Patrick_Devlin Posted June 23, 2004 Share Posted June 23, 2004 I was watching a stupid article on the news the other night where they wind tunnel tested those England flags that everyone has on their cars. The results showed that at a steady 60mph fitted to a Ford Focus, the drag coefficient went up from 0.32 to 0.36 with a flag on each front window. The reporter said that "drag increased by 10%" Is this true... does an increase of 10% in the drag coefficient mean a 10% increase in drag? I find it hard to believe that 2 little flags will cause that much increase in drag! Link to comment Share on other sites More sharing options...
400BHP Posted June 23, 2004 Share Posted June 23, 2004 You would be suprised what would cause drag on your car, I saw on top gear once, that driving with your windows open makes you use a significantly larger amount of fuel than if they were up, 1 chalk in the favour of A/C! Link to comment Share on other sites More sharing options...
Patrick_Devlin Posted June 23, 2004 Author Share Posted June 23, 2004 Thats odd, maybe I heard it wrong, but when Clarkson tested that diesel A8 he said that the A/C used more fuel than leaving a window open! 1 Link to comment Share on other sites More sharing options...
400BHP Posted June 23, 2004 Share Posted June 23, 2004 Maybe I did! I suppose it depends on the size of the car, an A8 is a bit of a monster! Sorry to digress! Link to comment Share on other sites More sharing options...
TLicense Posted June 23, 2004 Share Posted June 23, 2004 Originally posted by Patrick_Devlin I was watching a stupid article on the news the other night where they wind tunnel tested those England flags that everyone has on their cars. The results showed that at a steady 60mph fitted to a Ford Focus, the drag coefficient went up from 0.32 to 0.36 with a flag on each front window. The reporter said that "drag increased by 10%" Is this true... does an increase of 10% in the drag coefficient mean a 10% increase in drag? I find it hard to believe that 2 little flags will cause that much increase in drag! Patrick here's a little test. Buy one of those flags. Drive at say 50mph with flag in hand, hold flag out of window. See how hard it is to hold flag. I think you'll be quite surprised Tony 1 Link to comment Share on other sites More sharing options...
Patrick_Devlin Posted June 23, 2004 Author Share Posted June 23, 2004 But I'd need to hold one in each hand to recreate the test After seeing all these flags bent double on cars travelling down the motorway I was thinking about development cost of producing a flag thats wind tunnel tested So, no answers to the original question then? Link to comment Share on other sites More sharing options...
trooper Posted June 24, 2004 Share Posted June 24, 2004 Drag force = C*D*A*(V squared)/2 C=drag coefficient D=Air density A=object area V=air velocity normal to surface D, A & V are going to be pretty constant so if the drag coefficient increaces by 10% then I think it's fair to assume that the force holding the car back will also increace by 10%. So to try and answer the orriginal Question, yes. Drag coefficient doubles, drag doubles. 1 Link to comment Share on other sites More sharing options...
dangerous brain Posted June 24, 2004 Share Posted June 24, 2004 Originally posted by trooper Drag force = C*D*A*(V squared)/2 C=drag coefficient D=Air density A=object area V=air velocity normal to surface D, A & V are going to be pretty constant so if the drag coefficient increaces by 10% then I think it's fair to assume that the force holding the car back will also increace by 10%. So to try and answer the orriginal Question, yes. Drag coefficient doubles, drag doubles. And as drag is a relation of V squared the faster you go makes drag effect you exponentially so if you reduce your drag co-efficient (stick spoilers, on wind down windows, put silly placcy flags on etc) then it will increase the drag exponentially the faster you go. So at a set speed (depending on the speed you pick) drag can be increased by 10%. If you go further up the speed curve you could end up doubling the drag (up until you hit the speed of sound when it all changes again). The drag co-efficient is a worked out figure to be able to compare drags (i forget what the formula for that is as its 13 years since i sat my aerodynamics classes). Like said above quite a lot of stuff stuck on the outside of cars really does effect the drag co-efficient which is why on test runs I am sure manufacturers used to tape over door handles and ensure aerials were down etc to get a better quoted mpg rating and higher top speeds. 1 Link to comment Share on other sites More sharing options...
Aerotop Dave Posted June 24, 2004 Share Posted June 24, 2004 I know that roof racks add about 20% drag. And the Supra rear wing adds a bit too. One up for the wingless boys... 1 Link to comment Share on other sites More sharing options...
SKI Posted June 24, 2004 Share Posted June 24, 2004 Stick the flag on the inside of the back windscreen... Problem solve Link to comment Share on other sites More sharing options...
Syed Shah Posted June 24, 2004 Share Posted June 24, 2004 Originally posted by 200SX Boy And the Supra rear wing adds a bit too. One up for the wingless boys... Actually, the quoted CD for a spoilerless Supra is 0.34, while it is 0.33 with the spoiler. Weird. Link to comment Share on other sites More sharing options...
Doughie Posted June 24, 2004 Share Posted June 24, 2004 Originally posted by Syed Shah Actually, the quoted CD for a spoilerless Supra is 0.34, while it is 0.33 with the spoiler. Weird. you get lots of nasty vortices and "dirty" air flow behind a car, so i guess it's possible the wing smooths some of that out which may mean there's actually less drag with it on, as the drag coeffs. imply. Also - in NASCAR, 2 cars can travel at a higher speed when they're nose-to-tail. You may say "yeah of course mate - it's called slipstreaming". but what is interesting is that the FRONT car can go faster when a car BEHIND it is right up on it's bumper. this tells quite a bit about smoother air-flow behind an object can help reduce it's drag. Link to comment Share on other sites More sharing options...
Syed Shah Posted June 24, 2004 Share Posted June 24, 2004 Yes, that is how it was explained, there is a diagram for it somewhere on http://www.mkiv.com though I can't seem to find it! Link to comment Share on other sites More sharing options...
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