Air deflectors on trucks/lorries

[Piwoslaw]

I've seen air deflectors on the front corners of trucks for years and have always wondered whether they really work.



The above picture shows deflectors not only on the sides, but also above the windshield. Does this help by deflecting towards the rear air that would normally be pushed to the side, thereby gaining forward momentum (sort of like a sail), or is this outweighed by the penalty of extra drag? I've also seen deflectors/scoops on the rear of vehicles, scooping air into the lowpressure zone immediately behind the truck. Would this make sense, or would it act more like a parachute?

I'll add that trucks aren't the only places I've seen side deflectors:

German-built Ty2-50 steam loco.

[winkosmosis]

I seem to recall seeing discussion on these things here before. They're supposed to keep airflow attached instead of fanning out and making a virtual parachute (like when you hold a spoon under flowing water)

[Cd]

The Plymouth Superbird and Dodge Daytona had something similar on its' A pillars. ( That is the chrome piece that you can see in the picture :



http://aerowarriors.com/jpgs/88rd30.jpg

I found it interesting that they were removed when the car was run for top speed at Bonneville. Strange, since when raced on the speedways, they kept them on.

Bobby Isaac and his Dodge Daytona at Bonneville-1970 on Flickr - Photo Sharing!

( The above picture is a poor example, but I saw a better picture that clearly showed that there was no chrome deflector there. )

I wonder if the visor style extention did anything good at all for aero. :



Thanks for posting this question - I've wondered the same thing for years.

[winkosmosis]

That visor has a much smaller outlet than inlet. It takes energy to compress air.


I wonder why they took the things off the Dodge to do the speed run. Obviously they had them on for a reason

[Piwoslaw]

Originally Posted by winkosmosis

I wonder why they took the things off the Dodge to do the speed run. Obviously they had them on for a reason

Maybe they only work up to a certain speed, above that there's more drag than gain?

[Piwoslaw]

I looked into my little book and found a short paragraph on deflectors. It states that these deflectors have the same effect as increasing the radius of the leading edges. In [2], Hoerner gives a reduction of Cx from 0.71 to 0.26 for deflectors on the leading edges of a bluntnose body, and a reduction from 0.58 to 0.27 when used at the rear. Unfortunately, these are only for lab models, and the rear mounted deflectors showed little or no improvement when used on road vehicles.

The effect on drag for front mounted deflectors depends on the aerodynamic situation between the tractor and the trailor. Buckley et al. ([1]) give a 0.05 reduction of drag for a cab with sharp edges and deflectors with 0 degrees of deflection (i.e. parallel to the direction of movement). This reduction increases to 0.25 when the angle is increased to 15 degrees.

In [3] Wysocki researched the aerodynamics of a long-distance bus. Other than changing the front angle and finding the penalty of using side mirrors (8%), he also tested a deflector on the rear, used to scoop fresh air down onto the rear window to keep it clean, finding it increased drag by about 3%.



[1] Buckley F.T., Marks C.H., Walston W.N., A study of aerodynamic methods for improving feul economy, US National Science Foundation, final report SIA 74 14843, University of Maryland, Dept. of Mech. Engineering, 1978.

[2] Hoerner S.F., Fluid Dynamic Drag, Hoerner Fluid Dynamics, PO Box342, Brick Town, N.J. 08723, USA, 1965. (I believe there is a new edition of this book.)

[3] Wysocki Z., Badania aerodynamiczne autobusu PR-110U Jelcz-Berliet - rozkłady ciśnień, Spr. Inst. Lot. 9/BA/78, Warszawa 1978.