ipse dixit

Yes, and in particular downforce. That is, if you drink the Porsche Kool-Aid.

stuntman

The 911s shape isn't ideal aerodynamically and generates a lot of lift. Combined with a large rear polar moment from the entire engine behind the rear axle, rear downforce is needed through wings, which is why base 911s have had deployable wings for decades.

The rear engine layout isn't ideal for packaging diffusers, which is also a problem for Porsches motorsport racecars. I guess those two major inherent issues are reasons Porsche is going mid-engine with their new 911 racecar.

0.02

STG

^^^^ makes sense. The 911 design and rear engine layout is very unique among the rest of the cars out there. All the other exotics being mid engine have an advantage in many respects.

silverrules

Very informative. Wonder why the engine on 911 hasn't moved to center all this time. They had no problem replacing the 6-spd to PDK.

awew911

Nailed it...

MaxLTV

Ferrari and Corvette use underbody aero, which is better shaped due to longer trunk with no engine in it. They also use upward sloping rear lids, which basically create wing shape, and then add gurney flaps on top of that. So basically, because the rear part of the car is less densely packaged there (no engine or rear seats), it's shaped slightly better aerodynamically. Plus both use gurney flaps rather than wings. Gurney flaps are comparable to wings when you need little downforce, but get very drag-inefficient for high-downforce applications - hence race cars doing big wings with very small flaps or no flaps. Look at the rear end of this Speciale - you can see a huge flap-like element on its trunk: http://performancedrive.com.au/wp-content/uploads/2014/09/Ferrari-458-Speciale-Aperta-roof-down.jpg Very likely that they could have gotten more drag-efficient downforce with the wing, but it would not work for their design goals. Z06 similarly has a huge gurney flap with an adjustable portion: http://www.chevrolet.com/content/dam/Chevrolet/northamerica/usa/nscwebsite/en/Home/Vehicles/Cars/2016_Corvette_Z06/Model_Overview/01_images/2016-chevrolet-corvette-z06-sports-car-mo-design-1480x551-01.jpg

I'm not an expert, but some papers I've seen suggest that gurney flaps are as efficient as wings or even better in some scenarios but only to the size of about 1/2". You can see corvette and ferrari with flaps way beyond that size.

silverrules

^^Thanks Max and all. I learn something new every day. It all makes sense now.

stuntman

Are you confusing gurney flaps with a "spoiler"?

"The Gurney Flap (or wickerbill) is a small tab projecting from the trailing edge of a wing"

Hoopumpers

^ I've witnessed first hand the effects of a ~1/2 inch wicker on my 997 cup car at COTA. In qualifying session, had the wicker fly off the rear wing after a couple of laps (it's stuck on with tape), and the car was noticeably looser in high speed turns, especially 16,17,18 where it continued to step out on me (with sticker tires). I was really perplexed until I came into the paddock and we realized it had flown off. I've always heard that that tiny wicker effectively lengthens the wing by a substantial amount, but didn't fully believe it until feeling it myself. Pretty wild.

Anyone who thinks the massive wing on the 991 RS is for show hasn't spent any substantive time on track

Hoopumpers

Here's the best explanation for what happens when the rear wing doesn't exist. Goossens doesn't realize it's torn off in the contact earlier and tries to take next corner at speed with no rear downforce...

signes

It was the first thing I noticed on the RS, the feel and roar of wind noise from the wing over 100mph. Also notice the aero grip going faster rough corners.

MaxLTV

The cars that I pictured have "spoilers" sticking up perpendicular to the surface of the tail rather than parallel to it, which makes the spoiler a giant gurney flap. The reason race cars have small gurney flaps is that 1) they can have wings; 2) on smaller scale, gurney flaps are as efficient or more efficient than wings.

On Corvette, it's very obvious - this thing sticks straight up and is responsible for 80% of the rear downforce, according to their press kit:


The photo makes it look smaller than it is - it's almost as tall as a taillight.

On Speciale, it's more visually integrated but is very much the same thing - a big vertical plane on the trailing edge of the trunk.

stuntman

Gurney Flaps, Spoilers, and Wings are not synonyms, they are specific aero devices.

Gurney Flaps (aka: wickerbills) are attachments or projections on the trailing edge of a wing. A vertical projection from a body panel is usually defined as a "Spoiler" and not a "Gurney Flap" or "Wing".

Chevrolet uses a lot of Nascar-style spoilers because they are simple, cheap, and very effective at producing downforce. There are countless descriptions of "lips", "duck-tails", etc... for the shape of the trailing edge of cars but for all purposes, "spoilers" are usually not incorporated into the bodywork and are usually bolt-on protrusions.

0.02

JCtx

I like the integrated way of producing downforce. It's more aesthetically pleasing to me, the way Ferrari (and others) does it. I could still pass an integrated tail, like the GT3. But a big a$$ wing, like ACRs, RSs, etc., I simply don't like on a road car. But there's no denying their effectiveness, especially if adjustable. And that's why even Ferrari adds it to their racing cars.

MaxLTV

Well, I did not mean to get into a terminology discussion but merely described the aero effect.

Spoilers are pretty much anything attached to a trunk, with diverse shapes and aero effects (most are just to calm airflow rather than create downforce), so saying "spoiler" would not communicate the point. While gurney flap is something that refers to a very specific design and very specific aerodynamic effect, which I wanted to describe. Something like this but upside down:




I'm not a terminology expert but I think something can be both a spoiler and a gurney flap. NASA and physics textbooks/papers define gurney flap as a flat plate on the order of 1 to 3-4% of the airfoil chord in length, oriented perpendicular to the chord line on the airfoil windward side at the trailing edge. And airfoil is defined as any shape designed to produce lift or downforce when moving through the air. So it seems that car bodywork qualifies as airfoil - it does produce downforce - so any spolier that's a flat plate perpendicular to bodywork is a gurney flap. FWIW, GM calls it gurney flap in Z06 literature. But IMO that's not important, at least to the point I was trying to make.