Airspeed indicator for your car

[rh77]

Originally Posted by krousdb

A true pitot tube (by the way, I used to work for the company that invented the pitot tube, Pitometer Associates was the name)measures the pressure of the air as it is forced into the forward mounted orifice and also measures the negative pressure with a reverse mounted orifice as the air goes by. It then adds the two together to give you a reading.

Neato - you worked for the company that invented the Pitot Tube. I had a basic understanding of how it works, but didn't know about the 2 measurements - cool (hot I mean). So I assume the heat is used to get a consistant reading at higher altitudes or is it more just overall temperature of the air? To confess, I have a passion for flight as well as FE. I'm a passenger at least 2 times a week (up to 8 or 9) and I still love to fly. I'd like to get my pilot's license, but it's just too expensive of a hobby (the training, plane rental/insurance, CE hours). For now I'm happy flying on the computer. Cheaper and safer.

The product sounds like a good way to measure wind velocity and headwinds (I suppose you subtract your speed from the meter to get a headwind value). Let us know how it works (and if there are limits to the product's range, like if you end up with going 65 with 35 mph gusts, that would be 100 mph wind speed, potentially).

Regarding the XM -- I mounted my antenna on the hood of the TL. It's the little, flat magnetic one that's about 2" L x W with an aerodynamic top. Wiring was simple and hidden. I previously had the big roof-mounted one back when XM was still pretty new. I'm sure that created some drag...

RH77

[krousdb]

Originally Posted by rh77

Neato - you worked for the company that invented the Pitot Tube. I had a basic understanding of how it works, but didn't know about the 2 measurements - cool (hot I mean).

The pitot tube was invented in 1896 IIRC. I used them to measure water velocity in large munucipal water mains, up to 60" in diameter. The tube is inserted into the pipe and oriented such that one end points toward the flow and the other end away from the flow. Flexible hose was connected to each pitot orifice and connected to a glass U tube that is filled with a substance that has a higher specific gravity than water. The higher the velocity, the heavier the fluid you would use. The flow would cause the fluid to deflect and the measurement of deflection would translate to a velocity for each different fluid used. Typically I would use carbon tetra chloride (SG 1.60), benzene (SG 2.90) or Mercury (SG 13.58). All are seriously carcenogenic.

Anyway once you got the velocity in the center of the pipe you would take readings at different maybe 1" increments to get the flow profile within the pipe. Armed with all of that data, you can calculate the flow rate within +/- 1.5%. You would then compare the flow rate to the metered flow rate and then calibrate the meter to match the pitot reading.

Originally Posted by rh77

So I assume the heat is used to get a consistant reading at higher altitudes or is it more just overall temperature of the air?

For airplanes, it sounds like the measurement is electronic from what you have said so the heating probably has to do with stabilizing the reading. I can't be sure though.

Originally Posted by rh77

The product sounds like a good way to measure wind velocity and headwinds (I suppose you subtract your speed from the meter to get a headwind value). Let us know how it works (and if there are limits to the product's range, like if you end up with going 65 with 35 mph gusts, that would be 100 mph wind speed, potentially).

Back to the airspeed indicator..... Well it works great! The readings are about what I had expected, jumpy but still useful. I headed out for a test drive starting against the wind. At 55, the gauge showed 62. In the other direction at 55, the gauge read 50. So there was a 5-7 MPH wind.

The readings are fairly smooth except during wind gusts or when cars go by. As expected, cars passing in the opposite direction cause a spike in airspeed, 5-10 MPH. But what is surprising is that the increase lasts long after the car passes. When cars pass in the same direction you get a nice drop, again 5-10 MPH. It was fun drafting a tractor trailer. At 45 MPH, the air speed dropped to zero. Since the gauge does not register below 20 Im not sure what the actual number was. At 65, following at one car length, the gauge bounced between 20-30MPH. At a safer distance, maybe 4 car lengths, the gauge bounced between 35-45. Even at 10-15 car lengths there was still some benefit, about 55.

So this gauge will be fun to have and I will be recording relative wind speeds with my daily commute data. It should help to quantify the effect of wind speed on my FE. I will try to get some pics posted today.

[krousdb]

Here are some pictures:

Front view above: Yes, it does need a wash.


Tube mounting. Prius in the background.


Gauge Mounting. It is starting to look like a cockpit in there.

[SVOboy]

Dremel down the mounting area so it looks more snug,

It seems angry to be squar pegged into a round hole.

[rh77]

Originally Posted by krousdb

I used them to measure water velocity in large munucipal water mains, up to 60" in diameter.

That's a lot of water! Was there a lot of pressure on the main?

Originally Posted by krousdb

Typically I would use carbon tetra chloride (SG 1.60), benzene (SG 2.90) or Mercury (SG 13.58). All are seriously carcenogenic.

Yikes, exactly. Did they provide you with personal protective equipment? Were you nervous to have these chemicals very near to the public water supply?

Originally Posted by krousdb

The readings are fairly smooth except during wind gusts or when cars go by. As expected, cars passing in the opposite direction cause a spike in airspeed, 5-10 MPH. But what is surprising is that the increase lasts long after the car passes. When cars pass in the same direction you get a nice drop, again 5-10 MPH. It was fun drafting a tractor trailer. At 45 MPH, the air speed dropped to zero. Since the gauge does not register below 20 Im not sure what the actual number was. At 65, following at one car length, the gauge bounced between 20-30MPH. At a safer distance, maybe 4 car lengths, the gauge bounced between 35-45. Even at 10-15 car lengths there was still some benefit, about 55.

This is very useful information. A few questions, as usual. I assume you were directly behind the truck. Did you feel wind buffeting the car at 4 car lengths? I can usually feel it at that distance, and figured that it was detrimental to FE. Did you draft at all from the back corner of the trailer in another lane (safer in case of emergency stops, but unknown if useful). A while back MetroMPG noticed an increase in FE on his ScanGauge when cars passed him on the highway, or cruising in a heavy flow of traffic (the econ dropped when exiting the highway). The theory asked whether it was better to take the 2-lane road, or the "Interstate" (or Canadian equivalent). I can't recall the thread, but there was discussion that heavy traffic provides an airflow beneficial to FE vs. a desolate road, and if a continuous pocket of air existed to cruise in. Since the oppositely passing car effected you for longer than expected, there may be some weight to the theory. Good work!

RH77

[SVOboy]

There is also the theory of surfing that I would like tested: driving next to a semi so that it's wake is sorta behind you which somehow pushes you forward? I forget for sure the idea.

[rh77]

Originally Posted by SVOboy

There is also the theory of surfing that I would like tested: driving next to a semi so that it's wake is sorta behind you which somehow pushes you forward? I forget for sure the idea.

Unfortunately, truckers hate that. I was told by one that if he sees a vehicle in his mirror and expects it to pass, but instead hangs out there, it's risky becuase of emergency lane change possibilities, or if the trailer brakes lock-up it might fishtail into you. It's kinda like driving in someone's blind spot. If you piss them off enough, then they simply get on the CB and another truck can do the squeeze-play from ahead to get you out of there. If it continues, the piss-off potential increases, and with road rage these days, I'd recommend another form of FE increase.

Road Rage Incident:
I didn't know if I should post this, but maybe a lesson can be learned out there if you get into a road rage incident. About 2 weeks ago my wife and I were driving along in the city here and this guy cuts us off (no blinker) to the point where I had to really hit the brakes. Instinct had me honk the horn, then he gave us the finger. I just smiled and didn't let it get to me (I used to get all bent out of shape and would've gotten really angry, but with the slower driving, I've mellowed out). Long story short, the guy later tried to run us off the road (at that point my wife called the cops), then when we stopped, he gets out of the car with a 5" knife and knocks it on the driver's-side window at me. Meanwhile the dispatcher is hearing all of this going on and we peel out of there and wait for the Police to intercept. They go and arrest the guy on Felony charges at his house. No priors - the guy just lost it, and after he calmed down, he admitted that he did something wrong. Then he said that he thought we had a gun. The officer was like "Yeah right, so you get out of your car with a knife to attack the people with the gun. Right."

Anymore, I just let it go. It's sad. The Police said they have an average of 4 calls per day on road rage. People get shot anymore with this stuff. Anyways, might have to go to court and all that crap.

Just be careful out there folks...

RH77

[philmcneal]

Originally Posted by rh77

Unfortunately, truckers hate that. I was told by one that if he sees a vehicle in his mirror and expects it to pass, but instead hangs out there, it's risky becuase of emergency lane change possibilities, or if the trailer brakes lock-up it might fishtail into you. It's kinda like driving in someone's blind spot. If you piss them off enough, then they simply get on the CB and another truck can do the squeeze-play from ahead to get you out of there. If it continues, the piss-off potential increases, and with road rage these days, I'd recommend another form of FE increase.

Road Rage Incident:
I didn't know if I should post this, but maybe a lesson can be learned out there if you get into a road rage incident. About 2 weeks ago my wife and I were driving along in the city here and this guy cuts us off (no blinker) to the point where I had to really hit the brakes. Instinct had me honk the horn, then he gave us the finger. I just smiled and didn't let it get to me (I used to get all bent out of shape and would've gotten really angry, but with the slower driving, I've mellowed out). Long story short, the guy later tried to run us off the road (at that point my wife called the cops), then when we stopped, he gets out of the car with a 5" knife and knocks it on the driver's-side window at me. Meanwhile the dispatcher is hearing all of this going on and we peel out of there and wait for the Police to intercept. They go and arrest the guy on Felony charges at his house. No priors - the guy just lost it, and after he calmed down, he admitted that he did something wrong. Then he said that he thought we had a gun. The officer was like "Yeah right, so you get out of your car with a knife to attack the people with the gun. Right."

Anymore, I just let it go. It's sad. The Police said they have an average of 4 calls per day on road rage. People get shot anymore with this stuff. Anyways, might have to go to court and all that crap.

Just be careful out there folks...

RH77

lol wow what a story... makes me wonder if should get that "**** ***" hat if people were to eye me down because my hypermiling didn't let them be first in line at the red light.

[krousdb]

The water pressure ranged from 40 to 100 PSI. It all depends on the hieght of the local water towers.

I had no protective equipment but I wasn't particularly concerned. I was in my early 20's and stupid. It would be difficult for the chemicals to actually get into the water supply, but even if they did, the concentration would be extremely low. Still, it is scary to think about what someone could do to the water supply if they really wanted to.

Originally Posted by rh77

This is very useful information. A few questions, as usual. I assume you were directly behind the truck. Did you feel wind buffeting the car at 4 car lengths? I can usually feel it at that distance, and figured that it was detrimental to FE. Did you draft at all from the back corner of the trailer in another lane (safer in case of emergency stops, but unknown if useful). A while back MetroMPG noticed an increase in FE on his ScanGauge when cars passed him on the highway, or cruising in a heavy flow of traffic (the econ dropped when exiting the highway). The theory asked whether it was better to take the 2-lane road, or the "Interstate" (or Canadian equivalent). I can't recall the thread, but there was discussion that heavy traffic provides an airflow beneficial to FE vs. a desolate road, and if a continuous pocket of air existed to cruise in. Since the oppositely passing car effected you for longer than expected, there may be some weight to the theory. Good work!

RH77

I was directly behind the truck and there was buffeting, but not bad enough to hurt FE. Actually FE was noticibly higher while drafting. I cant really follow in another lane because there are only two lanes and I will be blocking the fast lane. I have had discussions on other forums about what you are referencing and in that case it was called the "corridor effect". I believe that there is something to it but you have to drive fast enough to stay up with traffic. It might be worth a try, but due to my short gearing, my rpms will be high. Prolly better results when I get my CX tranny installed. Anyway, it deserves a try, for at least the highway portion of the commute.

[krousdb]

Originally Posted by rh77

A while back MetroMPG noticed an increase in FE on his ScanGauge when cars passed him on the highway, or cruising in a heavy flow of traffic (the econ dropped when exiting the highway). The theory asked whether it was better to take the 2-lane road, or the "Interstate" (or Canadian equivalent). I can't recall the thread, but there was discussion that heavy traffic provides an airflow beneficial to FE vs. a desolate road, and if a continuous pocket of air existed to cruise in. Since the oppositely passing car effected you for longer than expected, there may be some weight to the theory. Good work!

RH77

I tested this theory today. Previously my best trip to work was 60.41 MPG at 30F, Dry roads and a 9MPH tail wind. Ths was pulling out all of the stops and taking the "road less traveled" route which takes 55 minutes. The distance was 40.57 km, 1.580 L, engine on 67% of distance.

Today I took the "road that more traveled" route which includes 6 more miles of highway and three fewer miles of engine off coasting. It took ten fewer minutes that the normal route. I didn't do any drafting, just took advantage of the wake left behind by cars in front of me and cars passing in the left lane. The windspeed / speedometer difference was -10 to -20 mph as a result.

The temp was 44F, wet roads, light and variable winds. The result was 59.59 MPG, 39.60 km, 1.563 L, engine on 81% of distance. So the MPG was slightly less, but the fuel used was also less because of the shorter distance.

One other benefit of the faster drive is that when I get off of the highway, there is less traffic and I made all of the lights. Had I not done that I would have been 2-3 MPG lower.

This deserves further study. More aerodynamic mods could help further now that I might be spending more miles at higher speeds.