To generate high lift during takeoff and landing, airplanes use flaps — a hinged panel at the trailing edge of the wing that deflects downward. The flap rotation about the hinge is a sharp, discrete deflection. Now, what if instead of that sharp bend, the entire trailing portion of the wing curved smoothly, more like how a bird adjusts its wing? That is the idea behind smooth camber morphing.
There’s already good evidence — from both simulations and experiments — that smooth camber morphing reduces drag compared to a conventional hinged flap, at the wing level. But does this advantage carry over to airplane level, wherein there is also a tail to keep the airplane trimmed?
Ravi, a PhD student co-advised by Prof. Santanu Ghosh and me, looked at this as part of his thesis. The short answer: yes, smooth camber morphing does remain advantageous at the aircraft level — but mainly during takeoff, landing, and low-speed cruise.
PhD is commonly considered as the pinnacle of education — the highest degree and the final destination. Most people picture it as the grand finale — the end of all of that studying! But that is the wrong way to think about PhD for anyone considering this path. Think of a PhD less as graduation and more as admission — specifically, admission into the very beginning of an academic career. Think of it as the kindergarten of that world.
The traditional education system — with its syllabuses, timetables, and standardised exams — ends with your master’s degree. What comes next is a completely different game.
Whether it’s military reconnaissance or wedding photography, using quadrotors (also known as multirotors) has become the standard. However, one of the primary limitations of today’s electric motor multirotors is their limited endurance, which is primarily due to battery capacity.
Using internal combustion (IC) engines would be much more efficient. While gasoline offers more than 25 times the energy density of lithium-polymer batteries, IC engines respond sluggishly compared to electric motors. Anyone who has driven a car with an IC engine knows the lag between pressing the accelerator and feeling the engine respond—a delay that would be catastrophic for a quadrotor trying to maintain stable flight.
Quadrotors are inherently unstable flying machines. They stay airborne and level only because their electric motors can change speed in milliseconds, constantly adjusting thrust to maintain balance. Owing to delays in carburetors, fuel-air mixing, and combustion cycles, an IC engine seems fundamentally incompatible with the rapid response required to stabilize an IC engine-powered quadrotor.
IC engine-based bi-rotor on a test stand
Equipping IC engines for quadrotor control is akin to transforming a marathon runner into a 100-meter sprint champion. That’s the engineering challenge Ajith, a PhD student whom I am co-guiding with Prof. Ramakrishna, tackled in his research, the results of which are published in an article titled “Throttle-controlled internal combustion engines as propulsion and control units for high endurance quadrotors: a feasibility study,” recently published in Aerospace Science and Technology.
When an aircraft enters a spin—a motion wherein a stalled aircraft spirals downward—predicting its behavior becomes extraordinarily challenging. The aerodynamics are nonlinear and unsteady, and depend not only on what’s happening now but also on what happened moments before. Traditionally, researchers used extensive wind tunnel testing to build aerodynamic models for aircraft spin. These models are significantly more complex than the aerodynamic models required to predict flight during, for example, cruise or turn. Furthermore, this approach is time-consuming and expensive, often yielding models with limited fidelity.
Data-driven modeling offers a promising alternative, with techniques such as Dynamic Mode Decomposition (DMD) leading the way. However, these methods do not directly apply to real flight data, wherein the measurements of outputs as well as inputs (such as elevator deflection) are noisy. Also, the sensors used in aircraft have vastly different noise characteristics. Standard DMD methods fail to produce correct and reliable models in such cases. That’s exactly the problem my PhD student, Balakumaran, tackled in his research on robust aircraft spin modeling using enhanced Hankel Dynamic Mode Decomposition with error compensation—the results of which are published in the Aerospace journal.
In August 2023, Chandrayaan-3’s Vikram lander touched down softly on the lunar surface, demonstrating its precise landing technology. As we continue to celebrate this triumph, it’s exciting to consider how emerging technologies might shape the future of landings - both in space and on Earth.
Whether it’s a lunar lander gracefully descending on the Moon’s surface, a Mars explorer touching down on the Red Planet, or a cutting-edge vertical takeoff and landing (VTOL) aircraft on Earth, the ability to land softly and safely is a complex yet crucial challenge.
Historically, liquid rocket engines have been the go-to for achieving VTOL capabilities in planetary vehicles, largely due to their ability to provide controllable thrust. However, VTOL applications on Earth necessitate a safer option. Enter hybrid rockets, which could prove to be a superior alternative for VTOL operations within Earth’s atmosphere and in space, offering advantages that extend well beyond just safety.
In another post, I had described my PhD student Anandu Bhadran’s work on establishing the thrust controllability of hybrid rocket motors. Leveraging on that, we embarked on a journey to show that hybrid rocket motors can be used for soft landings. Beyond simulations, we wanted to demonstrate soft landing using hybrid rocket motors. However, we did not have the bandwidth, in terms of time and resources, to develop a complete platform for this.
Hence, Anandu, along with Prof. Ramakrishna and I, delved into an innovative approach to this problem – we demonstrated the practical feasibility of using hybrid rocket thrusters in landing platforms with a technique called hardware-in-the-loop simulation (HILS). We reported our studies in the International Journal of Aeronautical and Space Sciences.
IC engine-based bi-rotor on a test stand