UP until the end of World War. II, little was said about. "odd-ball" tube sizes, but with the growing trend of sheet metal fabrication, which is the prime factor in ...
UP until the end of World War II, little was said about "odd-ball" tube sizes, but with the growing trend of sheet metal fabrication, which is the prime factor in today's methods of airframe construction, the demand has increased for great quantities of aircraft tubing and
this has resulted in reduced distributors' tube stocks. If you have purchased a set of amateur aircraft plans and find you have difficulty in pur-
chasing certain size diameter and wall thickness tubing, don't give up the project. For example, if the plans indicate a special tube size of 5/16 in. OD X .028 in. wall and you find
you have trouble in finding a source to purchase this size, then
your alternate recourse to this problem will be as follows: (A) Use 5/16 in. x .035 in.
wall tube - .104#/Ft. (B'- Use % in. x .035 in. wall
could be the start of an expensive scrap heap. To those of you who are not familiar with purchasing aircraft quality seamless tubing, I will describe briefly the general method to evaluate quality tub-
ing, look for the following defects: 1. Thin fins of metal called "laps are folded over the adjacent metal of the tube. They are formed in the piercing mill
operation. 2. "Pits" are small depressions.
They are formed by
rolling grit into the tube surface or by over-pickling the
tube when cleaning scale off preparatory to drawing. 3. "Tears" are ragged openings in the interior or exterior
surface of the tube which are caused by mandrel or die picking up hard or weak spots in the metal during drawing operations. Small "tears" are
grit in the lubricant; also by
er. Make sure he is willing to put in writing material specifications saying the material meets with Government Aircraft Tubing Requirements. If the seller is reluctant to comply with
aircraft quality when he approves your ship prior to cover-
insufficient lubrication.
this request, the buyer should
ing.
tube -M27#/Ft.
referred to as "checks".
Before proceeding further, it
4. "Scratches" are made by rough dies or mandrels, or by
will be necessary that you recheck the plans to see if any brackets or fittings are welded
to these tubing sizes that were changed. If you use alternate (B) it will be necessary that provisions are made on all fittings and brackets to account for change in radii and diameter cut-outs to mate properly in
respect to tube diameter. I would like to suggest to those individuals who are now in the process of designing their own aircraft and those contemplating doing so, to stay clear of "odd-ball" tube sizes. This will lessen the difficulty in locating source and purchase.
As a further suggestion, I would advise not to use tube wall thicknesses of .022 in. and
.028 in. as these particular wall
5. "Sinks" are depressions or collars extending around the inside of the tube caused by a displaced mandrel, which permits drawing the tube to a
smaller inside diameter. 6. "Rings" are transverse corrugations in the wall of tubing
produced by insufficient lub r i c a t i o n a n d subsequent
jumping of the tube during drawing. 7. "Wall - thickness variations" are caused by inaccurate piercing or worn mandrel dies. Government specifications permit a variation of nominal wall thicknesses, (see chart). Aircraft tubing purchased from reliable distributors will meet all rigid requirements.
thicknesses are in very thin wall range, and it is fairly easy to burn through this wall thickIf you purchase tubing adverness during welding, thus destised as surplus or used aircraft troying its initial strength. In tubing, caution should be exerall probability it will have to be cised when dealing with the sellreplaced with a new tube. This
then beware of deals of this nature. Remember .this, the CAA inspector does not bite or taste the tubing you use to see if it's
