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'[EE]: Force sensors'
2002\01\27@012029 by Tony Goetz

picon face
Hey, can anyone recommend some force sensors, capable of being interfaced to
a Pic? I need a few for the wind tunnel we're building (which is coming along
quite nicely, BTW). I'm not sure how big the forces are we'll be measuring,
but they shouldn't be too great. Also, any ideas for a pitot tube? If not, we
can make our own I suppose, though I don't know how accurate it will be.

Thanks,

-Tony

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2002\01\27@150431 by Dave Dilatush

picon face
Tony Goetz wrote...

>Hey, can anyone recommend some force sensors, capable of being interfaced to
>a Pic? I need a few for the wind tunnel we're building (which is coming along
>quite nicely, BTW). I'm not sure how big the forces are we'll be measuring,
>but they shouldn't be too great.

When it comes to measuring force, I suspect you're stuck in "industrial
territory".  Pressure, temperature, and acceleration can be measured
with sensors that can be had at reasonable prices (for the
experimenter/hobbyist, that is) but force is something I've never seen
done in a cheap, easy-to-use package.  Entran has load cells and strain
gages, but these cost US$350 and up.  Honeywell has some solid-state
force sensors (http://content.honeywell.com/sensing/prodinfo/force/) but
I don't know the price or who distributes them.

You can Google for a bunch more, but I don't see anything affordable
unless you've got big bucks.

Dave

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2002\01\27@152337 by Jinx

face picon face
Perhaps you could bodge something mechanical that
you could test empirically with weights

Measure the distance a vane moves while pulling on a
light spring ?

Or even an actual spring balance with a linear pot attached ?

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2002\01\27@153016 by Jinx

face picon face
As you're working with moving air anyway, what about
a pitot tube made from a pressure sensor

Wouldn't the air speed at a point on the object imply the
(perpendicular) force. You could measure wind speed
at various point using cheap DC motors as anemometers

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2002\01\27@155417 by Dave Dilatush

picon face
Jinx wrote...

>Perhaps you could bodge something mechanical that
>you could test empirically with weights
>
>Measure the distance a vane moves while pulling on a
>light spring ?
>
>Or even an actual spring balance with a linear pot attached ?

A while back (maybe a LONG while back), Scientific American magazine had
a feature in their since-discontinued "Amateur Scientist" column that
described some experiments with a home-built wind tunnel apparatus.  For
measuring the forces on their airfoils, they built their own
microbalances using the guts from analog meter movements.  They glued a
small vane onto the meter pointer and used a slotted optocoupler to
sense the position of the vane; a servo circuit consisting of an opamp
and a few other components sensed the output of the optocoupler and
drove the meter coil to keep the vane position constant.

The vane and pointer were linked to the airfoil surface being monitored,
and the meter coil current thus served to indicate the force acting on
the airfoil.  IIRC, they were dealing with forces on the order of a few
hundred milligrams and resolving them down to a microgram or two.  I
don't know what forces Tony is interested in measuring, but they're
probably a bit higher than that.

Dave

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2002\01\27@161013 by Herbert Graf

flavicon
face
What about a piezo transducer? Would probably require some heavy
amplification but it should work. TTYL

> {Original Message removed}

2002\01\27@162559 by Tony Goetz

picon face
We plan to measure two principal forces - lift and drag. These will be
measured off a lever attached to the test model and protruding through the
side of the tunnel. I really have no idea how much force to expect at the
speeds we'll be running (maybe up to 40mph), but high precision isn't
necessary.

I did some web searches a few weeks ago but found basically what you guys
seem to have confirmed: these things aren't cheap! I was hoping I just hadn't
been looking in the right places. The thing with using electronic scales vs.
mechanical springs and weights is that, primarily, the output display would
be easier to read since the project should be usable by people who have no
idea how to use the wind tunnel in the first place. (since it also gives me a
chance to finally use a Pic in a practical application, why not?)

What about a piston type arrangement, where the lever pushes on an air filled
piston and the pressure inside is measured? Any thoughts on how complex this
kind of thing might be?

<< A while back (maybe a LONG while back), Scientific American magazine had
a feature in their since-discontinued "Amateur Scientist" column that
described some experiments with a home-built wind tunnel apparatus. >>

I've got the CD archive of "Amateur Scientist," and know the article(s) you
mean. For gauging airspeed, the main problem with using a homebuilt
static/dynamic pressure differential system is calibration. It's fairly easy
to calculate what the airspeed should be at different measured pressures but
it should still be calibrated, and heading up to a "real" wind tunnel for
calibration seems a bit expensive and time consuming, even if we manage to
get the home made pitot tube working right the first time. It's because of
this that some ready made unit would be preferable.


thanks for your time
-Tony

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2002\01\27@163426 by Dave Dilatush

picon face
Herbert Graf wrote...

>What about a piezo transducer? Would probably require some heavy
>amplification but it should work. TTYL

That'd work if he was interested in measuring vibration; but piezos
don't respond down to DC so they can't be used for measuring static
force.

Dave

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2002\01\27@164405 by Dave Dilatush

picon face
Tony Goetz wrote...

>We plan to measure two principal forces - lift and drag. These will be
>measured off a lever attached to the test model and protruding through the
>side of the tunnel. I really have no idea how much force to expect at the
>speeds we'll be running (maybe up to 40mph), but high precision isn't
>necessary.

At those speeds you might be talking about forces of several pounds, I'd
guess, depending on your model size.  Tough job...

{Quote hidden}

You'd need a piston with very low friction, short throw (i.e., very
small chamber displacement so your model doesn't move a lot) and a very
tight seal, which could be a nightmare.

How about ripping the guts out of an electronic bathroom scale and using
them to measure your forces?  Might take some experimenting around, but
in the end it could be the easiest and cheapest way to get the job done.

I'm working on a general-purpose, el-cheapo force sensor for some hobby
projects of my own, but I'm not going to have it ready any time soon;
too many other projects (like a PID controller for my toaster oven so I
can do some REALLY precise frozen-food work) that are higher up on my
"TO DO" list...

Dave

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2002\01\27@171810 by Tony Goetz

picon face
<< >What about a piston type arrangement, where the lever pushes on an air
filled
>piston and the pressure inside is measured? Any thoughts on how complex this
>kind of thing might be?

You'd need a piston with very low friction, short throw (i.e., very
small chamber displacement so your model doesn't move a lot) and a very
tight seal, which could be a nightmare.

How about ripping the guts out of an electronic bathroom scale and using
them to measure your forces?  Might take some experimenting around, but
in the end it could be the easiest and cheapest way to get the job done.

I'm working on a general-purpose, el-cheapo force sensor for some hobby
projects of my own, but I'm not going to have it ready any time soon;
too many other projects (like a PID controller for my toaster oven so I
can do some REALLY precise frozen-food work) that are higher up on my
"TO DO" list...

Dave >>


Maybe an electronic scale wouldn't be such a bad idea. If I wanted to
interface to it, since the lever would produce a force proportional to the
distance from the fulcrum instead of the actual force on the model, it may be
a pain in the butt. But then I guess the readout could always say next to it
"1/2 of actual force." Or maybe it has some sort of hackable circuit that
could be interfaced already...

Out of curiosity, how far in the future are you talking about for your
project? on the order of months, or years?

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2002\01\27@171821 by Tony Goetz

picon face
<< >What about a piezo transducer? Would probably require some heavy
>amplification but it should work. TTYL

That'd work if he was interested in measuring vibration; but piezos
don't respond down to DC so they can't be used for measuring static
force.

Dave >>

While vibration would be an interesting force to measure, it doesn't look
like the project will deal with that. For one thing, the fan itself presents
a fair amount of vibration, enough to introduce quite a bit of error in those
measurements. Mainly we're focusing on lift, drag, and flow visualization.

Maybe on wind tunnel version 2.0.........

-Tony

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2002\01\27@174922 by John Ferrell

flavicon
face
Here is an idea I have not managed to act upon:

An RC servo with a fixed pulse input will try to hold a given position. The
input current goes up with the effort required on the servo arm.  About $15
plus the pulse generation costs. BTW, I have never been able to PRECISELY
maintain a servo position with a 555 timer... Hopefully a PIC will do
better.

John Ferrell
6241 Phillippi Rd
Julian NC 27283
Phone: (336)685-9606
Dixie Competition Products
NSRCA 479 AMA 4190  W8CCW
"My Competition is Not My Enemy"



{Original Message removed}

2002\01\27@180040 by Dave Dilatush

picon face
Tony Goetz wrote...

>Out of curiosity, how far in the future are you talking about for your
>project? on the order of months, or years?

Probably a couple of weeks, or a month, or two, or...

My problem is, I have **WAY** too many projects I'm interested in
working on, and not enough time to accomplish them all when I'd like.
Plus, doing this stuff all day for $$$ it sometimes feels too much like
a "busman's holiday" doing it in the evening too.  So I always seem to
end up spending more time vegging than I intend.

Let's put it at "a month or so".

Dave

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2002\01\27@182438 by Robert Rolf

picon face
The quick and dirty solution is to use a couple of cheap ($50) digital
postal scales (or equivalent. I saw a digital fish scale for $28C some
years back).
You can easily pull out the load cell and remote it with some heavy guage 4
conductor shielded wire. You can read the LCD data by XORing the segment bits
with the BP signal (effectively removing the driving square wave). Use a PIC
to dedode the segments. IIRC you only need 5 segments to uniquely
identify what digit is being displayed.

There are also cheap ($15C) digital tire pressure guages that could be hacked
to stay on all the time. You may want to crank up the gain to get finer
resolution.

A varient on the current balancing of meter movements to measure microgram
forces is to use a small loudspeaker with linear optointerrupter for feedback.
An 8" woofer can generate 10's of grams of force continuously if you blow air
through the coil gap.

What's your time worth compared to buying a commercial nearly turnkey solution?

Robert

Dave Dilatush wrote:
{Quote hidden}

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2002\01\27@182904 by Tony Goetz

picon face
<< >Out of curiosity, how far in the future are you talking about for your
>project? on the order of months, or years?

Probably a couple of weeks, or a month, or two, or...

My problem is, I have **WAY** too many projects I'm interested in
working on, and not enough time to accomplish them all when I'd like.
Plus, doing this stuff all day for $$$ it sometimes feels too much like
a "busman's holiday" doing it in the evening too.  So I always seem to
end up spending more time vegging than I intend.

Let's put it at "a month or so".

Dave >>

LoL, I know what you mean. As soon as I get inspired enough to work on one
project, another one catches my eye. Result? I sit there, staring into space,
trying to decide just what to do with my time. Our wind tunnel doesn't have
to be completed until May or so, though we're trying to get it done as soon
as possible. Most likely it'll also take "about a month or so." Probably
more. Either way, I'd still be interested in your el-cheapo force sensor
project. Please let me know how that goes.

I hope to get a small page up online documenting our progress on the wind
tunnel so others can get an idea of what we're doing. I'll post it when I get
it up.

-Tony

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2002\01\27@201655 by Tony Goetz

picon face
Those are pretty good solutions, and I'm becoming more and more optimistic
here. And, to answer the question, time is a bit more expendable than money.
Make that alot more. We have several months to build this thing and are down
to our last dollars for now, and about the only obstacle left is force
measurement. Actual construction on the tunnel itself is going so smoothly
we're certain we've done something wrong. The diffuser is finished, the base
structure is just about finished, the fan is mounted, and the test section is
well under way. A trial run with the fan and diffuser went well. Frightening.

-Tony

<< The quick and dirty solution is to use a couple of cheap ($50) digital
postal scales (or equivalent. I saw a digital fish scale for $28C some
years back).
You can easily pull out the load cell and remote it with some heavy guage 4
conductor shielded wire. You can read the LCD data by XORing the segment bits
with the BP signal (effectively removing the driving square wave). Use a PIC
to dedode the segments. IIRC you only need 5 segments to uniquely
identify what digit is being displayed.

There are also cheap ($15C) digital tire pressure guages that could be hacked
to stay on all the time. You may want to crank up the gain to get finer
resolution.

A varient on the current balancing of meter movements to measure microgram
forces is to use a small loudspeaker with linear optointerrupter for
feedback.
An 8" woofer can generate 10's of grams of force continuously if you blow air
through the coil gap.

What's your time worth compared to buying a commercial nearly turnkey
solution?

Robert
 >>

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2002\01\27@201920 by Peter Meleschko

flavicon
face
How about using a variable resistor connected over a spring. I know some
MIT roboguys uses this to measure force in linear-actuators.
yobotics.com/actuators/description/description.htm
www.ai.mit.edu/people/dwrobin/projects/freac/
Simple and cheap...

PeterM

søn, 2002-01-27 kl. 07:18 skrev Tony Goetz:
{Quote hidden}

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2002\01\27@205724 by Tony Goetz

picon face
This would be a nice, cheap, easy way to go. Unfortunately, enough movement
to cause a change in a variable resistor would cause a change in the
position/rotation of the test model. Which would cause new forces to act on
it, which would further move it and the resistor. It would be great if it
were that simple....

Still interesting sites, though...!

-Tony


<< How about using a variable resistor connected over a spring. I know some
MIT roboguys uses this to measure force in linear-actuators.
yobotics.com/actuators/description/description.htm
www.ai.mit.edu/people/dwrobin/projects/freac/
Simple and cheap...

PeterM >>

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2002\01\27@222011 by Bob Ammerman

picon face
How 'bout a simple spring loaded arm that turns a pot?

Bob Ammerman
RAm Systems


----- Original Message -----
From: "Dave Dilatush" <spam_OUTdilatushTakeThisOuTspamHOME.COM>
To: <.....PICLISTKILLspamspam@spam@MITVMA.MIT.EDU>
Sent: Sunday, January 27, 2002 12:02 PM
Subject: Re: [EE]: Force sensors


Tony Goetz wrote...

>Hey, can anyone recommend some force sensors, capable of being interfaced
to
>a Pic? I need a few for the wind tunnel we're building (which is coming
along
>quite nicely, BTW). I'm not sure how big the forces are we'll be measuring,
>but they shouldn't be too great.

When it comes to measuring force, I suspect you're stuck in "industrial
territory".  Pressure, temperature, and acceleration can be measured
with sensors that can be had at reasonable prices (for the
experimenter/hobbyist, that is) but force is something I've never seen
done in a cheap, easy-to-use package.  Entran has load cells and strain
gages, but these cost US$350 and up.  Honeywell has some solid-state
force sensors (http://content.honeywell.com/sensing/prodinfo/force/) but
I don't know the price or who distributes them.

You can Google for a bunch more, but I don't see anything affordable
unless you've got big bucks.

Dave

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2002\01\27@222031 by Dave Dilatush

picon face
Tony Goetz wrote...

>...Either way, I'd still be interested in your el-cheapo force sensor
>project. Please let me know how that goes.

OK, will do.  Maybe, now that there's actually an application for this
thing, I'll have some motivation for moving it "up the queue" a bit.
That's always been the problem with that thing; it's an intriguing
design calisthenic, but I haven't really had any practical use for it so
it keeps yielding to other projects.

I'll keep you posted.

Dave

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2002\01\27@222419 by Dale Botkin

flavicon
face
On Sun, 27 Jan 2002, John Ferrell wrote:

> Here is an idea I have not managed to act upon:
>
> An RC servo with a fixed pulse input will try to hold a given position. The
> input current goes up with the effort required on the servo arm.  About $15
> plus the pulse generation costs. BTW, I have never been able to PRECISELY
> maintain a servo position with a 555 timer... Hopefully a PIC will do
> better.

It will, I can hold one ROCK steady with a PIC.

Also, you can hack the servo apart and find the pinout for the servo
driver chip.  There will be some usable signals there if you want to use
them; the motor is usually driven via PWM pulses to an H-bridge or some
similar arrangement.  I've used these signals with limited success
(ultimately a failure, but an interesting one anyway) to determine when
the servo is moving or settling.  You might be able to look at pulse width
and figure something out.

Dale
---
"Curiosity is the very basis of education and if you tell me that
curiosity killed the cat, I say only the cat died nobly."
         - Arnold Edinborough

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2002\01\28@011205 by Tony Goetz

picon face
Glad we could provide some motivation! I was real unsure of just what to do
as far as measuring the forces acting on our test models, since by the very
act of measuring them you change them in alot of cases. I hope we're both
successful.

-Tony

<<  OK, will do.  Maybe, now that there's actually an application for this
thing, I'll have some motivation for moving it "up the queue" a bit.
That's always been the problem with that thing; it's an intriguing
design calisthenic, but I haven't really had any practical use for it so
it keeps yielding to other projects.

I'll keep you posted.

Dave >>

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2002\01\28@061947 by Mike Blakey

flavicon
face
What size (diameter) is your tunnel? and what air speed. We have got a few tunnels
here ;-) so I'm happy to help. We / I have made several Pitot-static probes and
force sensors for calculating drag etc.



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2002\01\28@072152 by o-8859-1?Q?K=FCbek_Tony?=

flavicon
face
Hi again, I
forgot to include this link:

www.hbm.com/en/section/imt/index.asp?page=/en/section/imt/company/app
s/app_view.asp?task=show&menu=applications&cat=95&sol=5

( watch out for line breaks :) )

/Tony

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2002\01\28@072200 by o-8859-1?Q?K=FCbek_Tony?=

flavicon
face
Hi,
I definately think strain gauges are the way to go here,
either as someone pointed out hacking an cheap postal scale
or build your own. There are companies that sells the strain gauges
separately compared to complete loadcells ( not cheap as you've found out ).
The thing with loadcell are that they are fairly pricy as the also
( normally ) have temperature and linjearisation circutry inside,
not to mention that the mechanics ( housing ) is somewhat hard to
manufacture.
In your case you does not seem to have use for wide temperature spec.
So I would suggest purchasing separate strain gauges ( 4 recommemded )
amplify by an simple opamp ( perhaps with zero and gain pots for
'calibration' )
The maths to build an 'scale' from them are farily trivial and should pose
no major trouble.

Makes some searches for 'strain gauges' and it should turn up a few
directions.

Here's one to start with ( in Europe ):
http://www.hbm.com/

/Tony

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2002\01\28@145302 by Peter L. Peres

picon face
> vibration.

All fans vibrate. Insulate it from the tunnel using a slack acordeon
baffle made of rubberized cloth or similar and provide it with its own
stand (not the same as the tunnel's).

More vibration will come from stalling airfoils in the tunnel. This can
produce vibration with forces that exceed the maximum expected drag or
lift a few times. Plan for this in the metering system imho (so it will
not be destroyed).

The oldest pressure metering system uses tiny pitots and thin rubber
tubing with 'U' water manometers mounted on a vertical panel. Add
watercolor to the H20 and a movable ruler as scale. You can change water
for something much lighter for higher sensitivity (benzine will work but
it is flammable).

Motorola makes pressure gauges that work at the pressures you need, both
absolute and relative.

You can make a force sensor using a diaphragm on a closed vessel to which
vessel a pressure gauge (see above) is mounted. The diaphragm is also the
spring (or provide an additional external one). It has a problem with
temperature.  You can compensate this but it becomes complicated (better
compensate electronically).

I am not an expert on this, just did my homework once upon a time ...

Peter

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2002\01\28@145311 by Peter L. Peres

picon face
Check Omega for both items. The force sensors will likely be strain gages.

Peter

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2002\01\28@152009 by Alan Shinn

picon face
There has been a long thread(s) about force measurement on the arocket
list for static motor test stands. Richard Nakka has a website with
detailed design procedures for ($10.00) load cells (but there is a
minimum quantity for buying the strain guages -- 10pcs/$45.00 from Omega)
http://members.aol.com/riccnakk/strainlc.html   (direct link but AOL so
{very} slow - at least this morning)
http://www.nakka-rocketry.net/   (some sorta mirror site??)


As for Pitot tubes, make your own using the differential IC preassure
guages (~$35.00??) and calibrate with your car - (at least two people
here please - one to drive and one to attend to the instruments)

Also, you could use a preassure guage and a bellows for a low stick, low
hysteryses force sensor (liguid filled system of coarse) This should
elliminate the problems with a piston arrangement although those glass
dashpots are really low friction and may work fine (can they be filled
with fluid??).
--
Looking forward:
Alan Shinn


Experience the
beginnings of microscopy.
Make your own replica
of one of Antony van Leeuwenhoek's microscopes.
visit    http://www.mindspring.com/~alshinn/

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2002\01\28@164435 by Eoin Ross

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This is exactly how aircraft measures airspeed.
Older aircraft fed the air directly to a pressure gauge - if I remember rightly it needs to be differential pressure - the reference being normal atmospheric pressure.


>>> joecolquittspamKILLspamCLEAR.NET.NZ 01/27/02 03:28PM >>>
As you're working with moving air anyway, what about
a pitot tube made from a pressure sensor

Wouldn't the air speed at a point on the object imply the
(perpendicular) force. You could measure wind speed
at various point using cheap DC motors as anemometers

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2002\01\28@180647 by Russell McMahon

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> The oldest pressure metering system uses tiny pitots and thin rubber
> tubing with 'U' water manometers mounted on a vertical panel. Add
> watercolor to the H20 and a movable ruler as scale. You can change water
> for something much lighter for higher sensitivity (benzine will work but
> it is flammable).

Manometer sensitivity can also be increased by placing the "readout" arm at
an angle so that the same vertical height is covered by a longer arm length.
This could be measured electronically. Bandwidth/response rate will not be
large.

> Motorola makes pressure gauges that work at the pressures you need, both
> absolute and relative.

Far easier than the above but less visually impressive :-).



       RM

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2002\01\28@182407 by Tony Goetz

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The test section is a 17" cube. It's a suck down tunnel, with the diffuser
opening up to te 27" diameter of the 1/4hp fan we have. We're not entirely
sure how fast it should go, but a rough (very rough) estimate is 40mph. We
finished the diffuser Friday and hooked up the fan. Now we can truthfully say
that our project sucks! We're quite happy with it. Building a pitot tube is
not out of the question, but being sure it's producing accurate outputs is
kind of questionable. I'd be very interested in the designs you've built and
how they work.

I'm a bit uncertain about linkages for measuring the forces acting on the
models, though. We're planning on at least testing a few wing sections, which
are seemingly easy if mounted on a lever protruding out the side of the test
section. Still, it would be nice to have a few complete models to test, and I
do'nt know how good that setup would be for them. Searches online haven't
revealed much regarding how professional wind tunnels mount test articles.
Can you lend any advice in this area?

On a side note, if anyone's interested, I've built up a typical WWI airfoil,
based on an Albatros D.V wing, and will also make sections of general
aviation, airliner, laminar flow, and high speed wings to investigate their
characteristics at low speeds.


-Tony


In a message dated 1/28/02 3:20:52 AM Pacific Standard Time,
.....mike.blakeyKILLspamspam.....BAESYSTEMS.COM writes:

<< What size (diameter) is your tunnel? and what air speed. We have got a few
tunnels
here ;-) so I'm happy to help. We / I have made several Pitot-static probes
and
force sensors for calculating drag etc. >>

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2002\01\28@224335 by jim korman

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Tony Goetz wrote:

{Quote hidden}

<some stuff snipped>

Tony, As far as the pitot tube... Run the pitot through the wing so
that the open end is flush with the "upper surface". Connect it to a
tube with water and slowly bring up your "airspeed". You should see the
water in the measuring tube start to rise as the pressure decreases over
the wing surface. Your measurements in inches of water should be pretty
accurate. Then at constant "airspeed" you can vary the attack angle
of the wing and measure the variance of the pressure over the wing.
The kids should be able to figure the forces based on the mass of
the water lifted in the measuring tube.

Jim

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'[EE]: Force sensors'
2002\02\02@005233 by Tony Goetz
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Sorry it's taken me so long to respond - since Tuesday I've been pretty busy
and haven't had much time for answering e-mail. Still have to put school
first! Everyone here has been quite helpful and I don't want to just ignore
the responses. I've had alot on my mind and it's been busy, but a good kind
of busy. Not that kind of aggrivating-finals-week sort of busy I had last
week.


The responses I've gotten here have been, as always, real helpful. You guys
have posted good ideas and have put me on a path to a workable solution. Not
to mention providing some great websites. I really appreciate all you've done
and will try (and hopefully get around) to putting up a small website with
project updates. Sorry I didn't respond to each posting - there were so many,
I thought it might be better to address them like this instead of filling up
everyone's mail box with the individual replies. Thanks again guys!

-Tony

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