ASI.....bleed air adjustment

StanFoster

Active Member
Joined
Nov 16, 2003
Messages
17,139
Location
Paxton, Il
Aircraft
Helicycle N360SF
Total Flight Time
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There was some talk this morning in another thread about ASI reading wrong.
I chimed in and mentioned I would post a thread on a bleed air adjustment fitting I came up with to make my ASI read much closer to the correct airspeed.

Every rotorcraft I have owned had my ASI reading too fast. My SparrowHawk was the worst....it would read 8-10 mph too fast at cruise, and as much as 10-12 mph too fast at high speed runs.

I came up with the idea of making a valve that would bleed off just enough of the excess pitot tube pressure...making the ASI read accurately.

I know the whole problem stems from the static port not being located at neutral pressure....plus some other things I dont understand enough to fix. But I do know that bleeding off a little pitot tube pressure works great. This may be considered a 'bandaid' fix instead of properly configuring the static port.....and pitot tube....but it works for me. I hear so many times figures of gyros going way over 100 mph....and much higher figures...and I have always wondered what their true averaged GPS groundspeeds really are. My experience has had every one of mine read too fast.

I got fussy about my ASI being off in my SparrowHawk to do something about it. I would average my GPS groundspeed runs after flying straight and level into the wind...and then fly 180 the other way....noting the two groundspeeds...and averaging. Say my GPS showed me going 90 mph with the wind....and 70 mph against...I want my ASI to say 80 mph. My SparrowHawk would read 88-90....not acceptable! So...after my GPS run...I would just keep flying on the 2nd leg....and simply turn the screw on my airbleed valve...and watch the airspeed slowly go down till it read 80 mph. My experience with this varified that the ASI stayed accurate until there was a big change in temperature towards winter...then it was only off 2-3 mph...and a simple partial turn of the screw dialed her in again. I found that I only adjusted this thing basically twice a year, and I was always checking it just for the fun of it...and the pleasure knowing that ASI was giving me actual airspeed readings.

Here are some pictures. I took a wellnut....and took the screw out.....put a sloping notch in it with a hacksaw. One end of this wellnut was just the right size for a vinly tube to go into it.....then I T'd it into the pitot line.

The slotted screw was inserted in the back end of the wellnut...and when it was all the way screwed in....it was like before...the ASI was reading way too high. But as you back out the screw, there is a point when the slot just starts releasing a little air.....then the ASI starts to read less. As you turn the screw CCW, more of the slot gets exposed and more air gets bled out. This goes on until the ASI reading matches the GPS averaged groundspeed...and you are done adjusting.

My Helicycle has its static port positioned as the plans show...and when I start flying it...I will be conducting some GPS groundspeed runs. If my ASI doesnt read correctly, I will try a static port as Chuck Beaty suggested. But if that doesnt give me correct results...I will be having a bleed air adjustment screw in my instrument pod.

The last picture is a tee made just for the picture..its just a butt joint..whereas of course a real T would be inserted.

Stan
 

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Stan, I did this mod on my RAF some time back and it works great!
 
True airspeed and indicated airspeed (without instrument or installation errors) are two different things.

True airspeed is most easily obtained with a propeller type instrument. A propeller or rather a windmill indicator, without friction, is a screw and the air is a nut. It would indicate true speed whether in water or even in loose snow. It doesn’t care about air density.

Airspeed indicators based on the difference pressure between static and pitot ports can only indicate true airspeed under standard conditions of barometric pressure and temperature, a rare situation on this Earth. In most real life situations, indicated airspeed is higher than true airspeed because we rarely fly at sea level standard conditions: 59ºF with a barometer reading of 29.91 inches.

A flying machine responds in accordance with indicated airspeed and doesn’t care in the least about true airspeed. Stall speed, rate of climb, etc. are directly related to indicated airspeed (without errors).

Boats have sometimes used pitot type speed indicators. The readings are very different between fresh and salt water.

When an airspeed indicator is made to agree with a GPS, it is being calibrated for true airspeed. Might as well toss that $300 TSOed ASI and get a motorbike speedometer with a prop stuck on the end of it.

An ASI can be calibrated with the aid of a GPS but temperature and barometric pressure must be known and a set of tables for converting true to indicated airspeed must be on hand. I expect a Google search would turn up dozens of conversion applets.
 
I'm missing something here. Why not solve the problem by determining the right place to put the static port, so it could be repeated by anyone building the same machine?
 
I'm missing something here. Why not solve the problem by determining the right place to put the static port, so it could be repeated by anyone building the same machine?

I'm with you, Paul. It's so easy to do it properly, why all this "work around"?:noidea:
 
Tinkering is half the fun, Paul and Pete.

There is no place on a gyro where a Cessna style static port can be located to provide error free indication over the range of likely variables; doors on, doors off, yaw angles, pitch angles, etc.

If we exclude a proper NACA style static port, Stan’s bleed port is at least in part self compensating for the above variables.
 
All of the points made by CB and others are valid--ie just do it right to start with. According to my research one of the first things in "do'in it right" is to have the pitot and static source co-loscated in undisturbed air. I purchased such a unit from Aircraft Spruce and installed it on the nose of my RAF. Good thing I left the GPS hooked up as in calm air at 60mph on the gps the IA was 36!! I beliece in these enclcosed and semi enclosed machines (RAF with no doors) there is a area of abnormal low pressure created behind the instrument panal where many choose to locate the static source. Before doing Stans mod I was reading 8-10 mph too high. This mod takes less than an hour and is especially helpful in the lower AS ranges.
 
For those folks who must have airspeed agree with GPS, the thing in the attachment will do it.

It’s a 0-80mph speedometer driven by a propeller. It indicates true airspeed and is surprisingly accurate.

Flying by true airspeed can get you into a heap of trouble, particularly during hot/humid weather.

The round can below the ASI is a mechanical rotor tach.

Sorry for the fuzzy picture; it’s a blowup from a smaller photo.
 

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I beliece in these enclcosed and semi enclosed machines (RAF with no doors) there is a area of abnormal low pressure created behind the instrument panal where many choose to locate the static source...

Exactly. So, why do most builders choose to locate it there?

As near as I was able to determine, Sport Copter's system, which places both pitot and static on tubes sticking out from near the top of the mast, resulted in seeing clean enough air to produce reliable readings, for the price of some plastic tubing to plumb it to the panel.
 
Sorry Wayne won't let us look.
 
While Chuck's comments about true versus indicated airspeeds are true,
I have found it easier to get a good linear and reliable readout from my
prop/true ASI than from the Pitot system. I have both fitted.

Conditions dont vary as much where I fly, as they do some places, but,
when lift is poor, I reckon i know it within a few seconds of getting airborne,
if not before.

I suppose what I'm saying is, I would prefer an accurate true airspeed indicator,
than a dodgy indicated airspeed.

A significant contributor to Peewee Judge's accident, way back, was an
under-reading ASI.
He was not only flying too fast, he was flying substantially faster than he thought
was the case.

By the way, the inside of the keel tube is the best static source I've found so far.
 
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