Thread: Measuring stuff
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Old 05-15-2020, 11:06 PM   #2 (permalink)
JulianEdgar
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For me, the huge advantage of testing is that you can see what really happens, not what some people suggest might happen.

Testing can be quite salutary. When I first installed my GOE222 profile rear wing, I set it by eye at what I thought would be a good starting point angle of attack. But the wing did nothing - it produced no measurable downforce (or less lift).

I then did about ten tests using different attack angles, and found that the angle that gave best downforce 'looked' all wrong. What I hadn't considered was that the airflow coming down the rear hatch of the Insight was not parallel to the ground, so setting the wing angle according to the official lift/drag curves for the GOE222 profile gave poor results. Instead, it had to be set to provide that angle of attack to the true oncoming airflow direction. Obvious when you think about it - but I hadn't thought of it.

When I measured surface pressures on the XE Jaguar, I found that my results matched pretty well with Jaguar's CFD, except that I could not measure positive pressure at the base of the rear glass, as their CFD showed. I was in communication with Jaguar's chief aerodynamcist and we had an interesting conversation about the discrepancy. In short, he said it could be either the CFD in error, or my measuring in error. I was interested that he nominated both. (And he wasn't just being polite. He was certainly not backward in suggesting if I had written something wrong.)

When I first started using the Magnehelic gauge to measure pressures, about 16 years ago, I was trying to improve airflow through an under-hood intercooler. I was successful in improving its performance, but what I most remember is watching the gauge and seeing a further thing that influenced the actual pressure differential across the intercooler core - turbulent atmospheric airflow. When I was in the wake of another car, I could see the gauge flickering. And when I passed through small hills and valleys you could see the pressure differential across the intercooler change a lot in the gusty crosswinds.

In the same way, if you are measuring the pressure differential between the top-of-hood and under-hood (eg to best site vents), you can see major changes in the pressure differential with yaw (ie crosswinds). I always wondered that older Japanese cars used to place vents down each side of the hood (and so not in the area that normally has greatest pressure differential, which is towards the front of the car). But then when I did the measurements, I found that those side locations worked far better than you'd first expect because of the yaw component of the airflow that occurs in crosswinds.

However, by far the most exciting measurement technique I have used has been measuring suspension height - and so lift/downforce. It was a bit serendipitous - I had installed air suspension on my car, so needed front and rear height sensors. With the suspension installed and working, I wanted to monitor height, so fed the data into my MoTeC dash so I could watch it 'live' (Developing the control system for the air suspension took about a year and went through three major iterations.) Then it occurred to me - why not use this data to assess lift/downforce? With that incentive, I decided to develop proper front and rear undertrays with a rear upward section (ie diffuser). It also gave me an added impetus to research diffuser angles, that I was already doing for the book.

The first test drive at 100 km/h with the undertrays fitted caused my dash warning to flash - RIDE HEIGHT LOW! (I had programmed the dash to give warnings like this, in case the suspension control system went mad or leaked, etc.) The change in ride height caused by the downforce was enough to trigger that warning (I think I had it set for a ride height 10mm lower than normal.) So the undertrays were working, not only in reducing lift but in developing real downforce. But could I feel it - or was it one of those things that's undetectable? It was (and remains) quite obvious, and when I measured how many kg it is, that's not surprising. It's an uncanny feeling - the tyres pushing down harder on the road (and so having more grip) but without the car having any more mass. It also makes an obvious difference to straightline stability. I had both undertrays off about 12 months ago and the car was noticeably worse in stability and cornering.

I recently measured pressures under the Insight's front and rear undertrays at a single speed (70 km/h - you don't need to be going fast) to see if they improved (ie got lower) as ride height decreased. What I found was that going lower than 130mm (rear clearance) and 110mm (front clearance) gave no advantage - but at ride heights above those figures, downforce decreased rapidly. This is a perfect example of measuring what is really happening, not what people think might be happening, or what happened in textbook or tech paper references to other cars. Just measure your own.

My Edgarwits (aerofoil-based external air curtains) have measurably reduced drag. But I think that they've also done something to the undercar airflow. (The wool tuft testing showed the side airflow pattern altered, and it appeared the airflow from the car's side to the undercar was changed.) So what have they done to lift/downforce? I am not sure, but one day I'll spend 30 minutes and do some testing measuring undercar pressures with the Edgarwits on and off. Then I will know.

No theory, no guesswork, no opinion, no speculation - just what is really happening.
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