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MeteorGray · 06-09-2020, 08:35 AM

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?

06-09-2020, 08:35 AM	#31 (permalink)
MeteorGray Master EcoModder Join Date: Jul 2015 Location: Louisiana Posts: 361 Thanks: 275 Thanked 132 Times in 102 Posts	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? __________________