Flutter certification requests and methods of compliance are dictated by standards. Four European regulations and the German national standard are the most widely used in certification of light aeroplanes:
- EASA CS-VLA (Certification Specifications for Very Light Aeroplanes).
- EASA CS-LSA (Certification Specifications for Light Sport Aeroplanes).
- EASA CS-23 (Certification Specifications for Normal, Utility, Aerobatic, and Commuter Category Aeroplanes).
- EASA CS-22 (Certification Specifications for Sailplanes and Powered Sailplanes).
- LTF-UL (Lufttüchtigkeitsforderungen für Ultraleichtflugzeuge), which is managed by the German national authority DULV (Deutscher Ultraleichtflugverband e.V.), issued by the same organization, and recognized by DAeC (Deutscher Aero Club).
All the regulations and their respective amendments can be downloaded for free from the EASA website (https://www.easa.europa.eu/en/document-library/certification-specifications) or from the DULV website (https://www.dulv.de/sites/default/files/Downloads/NfL%20motorisierte%20UL%202-547-20.pdf), although the latter is only available in German.
Before examining in detail the flutter-related requirements of each regulation, let’s clearly identify which types of aircraft can be certified under each one.
CS-VLA applies to single-engine (spark- or compression-ignition) aeroplanes with no more than two seats, a Maximum Certificated Take-off Weight of no more than 750 kg, and a stalling speed in the landing configuration of no more than 83 km/h (45 knots) (CAS). These aircraft are approved for day-VFR only.
CS-LSA applies to Light Sport Aeroplanes with a single, non-turbine engine fitted with a propeller, a maximum seating capacity of two persons (including the pilot), a Maximum Take-Off Mass of no more than 600 kg for landplanes (or 650 kg for seaplanes), a maximum stalling speed (VS0) of no more than 83 km/h (45 knots) CAS, a non-pressurized cabin, and approval for day-VFR only.
CS-23 applies to aeroplanes in the normal, utility, and aerobatic categories with a seating configuration of nine or fewer (excluding pilot seat(s)) and a maximum certificated take-off weight of 5,670 kg (12,500 lb) or less, as well as to propeller-driven twin-engine aeroplanes in the commuter category with a seating configuration of nineteen or fewer (excluding pilot seat(s)) and a Maximum Certificated Take-Off Weight of 8,618 kg (19,000 lb) or less.
CS-22 applies to sailplanes and powered sailplanes in the utility (U) and aerobatic (A) categories. It includes sailplanes with a maximum weight of 750 kg, single-engine (spark- or compression-ignition) powered sailplanes with a design value W/b² (weight to span²) of no greater than 3 (W[kg], b[m]) and a maximum weight of 850 kg, as well as sailplanes and powered sailplanes with a maximum of two occupants.
LTF-UL applies to three-axis standard control Ultra-Light aircraft (UL) with a Maximum Certified Take-off Weight of 300 kg for a single-seat aircraft (plus an additional Rescue System) and 450 kg for a dual-seat aircraft (plus an additional Rescue System), both with a stalling speed VS0 of no more than 65 km/h.
Regardless of the regulation used, paragraph 629 deals with flutter and aeroelastic instability verifications. It contains the general requirement that: It must be shown (…) that the aeroplane is free from flutter, control reversal, and divergence for any condition of operation within the limit V-n envelope (…).
While the requirements are the same, the individual regulations differ in terms of how compliance can or must be demonstrated and any additional obligations that must be met.
The LTF-UL regulation, which applies to ultralight aircraft, requires systematic flight tests to induce flutter at speeds up to VDF (Maximum Demonstrated Flight Speed) in every case. These tests must show that there is no reduction in damping as VDF is approached. Furthermore, for aircraft with a VD (Design Speed) exceeding 200 km/h, a ground vibration test, including flutter calculations, must be performed before airborne flutter testing to ensure that the aircraft is flutter-free up to 1.2 VD.
Even the simplest regulation provides several key insights that apply to all other regulations:
- If properly executed, flight tests are considered the best method for verifying the absence of flutter within an aircraft’s flight envelope.
- Flight flutter tests are dangerous because, if flutter arises explosively, it can lead to the loss of the aircraft. Flutter is a phenomenon in which elastic and inertial forces interact with aerodynamic forces. The latter depend on dynamic pressure, which in turn is a function of velocity squared. As speed increases, the risk associated with flight tests rises sharply.
- When conducting flight tests, it is necessary to proceed incrementally, increasing the speed at which flutter is induced only after verifying that damping is not too low to safely continue. To obtain this information, the aircraft’s lifting surfaces, including horizontal and vertical tailplanes, must be appropriately instrumented, and a valid methodology must be used to assess damping at each flight speed.
- Preliminary flutter analyses reduce the risk of accidents during flight tests by predicting the onset of this phenomenon within the flight envelope with varying degrees of reliability. The use of a 1.2 safety factor on the speed to be analyzed provides a margin for the inherently complex and approximate nature of these calculations.
The LTF-UL, like all other regulations, leaves significant discretion regarding how flight tests are conducted and what methodologies should be used for GVT and flutter verification calculations. For example, no information is provided on the aircraft’s mass at the time of testing, the center of gravity position, surface balancing, or control stiffness, which must therefore be agreed upon in advance with the authorities.
The CS-VLA allows for verification of the absence of flutter within the limit V-n envelope and at all speeds up to the speed specified for the selected method, using one or a combination of the following three methods:
- A rational analysis may be used to show that the aeroplane is free from flutter if the analysis shows freedom from flutter for all speeds up to 1.2 VD (Design Dive Speed).
- Flight flutter tests may be used to show that the aeroplane is free from flutter if it is demonstrated that proper and adequate attempts to induce flutter have been made within the speed range up to VD; the vibratory response of the structure during the test indicates freedom from flutter; a proper margin of damping exists at VD, and there is no large and rapid reduction in damping as VD is approached.
- Compliance with the rigidity and mass balance criteria in Airframe and Equipment Engineering Report No. 45 (as corrected), “Simplified Flutter Prevention Criteria” (published by the Federal Aviation Administration).
Additionally, the regulation requires:
- Determining the natural frequencies of main structural components by vibration tests or other approved methods. (This measurement may be avoided if both methods 2 and 3 are used and the aircraft’s VD is less than 259 km/h (140 kt)).
- Demonstrating freedom from flutter after the failure, malfunction, or disconnection of any single element in any tab control system.
In addition to flight tests and an analytical study of flutter, the CS-VLA also allows certification through a third method. Report No. 45, available for download at (https://apps.dtic.mil/sti/citations/ADA955270). It was published in 1955, at a time when rational methods of flutter analysis were not available. It is based on a statistical comparison of the inertia and stiffness characteristics of the aircraft under examination with those of aircrafts that have exhibited flutter.
Despite the fact that the simplified criteria contained in it represent the least reliable method to adopt—due to the absence of aerodynamic effects in the calculation—and that in some cases they have proven to be unconservative, while in most cases a better design could be achieved by reducing or eliminating the need for non-structural balance weights, the application of these criteria to personal aircraft is considered adequate to ensure freedom from flutter and so is widely used due to its relative simplicity.
In any case, due to its statistical foundation, Report No. 45 cannot be applied to vehicles that do not have a conventional configuration. Therefore, its use is valid for certification only for an aircraft that:
- Does not have large mass concentrations (such as engines, floats, or fuel tanks in outer wing panels) along the wingspan.
- Does not have a T-tail, boom-tail, or V-tail.
- Does not have unusual mass distributions or other unconventional design features that affect the applicability of the criteria.
- Does not have a significant amount of sweep.
- Has fixed-fin and fixed-stabilizer surfaces.
As a note, at the time of writing this document, the authors are unaware of the usefulness of determining the natural frequencies of the aircraft in the case of certification through flight tests.
The structure and format of the CS-LSA differ from other Certification Specifications issued by EASA. The reason for this is that the CS-LSA is based on a specific revision of the existing industry standards issued by ASTM International.
Paragraph “4.6 – Vibrations” of the ASTM document “F2245-10c Design and Performance of a Light Sport Airplane” states that: flight testing shall not reveal, by pilot observation, (…) flutter (with proper attempts to induce it), (…), at any speed from VSO (Stalling Speed) to VDF (Demonstrated Flight Stalling Speed).
This requirement is integrated into the main document by specifying that, for aircraft with a Vne exceeding 200 km/h (108 kt), a ground vibration test with subsequent analysis of vibration modes, frequencies, and potential flutter cases must demonstrate that the aircraft is free from flutter before flight verification. (This ground vibration test and analysis may be omitted when there is clear justification to assume the airplane’s freedom from flutter due to compliance with all reasonable analyses or Report 45, when applicable).
The requirements of the CS-LSA therefore appear to be aligned with those of the LTF-UL, where only flight tests are considered valid for flutter certification, while analyses, or alternatively compliance with rigidity and mass balance criteria, are required only to reduce risks before testing.
The CS-23 follows the structure of the CS-VLA concerning the requirements for certification, but, unlike the latter, which required demonstrating the absence of flutter using one of the three previously described methods, the CS-23 prescribes both flight tests and the execution of calculations, whether they are a rational analysis or compliance with Report No. 45 (adding to the applicability requirements of the latter that the VD is less than 482 km/h (260 knots) (EAS) and less than Mach 0.5).
In addition to these fundamental requirements, the standard also prescribes that flutter requirements must also be met:
- With zero fuel in the wings.
- After the failure, malfunction, or disconnection of any single element in the primary flight control system, any tab control system, or any flutter damper (For aeroplanes other than those meeting the requirements for the use of Report No. 45).
- After fatigue failure, or obvious partial failure of a principal structural element (For aeroplanes showing compliance with the fail-safe criteria).
- With the extent of damage for which residual strength is demonstrated (For aeroplanes showing compliance with the damage-tolerance criteria).
For turbo-propeller powered aeroplanes, the dynamic evaluation must even include whirl mode degrees of freedom, which take into account the stability of the plane of rotation of the propeller and significant elastic, inertial, and aerodynamic forces; other than propeller, engine, engine mount, and aeroplane structure stiffness and damping variations appropriate to the particular configuration.
In CS-23, unlike other standards, means of compliance are proposed, which, although not mandatory, provide useful information and guidance on how to conduct flight tests, GVT, and analyses. These are reported in the FAA Advisory Circular No. 629-1B (https://www.faa.gov/regulations_policies/advisory_circulars/index.cfm/go/document.information/documentid/22454).
In this document, a clarification regarding whirl flutter is of particular importance for the certification of even light aircraft: Amendment 23-31 of § 23.629(e) now requires an investigation of the whirl mode phenomena for both single and multiengine turbopropeller airplanes. Although airframe influence may be negligible for fuselage-mounted single-engine tractor configurations, the potential for propeller whirl flutter still exists. (…) For this reason, contrary to common belief, the verification of potential whirl flutter must also be carried out for single-engine aircraft.
The category of sailplanes is the most sensitive to the possibility of flutter due to the high aspect ratio of their wings. This issue is reflected in the CS-22 standard, which requires both preliminary flutter calculations based on experimental investigation results and flight tests:
- A ground vibration test which includes an analysis and an evaluation of the established vibration modes and frequencies for the purpose of recognizing combinations critical for flutter by an analytical method, which will determine any critical speed in the range up to 1.2 VD.
- Systematic flight tests to induce flutter at speeds up to VDF. These tests must show that a suitable margin of damping is available and that there is no rapid reduction of damping as VDF is approached.
It is worth noting that, since the statistical base on which Report No. 45 was developed is based on light airplanes and not sailplanes, this method is not considered valid for certification.
In summary, we can state that, although the requirements of the various standards may seem similar regarding flutter certification, a closer inspection reveals that they differ significantly in terms of complexity and the effort required. The numerical part, although sometimes not mandatory, serves not only as verification but also to increase safety during flight tests, reducing the risks of manoeuvres that, by their nature, could be catastrophic.
Flight tests, on the other hand, are an essential constant, except for CS-VLA. However, from a technical perspective, the issue remains open regarding the reliability of the information obtained when the aircraft is excited by a hit to the control surfaces via the stick or pedals. The time with which the forcing function manifests in these cases is quite long, significantly differing from an ideal impulse, and has a frequency band that is rather limited, estimated at less than 10 Hz. The aerodynamic load is thus not capable of exciting the higher-frequency modes, and sometimes not even the first modes, making the acquired data of little utility.
About Vicoter
Vicoter is an Italian company established in 2009, specializing in structural measurements, testing, and analysis, primarily within the aerospace sector. Its activities range from vibration tests to stress tests, conducted both on the ground and in flight. Vicoter’s core expertise lies in performing Ground Vibration Tests (GVT) at the manufacturer’s site, which are critical for understanding the dynamic behavior of aircraft. In addition to GVT, Vicoter also provides detailed flutter analysis, essential for the certification of light aircraft. These analyses ensure the safety and stability of the aircraft during flight, enabling Vicoter to offer comprehensive support throughout the testing and certification process.
Read more about as Vicoter performs its GVT and flutter analyses in Certificazione a flutter or look at various previous certification activities in Flutter verification of APUS i-2, Ground Vibration Test and flutter verification of the Bristell Classic LSA, Ground Vibration Test and flutter verification of the new Bristell B8 aircraft, Flutter certification analysis of the Alpi Aviation P300 aircraft
