Design of Adjustable Spoiler to Reduce Drag and Increase Downforce

[CCTENG]

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.

 

[macmet]

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.

 

[patprimmer]

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

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[CCTENG]

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

 

[MikeHalloran]

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



Mike Halloran
Pembroke Pines, FL, USA

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[CCTENG]

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

 

[thruthefence]

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.

 

[MintJulep]

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.

 

[SomptingGuy]

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

 

[SomptingGuy]

and...

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

- Steve

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[macmet]

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

 

[patprimmer]

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 for tips on how to make the best use of Eng-Tips Fora.

 

[Bribyk]

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?

 

[KENAT]

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...

 

[GregLocock]

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

SIGlease see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[KENAT]

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...

 

[Tmoose]

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.

 

[Tmoose]

chin spoiler drag / lift optimization