For a conventional aircraft the moment created by the tail needs to be balanced by the moment created by the main wing. These moments are referenced to the center of gravity of the aircraft. Aircraft trim does not guarantee stability.
$$Wing\ Lift\times Wing\ Lever=Tail\ Downforce\times Tail\ Lever $$
As the AOA increases, the wing lift and the tail downforce also increase but the downwash from the wing means that the wing AOA increases more than the tail AOA.
Trim points are used to linearize the nonlinear aircraft dynamics for easier simulation and analysis. When this occurs the parameter us a small perturbation from the referenced trim state. For example
$$U=U_e+u$$
where $U_e$ represents the value at the trimmed state, while $u$ represents the small perturbation from the reference. For different trim points (speed) the model for the aircraft throughout its speed range can be calculated by forming a system matrix for each speed. For reversible control systems the stick force gradient decreases with increasing speed.[3] This means that as the trim speed increases, the ability to trim the aircraft decreases.[3] Some aircraft change the trim by moving the entire horizontal tail.[3]
[[Stability of Conventional Aircraft]]
[[F-18 OFP v10.1]] – F-18 had pre-set takeoff trim conditions
[[Aerodynamic Stability Derivatives]] – stability derivatives are evaluated around a trimmed state
[[T-38 Flight Control System]] – used actuators to adjust the neutral position of the artificial feel system bungees
[[Mechanical Flight Control Systems]] – The pilot would like to trim the stick force to 0 at a new stabilized flight condition.
[[Stick Trim]] – pilot adjusts the trim tab
[[A3J-1 Control System]] – Uses a bungee for trim
Artificial Feel Systems
[[Challenges of Oblique Wing Aircraft]] – trimmed by rolling to a bank angle and deflecting the controls in a coordinated fashion
[[AS-5]] – had 0 trim at LD_max
[[Superaugmented Aircraft]] – pitch attitude dynamics are appropriate for low frequency and trim.
[[RASCAL Baseline Control System]] – low-frequency integrator slowly trims the stick to center
[[F-22 Flight Control System]] – Used a pitch integrator to eliminate trimming
[[BP4.7]] – automatic trim functions are desirable
[[Forward Swept Wing]] – it is important to select a proper CG position to minimize trim drag
[[Space Shuttle Aerodynamics]] – body flaps are used as a longitudinal trim
[[F-18 Longitudinal Auto Flap Up]] – integrator provides constant trim for the aircraft
[[Superaugmentation Architecture]] – he \(\theta,\ \dot{\theta}\rightarrow \delta_e\) mechanization requires elevator trim when \(\theta\neq0\)
[[Superaugmented Disturbance Sensitivities]] – speed-based systems create pitching moments that are proportional to the change from a set trim.
[[L-1011 Tristar]] – used an all-moving tail surface for trim
F-14 Wing Pivot Selection– wing pivot point selection was critical for reducing trim drag.
[[Linear Model Extraction]] – creating a linear model around a trim point
[[F-8 DFBW CAS]] – forward path integrator provided zero steady-state error
[[SAAB Gripen Aerodynamics]] – balances trim with canards and elevons
[[Viggen Flight Control System]] – used to manage trim drag
[[Experimental Aircraft Program (EAP)]] – canards helped to minimize the trim drag
[[B-1 Pitch Control Channel]] – Initial trim system could overpower the pilot
[[B-1 Roll Control Channel]] – spoilers used a deadband to avoid trim drag
[[Relaxed Static Stability]] – has smaller trim forces on the tail
Sources
- [1] “John Roncz Presentation Airventure 2011 – YouTube.” Accessed: Apr. 22, 2023. [Online]. Available: https://www.youtube.com/
- [2] “CAA Paper 2009/02The Aerodynamics of Gyroplanes”.
- [3] Air Force Test Pilot School Edwards Afb Ca, “Volume II. Flying Qualities Phase. Chapter 14: Flight Control Systems.:,” Defense Technical Information Center, Fort Belvoir, VA, Dec. 1988. doi: 10.21236/ADA319984.
Backlinks
[[KC-135 Flight Control System]]
[[Linearized Flight Dynamics Around a Trim Point]]
[[Moments]]
[[Prediction of Accuracy of Aircraft Stability Derivatives]]