Anemometer error? "Foil effect"??

[Robert Wilson]

My "Skywatch" Xplorer 1 is, I think, accurate. But the other day (totally still/open countryside) whilst my car was being driven at exactly 40mph according to my speedometer I held my "Skywatch" well out of the window and above the car's roof-line.
I was surprised to see it reading well over 50mph.
Could this be the aerofoil effect created by my car's body shape? As with a sail or aeroplane wing where the air "over" the wing travels faster than under it thus creating lift.
If so, I wonder what the "lift" force is away from the road surface, which would substantially affect ground adhesion.
Car is a VW Golf 1.9TDi

 

[sarabande]

how long is your arm ? About 60cms? Still in the compressed air flow, I reckon.

 

[Ex-SolentBoy]

My "Skywatch" Xplorer 1 is, I think, accurate. But the other day (totally still/open countryside) whilst my car was being driven at exactly 40mph according to my speedometer I held my "Skywatch" well out of the window and above the car's roof-line.
I was surprised to see it reading well over 50mph.
Could this be the aerofoil effect created by my car's body shape? As with a sail or aeroplane wing where the air "over" the wing travels faster than under it thus creating lift.
If so, I wonder what the "lift" force is away from the road surface, which would substantially affect ground adhesion.
Car is a VW Golf 1.9TDi

In simple terms.

The air has to accelerate to go over the top of the car.

Unlike a wing the car is close to the ground so there is no significant low pressure area under the car and therefore no significant lift. In fact, the increased pressure above does the opposite and pushes the car into the ground.

 

[sarabande]

umm...?

"As the higher pressure air in front of the windshield travels over the glass, it accelerates, causing the pressure to drop. This lower pressure literally lifts on the car's roof as the air passes over it, while the air passing underneath the car adds additional lift"

(http://www.europeancarweb.com/tech/0610_ec_aerodynamics_tech_buyers_guide/index3.html)

Now I know Mr Bernouilli is sometimes gravely misquoted in relation to accelerated air flow, but when I last looked under my car (this morning) there was quite a lot of smooth plastic sheet under the engine bay, and it seemed that lots of expensive engineers had worked hard to make the undertray as smooth as possible.

 

[VicS]

My "Skywatch" Xplorer 1 is, I think, accurate. But the other day (totally still/open countryside) whilst my car was being driven at exactly 40mph according to my speedometer I held my "Skywatch" well out of the window and above the car's roof-line.
I was surprised to see it reading well over 50mph.
Could this be the aerofoil effect created by my car's body shape? As with a sail or aeroplane wing where the air "over" the wing travels faster than under it thus creating lift.
If so, I wonder what the "lift" force is away from the road surface, which would substantially affect ground adhesion.
Car is a VW Golf 1.9TDi

Too close to the car.

Get SWMBO to stand on the roof and hold it

 

[Robert Wilson]

In simple terms.

The air has to accelerate to go over the top of the car.

Unlike a wing the car is close to the ground so there is no significant low pressure area under the car and therefore no significant lift. In fact, the increased pressure above does the opposite and pushes the car into the ground.

I thought the increased airflow/speed caused by the foil's longer distance to travel reduced the pressure and created lift? which is why an aircraft can go up/stay up, and why a sail pulls a boat along (with the hull-form "foil" as well.

Sarabande seems to agree with my view and contradict Ex-SolentBoy's.

However, I can now see why my Skywatch reads much higher; and why the undertray helsp keep my car from "taking-off". I think !!

Thank you for your responses

 

[lw395]

The undertray is to reduce friction or drag.
The front air dam reduces the air pressure under the car.

And those anemometers are not that accurate in my experience.
I suspect a lot of them over-read to compensate for being used close to the ground where the wind is less.

But I would expect the airflow to be increased even a couple of feet above the car.

Then there is the possibility you were driving into the wind...

 

[sarabande]

(totally still/open countryside)

from the OP