Flapping-wing flight: modelling - 4 Flashcards
What assumption is quasi-steady modelling based on?
Assumes the instantaneous forces on a flapping wing are equal to the forces that would act on a wing moving steadily at the same free-stream velocity and angle of attack. Unsteady effects are neglected.
In what cases is quasi-steady modeling considered acceptable to model flapping-wing aerodynamics?
For a low reduced frequency of k < 0.1 and for high flapping frequency compared to body natural frequencies w_flap/w_body»_space; 1
What terms are included in a typical quasi-steady model for flapping-wing aerodynamics?
Centripetal forces (not aerodynamic), translational and rotational circulation, added mass, and viscous forces (drag).
Name 3 different approaches that can be used to model flapping-wing aerodynamics. What are the advantages and disadvantages of each?
Quasi-steady models, empirical models, and CFD models. Quasi-steady is highly efficient computationally-wise but not very accurate. Empirical models have no physical information but are very accurate. CFD models allow for physical information and are very accurate but computationally inefficient with no onboard applicability, so they cannot be used for control.
What three simplifications are often made when modeling flapping-wing dynamics?
Assume the system is a single rigid body, so there are no inertia effects. Assume the system comprises multiple bodies that can move concerning each other. Assume we have a time-varying dynamic system. The system responds differently to the same input at different times.
Explain the principle of time-scale separation in the context of flapping-wing flight.
The principle applies the averaging theorem by considering the FWMAVs as nonlinear time-periodic and converting them to time-invariant by averaging over each flapping cycle T. This means we assume the body dynamics to be decoupled from the flapping dynamics since they are characterized by very different frequencies.
What are the advantages and disadvantages of data-driven dynamic modeling?
Allows the identification of a complex model of the dynamics system from experimental data without the need for a priori knowledge. However, it provides less physical insight and is harder to generalize (depending on the test platform data).
What are the challenges of obtaining suitable experimental data on FWMAVs?
It’s hard to replicate the same conditions and create realistic conditions. Furthermore, information is hard to capture in high-frequencies.
What are the main advantages and disadvantages of free-flight and wind tunnel testing?
Wind tunnel testing is easy to repeat the same conditions, reduces the impact of external factors, and is feasible before the FWMAV can fly. However, force measurements might have errors since the FWMAV is fixed, which hinders oscillations that would occur in free flight. Also, only the steady state can be studied.
Free-flight testing is realistic and can study maneuvers. However, oscillations affect sensors; sensors might be hard to design for the requirements, difficult to capture, difficult to replicate, and must be able to fly.
What are the typical dynamic properties of (i) FWMAVs with a tailplane, (ii) FWMAVs without a tailplane, and (iii) flapping-wing insects?
(i) FWMAVs with a tailplane are passively stable
(ii) Tailess FWMAVs are open-loop unstable. Better performance but need stabilization/control.
(iii) FW Insects are open-loop unstable. Better performance but need stabilization/control.
