Pitot Tube Question

[BSAFER]

The Pitot tube that comes with the Sparrowhawk (Skydat) is expected to be a straight shot from the Skydat straight out the front (right where I want my heater core). Is there any reason I can't get longer tube or using some metal/copper fittings, reroute it so it's not a straight path?

Sorry for the pitot tube ignorance. Seems like it's just a tube to pass pressure/change, so I would expect I could do this without problems.

Thanks.
Brent

[jcarleto]

No problem at all. You just have to make sure your "plumbing" is leak-proof. DO NOT blow into it to see if it leaks with the altimeter attached. It can damage the instrument. Your static source and pressure source may be very remote. The "plumbing" in my Bonanza, for example, goes all over the place. Most FW have the pitot tube located way out on the wing somewhere and plumb back to the panel.

[StanFoster]

I would like to add that most SparrowHawks/ RAF's airspeed indicators read too high. Mine was 10-12 mph too much. I installed a bleed air valve that is a controlled air leak that brings the ASI down to more accurate readings. My Helicycle seems to also be reading about 7- 10 mph too fast as well. I am going to install a bleed air on it as well. When my ASI. Reads 100 mph, I want to be going 100 mph. Stan

[willisbr]

I did not know this Stan. This doesn't sound like a good feature to have when doing short field takeoffs. I don't want to be trying to take off at 35 MPH short field. How can I do some testing?

[jcarleto]

Easiest way to test an ASI is to compare against a GPS. Fly a heading for a minute, then fly 180 degrees the opposite heading for a minute to compensate for wind. The ASI should match the average of the two GPS ground speeds Make sure you do the pressure/altitude calculations before you decide to play with your plumbing.

[All_In]

It doesn't cost much to have them IFR certified at your local avionics shop.

We have to, however, it sure provides me major peace of mind while practicing emergency cross-control landings at minimum air speeds for the shortest soft-field landing.

[Mike484]

Originally Posted by jcarleto

Easiest way to test an ASI is to compare against a GPS. Fly a heading for a minute, then fly 180 degrees the opposite heading for a minute to compensate for wind. The ASI should match the average of the two GPS ground speeds Make sure you do the pressure/altitude calculations before you decide to play with your plumbing.

This would work with any GPS, even one made for a car, probably the easieast and cheapest way to check it.

[gyroplanes]

Originally Posted by All_In

It doesn't cost much to have them IFR certified at your local avionics shop.

John, I used to do IFR checks. All you do in the check is verify the instruments are working and that the system doesn't leak.
Pitot and static location errors are NOT accounted for during an IFR Cert..
It is taken for granted that the locations were corrected when the aircraft was certified.
With amateur-builts, you are on your own.

Slip a piece of surgical tuning over the pitot tube. Block the free end and roll up the tube.
Stop at a high airspeed and hold it for one minute, There should be no leakdown.
Do almost the same with the static port. A flush port can be tested by clay around the surgical tube holding it in place and sealing it. Draw a vacuum with your mouth and hold the hose sealed for 1 minute. No leaks, no problem.

Location is another matter. The pitot should protrude into an uninterrupted air flow.
The static must be "static" air. The safest is a rigid tube, sealed at the end, with a series of pinholes around the tube. The boundry layer effect allows this to work, even in the slipstream.
Inside the cabin or instrument pod, you may have a partial vacuum due to air passage speeding up over the leaky surface (Bernoulli) which makes the airspeed and altimeter read high.

[All_In]

Thank you Tom!

Ben and friends, if you guys can get down here and there aren't too many of you, my brother can probably use a friends of ours shop and equipment for free to check you altimeter and air speed indicator, just no log book entries.

[BSAFER]

Thanks for the information. Appreciate the feedback. Good stuff.

[okikuma]

Hi Brent,

There should be no problem in using longer tubing to accomplish what you want. My suggestion is to remain with the same inside diameter as the original.

There is a simple method to test the accurate of an analog airspeed indicator and separately an analog altimeter using the "Water Tube Manometer." In fact, this method was exclusively used by the US Army and US Navy to check the accuracy of airspeed indicators and altimeters from the 1920s up and through WW2. Way back when, I have used this method on my Piper Pacer to test the accuracy of the airspeed indicator and altimeter while removed from the panel and again while installed in the aircraft as a comparison.

Jim Weir often writes wonderful "how to" articles for Kitplane Magazine. The following link is such an article on airspeed calibration using the "Water Tube Manometer."

http://www.rst-engr.com/rst/articles/KP89JUL.pdf

Russ Erb of EAA Chapter 1000 has also written a wonderful paper using the same method. By the way, Russ is an instructor at the USAF Test Pilot School at Edwards AFB, an experienced Aerospace Engineer, and builder of a wonderful example of a Bear Hawk. By the way on a previous post on this forum, someone provided a video link on vacuum forming compound curved acrylic/Lexan covers. The gentleman in the video explaining the method in his own kitchen is Russ Erb. That video should be titled: "Cooking with Erb."

http://www.eaa1000.av.org/technicl/instcal/instcal.htm

I have often wondered how accurate the electronic airspeed indicators such as the SKYDAT are and it would be simple to use the "Water Tube Manometer."

Now the second part of the problem is the location of the pitot tube. We all know that the pitot tube must be in mounted in "clear air." What most people do not realize is that the location dictates the length of the pitot tube to remain in "clear air."

All aircraft (ones that obviously don't have their engine in front of the nose) create a "bow wake" just like a boat and ship does in water. Inside the boundary layer of this "bow wake," the air is at higher pressure than the ambient air pressure outside the wake. A short pitot tube that is located inside this "higher pressure zone" would read at a higher airspeed. This is why Stan was reading higher airspeed indications than his gyro was flying. The stock mount and length of the pitot tube on the Sparrowhawk is too short to be in clear air.

One solution for the Sparrowhawk is lengthening the pitot tube to be in clear air, however a 3 foot long pitot tube is dangerous on the ground whereas anyone can inadvertently walk into it and cause damage to the aircraft and themselves. The other solution would be to mount the pitot tube on top of the cabin. Take a look at pictures of UH-1 Hueys and UH-60 Blackhawks and see that the pitot tubes are mounted on top of the cabin. The method that Stan used to solve his high airspeed readings was ingenious and very adequate since he didn't want the added expense or major alteration in removing and relocating his pitot tube.

Tom mentioned the pitot/static tube combination and it is the best type for lower speed aircraft. It consists of a pitot tube with a second lower static tube with and "capped nose" and multiple small diameter holes on the side of the tubing. The following picture is such a pitot/static tube system that is mounted on a 1930s vintage Bowlus Baby Albatros that a friend of mine restored and flies.

Anyway Brent, that is my long version to your simple question.

Wayne

[Greg Lockhart]

Would down on the nose wheel strut of a SparrowHawk be a good place to locate the pitot tube... would that be in undisturbed air?
Thanks,
Greg Lockhart

[okikuma]

Hi Greg,

In my opinion, that wouldn't be a good idea only because having a pitot tube closer to the ground increases the chance of the tubing ingesting foreign material and other nasty bits of things one normally would not want to get in and clog up the system.

In seeing the picture of the Sparrowhawk in progress, I'm thinking that you are still building your machine. If so, you are the perfect person to experiment alternate pitot/static tube source and location to obtain a more accurate readings than the original.

If it were me in your shoes, I would first build myself a Water Tube Manometer to test the accuracy of the SKYDAT system prior to mounting it into the Sparrowhawk. As a comparison, I would also test the Water Tube Manometer with an analog airspeed indicator. Then would build two identical pitot/static tube systems based on the photo of the Bowlus Baby Albatros and mount both on a 3 or 4 foot long PVC pipe or piece of wood with one to the analog airspeed indicator and the other to the SKYDAT. With a help of a friend, hold the pole vertically up through a sun roof or outside the passenger widow of a car. Using a GPS to cross check, drive the car up and down a nice long straight section of road and then record the accuracy of both airspeed indicators compared to the GPS ground speed readings in both directions (and my vehicle's speedometer - might as well check it's accuracy too).

The more I write about this, the more I'm thinking of trying out this experiment myself with some of my friends with various airspeed indicators and pitot/static tube systems.

Wayne