Pull Test Equipment

Does anyone know where I can get plans for a pull testor that uses a weight to test tension (like used at VSC)?
Chris

Chris,

It shouldn't be very hard to lay out the proportions yourself. Check the AMA pull test chart for starters.

I'm not absolutely certain of the 'ratio' on ours in Tucson. Three-to-one seems about right, though.

What we made is an "L" shape, laid on its back. If the handle hooks on 12" above the hinge, and pull required is 30 pounds, a 10# weight 36" from the hinge matches the required torque when the long leg JUST lifts off the base. (30# x 1' = 10# x 3' -- torques match.)

The upright is braced to the horizontal piece with a diagonal -- plywood or whatever. The "L" is hinged onto a base board or plate.

Bob Emmett worked out the dimensions so that the long leg, with no weights added, gave us the pull spec for 1/2As. For other displacements, pegs that fit barbell weights are put on the "L"s long leg where they match the AMA specs. The pegs are marked to remind the pull tester how many weights go where for which displacement.

Stake or weight the base below the pivoting "L" -- .60 ships need a BUNCH of pull.

Any help?

Could someone please make a drawing of the pull tester? DOC

A bucket on a pully will also work. Fill a couple of buckets with sand to the required weight. Store them for the next contest, or throw them on the parking lot when done. (Of course, everyone continues to pull test before each flight, even when there is not a contest, right?)

>A bucket on a pully will also work. Fill a couple of
>buckets with sand to the required weight. Store them for
>the next contest, or throw them on the parking lot when
>done. (Of course, everyone continues to pull test before
>each flight, even when there is not a contest, right?)

Emmitt's machine is, I think, superior to a bucket-raise job - because the pull falls off naturally once the max pull has been reached. It's also much more compact.

I have one of those fish weighing scales in my truck..., the spring kind, with a piece of 1/8" landing gear wire bent to adapt to a handle. Small and easy to use.....and for giggles I checked it against the Nats stuff....it was right there as far as accuracy.

The wheeled contraptions are very easliy built/set up wrong, and can cause damage.

See Ya

Just to show it can be done, no matter how revolting and against my better judgment, I am in agreement with Peabody.

We have a Gary Letsinger-designed and -produced pull-test machine which is really cool. Until we neglect to set up our pits such that model/line sets are taken to the machine instead of the other way 'round.

For most of our needs what is generally referred to as a fish scale is a pretty good option.

As to pull-testing models while out practice flying, having had a line break immediately prior to second-round flight in PA at 2004 NW Regionals, I do it all the time. There's a suitable scale in my tool box if and when anyone wants to use it.

I'm even thinking of forcing Rush to use it, assuming installation of the second set of leadouts in his Impact done to same standards as first set.

(Can you imagine how disconcerting it must have been to feel neutral setting changing during a flight, this having been Howard's first clue something might be amiss?!)

End of Dirt-to-Peabody agreements...

There is a fundamental flaw in this type of pull test machine. It is true that the static force at the handle will be the weight times the ratio of the arm lengths, but to get the weight to rise you actually have to accelerate it. This is where the problem is. The moment of inertia goes by the square of the length, so for an arm length ratio of 3:1 it will take 9 times as much force (in excess of the set weight) to get the weight to lift as it would if the arm was 1:1.
This is why you have to pull very-very-very slowly.
The machine they had at Muncie for F2D has a ratio of about 10:1. You had to be extremely careful or you could snap your lines very easily.
All things considered I prefer the spring scale.
Pat MacKenzie
P.S. BTW,I encountered this design flaw in an industrial application.
A company had made a testing machine for wheels and casters. In order to make it easier to load and unload the large weights they used a pivoting arm. The caster was placed at 25% and the weight at the end, so they thought they would only have to use 1/4 of the usual weight. The wheel of the castor had a steel wheel running under it with various "bumps" welded to it. Every wheel they tested exploded in no time, at much less than the expected value. It took me a while to figure out why, and even longer to convince the "brass" that they hadn't taken all of the physics into account.

Let me remind you of a simple problem in trigometry.

The pull test equipment is usually close to the ground; probably not more than 6" from the ground. If you hold your model to do the "pull", it is your best interest to kneel down to 6" from the ground. Otherwise, you are pulling more than necessary! The sine/cosine of the angle between your handle and your model means that you are pulling more if you stand to do the pull test!

It is'nt much, but why put yoursself at a disadvantage?

Pat,

Good point! However, I can't recall anyone trying to 'ring the bell' like the circus midway hammer thing. For stunters, particularly, most guys ease up to the pull needed, and the pull 'official' will immediately signal the test passed when the long leg DOES lift off.

In short, I've never seen 'rate' enter -- just very gradual vernier incrementing until pass or line break. Good to keep rate and associated acceleration in mind, though.

Thanks.

Paul Smith

I made one of these years ago with a 33-pound steel weight, a 2-step wood step-ladder and a ball-bearing pulley. Two angle clips on the under side of the top step supported a bolt-axle for the pulley. Obviously, also a piece of steel cable from the weight to the handle-hook.

To eliminate the complaint of "acceleration", we would have an assistant lift the weight off the ground and the contest lowered it via his airplane, lines, and handle.

Contestants hated it. Too honest. Prefered to go easy with fish scale.

I have often been frustrated by the pull test. This is when the PTI (pull Test Individual) hooks a spring fish scale to your handle and hangs on to the other end of the scale. As I begin to pull the 40 lbs or more, the PTI naturally is pulled forward, negating my effort. To compensate, the PTI leans backward. The result: I pull about 150% of the required tension!

The best equipment I have seen is a simple dead weight and cable to a pulley about 12" above the base. There is no "acceleration" problem, and the pull is exactly equal to the weight. There is a problem with the pilot seeing 60'or so down-line to determine if the weight is actually off the ground. I have seen modifications of this design with a Micro-Switch and a light. As soon as the weight is lifted, the Micro-Switch turns on a light, seen by the "puller". That is complex for most contests, but it works.
Dr. Spark

I've got the Choppers' pull tester plans and will make a copy for you. Tried to scan them just now to no avail, apparantly didn't reinstall something correctly after my last round of computer chemo getting rid of cyber cancer (which worked really well by the way, it's still in remission).

I'll be gone all weekend but will try to check email ([email protected]) on Sun night for your address and try to remember to get it in the mail Mon morning.

Thank you Mike! I'll look forward to getting the plan.
Chris

>Thank you Mike! I'll look forward to getting the plan.
>Chris

Engineering drawing for pull tester is attached.

reference: A few Good Men, starring Jack Nicholson:

"the pull test? THE PULL TEST ??? You can't handle the pull test !!"

They had a 2:1 or 3:1 machine at the Cleveland stunt contest. It was not too bad.
The one at the F2D W/C felt like you had the handle tied to a brick wall. It was more like 10:1. Any other time I have pull tested an F2D model at a W/C it has always been a non-event. This was very different.
Pat MacKenzie

I can re create a drawing of a pull tester using a simple
"A" frame and pulley. This design uses an ordinary flashlight of 2 or 3 cell with modifications wih a Micro-Switch to tell the "puller" when the weight has been lifted.

If anyone is interested, I will send a simple diagram.

>
>For most of our needs what is generally referred to as a
>fish scale is a pretty good option.
>
As long as you are not extending the spring beyond the "linear regime"...which fishing scale designers are supposed to take into account for you anyway...then good old Hooke's law rules. Being the naive physicist that I am, I just assumed that everybody carried a cheap fishing scale in their toolbox. Good for weighing fish too...

{aside--whispered behind hand--great post Mr. Dirt!}

Andy ( F= -K*X ) T

With a weight-lift pull-test it is obvious to everyone that an honest test has been conducted. Then only way to OVER-pull is if the weight Bottoms-out against the pulley.

With a fish-scale there has been suspecion that maybe the official is going "easy" on contestants, or maybe over-testing them.

This is my favorite fishing scale, it works well at least 100 years ... and it goes to 100 kilo, so you can make pull test on 4 models at once