Aerodynamic Experiments of a Dual Ducted Fan Vehicle in Hover and Edgewise Flight

Open Access
Author:
Hook, Ryan Franklin
Graduate Program:
Aerospace Engineering
Degree:
Master of Science
Document Type:
Master Thesis
Date of Defense:
None
Committee Members:
  • Dr Dennis Mc Laughlin, Thesis Advisor
  • Dennis K Mc Laughlin, Thesis Advisor
Keywords:
  • Computational Fluid Dynamics
  • Aerodynamics
  • Experimental
  • Ducted Fan / Rotor / Propeller
  • Shrouded Fan / Rotor / Propeller
  • Dual Duct
  • Tandem
  • Wind Tunnel
Abstract:
Aircraft capable of vertical takeoff and landing, as well as hover, have a wide variety of defense and commercial applications. Unrestricted by runway access and the ability to hover on location for extended periods of time allow the vehicle to complete missions impossible for fixed-wing aircraft. In general, helicopters have filled this role, but new aircraft are continuously analyzed in search of an improved alternative aircraft. Recently, a class of unmanned air vehicles (UAVs) has utilized a single-ducted fan to provide lift and propulsion. Compared to the isolated rotor of a helicopter, placing a duct around the rotor increases the total thrust of a vehicle, allowing for the vehicle size to decrease while increasing payload capacity. The vehicle safety is also improved when operating in close quarters by shielding the rotors from strikes. There is also potential for reduced noise with the proper acoustic shielding applied to the duct. However, undesirable aerodynamic characteristics are also associated with the addition of the duct when the vehicle enters forward flight. High drag and large nose-up pitching moments are the two leading aerodynamic flaws experienced by a ducted lift fan as it enters forward flight and encounters edge-wise flow. A ducted fan wind tunnel model was designed and fabricated for the purpose of quantifying these unfavorable aerodynamic characteristics. Instead of a single-ducted fan vehicle, this research explores the unique concept of a tandem dual-ducted lift fan vehicle, which would greatly expand the payload capabilities over a single-fan. Measurements were first obtained from the dual-ducted fan model while operating in hover. Rough profiles of the velocity magnitudes above and below the forward and aft fans were obtained through measurements made with a mini-vane anemometer. The velocities were shown to be more ideal in the aft duct. The flow in the forward duct was further examined with a five-hole pitot probe above and below the rotor, allowing for measurement of the velocity magnitude and the three velocity components. The axial velocity was found to be the dominating velocity component above and below the rotor, while the down-stream and cross-stream components related to the slipstream contraction above and below the rotor. The total thrust of the model was measured with the use of a force balance and the max thrust coefficient was found to be 0.022. With the use of Penn State’s wind tunnel facilities, forward flight was simulated. Flow visualization performed with a single, vertical smoke wire revealed flow separation over the leading edge radius. Velocity magnitude measurements from a five-hole probe and kiel probe also recorded flow separation in the front half of the forward duct. The five-hole pitot probe also measured highly angled flow through the duct. A force and moment balance measured the aerodynamic lift, drag, and pitching moment in several forward flight configurations. The need for a computation tool for the design and analysis of these vehicles was recognized. Working with Dr. James Dreyer of the Applied Research Lab at Penn State, two types of simulations were designed and validated. The first model was a steady state simulation which used a momentum source in place of each rotor to simulate rotor thrust. While this proved capable of rapid-turnaround times, it was unreliable at higher forward flight velocities. The second model was an unsteady, time-dependent simulation which resolved the rotating rotors. This fully-resolved model returned lift and drag forces within 5% of the experimental values but demanded greater time resources in order to reach a steady state. A streamlined nose was fitted over the blunt nose of the original dual-ducted fan wind tunnel model in an attempt to reduce overall drag and flow separation off the leading edge radius. Flow around the streamlined model was analyzed with the use of smoke wire flow visualization and five-hole probe and kiel probe measurements within the forward duct. Decreased flow separation in the forward duct was made evident by the probe measurements due to increased flow velocities near the leading edge radius. Compared to the baseline blunt model, the drag was decreased by 4% in some cases and nose-up pitching moment was increased for most conditions.