The Family of Aircraft
Alloy -A mixture of metallic elements.
Anneal -To soften by heating, followed by either air or water quench.
Quench -To cool by rapid immersion in a solution, or cooling in still air.
Planish -Smoothing by light hammering or rolling.
Work Harden -To increase the hardness of a metal by hammering, rolling or forming. Most metals work harden,
such as gold, silver, aluminum and steel, but lead does not.
from Sport Aviation, September 1999
by Kent White
Aircraft... the word suggests various images of brightly painted fabric, varnished wood, black tubing, or a
fuselage twinkling across a grass strip. But say aluminum aircraft and rows of rivets come to mind, and
a smooth skin beckoning to be stroked. For the builder, however, those simple words may invoke images of
drilling holes and pounding rivets, or long hours spent forming that cowling to get the perfect fit.
Regarded today as simultaneously cheap and expensive, it has been 150 years since the metal was regarded
as precious enough for Napoleon to have a tea service made of it, knowing full
well that platinum was cheaper. Aluminum for aircraft use is light in weight, sometimes high in strength, and
offers both good capabilities and severe limitations for forming, straightening, and joining.
The Spartan Executive...or 2024 never
looked so good!
What are the aircraft aluminums? How are they different from each other? What are their limitations and
capabilities? In its pure elemental form, aluminum is a very usable metal. Let's take a look at how its
application changes as it is alloyed with other metals.
The aluminum alloy designation system was changed in October 1953
from the old two digit system to the present day four digit system. The old two digit number will be included
here in parentheses next to the current four digit for reference. To begin, let's divide the family into
two categories: the non-heat treatable and the heat treatable alloys. Some will be weldable, some will
Note: Be mindful that the rule of workability dictates that as strength (hardness and temper) increases,
Non Heat-Treatable Alloys
These alloys can only be strengthened by strain or work hardening applied to the sheet. Either rolling or
pulling the sheet includes the necessary hardness. This wrought hardness is indicated by an H, marked on
the sheet, or found in the accompanying paperwork. Remember, you can only harden it by Hammering (or by
otherwise working it in some fashion). Although there are numerical designations for hardness, H14, H18,
H32, H34, etc., most of us metal folk refer to the hardness as quarter-hard, half-hard, three-quarters,
and full, and know that friendly suppliers will follow suit. S-O means full soft, or no temper at all.
1100-(2S) is referred to as an alloy, although technically it is the element itself in its pure (99% or
greater) form. 1100 is quite simply the most shapeable alloy. It works readily into very complex parts,
and its low strength (5KSI-22KSI yield) and excellent weldability make it ideal for non-structural parts
like tubing, tanks, gear-leg fairings, strut fairings, cowl bumps, scoops, and welding rod.
This P-38 Lightning has many ducts made from press formed 3003, and the individual sections are then gas
welded to make a complete unit. (Owner: Erickson, Tillamook Air Museum.)