Nice low-speed & pretty cheap Pitot-Static Probe

[danstrider]

I have been using a pitot-static probe from Dwyer, meant originally for air conditioning ducts:
http://www.dwyer-inst.com/htdocs/PDFFILES/cat/airvelocity/160_cat.pdf

I have modified the relatively big/heavy a/c duct mounted version by dremeling off the probe about 2" beyond the first 90deg bend. Then, I very carefully dremel off the outer casing about 1" from the 90deg bend, leaving the inner pitot tube in tact. Then, I take a piece of scrap pitot tubing from the discarded end (may have to dremel a section off to get the tube from inside) and stuff this end inbetween the outside case and the inner pitot tube. Some magic JB Weld to make this all awesome airtight, and viola!

I propose this as an alternate to the Eagletree-style straight brass-tubing pitot and static probe. At school for several research model aircraft, we noted a relatively large (~20%) discrepancy between airspeeds when moving from the brass-tube to a real pitot-static probe. The Dwyer probe also says it is accurate within a recommended 15deg off-axis airflow, meaning local effects from sideslip or pulling alpha will at least marginally be acceptable (better than the brass-tube IMHO).

I'll have to take pictures sometime, but that involves taking the tail off my aircraft to get to the part... doh.

For anyone curious, I have been using this probe at speeds down to 16kts and up to 86kts and feel like the speeds have been consistent, even as my aircraft gets close to stall. I use the 1/8" diameter "pocket sized" 6" insertion length model.

Hope this helps somebody!
Dan

[bobthenuke]

Dan,

I think this will become a very informative and interesting thread. However, being an EE, not a fluids kind of guy, I'm a bit confused. Can you just show a picture of what you have now or a diagram of some sort? I'd like to think that your setup can be duplicated at home without having to purchase and modify an existing system - maybe I'm wrong about that?

Thanks,

-bob

PS - a little of topic, but when I was hang gliding we used to call the hand-held Dwyer airspeed measuring gadgets "Dwyer Liars" since it took the average of several to come to a conclusion of what the wind speed "may" have been. :D

[danstrider]

Hi Bob,

Here is some information about pitot-static probes:
http://en.wikipedia.org/wiki/Pitot-static_system

They are used to measure the pitot (aka total, aka stagnation) pressure. This means it is measuring the pressure of a streamline of air that hits the tip of the probe and comes to a complete stop.

They are also used to measure the static pressure, the ambient pressure of the air. The idea is that measuring a pressure parallel to streamlines is effectively the same as the air going surrounding the aircraft.

The idea of posting this link is to provide a standardized and certified part so folks don't have to build them. There are certainly several home-brew pitot-static probes that work acceptably. However, the Eagle-Tree suggested method of using a 1/8" brass tube sticking out 2" in front of the wing (or fuselage nose) is far less precise even than a home-brew one that looks like a normal pitot-static probe. In fact, by leaving the Eagle-Tree static port simply open inside the aircraft could cause major discrepancies in altitude and airspeed since the fuselage likely pressurizes due to cooling holes.

Dan

[bobthenuke]

Hi, Dan

I understand the the theory fine, just don't know how one pitot-static system will be basically different than another. I'm a bit confused as to exactly what you have after making the mods to the commercial unit. I had a Pitot-Static tube assembly from a WWI aircraft and it was nothing more than two tubes facing into the airstream, one open and the one below it plugged with the small holes in it for static. I'm sure that these have progressed a bit from then, but this is how I think of the pitot static tube assembly (not unlike what EagleTree shows).

The Wikopedia setup makes sense to me, but this again appears to be different than what I can out of the Dwyer system.

EagleTree is quite open about having an open static port inside the fuse with pressure variances posing a problem and recommends the two tubes. How does this differ from what the Dwyer system is composed of? I guess I'm particularly dense this morning.

-bob


EDIT: I've come to two conclusions, Dan -
1) You're thinking of the older version of the Eagle Tree that had a pressure port and static was open - the version I have has a pressure port, altitude port and static port. The Static and Altitude port are tied together and terminate in a static tube also mounted outside the airframe.
2) the Dwyer system comprises a tube within a tube - the outside tube is the static and the inner the pitot - is this correct?

[brnjones]

What type of pressure sensor are you using?

Thanks!

-Brandon

I have been using a pitot-static probe from Dwyer, meant originally for air conditioning ducts:
http://www.dwyer-inst.com/htdocs/PDFFILES/cat/airvelocity/160_cat.pdf

I have modified the relatively big/heavy a/c duct mounted version by dremeling off the probe about 2" beyond the first 90deg bend. Then, I very carefully dremel off the outer casing about 1" from the 90deg bend, leaving the inner pitot tube in tact. Then, I take a piece of scrap pitot tubing from the discarded end (may have to dremel a section off to get the tube from inside) and stuff this end inbetween the outside case and the inner pitot tube. Some magic JB Weld to make this all awesome airtight, and viola!

I propose this as an alternate to the Eagletree-style straight brass-tubing pitot and static probe. At school for several research model aircraft, we noted a relatively large (~20%) discrepancy between airspeeds when moving from the brass-tube to a real pitot-static probe. The Dwyer probe also says it is accurate within a recommended 15deg off-axis airflow, meaning local effects from sideslip or pulling alpha will at least marginally be acceptable (better than the brass-tube IMHO).

I'll have to take pictures sometime, but that involves taking the tail off my aircraft to get to the part... doh.

For anyone curious, I have been using this probe at speeds down to 16kts and up to 86kts and feel like the speeds have been consistent, even as my aircraft gets close to stall. I use the 1/8" diameter "pocket sized" 6" insertion length model.

Hope this helps somebody!
Dan

[danstrider]

Hi Brandon, I'm using a Piccolo II that has pressure transducers inside. I'm not sure of their model numbers exactly, but I have a picture of the Piccolo guts (will have to post tomorrow, on the other computer).

Hi Bob, thanks for clearing up the versions. I have personally used the old version where you have to cut label to get to the static port. The school has the version where both air tube nipples are available. #2 is correct.

It's late, so I'm not sure if I'm replying very well at the moment....
Dan

[bobthenuke]

Dan,

Where exactly did you order your Dwyer Pitot/Static tube from and how much did it run?...

bob

[danstrider]

Um, the one I'm using was given to me, so I don't know where to order them.

The price listing I found here:
http://www.dwyer-inst.com/htdocs/airvelocity/Series160Price.cfm

Looks like there is an order-now button too :-)

Hope this helps!
Dan

[TheThomas]

I can make pitot/static tube units for $50 if anyone is interested. They are used on larger airframes (+100") and weighs just about an ounce. Dimensions are roughly...1/2" diameter and 14" long with two tubes poking out the backside for both pressure ports.

PM me if anyone is interested.

-Tom