Lift and drag

[JulianEdgar]

Just a few general comments about lift. Coming into this discussion group from outside, as it were, I have been really surprised how no-one seems to pay much attention to lift.

In general, lift = drag. This is called 'induced drag'.

People will often say (eg about my car, that has measurable downforce), doesn't that downforce cause drag? The answer to that is 'yes' - but your car with lift also has drag!

That is, it doesn't matter whether the force is up or down; both cause drag.

Basically pretty well all cars on the road have lift. This can be measured very accurately with height position sensors on the suspension (my preferred approach)...



...or less accurately with electronic distance sensors looking down at the road. (I use ultrasonic sensors and an Arduino and a custom program I wrote in XOD visual programming language.)



So, unlike drag which is so hard to measure, lift/downforce is relatively easy to measure.

Many modifications that decrease wake area through long upper body curves (eg The Template) are likely, in my opinion, to generate a lot of lift. This can be measured, not only with the two above techniques, but also by directly measuring upper surface body pressures. This is easy and cheap to do with a surface pressure sensor (make it yourself), a Magnehelic gauge and a sealed reservoir.

I did testing of this sort on a Jaguar and then was able to compare my measurements with the official Jaguar CFD. They matched very well.

My measurements:



Jaguar CFD (hotter color = lower pressure):



Note the measured high lift on all the upper body curves eg windscreen header rail, roof, rear window. This pattern is measurable on all cars with these sort of upper body curves - and the longer they go for (eg The Template), the more lift (and so drag ) that is developed!

So it then becomes a balancing act, induced drag through lift versus less wake drag.

Unfortunately though, if you like driving, pretty well all lift is bad! However, an easy remedy is to use a very smooth undertray and so get high speed flow (and so low pressure) under the car. That counteracts lift, and of course reduces drag in its own right as well. (But only if you're not using a deep front airdam...)

As with upper body pressures, those under the car can also be directly measured, so you can see how well your undertray is working in developing low pressures.

Tl;dr:

• if your car has lift, it is getting drag from that source
• lots of techniques designed to reduce wake drag create lift
• but lift is easy to directly measure

[redneck]

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Please post some pictures of your measuring outfit (Magnehelic gauge and a sealed reservoir).




>

[M_a_t_t]

Originally Posted by JulianEdgar

Unfortunately though, if you like driving, pretty well all lift is bad! However, an easy remedy is to use a very smooth undertray and so get high speed flow (and so low pressure) under the car. That counteracts lift, and of course reduces drag in its own right as well. (But only if you're not using a deep front airdam...)

Can you cancel induced drag from lift by increasing downforce or is the induced drag from each treated like an absolute number?
The quoted section sounded like it was only talking about effects on stability and driving feel, so I wasn't sure if the same rules applied to drag. This was the best way I could figure out how to word the question.


In another thread he posted this picture while talking about the magnehelic gauge and reservoir

[JulianEdgar]

Originally Posted by M_a_t_t

Can you cancel induced drag from lift by increasing downforce or is the induced drag from each treated like an absolute number?

Lift/downforce are basically just a continuum (like thrust /drag). A positive upwards force is called lift, and a positive downwards force is called downforce.

Think of it like this. The upper surfaces of the car are developing (mostly) lift - upwards forces. To counteract that, we need to have forces that pull in the opposite direction - downwards. If, overall, 'up' is greater than 'down', we have 'lift'. If, overall, 'down' is greater than 'up', we have 'downforce'.

So we might do things that create down forces (eg a rear wing), but unless those down forces are greater than the up forces, we will still have overall lift.

Lowest induced drag comes from having neither lift or downforce. Therefore, if you can reduce lift without doing anything that at the same time increases drag, you're on a winner. (Dick Barnard, the aerodynamicist I worked with on the book, had a good example. He said,"A fence placed across the roof will reduce lift. But it will also massively increase drag!")

The easiest way of reducing lift without adding extra drag is a full length undertray and rear diffuser.

[JulianEdgar]

Here's an easy way of seeing it. It's a plot of pressures on the bodywork. Where the solid line is above the body, the pressures are low. Where the line is below the bodywork, the pressures are high. Note also the direction of the arrows that shows how those pressures are developing forces (ie they are vectors).

We can see that the attached flow over the car (right up to the separation that occurs at the base of the hatch) creates a lot of lift. (Only at the base of the windscreen, and a tiny bit at the trailing edge of the hatch, is that pressure creating any downwards force.)

You can also see from the magnitude of the lift forces that it's very hard to offset all of them with anything on top of the car. Therefore, we need to work under the car, where we have a large area - and so even lowering pressures a bit will have a big impact. (Obviously, this diagram doesn't show any of the pressures under the car - in those days, the undercar was basically ignored.)

Finally, note the magnitude of the lift pressures compared with the wake pressures - lift forces can be very large.

[JulianEdgar]

And, something I'd not thought about in the context of this diagram.

Let's imagine we put a full air dam on the front of the car, extending to the road (ie an extreme). Now the pressures we can see in the wake would also occur under the car.

We can see from the length of those wake arrows, that if they were placed under the car and pointed downwards, they would help decrease overall lift but could never counteract it entirely - the pressures aren't of great enough magnitude.

However, with a full length, smooth undertray and diffuser, we can get pressures lower than the wake. How low (ie how long an arrow)? I'd be guessing on this car but I'd say of similar magnitude to the 'up' arrows directly above the rear wheels.

[JulianEdgar]

So lift is primarily caused by airflow wrapping around curved upper surfaces. This can be clearly seen in both images:





Also able to be seen is that where the air is directed upwards (base of windscreen, little rear kick-up on the Jaguar) a higher pressure is developed.

Also obvious on the Jaguar is that the pressures caused by the wrapping-around of airflow are much lower than the pressure in the wake. (Therefore, the wake has nothing to do with these low 'lift' pressures, an incorrect theory that has been suggested elsewhere on this group).

The classic high-lift shape is the Porsche 911 - one long curve from the top of the windscreen header to the rear of the car. If there is attached flow (as you can see from the tufts there is), you can imagine the lift being created!



The ducktail spoiler Porsche then fitted disrupted this flow, reducing lift.

The long, sweeping curve of the Jaguar down to a relatively small wake area, all with attached flow, is ostensibly good for low drag. However, the penalty is lift and and so induced drag. But you can see how that has been in part addressed, with the undertrays and pseudo rear diffuser (it's basically just shaped mufflers and spare wheel well) giving low pressures under the car (much lower than could be achieved by a front air dam).

[M_a_t_t]

So you can't make the top of the car not produce lift but you can counteract it using other mods. For pure drag reduction you would want an overall neutral lift/downforce. It sounds like a neutral car body isn't very realistic (especially DIY) in terms of real world and the best result would still have some overall lift.

[JulianEdgar]

Originally Posted by M_a_t_t

So you can't make the top of the car not produce lift but you can counteract it using other mods. For pure drag reduction you would want an overall neutral lift/downforce. It sounds like a neutral car body isn't very realistic (especially DIY) in terms of real world and the best result would still have some overall lift.

No, it's actually quite easy. My Insight develops downforce from a very simple (but large) undertray and basic diffuser. If I wanted zero lift, I could easily reduce its size.

My Insight:







The important thing is to actually measure what you are doing. So, through measurement of lift/downforce, you can see what the impact is of the modifications you are performing.

[2000mc]

Originally Posted by M_a_t_t

So you can't make the top of the car not produce lift but you can counteract it using other mods. For pure drag reduction you would want an overall neutral lift/downforce. It sounds like a neutral car body isn't very realistic (especially DIY) in terms of real world and the best result would still have some overall lift.

Originally Posted by JulianEdgar

No, it's actually quite easy. My Insight develops downforce from a very simple (but large) undertray and basic diffuser. If I wanted zero lift, I could easily reduce its size.

If I could try to ask that a different way...
If the top of a car produced 100lbs of lift, would an undertray producing 100lbs of down force, or an undertray producing 0 lbs of down force net the car the lowest overall drag?