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 the most efficient way to adjust rear downforce is changing the wing angle in the 0-5 degree range. If you need more downforce, keep the 5-degree angle and add progressively larger Gurney flaps. Finally, if you still need more downforce, increase the wing angle until it stalls.
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 just released 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.
The CFD L/D ratio of 2.7:1 isn’t very good. In fact you can see that the single wing with large Gurney flap makes the same downforce as the dual wing at 35 degrees. And it does so with 1/3 the drag. Perhaps there’s something wrong with the values in the chart. 9LR claims up to 10:1 L/D ratio, and so something is clearly off. I’m also wondering if better numbers will come when they optimize the wing angles and slot position.
In Competition Car Aerodynamics, McBeath also examines dual-element wings. The dual element wing made of 60% more downforce than the single element wing, which is pretty close to the 9LR values above. So maybe the L/D ratio is correct, and there’s just a lot of drag when you add a second wing? My experience with a dual-element wing (a cheap one), wasn’t so great. I would say that unless you’re doing autocross (where drag won’t matter as much) or have a shit ton of power, a single wing is going to be a better bet.