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Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

(OP)

I would like to create an Adjustable Spoiler for an Automobile. My spoiler would be used for street vehicles for the purpose of reducing drag primarily (for fuel economy) and increasing down force (while braking or turning at high speeds)and would be completely automatically adjustable (angle of attack) based on speed.
 
I was wondering if there is anything you would recommend I could do or whether you could suggest what airfoil I should use and which ones I should look at. I have looked at some NACA and SELIG airfoils but I can't come to the consensus of the final design.
 
 
Any help or suggestions would be greatly appreciated. Thank you in advance.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Maybe it's just me, and since I'm not an automotive engineer I might be overlooking something, but is there really a need for downforce on a street car?  Where are you driving that you need downforce for turning at high speeds?  Are you  building an unltralight car?

I see the need to improve fuel efficiency but I would have thought changing the profile of the car itself would be more beneficial than putting a wing on it.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Wings that produce down force hurt fuel economy.

Adjustable wings are not new. Porsche did one on the 911 about 1990 I think and Chaparrel did one on on their sports prototype race cars in the 70s. The concept was  banned in racing due to fears of failure leading to crashes due to sudden loss of drag and down force, and due to fears of detached wings impacting crowd and other cars.

I think most OEM body engineers consider wings a styling aid with zero or less than zero technical merit.

Regards

eng-tips, by professional engineers for professional engineers
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RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

(OP)
Thank you both.

I understand that there is no dramatic need for down force on street cars. I was looking towards implementing a spoiler even simply for long term effects (i.e. reduce drag to save fuel).  I understand that it would be hard to create a generic spoiler for multi-vehicle purpose, but perhaps I would have to check to see a small list of vehicles that have large Drag Coefficients and implement my spoiler on those.

My spoiler should be controlled entirely autonomously via on-board sensors that would detect speed and change angle of attack; as well as detect traction control or slip and engage max down force at that time.

What my primary concern is to pick an airfoil design that would help me achieve this. How would I know which one would give me the least drag at different angles of attack for typical driving speeds?


Thanks for all your help again.

Regards,

Cumbysis

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

SPOILERS DO >>>>NOT<<<< REDUCE DRAG.



Mike Halloran
Pembroke Pines, FL, USA

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Maybe I was to subtle.

Mike is correct.

SPOILERS DO NO REDUCE DRAG.

It is a basic law of physics. The energy to create the down force comes from drag

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

(OP)
I guess I should not term the apparatus a "spoiler" since spoilers "spoil" the airflow around the vehicle to give it down force. Calling it a wing seems to be a better idea; I'm still focusing on reducing drag however.

Regards

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

In the airplane world this is called " induced drag". You know the horizontal stabilizer back on the tail of aircraft? It is an upside down wing, It's 'lift' is opposite of the "real wing's". It's lift is down, toward the ground, a force needed to contradict the natural nose down forces generated by the 'real' wing, when it generated "lift" in the 'up' direction. Want to make an aircraft fly faster, higher, more efficiently? Load it "tail heavy", or "aft CG". This reduces the 'work' the horizontal stab ( or spoiler, if you like) must do. At the expense of stability. There is no free lunch.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

So, the automotive manufacturers have access to small scale and full-size wind tunnels, and CFD.

The number of production cars with movable aerodynamic surfaces can be counted on your fingers, maybe only one hand's worth of them.

Think about that for a while.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

A spoiler/wing/etc will:

- Make your car a target for kids to steal.
- Increase your fuel consumption.
- Reduce you maximum speed.
- Make your rear view mirror useless (unless you were thinking of roof-mounting it (!).

- Steve

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

and...

In the UK it would probably invalidate your insurance, since the insurance drones would see it as a "performance enhancement modification".

- Steve

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

I worked on a very fast car once. After a couple of years they needed to tart it up, so the stylist added a wing. So the lads took it down to Nardo and after a couple of weeks driving in circles they finally came up with a setting that caused no problems, and didn't hurt the top speed. Aerodynamically it did not exist.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

You might have better luck in another forum on here.  I think there is an aerodynamics forum somewhere.  They might have more resources....

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

They will not have a resource to change the laws of conservation of energy.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Even if you could change the laws of physics and reduce your drag, say, 10%, your fuel economy would only improve about 2% (i.e. 20 MPG to 20.4 MPG). Driving 12,500 miles per year you would save 12.25 gal of fuel per year, saving you about $40.  How much will the spoiler cost?

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

OK, I think maybe I see what the OP is talking about.

Originally I was with the "SPOILERS DO >>>>NOT<<<< REDUCE DRAG." crowd but then recalled something from Aero at School.

CCTENG are you talking about an aerodynamic device to energize the flow so that it follows the car body shape at the rear?  Hence reducing the low pressure area behind it and hence drag?

At university my Prof was doing research on how this effect could be applied to the Engine pylons on airliners.  I think they called it a trip wire or something.  Basically a feature a bit before the end of the pylon that upset the boundary layer making it more turbulent.  The extra energy in the airflow causes it to actually follow the sharp drop off of the pylon at the rear, at least partially, actually reducing drag.

I don't know enough about it to give you any more specific advice.  My university was Southampton, Aeronautical department, the prof was either Dr Tutty or maybe Graham Bull, can't recal which as I had aero classes from both.

However, given what you're trying to achieve, a conventional aerofoil shape may not be what you need.

I do recall a TV show some time back with some guy working on stock cars or something and in his league or whatever wings were banned but he but a 'box' on the rear and it actually worked, can't recall details though.

Then again, the fact you also talk about down force makes me doubt you were thinking along these lines.

KENAT, probably the least qualified checker you'll ever meet...

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Flow modifiers have been used on trucks and so on, with some success.

But... the car manufacturer has altready optimised the shape of the tail within their constraints.

Cheers

Greg Locock

SIG:Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Greg, agreed.  I just thought I'd chip in with the one thing I could think of that kind of made sense.

As soon as I posted I realized that the shape of the rear of some vehicles was probably designed at least partially for this purpose.

I'm not sure that an adjustable feature for this purpose would make sense, haven't really thought about it though.

Plus like I said, that fact the OP talks about down force makes me think this may not be what he was talking about.

KENAT, probably the least qualified checker you'll ever meet...

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Not necessaruly the freestanding spoiler I think the OP was offering, but .................

From SAE Progress in technology series No 18 PT-18 - "Automotive fuel economy part 2" published ~ 1979.

VW engineers Hucho, Janssen and Emmelmann published "The optimization of body details - A method for reducing the aerodynamic drag of road vehicles"  All the test results quoted in this report were obtained in VW's wind tunnel using full scale models and actual vehicles with suspension unlocked and cooling airflow flowing . Their Stated Cd undertainty is +/- 0.002 to 0.003.

Bottom line, There are front and rear "spoilers" dimensions that reduce lift and drag. This squares well with some of the info in the Chevy Power books making claims that a chin and rear deck "spoiler" dimensions (and even modest vehicle rake) can reduce lift and drag.

I think this is because there is little about automotive aerodynamics that involves airfoil theory.  Perhaps It is all ground effects (some sources say is max when the "wing" is 1/2 span off the ground, or when my car has 24 inch ground clearance), or maybe because even swoopily styled cars are basically bluff bodies, so "drag" and "lift" are  the result of pressure creeping around and influencing any surface they can.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Porche whale tail and the Chrysler Crossfire are both rear spoilers that adjust with car acceleration and speed parameters.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Tmoose

I agree about chin spoilers and ground effects and rake so long as it is gained by lowering the front rather than raising the rear.

The OP mentions aerofoil shape which does not apply to chin spoilers or even rear deck upturns, but only applies to wing style devices.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Agreed, as to "chin spoilers", diaplanes, flow seperators, whatever...The more air flow that can be excluded from the under carriage and underside of the chassis the better for reduction of aerodynamic drag...With most cars nowadays being FWD, it probably won't upset handling (may even improve?) at high speed. The ultimate if impractical limit is to exclude ALL airflow from under the car to eliminate most of the drag and increase downforce.  Was done years back by Jim Hall and the 2-J but ruled illegal by most race organizations...totally  impractical for street.

As to wings, a waste of money for street, IMO.  For aircraft, I don't recall seeing a plane with a "reverse" empanage lately.  I designed a set of wings for a sailplane once a long time ago and the stab was a "full flying" symmetrical airfoil!

Rod

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Here is the simple pretty version from a former racer.  Wings, or whatever you want to call them, reduce top end speed, and generally at best generate about 300 pound of downforce @ 100 mph.  However the drag created by such wing drops your top end speed by about 10 mph, depending on your car.  Depending on what you classify as a high speed turn, you are not going to see a significant improvement in performance unless your "wing" is very large, 7 feet across for single element with a high angle of attack.  Street car performance is a pretty simple thing, don't worry about the aero effects, just improve your mechanical grip.  If you were talking about a purpose built race car, then you can talk about wings and downforce, until then everything else is for show.  

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

If I remember rightly, Porsche produced one vehicle with a rear spoiler that popped up at a certain speed ... a speed higher than the limit in most US states.  So the US version had the pop-up speed reduced.  What's the point of having useless flamboyant body-kit if you can't show it off.

The really odd thing about the whole business is that it's only men who body-kit their cars.  What are they doing?  Trying to impress other men?

- Steve

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

There are a couple of cars with movable front splitters & pop up wings.  Always looked like an "arrest me now" flag to me.  There are also a couple of high end cars that reduce their ride height at high speeds.  That is probably a more useful but more costly solution.  At the speeds they activate nobody is worried about fuel consumption.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Sompting.  I believe it was the 911 and the speed was 70mph.  The joke as I recall it in UK was that if a cop saw one with the spoiler up he'd pull it over.

KENAT, probably the least qualified checker you'll ever meet...

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

In my opinion the only real functions of wings and rear deck spoilers on road cars it to attract extra attention from police and to help avoid drivers from being distracted from anything behind them by obstructing such items from view.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Next, they'll be adding speed holes... shocked

That's a Simpson's reference, for those who don't know.

V

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

The Porsche Boxster also has a pop up aerodynamic device, I guess spoiler is maybe the best thing to call it.  Going from memory its about 60mm across, and extends about 10mm up from the rear at about 100-120kph.  It slides out from just behind the rear decklid rather than the 911's pop up wing that is hinged and kind of swings up.

Bob

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Anyone else remember "Speed Blocks" painted onto the tanks of old Yamaha bikes (e.g. the FS1E).  At least they didn't slow the bikes down.

- Steve

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Would changing the back end of a vehicle to make it pointed (cone shaped) improve drag? Isn't that four triangular spoilers.

It would be very Ugly yes. Hey, wasn't the "Pacer" a dull point in the back?
  
kch
 

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Q: Would changing the back end of a vehicle to make it pointed (cone shaped) improve drag?  

A: Simplistically yes but...  There are so many other factors that it's not a very practical solution.  Also the height of a car compared to it's length means that trying to manage airflow in this way is pretty impractical.  Some solar powered cars or those cars that go hundreds of miles on a gallon of gas, typically university research projects or the like, do use a shape a bit like this (essentially some kind of aerofoil/teardrop) though with the driver usually reclined. http://www.msnbc.msn.com/id/8737930/

Q: Isn't that four triangular spoilers.

A: No.   

KENAT, probably the least qualified checker you'll ever meet...

RE: Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

Several race cars of the past have tried the "long tailed" approach. E.g., Porsche's Le Mans efforts of years past.  Great theory...totally impractical, even for a race car and completely out of the question for street use.  However, you can knock yourself out up on the salt flats...those boys just love looooooong cars.

Rod

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