Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach
While it is natural for the users to focus on their mission goals, it will go without saying that they also expect a serviceable and reliable aircraft to be built. This will involve satisfying a whole raft of constraints that the designer must be aware of even if they do not at first occur to the eventual user. For example, it is no good producing a really low-drag design that on landing needs a very long runway because no brakes have been fitted or a very smooth surface because the landing speed is high, yet the user only has access to much less ideal facilities - the user simply assumes the aircraft must be able to land safely and without damage. The general requirement that an aircraft be fit for flight is normally termed “airworthiness.” There are various guides to establishing the airworthiness of new aircraft; perhaps the most relevant to the aircraft considered in this book is the NATO document STANAG
Those familiar with the Federal Aviation Authority requirements or the European Aviation Safety Authority certification specifications will recognize significant chunks of this document. The basic aim is to set out an “acceptable means of compliance” by which a new aircraft may be deemed airworthy. This will be made up of “detailed arguments” and “means of evidence.” STANAG 4703 gives considerable detail in a series of tables for each of the seven areas it covers.
For example, Essential Requirement ER.1.1.2 says: “The UA must be free from any aeroservo-elastic instability and excessive vibration.” The detailed arguments required for this are set out as
Aeroservoelastic effects - A rational compelling set of arguments must be provided to the satisfaction of the Certifying Authority, in order to show that the UA is free from flutter, control reversal, and divergence in all configurations. A margin >1.22 VD should be applied. Simplified analytical or computational conservative methods may be used. Though specific flutter flight tests with appropriate excitation are not mandatory, flight tests survey should not reveal excessive airframe vibrations, flutter, or control divergence at any speed within the design usage spectrum as per UL.0.
while the acceptable means of evidence are listed as “A combination of assumptions, tests and analyses.”
Typical constraints that must be checked in the earliest stages of design include the following:
These constraints arise mainly from a failure mode analysis of the aircraft, essentially a list of the things that might go wrong if sufficient design care is not taken.
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