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n8714d
04-30-2006, 07:23 PM
:) Guys/Galls,
For a pet project that I am working on, I need to modify an IAS meter in such way as to make it 3 times more sensitive. The standard IAS reads 120 mph max.What I need is for it to indicate 120 mph when,in reality, nothing is ha ppening faster than 40 mph.
Any ideas on how to do it?
Anybody out there skilled in the art?
Thanks,
Miguel Azevedo
N8714D
Pa22/20-150

Homer Landreth
05-01-2006, 09:02 AM
Hi Miguel;
Not very easily, but theoretiaclly possible. Airspeed indicators are a mechanical or analog solution to the equation. Airspeed = the square root of (2 x [Pitot pressure - static pressure] / air density). The instrument uses a round gear to mesh with a triangular shaped piece that has for the third side of the triangle an arc shaped gear toothed portion. In looking at the airspeed indicator you can tell that the pointer motion around the 360 degrees of the dial is not linear. 0MPH to 60 MPH makes the needle rotate through only a fraction of the arc of pointer rotation for say 60 MPH to 120 MPH. A rough guess is that 0 to 60 MPH rotates the indicator needle from 0 to the 85 degree position of the dial. But 60 to 120 MPH rotates the needle from the 85 degree position of the dial to something around the 220 degree position of the dial. The problem solution for your request (which incidentally, in consideration of how an airspeed indicator works, is not making it more sensitive, but is really making it less sensitive) is the fabrication of a new gear and a new arc length of the mechanical triangle piece. You didn't say what the range of the indicator needs to be, but as I recall, the gear rotates the number of times to make the trip along the arc piece from zero to the max reading, and the arc length determines how much the indicator needle moves in proportion around the scale. That is where the non-linearity comes in. If that isn't complicated enough, there is spring tension on the needle that factors into it, although at a pretty negligible amount. Sooooo the airspeed indicator caper would be pretty convoluted and tedious and kind of hit or miss. However if you are software literate, you could attain the scaling you need by using a portable GPS input into a PDA which is capable of displaying an airspeed indicator. (such as a Bluetooth Belkin unit talking to an HP PDA, and then you could frinkle with the software program (usually freeware) that you are running on the PDA. That would be easier and a lot more interesting. :) Anyway, if you pursue it let us know how it works out. Actually, with all of the above said, exactly what is it you are trying make this for, it is kind of intriguing ?

MikeM
05-01-2006, 10:23 AM
:) Guys/Galls,
For a pet project that I am working on, I need to modify an IAS meter in such way as to make it 3 times more sensitive.

Why not use a manometer? (curved tube filled with liquid)

How about an electronic solution? You can obtain relatively inexpensive integrated circuit micro-machined pressure sensors, some of which have two ports, i.e. a differential "gauge" sensor. Solid State Sensors used in modern aircraft Altitude Encoders are similar, but of the "absolute" type (only one port).

An $5 IC-chip Instrumentation amplifier and a $10 Digital Voltmeter module would read out the ram pressure from a Pitot. If you want it calibrated in mph or Kts, it would take a look-up table implemented in some digital circuits (like a microcontroller).

n8714d
05-01-2006, 10:24 AM
Homer,
Thanks for a quite authoritative reply.
Maybe using an IAS meter is a dead end for me, unless somebody in the instrument repair business has a quick and dirty answer up his sleeve. Will contact one here in MI.
Pet project is an on-board, real time BBY meter. FAA is already involved and their people are more enthusiastic about it than me...
Thanks again.
Miguel
N8714D

Gilbert Pierce
05-01-2006, 10:57 AM
Ultra-lites use an airspeed indicator that is nothing more than pitot inlet in the bottom of a glass or plastic tube with the top of the tube open to the atmosphere-static port. In the tube is a disk that rises and falls as the pitot pressure rises and falls. All that is left is to calibrate the tube against the floating disk. Avialable from Leading Edge Airfoils. Make your own and adjust the sensitivity by making the floating disk larger or smaller in relation to the inlet port.
Another form is the vane type used on many slow antique airplanes. Merely a vane attached to the strut with a pivot and pointer calibrated in mph. By adjusting the lenght of the pointer in relation to the vane you can adjust the sensitivity.

Aircraft Spruce has a winddriven turbine generator AS for hang gliders-10 to 70 mph.

13010
05-01-2006, 12:48 PM
There are available aneometer either fan, prop or whirling cups thet work quite well in the low range. A lot of parasailers od trikes use them. Look in the ultralight camp and you will find what you need fairly cheap.
J.D.

andya
05-01-2006, 02:27 PM
I have an airspeed indicator that came out of a "duster" aircraft of some sort.
It has a wider distance between the low end numbers (40-70) than the original gage. Maybe something like that would work. Also I'm told that airspeeds for helicopters are similarly scaled in that wider distance between marks at the low end compared to the average airplane indicator.

Gilbert Pierce
05-01-2006, 09:35 PM
:rolleyes: Homer,
In my book the most elegant engineering solution to a problem is the simplest one you can find. Rather than extensive mod required to change the gear ratio on an existing AS why not use the simple solutions that are already avialable? It's called thinking out of the box or in this case the Airspeed indicator case.
Anyway after reading your explanation of how to regear it I thought I would add some simpler solutions to whatever he is try to accomplish.

n8714d
05-01-2006, 09:35 PM
Andy,
Hey, this is a good sugestion!
The helicopter thing. I'll stop by U of M Lifeguard tomorrow an check it out.
Thanks,
Miguel
N8714D

n8714d
05-01-2006, 09:49 PM
Mike,
Yes, I've been rummaging the same line of thought. Just not so skilled with electronics. I'll need someone to back me up. As mentioned, FAA is already involved and are keen on having something like a std. IAS on panel. I'm sure they would accept the electronic version if they feel confortable with it.
Thanks,
Miguel
N8714D

n8714d
05-01-2006, 10:18 PM
Homer,
I must be misunderstanding your latest posting!
Quite on the contrary, I cannot accept "...any old piece of crap found in magazines". Remember, FAA is already involved. I did understand and appreciated your appraisal of the problem I'm faced with. Indeed, if I were endowed with mechanical skills to the level required, yours would be the solution adopted. However, the latter is not the case and I have time running against me.
Once again, I thank you for trying to help.
Miguel
N8714D

n8714d
05-01-2006, 10:37 PM
Gil,
What I am trying to measure is the volume flow of bby, the stream of gases spewing out of the crankcase breathing tube. The pulsating nature and toxicity of this gas precludes the use of a Hall-type (open) AS meter. The vane (turbine) type is an excellent solution but, in its hang gliding form, is severely limited in operational temperatures. Turbine flow meters with electronic indicators do exist and would be ideal, if it were not for the price charged (expensive). I am trying to design something that is purely "mechanical". No power, no vacuum-dependent. A Pitot tube is the best I found so far. Problem remains on how to display the fraction of an inch of water collumn pressure signal. Hence the AIS solution.
Thanks for contributing.
Miguel
N8714D

Homer Landreth
05-01-2006, 10:39 PM
Hi Miguel;
I know you need a better solution and just felt you were not getting quality responses. It was a little frustrating to see your request for help get ported off into the flying kites arena with a suggestion to use a plastic tube filled with water. I sort of felt that if that was what you could use you would have asked for it, and you asked for an airspeed indicator. Hope you get it solved.

Gilbert Pierce
05-01-2006, 11:15 PM
Miguel,
I understand now what you are trying to do. I have used a regular airspeed indicator to trouble shoot blowby problems on aircraft engines, it worked great. If you have enough blowby to cause a problem it definitely shows up on the AS indicator.
In a previous life I designed test cells for production diesel engines. We used a water manometer across a calibrated orifice so that the blowby gases could exit and we got pressure reading across the orifice that was indicative of the volume of gas being expelled. In this application we could definitley see the rings seat in as we ran the engine at full horsepower. A fraction of an inch of water was significant as we tested every engine before shipment and wanted to insure a quality engine. Later on we changed this to an electronic transducer that measured the pressure, converted it to a digital signal and fed it to the computer. The transducer was a couple of hundred bucks and a ANALOG DEVICES A to D converter was pretty cheap.

n8714d
05-08-2006, 10:59 PM
Gil,
Sorry for the tardy answer!
And thanks for sharing your expertise. We have much in common, as I also designed test cells in a previous life. And, indeed, water collumns are a really neat solution to a tight budget. I agree with you. If you have a problem, the IAS will pick it up. However, take my engine for example (0320). A nice, dry, tight - as bby goes - oldie. Max BBY at 2500 rpm is no more than 0.12 in WC. A normal IAS will not pick it up. If we set 1.5% of Vasp as a limit, that corresponds to ~100 l/min. or roughly 32mph. A 0-120 mph AS will detect it OK. My engine? It is equivalent to hardly 7 mph, which is within the dead band!
Thanks!
Miguel

n8714d
05-08-2006, 11:20 PM
Homer,
Thanks!
Sometimes I fail to come up with a clear understanding of what people really want to convey to me. Not born here, will forever carry this handicap, I suppose! But thanks again. Now I see you were advocating in my behalf.
Tell you what I did. Bought a 0-0.5 in WC Minihelic, from Dwyer, the Lab. Equip. maker.
On a test bench, works marvelously. On the instrument panel and under static testing (on the ground), it is not sensitive enough. Maybe it is due to the natural posture of a taildrager. The gage comes with a a warning that it will only indicate correctly if installed in a vertical plane. Will wait FAA inspection and Form 337, then fly the thing and see what happens. Too much trouble rigging the tail up etc.
It would be neat to have a suitable IAS though...
Miguel
N8714D

13010
05-09-2006, 09:04 AM
I also failed to read the whole thing before suggesting the ultralight stuff. If you use an inclined water collum the change is more pronounced. That is the change in heigth also moves horizontally. This implies not flying useage. The other way to measure flow is differential pressure across a restriction. This is really what you are doing if you are measuring pressure in the vent line vs. atmospheric pressure. A calibrated orifice (washer) would have little nonrecoverable flow restriction and you can place a differential pressure gauge across it (airspeed indicator). The drop can be calibrated with a known flow at a given temprature. This is what would be used in dirty flow in an industrial application. The pressure change near the restriction would be more repeatable and more easily measured.
J.D.