Measured aero pressures on Insight

[MeteorGray]

Julian, you're a great resource, a bona fide testing guru!

Thanks for all the testing information you are contributing to this site!

One suggestion and one question:

The suggestion: You might want to try a simpler way to mount the pitot tube. I've been using a homemade pitot tube (a simple copper pipe without any side holes) for the past 20 years in the grill of my F250 truck, which sticks out maybe three or four inches in front of the grill. This unofficial, self-named pitot tube is connected to an aircraft airspeed indicator via a plastic tube that is run through the engine bay into the cabin where the gauge resides.

When I first installed the airspeed indicator, I left the static port open in the cabin, but as you noted, I too found this will not give accurate results due to the varying pressures in the cabin. So, I ran a plastic tube from the gauge's static port into the engine bay and got good results. I put a piece of open foam in the end of the tube to keep the mud dabbers out, and this may or may not mitigate any effects of air currents in the bay at speed.

I will note that I do have louvers in the hood to bleed off some of the intense engine bay heat produced by the turbocharged diesel engine. This helps preserve the battery and other heat-sensitive things that must live near the hot engine.

I know the pitot tube is working as intended via observing the same MPHs on the airspeed indicator as is demonstrated on the vehicle speedometer and also a GPS unit, but they are the same only in still air. If there is a headwind, I see appropriate increases in airspeed compared with the other two indicators of groundspeed. If there is a tailwind, the reverse is true. If there are sidewinds, the airspeed differential varies with the wind swirls. Also, when in traffic, all the moving vortex generators around me, aka cars, trucks and busses, make the airspeed indicator jump up and down like a neurotic cat on a hot stove.

All this to say that you might want to try a less elaborate way to mount the pitot tube. It doesn't bother my air speed indicator to simply place it in the grill.

Now, the question I have: The "pressure sensitive" colors on the hood of your car shown in the video seem to imply that the hood is experiencing high pressures not only at the base of the windshield, but also in the center of the hood. The reason I question this is I have installed hood louvers on several of my vehicles, and I have found the centers of my hoods see relative low pressure on the top as compared with higher pressures on the bottom (from the engine bay), which allows excessive heat to flow out and up through the hood at speed (and also when stopped, of course). An exception to this pressure differential would be at the base of the windshield, which does see high pressures due to the windshield vs hood confluence there.

So, are you actually seeing high pressures on the center of the hood as the colors in your video seem to show, or am I misinterpreting it?

[cr45]

Originally Posted by MeteorGray

All this to say that you might want to try a less elaborate way to mount the pitot tube. It doesn't bother my air speed indicator to simply place it in the grill.

I would be confident that in the fullness of time Julian will find that he can lower the pitot tube. However, as a starting point putting the pitot tube at a 2.2m height means that he can have 100% confidence in the measured static pressure.

The major difference between your use of a pitot tube and Julians use is that he is measuring static pressure and you are measuring total pressure. The air is almost certainly slowing down before it reaches your pitot resulting in a rise of pressure. This is not an issue if you are only measuring total pressure.

However it would cause a static reading to be too high and this discrepancy would increase with the vehicle speed. Pitot tubes have to be in free stream air in order to get accurate static pressure readings.

[MeteorGray]

I'm sure Julian's requirements are more sensitive than mine.

[JulianEdgar]

Originally Posted by MeteorGray

Now, the question I have: The "pressure sensitive" colors on the hood of your car shown in the video seem to imply that the hood is experiencing high pressures not only at the base of the windshield, but also in the center of the hood. The reason I question this is I have installed hood louvers on several of my vehicles, and I have found the centers of my hoods see relative low pressure on the top as compared with higher pressures on the bottom (from the engine bay), which allows excessive heat to flow out and up through the hood at speed (and also when stopped, of course). An exception to this pressure differential would be at the base of the windshield, which does see high pressures due to the windshield vs hood confluence there.

So, are you actually seeing high pressures on the center of the hood as the colors in your video seem to show, or am I misinterpreting it?

That view is of the back of the Jaguar, not the front!

Typically, there is a low pressure across the leading edge of the hood (around the front curve) and a high pressure at the rear of the hood (base of windscreen, as you say). As you'd then expect, there is a gradient from low to high pressure across the hood, and at one zone the pressure on top is the same as ambient.

However, the influence of any yaw (cross wind component) can cause some changes in this pattern, as the airflow wraps around a 'side corner' (ie from the fender onto the hood).

To site hood vents, it's best to use a differential pressure gauge (eg Magnehelic) to measure the pressures under, and over, the hood simultaneously at the different possible vent locations. To vent the engine bay you want the biggest difference (highest under, minus lowest over). In the last car I did this with, a forward location on the hood was best.

[JulianEdgar]

Originally Posted by MeteorGray

You might want to try a simpler way to mount the pitot tube.

Yes, as cr45 said, the requirements when measuring just airspeed are a bit different - and easier.

Incidentally, I liked your report on how measured airspeed varies in the different conditions eg in wakes of other vehicles.

I found this when I was measuring pressures on the top surface of an under-hood intercooler - the figures danced around a lot when I was behind other cars. Even, if memory serves me correctly, 20-40 metres behind other cars!

[JulianEdgar]

Originally Posted by cr45

I would be confident that in the fullness of time Julian will find that he can lower the pitot tube. However, as a starting point putting the pitot tube at a 2.2m height means that he can have 100% confidence in the measured static pressure.

I was thinking about this. I think I'll try to find some really secure large suction caps that I can use to stick an upright to a window, with the pitot tube on top. That way the system can be easily swapped from car to car. (And then not measure stuff on that side of the car!)

[cr45]

If you have a second pitot then you could test it in an alternative position but keep the present pitot at 2.2m and input the two static outputs to the one manometer - ideally your most sensitive one.

If the new position is accurate, the meter will give a zero pressure reading at a range of road speeds.

[JulianEdgar]

Originally Posted by cr45

If you have a second pitot then you could test it in an alternative position but keep the present pitot at 2.2m and input the two static outputs to the one manometer - ideally your most sensitive one.

If the new position is accurate, the meter will give a zero pressure reading at a range of road speeds.

Yes I do have two pitot tubes. Another really good idea!

I have a suction cap thingy coming and I'll do comparison readings at different heights when that arrives.

[MeteorGray]

I wonder how far from the front of the grill will one find still air that isn't influenced by the car.

As I noted, the pitot tube for my air speed indicator is just three or four inches ahead of the grill of my F250 truck, and in still air with no vehicles around, at 60mph ground speed, re the speedometer and the GPS, the air speed is also 60mph, re the air speed indicator with a smooth, steady needle.

So, I interpret this to mean the location of my pitot tube is OK, and that there is no forward air agitation going on.

[MeteorGray]

Quote: "That view is of the back of the Jaguar, not the front!"

Ooops, it's my assbackwardness coming out again.

I guess I'm still surprised there would be high pressure on the lid of the trunk as well, rather than some lift.