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Thrust Measuring Scale

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Old 11-29-2002, 10:17 AM
  #1
TommyWatson
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Default Thrust Measuring Scale

Does anyone know where I can get a small scale to measure thrust. I have made a sliding test bench using drawer runners, and now I need a scale to measure up to about 20 pounds. I can't seem to find any here in Australia.
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Old 11-29-2002, 10:30 AM
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Default Thrust Measuring Scale

Not sure how accurate you're trying for... most of the time I'd use something like
that merely for comparing differences between props or mufflers, myself.

They don't sell fishing scales down under? If not, it should be trivial to order one online
Like this one , if you want digital, for about $25.
Or this one , if you don't need digital, for $5 or so.
(Just a couple I ran across on a quick search)
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Old 11-29-2002, 09:53 PM
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Default Thrust Measuring Scale

Ok,
I do have the first one (the digital fish scale).

I have it tied to the turbine mount, around a pully then on to the digital scale. The wire pulls on the fish scale.

However all of the comercial test beds I have seen, have a scale that mounts on the front of the mount and the thrust "pushes" on the scale.

While the setup I have works, and will give me a relative reading, I wonder about its accuracy. Are there losses in the pully system?.

So hence the search for a scale that is small enough to mount on the front.

Of course all of this is academic. The proof of the pudding is in the flying. I just want to satisfy my curiosity.

Thanks

Tom Watson
Sydney Australia
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Old 11-30-2002, 12:32 AM
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JimTrainor
 
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Default Thrust Measuring Scale

Why not just calibrate it with some known weights.

One liter of water weighs one kilogram. That's a start, if you don't
have other known weights around.
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Old 11-30-2002, 03:07 AM
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Default Thrust Measuring Scale

What thrust are you showing for your MW54 ?
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Old 11-30-2002, 03:44 AM
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TommyWatson
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Default Thrust Measuring Scale

With the two piece cone, I show just 10 pounds, This is the newer engine that has only had about 10 minutes running and has not been ballanced by wren.
The other engine with the one piece cone I get 11 pounds. I am still convinced that there are losses in the pully system.

I havn't tried to swap the exhaust cones and see if it is the engine or the cone.
Both of my engines start and run beautifully. Both run cool <550 degrees.
I couldnt be happier with the wren.
I cant wait till the Total fire bans here in Sydney are over so I can at last fly the thing.


Tom Watson

Sydney Australia
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Old 11-30-2002, 04:00 AM
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Default Thrust Measuring Scale

Looking at your set-up, if the slides are free and the table is level I would say that you don't have much in the way of losses. My MW54 only puts out a little more than 10 lbs with the 2 piece cone. I just ordered a spare cone pre squeezed to 51mm. I haven't tried it out yet . The weather is lousy !
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Old 11-30-2002, 04:11 AM
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Default Thrust Measuring Scale

What sort of thrust measurment device do you use??
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Old 11-30-2002, 04:25 AM
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Default Thrust Measuring Scale

A digital fish scale as well. I calibrated it with known weights and it was bang on. I have done my test stand a little differently and get a straight pull although I don't think you are incurring losses with your single pulley.
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Old 11-30-2002, 04:42 AM
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Default Hi Tom

As I build and sell Turbine Test Benches, I agree with what all the gentlemen above had stated to you. You aren't going to notice much of a difference in measurements unless your down to splitting hairs and are looking for a 3 decimal place value - which, of course, you are not. If your concerned about how accurate your scale is, it's best to get an item with a specific weight value and test your scale against that. I have an expensive Load Cell with Digital read out that goes to 3 decimal places on my Mobil Turbine Test Cell. The digital like you have and the analog scales that others use - including those used on the Test Benches I sell, are close enough in accuracy and are a real bargain for the price as the unit on my Mobil Turbine Test Cell cost me well over $1,000. Will be glad to help you out any way I can. Basically, there really isn't anything wrong with your set up, Tom.

Todd J. Walters
GT-MAXX Inc.
E-mail: gtmaxx@sky-access.com
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Old 11-30-2002, 04:52 AM
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Default Thrust Measuring Scale

Thanks for the replys,

I guess my question, and it had been a subject of much discussion between myself and a few friends, is there any difference between the thrust being measured by pulling the fish scale or by pushing a normal scale. Logic says there is no difference, but I I don't know!!!.

Tom Watson
Sydney Australia
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Old 11-30-2002, 04:58 AM
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Default Thrust Measuring Scale

My vote is no !
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Old 11-30-2002, 05:15 AM
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Default Tom - there's no difference.

Here's a couple more pics for you to just help put your mind more at ease, mate. Here's the setup that I told you about. The one picture is of the Linear Bearing Block with Precision Rail. In the other picture, you can see the Load Cell. If you look just on the other side of the Turbine you can see a bit of the White Case - that's the digital readout. The reason I'm telling and showing you all of this is - Push, Pull - it all comes out the same. Your setup is fine and dandy, unless you want to do what I did and shell out well over a grand. With what I'm showing you here and what you have - you wouldn't notice that much of a difference to even worry about unless your scale is off. Just get something that has a set and proven weight value, and test your scale, that's how I check my scales on the little Turbine Test Benches I sell that I posted the first picture in. For the "Beastie" on the Mobile Turbine Test Cell - different story, I take it in to be calibrated.

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Old 11-30-2002, 05:18 AM
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Default 2nd picture

Here's the 2nd picture with the setup on the Mobile Turbine Test Cell.
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Old 11-30-2002, 05:25 AM
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Default Mobile Turbine Test Cell

.
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Old 12-12-2002, 05:36 AM
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Default Thrust Measuring Scale

Wow...you don't need a $1,000 piece of equipment to measure your thrust. Any type of horizonal measuring devise is going to add resistance to your total thrust. Here's an easy way to measure your thurst...

Simply suspend your aircraft from a basic digital scale. (http://www.tackletour.com/reviewberkleyscale.html)

Note the total weight of your aircraft. Once you've suspended your aircraft...run your engine while it is suspended from the digital scale. Measure your thrust. (Total scale reading while engine is running at selected speed - Aircraft weight before starting your engine = total thrust. Thats it!

**You may also build a simple "engine mount" and suspend it from your scale instead of attaching an entire aircraft to the scale...but obviously you need to secure your aircraft with some sort of devise so it doesn't try and fly around the room. Take into account the weight of these securing devises as well.
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Old 12-12-2002, 06:53 AM
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Default Thrust

Quote:
Originally posted by flyrc
(Total scale reading while engine is running at selected speed - Aircraft weight before starting your engine = total thrust. Thats it!
It's actually a vector sum, not a simple addition.

For example:

if the weight is 1 pound, and the thrust is 1 pound, the scale will read 1.414 pounds (the square root of 2 since in this case a right triangle will be formed, a simple case)

If you read the scale and simply subtracted the weight you would think the thrust is .414 pound, when it actually is 1 pound.

Otherwise it works fine


Mark
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Old 12-12-2002, 11:51 AM
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Default Thrust Measurement

I use a digital fish scale. However, they should be calibrated using a known weight. Mine was off by 11%, not a trivial error.
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Old 12-12-2002, 03:19 PM
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Default Thrust Measuring Scale

It's a vector sum? come again?!?! The weight and thrust are in the same direction... down. What am I missing?
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Old 12-12-2002, 03:46 PM
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Default thrust

I did not think he was talking about holding the plane nose down and blowing the exhaust up into the guage and the mounting fixture?

I though he meant to hold it and let it swing like a trapeze.

Come to think of it, with a trapeze set up I don't even think you need the scale, you can figure the thrust from the deflection and the weight.
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Old 12-12-2002, 03:54 PM
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Default Thrust Measuring Scale

I see.

That's a thought though... just swing a big wieght, measure the angle, do your sines and cosines, and voila you have your thrust.... no fishing scale required. Just have to make sure the weight is larger than the hight thrust you expect.

I'd worry about frying the scale using the hang-em straight down from from the scale approach.
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Old 12-12-2002, 05:24 PM
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Default Details on Suspension scale method

Ok guys...here is the secret....

Yes...you "hang" the aircraft/engine from the scale device. Exhaust is deflected by a simple hose fitting.

1. Using a metal shelf (mine is mounted about 6ft from the floor, and extends from the wall approx 2 feet), suspend the digital scale from the metal shelf.

2. There are (2) metal rods attached to the shelf as well that extend from the shelf to the floor. (I simply bolted the to the shelving. (These "rods" are actually 1in metal pipe.)

3. I attach my aircraft to a "mounting board" via bungee cords. The mounting board has (4) 0-rings attached to it which slide up and down the two metal rods. (The best example of this technique is a rowing machine. As you set on a rowing machine the seat slides forward and backwards.) This secures the aircraft during the thrust measuring process.

4. The digital scale is attached to this "mounting board" via a cable. (a small amount of weight was added to the mounting board to arrive at exactly 10lbs. once suspended from the scale.

Thats it! No complicated methods of calculating thrust. This is a pull-pull system with a single force axis. The grand total for this thrust measuring system came to a whopping $68.
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Old 12-12-2002, 05:33 PM
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Default Thrust Measuring Scale

What's complicated about a cosine?

My local grocery store sells scientific calculators at the check out for $10.

I'm sure I can do it for $15... including the calculator.
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Old 12-12-2002, 05:50 PM
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Default Thanks for the tip on buying a calculator...the rest of us were discussing thrust

This device serves multiple purposes. Not only thust...but total aircraft weight. (Sorry...a calculator won't help you with that, you do need a scale to accomplish this)

Anyway...it was my wish to share alternative ways for measuring thrust...this particular method has proven to be the most accurate method I have found as of yet. But I'm always looking for great ideas. Thanks again to all.
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Old 01-31-2003, 06:59 PM
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Default Thrust Measuring Scale

Tom.

Quote:
Are there losses in the pully system?. .....I am still convinced that there are losses in the pully system...... is there any difference between the thrust being measured by pulling the fish scale or by pushing a normal scale. Logic says there is no difference, but I I don't know!!!.

Blocks are by design and use tools to take loads off of lines. The losses in the blocks bearings will be minimal , limited rotation, and I would expect very little in your drawer rollers as well.
The difference on an angular pull is taken in the block. (Call up one of those famous Aussie sailors and he can tell you all about it!!)
Simply put the pull thru a block at an angle will be less than a direct line pull; how much is determined by angle change of the line. Check out site below:

Block loading at various angle charted and examples

http://www.thecrosbygroup.com/Crosby/body_229.htm


This gives the basic ideas but the examples given are for the load TAKEN BY THE BLOCK AT VARIOUS DEFLEXION ANGLES OF THE LINE. If the load is TAKEN by the block > It needs to be added to the reading on the scale.
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