Research Memorandum

... ." I RESEARCH MEMORANDUM AN ANALYTICAL EVALUATION OF TEE EFFECTS OF AM AERODYNAMIC MODIFICATION AND OF STABILITY AUGMENTERS ON THE ". FOR._ AEROhlAUTlCS 0 3 c;' WASHINGTON April 21, 1958 L NclCA RM A57K07 4 <- I AN ANALYTICAL EVALUATION OF TEE EFFECTS OF AI? AWODYNAEQIC MODIFICATION AND OF STABILITY AU- ON TEE PITCH-UP B3HAVIOR AND PR0BABI;E: PILOT OPINION OF TWO FIGETEZ Am By Melvin Sadoff andJohn D. Stewart The effectsof a wing modification andof stability augmentation on the computed longitudinal behavior in the pitch-up region and probable pilot opinionof the pitch-up characteristicsof two current fighter airplanes are presented. The cornputations indicated that the additionof a wing leading-edge extension to oneof these airplanes would(1) reduce the peak and overshoot anglesof attack forall flight conditions hvestfgated, with the exception of those at take-off speeds, (2)and ahould lmprove probable pilot opinion of the pitch-up behavior from unacceptableto unsatisfactory - the category of the elevator-controlledF-84F and F-& airplanes. Added pitch damping provideda simple by pitch damper with constant gain did not materially reduce the response overshoots and would not be expectedto improve pilot opinionm this airplane. One beneficial effect attributable to the damper, pitch however, was a reduc- tion in the peak positive maneuvering tail-load increments. For the ather fighter airplane, the computations indicated that added pitch damping provideda smleby pitch damper withnonlinear gaFn reduced thepeak: and overshoot valuesof angle of attack significantly for all flight conditions considered but the landing approach. !This should improve the probable pilot opinionof the pitch-up behavior for this airplane from unacceptable or unsatisfactory to unsatisfactory but acceptable - the rating category for YE"86D the and F-86F airplanes. The results for this airplane equippeda stick with pusher, which ais device intentiedto prevent pitch-up altogether, indicated that the pitch- up regionwas generally avoided with this deviceoperata, even far extremely rapid maneuvers. Canparisonof two versions of this device 2 NACA RM A57KO7 showed that care must be intaken the designof such systema to minimize reductions in the maneuvering capabilitiesof the airplane. INTROlXTCTION Desi@ considerations for supersonic fighter airplanesmay, in some cases, lead to configurations which would be expected a severeto have pitch-up problem at high anglesof attack. Three possible approachesto this problem are the use of aerodynamic modifications, stability augmenters, and devices for preventing entry into the pitch-up region. Gener- ally, the approach selected would be onbased carefa weighing of several important factors, such as possible performance lossesto extensive due aerodynamic modification, the comglexity of the stability augmenters or pitch-up preventers required, and the magnitudeof the basic pitch-up ". problem. In reference 1 aa analytical studyof the comparative pitch-up . .. behavior of several airplanes is presented and the computed axe results .. correlated with documented pilot opinion.By comparison of the cm- puted pitch-up characteristicsof new airplane designs with the corre- sponding results from reference1, the probablerelative severity of pitch-up and the associated probable pilot ofopinion pitch-up may be determined prior to actual flight experience. Appliedin this manner, . .. the method is alsouseful for investigating the effectivenessof aerodynamic modifications orof automatic control devices in altering the severity of pitch-up on existingairplanes. This method for estimating - probable relative severityof pitch-up is basedon only the longitudinal dynamic behavior in the pitch-up region: The effectsof othecmodes of motion such as roll-off, airectional divergence, and spin not entry are considered. Although beyond the scopeof the presentstudy, some con- sideration should be given to the possible adverseon effectspilot -1 - opinion of these other modesof motion; this could be basedan inspec-on tion of wind-tunnelrolling- and yawing-moment datain the pitch-up region. 1 In the present report the three approaches.the to pitch-up problem . noted above, that is, theof useaerodynamic modifications, stabillty augmenters, and devices for preventing entry into the pitch-up region, are assessed in the lightof their specific applicationto two example supersonic airplanes - the F-1OlA and F-l&A airplanes. The methods of references 1 and 2 are usedto eyduate the effects on the pitch-up behavior and on probable pilotophion of pitch-up of a wing leading-edge modification and of added pitch damping F-1Ol.Aon the airplane and of' added pitch dampingon the F-l&A airplane. Also presented are the NACA RM A5Z07 3 3 results of an analog study of the effectivenessof a limiting device, c referred to as a stick pusher, for preventing pitch-up altogether on the F-104A airplane. I- DESCRIPTION OF AIRPIAXES Basic Airplanes The two example supersonicfighter airplanes consideredin the present studyare the McDonnell F-lU and the LockheedF,-l&A airplanes. Two-view drawings of these airplanes, hereinafter referred air-to as planes A and B, respectively, and their pertinent physical characteristics are presented in figure1 and tableI, respectively. Wing Modification In aadition tothe basic airplane A, a configm-ationwith the wing leading-edge extension modification shownin figwe 2 was also studied. Stabflity Augutenters Pitch dampers.-Two types of pitch dampers were ccmsideredin the present study. For airplaneA, a pitch damperwas considered which *.; increased thetotal pitch damping to five timesthat of the basic airplane. In this case, itwas assumed themer was operative atall times witha gain constantof 52t/V. (See AppencuX A for definition of symbols.) A block diagram of this lFnear pitch damper is presentedFn figure 3. For airplaneB, a pitch damper witha gain of 52t/V was again assumed. For this case, however, it was assumedthe danper becomes operative onlyin the pitch-up region.The set-up assumed for theanalog cornputations for this nonlinear dampershown is In block-diagram form in figure 4. Stick pusher.- The stick pusheris a device designed by Lockheed Aircraft Corporationto limit the attainable airplaneangle of attack to values below those at which pitch-up occurs. The assumed operational envelope for the stick pusher, providedby Lockheed, is shown figurein 5. When the combinedsi-s from sensors sensitive to pitching velocityand I angle of attack reacha predetermined value establishedby this envelope, an action signal is transmitted to the solenoid valve actuator assembly. which in turn moves the stabilizer and cockpit stick into unisonthe trim position (i.e., the position forstick zero force .in steady level flight). If the pilot does not attempt to override the device, the stabilizer returns to trim at the of rateabout 20' per second. A block diagram, assumedto represent the stick pusherfor the analog cmputa- tions, is shown in figure 6. The dynamic behavior of airplanes A and B in the pitch-up region was computed bymeans of the evaluationmeuver and the basic equations of motion presentedin referenc-e 1. A representative time historyof the evaluationmaneuver from reference 1 is reproduced in figure 7. For the present study-, since wfnd-tunneldata indicated a large decreasein control effectivenessin the pitch-up region for twothese airplanes, these equationsof motion were modifiedto include this effect.Also, the equations were rewritten in ofterms absolute rather than incremental (from n = lg) values of &(u) and Cm(u) since it was desired to record absolute values of a and n. The modif'ied equations are: - -mv(6 - dL) = + %S (a)2t 8.1 qS + w (1) and Several flight conditions were selected for foranalysis each airplane. These are presented in the table beluw. Center of gravity location,- I I I C 1 1 A 0.25 Sea level Take-off 0.286 85 35,000 Cl!=€Ul ,286 1.20 35,000 Clean .2% B 23 Sea level Landing .20 approach .&I 353 OOo Clean 9 07 975 35,000 Clean - 07 *9 I. EACA RM A57K07 5 c - The basic aerodynamicdata for these flight conditions were obtained from wind-tunnel measurements provided by LockheedMcDomell and and are presented in figure 8 and in tableI. This section is' dividedinto two main subsections. In the first, the computed pitch-up behavior of the two example airplanesall the for flight conditions consideredis discussed insane detail. Particular emphasis is placedon the effectiveness of thewing modification andof the linear pitchdAmper QU. alrp7e A and of thenonlinear pitch damper on airplane B in minimizing thepeak angles of attack and thewing and tail loadsin pitch-up maneuvers. Also, the effectivenessof a prellmi- nary version of the Lockheed stick forpusher preventing entry into the pitch-up region is assessed.In the second sectlon, the effectsof the wing modification and of the pitch damperson probable pilot opinionof the pitch-up behavior of airplanesA and I3 are presented and discussed. With the exceptionof the take-offand landing-approach condLtions, all results are presented for initial stick-deflectionr-s correspond- ing to an average load-factor entry rateinto the pitch-up region of approximately 0.5g per second. For the low-speed flight conditions, the initial stick-deflectionramps were programnedto provide a gradual stall entry comparableto that used Amesby pilots in evaluating lg stall characteristics. Computed Pitch-UpComputed Bhaxior \ The results of the computations are presentedin figures 9 through 17. Computed

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