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Airworthiness

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

  • 4703.[1] This identifies a series of essential requirements (ERs) broken down into seven main headings:
    • ER.1 System integrity. System integrity must be assured for all anticipated flight conditions and ground operations for the operational life of the unmanned air system (UAS). Compliance with all requirements must be shown by assessment or analysis, supported, where necessary, by tests;
    • ER.1.1 Structures and materials. The integrity of the structure must be ensured throughout, and by a defined margin beyond, the operational envelope for the UA, including its propulsion system, and maintained for the operational life of the UA;
    • ER.1.2 Propulsion. The integrity of the propulsion system (i.e., engine and, where appropriate, propeller) must be demonstrated throughout, and by a defined margin beyond, the operational envelope of the propulsion system and must be maintained for the operational life of the propulsion system;
    • • ER.1.3 Systems and equipment;
    • • ER.1.4 Continued airworthiness of the UAS;
    • • ER.2 Airworthiness aspects of system operation;
    • • ER.3 Organizations.

Those familiar with the Federal Aviation Authority requirements[2] or the European Aviation Safety Authority certification specifications[3] 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:

  • 1. Sufficient flight stability and control authority to carry out desired maneuvers (static margin in pitch is a key factor here);
  • 2. Landing speed low enough to enable repeated damage-free landings on the available airstrips;
  • 3. Suitably reliable propulsion system with sufficient installed power and thrust to be able to take off and maneuver in likely adverse conditions;
  • 4. Suitable fuel tanks can be fitted that will safely contain the required amount of fuel and supply it in a controlled and reliable manner to the engine (while not upsetting center-of-gravity (CoG) requirements);
  • 5. Structurally sound airframe able to withstand likely gust and maneuver loads and to avoid aeroelastic problems such as flutter, over the life of the aircraft;
  • 6. Avionics capable of maintaining safe command and control during missions (sufficient radio range, autopilot capability, etc.);
  • 7. Sufficient on-board battery life or generator capacity to meet mission endurance achievable from fully fueled aircraft;
  • 8. Repeatable manufacturing processes making use of suitable materials that make due allowance for fatigue, corrosion, contact with fuels, maintenance, repair, and so on;
  • 9. Construction and assembly standards robust enough to ensure reliable operation given user capabilities;
  • 10. The possibility of the airframe to be sufficiently broken down for ground transport and storage.

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.

  • [1] NATO Standard AEP-83, Light Unmanned Aircraft Systems Airworthiness Requirements: https://www.gov.uk/ government/uploads/system/uploads/attachment_data/file/391827/20140916- STANAG- 4703_AEP- 83_A_1_.pdf.
  • [2] Such as FAR-23 Small Airplanes Regulations, Policies & Guidance, Part 23, Airworthiness Standards: Normal,Utility, Acrobatic, and Commuter Category Airplanes.
  • [3] Such as CS-23 for Normal, Utility, Aerobatic and Commuter Aeroplanes.
 
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