In my previous post, NACA Wing Shapes and Airfoil Tools, I compared the 9 Lives Racing “Big Wang” to a NACA 6412 wing. The NACA wing was most efficient in the 5-7 degree range, and would begin to stall above 10 degrees. Based on fooling around with camber and thickness in the NACA tools, I’d guess the 9LR Wang will operate best in a slightly lower range. To make more downforce, you can add a Gurney flap or a second element.
How a Gurney flap works depends on who you ask. Some sources say that the flap changes the location where the air above and below the wing meet, making that point further away from the leading edge. Effectively, the Gurney flap makes the wing chord (front to back length) longer.
And other sources say that a Gurney flap works by keeping airflow from separating at higher angles of attack.
Either way, Gurney flaps allow you to make more downforce. Typically Gurney flaps are about 1-5% of the chord length. 9 Lives makes flaps in 1/4″, 1/2″, and 3/4″. Given the 9LR chord of about 10″, the 1/2″ flap is 5%, and this is on the large side. I suspect that the 1/4″ flap is the one to have. This chart breaks it down nicely for you, but doesn’t list the smallest flap.
The chart doesn’t list the lift-drag ratio, but that’s easy enough to calculate from the 9LR numbers. I’ll use just the 100 mph values and make my own chart (below). When I then sort by L/D ratio, you can see that the single-element wing at zero degrees is the most efficient, with a 14:1 lift/drag ratio.
If you crunch the numbers further, you’ll see that adjusting the wing in the 0-5 degree range is pretty efficient, and that a Gurney flap is always effective, even at low angles of attack. If you need more downforce, keep the 5-degree angle and add progressively larger Gurney flaps. If you still need more downforce, rake the wing until you get to 10 degrees or so, but note the diminishing returns of downforce, and drag is going way up.
But aero isn’s just about efficiency, but balance. Let’s say you have an airdam and splitter making 200 lbs of downforce at 100 mph (which is what the Hancha Group CFD showed in Miata Airdam and Splitter). If you want the same downforce at the rear, you can generate that by either running the 9LR wing at 10 degrees AOA, or at 5-degrees AOA with a 1/2″ Gurney flap. The latter does so with 10% less drag.
More wings, more downforce
Time attack Miatas use wide and complex splitters, with multiple dive planes and surfaces to create more front downforce. And they turbo or swap the engine, so they don’t care so much about drag. For this specialized application, they need more rear downforce, and a multi-element wing is one way to achieve that.
Multi-element wings effectively increase the camber of the wing, and can therefore be used at a higher angle of attack. The more wing elements you add, the more downforce you get, and with that comes more drag and reduced efficiency.
But I’ll just comment on the dual-element wing. The placement of the second wing is critical, which should create a convergent slot (larger in the front, tapering to the rear), to accelerate air.
According to Katz, in Race Car Aerodynamics, the main wing should be run close to zero degrees, and the second wing at an angle up to, but not exceeding 40 degrees. 9LR has a dual element wing that can be added to their standard wing and their CFD also shows that greater than 40 degrees is a mistake. You can see that the downforce is greater at 40 degrees than 45 degrees.
Let’s say you have a 9LR single wing and you want to get the maximum downforce, so you run it at 10 degrees and use a 3/4″ wicker. That gets you 256 lbs of downforce, but the wing isn’t very efficient at those extremes. A double wing set at the most conservative angle of 35 degrees generates the same downforce and a lot less drag.