Wing It: Simple Talk on the Complex Subject of Aerodynamics

In reply to stafford1500 :

now I just need to decide whether it is worth my time to design one to fit my car.

Struts create horrible amounts of drag.
A round strut is about 9X the drag of an aero strut.

https://www.youtube.com/watch?v=ftq8jTQ8ANE

The car with the big huge wing up front on a bunch of bracing struts is a horrible design.

Aerodynamics is not simple... a lot of what you have in this article is ok, but not really great
because it is so easy to take a concept and implement it wrongly.

The multi segment wing shown on the A Mod Autocrosser has the airfoil segments so close together
that it will behave like a single curved sheet (single panel).
By separating the segments more, you get a slot-effect... the air between the two segments gets accelerated, which makes them more effective.

Also, the maximum flow speed on the car can be more than the speed of the car through the air.
So the range is more than just from zero to the driving speed of the vehicle (assuming there's no wind). 

But without a degree in fluid dynamics, there is only so much that you can do... and with a degree in fluid dynamics, there is not a lot more you can do, but you will have studied the pitfalls of "Common Sense" applied to aerodynamics.

I have a Flat type Spoiler on my Vette it is a Larry Shinoda design that was Installed at Rick Mears and this being a C4 the nose looks like a C5/6, I intend to smooth out the Bottom and try to get the Air from the Bottom to pull against the under side of the Flat Spoiler / In theroy making it a Taper from the top of the Spoiler to the Air coming out under the Car, And eliminating the Pocket that creates Turbulance. And i know it cant be compleatly eleminated but not be a Kamm effect Either.

Also... if you look at the struts supporting this wing... if you left them as plates instead of cutting out material from the flat plates, the reduction in drag would be worth the weight penalty.

Mark_42 said:

Also... if you look at the struts supporting this wing... if you left them as plates instead of cutting out material from the flat plates, the reduction in drag would be worth the weight penalty.

 

Since I wrote the article linked, I will weigh in on this.

While you are correct in the simplified 2D flow around a flat plate compared to short chord-length struts, this is a VERY non-2D situation. The flow around the back of the car pictured will be very much lateral as well as vertical, with the addition of some vorticity. Removing the 'windows' in the wing pylons may actually allow the flow under the wing to stay attached longer, especially in the condition with the car running at anything other than straight ahead (with no cross-winds). The straight line drag change between the pylon being flat plate and 'windowed' is going to be quite small in the grand scheme of the drag on the entire car assembly. Things must be taken in context, such as the car in question has added aero aids in order to improve downforce (to increase average cornering speeds and therfore increased overall lap speed), not so much to reduced drag as the aero changes for that effect would have looked quite different.

Aero IS very complicated and like every other aspect of life can be investigated and formulated to the tiniest facets.Conversely, it can be simplified in many aspects and still manage better results than doing nothing. The article is from a few years back and even then I noted that an article was just the smallest of windows into the world of aerodynamics. Entire books (even libraries) have been written on the subject, so there is plenty to learn from, if you take enough time.

From your first post: Slot gap sizing is a fairly complicated affair, and is generally done with a single design target in mind (speed, angle of incidence, etc). Without knowing exactly what those design parameters are it is difficult for anyone else to make critiques on something as sensitive as slot gap sizing. Second, local speed around a car will vary considerably with respect to the actual road speed. This is a feature of a 3D body moving thru a fluid, amplified by the local shape of the body.

Thanks,

Steve Stafford

Aerodynamicist

In reply to stafford1500 :

It's a well written article with a lot of good information. I hope I didn't come across too negatively.
I appreciate your answer, and that you worded it so diplomatically (something I could use improvement upon)
I'm not an aerodynamicist by trade, and fluid dynamics is only one facet of my degree major (one of my favorites)

I do understand that the car in the shown picture is more concerned with increasing downforce than reducing drag, but I also see the trusses as a potential place to reduce drag without a lot of cost. It's probably minimal, but would be worth studying how much improvement could be had by using flat plates for the supports.

The car with all of the struts that looked really bad to my eye was this one:
(I realize there are trade offs, but it seems that there would be a more elegant solution)

There is a lot of experimentation and analysis that goes into those cars, and there are some extremely bright
engineers doing a lot of research on race cars these days. But there still could be simple improvements that
might be getting overlooked. I think the truss structures may also be preferred on street cars and autocrossers
because it has a cool looking aesthetic, and I do see the flat plates in use on some of the high level open wheel (e.g. F1) cars.

When Smokey built his infamous NASCAR Chevelle, he flipped the car over and streamlined every leading edge with Bondo.  Bellypans were illegal but he had the next best thing!