More Pitot tube

jim_geo · Post by **jim_geo** » Tue Dec 13, 2005 1:00 am

I'm going with the old fashioned Pitot tube. Of course, stainless steel for me. My question is one of plumbing. Looking at drawing 15A shows the business of the tube just fine but what of the wing root end. I've looked high and low for some kind of description of what happens there and find nothing. I'm thinking of putting a bulk head connector on the root end rib but the problem I foresee is that I won't be able to get the tube across the gap. One small reason being there may not be enough room to get a wrench between the wing and fuse and the other being that right across from that is a fairly substantial construct being that of the wing spar carry through. This seems to be one of those construction details that just get dropped off into space. I would sure love it if someone out there could post a picture of this area or post a link that would give some detail to the problem I'm having.

jim_geo · Post by **jim_geo** » Tue Dec 13, 2005 12:02 pm

Well after sleeping on it over night I decided that I wouldn't use a standard Pitot system on my plane. Instead I'm going to use a cup anemometer. This way all I have to run are some wires. Harkens back to the good ol days.

. Just kidding of course. I found after a whole lot of searching on Dan Checkoway's web site that he had the exact same problem. Instead of using a straight bulkhead connector at the wing root he ended up with a ninety degree bend connector. I'd still like to see any other way of doing this.

Guest · Post by **Guest** » Tue Dec 13, 2005 10:23 pm

Jim,
I used a 90 deg fitting on the wing root for both the pitot tube and airspeed tube and used 90 deg. bulkhead fittings on the fuselage behind the aileron control tube access hole to enter the fuselage. I made up short tubes to connect the two together. When I mounted the wings the final time I tightened the fittings before I fully inserted the spar. The connecting tubes are long enough and U shaped to flex so they would still be tight when the spar is fully seated. It worked great for me.
Dan N742DA

jim_geo · Post by **jim_geo** » Tue Dec 13, 2005 10:41 pm

Thanks Dan, Your method sounds like a good way to work around the problem. I really like the idea of the U-shape thing you've got goin.

N537TM · Post by **N537TM** » Tue Dec 20, 2005 7:58 pm

[img]

Jim

we just used a tubing bender and an aircraft flare. On the outside we used the optional Vans SS pitot tube..

Mike [/img]

jim_geo · Post by **jim_geo** » Tue Dec 20, 2005 10:54 pm

So Dan, am I to understand that you also used a ninety degree bent tube for the static air ports? I'm wondering about this because I've given some thought to doing mine that way. Seems to me it's fast and fairly clean. I'm mostly curious about air pressure directly under the wing and if it has any effect that you can detect.

4kilo · Post by **4kilo** » Tue Dec 20, 2005 11:36 pm

The increased air pressure directly under the wing is certainly detectable. It is called lift. Seriously, the area near the wing is a horrible place to get static pressure, because the pressure will change with changing angle of attack as well as airspeed. Although you may be able to get a somewhat reasonable reading by manipulating the shape of your static pick-up, the readings will never be as good as what you could get farther from the pressure disturbances caused by the wing.

Although there are other problems with fuselage static ports, those are what I will be using. Having spent many many hours trying to get the instrumentation on competition sailplanes to work just right, I just don't think there is a better location on an RV for a static port than the aft fuselage.

Pat

Dan A · Post by **Dan A** » Wed Dec 21, 2005 3:45 pm

I bought a heated pitot tube from ACS back when they were $200. I mounted it where Vans plan suggest. It sticks about 5 or 6 inches below the wing so is not affected by the laminar flow on the wing. The root of the wing where it attaches to the fuselage is where I used the 90 deg. fittings. My airspeed and static air seems to be good at the panel.
Dan

4kilo · Post by **4kilo** » Wed Dec 21, 2005 6:31 pm

Dan,

Just for clarification, I was talking about a static pressure source. Although the change in dynamic pressure in the pressure field around the wing is measurable, it is not significant enough to be a problem for normal instrumentation purposes, especially if the pitot head is some distance from the surface as you describe. The change in static pressure IS significant, even well outside the boundary layer.

Where exactly is your static source? I am planning to use exactly the same pitot setup you are using, with a standard rear fuselage static source. How did you mount the pitot to the wing (ie, fastened to outside or with a mounting bracket inside the wing with the mount extending out through the skin)?

Pat

Dan A · Post by **Dan A** » Wed Dec 21, 2005 8:34 pm

Pat,
My static source is in my pitot tube assembly ACS # AN5814 The static source is on the side of the pitot tube. I used the Gretz Areo bracket to mount it to the wing. It goes thru the skin and I have access to the connections through the outboard inspection cover. I attached it to the aluminum tubing with nylon tubing as it is flexable. I used a piece of .060 flat stock to make a doubler to reinforce it. If you use the 5812 pitot tube you will have to use the fuselage static ports. It only has the air speed in it. That will save running two aluminun tubes thru the wing like I did.

I do think Vans bent tube will work just as good and is a whole lot less expensive. I also have Vans static source kit but haven't put it in and it appears like I won't need it.
Good luck,
Dan

4kilo · Post by **4kilo** » Wed Dec 21, 2005 9:14 pm

Dan,

I will be installing my pitot system in the wing shortly. My heated pitot is strictly a pitot tube, not a Prandtl tube, so I will need a seperate static source. At least there is only one tube to run through the wing.

I would have some concern about using a static source so close to the wing, but if you are getting good results, then your installation obviously works as it should.

Thanks again for the info,

Pat

jim_geo · Post by **jim_geo** » Sat Jan 28, 2006 12:09 am

Airplanes suck!

cjensen · Post by **cjensen** » Sat Jan 28, 2006 1:34 pm

ha ha!! suck they do!