+++ #2820 From: Russ Erb Subject: Possibly Useful Pitot Tube Info On 4 July, I tested my Pitot-static tube (AN5816-1, Shark fin design, 12v dc) to find out how much current the heater would draw. I needed to know this for the design of my electrical system. This wasn't as simple as you might think, since 1) I had guessed the answer would be about 20 amps, and my ammeter only goes up to 15 2) Heating up the Pitot tube when there is no slipstream to cool it is a good way to burn out the element Anyway, the answer was (for any of you who may decide to use this tube) steady state current was 15.12 amps, with a maximum inrush current of 28 amps. The maximum inrush current occured immediately after applying power to the heater, and the current dropped as the heater warmed up. If someone is interested in how I set up this test, say so and I'll write it up for the group someday. Over 3 days of last weekend, a friend helped me rivet the skin to the top of the left wing. Tomorrow we'll rivet the lower side of the nose section. It's starting to look like a wing. As you may or may not know, I'm working in a very confined space (1 car garage). The Metal Prime dries to a surface much like fine grit sandpaper. It's entertaining to walk by the wing and have your shirt get caught on the rough surface. It's as though the wing is reaching out to grab you and say "Stop! Admire Me!" +++ #2821 From: Rod Smith Subject: Re: Possibly Useful Pitot Tube Info > steady state current was 15.12 amps, with a maximum inrush current of > 28 amps. The maximum inrush current occured immediately after applying > power to the heater, and the current dropped as the heater warmed up. > > If someone is interested in how I set up this test, say so and I'll > write it up for the group someday. Yes I would be interested. Seems like a lot of amperage. I wonder if the heated pitot only, separate static vents on fuselage would use much less current. How did you decide on using a leading edge pitot/static. I'm thinking of doing the same, I like the looks, and it isn't a head knocker. +++ #2823 From: Subject: Re: [Bearhawk] Possibly Useful Pitot Tube Info Yes, usefull info. 15 amps steady state will most likely go up in actual application. Russ do you wish to repeat the test with the mast head packed in ice? Pull away the heat, operating temp goes down, resistance goes down, current goes up for a fixed supply voltage. The purpose is to melt ice in an airstream that is near freezing. Might be closer to 20 amps? +++ #2824 From: Russ Erb Subject: Re: [Bearhawk] Possibly Useful Pitot Tube Info Very interesting point, Kevin. It shouldn't be a problem for me since I'm keeping the wiring design that I created when I was guessing it would be 20 amps. The wiring is sized for an acceptable voltage drop in the wire, and would be able to handle more than 40 amps before having a temperature rise of 35 degrees C. Actually the Molex connectors would limit it to 24 amps by their ratings. I'm not sure what that is based on, but I suspect it is a temperature rise. In case you're wondering what sort of huge wire I'm using, I'm using two parallel circuits feeding the heater, both of 12AWG. That's two positive wires and two ground wires. The two circuits meet just short of the Pitot-static tube connector. The result is less than a 5 percent power loss in the wires. +++ #2825 From: Subject: Re: [Bearhawk] Possibly Useful Pitot Tube Info I was told that my AN 5812 pitot tube (on my RV-8) draws 15 amps. I have not tested it myself. I was also told to use no smaller that #12 wire to it. Hope this helps someone. +++ #3600 From: Rob Gaddy Subject: Copperstate observations I also checked out several Pitot Tube locations. The "standard" RV installation seems to be a simple tubing fitting located in the leading edge, or just under the wing leading edge with the pitot tube bent forward. Sometines the fitting was just "screwed" directly into the leading edge, and sometimes ther was an auxilliary plate (conformal to the leading edge shape) screwed to the leading edge with four screws. I don't plan a heated Pitot tube so was thinking about a simple installation on the leading edge, removable from outside. I probably will use the auxiliary plate and mount the pitot and static tubes to the backside of the plate. This way only the tubes (not the fittings) will be visable from the outside. The mounting plate would be water tight. I don't think that mounting the base fitting to the spar will be necessary. Any imput from the group? +++ #4651 From: Tim Subject: Project Description and System Schematics Been surfing around the net, looking for some stuff for the Group...Found this builders page with very nice schematics of Vacuum/Fuel/ Electrical and Pitot/Static System drawings.....Drawings start .5 way dwn pg. +++ #6612 Date: Fri, 22 Jun 2001 16:21:18 -0500 From: "Tim Anderson" fuzzet@ Subject: PV=nRT (airspeed pitot tube) Been working on getting my pitot tube done simular to bob's installation, I'm not intending on IFR operation, but it's nice to have a system that is calibrated as good as one can get. Bob lists using 3/8 X 0.035 alum tube for the pitot tube, with 1/4 inch OD tube to the airspeed indicator. A fairly common use, infact the pipe sizes on the airspeed indicator is 1/8 pipe. The inside diameter of a standard 1/8 pipe to 1/4 OD nylon fitting is about 0.022. The ID diameter of 3/8 X 035 is about 0.3 inches. Based on the good old gas equation it would seem that one should be carefull to match the pitot tube size with what get's fed to the airspeed indicator, at the instrument, ie the fitting. The 1/4 inch OD connecting line is very tiny, but it is not relavent in my pea brain. Input =output, and what goes on in between is not relavant. A large differnece in pitot tube size and airspeed line fitting could lead to calabration errors. I ordered 3/8 X 065 alum tube, which has a ID of about 0.025, much closer to the 0.022 fitting ID. Maybe i'm making a big deal out of nothing, maybe not. I do know that in my kitfox, my airspeed indicator is off by a bunch. Some of the error may be from using a non-TSO'd el-cheapo airspeed indicator, but it works well enough for the purpose intended. Any comments? Tim #398 +++ #6613 ate: Fri, 22 Jun 2001 21:58:03 -0000 From: zipppydoggg@ Subject: Re: PV=nRT (airspeed pitot tube) > A large differnece in pitot tube size and airspeed line > fitting could lead to calabration errors. If you look at the input to the airspeed indicator you will (probably) see a little hole. This is where the air "goes in". The hole is small to dampen the action of the meter's needle. There is no "flow" in the airspeed/altitiue system to speak of, so size and fittings and all the stuff you pay attention to in designing piping is not very important. If you were to use very small tubes, the system will be sluggish, but just as "accurate". IMHO, accuracy problems are probably due to static pressure errors. Bob (if think) uses the cabin air as the static pressure sorce, running no static lines at all. I am planning on running a static and pitot line to the pitot tube (unheated). This type of pitot tube looks line a 2 pronged fork, with one of the prongs bent. For this type of pitot tube, you will see two tubes rather than one for a "normal" pitot tube. One is open in the front, the pitot tube, and one is closed in the front, and has small holes along the sides of the tube. Often there is a little colar on this tube held with a set screw. This is the "static" tube. The placement of this colar is set to give accurate static pressures over a wide range of angle of attack. In running the pipes, don't forget to put a "T" at the low point, with a little tube extending below the main tube. This is the place water "should" collect, where it can be drained out. This goes a long way of preventing ice in the pitot tube, no airspeed indication. Been there, done that. Rob "spent my $.02" Gaddy +++ #6623 Date: Sat, 23 Jun 2001 19:54:43 pst From: "Russ Erb" erbman@ Subject: Re: PV=nRT (airspeed pitot tube) As the Pitot-statics guy at the USAF Test Pilot School, I'll say you're making a mountain out of a molehill. The size of the tubing will only affect the lag in your system (how fast it reacts), as the pressure will always stabilize with time. Smaller tubes cause more lag due to friction. I'd use a bigger tube, at least 3/8". The Pitot side is going to be the least of your worries. 99% of the problems come from the static side. If your Kitfox is like others I've seen, you don't have a static system--the altimeter is just open to the cockpit as is part of the airspeed indicator. Without a true static port, you'll never get close to the right answer. Open a car window while driving and feel your ears pop and you'll understand why--the pressure in the cabin is not what is outside. The Bob has the prototypes set up that way and I am very much in disagreement with him on that point. That's why I intend to install a Pitot-static tube on #164. I teach this stuff to budding test pilots, but I don't think you want to know that much. Russ Erb +++ #6636 Date: Mon, 25 Jun 2001 09:01:01 -0400 From: lisankejm@ Subject: RE: Re: PV=nRT (airspeed pitot tube) I have an aircraft spruce pitot-static tube on a Fisher Horizon homebuilt. It usually reads 5 to 8 mph slower than what it should. It presently doesn't have the collar on the static tube. Where should this collar be located in relation to the hole to compensate for the error? Also, will the correction affect the altimeter setting in an adverse way. IOW, will correcting one gauge throw the other one out of whack and viceversa? Joe Lisanke +++ #6650 Date: Mon, 25 Jun 2001 07:37:59 pst From: "Russ Erb" erbman@ Subject: Re: PV=nRT (airspeed pitot tube) Your best bet is to start studying how other airplanes have it installed. Tony Bingelis' books have some info too (if you don't have those you're working in too big of a vacuum). The point of a Pitot-static tube is that both ports are contained in the same probe--you just have to plumb two lines back to the instruments. Russ Erb +++ #6661 Date: Tue, 26 Jun 2001 06:37:36 pst From: "Russ Erb" erbman@ Subject: RE: Re: PV=nRT (airspeed pitot tube) > I have an aircraft spruce pitot-static tube on a Fisher > Horizon homebuilt. It usually reads 5 to 8 mph slower than > what it should. First of all we will assume that the error is in static pressure, not in total (Pitot) pressure, as this is typically the case (assuming you didn't totally buffoon the installation). If your airspeed is reading slow (compared to what?), then the difference between the total and static pressure is too small. To increase this difference, you need a lower pressure at the static port. Therefore, put the collar in front of the static port, which will cause a low pressure area in the eddy behind the collar. If the collar is too far from the static ports it will have no effect. You'll have to experiment with its location (trial and error) to get the desired results. As for the altimeter, if your Pitot pressure is correct and the problem in your airspeed is the static pressure, then your altimeter is wrong too. If the airspeed is low, then your altimeter is reading low as well. Fixing (compensating) the static pressure will fix both problems. Russ Erb +++ #6667 Date: Tue, 26 Jun 2001 16:05:52 -0500 From: "Tim Anderson" fuzzet@ Subject: Re: PV=nRT (airspeed pitot tube) > As the Pitot-statics guy at the USAF Test Pilot School, I'll > say you're making a mountain out of a molehill. thanks - Actually, I have a static port on the kitfox, it was placed where they had it on one of the demonstrators I saw. I know finding the perfect placement can be a bit of a trial and error process. My brother initally used a combined pitot/static probe on his lancair and it gave him all kinds of problems. +++ #6669 Date: Tue, 26 Jun 2001 19:12:29 pst From: "Russ Erb" erbman@ Subject: Re: PV=nRT (airspeed pitot tube) > thanks - Actually, I have a static port on the kitfox, it was > placed where they had it on one of the demonstrators I saw. All I can say without seeing the subject aircraft is that Pitot-statics installations are far more of an art than science, and I figure the big boys spend gobs of money in wind tunnel or flight test time to get it figured out. You can play with it and try to make it better, or just decide to leave it as it is. A few years from now I'll be able to report on how well my setup works. I hope it will work well, but I know enough not to bet and money on it. We'll see. Russ Erb +++ #6686 Date: Thu, 28 Jun 2001 00:45:09 EDT From: bearhwk272@ Subject: Pitot Static Comment The Art of Pitot Tubing does have a few constants that should be observed for best results. The items outlined here are framed with the Bearhawk in mind, applicability to other aircraft is by hit or miss. 1. Locate the pitot tube and the static port at the same wing mounted location. 2. Locate the mast at the leading edge outboard of the flap / aileron junction. ( Flap extension does back air up under the wing increasing local pressures causing a change in airflow around the front of the wing.) Note: there will be an induced error when turning due to the different velocity of the wings. This is a bigger effect than you might think; however, mount in too close to the prop arc and that becomes interesting also. 3. The mast should extend as a minimum 18 inches in front of the wing LE. Underwing locations ala Cessna and Piper work poorly on a flat bottomed airfoil. 4. The static ports should be 4 holes (start at about .040 Dia) located radially to the mast. 90 deg spacing. Holes should have orientation with the 1st hole at the 45 deg position. Holes drilled into a common plenum. (minimizes pitch and yaw effects) Holes should be located in an annulus or behind a stepped cut of about .030 deep. 5. Tubing runs should be of good practice, large sweeps, slope to low point. no leaks, constant tubing size, no fitting restrictions. 6. The Pitot hole is magic, hole should be round, square to the airstream and have sharp edges ID and OD. Size does make a difference. Consult with the mfg. of your airspeed indicator, the speed range of the aircraft does affect to a small degree the hole size and system accuracy. 7. Lots of little things do add up. All for now, back to the 110 temps. Kevin Bearhawk #272 +++ #6689 Date: Thu, 28 Jun 2001 06:13:42 -0700 From: Budd Davisson buddairbum@ Subject: Re: Pitot Static Comment A note about tuning airspeed indicators with the single-mast system: it's common practice amongst Pitts guys who are having IAS problems to fine tune the static situation by putting a small "O" ring around the mast ahead of the ports and moving it fore and aft to get the desired effect. This assumes the step in the mast isn't properly placed. bd +++ #6692 Date: Thu, 28 Jun 2001 12:42:00 -0700 From: Benton Holzwarth bcgh@ Subject: re: Pitot Static Comment > ...it's common practice amongst Pitts guys who are having IAS > problems to fine tune the static situation by putting a small > "O" ring around the mast ahead of the ports and moving it fore > and aft The Champ I've been flying has two forward projecting tubes (vs. a 'single mast') about an inch apart. The static line is capped off and has the static ports drilled as described previously about an inch back from the tip maybe; and a ring, about 3/16" wide, and about a 3/16" step up from the tube diameter, with a set-screw to lock it down, just foreward of the ports. Benton 28jun01 +++ $Id: 2.5.3-Systems-PitotStatic,v 1.5 2001/09/26 04:24:35 bentonh Exp $