What do the F-14, the L-1011 and the C-17 have in common?
You guessed it: their flight controls involve direct lift control [1]. More recent examples are the F/A-18E/F Super Hornet and F-35C [2]. While all these aircraft make use of the same fundamental concept, the implementations vary greatly. That should be enough reason for us to take a look at this fascinating way of controlling an airplane.
The DLC concept
In order to understand how DLC works, we first need to review how a conventional airplane is maneuvered to change the flight path angle (FPA).
It turns out, maintaining an airplane on a desired flight path is in fact an intriguing interaction of several dynamic processes.
The main wing angle of attack is modulated using the elevator control surfaces at the tail of the airplane. Let us understand how this plays out:
Elevator deflections are used to change the pitching moment, which in turn results in a pitch acceleration and pitch rate that change the angle of attack (AOA) of the wing, which ultimately results in a change of lift coefficient. This yields a change in FPA, which changes the AOA again…
All of these come with some specific dynamics that have been extensively reviewed in the literature [3] [4]. Sometimes, this concept is referred to as the Momentum Control Technique (MCT) [5].
Fun fact: With this concept, the flight path angle always changes “in the wrong direction” first, before settling in the desired direction.
Engineers refer to this as “non-minimum-phase” behavior [1]. The reason for this name lies in the frequency response diagram of such as system. Conceptually, this can be observed in Figure 1: The elevator deflection (trailing edge up) initially leads to a reduction of the total lift before the lift begins to increase again due to the AOA change of the wing. Note that this behavior cannot typically be observed on a pilot’s altimeter, as the extent is very small. When changing from one altitude to another, the non-minimum-phase behavior of an elevator causes the yellow trace in Figure 2. Again, the extent is exaggerated for clarity. DLC, as described later, does not exhibit this behavior.
This immediately leads to the following observation: The ability of an airplane to make flight path angle adjustments is significantly affected by its pitch inertia and the level of passenger comfort required. Furthermore, if the airplane is affected by a wind gust (horizontal or vertical), the pitch needs to be adjusted in order to maintain a given flight path angle.
What if we could change the flight path angle without changing the pitch?
Enter: Direct Lift Control (DLC)
The fundamental idea behind DLC is to directly modulate the lift coefficient cl using some form of control surface. This is most efficiently achieved using trailing edge flaps or ailerons, however some designs also used partially deployed spoilers [2] [6].
Notice how the modulation of the DLC flap around its neutral position enables a significant range of lift coefficients to be achieved. The equivalent AOA range is shown for illustration, if the profile section was fixed and the pitch would be modified to achieve the same change in lift coefficient.
Here lies one of the benefits of DLC: Provided the actuator rate is high enough, the lift coefficient can be modulated much faster and more accurately than using the momentum control technique.
As not all aircraft employ fast actuators for the flaps, some manufacturers elected to use spoilers and/or ailerons instead. Further, it should be noted that the DLC flaps themselves create a pitching moment, which needs to be compensated by the elevators. This is achieved by a proper control law design or by coupling DLC deflection to elevator deflection in a more analog world.
This concept gives rise to a new set of control regimes as depicted in Figure 4.
In a sense, the pitch and flight path angle (FPA) have been decoupled here. This might seem odd, however when exploring the possible applications in a combat scenario or a precision flying task (such as landing approach) the benefits are obvious [7]. It is the landing approach using a constant pitch that we are interested in for our discussion.
History
USAF and NASA have carried out DLC experiments in the 1960’s and 1970’s using F-8 and F-100C aircraft [2][8][6]. In Germany, there was extensive work done in this area involving the HFB 320 and VFW 614 ATTAS research aircraft [6]. The HFB 320 aircraft was evaluated with DLC flaps as well as DLC spoilers and incorporated a “thumb wheel” where the pilot was able to directly control the trajectory [6]. The VFW 614 ATTAS was able to demonstrate a more accurate flight path control using DLC, as well as active turbulence damping [1][9].
The L-1011 could do it all
Apart from research aircraft, there were also very successful applications of DLC in production aircraft, such as the L-1011 Tristar or the F-14. The L-1011 was years ahead of its time and incorporated DLC for the landing approach [10].
The Tristar deployed some of its spoilers during the landing approach to a DLC “null position” of 11°. The spoilers then reacted to longitudinal stick-inputs and significantly reduced pitch changes while improving response times [10].
The engineers of Lockheed had performed a crucial step: They incorporated DLC without adding an additional control inceptor.
This significantly facilitates the task of the pilot. Taking this idea to the realm of modern Higher-Order-Flight-Controls (HOFC) aircraft, DLC is just seeing a revival in more recent times…
Modern application: US navy project MAGIC CARPET
Under the “Maritime Augmented Guidance with Integrated Controls for Carrier Approach and Recovery Precision Enabling Technologies (MAGIC CARPET) project the US navy has significantly modified the flight control law and Head Up Display (HUD) of the F/A-18E/F super hornet fleet (shoutout to whoever made up this acronym). The new control laws incorporate DLC together with HOFC. In other words, longitudinal stick deflections translate into eighter a Flight Path Angle Rate Command (FPARC) or a Delta Flight Path Command (Delta Path). Herein lies the beauty of the concept: The pilot can simply adjust the flight path using longitudinal stick inputs and the DLC flaps will ensure that pitch remains constant (within some boundaries). In the FPARC mode, the pilot changes the FPA, as long as a stick deflection is present. In “Delta Path” mode, the aircraft even compensates for ship speed (!) and maintains a preselected desired path angle.
Additionally, the pilot sees the Ship Relative Velocity Vector (SRVV) on the HUD, which greatly simplifies the aiming task.
The concept can be seen at work in this video.
Observe the flap movements during the landing approach. The MAGIC CARPET project demonstrated the massive benefits of HOFC in combination with DLC. The handling quality ratings (HQR) were significantly better using the new concept and touchdown dispersion was reduced by more than 50% [2].
It will be interesting to see if the benefits observed in this project will trigger similar ideas in the transport category aircraft domain.
References
[1] T. Lombaerts und G. Looye, «Design and flight testing of nonlinear autoflight control laws incorporating direct lift control,» in Proceedings of the EuroGNC 2013, 2nd CEAS Specialist Conference on Guidance, Navigation & Control, Delft University of Technology, Delft, 2013.
[2] J. Denham, «Project MAGIC CARPET: Advanced Controls and Displays for Precision Carrier Landings,» in 54th AIAA Aerospace Sciences Meeting, San Diego, California, USA, 2016.
[3] R. Brockhaus, W. Alles und R. Luckner, Flugregelung, 3 Hrsg., Springer, 2011.
[4] M. Cook, Flight Dynamics principles, Butterworth Heinemann, 2007.
[5] M. Gerrits, «Direct Lift Control for Cessna Citation II,» TU Delft, 1995.
[6] P. Hamel, In-Flight Simulators and Fly-by-Wire/Light Demonstrators, Springer, 2017.
[7] M. Astrand und P. Oehrn, «Direct Lift Control of Fighter Aircraft,» Linköping University, 2019.
[8] NASA, «A flight study of the use of direct-lift-control flaps to improve station keeping during in-flight refueling,» 1973.
[9] K.-U. Hahn und C. Schwarz, «ALLEVIATION OF ATMOSPHERIC FLOW DISTURBANCE EFFECTS ON AIRCRAFT RESPONSE,» in 26TH INTERNATIONAL CONGRESS OF THE AERONAUTICAL SCIENCES, 2008.
[10] Lockheed, «L-1011-500 Tristar Technical Profile».