[billdianne]

Vans calls for a dimension of 6 to 7 inches from the bottom of the wing to the opening of the pitot tube. I have a blade type pitot that I would like to use that measures 4.75 inches. Is there any problem with having it closer than is called for in the plans?

 

[Ironflight]

Bill,

I am using a Piper blade style pitot that has about that same dimension - I have very accurate airspeed data.

Caveat - I am using the standard Van's static ports (rivets!) and location!

Paul

 

[Kevin Horton]

No problem. The laws of physics work in our favour with pitot tubes, as they are measuring total pressure, not ambient pressure. Bernoulli's law tells us that while the ambient pressure varies as the air speeds up and slows down, the total pressure remains unchanged. Your pitot tube should read accurately, as long as you don't put it's opening in the boundary layer, or in the wake of something else, or it is in the prop wash, or it is grossly misaligned with the local flow. The bottom surface of the wing guides the local flow in that area, by forcing the airflow at the wing surface to be parallel to that surface.

But, be very hesitant about using the static source, as there is a pretty small chance that you will end up with an accurate static source there. If someone does actually have an accurate static source from a Piper pitot, you would need to copy his installation accurately (exact same location, same mounting angle, and same part number pitot probe - different part numbers have different angles on the bottom face, which affects the static source position error). Vary any of these items, and you will get a different accuracy on the static source.

 

[billdianne]

Paul and Kevin, thank you for your quick replies, the value of this forum is inestimable.

Being mathematically challenged (having been told by my algebra teacher that he would pass me only if I promised to never take another of his classes) generally speaking how thick is the boundary layer?

thanks again

Bill

 

[Kevin Horton]

generally speaking how thick is the boundary layer?

I'm not sure exactly. I've seen one web page that claims the boundary layer thickness will be roughly 1% of the distance back from the leading edge. E.g., if the pitot tube is 18" back from the leading edge, the boundary layer thickness would be about 0.18". This feels like about the right order of magnitude, but I have no hard evidence to support this, so it is worth what you paid for it.

 

[Mark Albery]

It's not the boundary layer that's of primary concern, but the potential field of the aerofoil that can influence the pitot pressure.

In simple terms it's the "bow-wave" of the aerofoil - i.e. that part of the flow field that is influenced by the reflected pressure wave from the airfoil which is a function of Mach number and leading edge radius.

The fact that Van's have determined that 6-7" is satisfactory doesn't mean that closer won't work, but I'd be hesitant to go much closer.

 

[tin man]

No problem. The laws of physics work in our favour with pitot tubes, as they are measuring total pressure, not ambient pressure. Bernoulli's law tells us that while the ambient pressure varies as the air speeds up and slows down, the total pressure remains unchanged. Your pitot tube should read accurately, as long as you don't put it's opening in the boundary layer, or in the wake of something else, or it is in the prop wash, or it is grossly misaligned with the local flow. The bottom surface of the wing guides the local flow in that area, by forcing the airflow at the wing surface to be parallel to that surface.

But, be very hesitant about using the static source, as there is a pretty small chance that you will end up with an accurate static source there. If someone does actually have an accurate static source from a Piper pitot, you would need to copy his installation accurately (exact same location, same mounting angle, and same part number pitot probe - different part numbers have different angles on the bottom face, which affects the static source position error). Vary any of these items, and you will get a different accuracy on the static source.

Don't know what Kevin Horton has against piper masts but a lot of us are using them with great success. Previous discussion should be archived.

 

[Kevin Horton]

It's not the boundary layer that's of primary concern, but the potential field of the aerofoil that can influence the pitot pressure.

In simple terms it's the "bow-wave" of the aerofoil - i.e. that part of the flow field that is influenced by the reflected pressure wave from the airfoil which is a function of Mach number and leading edge radius.

The fact that Van's have determined that 6-7" is satisfactory doesn't mean that closer won't work, but I'd be hesitant to go much closer.

Nope. The pitot probe measures total pressure, not static pressure. The effect of the airfoil on the static pressure is of no concern at all.

Bernoulli's Law applies here. Let's place ourselves in the same frame of reference as the aircraft. The air way ahead of the aircraft is moving with respect to the pitot tube, with a speed equal to the TAS. The air pressure way ahead of the aircraft is equal to the ambient pressure at this altitude. As this piece of air approaches the aircraft, it will be affected by the presence of the aircraft, and its speed and pressure will change. Bernoulli's Law describes the relationship between the pressure and speed as this air moves around the aircraft. That piece of air comes to a stop at the entrance to the pitot tube - i.e. the speed is zero, in the frame of reference of the aircraft. Bernoulli's Law tells us what the air pressure will be here - it is the total pressure. And, it doesn't matter what speeds and pressures that piece of air had on the route between way ahead of the aircraft and the entrance of the pitot tube - the pressure will be the same once it is at rest with respect to the pitot tube.

 

[Kevin Horton]

Don't know what Kevin Horton has against piper masts but a lot of us are using them with great success. Previous discussion should be archived.

I have nothing against Piper pitot masts, when used to measure pitot pressure. They work great.

But, it is a real trick to find a good location to measure static pressure near a wing. A static pressure source needs to be at a location that has the same pressure as the undisturbed air at your altitude. If not, this causes errors in the indicated airspeed and indicated altitude. But, a wing, by design, has high pressure below it, and low pressure above it. If it didn't, our aircraft would not fly. So, it takes a lot of trial and error to find a location near a wing where the static pressure is correct. It can be done, but it probably won't happen on the first attempt.

Are you using the static source from a Piper pitot mast? If you have an accurate static source, please tell us the exact location of the pitot mast, and its part number. The part number is critical, as the different part numbers have different angles on the bottom face. The different angles are used to tune the accuracy of the static source, to compensate for the fact that the pressure at the chosen location is not the same as the ambient pressure.