flydjd

flydjd

Well Known Member
Hi all,

1st the disclaimer - I must have been in Homer's Maths class at school - if he was around about 35 years ago !!!!!

I am just starting to bend the fuel vent tubing and need some help with the maths involved - although I expect (know) I will be ordering extra tubing in the near future I would like to minimise the trial and error method.

For example with the 2 x 90? opposite bends on the 7 where they exit the fuselage floor I made the 1st bend out from the fuselage wall and then measured the distance from the wall to overhead the fitting on the floor and subtracted 1" for the bend as a start point for the 2nd 90? - DOH ! it came out about 1/4" short !
I don't even want to think about other bend angles

Help please
 
Can't really help with the math, but whenever possible I made mockups for the tubing out of coathanger wire.
 
Some easy rules...

Buy some extra tube and play with it....:)

But, as a starting point, these rules might help...

Calculating Lengths - To determine required lengths of a tube before bending, the lengths of the straight section lengths are added to curved section lengths, to arrive at proper allowance for bends.

Follow these rules to determine curved section lengths: (For these calculations, the radius is measured to the center of the tube):Rule 1: to find length of a 90? bend, multiply the radius of the bend by 1.57.

Rule 2: to find the length of a 180? bend or U bend, multiply the radius of the bend by 3.14.

Rule 3: to find lengths of sections having curvatures of other than 90? and 180?; multiply the bend radius by the included angle, in degrees, then multiply the product by the constant 0.0175. The results length of the curved section.

Stolen from... http://www.precisiontube.com/html/design_tips.html

It's a common problem for electricians laying out conduit... but conduit benders are nicely marked to allow for the "loss" of length you experienced...:)

gil in Tucson
 
use bulkhead fittings

after scrapping about 20ft of tubing trying to get the bends right I followed other builders tips and put bulkhead fittings in line and used banjo fittings to the andair fuel valve. Made life a lot easier and a lot less frustrating ;)
 
I like the coat hanger idea . I made up mock ups from the pieces of tubing that I pitched across the shop after they didn't fit ( there was a lot of it to work with ) . The hardest for me was the piece that goes from the selector valve to the R wing on the -8. These bends are a lot easier to do in sections ,ducktape them together then do the bending on the bench using the mock up as a template . Soft tubing is cheep . Have Fun
 
If you want to calculate the total distance, you add the straight lengths with the arc lengths of each bend to get the total distance.

The arc lengths are calculated by multiplying the bend radius by the angle in radians. For a 90 degree bend, that is PI/2 or 1.57 times the bend radius. If your bend radius is 1/2 of an inch, then the arc length would be 0.785 inches.
 
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