A sense of porpoise
Porpoising courted the column inches during the first week of pre-season testing at Barcelona. As the cars dropped down to the first corner, they began to shake up and down visibly, pacified only by the application of DRS or by the driver lifting off.
The appearance of porpoising appeared to catch many by surprise – even the teams, who had not encountered the sudden onset of an oscillating heave motion during their simulations. And although a few teams such as Mclaren and Ferrari weren’t quite so afflicted by it in Bahrain, there was still a noticeable vertical shake along the straights from the majority of cars, especially the Mercedes.
Former F1 aerodynamicist Jean-claude Migeot, who worked with Renault in the original ground-effects era in the 1980s, believes porpoising occurs when the ground-effect aerodynamics accelerate the airflow enough to induce the car’s natural heave frequency. This leaves the car susceptible to resonance, where the car is in a position where it is effectively undamped, particularly with the long chord length of the floor.
“When the car is shaking up and down quickly it can generate other forces,” says Migeot. “It’s the same for an aeroplane when it’s climbing. When the pilot is pulling the stick there is an oscillation of incidence and there is a first link to the speed of pitch. But this coefficient is stabilising so in fact, the movement of the aeroplane is completely stable.
“For an F1 car it’s destabilising because it increases the downforce when the car goes down and it’s diminishing it when the car is moving up. That’s the first step. The second step is these forces are big only in certain circumstances. Up to last year, they were ridiculous; they were so low, that it was not even worth measuring them because they had no influence. But when you have a system which is almost a closed channel, which used to be the case with skirts and which looks like the case now when the perimeter of the floor is very close to the ground, the amplitude [of the forces] start to be annoying, really annoying. And this happens because there is an interaction.
“This starts because we’re talking about a long chord [floor] – a length of several metres, which is in the same frequency range as the natural frequency of the car. The car has two natural frequencies (actually, there are four, but let’s put the wheels bouncing outside of the picture because that’s where they’re controlled by the dampers): the heave frequency, which is the
movement you see [in porpoising], and the pitch frequency.
“If the floor is on a very short chord like a wing, because the phenomenon could be the same under the front wing, for instance, when it’s very low [to the ground], you don’t get the same effect. But the chord being so short, its ‘exciting’ frequency is much higher than the car frequency. It’s completely filtered.”
Migeot says that the now-banned inerters – or ‘third damper’ – would have helped teams address porpoising in the car but, with limited suspension parameters, gains will have to be found in the windtunnel. For now, teams are having to raise rideheights to mitigate the effect of porpoising in the car, but at the hefty cost of underbody performance from the venturi tunnels underneath.