WINGS

[mech5496]

Z,

There is a paper on aero drag reduction by vortex generators which could give an idea on how they work. IMO the side swirls on the DeltaWing do a fair amount on sealing the underbody preventing vacuum loss without skirts touching the ground. Regarding your last design, it has an inlet converging "throat" which forces air under the car. I was wondering if this is needed or a "shoebox" style splitter could work the same. I.e. do you think it is better to increase airflow under the car or block it?!

[Z]

Harry,

"... paper on aero drag reduction by vortex generators..."

Those vortex generators (VGs) are very useful for controlling all sorts of flows. The two Hoerner books referenced in the paper cover experimental aero work done 50++ years ago, and show that VGs were widely used back then (ie. old tech!). FSAE teams could probably get significantly more downforce from their conventional wings by using appropriately placed VGs.
~o0o~

"Regarding your last design, it has an inlet converging "throat" which forces air under the car. I was wondering if this is needed or a "shoebox" style splitter could work the same. I.e. do you think it is better to increase airflow under the car or block it?!"

Interesting subject!

I will state here categorically that, IMO, JUST AS MUCH DOWNFORCE can be got with a "Shoebox" style underbody, as with the smoother underbody shown in the last two images.

So take the "Shoebox" shape, remove the rear Gurney, bend the back of the shoebox down to ~5 cm above ground, and then add two flaps as in the last images, to get similar overall downforce. A "blue-bubble" will fill the underside of the Shoebox and make it behave as if it has a smooth, low friction, bottom (eg. see BB under nose of Shoebox).

The reason I didn't post this solution is that the magical box of "Crayons For Drawing" that I borrowed from the fairies is NOT VERY GOOD at depicting real flows! Some of the problems are that these magical Crayons are very slow if you sharpen their points (ie. use small cell size), are even slower when trying to draw 3-D flows, and in their standard form are utterly incapable of modelling unsteady flows. Oh, and they have an extremely UNFRIENDLY "user-interface". (Aaaarghh!!! Engineers should NOT be allowed to write computer interfaces!!!).

Bottom line here is that the Crayons kept drawing "blue-bubbles" closely following a Shoebox-and-Double-Flap style underbody, with these BBs lowering overall downforce (ie. they block flow behind the flaps, lessening the peak "suction" under the first flap). Providing a smoother underbody to the main element convinced the Crayons to let the BBs slip away.

In practice (= in the real world), the best method of shaking-off the BBs would be a few appropriately placed VGs. But to draw that would have required me "sharpening" the Crayons (to ~1 mm cell size?), and doing a "3-D sketch" (= squillions of cells), for which I had no patience.

I note that even if I had tried to model VGs with the magical Crayons, I doubt that it would have given realistic results. This is partly because of issues such as "numerical diffusion", but more so because the Crayons implicitly expect the flow to be "steady", even though most real flows are thoroughly unsteady!

The disappointing bottom line here is that even though VGs are old tech and have been proven to work very effectively (eg. golf balls), the modern approach to designing aero with magical CFD, rather than with "real" experiments, might suggest to the designer that VGs do not work, and messy underbody flows cannot generate significant downforce. So Crayon wielding students (such as Julian?) might tell you that FSAE cars can never get to 3G-lateral on the Skid-Pad, because "It is impossible! The magical Crayons tell us so!!!"

Such is progress!

Z

(BTW, if anyone has detailed numbers or links to papers++ on the DeltaWing aero, BLAT, etc., then please post them! )

[mech5496]

Nothing detailed in those numbers, but the claimed figures for DW are 12000N of downforce for 2500N of drag at 90m/sec, with the downforce distribution being 25/75 front to rear.

Not quite sure if you are familiar with the '81 AAR Eagle that pioneered BLAT. It sure reminds me of your proposed double flap undertray that narrows up front.









(Note the interesting front upright/hub/a-arm attachment)

[Z]

Harry,

Yes, and the 75% rear aero distribution is because the DW has ~72% rear weight (IIRC).

Anyone worrying that since the DW can only get CL.A = ~2.4 (from above numbers), "... then how can we poor students get any more?", should keep in mind that the DW races on circuits where it has to do ~90 m/s on the straights. That is 324kph or ~200mph. The quoted downforce is already about twice the static weight of the car, and since no "active aero" is allowed, any higher CL.A and the car would probably bust its axles, or pop tyres...
~~~o0o~~~

Some comments about the 1980/81 AAR Eagles.

* The top pic seems to show the car pre-BLAT. The bottom two pics have the "extra little bit" that creates the side-vortices.

* F&R "winglets" are probably mainly for trim control (ie. adjusting F:R aero balance).

* Overall the cars look quite low drag. Nicely streamlined engine cover, wheelpods to control flow around the rear...

* The ~triangular plan-view underbody shape conveniently gives F:R aero% = ~mass% = ~tyre%.

* Front suspension is double-wishbone, but the upper-wishbone-outer-BJ is fitted directly IN the axle-centre, for good load paths and lower CG.

* The total package was obviously TOO GOOD (blew the opposition away), because it was banned within a few years. Ahhh... motorsport!

Bottom line, I reckon a good car for FSAEers to draw some inspiration...

Z

[mech5496]

Z, the first pic is actually from a model car, so they must have missed that detail. I just included it because it shows very well the triangular plan view shape of the car. On the success of the design, keep in mind that the 81 Eagles were powered by a small block NA Chevy engine while all others used turbocharged Cosworth engines (660 vs 750+ hp).

"On the aerodynamic side, rather than the sidepod tunnels being fitted to most Indycars at that time, the Eagle carried its low-pressure-creation apparatus in what was essentially a box built around the engine and rear suspension. With downforce from bodywork shaped specifically to generate powerful vortices beneath it, the ’81 Eagle became one of those racecars that turns out greater than the sum of its parts. "

"The ’81 Eagle turned out to be exceptionally good in practice as well as theory. In Mike Mosley’s capable hands the dart-shaped yellow and white machine qualified second fastest at Indianapolis, while earning the prestigious Louis Schwitzer Award for engineering excellence from the Indiana chapter of the Society of Automotive Engineers.
On race day, however, the hope generated by that front-row starting position went unrealized when the engine broke a connecting rod early in the race. The following week at Milwaukee, however, Mosley started from 28th and last grid slot as a promoter’s option after missing qualifications, then fought his way through the entire field to score a brilliant victory with the last car ever to win a National Championship race using a true stock-block engine.
After Mosley’s heroics at Indy and Milwaukee, Geoff Brabham qualified the road-race car on the pole at Riverside, more than a second ahead of the field, then led the race handily until a wheel nut was cross-threaded during a planned pit stop and the euphoria of the moment evaporated. Rocky Moran gave a similar performance at Watkins Glen, only to be derailed by a fuel hose problem." And after that a rule change came...

(Text in quotation marks from www.vintageracecars.com)

[Cunningham]

All, I've done some searching--without success--for clarification on rule T9.3:

T9.3 Minimum Radii of Edges of Aerodynamic Devices
T9.3.1 All wing edges including wings, end plates, Gurney flaps, wicker bills and undertrays that could
contact a pedestrian must have a minimum radius of 1.5 mm (0.060 inch).

Would one of you please enlighten me on what is considered an edge that could contact pedestrians? Do this include trailing edges of wings and endplates? Gurney flaps on upper elements of front wings?

My team is new to aero this year and as such, we do not have the benefit of discussing these questions with the technical inspectors.

Thanks.

[mech5496]

Cunningham,

I suppose you could ask via e-mail; however, just to be sure, I would keep all radii above 1.5mm

[JulianH]

I'll second what Harry said.
We got different answers to that question at all competitions.

In the beginning (UK 2012) we said "All the trailing edges cannot contact a pedestrian so we have them smaller". In scrutineering we had to adjust all of them.
Afterwards we started to ask for rules clarification and at least in the UK and Germany the "just all radii above 1.5mm" seems to be the best solution.

[404namenotfound]

Has anyone else seen the GFR wings this year? I saw a photo from their unveiling today and was surprised with their design. I'll admit my knowledge of aero is limited but it seems the double front wing wouldn't be effective given how close the elements are to each other. Thoughts?

[Some Guy]

I think we have started to get to the point where aero>everything else in terms of vehicle design. Certainly when it comes to "top" teams that have most of the other vehicle aspects well sorted. I'm not complaining (much), if the aero rules get clamped down you would probably just see those resources got to other areas like composites or kinematics.

Even if it's only a 10% down force increase (likely more), there are lots of other gains to be had in terms of countering pitching moment due to the rear wing and overall vehicle balance. In other words you gain a lot from a little.

I thought Ann-Arbor's aero was more interesting personally. Definitely a liberal interpretation of "open wheel" in the rear.