Lift

[aerohead]

Originally Posted by JulianEdgar

Rob Palin's comments, and the SAE paper research I did for the book, start to show why lift/downforce is so important.

Having received his comments, I am tempted to start doing some high-speed logging of body pressures.

High-speed pressure logging would be quite cheap and fairly easy to do... but then I think - why would I bother? None of the car modification community (of any complexion) seems to be much interested in real aero data.

After all, even basics like simple averaging pressure measurements with an analog gauge apparently is like rocket science, beyond the reach of normal mortals. (Has even one person on this group done it? It costs under US$100 - for equipment that will last a life-time - and takes about 10 minutes. No, so much better to believe in the absolute BS being constantly spouted than find out the truth for yourself.)

It's all a bit like estimating the weight of an elephant by guessing its volume, its density, and the measurements taken from other animals. Like, you don't have scales to weigh the darn elephant? Oh I see, you can get them - but you can't be bothered.

After all - and get this - here even wool tuft testing is the stuff of mystery and fallacious interpretation! Fxxk me.

So I think: let someone else actually do some actual real on-road testing of high speed pressure variations. Just maybe you might find out new and really useful information. Hell, that would never do, would it...

Some of the professionally-generated studies represent 42- centerline taps, measured simultaneously, over and under the test vehicle, under clinical conditions, and all data normalized to standard SAE air density. Centerline pressures do not represent the entire pressure distribution over the surfaces, which are better represented in the data indicating forces at both axles.
The density altitude is a significant issue. Testing in Denver, Colorado, on a hot summer day would not represent values derived along Pacific Coast Highway, a few feet above sea-level, on a cool fall day of 60-F. This context is really important.
I appreciate your investigations. I really do. It's just not something that ever captured my interest. I'll concentrate on the elephant in the living room.

[JulianEdgar]

Originally Posted by aerohead

-----------------------------------------------------------------------------------
' Lift by itself is not a serious problem in the current passenger vehicle at highway speeds ( 70-mph ).' Kent B. Kelly, Harry J. Holcombe, General Motors Styling Staff, SAE Paper No. 649A, 1963, SAE Congress.

Some of us are using references that are more recent than 57 years old.

Aerodynamics of Road Vehicles, 5th Edition, 2016, Page 449:

"The lift forces acting on a vehicle are extremely significant in road dynamic terms, since they can influence self-steering and braking behaviour, and thus the driver's ability to control the vehicle."

[M_a_t_t]

Originally Posted by JulianEdgar

(Has even one person on this group done it? It costs under US$100 - for equipment that will last a life-time - and takes about 10 minutes. No, so much better to believe in the absolute BS being constantly spouted than find out the truth for yourself.)

I have the book, the gauge, made a reserve tank, and bought a pitot tube. I went to start messing with the radiator pressures (cooling efficiency, etc.) and then realized the hose I had on hand was too short. Been busy elsewhere since then.


My project(s) progress lately:

[JulianEdgar]

Rob Palin has now written more than 5000 words of feedback on my book, chapter by chapter. That's a stunning commitment to an amateur like me, and one for which I am truly grateful. (No mistakes identified, but a lot of amplification.)

Unlike some some people on this discussion group (ie Aerohead) I don't pretend that I know more than the experts.

So for people who want to actually learn, here are some of his other recent comments on lift. (I have de-identified some car companies.)

The Audi TT [mentioned in the book for its scary rear lift results - they crashed] had a CLR that wasn't completely ridiculous - circa 0.11-0.13, I think. It did meet Audi's vehicle stability criteria at the time, but they got stricter afterward. XXXXXX applied the same standard, which was CLF>CLR up to 15 degrees of yaw, and the actual forces at Vmax need to be less than 10% of the axle weight. The TT became kind of a scare story within the aero community, and the name had incredible power within the VAG. Certainly, cars like the Mercedes CLS of that era were much worse (0.18-0.19 or so), but didn't gain any kind of notoriety.

My own view is that the reality is buried in the little-regarded fact that mean lift values leave out a hugely important part of the story, which is the actual frequency spectrum of the lift forces. In research I did back at MIRA, we saw easily 100% fluctuations in lift forces, on all manner of vehicles, with the associated spectrums varying wildly between vehicle shapes, and with individual modifications. A rear spoiler, for example, may have zero net effect on the mean lift, yet reduce the standard deviation of the forces away from that mean quite dramatically - and vice versa.

In the case of the TT, with its curved rump, I would imagine the fairly moderate mean value of CLR didn't reveal that the peak values could be >0.2, nor perhaps that such the underlying flow behaviours could be coherent, or prolonged, or at least of frequencies that perhaps interacted with some resonant modes of the suspension. Ordinarily, lift forces fluctuate at around 30Hz, which should be much higher than the suspension's natural frequencies, but maybe there was a higher order mode which overlapped. Frankly, I doubt anyone knows.

It's very difficult to examine such behaviour in the wind tunnel, due to conflicting demands of consistency of measurement and clarity of signal. Now that most CFD is done transiently, however, the awareness of the true nature of the forces is right in our faces every day, and there's more awareness of just how variable these forces are. Sadly, that also brings a growing realisation that the desire to run transient CFD for just 1-2 seconds of real time (to be practically feasible) is in terrible conflict with the reality of huge force variations as low as fractions of a Hertz. Even sampling for 10 seconds of real time is insufficient on a road car, yet we have to make do with ~3 seconds in CFD or we risk waiting many days for each result.

Stuff from a real, practicing expert.

Dr Wolf of Porsche was equalling fascinating. Note the lack of definiteness - something Dr Wolf was also happy to state on topics like lift. The real experts don't pretend everything was decided in the 1930s...

[freebeard]

Similar to what is happening in astronomy. Increasing the resolution of measurement reveals the jankiness of the model.

Still pictures won't tell you how the movie turns out.

[Grant-53]

Our 1970 Ford Falcon 6 cly. suffered from being blown about when passed by tractor trailers at 65 mph. The wake pushed the car about 3 ft to the right. I attached an 8 inch rubber air dam to the front pan. After that the wake gave only a slight push fore and aft.

[JulianEdgar]

Originally Posted by Grant-53

Our 1970 Ford Falcon 6 cly. suffered from being blown about when passed by tractor trailers at 65 mph. The wake pushed the car about 3 ft to the right. I attached an 8 inch rubber air dam to the front pan. After that the wake gave only a slight push fore and aft.

Yes, in old cars, a front air dam can be very effective at reducing lift.

In more modern cars with much flatter undersides, it's better to further improve the underside smoothness to reduce lift.

[skyking]

I'll be damming the truck down to the height of the front axle, and skirting to the same level between the wheels with conveyor belting that can take a hit if needed. Smoothing it out is really not practical. I also rarely exceed 100 Kph/62 mph when towing. Within 50 miles of home all speed limits are at 60, towing or not.
In the 3 western states the towing limit is at 60. It does simplify aeromodding planning, knowing I won't be at higher speeds.

[aerohead]

Originally Posted by JulianEdgar

Some of us are using references that are more recent than 57 years old.

Aerodynamics of Road Vehicles, 5th Edition, 2016, Page 449:

"The lift forces acting on a vehicle are extremely significant in road dynamic terms, since they can influence self-steering and braking behaviour, and thus the driver's ability to control the vehicle."

Context? 250-mph in a McLAREN Speedtail on the autobahn, or 70-mph in a Chevy Spark?
70-mph is 70-mph.

[aerohead]

Originally Posted by JulianEdgar

Rob Palin has now written more than 5000 words of feedback on my book, chapter by chapter. That's a stunning commitment to an amateur like me, and one for which I am truly grateful. (No mistakes identified, but a lot of amplification.)

Unlike some some people on this discussion group (ie Aerohead) I don't pretend that I know more than the experts.

So for people who want to actually learn, here are some of his other recent comments on lift. (I have de-identified some car companies.)

The Audi TT [mentioned in the book for its scary rear lift results - they crashed] had a CLR that wasn't completely ridiculous - circa 0.11-0.13, I think. It did meet Audi's vehicle stability criteria at the time, but they got stricter afterward. XXXXXX applied the same standard, which was CLF>CLR up to 15 degrees of yaw, and the actual forces at Vmax need to be less than 10% of the axle weight. The TT became kind of a scare story within the aero community, and the name had incredible power within the VAG. Certainly, cars like the Mercedes CLS of that era were much worse (0.18-0.19 or so), but didn't gain any kind of notoriety.

My own view is that the reality is buried in the little-regarded fact that mean lift values leave out a hugely important part of the story, which is the actual frequency spectrum of the lift forces. In research I did back at MIRA, we saw easily 100% fluctuations in lift forces, on all manner of vehicles, with the associated spectrums varying wildly between vehicle shapes, and with individual modifications. A rear spoiler, for example, may have zero net effect on the mean lift, yet reduce the standard deviation of the forces away from that mean quite dramatically - and vice versa.

In the case of the TT, with its curved rump, I would imagine the fairly moderate mean value of CLR didn't reveal that the peak values could be >0.2, nor perhaps that such the underlying flow behaviours could be coherent, or prolonged, or at least of frequencies that perhaps interacted with some resonant modes of the suspension. Ordinarily, lift forces fluctuate at around 30Hz, which should be much higher than the suspension's natural frequencies, but maybe there was a higher order mode which overlapped. Frankly, I doubt anyone knows.

It's very difficult to examine such behaviour in the wind tunnel, due to conflicting demands of consistency of measurement and clarity of signal. Now that most CFD is done transiently, however, the awareness of the true nature of the forces is right in our faces every day, and there's more awareness of just how variable these forces are. Sadly, that also brings a growing realisation that the desire to run transient CFD for just 1-2 seconds of real time (to be practically feasible) is in terrible conflict with the reality of huge force variations as low as fractions of a Hertz. Even sampling for 10 seconds of real time is insufficient on a road car, yet we have to make do with ~3 seconds in CFD or we risk waiting many days for each result.

Stuff from a real, practicing expert.

Dr Wolf of Porsche was equalling fascinating. Note the lack of definiteness - something Dr Wolf was also happy to state on topics like lift. The real experts don't pretend everything was decided in the 1930s...

The actual lift forces would be a function of the square of the velocity. A what velocity were the fatal events taking place? 70-mph? 170-mph?