Airspeed Indicator Testing, Rube Goldberg Method

[VictorBravo]

Gang, as you may remember I have been chasing some perplexing airspeed indicator problems lately. I did something stupid and took the pitot and static lines out of the wing when the wings were uncovered. After my bright idea about using the cockpit as the static air source proved wrong, I started trying to fingure out how to get an accurate ASI reading. After five attempts at a static probe, two sets of new tubes in the wing, and several flight tests where the ASI read 19 miles an hour low or 15 miles an hour high, here is the latest fiasco...

The other night I rigged up a test fixture to test my airspeed system. I mounted two sets of pitot and static ports on top of a metal pole, and the airspeed indicator on the other end. My wife drove the convertible up and down the street at various speeds with this hilarious pole and air probes sticking up out the back seat 4 feet above the car, and I cross checked road speeds with GPS. I did this test at midnight where there was no wind at all. I switched the tubes from one set of probes to the other to test both systems. I will try to attach photos below. The thin brass tube is the UAV pitot-static probe I bought. The high quality fin at the top was to keep the probes pointing into the wind. The ASI is clamped on to the pole with Home Depot's Aerospace Equipment Dep't. hose clamps.

Some idiot in a Buick pulled up and looked at us like we were Martians. So I said to him "We're scientific storm chasers... the Tornado is on it's way in from Hawaii and you'd better board up your house so all the fish whirling around in the tornado don't break your windows!" He drove away shaking his head...

I found that the small UAV pitot-static probe I bought off the internet was more accurate than the ones that came off the airplane. However the new probe had larger errors the faster we went. At 20, 30 and 40 mph the ASI was within 1 or 2 mph of the car and GPS. At 60 and 70 mph the ASI read 4 miles an hour higher than everything else. Alycia didn't want to drive any faster than 70 on city streets so the official NASA aerospace flight test ended.

I got the impression that at 90 or 100 mph the airspeed indicator would read 8-10 miles an hour high, but I will have to wait for my turn in the NASA wind tunnel to see.

So I decided to take my airspeed indicator in to Sky West Instrument today. The Administrator will be happy to know that I am having the instrument checked and calibrated by a certified shop, and they will put on the correct color markings to match the numbers on the type certificate. Max level cruise 105 mph (top of the green arc), yellow from 105 to 131, and redline at 131 mph.

I also put in an order for a really nice German pitot-static probe for use in competition gliders. It is a much higher quality thing than any of the stuff put into production lightplanes. This will guarantee me a good air reading. I just have to figure out a good way to mount it on the airplane since there is no bracket. It is supposed to be glued into a fiberglass wing when you build the glider, but there's nothing to glue it to on a Taylorcraft. I'll have to come up with some kind of Adel clamp bracket gizmo.

I am wondering if anyone has any more light to shed on the question of what was the original, certified, factory pitot-static location on the 1940 BC-65? Some people told me that the factory ran the static tube into the wing and left it. The previous Taylorcrafts I have owned had two copper tubes coming out the bottom of the wing at the jury strut, bent 90 degrees forward.

Can anyone verify whether the original design airispeed system worked accurately or not? I've heard rumors that it didn't, and rumors that the factory "adjusted" the sysstem so that the ASI showed the performance of the brochure

Photos of my test righ are attached for your enjoyment...


Bill

[NY86]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Most airspeed indicators read low at low speeds and high at high speeds. This gives you a margin of safety on both ends.

A quick test for the indicator can be found here

Bear in mind, also, that as your airspeed changes, so likely does the pitot tube's AOA.

I like the line about the Hawaiian tornadoes...

[jgerard]

Re: Airspeed Indicator Testing, Rube Goldberg Method

I say turn off your brain and turn on your butt (aka seat of the pants)

Just kidding, Honestly I like reading the tech questions and research stuff you post here and your ideas on ways to improve these old planes. Please keep us informed with the results from your Tcraft skunk works!

[N96337]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Bill, a few years ago at Arlington, there was a plane called the Mountain Goat or something like that. It had a pitot tube that had basically a balanced, pivoting end on it, and what looked alot like the tail on a windmill, to keep the end of the tube directly into the relative wind, no matter what the angle of attack of the aircraft was.... it was pretty interesting.... but what made me think of this was talking to him, he was telling me what trouble they had getting their ASI to read correctly...if I remember right, it was reading low on the bottom and high on the top... sound familiar?
JH

[VictorBravo]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Well, two things. First, I thought I had come up with the idea of a balanced pitot probe with fins on it! Dang! I may just try that... it really does seem like a good idea for an airplane that is side-slipped a lot. I was literally thinking about trying that real soon.

(Pardon me for getting long winded, but you reminded me of the Mountain Goat and its illustrious designer and promoter... this is a true story...)

The Mountain Goat is an airplane being built, or sold, or promoted by a guy named Bill Montagne. Imagine Jim Bede WITHOUT an engineering degree.

Montagne and Skip Holm approached me one fine day at the Reno Air Races, I think 1988 or 89, trying to get me to put up the money for Montagne to build a new racing wing for my Cassutt. They wanted ten or twenty grand right there on the spot, and if I'd just hurry up and write a check Montagne would get right to work building the supersonic Cassutt wing. These two Bozos were sizing me up like a holiday turkey and it didn't take a genius to smell a big rat. Skip Holm was an unlimited racer, and I couldn't figure out what the hell he was doing walking around the F-1 pit looking for me or any other F-1 racer.

Then someone pointed out the fact that Montagne had built this huge composite racing propeller for a Merlin engine, and he had convinced Skip Holm that it would let him win Reno at half throttle. So Skip was trying to buy this propeller, or have Montagne build one for him, and he had to put some money in Montagne's hands immediately to get the "Sky-Prop" for himself so no other racers could get it. All Skip had to do was find some idiot to give him money... for any reason legitimate or otherwise... and that's where I was supposed to come in I guess. So they cooked up this idea I assume, to tell me whatever they thought I needed to hear to cough up a bunch of money, and the money would go into Montagne's hands earmarked for Skip's windmill, and most likely all I'd get out of the deal would be a series of BS promises.

I was pretty dumb then and still kinda dumb now, but not that dumb. Show business is full of BS artists, liars, snakes, and frauds... don't ask me how I know. I walked away laughing at the both of them, feeling sorry that such a highly skilled airplane driver and famous test pilot like Skip Holm was such a schmuck when he wasn't sitting in an airplane.

Now here's the punch line.... Montagne actually got money form someone else (I think it was John Sandberg, who had built the Tsunami racer), and they installed Montagne's huge white carbon space age propeller on the front of one of Sandberg's racing Merlin engines... and put it in the test cell.

As the story was told later, at about 1/3 power, the propeller exploded, destroyed a $50K Merlin engine, and just about trashed the test cell. When they found a piece of the propeller, they saw that it was white styrofoam, hot-wired like a homebuilt Vari-Eze wing, with a layer or two of carbon fiber cloth on each side. No spar, no ribs, no engineering, no high strength attach bushings, nothing. It was as if he made a prop out of four surfboards. It would not have been strong enough to handle 450 horsepower, let alone 3,000.

Bill Montagne was "run out of town" in the air racing world post haste.

He had also been promoting a piston engine, propeller driven experimental racing plane which was going to be the first peopeller driven aircraft to break the sound barrier. The "Mach Buster" was it's name, and it looked like a little high wing T-38 with one of Montagne's hi-tech un-ducted fan propellers on the back. Mercifully for whoever was going to fly it, and more mercifully for whoever was going to invest in it, it fizzled before anyone's wallet or life was in too much danger.

Montagne decided to make his talents available elsewhere, and I believe it was at that time he decided to build the world's greatest bush plane.

Perhaps some of the Taylorcraft Forum members in Alaska can mention the Mountain Goat and Bill Montagne to any of the well-connected Super Cub drivers up there and ask what they think of it. Based on what a local Super Cub driver at my airport has told me, it's a source of endless amusement and bar room jokes with the Super Cub guys.

OK, sorry for the rant... the mention of the Mountain Goat was just too juicy to pass up

[VictorBravo]

Re: Airspeed Indicator Testing, Rube Goldberg Method

BTW, today I got my ASI back from the calibration shop. It was within 2 mph at all flight speeds, and now it is within 1 mph.

So the indicator was not my problem after all.

An educated friend of mine suggetsed that the static and pitot probes were possibly being affected by the airflow stagnation close to the wing leading edge, and reading too little dynamic pressure.

Meantime I removed almost all of the stuff I had tried, both new vinyl tubes in the wing and both new aluminum probe tubes. I'm gonna try another location for mounting the probes further from the wing.

[Len Petterson]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Finicky instruments Ahh.
Yeasterday I was talking to the best aircraft welder I know, just to have a small tab tacked on to the gear leg, and I was told this:
If you arc stick, tig or migweld on a airframe, rag and tube or aluminum makes no differnce,
a magnetic field is induced in airframe with a strenget what can affect airspeed and altimeter to point what the are not relable at all. Never mind what it will do to the compass.
He was talking from his own experience.
This may not be Bill's problem but maybe we avoid what some one else doing this misstake as I was close to having it dune to my plane.
I just did not know about it!
Len

PS: Bill, I take it the mountin goat is not as good climber as the one Mother Nature made.

[N96337]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Yup, that's the guy Bill.... you hit the nail right on the head... but I was trying to be nice. Hahahahaha.. which brings up the question..WHY? hahhaa
I agree with you totally...for some reason aviation seems to attract phonies, and BS artists like moths to a flame... but I still liked the pitot tube he had.. although it was the only thing on the airplane I liked. I was thinking of trying to build one of those for my plane as it goes back togehter again...
JH

[Carl Ellis]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Mountain Goat web site

http://www.bushplanes.com/

[VictorBravo]

Re: Airspeed Indicator Testing, Rube Goldberg Method

but I still liked the pitot tube he had.. although it was the only thing on the airplane I liked. I was thinking of trying to build one of those for my plane as it goes back togehter again...
JH

Depending on how "experimental-looking" you are willing to have your floating pitot tube......

I had actually daydreamed of a way to have it float in both pitch and yaw. The R/C model airplane world (my alma mater) has what they call "quarter scale ball links" of various configurations. These are actually well made light duty spherical bearing mounts, with a 4-40 threaded standoff integral to the ball and a threaded hole integral to the housing. By putting the threaded standoff into a small Adel clamp, in which the pitot tube is held, the tube can swivel maybe 20 or 30 degrees in any direction.

My plan was to have it balanced so the pitot tube hung nose-down at rest to discourage water from getting in. A few thin "beer can aluminum" fins soldered or epoxied on the back, and a section of very flexible tubing in the pivoting area, and voila!

The only thing that would look funky would be mounting it far enough away from the jury strut so the tube could pivot without hitting the strut. A small rubber tube bumper on the pitot at the right location would prevent scratching the paint.

Whichever one of us that gets it done first, post photos!

I ordered a German calibrated sailplane pitot-static tube, to once and for all put an end to my airspeed indicator woes. In the meantime I want to continue testing the tiny R/C and UAV probe I bought, in another location that is further away from the wing. The location I chose is eight inches in front of the jury strut / wing strut junction. This will give me good air, but it will also be VERY prone to getting walked into or bent by accident.

So, since this is more or less a temporary test installation, I'm mounting the pitot tube on a slightly modified wooden clothespin that will be clamped around the lower jury strut ! This way I can walk into it and simply knock it off the strut without bending it.

If I had the balls, I'd put it someplace an FAA field inspector could see it , just to take a photo of his face. "Honest, Sir, it's a Spruce clothespin...."

Nahhh, I don't have THAT kind of balls

[VictorBravo]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Yesterday's test flight was interesting. At 5000 feet I did a series of junior birdman test pilot runs with the GPS and the newly calibrated ASI. I had mounted the pitot/static probe on a long carbon fiber tube wired to the jury strut-wing strut intersection, with the tip of the probe a foot forward of the leading edge and two feet under it. A pitot leak test was done, showing no leakage. An alternate static port (fuselage side Cessna type) had been installed in the hole where I had removed the instrument venturi.

The four high speed runs (at 90 degree course deviations) were 92-101 GPS and 82-84 IAS, not much better than before.

The stall runs were about 40-48 GPS and right at 30 mph IAS. I assume the high GPS stall speed was because of flying on a warm day at 5000 MSL. The airplane was not weighed, but had only one plump test pilot and about 6 gallons of fuel aboard.

As I was heading back, I remembered that I had hooked up the other static port to a loose tube. Luckily I had also left the glareshield off due to all the screwing around with instruments. So, I leaned over the instrument panel and started un-plugging air lines and fishing around for the line from the other static port, wrestling with the tubes as the little Taylorcraft did a couple of interesting gyrations when I leaned against the yoke.

Finally the new static was plugged in to the airspeed, and there was a change in the reading immediately.

So we went back up to 5000, and did all the runs again. With the static on the side of the fuselage, the "high speed" runs showed 92-102 mph GPS and 90-92 on the airspeed indicator. Stall speed went up to 35 mph IAS.

So now it's getting a little closer to reality, but it still needs some fiddling. Today I expect the arrival of a better quality sailplane pitot-static probe. If daylight permits, this probe will be lashed and bound to the same location as the previous UAV air probe and tested.

One of these days I'll get it right!

[Richard Boyer]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Bill,

So are you reeeeaaaallllllyyyy bored??

When we went to SWRFI a few weeks ago I was pretty much loaded to the gills. I bounced my way down the runway and pulled back when it felt like it was time to fly. I noticed that my Airspeed Indicator was showing a stunning 0. I was defying Physics!!!!! Took me a second to realize that the "anti-mud daubber" flap was stuck closed. Some yahoo suggested stalling the plane to see if it would unstick it. I figured I had better things to do at gross weight.

Flew and landed just fine.....

Your adventure makes for a fun read though!!

[N96337]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Hey Bill, how about moving the pitot tube outboard on the wing...like within a foot or two from the tip?
JH

[VictorBravo]

Re: Airspeed Indicator Testing, Rube Goldberg Method

Made another stealth test flight today with yet another supposedly good pitot-static probe. It was still reading 12+ miles an hour less than what the GPS said.

In frustration I jut pulled the static line off the back of the ASI, and it started reading pretty close to the GPS value!

Now I don't know WHAT the hell to do!