|Myth 2 - Composites are lighter than Aluminum.|
It is certainly true that a testing lab can take coupons of heat treated aircraft Aluminum alloy and an equal sized coupon of composite material and test to failure.
The composites are likely to win by a large margin. This is the promise that fuels interest in composite aircraft.
But as Stephan Wilkinson wrote in the April 1995 issue of Aviation Consumer:
You can reduce weight and you can reduce cost, but neither in 1968 nor in 1995 is it possible to do both simultaneously with composites.
Only if a company is willing to invest in the very expensive processes like Beech to machine wrap graphite ribbon to fabricate the Premier or Cessna to Resin Transfer Mold horizontal tails, can light weight and strong designs can be produced.
But to use the layup methods employed by Cirrus or the new startup, Adams Aircraft, the structural weight of these airframes will exceed those of Aluminum aircraft.
Whatever method is used, several things erode the composite advantage. Door hinges, wing-to-fuselage attach, control surface attach, control surfaces, bellcranks, pulleys, engine mounts all have stress concentrations that require steel fasteners and bushed holes.
There are several ways to deal with these highly loaded areas, but all add weight and add to the cost of building composite aircraft.
Paul Whetstones web site addresses some additional issues that increase weight and cost to composite aircraft such as:
Adding lightning strike protection.
Requirement for a ground buss to all electrical components. Not required in sheet metal aircraft since the metal structure is the ground buss.
YOU BE THE JUDGE!
The ratio of empty to gross weight is one of the most telling measures of structural efficiency.
The equations are basic:
EMPTY WEIGHT + PAYLOAD = GROSS WEIGHT
Reduce empty weight by 100 lbs and the pilot can load an extra 100 lbs of payload, fuel/ baggage/ people.
EMPTY WEIGHT/GROSS WEIGHT = WEIGHT EFFICIENCY RATIO
The lower the ratio, the more efficient the design.
There is more included in the empty weight number than just structure. Engines, furnishings, systems etc. are also a part. To keep the weight under control, project management must select components as light as possible, and design a structure as light as possible.
Weight is like aircraft drag. A good designer can minimize but not eliminate drag nor some finite empty weight. An inept design that squanders either, results in an airplane with no value in the marketplace.
There are two aircraft certified with composite fuselages and conventional Aluminum wings. Let's see how their numbers compare with comparable Aluminum aircraft. The numbers are from Aviation Week's 2004 directory for turbofan aircraft:
DO COMPOSITE AIRCRAFT SHOW ANY SIGNIFICANT WEIGHT ADVANTAGE OVER ALUMINUM AIRCRAFT?