The reason why NASCAR's roof flaps don't work is the same reason why airplanes sometimes crash

"No, I don't thank you for the fish at all" (notindetroit) 05/02/2016 at 21:01 • Filed to: None

2

3

I finally got around to reading !!!error: Indecipherable SUB-paragraph formatting!!! on why NASCAR’s new roof flaps don’t work, and I’ve developed a take of my own from his takeaway. If I understand him correctly, the reason why the roof flaps don’t work is because certain conditions need to be met during a crash in order for the roof flaps to properly function, and those conditions don’t always occur on every track crash (in fact, the takeaway I got is that they rarely do). I think there’s an even more basic underlying issue towards roof flaps: you’re asking a piece of aerodynamic equipment to work and only work precisely when the operating conditions of absolutely else are iffy at best.

I personally come from an aviation background, both from a practical (i.e. learning to fly) standpoint and from an engineering standpoint. There are people on OppositeLock much more qualified in ether position individually, or even both simultaneously, but I at least know “enough to be dangerous” as they say. One of the stories I remember being told is of a student and instructor pilot who were figuring out what to do after a large duck or goose impacted their aircraft, resulting in exactly the type of cartoon-styled dent you’d imagine on the forward edge (leading edge as we’d say in the industry) of the wing. They decided that since the home base airport wasn’t too far off anyway, they’d try to make it back and relayed this to said airport’s air traffic control, which is how we know of this. I add that last qualifier because those two never made it back to clarify afterwards. What this amounts to is a lesson in how objects, aerodynamically speaking, behave in predictable patterns only because they’re specifically designed to do so, or have been studied extensively to figure out what those patterns are - and when those shapes deviate from the known (i.e. get damaged), well, they behave very unpredictably.

The roof flaps of NASCAR effectively function as “anti-flaps” as understood in the aeronautical world. Flaps on an aircraft reduce the angle of attack necessary to maintain proper low-pressure airflow over the wing - to put that in inaccurate but somewhat understandable terms, they magically make the wing behave bigger than what it actually is (at the cost of massive drag, which is why airplanes don’t just fly with their flaps down all the time). NASCAR’s roof flaps have the opposite effect - they keep the car grounded by spoiling the smooth airflow necessary to keep that airflow at a low pressure. The only problem is, like on an aircraft, they don’t quite function if the car isn’t operating predictably.

The first issue is, as with the aeronautical example I illustrated, aerodynamic control surfaces plain old don’t tend to work when a vehicle is damaged, even if the control system itself is not. Aerodynamic control systems are very much dependent on the rest of the vehicle. In the previous example, the aircraft’s ailerons no longer worked because the dent in the wing itself was sufficient enough to effectively block proper airflow to the ailerons. The dent also eliminated positive aerodynamic flow to a huge chunk of the wing, effectively making that part of the wing “dead” and in turn remapping the stall characteristics of the aircraft in a very dangerous and ultimately deadly manner (when a catastrophic failure starts effecting the stall characteristics, it’s almost always very negative). When a NASCAR racer is involved in a wreck, the aerodynamics of that car behave in a manner that can be predicted only if the wreck could be 3D scanned at that exact moment so every crease and skin wrinkle could be analyzed and determined as to what the aerodynamic effect would be. The possibilities include suddenly having remapped aerodynamic properties that overcome and nullify the effect of roof flaps, or block the airflow that make the roof flaps function.

In fact there was a whole type of aircraft that was prone to exactly this sort of thing, just as, say, Ferraris have a reputation for making great BBQ cookouts. In the 1960s-80s Mitsubishi (yes as in the same people that made your Evo) made an airplane called the Mu-2, and it was a hot aircraft all right capable of cruising at NASCAR speeds all day - and employed an aerodynamic control system not unlike NASCAR’s roof flaps in principal. It also had a very dodgy reputation for being a very accident-prone aircraft. You can read more about that aircraft and its control systems !!!error: Indecipherable SUB-paragraph formatting!!!

The second issue is understanding that aerodynamic force isn’t the only force being involved. In fact (according to my poor aerodynamic understanding - hey, there’s a reason why I don’t do this for a living, I’ll readily admit I’m not exactly at the top of my class) aerodynamic forces end up being mostly an assist to other forces at work on the car. This goes back to the concept of “net force” you hopefully learned about in middle or high school. The “net force” that causes a NASCAR racer to go airborne can be aerodynamic - or it can just be the brute force (pun intended) effect of being hit at 180 MPH by another car traveling at 180 MPH and then slamming into a wall. That’s literally tons and tons of force and momentum that needs to go somewhere , and somewhere often ends up going airborne. Roof flaps - in fact any aerodynamic control surface - are not a momentum dissipation device. I’m not even sure if such a thing is physically possible in accordance to thermodynamics.

The third issue is the, at least to me, rather insane requirement that the car must be going backwards in order for the roof flaps to function. As bwp240 demonstrated, cars are basically facing all sorts of directions in a matter of seconds in a crash, and whatever little time it’s facing backwards is far insufficient time to establish the type of airflow needed to get the roof flaps to even amount to a damn. In fact by my own determination the instances in which a car will be going backwards in a manner that will allow the flaps to even work are relatively rare, or at least woefully insufficient to prevent airborne cars in the manner NASCAR and the France family is hoping and figuring for.

In conclusion - I’m seriously wondering what kind of aerodynamicst they even consulted for this scheme. Either I’m even more mediocre than I thought in this particular skills set or that’s just really depressing.

OpposResidentLexusGuy - USE20, XF20, XU30 and Press Cars > No, I don't thank you for the fish at all 05/02/2016 at 21:07

2

Great write up.

jvirgs drives a Subaru > No, I don't thank you for the fish at all 05/02/2016 at 21:28

2

The car doesn’t need to be going backwards for the flaps to function. The roof flaps are not designed for a crash at the angle the 34 spun. The right flap is on a angle and the left flat is horizontal to the rear spoiler. The right flap is angled in such a way that once a car begins (it doesn’t even have to be fully sideways) to get sideways, the right flap will open. There is an air channel that connects to the left flap that as the car continues to rotate the left flap will then open. The issue is when the car rotates the opposite direction, the left flap will not open until the car is pretty much backwards. At any other track this is an extremely rare occurrence. At Talladega and Daytona there is a higher likelihood that a car can be turned toward the right due to the pack and the speeds are sustained around 200mph.

With Kenseth’s wreck, the hit on the front knocked the front of the car up in the air and opened a large hole in the nose allowing increased airflow under the car creating lift. by the time the car started to rotate it was already lifted to the point were the roof flaps, even though they were open, were ineffective.

You can find videos of spins were the car doesn’t even get fully sideways and the roof flaps are starting to open.

bwp240 > No, I don't thank you for the fish at all 11/15/2020 at 01:30

0

Full disclosure, I did not consult any aerodynamicists . I just made inferences from observations. I am a meteorologist by education (at the time) , so I can only really answer for the wind and nothing mechanically. I wish I would have found this writeup earlier so I could have learned more at the time.