Measured aero pressures on Insight

[cr45]

I actually own two +/- 2psi manometers myself and hence the reason for the question. They have 0.01" H2O resolution.

The funny thing is I went looking for a +/- 1 psi hoping for better accuracy but with the +/- 1 psi having a 1.0 % fsd accuracy and the +/- 2 psi having a 0.3 % fsd accuracy, then the latter would appear to have higher accuracy.

I paid a very similar price to yourself for mine.

[JulianEdgar]

Originally Posted by cr45

I actually own two +/- 2psi manometers myself and hence the reason for the question. They have 0.01" H2O resolution.

The funny thing is I went looking for a +/- 1 psi hoping for better accuracy but with the +/- 1 psi having a 1.0 % fsd accuracy and the +/- 2 psi having a 0.3 % fsd accuracy, then the latter would appear to have higher accuracy.

I paid a very similar price to yourself for mine.

Have you tried using them for measuring aero pressures? For 'simple stuff' like before/after the radiator, just place the ends of the hoses in the approriate locations. For 'complex stuff' (like panel pressures), you'll need to make a sensing puck.

[cr45]

At this moment in time I use mine for doing engine port flow measurements using a venturi and hence the 2psi works well.

For car aero measurements - particularly at the front of the car - I was wondering if there would be any merit in using a pitot tube protruding in front of the car to get a static reading to be used as one input to the differential manometer. This would be a substitute for the pressure box. How far the pitot would need to protrude in front of the car to get a clean static pressure is the question.

[JulianEdgar]

Originally Posted by cr45

At this moment in time I use mine for doing engine port flow measurements using a venturi and hence the 2psi works well.

For car aero measurements - particularly at the front of the car - I was wondering if there would be any merit in using a pitot tube protruding in front of the car to get a static reading to be used as one input to the differential manometer. This would be a substitute for the pressure box. How far the pitot would need to protrude in front of the car to get a clean static pressure is the question.

That's a very interesting idea - one I'd certainly not thought of.

The two issues I can see though is that I think the static port would need to be a long way ahead of the car (eg 1+ metres) and such a support could change flow patterns on the car.

But, depending on how quiet your roads are, I think both are not major deficiencies. I'll give it a go today and report back.

[JulianEdgar]

Initial measurement - it's looking very good indeed.

I decided to mount the pitot tube above the car - that's legal here whereas in front of the car wouldn't be. It's 2.2 metres above the ground.

Using the pitot tube as the static reference, the stagnation pressure on front numberplate measured 1.2 inches of water (300 Pa). That compares with the calculated stagnation pressure (see cr45's post above) of 308 Pa.

The reading also does not drift as it does when using the sealed vessel as the reference (it's impossible to keep temps exactly constant, so the volume expands/contracts a bit).

So well done cr45 for the great idea.

Now I need to do all the measurements - and revise the data I showed previously!

[JulianEdgar]

So I think that using the pitot tube as a static reference is a much better approach than using the sealed vessel as the reference. Thanks again to cr45 for the excellent suggestion.

I have redone all of the Honda's measurements. Note that the basic pattern remains the same as with the previous measurements - for example, no negative measurements became positive or anything like that! However, I think with the pitot tube static reference, the absolute figures are much more accurate.

I also did an additional measurement in the middle of the roof because I suspected lots of lift might be being generated here because of the roof curvature (and I was right).

Test:

• Pitot tube static reference located 2.2 metres above ground
• Magnehelic 0-3 inch of water gauge
• Custom measuring puck (easy to make yourself)
• 11 degrees C, fine and clear
• Light winds
• 80 km/h test speed
• Average of two measurements in different directions
• Pascals rounded to 5

Note: the Insight runs full front and rear undertrays, with the rear undertray incorporating a 10 degree diffuser.

[JulianEdgar]

With the pitot reference giving such stable readings, I was able to do a test on the Insight that with the previous sealed reservoir approach, was difficult/impossible. I used a 0-120 Pa Magnehelic gauge (ie 0-0.5 inches of water FSD - so very sensitive).

I placed the measuring puck on the bumper corner and recorded a two-way average of -30 Pa at 80 km/h.



I then added a large separation edge (always go large when testing - if it works, you can always go smaller.) Note that the measuring puck was not moved.

The pressure increased to -17 Pa (similar to the pressure previously recorded in the middle of the wake).



So the separation edge measurably and clearly reduced the vertical suction peak down the side of the bumper.

[jakobnev]

The pressure increased to -17 Pa (similar to the pressure previously recorded in the middle of the wake).
...
So the separation edge measurably and clearly reduced the vertical suction peak down the side of the bumper.

Could you measure in the middle again? (with the separation edges on the bumper) The pressure should have increased there too.

And I'm curious if an improvement could be measured with your throttle stop testing.

[JulianEdgar]

Originally Posted by jakobnev

Could you measure in the middle again? (with the separation edges on the bumper) The pressure should have increased there too.

And I'm curious if an improvement could be measured with your throttle stop testing.

I have some lip-style rear spoiler molding coming to make the proper separation edges (like this)



When it arrives - two to four weeks I imagine - then I'll do a bunch of measurements. My guess is that the separation edges will impact only the pressures near the edges, not generally within the wake.... but we will see!

[cr45]

This looks very promising indeed and stagnation pressure is now where it would be expected to be.

I decided to have a look at the sensitivity of pressure to temperature change for a pressure box.

Using Gay-Lussac's law for a closed container then P2/T2 = P1/T1

Hence P2 = (T2/T1) * P1

Lets assume that the atmos pressure is 101325 Pa and the temperature is 10 C.

Also lets assume that the temperature of the air rises by 0.1 C by the time you take your aero measurements.

The new value for the pressure in the box will be

P2 = ((10.1 +273.15) / (10.0 + 273.15)) * 101325 = 101361

That is a difference in pressure of 36 Pa which is very significant considering the magnitude of the pressure differentials you are trying to measure.