Measuring aerodynamic car panel pressures on the road

Text and pics: Julian Edgar

Highly effective aerodynamic test and development can be undertaken on the road or track by the direct measurement of surface body pressures. Here an Evolution Measurement EvoScann P16A data logger is being used, but I have also done similar testing using a low-cost Magnehelic gauge.

Evolution Measurement EvoScann P16A pressure scanner, CANDI CANbus to USB converter and laptop. Note the surface pressure measuring patch next to the P16A logger. Multiple patches were used in the testing described below, with 90 samples (at 10Hz) taken in two directions and then averaged.

The test vehicle in this case was my Gen 1 Honda Insight - a 20+ year old car but still interesting with its aluminium and plastic body, and claimed original Cd of 0.25. During this testing I was developing a rear spoiler and fins for the car. The aim was to decrease rear lift (achieved), improve straight line stability (achieved) and decrease overall drag (achieved - but the testing of that isn't covered in this article).

The Honda Insight fitted with ducktail spoiler and rear fins. Not covered in this article are the front external air guides, rear separation edges and full length undertray with diffuser.

Upper surface pressures were measured using the aforesaid EvoScann P16A and Scanivalve surface measuring patches, with a static pressure reference provided by a pitot tube mounted on an 800mm high pole at the leading edge of the bonnet (visible in the pics below). Test speed was 80 km/h and testing was conducted at 550 metres altitude. Test data is in gauge Pascals.

(Above) The measured upper body pressures without the rear spoiler and fins in place (while they are shown in the photo, they were removed for testing).

(Above) The pressures with the rear spoiler and fins in place. All pressures have increased (i.e. they are "less low"), and now there is a positive downwards pressure at the back of the car. 

This graph shows in more detail the measurements that were made.

Three different tests were undertaken – without the spoiler and fins, with the spoiler, and with the spoiler and fins. Each group of bars reflects the number of lateral locations where measurements were made. So for example, three pressure measuring pucks were used at the rear spoiler/end of hatch, one at the left-hand side, one in the middle and one at the right-hand side. Two locations were used for the lower hatch (left-hand side and right-hand side), and so on. 

Leaving aside the wake for the moment, each measuring location shows an increase in pressure when the spoiler was fitted, and a further increase in pressures when the fins were added.

In the wake, the base pressures fell (bad for drag) when the spoiler was fitted but recovered slightly with the added fins. However, the base pressure remained lower than without the spoiler and fins being fitted. Testing of overall drag, though, showed that the fins and spoiler reduced drag – the increased pressures on the upper rear surfaces (and potential disruption of trailing vortices from the C-pillars) apparently more than making up for the lower wake pressure. 

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