What a Drag!

[Paul Ranson]

CdA is your 'Coefficient of drag' multiplied by your 'Area'. When you measure the drag that's the number you actually end up with. Normalising to a Cd, to allow comparisons with different shapes or sizes, requires some definition of what 'Area' means.

 

If we're comparing spheres of different sizes, or scale models of a Seven, then we have a good basis for comparison, the Cd will remain constant but the area will change. But what effect does removing the windscreen from a Seven have? Is it a change of frontal area? A change of Cd?

 

Paul

[Lawrence_Z]

Having established that removing the windscreen will have the effect of adding 20 bhp at 100mph

 

Can the panel please advise how much the weight or downforce of the car changes at the same time?

 

Lawrence

[Joe 90]

CdA is the important everyday figure if you want to know how much power you need to reach a given speed. Cd is the more useful quantity at the design stage, because it can be plugged into calculations where the area is adjustable. As an example, the drag of an aerofoil can be estimated from it's size and Cd curves.

 

 

 

99,000 miles so far

[Tony C]

One of my students has a contact in one of the F1 teams. I'll try to arrange for a seven to be wind tunnel tested in various configurations, in order to come-up with some definite figures for both Lift and Drag.

From the point of view of a basic understanding of an extremely complex subject, I think it is fair to say that an improvement in one area of aerodynamic efficiency frequently leads to a reduction in efficiency somewhere else - look at the trouble the Jaguar team are having with their F1 aerodynamics.

 

The Se7en's inherent efficiency is due to its excellent power/weight ratio, weight distribution and suspension geometry. It is worth considering that any attempt to significantly alter its aerodynamic performance would require modifying it so extensively that it would no longer be a Se7en - QED.

 

Cooling drag, I would imagine, is a significan contributor to its Total Drag. I've seen some discussion concerning partially fairing-in the bottom of the engine compartment, leaving a small opening to allow efficient cooling airflow. The cooling exhaust grille's on the top of the bonnet would be another source of Drag.

 

Sorry to talk aircraft again, but the P51 "Mustang" of WW2 vintage was significantly faster than its peer fighter, later Marks of Spitfire. This was due to two things. Firstly its laminar flow wing, which is of no significance here, but also to its reduction in cooling drag, entirely from the clever design of its radiator inlet and exhaust ducting. This was designed in such a way to generate "negative drag". It consists of various cross sectional areas of inlet duct and a variable area exhaust (to cater for different 'speeds'). I'm not suggesting for a moment that such a complicated system be considered for a Se7en, the weight increase alone would negate any saving in drag, but an accurately calculated exhaust for the cooling air from the engine compartment might give a worthwhile reduction in cooling drag.

 

Something like this would need extensive trial and error and use of a wind tunnel - so I guess it's not really practical.

 

I think we all have to accept that using empirical values for Drag is possibly going to lead to false assumptions - has the quoted advantage from removing the standard windscreen been verified, or is it an empirical value. Bear in mind that just calculating the result of the reduction in cross sectional area is possibly going to result in an optimistic (that's not spelt right!) assumption.

 

The car has the aerodynamics of a brick, but if removing the standard windscreen and fitting an aero screen gives a perceived advantage - as I'm sure it does - wack-oh!!!

 

At least this discussion gives me something worthwhile to do while waiting for my SV to be delivered.

 

If you want me to shut-up about it - all you have to do is ask - I'm not too sensitive.

 

Best Regards

TFC

[EFA]

In answer to whoever asked about pannelling the underside and effect on speed, a SLR race car which was fitted with data logging was tested at Sneterton by an nunamed individual.

 

Comparisons of the logs with the front/rear or front and rear undersides panelled gave the following results:

 

Front (rear of nosecone to footwells) +3mph

Rear (a frame and triangular sections to side of a-frame +2mph

 

Front and rear together +7mph.

 

The beseline speed was around 120mph.

 

These results were generated along Revett into the Bombhole on a flat throttle.

 

I bolted the same bits onto my car and got a definate increase of around 5mph before the "air brake" starts to work.

 

Fat Arn

Visit the K2 RUM website

See the Lotus Seven Club 4 Counties Area Website here

 

[Julian Thompson]

Tony

 

There has been a seven in the tunnel before; a search of the archives looking for aerodynamics will bring it up. In addition to the usual things like bringing in the arches closer to the wheels (15kg lift reduction as I recall) the test went on (and I have heard this before) to explain that the car had approx 50 or 60 kilos of lift at the front. Rear lift was less (I have heard 4 kg being mentioned?).

 

I am working on a front diffuser and airdam design at the moment and to help me work out what is going on I have made up a pressure testing rig which consists of a very tiny, easily placable pressure sensor and appropriate power circuit and readout in the car.

 

I intend to do some testing to put some numbers to my thinking with regard to locating high and low pressure areas on the car; I also intend to work out best placement for the cold air intake and do some study on the amount of rad flow.

 

I am not university schooled in aerodynamics so my results will be very common sense, I'm afraid, but nonetheless I will post them down when we get some nice weather.

 

I have done some test runs in "ordinary" cars and a mate's Westfield so I have some intersting numbers to compare against.

[Trevor Phillips]

Julian,

I'd be fascinated to see the results and any pictures afterwards of the design.

On the front wheel arches I note that Lol Pilfold's extremely rapid 7 has a wrap round arch to stop airflow getting between the arch and the tyre - potentially an area of lift.

[Mark H]

Has anyone ever put louvers in there front cycle wings ,it could help reduce the negative pressure and still be road legal,

 

 

[edmandsd]

If you completely panel the underside of a seven so it is effectively sealed (and don't have vents in your bonnet) I would have thought two things would happen:

Air passing through the nosecone/rad will not be able to escape meaning you might aswell cover the nosecone with a flat sheet which can't aid areodynamics can it ? Secondly your engine will overheat.

It was for this reason that I left the rear of the engine compartment open and didn't seal the rear undertray to the back of the car - instead leaving a 1 inch gap to relieve air passing through the transmission tunnel. To be honest the results were probably not worth all the effort and probably gave me 1.5 mph extra terminal speed over the quarter mile at around 130mph.

I wouldn't want people rushing out and doing this in the belief it gives a significant performance gain when I honestly don't think it does.

Maybe I should completely seal the underside, lose the bonnet vents and vent the rad out of the top of the nosecone to get another 5 mph but I don't think I'll bother !

 

 

Home of BDR700

[Tony C]

Julian,

Thanks for the info. I remember seeing the wind tunnel thread, but couldn't see any data.

Basically, that was what I was after.

 

As far as your not being "university schooled in aerodynamics (by your own comments) so my results will be very common sense, I'm afraid, but nonetheless I will post them down when we get some nice weather".

 

Me either; I just teach pilots and try to stick to the "hand out the window of your car" variety of instruction.

 

I've managed to work backwards through Peter's data and can now see what is what. I appologise to everyone if I gave the inpression of being a "smart Arsenel" newcomer.

 

I too will be interested in your findings - more power to you!

 

It turns out that the student with a contact with a Formula 1 team was exaggerating, just a little.

 

Regards

TFC