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  #21  
Old 04-14-2016, 04:49 PM
f1rocket's Avatar
f1rocket f1rocket is offline
 
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Quote:
Originally Posted by bret View Post
For the side load thing, take a HD plastic trash bag, fold it a few times to get 8-10 layers, and set the tire down on it, the CF of the plastic sliding on its self will allow the gear to flex outward when lowering, I do this trick for taking the pre load off the steering components on big trucks doing a field front end alignment check.
Brilliant!
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  #22  
Old 04-15-2016, 02:41 AM
Yen Yen is offline
 
Join Date: May 2010
Location: Benaraby Queensland. Australia
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I weighed mine with U beaut scales borrowed from SAAA in Australia. Freight was a rip off but otherwise good.
I tried to weigh in the hangar with one door open, that is about 3 metres wide. I couldn't get a stable weight because of wind creating lift.
Closed the door and I got 3 identical weighings.
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  #23  
Old 04-15-2016, 08:37 AM
Pukauma Pukauma is offline
 
Join Date: Jul 2014
Location: West Covina, Ca
Posts: 52
Default Digital scale

This is what I have been thinking......

http://www.ebay.com/itm/660lbs-LCD-A...4AAOSwA4dWGVSG

660 lb. capacity for $54. Should do the trick?
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Cliff Langlois
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  #24  
Old 04-15-2016, 01:12 PM
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ssmdive ssmdive is offline
 
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Quote:
Originally Posted by f1rocket View Post
The total weight should be the same but the center of gravity will be aft of the proper location. You want to know the CG when the aircraft is in level flight. The CG is the point around which the airplane will pitch in flight and knowing it exactly is critical to keeping the airplane within the flight envelope. This is very important stuff and should be performed very carefully and according to accepted methods.
I know the importance of CG, was a loadmaster years ago.

But maybe I didn't explain my question well enough... I'll try again.

So Arm X Weight=Moment and CG envelope is based on these calculations.
So let's just use the tailwheel weight since it has the most arm (~240).

I level the plane and the TW weight is ~50#. And the mains are about 430# each giving us 910# total weight and a TW moment of 12,000.

So we weigh it in a three point attitude and we get:
Tail 70# and 420 each for the mains.... The TW moment would READ 16,800 but the plane is STILL in CG.... Dead center CG as a matter of fact. The total AC weight would still be 910, we would just need to shift the CG numbers on paper. So dead center is now 16,800 instead of 12,000.

I think I know the answer... And that it is gear differences could change the numbers.
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  #25  
Old 04-15-2016, 01:52 PM
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gyoung gyoung is offline
 
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Location: Spring, TX
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In the 3 point attitude the arms are not the same as when level. You've changed the geometry so the distance from the datum to each wheel contact point is shorter.
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  #26  
Old 04-15-2016, 01:57 PM
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Snowflake Snowflake is offline
 
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Location: Victoria, BC, Canada
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Quote:
Originally Posted by ssmdive View Post
So just a question..... Why have to level the plane?
The short answer is that you need to turn the measurement into a linear measurement parallel to the canopy rail... To do that, you'd be doing a lot of trigonometry to work out you CG location, and every time you make one of those calculations you risk making an error. You also need to know your exact mechanical dimensions more accurately, as small deviations from the "design ideal" could be magnified by each calculation.

My procedure for measuring tailwheels, which I have used on a large number of tailwheel aircraft including my own:

1. Weigh the entire plane in 3-point attitude. This is your empty weight, and is independent of attitude.
2. Raise the tail until the plane is level, and read the weights on the two main gear. Don't bother trying to weigh the tail.
3. Add the two main gear weights from (2), and subtract that total from the empty weight (from 1). This gives you the weight on the tail when level.

You now have the weights on the mains, the weight on the tail, and the empty weight. My personal opinion is that using the "book values" for the locations of your main gear and tailwheel will be just fine, provided you have built the plane nominally to the plans. Even if you've used different-sized wheels, the axles will still be in the same places when the plane is level.
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  #27  
Old 04-15-2016, 03:13 PM
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ssmdive ssmdive is offline
 
Join Date: Jul 2012
Location: FLL
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Quote:
Originally Posted by Snowflake View Post
The short answer is that you need to turn the measurement into a linear measurement parallel to the canopy rail... To do that, you'd be doing a lot of trigonometry to work out you CG location, and every time you make one of those calculations you risk making an error. You also need to know your exact mechanical dimensions more accurately, as small deviations from the "design ideal" could be magnified by each calculation.

My procedure for measuring tailwheels, which I have used on a large number of tailwheel aircraft including my own:

1. Weigh the entire plane in 3-point attitude. This is your empty weight, and is independent of attitude.
2. Raise the tail until the plane is level, and read the weights on the two main gear. Don't bother trying to weigh the tail.
3. Add the two main gear weights from (2), and subtract that total from the empty weight (from 1). This gives you the weight on the tail when level.

You now have the weights on the mains, the weight on the tail, and the empty weight. My personal opinion is that using the "book values" for the locations of your main gear and tailwheel will be just fine, provided you have built the plane nominally to the plans. Even if you've used different-sized wheels, the axles will still be in the same places when the plane is level.
Great answer.

Thanks.
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  #28  
Old 04-15-2016, 06:38 PM
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John C John C is offline
 
Join Date: Mar 2005
Location: Moundridge, KS
Posts: 149
Default Vertical CG?

You would need to know to Vertical CG to make the trig work. We normally work the other way, three swings to get the vertical CG. Nose up, level (for the CG) then nose down. We needed the vertical CG to put an off center JATO bottle on a target drone.

For a tail dragger:
1. find the CG at the surface of a level airplane (this is what we normally do) and draw a line through the CG, perpendicular to the ground.

2. Draw a line from the mains to the tail for a new reference line. Now determine the CG along the new line with the tail on the ground. Draw a line through the CG, perpendicular to the (new) ground reference line.

The two perpendicular lines will intersect at the airplanes vertical CG.
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