General Flight Test Theory Applied to Aircraft Modifications

General Flight Test TheoryTUTORIAL Applied to Aircraft Modifications GENERAL FLIGHT TEST THEORY APPLIED TO AIRCRAFT MODIFICATIONS Lt Col Lionel D. Alford, Jr., USAF and Robert C. Knarr Any external aircraft modification has potentially far-reaching effects on the capability of the aircraft to succeed or fail in its mission. The authors take a systematic look at the effects that small changes can have upon the whole, with a series of examples that demonstrate why careful review of data or testing is often vital in the assessment of system modifications. new design aircraft program always external aircraft modification. The aircraft includes an instrumented test to design problems covered here represent Avalidate the analyses. But a modifi- the fundamental characteristics by which cation program may rely instead on pre- aircraft capability is judged. These design viously collected data for model valida- problems, when not properly analyzed and tion. Such a program must adequately tested (if required), have historically address the effects of the modification on resulted in significant degradation of air the aircraft and its mission. The user must worthiness. We define the subject area and judge these effects for their desirability— explain the importance of each problem especially when they degrade mission by discussing the rationale behind stan- capability. But, to be judged, they must dard design practices and air worthiness be fully understood. Reviewing historical and operational considerations for the fleet data or conducting a test are two ways to aircraft. Concrete examples illustrate each validate the data by which these effects case. Although only effects to the C–130 on aircraft capability are judged. aircraft are discussed in detail, these In this article, we address eight critical principles and observations apply to any test and evaluation considerations for an aircraft. 157 Acquisition Review Quarterly—Spring 1999 STRUCTURAL (STRESS AND The ultimate result when an aircraft is LOADS ANALYSES) not designed to standard engineering practice (or verified by test) is increased like- Rationale. When structural strength lihood of component or structural failure. proof tests are not performed, it is a stan- An example of this is skin surface antenna dard engineering practice to specify that mounts that come off in flight due to aircraft modifications be designed for a repeated flights at high airspeeds. Struc- 25 percent or greater static margin of tural modifications that pierce the pres- safety using a factor of safety of 1.5. The sure vessel and are grounded in the load- modified airframe will then have the bearing components of the aircraft are a strength capability to be released to fly at special threat. This is because those com- 100 percent of design capability. ponents, when they fail, have a tendency However, if to cause the failure of other load-bearing analyses show structures. This domino (a.k.a. zipper) “In fact, the most that an aircraft effect can result in the loss of an aircraft. prolific sponsor has a margin of The loss of a modified KC–135 aircraft in of research competi- safety between the early 1970s was probably attributable tions is the federal 0 and 25 per- to such a failure in a fuselage-mounted government, and cent, then the radome. Another problem symmetric in particular the aircraft must be modification can create is asymmetric Department of loading. As a result of even the most be- Defense.” tested with suf- ficient instru- nign maneuvers, the modification may be mentation to subject to airloads that cause oscillations ensure a positive margin of safety for the in the fuselage. This can result in fatigue ultimate design conditions in order to pre- failure of structures well forward of the vent flight envelope restrictions. Finally, modification. The Beech V-tailed Bonanza if analyses reveal a negative margin of is a classic example of this; the shape of safety or failure occurs during testing, ei- the tail caused fishtailing that eventually ther the deficient structure must be rede- resulted in fuselage failure. signed or aircraft flight envelope restrictions must be imposed. Air worthiness and operational con- PRESSURIZATION siderations. Reduction of the flight envelope means the aircraft must be re- Rationale. Pressurization is directly re- stricted in airspeed, symmetric or maneu- lated to the previous discussion. It is in its ver G-loading, sideslip, or payload to pre- own category because it is a common and vent a design load limit (DLL) from be- potentially catastrophic failure mode in ing exceeded. Limiting the C–130 flight modifications. Generally, when the aircraft envelope as a result of any modification pressure vessel is penetrated, for whatever will significantly affect the aircraft mis- reason, a full pressure test series (proof sion capability. This is to be avoided at all and leakage rate) is made on the aircraft. cost. Following a significant modification, a full pressure test must be completed prior to 158 General Flight Test Theory Applied to Aircraft Modifications the first flight during which the aircraft higher frequency local modes as driven will be pressurized. The pressurized por- by jet noise, aerodynamic turbulence, tion of the aircraft must be capable of unbalance in rotating equipment, propeller withstanding proof-pressure testing at a or rotor blade aerodynamic disturbances, level 1.33 times the maximum setting plus gun blast, etc. They can also cause con- tolerance on the safety valve. This test trol problems (which will be discussed in should be performed on each modified the section on handling qualities). aircraft. Flutter is the dynamic instability of an Air worthiness and operational con- elastic body in an airstream. Flutter speed siderations. The importance of verifying (Uf) and the corresponding frequency (vf) pressure vessel integrity is evident from are defined as the standpoint of the potential conse- the lowest air- “Flutter is the quences of a modification failure which speed and fre- dynamic instability breaches the pressure vessel. Pressure ves- quency at which of an elastic body sel failures have the potential to cause the a flying struc- in an airstream.” loss of an aircraft due to an explosive de- ture will exhibit compression. An example of this is the C– sustained, simple harmonic oscillations. 130 flying near Iceland that had a breach Flutter is a dynamic instability (self- near the wing root; it lost most of the top sustaining and increasing) that may result of the center wing and some of the fuse- in failure of the structure. In aircraft, the lage. This aircraft made it back safely; failure of a main structure generally re- many crews have not been so lucky. With sults in the loss of the aircraft. Aircraft are any depressurization there are additional designed such that their airframe flutter safety hazards to the crew as well. will occur at airspeeds and conditions outside the aircraft envelope by a safety margin of at least 15 percent. Modifica- FLUTTER, BUFFETING, AND VIBRATION tions that change the vibrational modes of an aircraft cause the flutter speed to Rationale. Airframe vibration com- change. prises three distinct areas: flutter and The frequency and airspeed at which aeroelastic instabilities, dynamic loads, flutter occurs generally increases with and vibroacoustics. Flutter deals with increased structural stiffness. However, dynamically unstable elastic coupling of many times increased stiffness in a struc- the airframe with the air stream, and tural component changes the vibrational occurs primarily in the lowest frequency frequencies of that component and result airframe elastic modes. Dynamic loads in changes of frequencies in the overall deal with the forced vibration resulting aircraft structures. These changes can from buffeting, atmospheric turbulence cause unforeseen consequences such as (gust), landing impact, sharp maneuvers, vibration or flutter, and their effect must heavy store release, and other factors, be evaluated by analyses or testing. Usu- again in the lowest frequency airframe ally, a ground vibration test is made to elastic modes. Vibroacoustics deals with determine changes in the vibrational the forced vibrations of the airframe in the modes of a modified airframe. These 159 Acquisition Review Quarterly—Spring 1999 modes are used to validate or update the Air worthiness and operational con- structural dynamic analysis model that siderations. Flutter is a special concern determines the flutter speeds and for the C–130 empennage and can be a frequencies. problem for any wing- or empennage- Buffet is the elastic structural response mounted modification. The A model of the of the airframe in the lower frequency aircraft was analyzed with a 15 percent structural modes to aerodynamic flow flutter safety margin, but exhibited ap- separation or shed vortices. Flight surfaces proaching flutter during high-speed flight. (wings, tail surfaces, etc.) buffet due to The aircraft was limited in airspeed due the oscillating forces as flow separates and to this problem. The B model was rede- reattaches over local areas. Buffet also signed with greater rudder and elevator tip occurs when surfaces downstream of flow weights to change the frequency of the separations are elasticity excited by the surface bending and fuselage torsion and flow turbulence or by shed vortices. If get back to the 15 percent safety margin. buffeting occurs or if it is considered likely Any modification that changes the fuse- (there is no ana- lage torsion or fin-bending modes has a lytical proce- potential to cause flutter in the C–130 “[Vibration] can dure to predict empennage. Because of this, special care result in fatigue these phenom- should be taken to ensure that modifica- failure of structures, particularly light- ena), the sur- tions do not negatively affect the aircraft’s weight structures face must be in- flutter safety margin. With the advent of directly in the strumented and high-speed digital computers and the slipstream, such flight tested.

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