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TRANSIENT FLIGHT FLUTTER TEST OF A W I N G WITH TIP T A N K S
R. J . W e r d e s - McDonnell Aircraft Corporation, St. Louis, Missouri Abstract Wing flutter was encountered during flight test- ing of the F2H-2 airplane with full wing tip tanks.
As a result, more refined theoretical analysis as well as flight flutter tests were initated to establish cor- rective measures and to experimentally verify the stability of the improved system. The results from the flight flutter tests, utilizing the transient response are presented. The method of excitation technique, consisted of abrupt deflections of the ailerons result- ing from "stick bangs" and data were measured by wing tip accelerometers.
Figure 1. McDonnell Model F2H-2 with 200 Gallon A comparison of the results with theoreticalpre- Wing Tip Tanks dictions is presented and indicates that reasonably good correlation w a s obtained. The influence on wing flutter of tip tank fuel transfer cycle, which was in- For flight testing the aeroelastic properties corporated to control the center of gravity range of of the wing, two accelerometers were installed in each the tank during defueling, is indicated by the measured wing tip, one located forward and one aft a s shown in Figure 2. The outputs from these accelerometers results and compared with the theory. The final were recorded on an oscillograph. By comparison configuration utilized a transfer cycle which was of the magnitude and phase of the various records, proven stable as a result of flight flutter testing. It is concluded that transient response measurements wing motion could be identified as symmetrical or resulting from stick bangs provide a reasonably r e - asymmetrical, and some idea of the magnitude of liable and safe technique of flight flutter testing for bending and torsion at the wing tip could be deter- mined.
wings wlth external tanks or heavy stores.
The means of excitation - of inducing oscilla-
tions of the wing - was provided by the pilot. An
asymmetrical pulse was induced by a sharp lateral INTRODUCTION blow on the control column by the pilot's fist. A symmetrical pulse was induced in the same manner The Model F2H-2 Airplane is a single place, carrier-based, two-engine jet fighter. Its gross weight by the pilot striking the control column forward or aft. The. pilot excited the system by these "stick is approximately 20,000 pounds and it was designed bangs" a t each small increment in speed for a con- to fly in the high subsonic region. All controls are manual except for the power-boosted ailerons. It stant fuel loading condition, or at each small incre- differs from its predecessor, the F2H-1, in that it mental change in fuel loading for a constant speed c a r r i e s 200-gallon fuel tanks on each wing tip. condition.
Figure 3. 200 Gallon Wing Tip Tank Geometry Showing Compartments nearly neutrally stable for several tip tank fuel con- Figure 2. Three View F2H-2 ditions during initial flight flutter testing.
The empty tank was the first configuration to be tested. Adequate stability was demonstrated and Since the rate of transition from a stable to an is seen in Figure 4 to be in fair agreement with the unstable condition with increase in speed was quite results of theoretical analysis which are also shown.
low, as predicted by the initial theoretical analysis, All theoretical results are based on the use of in- this was considered to be a reasonably safe technique. compressible flow three -dimensional strip theory and were conducted for the test altitude of 10,000 feet.
The oscillograph records obtained in this fashion The aerodynamic properties of the tip tank were rep- were analyzed to establish the rate of decay, fre- resented by an equivalent rectangle which produced quency, and mode of the wing oscillatory motion, the same steady aerodynamic force and moment co- as a means of defining the wing aeroelastic stability. efficients relative to the wing elastic axis as deter- Most of the flight testing was performed at approxi- mined by wind tunnel tests. Fuel was considered as mately 10,000 feet altitude in order to test to the a solid mass.
highest q possible for this Mach number limited air- plane.
DISCUSSION Prior to the flight of the production model of the F2H-2, its prototype, the XF2H-1, modified to c a r r y 200-gallon wing tip tanks having slightly smaller diameter and slighly greater length, had been thor- oughly flight tested and had demonstrated adequate aeroelastic stability for all tank fuel contents from full to empty. Because of this, no problem a r e a s were anticipated for the F2H-2 configuration, the two airplanes being considered fairly similar dynamically.
Each tip tank for the F2H-2, as well as for the XF2H-1, was divided into three compartments as shown in Figure 3, and by means of internal plumbing was defueled ir. a forward-aft-center (F-A-C) se- quence by means of pressurized air. This defueling Figure 4. Flight Test Correlation Empty Tip Tanks sequence was selected since it kept the tank center Fwd Tank Fitting Preloaded of gravity travel at a minimum, generally in a for- ward location with respect to the wing elastic axis which w'as considered stabilizing, and did not impose The next fuel configuration tested was the full maneuvering load restrictions on the airplane. tank. Near neutral stability was encountered at 450 knots equivalent airspeed in the asymmetric mode.
Because the symmetric wing mode exhibited It can be seen in Figure 5 that the test points show extremely good aeroelastic stability properties for less stability than the theory at the higher speed end.
all tip tank fuel conditions, the following discussion is This is probably due to the system being so nearly confined to the asymmetric wing mode which became neutrally stable that any external disturbance would quency. The minimum point is associated with a ratio of wing torsion frequency to wing asymmetrical bending frequency equal to one. A comparison of the operating conditions of the XF2H-1 and F2H-2 Air- planes i n Figure 6 shows clearly why near-neutral stability was encountered at 450 knots equivalent airspeed for the F2H-2 while the XF2H-1 was ade- quately stable. One means of improving the F2H-2 stability is also indicated here. If the wing torsional frequency could be reduced in some way, it would approach the more stable XF2H-1 operating condi- tion. This, as will be seen, is exactly what was done.
Tests were conducted to compare the tank-to- wing attachment stiffnesses of the F2H-2 and XF2H-1 Airplanes and the forward attachment of the F2E-2 Figure 5. Flight Test Correlation Full Wing Tip was found to be much stiffer than that of the XF2H-1.
Tanks Fwd Tank Fitting Preloaded By rigging the forward tank-to-wing attachment, rep- resented schematically in Figure 7, so as to permit some motion between the upper ball-socket arrange- necessarily be amplified under this condition. The ment which had previously been pre-loaded to an theory shown here is the result of an extensive pro- equivalent of 3 g's normal force on the tank, the stiff- gram of ground testing and a prodigious amount of ness contribution of the attachment was effectively theoretical analysis which was initiated subsequent reduced.
to this incident and which continued during and after the flight testing program had been concluded.
In examining the flight test resultsfor the F2H-2 and the XF2H-1 Airplanes with full and empty tip tanks, it was noted that the frequency of the critical asymmetrical mode was ten to fifteen percent higher f o r the F2H-2 Airplane than for the XF2H-1 Air- plane. A similar difference was noted during the tests, but the full significance of ground vibration this difference was not indicated by the relatively limited theoretical analysis for the XF2H-1 Air- plane.
When more extensive analyses were conducted Figure 7. Schematic Diagram of Forward Wing-to- for the F2H-2 Airplane and the effect of a wide vari- Tank Attachment Fitting ation in wing torsional frequency was studied, the primary difficulty was uncovered. As shown in Figure The system, so modified, was flight tested and 6, a region of relatively low flutter speed is en- with adequate looseness in the forward fitting as countered for certain values of wing torsional fre- established by trial, proved to be a satisfactory configuration. A comparison of experimental and theoretical flutter stability for this configuration is shown in Figure 8. It was found that the frequency of the critical mode which had been theoretically shown to be proportional to the wing torsional fre- quency, had decreased by 12 to 15 percent as a re- sult of loosening the forward fitting. This effect can be seen by a comparison of Figures 5 and 8.
Having improved the full tank stability suffici- ently, testing was continued at gradually increasing speeds, with the tank fuel decreasing from full to empty at each speed. At 450 knots equivalent air- speed, a condition of low damping was foundfor a fuel content of from 150 to 120 gallons. This region is shown by theory in Figure 9. Figure 10 shows a Figure 6. Stability Diagram for Full Tip Tank Config- "slice" taken through Figure 9 where the variation of damping with minutes of fuel transfer measured uration of XF2H-1 and Original F2H-2 Airplanes Figure 8 . Flight Test Correlation Full Wing Tip Tanks Fwd Tank Fitting Loose Figure 10. Correlation of Flight Test Data Logarith- mic Decrement Versus Minutes of Fuel Transfer - Cycle F-A-C Ve= 464 Knots EAS for a flight velocity of 465 knots is compared with is seen to theoretical results. Good correlation greater than unity (1.44). It follows, then, that some- exist. (It might be mentioned here that it took about where along the fuel transfer cycle, the operating 27 or 28 minutes to transfer the 200 gallons of fuel frequency ratio must approach and pass through a from each tank.)
value of unity, traversing the characteristic dip in the stability boundary. Whether or not this results in an unsatisfactory condition depends on the value of the minimum velocity of the dip.
It was found from theoretical analysis that the minimum velocities of the dip in the stability bound- ary were lower in the early stages of the fuel transfer
- when the tip tanks contained a large quantity
cycle of fuel - than in the latter stages of the fuel cycle.
This indicated the desirability of making the trans-
ition through the critical frequency - which was un-
avoidable - late ih the cycle when the tip tanks were
nearly empty. The essential short-comings of the original fuel cycle (forward-aft-center) w a s that it did not accomplish this. The transition through the characteristic dip in the stability boundary occurred quite early in the cycle when the minimum velocity of the critical region was well within the operating Figure 9. Variation of Flutter Speed Along Original speed range of the airplane.
Fuel Transfer Cycle - Cycle F-A-C
By altering the internal plumbing of the tip tank the sequence in which the three fuel compart- The region of relatively low speed instability ments of the tank were emptied could be changed.
was found, from a theoretical analysis, to be caused Without modifying the compartmentatiofi ~f %e : a r k by the wing torsion to asymmetric bending frequency there were just two alternate fuel transfer cycles ratio being close to unity. Though the hbility boundaries for various fuel loadings do not follow the same variation with change in the wing torsional did not yield a value of the wing torsion to frequency as shown for the full tank condition, Figure asymmetric bending frequency ratio similar 6, the boundaries do have the common characteristic to that of the original cycle in the region of of a rapid transition in the oritical velocity of the 120 to 150 gallons, and system as the torsion to asymmetric bending fre- quency ratio of the wing approaches and passes through maintained a stabilizing tank center of a value near unity. In the full tip tank configuration gravity location well forward of the wing the torsion to asymmetric bending frequency ratio is elastic axis.
somewhat less than unity ( . 8 5 ) , while in the empty tank configuration the frequency ratio is somewhat 1 0 0 As seen in Figure 11, the tank moment of in- ertia for both the intermediate cycle (A-C-F) and the final cycle (c-A-F) i n the range of fuel content 120 to 150 gallons is substantially greater than from that for the original cycle (F-A-C) and consequently each produces a lower ratio of wing torsional fre- quency to wing asymmetric bending frequency, that is, in the direction of increased stability.
Figure 13. Correlation of Flight Test Data Logarith- mic Decrement Versus Minutes of Fuel Transfer
- Cycle A-C-F Ve = 470 Knots
Cycle The theoretical variation of flutter speed with fuel usage is shown in Figure 14 for the final fuel cycle. It exhibited the greatest stability of the three cycles becoming neutrally stable at about 600 knots Figure 11. Wing Tip Tank Inertia Properties for equivalent airspeed, which was far i n excess of the Three Fuel Transfer Cycles maximum velocity for this airplane. Here again in Figure 15 flight test stability data in the form of log- arithmic decrement versus minutes of fuel transfer Both fuel cycles were flight tested.
The theo- obtained for the final fuel cycle at a velocity of 470 retical variation of flutter speed with fuel usage is knots equivalent airspeed is compared with theoretical shown in Figure 12 for the intermediate fuel cycle.
results. It is to be noted that good agreement has is seen to exist to about 500 knots Adequate stability been obtained here between the theoretical and test equivalent airspeed. In Figure 13 flight test stability data for both the value of damping and frequency of data i n the form of logarithmic decrement versus the lowest damped mode. This fuel transfer cycle minutes of fuel transfer obtained for the intermediate was incorporated a s the final fuel sequence configura- fuel cycle at a velocity of 470 knots equivalent air- tion because of its greater stability plus the fact that speed is compared with theoretical results. Good it did not impose flight load restrictions on the air- agreement i s seen to exist.
Figure 12. Variation of Flutter Speed Along Inter- Figure 14. Variation of Flutter Speed Along Final
mediate Fuel Transfer Cycle - Cycle A-C-F
Fuel Transfer Cycle - Cycle C-A-F
plane since the centers of gravity for conditions f o r large fuel contents were always relatively close to the wing elastic axis.
It has been shown how flight flutter testing by the transient response technique provided a reliable measure of the flutter stability of the wing tank con- figuration when employed in conjunction with theoreti- cal analysis. It is concluded that transient response from "stick bangs" can provide a reasonably reliable and safe technique of flight flutter testing for wings with external tanks or heavy stores.
Figure 15. Correlation of Flight Test Data Logarith- mic Decrement Versus Minutes of Fuel Transfer - Cycle C-A-F Ve = 470 Knots