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FLUTTER STUDY

Studies on flutter included extensive investigation of wing modes, fin and rudder modes, and control surface buzz (Fig. 10(a)).

FIGURE 10(a). Flutter model.

 

 

 

WING

  The wing was treated as a plate, because of the low aspect ratio, and the modes were calculated for the complete aircraft, since it was obvious that it would be impossible to separate the wing and fuselage in a structure of this type. Cantilever modes were also calculated, as a check. Calculations involving the complete aircraft in the symmetrical case involved 6 degrees of freedom, plus control effects.
  Vibration modes were calculated by a matrix iteration method from a 60-point matrix. The frequencies were found to be quite low, and it was later decided to include all wing modes up to the anticipated control surface frequencies. A number of methods of calculating flutter were tried, and we came to the conclusion that a conventional strip theory analysis using two-dimensional derivatives was inadequate for highly swept wings, and that a form of lifting surface theory was required. The aircraft's flutter speed appeared dependent on the fuselage bending and torsional stiffness, with the wing torsional stiffness playing a secondary role.

FIN AND RUDDER

  Three degrees of fin freedom were included in the analysis, together with the rudder fundamental mode. A wide range of rudder frequencies was covered to establish the stiffness required of the control circuit for flutter prevention.
  The results showed that flutter should be no problem on the fin, providing the rudder frequency was kept to twice the fundamental bending frequency. A stream wise strip method was used for the supersonic analysis and no flutter speeds were encountered.
  The low speed model programme demonstrated except for very low rudder frequencies, the calculations were conservative. A very high margin was obtained and the flutter point agreed well with N.A.C.A. data on similar plan forms. In view of the high margin it was considered worthwhile to proceed with a transonic model programme.

CONTROL SURFACE BUZZ

  Three types of control surface buzz were considered, the first being the shock wave boundary layer interaction problem which occurs at a speed slightly higher than the wing or fin critical Mach number. An oscillatory condition arises when the shock waves move rapidly back and forth across the control surface hinge line, influenced by the trailing edge shape of the control surface, and the particular flight manoeuvre being made.
  Another type arises from the interaction between the structure and the integrated electronic system and auto-pilot. The accelerometers in the auto-pilot sense airframe vibrations, as well as primary motion, control surface movement results. Work was done to arrange sensor location to minimise these false signals. T'he third type of buzz is associated with a one degree of freedom flutter due to the theoretical loss of damping at low supersonic Mach numbers.

GROUND RESONANCE

  Ground resonance was an important factor, since with an aircraft the size and weight of the Arrow, the complete dynamic structural characteristics had to be completely investigated on the three-point under-carriage suspension. The tests showed that the aircraft, fuselage, and the wing were better than calculated. However, the fin appeared to be less easily predictable, since there was considerable rudder torsion coupling. Further fin flutter tests have been made to check the predictions.

FLIGHT FLUTTER TESTING

  The modes of interest are easily excited by the controls in flight, and we have done considerable stick tapping in an attempt to obtain records throughout the speed envelope. From this, by telemetry, the time and magnitude of damping are checked. Fig. 10(b) shows the excellent damping recorded from these tests.

MATERIALS

  To eliminate time delays, due to development of unfamiliar production processes, we did not go in for extensive use of the newer techniques, such as steel honeycomb, and used titanium only where its high temperature properties were required, rather than from a strength consideration. However, a number of the newer materials were used.
  The inner wing skins are machined out of a solid billet, using a 75ST aluminium alloy. This was stretched a nominal 2 per cent immediately after solution treatment and before artificial ageing, to produce an essentially stress-free condition. We felt that fully heat-treated alloy would contain residual stresses which would be relieved during machining and result in distortion.
The stress relieved plate is also much less susceptible to stress corrosion.
  For, components machined from hand forgings, we used a new aluminium alloy 7079, which has a chemical composition and heat treatment which is guaranteed to have a minimum transverse elongation of 4 per cent, and which can be heat-treated to achieve its maximum mechanical properties with almost negligible stress, even on sections up to 6 in. in thickness.
  On certain of the external surfaces we had a need for magnesium alloy sheet, which was determined by the necessity of achieving the required degree of stiffness, as well as maintaining the highest possible strength to weight ratio. The standard magnesium alloys of the aluminium zinc type were found to lose much of their strength at the temperature corresponding to a Mach number of 2, i.e. approximately 250'F, and they failed to recover their original properties on return to room temperature.
  The specification finally used was ZE41H24, and we had little difficulty with it after establishing the proper techniques of hot forming.
  ZH62, a new casting alloy, was used on the windshield and canopy castings, which are complex and difficult to cast in any magnesium alloy. Strength is maintained quite satisfactorily in this alloy to over 300'F, with almost complete recovery. The alloy is weldable, which makes possible the salvaging of complicated castings which might be scrapped if they happened to be unacceptable through minor surface defects, such as miss-runs or local surface porosity.
  On the undercarriage, which had to be relatively slim to go into the thin wing section, the manufacturers of this component went to 280,000 p.s.i. high strength steel. Extensive investigations had been made on several steels to determine the best compromise between high tensile strength and an inevitable reduction in ductility and impact strength, which results from the use of high tensile strength. Great care had to be taken to reduce the hydrogen content, since hydrogen embrittlement has a marked effect on the fatigue strength and impact properties.
  Titanium was used in sheet form extensively in the shrouds and portions of the structure adjacent to the jet pipe, where the low weight and high strength at temperatures up to 800'F are required. This is mainly commercially pure titanium.
  Extensive investigations were made into the possible use of titanium as a structural material, but although at that time some of the titanium alloys appeared to be promising as a replacement of high strength aluminium alloys in the structure, the manufacturers were unable to obtain uniformity of the product to ensure that the higher strength was in fact available.
  Titanium is being used as a replacement for steel in some joints and fastenings, since there is a distinct weight saving from this.

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