded as having tested such technology was chassis 906-141, a 906 Carrera 6 which was used as a factory guinea pig for years and would later become known as “the flap car”. Unlike our subject 908LH, the flap car had movable aero elements at both the front and the rear of the car, retractable canards, and a pair of horizontal flaps recessed into the rear bodywork. Additionally, while the 908LH seen here has suspension-articulated ailerons, the flaps and canards of the 906 were driver-operated by a lever inside the cockpit.

In a very intense test at Porsche’s famous Weissach circular test pad, the Flap Car was the proof in the pudding. Without any of the flaps actuated, the Porsche measured 1.05 g. With just the rear flaps deployed, there was little difference in cornering speed, similar results were recorded with just the front flaps deployed as well. With both sets of flaps deployed, however, the Porsche managed 1.11 g on the skid pad. In testing at the Hockenheimring, a test driver confirmed the findings, telling the engineers that the Porsche was most confidence inspiring with all four flaps deployed. Unfortunately, this method produced the slowest overall lap time, as the small 2-liter Porsche could not power through the air as well with the flaps deployed. Even through the fast east curve at Hockenheim, the Porsche was slower, and this was only exacerbated when the car was pulling down long straights.

The test did come up with some positive results, however. By exhausting nearly all available options, Porsche learned through this test that it was most effective to deploy the flaps to increase the load on the inside wheels. They also determined that having a way to reliably and quickly retract and deploy the aerofoils, so they were active during cornering, and tucked away out of the drag during acceleration and down the straights would make the benefit all the more. Controlling these actions was determined to be too much for the driver to handle during aggressive driving, so they needed to come up with a solution that was passive, rather than driver activated as on the flap car. In addition to all of this, Porsche knew that they’d have to abandon their tried-and-true 2-liter flat-six in order to find more speed. In a long roundabout way, this developed into the 908’s 3-liter flat-eight.

As with most of Porsche’s aerodynamic exercises, the intended outcome was increased performance at Le Mans. If only there were some way to increase a car’s top-speed along the *long* Mulsanne straight, while also maintaining a balanced downforce profile in the curves, esses, corners, and kinks of the rest of the circuit. Porsche, through a lot of testing, and their infinite wisdom, figured out a way to increase the angle of attack of a wing when the car’s suspension was compressed, and that plan was eventually instituted in real terms on the 1968 908 LangHeck car. With what they learned from the flap car testing at Hockenheim, Porsche eventually developed the racing car that would carry out all of the lessons learned in this 908LH. Suspension articulated ailerons were experimented with Porsche’s short-lived hill-climb special, the 909 Bergspyder, as well as a 908 short-tail, but were perfected for the 908LH’s attack on Le Mans.

As you can see in the .gif above, the 908LH employs two vertical fins for high speed stabilization connected by a long horizontal aerofoil, with a pair of articulated flaps, one at each end of the rear aero assembly. During practice for Le Mans, the 908 LangHeck was tried with and without this aerofoil assembly, and it appeared that no difference was made in top speed. In either configuration, the 908LH achieved the same revolutions down the long straight, but without the aero assembly fitted, the Porsche was much less stable at high speed. After this result, the team wanted further confirmation, and ran the Porsche with cotton strings attached to the car to determine aero flow. Without the aerofoil fitted, the air along the rear deck tended to become quite turbulent and detach before the end of the car. With the assembly fitted, the air was more inclined to stick to the Porsche for better management of flow. Additionally, the aerofoil, in conjunction with its pair of articulated ailerons, helped the Porsche achieve higher cornering speeds through the lower-speed sections of the course, reducing overall lap-time significantly. At the cost of a little bit of aero flow, a pair of NACA-style ducts were added to the rear decklid of the 908 in order to provide increased cooling air to the Porsche’s gearbox for the purposes of longevity.

Most importantly, perhaps, was the issue of front downforce. Porsche tried a number of methods to increase front downforce on the 908, but without matching rear downforce could not make it work. Balancing downforce and drag at Le Mans is critical, and the best result was shown with the rear ailerons at a resting angle of attack of negative 18 degrees, and a small front canard attached. This provided a front downforce of 5 kilograms and a rear downforce of 10 kilograms at speed. Just enough to keep the car stable, but not enough to perceptibly impact the Porsche’s top-speed.

As you can see from this series of photos, the rear ailerons are operated by a series of shafts connecting it to the rear lower suspension wishbone. As the wishbone moves upward in relation to the bodywork (suspension compression), the shafts articulate the wing in a way that pulls the movable aero inward. Then, when the suspension elongates in relation to the body (as the inside wheel in a corner or both wheels extending over a rise might) the articulation rods would move in a way to push the movable aero upward and create rear downforce. There are a number of rods, bellcranks, and articulating ball connections that make up the full system, but the end result was obviously a benefit.

At Le Mans in 1968, Porsche arrived with a squadron of four 908 Long Tail cars (Siffert/Herrmann, Elford/Mitter, Neerspach/Stommelen, and Buzzetta/Patrick), ultimately scoring a much-needed 3rd place podium position, behind Ford’s 5-liter GT40 and a privateer 3-liter Porsche 907 longtail, with the Neerspach/Stommelen car. While what was proven was intended for Le Mans, the consequence was a Porsche that was also good at other tracks with long periods of sustained high-speed, like Daytona, Spa, and Monza.

The Porsche seen in these photographs is a member of the Collier collection, as displayed by the Revs Institute at Rennsport Reunion V just a few weeks ago. This is 908 chassis number 025, an aluminum chassis car that served Porsche well throughout the early part of 1969. Aside from a DNF at Daytona in 1969, the Porsche went on to place second overall at Monza with Elford/Ahrens/Attwood sharing the car, and scored a lap record and an overall victory at Spa with Siffert/Redman (Redman scored the impressively fast lap average of 145.28).

After consideration of 1968’s event, the organizers of the world championship decided to outlaw the movable rear winglets from May of 1969 onward. The 908 Langheck program was already scheduled to be replaced by a brand new 917 Langheck, also making use of movable rear ailerons. The 917, meanwhile, had been developed from the outset with this aero trick up its sleeve, and when it was outlawed, the 917 aerodynamic profile had to go back to the drawing board. That, unfortunately, is a story for another time.

Today, 908-025 pictured here, has been returned to its 1969 Monza-winning beauty, complete with suspension-movable aero. The Revs Institute brought the car out to Rennsport Reunion for a very talented Mr. Gunnar Jeanette to drive, competing against larger displacement and higher powered 917s. Gunnar managed to hold his own, running near the front all weekend. He claimed that the movable aero didn’t help much around a short tight circuit like Laguna Seca, but from the video that they provided us, it is easy to see that the little winglets are rarely not in play.

Even on Laguna’s long and curved front straight, the little ailerons are seen strutting their stuff. While the video has lots of interesting things going on, take the time to actually watch the ailerons, paying particular attention to where and when they are employed. You can see a video that the Revs Institute produced featuring a short interview with Gunnar about the active aero below, followed by a multi-camera view of the car’s first lap around Laguna Seca during Sunday’s racing action.

I want to extend a special thank you to The Revs Institute and the Collier Collection for helping us out with photos and video of the car and it’s unique aero.

Videos and Static Images supplied by The Revs Institute. On-track images ©2015 FlatSixes.com/Bradley C. Brownell, All Rights Reserved.

Information sources: “Porsche 908: The Long Distance Runner” by Foedisch, Nesshoever, Rossbach, Rossbach, and Schwarz, “The Racing Porsches” by Frere, and “Porsche: Excellence Was Expected” by Ludvigsen.