Reattaching wake and drag penalty

[Cd]

I'd like to ask all of you here about what kind of drag penalty that results from using a spoiler ( or trunk lid ) to reattach airflow on a car with a rear taper that is not the ideal angle.



Note that the Camaro had a worse Cd without that spoiler, even though it kicks up at an angle ( The same results can be found in upward tilting spoilers on the R&T project cars - the Crisis Fighter Pinto and Crisis Fighter Datsun Z car.



On cars such as this Fusion, if you follow the roof line out at a 15 degree angle, you eventually meet up with the edge of the deck lid spoiler. I am guessing that this acts as a way of grabbing the airflow and bringing it back into the ideal trajectory.

For those folks on this site not willing to create a huge boat tail on the end of their prized car, why not create a large shelf spoiler that is easily removed ?
From the examples above, it seems as though adding a spoiler can sometimes have a drastic reduction in drag.... but what are the penalties ?

Thanks !

[Frank Lee]

Isn't that something? I've been thinking of this exact thing now, ever since Metro put up that F150 aero thread.

I've been outside pondering the best aeromods for my Tempo. Even have a crude mock-up on it.

Boattail doesn't appeal to me for several reasons.

No top chopping; I don't want to drop the seats onto the floor because I want to retain rear seat functionality too, and I need the headroom.

No fastback window and bodywork; too much work for too little gain.

And so on... until I settled upon the notion of a "Bonneville Style" spoiler remarkably similar to the one pictured on the Fusion! So I've been reviewing my Hucho book and surfing for aero articles. Yes, that's why my recent posts reference some Hot Rod magazine stuff.

I want my spoiler to go out rather than up to achieve the imaginary 14 deg drop from the roof. This minimizes the area of the body that leaves a wake zone. The Camaro would have been even more slippery with a "Bonneville Style" spoiler.

[Cd]

Yo
Frankie, whyoncha extend it all the way out and tilt it down like the trunk on the ESX ?
Not only would the airflow connect, but it could follow the angle downwards and make your wake even smaller.
Your back glass is really steep. I wonder if a small lip spoiler would be helpful in convincing the air to meet the big spoiler without too much turbulence.

[JackMcCornack]

I thought the reason a spoiler (can) reduce drag on (some) so-called "fastbacks" is that the lift generated by the attached air following the slope of the body created vortecies at the sides of the roof...and at some angle the benefits of filling in the vacuum behind the car (with air coming down over the fastback) were outweighed by the drag penalty of the vortexes (see, I can spell it either way). I don't have my Hucho handy, but I'm surprised the optimum angle is as low as 14 degrees...not the first time wind tunnel results have surprised me, but still.

[dwtaylorpdx]

On a lot of sedan race cars we add spacers under the hood hinges to let the high pressure area in front of the firewall loose. It often ads 4 or 5 mph down the straightaway. Anybody tried it for mileage?

Dave

[Frank Lee]

Ummm... what?

I'm told what you'll have by raising the rear of the hood is flow into the engine compartment.

[Frank Lee]

Originally Posted by Cd

Yo
Frankie, whyoncha extend it all the way out and tilt it down like the trunk on the ESX ?
Not only would the airflow connect, but it could follow the angle downwards and make your wake even smaller.
Your back glass is really steep. I wonder if a small lip spoiler would be helpful in convincing the air to meet the big spoiler without too much turbulence.

I think you are on the right track- thanks for the workup!

I need to think about how far out the back I'm willing to go with these appendages.

[Piwoslaw]

I found a wedge-shaped spoiler in a book and posted the translation here. I guess it's the next best thing to a full length roof extension.

[aerohead]

I'm guessing that the Fusion is FWD? The Camaro is RWD,yes? Since these are both Bonneville cars and they're running parachutes,then they're both looking at at least 175-mph.Some Bonneville cars have enough horsepower to spin their tires when they hit an "aerodynamic wall" ( the car has more power but cannot attain higher speed due to air drag) and can drift into a most dangerous spin.--------- I think these spoilers are a compromise between drag reduction and downforce and not necessarily the best designs for "street" cars.------------------ Any design which has an exit tip which falls along Kamm's roofline profile,whether an up-kick or straight back,will both do the job.------------- All spoiler setups pay a mpg price due to the attached vortex directly above them since the air is making more than one "pass" over the body.And you have to pay for this circulation.-------------- If you do the straight back shelf be mindful of unsuspecting pedestrians walking up from behind.Maybe cushion the trailing edge as Porsche did with the Carrera Whale-tail.------------- I'm partial to Ford's bi-wing spoiler which appeared on the Sierra concept car and Merkur XR4Ti.It gets the job done and with less drag.

[Frank Lee]

Hmmm... good idea. I will make mine razor sharp.