Lift and drag

[JulianEdgar]

Originally Posted by 2000mc

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?

100lbs of "downforce" from the undertray, so actually giving no net downforce or lift.

But I do think you're making it hard for yourself. When you can measure whether the suspension is being lifted (or compressed) on its springs, it makes it all very easy to understand.

[2000mc]

So I’m thinking if I had a giant double wing, if one wing was angled to make 100lbs of down force, and the other was angled to make 100lbs of lift, I feel like it would cause more drag than if only one was set flat to make no lift, or if both were set flat to make no lift.
Do you think one wing flat and one angled would be the highest drag option of the 3 mentioned?
Or have I created a totally different scenario?

[JulianEdgar]

Originally Posted by 2000mc

So I’m thinking if I had a giant double wing, if one wing was angled to make 100lbs of down force, and the other was angled to make 100lbs of lift, I feel like it would cause more drag than if only one was set flat to make no lift, or if both were set flat to make no lift.
Do you think one wing flat and one angled would be the highest drag option of the 3 mentioned?
Or have I created a totally different scenario?

I think you're making a very weird hypothetical situation. The actual mechanism by which lift-induced drag is created in cars is not particularly simple, as far as I can tell. (In fact Barnard in his book states that specifically, as he said to me more than a few times.)

If we stick to cars developing lift through their normal body shape, if the upper surface's lift are completely offset by the lower surface's downforce, then there will be less drag than if the situation were different.

When I was researching my book, I ended up using The Leading Edge (Goro Tamai), which is a very good book on solar racing car aerodynamics. It's one of the very few references where I used 'race car' books, but solar race cars are the vehicles in the world chasing lowest drag at (basically) any cost. He states that no vertical force (ie no lift/downforce) is best for lowest drag, which of course makes sense in terms of induced drag.

[2000mc]

“no vertical force (ie no lift/downforce) is best for lowest drag” is easy for me to agree with, but I would think that would apply to any given surface, rather than net lift/downforce of the entire vehicle

if the upper surface's lift are completely offset by the lower surface's downforce, then there will be less drag than if the situation were different.

To me, inducing additional downforce goes against “no vertical force (ie no lift/downforce) is best for lowest drag”

[JulianEdgar]

Originally Posted by 2000mc

“no vertical force (ie no lift/downforce) is best for lowest drag” is easy for me to agree with, but I would think that would apply to any given surface, rather than net lift/downforce of the entire vehicle

if the upper surface's lift are completely offset by the lower surface's downforce, then there will be less drag than if the situation were different.

To me, inducing additional downforce goes against “no vertical force (ie no lift/downforce) is best for lowest drag”

Huh?

We want the downforce to be completely offset by the lift. Or the lift to be completely offset by the downforce. That is, no net lift or downforce. Of the whole vehicle. It's one entity!

It seems pretty simple to me!

Net = key point.

It's one reason that cars with very low drag (eg Tesla Model S) have such low coefficients of front and rear lift.

Expansion:

Consider an object in space. We can apply a force in one direction, let's call that 'lift'. We can apply a force in the opposite direction - let's call that 'downforce'.

If we have 'lift' forces occurring, and we don't want the body to accelerate, we need to apply 'downforce'. When lift and downforce are equal, we have no body acceleration. We don't then regard 'downforce' as significant; it's just been balanced by 'lift'. The body is then in equilibrium.

And in regard to cars, in this case the net pressures on the top surfaces are the equal and opposite the net pressures on the bottom.

I think - and this is an educated guess - with no lift/downforce net differential, the strength of trailing vortices is much reduced, so explaining the reduction (to nil in this case) of induced drag.

[aerohead]

Originally Posted by JulianEdgar

Huh?

We want the downforce to be completely offset by the lift. Or the lift to be completely offset by the downforce. That is, no net lift or downforce. Of the whole vehicle. It's one entity!

It seems pretty simple to me!

Net = key point.

It's one reason that cars with very low drag (eg Tesla Model S) have such low coefficients of front and rear lift.

Expansion:

Consider an object in space. We can apply a force in one direction, let's call that 'lift'. We can apply a force in the opposite direction - let's call that 'downforce'.

If we have 'lift' forces occurring, and we don't want the body to accelerate, we need to apply 'downforce'. When lift and downforce are equal, we have no body acceleration. We don't then regard 'downforce' as significant; it's just been balanced by 'lift'. The body is then in equilibrium.

And in regard to cars, in this case the net pressures on the top surfaces are the equal and opposite the net pressures on the bottom.

I think - and this is an educated guess - with no lift/downforce net differential, the strength of trailing vortices is much reduced, so explaining the reduction (to nil in this case) of induced drag.

They would be a function of a pressure/velocity discrepancy between any two adjoining flow fields (like body sides and roof) and how the differentials induced span-wise flow under a physical imperative to attain equilibrium.
On Ahmed body investigations,all vorticity could be eliminated by tuning the rear roof slope,boat-tailing,and diffuser angle.Same way Cybertruck may come in at around Cd 0.28.Without mirrors of course.

[JulianEdgar]

Regarding lift, you can think of it like this.

1. We want the car to not be a lifting body, as this causes induced drag.

2. Cars develop lift over their upper curved surfaces, and the longer that flow stays attached, the greater that overall lift. (Refer to the pressure measurements shown earlier in this thread to see this in action.)

3. But we have two surfaces we can work with, and if they each develop low pressures, they pull in opposite directions.

4. Therefore, if we develop sufficiently low pressures under the car over a sufficiently large area, the car ceases to be a lifting body.

5. This is most easily achieved with a flat underfloor and simple rear diffuser.

6. Such a flat undercar body is also good for reducing drag (not just induced drag but also pressure drag).

I think maybe some of the misconceptions here about this come from:

- The idea that downforce = induced drag (but of course, an equal measure of lift will cause an equal measure of induced drag)

- Bodies with attached flow don't develop lift. This is of course completely wrong, as picturing the lift being generated by an aircraft wing will soon show you.

The Tesla Model S is a good example of all this.

Attached flow on upper surfaces from nose to tail:



But offset by a truly excellent under car flat floor and diffuser:



Resulting in low lift figures:

CL (front: 0.1
CL (rear): 0.1

[freebeard]

"Boy, you gotta carry that weight"

To paraphrase the Old Farmer, 'you can lift in one hand and downforce in the other and see which one fills up first.'

My solution for the Beetle (similar to Porsche) was to graft on a Tropfenwagen tail.


Center of pressure moves back. I should redo this with the new software I'm learning. The parts could be Booleaned together, it will probably bork the materials assignments.

edit:
This one has more aggressive taper because the engine cooling air is introduced into a vertical trough, Coanda nozzle.

[JulianEdgar]

Originally Posted by freebeard

"Boy, you gotta carry that weight"

To paraphrase the Old Farmer, 'you can lift in one hand and downforce in the other and see which one fills up first.'

My solution for the Beetle (similar to Porsche) was to graft on a Tropfenwagen tail.


Center of pressure moves back. I should redo this with the new software I'm learning. The parts could be Booleaned together, it will probably bork the materials assignments.

edit:
This one has more aggressive taper because the engine cooling air is introduced into a vertical trough, Coanda nozzle.

I think anyone can speculate, and lots of people do so here with all sorts of weird and wacky theories. None of its counts for anything much.

I don't know if Americans use this saying, but without actually building and testing, it's all just pissing in the wind. Harmless but pointless.

A few hours on the road testing a mock-up is worth an infinite amount of such pissing. Then we have measurements to discuss, not speculation.

[freebeard]

One does what one can.



This was a mock-up to establish the dimensions. I lost the workspace.