What causes car drag?

[JulianEdgar]

Originally Posted by aerohead

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QUESTION :
at what velocity were the Insight measurements obtained? I've been adjusting all your values to 70-mph, as other automotive messengers are reporting figures obtained at 70-mph.
Second question. In other reporting, forward stagnation pressures = to local density altitude / station pressure are indicated, normalized to SAE standard = 101,327-Pascals.
When I adjust the 50-mph and 43-mph data to 70-mph, none the stagnation pressures indicate even a fraction of that.

I would have thought a qualified engineer would find this all very easy. But let the sociologist / geographer / journalist help you!

550 metres altitude with a sea level pressure of 1013.25mB = 951mB

951mB at (say) 25 degrees C = air density of 1.1 kg/cubic metre

80 km/h (22.2 m/s)

Calculated stagnation pressure = 271 Pa.

Pressures measured on car:

[aerohead]

Originally Posted by JulianEdgar

I would have thought a qualified engineer would find this all very easy. But let the sociologist / geographer / journalist help you!

550 metres altitude with a sea level pressure of 1013.25mB = 951mB

951mB at (say) 25 degrees C = air density of 1.1 kg/cubic metre

80 km/h (22.2 m/s)

Calculated stagnation pressure = 271 Pa.

Pressures measured on car:

My apology, I just now saw the 80 km/h in the caption. Thanks!

[JulianEdgar]

Originally Posted by aerohead

1) ' pressure drag is the largest component in the aerodynamic drag. Its minimization is the true objective of motor vehicle aerodynamics.'
Hucho, page- 110, top of page.
2) ' The main contributions to the drag force originate from the tear part of the body,' Hucho, page- 61
3) ' low drag can only be achieved when the separation at the rear is eliminated.' Hucho, page- 16.
4) ' Optimum shapes are 'streamlined' bodies having a very slender rear part.'
Hucho, page-61.

All quite correct!

But none of it addresses the point that attached flow can cause drag (and of course, also thrust).

[aerohead]

Originally Posted by JulianEdgar

All quite correct!

But none of it addresses the point that attached flow can cause drag (and of course, also thrust).

So, in light of the knowledge that attached flow causes drag / thrust, what would we do with that information?
Hucho emphasized reducing, or eliminating separation. Is that off the table now?

[JulianEdgar]

Originally Posted by aerohead

So, in light of the knowledge that attached flow causes drag / thrust, what would we do with that information?

Um, develop separation edges, use rear spoilers to increase rear pressures on inclined surfaces and so reduce drag and lift, optimise thrust on forward-facing surfaces - they might do for a start.

And are you now acknowledging your complete error in this area, and your subsequent misleading of people here for many years on this topic?

[aerohead]

Originally Posted by JulianEdgar

Um, develop separation edges, use rear spoilers to increase rear pressures on inclined surfaces and so reduce drag and lift, optimise thrust on forward-facing surfaces - they might do for a start.

And are you now acknowledging your complete error in this area, and your subsequent misleading of people here for many years on this topic?

1) separation edges are out. They were invented at the FKFS, so completely unreliable technology.
2) rear spoilers were researched at the FKFS, so they're also out.
3) optimize thrust on forward-facing surfaces. 90-degree edges at the windshield header ?
4) splitters are out. FKFS.
5) wings are out. FKFS.
6) wing capping plates. Lanchester probably got it all wrong.
7) Insight or Prius rooflines. Forget it, just more FKFS B. S..
8) Low-drag cooling systems. Wrong. FKFS.
9) If I believed in luck, I'd say 'good luck' approaching Cd 0.09.

[JulianEdgar]

Originally Posted by aerohead

1) separation edges are out. They were invented at the FKFS, so completely unreliable technology.
2) rear spoilers were researched at the FKFS, so they're also out.
3) optimize thrust on forward-facing surfaces. 90-degree edges at the windshield header ?
4) splitters are out. FKFS.
5) wings are out. FKFS.
6) wing capping plates. Lanchester probably got it all wrong.
7) Insight or Prius rooflines. Forget it, just more FKFS B. S..
8) Low-drag cooling systems. Wrong. FKFS.
9) If I believed in luck, I'd say 'good luck' approaching Cd 0.09.

To remind you, we're talking about the fact that attached flow can cause drag, something you said did not occur.

[aerohead]

Originally Posted by JulianEdgar

To remind you, we're talking about the fact that attached flow can cause drag, something you said did not occur.

My takeaway from Hucho and all the others in fluid mechanics is, that any drag associated with resultant forces pale in comparison to low base pressure aggravated by separation.
Most attached flow drag is associated with viscous shear and its friction drag.

[JulianEdgar]

Originally Posted by aerohead

My takeaway from Hucho and all the others in fluid mechanics is, that any drag associated with resultant forces pale in comparison to low base pressure aggravated by separation.
Most attached flow drag is associated with viscous shear and its friction drag.

You claim to be an engineer. Do some simple vectors and resultant drag and thrust forces based on these pressures and angles:



You'll be surprised how much attached flow pressures matter.

(They mattered less 30 years ago when there was so much rear separation. Update your references.)

[aerohead]

Originally Posted by JulianEdgar

You claim to be an engineer. Do some simple vectors and resultant drag forces based on these pressures and angles:



You'll be surprised how much attached flow pressures matter.

(They mattered less 30 years ago when there was so much rear separation. Update your references.)

Again, what in the world will measuring the pressure profile accomplish towards lowering the Cd 0.25, down to Cd 0.09? Possession of a pressure profile implies that we have something of comparative nature with which to guide us. Up, down, or sideways.
If we agree that the roofline on the Insight is 'streamlined', we ought to leave it alone, other than extending its length, while boat-tailing the sides and diffuser, to achieve even higher pressure regain, higher base pressure, lower pressure drag, and lower drag.
A BEV Insight would lose the grille opening almost completely. The belly could be perfect. Tesla mirrors would help. Front skirts. Boat-tail. ( Three-Wheeler made a fine one! ) 80-mpg is not uncommon on a good day, at real highway velocity.