Blown Rear wings: seperating and stalling

NOTE: Update on McLarens SnorkelRear wing here 

Renaults CFD shows how the flow passes around a multiple element rear wings

For an F1 car the rear wing creates around a third of the cars downforce.  But running at high speed the drag from the rear wing is tremendous.  Anything that reduces the drag of the rear wing will aid top speed.  If this can be done in a non linear way, that is; high downforcedrag at lower speeds increasing towards top speed and then less drag only at speeds where car is in a straight line and doesn’t need downforce, then laptimes will show an improvement.

A single element wing sees the flow separate (circle) at steep angles

As airflows over the surface of a wing it has a tendency to slow down and separate from the wing.  Particularly underneath the wing which runs at a lower pressure than the top surface.  This separation initially reduces efficiency by adding drag to the wing, before the airflow totally breaks up and the wing stalls.  When a wing stalls the wing loses most of its downforce and drag. 

A single element wing will then stall, as the flow breaks up under the wing

The steeper a wings angle, the greater chance of separation.  To combat this aerodynamicists need to speed up the flow near the wings surface, to do this they split the wing into separate elements, this creates a slot.  Which sends high pressure air from above the wing through the slot, which then speeds the local flow underneath the wing.  The more slots the steeper the wing can run. 

With a two element wing, flow passes through the slot to prevent seperation

In the nineties teams were unlimited in the number of elements they could use.  Slowly the rulemakers sought to reduce the wings potential for downforce and reduced the number of elements (defined as ‘closed sections’ within the rules), initially to four then three and currently two.  Modern rear wings are made up to two elements, a main plane (the forward section of wing) and a flap (which sits behind it).  Thus the wing is intended only to have a single slot and hence only one place to speed up the flow under the wing.  However the rules are typically vague, thus a small 15cm section in the middle of the wing is exempt from this rule, teams have been adding a slot in this area for several years now.  This slot is the same dimension on the front as it is on the back of the wing, so there has been no issues of legality within the rules, most team run a wing of this configuration.

Last year BMW Sauber and McLaren ran wings with the narrow 15cm opening on the front of the wing, but this inlet diverged to make a slot the full width of the rear wing (normally within the main plane).  This slot was aligned to send its airflow at an acute angle, roughly inline with the general flow over the wing.  Again this was deemed legal as the slot made the wing profile an ‘open section’ only in the middle of the wing, where as the outers spans remained a ‘closed section’ albeit one with a “U” shape.  With this design the slot could allow the entire wing to be steeper and not just the geometry in the middle 15cm of the wing.  This year Williams have joined the group running these sorts of wings.

With a blown wing, the extra inletoutlet creates a legal second slot

Again previously teams have sought to use the wing stalling to gain top speed (from the reduced drag).  By flexing the wings at higher speed, the wings move to create smaller slot gaps and this leads to the wings stalling.  The FIA has acted with both load tests and in the past few year slot gap separators to prevent this practice.  Slot gap separators are now mandated for the rear wing, and appear a plate fitted around the profile of the two wing elements to prevent them moving.

The McLaren 2010 wing uses a slot in the flap (not the main plane), this time fed by the shark fin and an opening above the drivers head.  If the teams’ protests about its legality are true, then the issue is that McLaren are using the slot to stall the wing. 

A slot in the flap could break up the airflow and allow the wing to stall

This could be possible in several ways; one could be having the slot orientated differently to the airflow over the wing, if it were at nearer right angles to the flow it could blow hard enough to disrupt the airflow enough to stall the wing.  Another solution might be that the slot blows at lower speed maintaining a clean airflow over the wing, then at higher speed the slot chokes with the greater airflow trying to pass through it, the slot no longer blowing stalls the wing.

These approaches would have to be tuned to have no effect at speeds lower than the top speed on the straight, thus the wing would provide normal downforce until near top speed.  Then near top speed the flow through the slot would start disrupt the wings flow and stall the wing.  The difficulty in getting this tuning to work is what’s given rise to the rumour about the driver operated snorkel duct on the McLaren.

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