Control and Stability in Aircraft Conceptual Design

Transcription

1 Gnat Control and Stability in Aircraft Conceptual Design W. H. Mason YF-17 graphics from Joe Chambers F-104 T-45 Based on AIAA Paper , Control Authority Assessment in Aircraft Conceptual Design, by Jacob Kay, W. H. Mason, W. Durham, and F. Lutze, Virginia Tech and: VPI-Aero-200, November 1993, available on the web as a pdf file, see the reference on the class web page. Aerospace and Ocean Engineering Slide 1 9/30/05

2 The Problem In Conceptual Design The Flight Controls Guys (if they re even there, and worse, they may be EEs!): We need a complete 6 DOF, with an aero math model from -90 to + 90 or else forget it The Conceptual Designers: Just Use the Usual Tail Volume Coefficient Exaggerated? Not That Much! This class requires a reasonable middle ground between these extreme views Slide 2 9/30/05

3 What s a Tail Volume Coefficient? (Hopefully a review) c HT = L HT S HT c w S W, c VT = L VT S VT b W S W L HT C L c W quarter chord S W - wing area S HT - tail area See Raymer, pages , typical values: c HT : 0.5 to 1, c VT : Slide 3 9/30/05

4 What you need to know and do Control and Stability are distinctly different You have to develop a policy for each axis: - stable or unstable? Why? You have to decide how you want to control the vehicle including the control system concept design You have to establish the criteria to determine the amount of control needed You have to have an assessment plan: -How do you know you have adequate control power? The story for each design is different, there are no universal cookbook answers Slide 4 9/30/05

5 To do this you need stability derivatives control derivatives weight and mass properties - the cg range - the moments of inertia flight envelope - where are the critical conditions? Slide 5 9/30/05

6 Some Guidance FAR Part 25 (commercial) and Part 23 (general aviation) tell you where and under what conditions you have to demonstrate adequate trim and control MIL STD 1797 (replacing the MIL SPEC 8785) provides quantitative guidance for handling qualities requirements Some control requirements are performance based rotation at takeoff (trim for seaplanes) We have some programs to estimate some control and stability derivatives, and a spreadsheet to assess Slide 6 9/30/05

7 Typical Conceptual & PD Considerations I Equilibrium/Performance Considerations Normal Trimmed Flight: Classical 1G trim Longitudinal Maneuvering Flight Steady Sideslip Engine-Out Trim Crosswind landing Slide 7 9/30/05

8 Typical Conceptual & PD Considerations II Dynamic Considerations Takeoff and Landing Rotation Time-to-Bank Inertia Coupling - Pitch Due to Roll and Yaw Due to Loaded Roll Coordinated Velocity Axis Roll Short Period and CAP Requirements High Angle-of-Attack/Departure Slide 8 9/30/05

9 Typical Conceptual & PD Considerations III Other Considerations: Gust Non-linear Aerodynamics - High angle of attack Aeroelasticity Control Allocation for multiple controls Special Requirements: weapons separation, stealth, etc. Slide 9 9/30/05

10 Control Authority Assessment Sequence Design Concept: Geometry Mass Properties Aerodynamics DATCOM Computational Aero (i.e., vortex lattice) Flight Conditions: weight, cg location speed, altitude thrust, load factor Control Power Evaluation Requirements: Pass/Fail and why? Slide 10 9/30/05

11 Some PC Tools simple flight condition definitions tool a vlm code: JKayVLM longitudinal (& poor lat/dir) S & C derivatives three surface & two surf + thrust vector trim code a first-cut spreadsheet evaluation of control power. New: Drela s AVL Extended VLM code Slide 11 9/30/05

12 JKayVLM the Vortex Lattice Method follows Katz and Plotkin: vortex rings includes ground effects define longitudinal & lateral surface separately - lateral is very crude approximation define config as a collection of panels, each with a constant % chord LE & TE device puts rings on each panel let code step using finite differences to estimate both stability and control derivatives. Slide 12 9/30/05

13 Single panel: Point 1 %c from LE %c from TE slat Hinge Line flap Point 4 Point 2 Point 3 Note: panel does not have to be in a coordinate plane Slide 13 9/30/05

14 Combine Panel to Model Plane X Y Section No. Wing1 2 Wing2 3 Tail1 4 Tail2 4 X Z Two Tails (my Dir.) Total of Four Sections 4 Longitudinal Model (Top View) Lateral/Directional Model (Side View) Slide 14 9/30/05

15 JKayVLM Validation The following bar charts show the predictions of JKayVLM with DATCOM and actual values for an F-18 type configuration Slide 15 9/30/05

16 VLM Code Accuracy: α Derivatives Data C Lα per rad lift curve slope VLM DATCOM Cm α per rad. Data VLM DATCOM pitching moment slope Mach.2 Mach.6 Mach.2 Mach.6 static margin, % mean chord Data VLM DATCOM static margin Mach.2 Mach.6 Slide 16 9/30/05

17 VLM Code Accuracy: Pitch Rate Derivatives Data C 6.00 L q Lift Due to Pitch Rate VLM DATCOM C m q Pitch Damping Data VLM DATCOM Mach.2 Mach.6-1 Mach.2 Mach.6 Slide 17 9/30/05

18 C Y β VLM Code Accuracy: β Derivatives side force due to sideslip Data VLM DATCOM Data 0.02 C l β Mach.2 Mach.6 Mach.2 yawing moment due to sideslip Data 0.15 VLM rolling moment due to sideslip VLM DATCOM DATCOM Mach.6 C n β Mach.2 Mach.6 Slide 18 9/30/05

19 VLM Code Accuracy: Roll Rate Derivatives Roll Damping Yaw Due to Roll Rate Data 0.10 VLM DATCOM Data 0.05 VLM DATCOM C l p C n p Mach.2 Mach Mach.2 Mach.6 Slide 19 9/30/05

20 VLM Code Accuracy: Yaw Rate Derivatives 0.20 Data yaw damping VLM DATCOM 0.08 roll due to yaw rate Data VLM DATCOM C n r C l r Mach.2 Mach.6 Mach.2 Mach.6 side force due to yaw rate Data 0.60 VLM DATCOM C Y r Mach.2 Mach.6 Slide 20 9/30/05

21 C L δ e VLM Code Accuracy: Elevator Effectiveness Data lift due to elevator VLM DATCOM C m δ e Data VLM 0.15 pitching moment due to elevator Data VLM DATCOM Mach.2 Mach.6 Mach.2 Mach.6 roll due to differential elevator 0.10 C l δ e 0.05 Mach.2 Mach.6 Slide 21 9/30/05

22 VLM Code Accuracy: Flap Effectiveness C L δ flp 1.00 flap effect on lift flap effect on pitching moment Data VLM DATCOM 0.50 Mach.2 Mach.6 C l δ flp C m δ flp rolling moment due to differential flap Data VLM Mach.2 Mach.6 Data VLM 0.05 Mach.2 Mach.6 Slide 22 9/30/05

23 VLM Code Accuracy: Aileron Effectiveness Data C l δ ail VLM DATCOM Mach.2 Mach.6 Slide 23 9/30/05

24 VLM Code Accuracy: Rudder Effectiveness 0.20 side force due to rudder Data VLM DATCOM rolling moment due to rudder Data VLM DATCOM C Y δ r Mach.2 Mach.6 C n δ r C l δ r yawing moment due to rudder Data VLM DATCOM Mach.2 Mach.6 Mach.2 Mach.6 Slide 24 9/30/05

25 A Similar Evaluation of AVL Required Can t trust that you know how to use the code Can t understand code limitations Until you do a complete evaluation as shown above for JKayVLM Slide 25 9/30/05

26 Aircraft Assessment Spreadsheet For several typical flight situations, a spreadsheet containing 11 different cases has been put together. The spreadsheet actually computes the required control deflection or time required to do the maneuver. To use it you need to enter: the flight conditions and the mass properties, both at heavily loaded and lightly loaded conditions, the full range of cg locations and the inertias the stability and control derivatives - corresponding to the each cg position To do the assessment: check that the required control deflection is acceptable check that the time required meets the requirement Slide 26 9/30/05

27 EXCEL Spreadsheets, 11 Worksheets: 1. Nose-wheel Lift-off 2. Nose-down Rotation During Landing Rollout 3. Trimmed 1-G Flight 4. Maneuvering Flight (Pull-up) 5. Short Period & Control Anticipation Parameter (CAP) 6. Pitch Due to Roll Inertial Coupling 7. Time-to-Bank Performance 8. Steady Sideslip Flights (Aileron & Rudder Deflections) 9. Engine-out Trim (Aileron & Rudder Deflections) 10. Roll Pullout 11. Initiate & Maintain Coordinated Velocity Axis Roll Slide 27 9/30/05

28 Just One Simple Example: *********************************************************************** Trimmed 1-G Flight *********************************************************************** Input: Weight (lbs) Reference Area (ft^2) 400 Speed (ft/s) 400 Air Density (slug/ft^3) 2376 C-m C-m-delta E (/rad) C-L C-L-delta E (/rad) C-m / C-L (-Static Margin) -0.1 C-L-alpha (/rad) 4 Output: C-L Required for 1-g trim Elevator Deflection for Trim (deg) AOA Required for 1-g Trim (deg) Slide 28 9/30/05

29 Tested Against Existing Airplane methodology applied to a known airplane results generally good Slide 29 9/30/05

30 Review: The Pitching Moment Trim Story Slide 30 9/30/05

31 Sizing Control Surfaces: the X-plot A rational basis for H-Tail sizing: bigger tip back limit required cg range Tail Size Minimum Tail Size nose up limit nose down limit (unstable a/c aft cg limit) static stability limit (stable a/c aft cg limit) fwd cg location, usually given as %mac aft simplified for illustration: critical aft limit depends on design Slide 31 9/30/05

32 High Angle of Attack Aerodynamics are nonlinear - complicated component interactions (vortices) - depends heavily on WT data for analysis Motion is highly dynamic Exact requirements are still being developed Keys issues: - adequate nose down pitching moment to recover - roll rate at high alpha - departure avoidance - adequate yaw control power - the role of thrust vectoring Slide 32 9/30/05

33 Typical unstable modern fighter + The Hi-α Story Longitudinal Max nose up moment Cm Pinch often around α ~ Cm* α 90 - Max nose down moment Minimum Cm* allowable is an open question Issue of including credit for thrust vectoring Slide 33 9/30/05

34 Typical Swept Wing C m Characteristics C m data from NACA RM A50K27 AR = 10, Λ c/4 = 35, λ = 0.5 Re = 10 million x ref = c/ VLMpc calculation C L Slide 34 9/30/05

35 Real World Pitchup DC-9 F-16 Max Nose Up Max Nose Down Shevell and Schaufele,, Journal of Aircraft, Vol. 3, No. 6, pp (1966) W. H. Mason, Stability and Control in Computational Simulations for Conceptual and Preliminary Design of Aircraft: the past, today, and future?, NASA/CP /PT1, April 2004, pp Slide 35 9/30/05

36 Design Chart to Avoid Pitchup: A starting point Pitching moment characteristics as separation occurs must be controllable. Requires careful aero design. Horizontal tail location is critical Aspect Ratio historical trends from early wind tunnel data Probably OK NASA TM X-26 Probably Pitchup Prone Fighters Transports Quarter Chord Sweep Note: DATCOM has a more detailed chart Slide 36 9/30/05

37 The Hi-α Story Directional + Stable Forebody Effect Cn β w/ Vertical Tail V-Tail in wake Unstable Config w/o V-Tail α 90 Directional problem often around 30 for fighters Slide 37 9/30/05

38 The Hi-α Story Lateral + Clβ Drooping LE devices helps control severity Unstable 0 0 Dihedral Effect (also due to sweep) - α 90 Flow Separates on wing Stable Slide 38 9/30/05

39 Comment on Thrust Vectoring Thrust vectoring mainly provides moments at high thrust - a problem if you don t want lots of thrust! Thrust vectoring moment is near constant with speed Ratio of aero to propulsive moments - propulsion dominates at low q - aero dominates at high q Slide 39 9/30/05

40 Special Issues for Supersonic Flight and Related Planforms Pitchup See Alex Benoliel s Thesis for a survey and estimation method Aerodynamic Center Shift See Paul Crisafulli s Thesis. One chapter addresses ac shift Both available at Mason s web site under: Thesis/Dissertation Titles and Placement. Alex s Thesis is also under design related reports on our web site Slide 40 9/30/05

41 Conclusion We ve outlined the issues, typical criteria and procedures We ve established some useful tools. Each project is different in the details Each individual controls person has to develop the detailed approach for their particular design EA-6B F-15 Aerospace and Ocean Engineering Slide 41 9/30/05