Tip: Mounting a Piper Blade Pitot

Ironflight · #1 02-13-2007, 05:20 PM

I got a Private Message asking about how I mounted the Piper Blade-style Pitot on the Valkyrie, and as this was not the first time I had been asked, I figured that I might as well answer where it can be seen by others. The bottom line was that this was really easy to do. The blade is designed to mount on a flat surface, so all I needed to do was pick the surface and make sure it was stable. I decided that I wanted to keep it close to Van's design pitot position, and the best place I could find was just outboard of the rib that is just outboard of the aileron bellcrank access. I simple made a doubler for the skin (before the skin was mounted), and test-fit the pitot. I added nut plates (not in these photos), and the structural part was done. (You can see the doubler plate and hole for the pitot.)

Since this was a heated pitot, I was a little concerned about hooking up a nylon or plastic line to the metal assembly, so I rigged a test by putting on a short segment of line and hooking the pitot up to the power supply (with the resistor I planned to use to control the current. In a few seconds, it got hot enough to smell. In thirty seconds, the nylon melted off. I decided to use a metal tube all the way to the wing root! This photo shows the roughed in connection I made. I ran the line next to the spar, under the bellcrank pivot to make sure there was no danger of interfering with the controls. At the first Condition Inspection, everything was still in good shape.

Like I said up front - pretty simple!

Paul

jonbakerok · #2 02-14-2007, 06:28 AM

I also used a Piper pitot blade. It was a heck of a lot cheaper than any other heated pitot and it even includes static port heat.

Which brings me to my point. I used the static port on the blade and it worked fine, or at least good enough for me. It's accurate in level flight but in a power-on stall or full flap stall it indicates 45 MPH at the break instead of 50. Power-off, clean, it stalls at 55 like any other RV6.

However, I've read of other people using a Piper blade who had an error of as much as 20 mph even in level flight and had to switch to the standard static ports. The difference seems to be the angle at the bottom of the blade. I have no idea which one I've got or what model it came off of.

It looks like you didn't use the static port. Did you try and it didn't work in your case? We could compare the angles.

IowaRV9Dreamer · #3 02-14-2007, 06:48 AM

Paul, Thanks for posting this - it helps a LOT!

I bought a Piper heated pitot tube from ebay (supposedly from a Lance). It has 2 elements - I thought they were "Low" and "High" not heated pitot and heated static? I measured the resistance (don't have the numbers handy) and it looked to me like it was low/high. I haven't yet turned it on.

Two questions:
Why are you using a dropping resistor (it must get hot too)?

Would it be better if the doubler plate extended up under the the spar or rib and picked up those rivets? That would make it sturdier but perhaps it is OK just on this skin? I'm just wondering since the bottom skins are so thin.

To all you experienced builders - keep posting these tips. It makes it easier on those who come behind and will push that Vans Hobbs meter up even higher!

Ironflight · #4 02-14-2007, 07:21 AM

Jon - I didn't even try using the static port because I had heard a number of folks say that they didn't have good luck with it. It's good to hear that it is fairly accurate for you - I just decided not to gamble on it, and might have been over conservative. I had enough exposure to the potential horrors of finding good pitot and static locations over the years that I figured I'd use the tried and true for this plane.

Dave - I used the dropping resistor because when I first hooked the thing up it drew a lot more current than I wanted. I seem to remember doing some research on what Piper did, and decided to current limit it with an appropriate series resistor.

It certainly wouldn't hurt to extend the doubler plate to pick up stronger attach points, but since I had a QB kit, I really didn't have an easy way to do that. In any case, I've got 380 hours on the installation, and no signs of deformation or fatigue. I think it's sufficient.

Paul

rvbuilder2002 · #5 02-14-2007, 09:46 PM

One thing RV builders should be aware of is that there is many different part #'s for the piper blade style pitot.
At least one of the differences is the angle on the bottom of the blade which contains the static port.
All of the blade pitot installations on RV's that I am aware of, have erroneous airspeed readings at the low end. It is possible that one of the specific part #'s works correctly on an RV but I am not aware of one.

Ironflight · #6 02-14-2007, 10:04 PM

Quote:

Originally Posted by rvbuilder2002

One thing RV builders should be aware of is that there is many different part #'s for the piper blade style pitot.
At least one of the differences is the angle on the bottom of the blade which contains the static port.
All of the blade pitot installations on RV's that I am aware of, have erroneous airspeed readings at the low end. It is possible that one of the specific part #'s works correctly on an RV but I am not aware of one.

You'e correct Scott - there are quite a few differnt models. I actually figured out which one I have one time, but can't find that particular scrap of paper right now - I'll have to try and dig it out some time. I can tell you that whichever one I have, my airspeed is very accurate throughout the range, with a stall right where the specs say it will be (about 53 knots at gross). Just lucky I guess.

Paul

Kevin Horton · #7 02-15-2007, 04:08 AM

Quote:

Originally Posted by Ironflight

You'e correct Scott - there are quite a few differnt models. I actually figured out which one I have one time, but can't find that particular scrap of paper right now - I'll have to try and dig it out some time. I can tell you that whichever one I have, my airspeed is very accurate throughout the range, with a stall right where the specs say it will be (about 53 knots at gross). Just lucky I guess.

The different angles on the bottoms of the various Piper pitots will affect the static pressure from them, but it shouldn't affect the Pitot pressure.

Pitot tubes are pretty hard to mess up, as the laws of physics are working in our favour. They measure the total pressure, and Bernoulli's Law tells us that the total pressure stays the same, even as the local velocity and local static pressure change. As long as a pitot tube is mounted well clear of the boundary layer, out of the prop wash, not in the wake of anything else, and roughly aligned with the air flow it should work properly, and give the expected accuracy. It doesn't matter whether it is a Piper pitot, a Dynon pitot, or a bent piece of aluminum tube, etc.

Static ports are a whole different problem. Now Bernoulli's Law is working against us, as it tells us that the local static pressure changes as the air accelerates and decelerates around the aircraft.

So, given that Paul used Van's standard static source, the fact that he has good airspeed accuracy with a Piper pitot should not be a surprise.

rvbuilder2002 · #8 02-15-2007, 05:33 PM

Quote:

Originally Posted by jonbakerok

Which brings me to my point. I used the static port on the blade and it worked fine, or at least good enough for me. It's accurate in level flight but in a power-on stall or full flap stall it indicates 45 MPH at the break instead of 50. Power-off, clean, it stalls at 55 like any other RV6.

However, I've read of other people using a Piper blade who had an error of as much as 20 mph even in level flight and had to switch to the standard static ports. The difference seems to be the angle at the bottom of the blade. I have no idea which one I've got or what model it came off of.

Ah Yes...
I should have been more specific.
I was primarily relating to Jon's post (above) about airspeed indications being incorrect at the low end.
What I should have said was "All of the blade pitot installations on RV's that I am aware of, that have used the internal static port, have erroneous airspeed readings at the low end."
I agree with Kevin. Pitot is pitot. As long as you stay out of accelerated or disturbed airflow (prop. slip streem, etc.)

Now there may be a specific part # Piper blade that will work using the internal static port but I am not aware of one.

The best way to be sure that your system will be accurate, is to use the blind rivet ports supplied by Van's.

Tom Navar · #9 02-20-2007, 07:00 PM

Paul: I would like to know if the wire bundle that is shown in the picture of your pitot setup carries enough wiring to power your nav lights, landing lights, and strobes. My wing is also QB, and I don't feel compelled to install a conduit, as the grommets are already installed.
TN

Ironflight · #10 02-20-2007, 07:14 PM

Quote:

Originally Posted by Tom Navar

Paul: I would like to know if the wire bundle that is shown in the picture of your pitot setup carries enough wiring to power your nav lights, landing lights, and strobes. My wing is also QB, and I don't feel compelled to install a conduit, as the grommets are already installed.
TN

Tom,

Sorry - that wire you see in the photo is only the strobe cable. I wanted that separate from the rest of the wing wiring (like antenna Coax), so I ran the strobe through the grommets that Van had already installed in my QB wing, and ran a separate wire tray for everything else.

Paul