Idea for pull test rig..

[Bob Reeves]

Toying with the idea of building a pull test rig for next season.. Know I am opening my idea up to all kinds of criticism but what the heck I can take it... Bring it on 

With the new 10g rule I thought it would be nice to have something calibrated by airplane weight.. Also would be nice to take at least one human factor out of the equation.. Believe me if someone pulls a bellcrank out or a leadout breaks it doesn't matter what was at fault the guy holding the scale takes the heat. I also didn't want anything with cable or anything that might hang up and make the pull harder than it really should be.

This is what we have gathered up so far.. This idea is to have a sliding weight on the horizontal leg and a handle hook on the upright. A scale calibrated to the weight of the airplane with a pointer attached to the weight.. Just move the weight till the pointer matches the weight of the airplane and pull.. As soon as the weight is lifted your done..

Each one of the 3 chunks of steel in the photo weighs 30 pounds.. Discounting any friction in the hinge and the weight of the horizontal leg.. If the 30 pound weight is the same distance from the hinge point as the handle the pull will be 30 pounds.. Move it closer and it will be less move it farther out and it will be more.

Thinking of using bearings instead of the hinge and suspending the weight using bearings to make it slide easier. It will also obviously have some means to lock the weight into position once it's set. We have 3 pit areas so once the design is finalized I will have to make two more...

So OK fire away....

[Russell Shaffer]

I say to go ahead and build it, but don't spend any more than you have to in labor or money.  Once it's done, you will modify it several times and end up building 1 or 2 more just to get everything right.  After that, you can make some nice ones.  There are always bugs and afterthoughts when designing and building any new contraption. 

[Howard Rush]

How about a device that uses the weight of the airplane to set the pull force?  For example, one could put the plane in a box connected to a pair of levers at right angles.  The arm from which the plane hangs is ten times as long as the upright arm, to which is attached a rope that goes to the pull tester.  Put the plane in the box and set the slider on the pull tester to match the force on the rope.  This is a silly mechanism, but may give an idea to somebody like Derek Moran, who will come up with something really cool. 

I'll post this on SSW, too.

[Zuriel Armstrong]

Bob,

I like the simple idea of a pull tester.  The thign that stands out to me, however, is the numbers on the floor...your shop is too big if you can cut lines on the floor.

[Bob Reeves]

Not really that big, 40 feet.. I loop the lines around a reel fastened to the back wall.. The hard part is keeping the isle down the middle cleared of junk I drag in...

[Rudy Taube]

Hi Bob,

I think your on the right track. 

1. Before you reinvent the wheel you may want to contact Jerry Silver of the Valley Circle Burners in SoCal. I think it is Jerry who designed and built the two almost perfect pull test machines that he lets us use at our big contests. They are just like you described in your post. I am sending you Jerry's email via your mailbox. He may have some photos, or drawings that may be of use to you. Jerry is a great guy and the CD of last weeks great Cal Champs contest!

2. I think the horizontal leg is about 2X the vertical leg. This cuts your weight required in half, a good thing.

3. Your slider idea is perfect. A few weeks ago I made some drawings to modify the ones (Jerry's) we use at contests. Like you, I also want to use a slider for two reasons. One is what you already said, mark the horizontal leg in "model weights in oz" then slide the weight to that point and pull.
      The 2nd reason is that when the pull is made, the horizontal leg is tilted toward the plane. If the slider is smooth enough it will move the weight toward the plane and will LESSON the amount of force, thus preventing ANY chance for an "over pull". ....... At least that is my theory? 
       I plan on using a TRAVERLER CAR (with the weight on it) and TRAVERLER CAR TRACK from the sailing world for the "slider" assembly. This car has excellent bearings and will slide easily, both for adjustment of weight, and to relieve the force. Maybe too easily??? But it can be adjusted to have the right amount of friction. (BTW, there are many cheaper/easier ways to make this slider and I'm sure you will come up with one! :-)

4. The only other mod I would make is to have a RED LIGHT connected to a micro sw under the horizontal leg. This would allow the pilot (who is pulling the plane) to instantly see when the pull is complete. (placing a hood over the tail light may help).

5. I think when you see Jerry's excellent system you will save yourself a lot of work and time by copying his design, with a slight mod for the new weight/pull rules.

6. As someone who pulled TWO line breaks at last weeks contest, (one at 10 lbs. and one at 15 lbs.), I would like to have a system that ONLY has the pilot involved in the actual pull and NOT the pit boss. (we used the old fashioned fish scales to pull, we did not have Jerry's system at the contest) This would require the test unit to be staked down in grass, or fixed somehow on pavement. This would be fast, safe, smooth, and less stress on the pit boss when something breaks! 

Thanks for making your post. It will be nice to see how different people/clubs solve this problem.  We have many smart guys on this list, the other ideas should be awesome!!!! 

Regards,  

[Bob Reeves]

Thanks for the heads up on Jerry, would like to see a photo...

Actually if you think about it the hardest pull even with a fixed weight will be at the point just as it moves. Once the weight is lifted, leverage is in your favor and the pull then becomes less and less as the weight is lifted. My personal feeling is that if you provide a means to lock the weight into position you lessen the chance of the weight moving the other way and making the pull harder than it should be.

Being in electronics my entire working carrier, bells, lights etc. was the first thing I thought of but this would just add more stuff to go wrong or break at the most inopportune time. It would also add a battery which would need to be replaced or maintained.. A spring loaded flag should be fairly easy to design and be low maintenance.

Not familiar with the Traveler car you are talking about will have to goggle it. I am thinking of making a carrier using roller skate bearings which I understand are really good bearings that can be bought cheap. From way back when I was involved in boats anything that has to do with a boat isn't cheap but worth a look.

Yep, $100.00 + for the car and $75.00+ for the track.. Will have to pass, pack of 10 roller skate bearings is $20.00 for good ones or 20 for $20.00 for OK ones...

[john e. holliday]

I don't know if the pul test scales used at Tuscon are the sames as mentioned, but, they work very smooth.  At times you think you are not going to lift the weight then all of a sudden it comes up.   At the NATS one year they did have a light system that the light came on when weight was lifted.  The old bucket method over a pully wheel was/is used in Navy Carrier, but, you have the full weight of the sand/bucket on the lines/control system until you let off pulling.  Have fun,  DOC Holliday


PS:The biggest thing wrong with pull tests is the contestants jerk on the lines instead of a gradual pull.  jeh