NURFLÜGLER is the German translation of the term flying wing and distinctively states the projects overall objective: the full integration of aerodynamics, mechanics into a safe and useable wing design. The transition to blended wing bodys is seamless and as the design was iterated, a thickended body midsection was deemed practicial, thus the project evolved into a blended wing-body design.
In this project, we follow a hybrid approach to UAV design - the mix of an iterative aerospace design process and an industrial product design thinking process.
Aerodynamics and main body design inspiration
The flying wing design was chosen early in the design process, as it was deemed an efficient, lightweight principle that could well be applied to the requirements. Compared to conventional designs, it provides the possibility to integrate the hover propulsion system into the main body, thus reducing possible dangerous implications of free, unshrouded hover propellers.
Achieving stability is a challenge for flying wings as they lack the fuselage that provides lever arms to the control surfaces. NURFLÜGLER incorporates an elevator in the tail section and rudders in the winglets to provide a basic amount of controllability. However, as the main mission profile requirement lies in flying straight between launch sites, control surfaces are mainly provided to counter gusts and achieve loitering capabilities.
Without an empenage, flying wings do not provide means to counteract a conventional airfoil’s downwash.Different airfoils or a swept-back wing together with an adequate location of center of gravitiy may be chosen to achieve stability.XFLR5 tests provided a solution in using slightly reflexed cambered profiles for the outer wing sections as they are often found in plank flying wings. The upwash of the airfoil “RONCZ” was reduced by creating a custom airfoil blend from “RONCZ” and MH61 airfoil.Stability is still a challange, but by placing the heavy components like batteries in the aircraft’s nose, NURFLÜGLER achieves a small but manageable static margin of around 2-3%.
Hover propulsion system.
NURFLÜGLER incorporates four main hover rotors and two auxiliary actuators. the auxiliary actuators are small, five-inch rotors located in the wing tips near the winglets. They do not contribute to the system’s overall lift. Instead their small size and fast reaction to control inputs enhances the control authority mainly for the roll axis. By using bi-directional propellers for these auxiliary actuators, a certain redundancy for roll control is given.
Cruise Propulsion SystemEstimations using Javaprop and Propcalc have shown that a custom propeller with high pitch is favorable to achieve desired efficiency.
The main internal structure carrying most of the loads is laid out using carbon rods. They interconnect the inner body section to the propulsion sections and also to the wings.
The installation of the components in the main body section is supported by a 3-D printable honeycomb structure made frome durable ABS(Acrylonitrile butadiene styrene). This allows for a ligtweight installation of the payload bay, avionics compartment, and battery compartment.
Using thin walls of insulation Depron, the individual compartments may be separated against each other. Avionics and battery compartment may be designed allowing waste heat transfer from the avionics to the battery compartment for operation in very cold conditions.
The outer shell of the center and propulsion section will be made from CFK or CFK honeycomb sandwich materials.
For fast prototyping reasons the outer wing sections will be made from foam materials commonly used for radio controlled planes like Elapor or Styrodur. Using protective coatings, these materials are sufficiently resistant against rain.
- two hatches for upper and lower payload bay access- internal moveable walls and additional belt to secure smaller payloads
Landing Gear- convienient access to cargo due to long landing gear- legs seamlessly integrated into aircraft body for drag reduction in forward flight- pressure sensors in legs for automated propeller shutdown on ground
- integrated information display and user buttons in front of aircraft nose above main camera for interaction- additional rear view camera integrated in top hatch for safe 360-degree view during take-off
- NURFLÜGLER disassembles into 5 parts, and fits into a box with EUR-pallet footprint
Deviating from the recommendations, NURFLÜGLER incorporates a different parachute recovery system to the proposed Skycat system. The Skycat sytem is specified for use in UAV from 10-18kg, but in no case exceeding 23kg according to the datasheet.The chosen system from Fruitychutes is capable of recovering UAV up to 29kg, thus will be able to provide safety for the 24-25kg-NURFLÜGLER.
The specified operating temperature range is one of the most challenging requirements. NURFLÜGLER incorporates powerful hover motors from KDE that are specifically built to work in severe condition. The feature high-temperature capable magnets and copper windings. The stress on these motors during take-off and landing is substantial but within the motors specifications.The T-motors chosen for the cruise propulsion system are working continouously at a power output of around 270W, which is considerably lower than the specified maximum short-time output of 750W.Special attention must be paid to the operating temperature of the batteries. Due to the parallel installation of large packs, overheating is not considered a critical issue for normal operation. Instead, cold weather may inversly affect performance. Since the avionics are located close to the batteries, it may be feasible to use their waste heat to heat the battery compartment
- large wing area provides possibility to incorporate solar cells for further efficiency improvements
NURFLÜGLERStephan Mayoungou & Christian Tremplernurfluegler@gmail.com