• 20 Mar 2020 10:56 PM
• 0

Glossary

Definitions

• Analysis: Analysis of parts is done with approximations of the design as well. Whether computational structural dynamics (CSD), computational fluid dynamics (CFD) or computational electromagnetics (CEM), the surfaces and solids are gridded into finite elements to represent the design. Ideally, these include high-density meshes when the geometry is complex or attributes (e.g., loads, turbulence or scattering) are intense to capture the important details that drive the relevant performance. Engineers hope to finish the analysis or test in time to affect the relevant decisions with a design that fits with all the other changes. But the analysis only explores the phenomena and interactions the engineers know and try to model, so they generate as-analyzed performance to predict that of “The Design,” with confidence that comes from correlation with testing.

• A Standard Airworthiness Certification is the goal for aircraft intended to operate broadly because it allows minimal flying restrictions within the national airspace system (NAS). Earning this involves first obtaining a Type Certificate (TC), which denotes full compliance with publicly vetted design requirements, such as the established 14 Code of Federal Regulations (CFR) Part 23 airplane or Part 27 rotorcraft rule sets. Retaining validity requires maintenance that complies with the applicable rules [CFR Parts 21, 43 and 91]. Issued per tail number, an airworthiness certificate can be revoked if the aircraft deviates from the approved design or if regulators consider it not airworthy. Most aircraft have a standard airworthiness certificate issued within a specific operational category (e.g., normal, utility, acrobatic, commuter, transport and manned free balloons). Other, Special Class aircraft like VTOL aircraft are outside the established rule sets but can use an equivalent process and be eligible upon earning a TC. This opens paths to volume production and extensive use, including revenue operations transporting people or goods (“common carriage”).

• An Export Airworthiness Certification shows an aircraft or article (e.g., engine, propeller or other equipment) meets a specific standard before allowing it to be exported to a foreign country.

• A Special Airworthiness Certification can be issued to aircraft that have not (yet) shown compliance with all the applicable design regulations needed for a standard airworthiness certification. To enable flying such aircraft while ensuring NAS efficiency, public safety is typically assured via operational restrictions. To accommodate the needs of various operators, the FAA offers different categories. Some (Primary, Light Sport Aircraft, Limited, Multiple, and Special Flight Permit and Authorization) are not particularly relevant or newsworthy for novel aircraft, but three other special airworthiness certifications might generate excitement.
          o Restricted applies to aircraft specifically built or modified for a special, narrow purpose, such as agricultural spraying, external load operation, survey, etc.
          o Provisional is available to an aircraft or engine manufacturer or air carrier in the TC process. This allows special operations, such as flight crew training, manufacturer demonstration flights and aircraft service testing.
          o Experimental certificates can be issued to any aircraft and are a kind of catch-all for those not covered by the above. Within “Experimental,” regulators again accommodate by authorizing narrower purposes such as exhibition, air racing, market survey, crew training, and operating amateur-built, kit-built and light sport aircraft. Reflecting design progress are two other authorizations: showing compliance and conducting research and development (R&D).       

• A Special Airworthiness Certificate for Showing Compliance with Regulations may be requested by a TC applicant that is confident their aircraft build conforms with the design drawings and will pass the required certification tests.

• A Special Airworthiness Certificate for Experimental R&D applies to any combination of pure research, development and pre-flying tests needed for TC. It is needed for testing new designs, equipment, operating techniques, certified aircraft with substantial design changes and certain military designs. Because regulators protect the public primarily by restricting flight operations, an R&D issuance has minimal criteria for aircraft design or build quality.
   
• Autogiro: The original term, trademarked and licensed by Juan de la Cierva (Spain), for an aircraft using an autorotating rotor for lift plus one or more propellers for thrust.

• Autogyro: The general term for an autorotating aircraft using an unpowered rotor for lift and one or more propellers for forward flight and one that was not a licensed Cierva Autogiro. The US FAA recognizes the name “gyroplane” instead.

• Compound helicopter: According to Principles of Helicopter Aerodynamics, by Prof. J. Gordon Leishman (Cambridge University Press, 2003): “A compound helicopter involves a lifting wing in addition to the main rotor (lift compounding) or the addition of a separate source of thrust for propulsion (thrust compounding)…. The idea is to enhance the basic performance metrics of the helicopter, such as lift-to-drag ratio, propulsive efficiency, and maneuverability. The general benefit can be an expansion of the flight envelope compared to a conventional helicopter. In other words, a "compound" may involve both lifting and/or propulsive compounding. The goal is to "off-load" the rotor from its normal lifting and propulsive duties. This can be done by using a wing and/or auxiliary propulsion.”

• CTOL: conventional takeoff and landing. A runway is used for takeoff and landing of fixed-wing aircraft. The term “conventional” stems from the fact that airplanes came first.
   
• Electric Rotorcraft: An eVTOL aircraft that utilizes a rotor, such as an electric helicopter or electric autogiro.
   
• ESTOL: extreme short takeoff and landing. The required runway length can be reduced by increasing the installed power and thrust for more rapid acceleration and/or by increasing the wing lift.
   
• Fan: A type of open thruster. Axial thrust device, thrust control only, features for axial flow only, solidity >0.5.
   
• Gyrocopter: This term was trademarked by Igor Bensen and the Bensen Aircraft Corp. for its gyroplanes.
   
• Gyroglider: A Bensen trademarked name for its towed autorotating gyroplanes.
   
• Gyrocraft: A general term for all autorotating aircraft.
   
• Gyrodyne: An autogyro that is capable of VTOL and/or hovering, as well as extended forward flight in autorotation (i.e. a powered gyroplane).
   
• Gyronautics: A term coined by Skyworks Global for “the science of sustained autorotative flight”.
   
• Gyroplane: A general term for an aircraft that cruises in autorotative flight (aka an “autogyro”).
   
• Heliplane: A US Defense Advanced Research Projects Agency (DARPA) program from 2005 to 2009 for a high-speed tip-jet rotor gyrodyne.
   
• Hover Bikes/Personal Flying Devices: These single-person eVTOL aircraft are considered to be in the general class of hover bikes or personal flying devices with the primary differentiation being that the pilot sits on a saddle or is standing, or something similar. Nearly all are multicoptertype wingless configurations.
   
• LARS: The need for a runway can be eliminated for a fixed-wing aircraft by launching and recovering using off-board a launch and recovery system.
   
• Lift + Cruise: Completely independent thrusters used for cruise vs. for lift without any thrust vectoring.
   
• Material Testing: Testing relies on approximation as well. Every material specimen cannot be tested to failure to be sure of its properties, or there’d be none left to use. So, material allowables are developed using a small sample of test data with “knock-downs” from the tested sample to account for material variation, with a strong relationship between sample size and knock-down. Analyses of new and complex phenomena (e.g., shape memory alloys) and complex joints (e.g., bolted bonded composites) and innovative configurations (e.g., multi-rotors) are without the depth of test data to provide the same confidence in predictions as for conventional designs.
   
• Open Rotor: A type of open thruster. Rolls-Royce name for a high-solidity propeller (EASA Notice of Proposed Amendment 2015-22, 2015).
   
• Powered-Lift: A heavier-than-air aircraft capable of vertical takeoff, vertical landing, and low-speed flight that depends principally on engine-driven lift devices or engine thrust for lift during these flight regimes and on nonrotating airfoil(s) for lift during horizontal flight.
   
• Production: Fabrication of parts and assemblies encounters tolerances, with the acceptable tolerances for each part based on affordability. So, the as-built product represents the design, but every one differs. This is very pronounced for early helicopters with aluminum parts with compound-curvature built from hand-drafted lofted surfaces. The mechanics knelt on the parts to successfully buck the rivets to fit the frames, and the aircraft worked well enough. No one worried that the as-built product didn’t match the as designed lofts, until subsequent computer-aided design (CAD) models from digitized mylar drawings were used for new parts that didn’t fit the as-built aircraft.
   

• Product Testing: Product or component testing is an approximation as well; only final compliance has the actual finished product available to test. For early tests that are constrained by cost, schedule and knowledge of the design, parts and interfaces are represented as well as they can be. So, the as-tested configuration also differs from “The Design.”

  Confidence in analysis is based upon correlation of as-tested data with the as-analyzed predictions, both approximations of “The Design.” This lack of as-tested data is one of the challenges with innovation, whose delays and costs terraform the so-called “Valley of Death” between “good ideas” and fielded products.

  It is said that “no one believes analysis but those who did it” (as they know the actual level of detail that was used) but “everyone believes test except those who did it” (as they know the amount of putty and tape used to get the testing done on time and budget). Compliance is confirmed by comparing the tested (or predicted) performance to the requirements. So, “The Design” is still only the plan.

• Promotion: Articles, brochures and proposals fuel the imagination with artists’ renderings that present the wish list for design, the performance and the operational capability. These are not a commitment but a vision — making claims of performance are just that, claims. Caveat emptor: beware the promise that “it does” or “it can.”

  At every stage in the creation and development and production of a product or system, there are many designs, from initial to current to hoped. The important information comes from all the subsequent activities that use the design but depart from it — all for good reason.

  So, when someone talks about “The Design,” keep in mind that it’s a reference point… but it’s only the plan. 

• Propeller: A type of open thruster. Axial thrust device, thrust control only, features for axial flow only, solidity <0.5.
   
• Propeller-rotor: A type of open thruster. A niche term for a device that balances rotor and propeller functionality used in the context of Boeing's tilt wing concept (Young and Lytwyn, AHS Forum, 1967).
   
• Prop-fan: A type of open thruster. United Technologies Corp./Pratt & Whitney name for a high-solidity propeller (US Patent 4171183, 1979).
   
• Proprotor: A type of open thruster. An abbreviation of propeller-rotor, a device that balances rotor and propeller functionality that gained popularity through Bell's tiltrotor developments (Troy Gaffey, JAHS, 1969).
   
• RIA: runway independent aircraft. The capability to operate without a takeoff or landing roll. It was first used formally by NASA in program plans and budget documents in the late 1990s.
   
• Rotor: A type of open thruster. Designed to use cyclic variation of blade pitch and blade flapping to generate axial and non-axial (out of plane) forces and moments, and/or design provisions for edgewise flow.
   
• Rotorcraft: Rotorcraft means a heavier-than-air aircraft that depends principally for its support in flight on the lift generated by one or more rotors.
   
• Rotor-propeller: A type of open thruster. Once used for propeller-rotor (Ham et al, NASA CR-137756, 1975).
   
• Runway independence: the capability to operate without a takeoff or landing roll, performed by "runway independent aircraft" (RIA).
   
• STOL: short takeoff and landing. The required runway length can be reduced by increasing the installed power and thrust for more rapid acceleration and/or by increasing the wing lift.
   
• SSTOL: super short takeoff and landing. The required runway length can be reduced by increasing the installed power and thrust for more rapid acceleration and/or by increasing the wing lift.
   
• Unducted fan: A type of open thruster. General Electric name for a high-solidity propeller (NASA-CR-180867, 1987).
   
• USTOL: ultra-short takeoff and landing. The required runway length can be reduced by increasing the installed power and thrust for more rapid acceleration and/or by increasing the wing lift.
   
• Vectored Thrust: An eVTOL aircraft that uses any of its thrusters for lift and cruise.
   
• VTOL: vertical takeoff and landing. Enabling the takeoff roll to be reduced to zero eliminates the need for a runway altogether. Vertical takeoff and landing (VTOL) aircraft can land virtually anywhere. It is not just the end of a runway length continuum, but it represents a nonlinear change in capability, as it provides unique freedom from surface conditions such as slope, positive obstacles (stumps) and negative (holes), and in surface quality. Takeoff and landing with no forward velocity also offers some independence from wind, providing real-time freedom in the direction of approach and departure, unavailable with prepared runways.
   
• Wingless (Multicopter): No thruster for cruise — only for lift.