Preliminary Detailed Design
Team Vision for Preliminary Detailed Design PhaseThis phase brought with it an initial plan to design most of the aircraft and procure equipment to perform a static thrust test to verify the equations we are using to make our speed estimates. Airspeed is a critical component of the calculation of aerodynamic forces wherein the force is proportional to the dynamic pressure times an effective area term. Likewise design work during the second semester is not optimal by any means. We will have a lot of construction and want to leave as much time for that as possible, so the goal was to develop the wings, wingbox, landing gear, fuselage and electrical system by the review on the 19th. The intention was then to leave the last two weeks for the revision of the main lifting and carrying structures and to complete the control and stability surfaces such as the tail, ailerons, and elevators.
On the 12th we experienced a setback. The static test fixture was not welded in accordance with the drawing provided to the shop which resulted in improper fitup. The flat plate experienced substantial warping due to the excessive localized heating of sloppy MIG welding work which drew it out of the tolerance to interface with our sensor. Substantial work will be required to separate the parts and restore them to a working state. It may be more time efficient to simply remake the pieces, but in light of other obligations and setbacks during the week of the 16th no progress was made on the restoration/replacement efforts. This substantially adjusts our testing schedule and will potentially have repercussions during MSD II. We have also adjusted our design mentality to avoid welding at all costs on the Smoke and Mirrors airframe. A failure of this magnitude would present exceptional challenges to our chances of success if it were to strike the airframe.
Design progress was proceeding steadily until the 16th. We expected to have a complete fuselage, wingbox, and landing gear design by the review. The wing was expected to be complete but unintegrated with control surfaces in accordance with plan. As work was concluding on the 16th the storage device containing the project shorted out and we lost approximately two weeks of data. Efforts began on the 17th to reconstruct our work. This was greatly aided by our notes and so we were able to reproduce our work with greater speed than we had originally done it. The current model also benefits from being an "experiential revision" in that we are more experienced with the ins-and-outs of the design. We have made several improvements on this new draft and have driven down the part count. We have managed to get to about 80% of our desired state by the time of the review. We have satisfied, at the bare bones level, all of our hard design requirements but it is rather less refined, analyzed, and documented than we would like it to be.
In light of these setbacks we have updated our plan. The following Gantt Chart should describe an achievable course of action. Our engineering requirements are unchanged and can be downloaded as an .xls.
Final Aerodynamic Design and SizingThe majority of design and analysis concerning aerodynamics, stability,and performance were outlined in detail in previous design phases, with a great amount documented in the subsystems design phase.This iteration concerned aerodynamic design optimization, and the following images below display the outcomes. Directly below are design documentation for drag calculation and landing gear placement, as well as MATLAB script files used to aid in the design and analysis(Right Click-->Save As)Zero-Lift Parasite Drag Calculations
Drawings, Schematics, Flow Charts, Simulations
System Level ViewWe are not currently in possession of a complete system level design but we have made substantial progress towards that objective. The sizing of the tail and control surfaces, while critical, is dependent on many geometric features of the rest of the aircraft. For this reason we have elected to design these features after finalizing the rest of the design. We have had a "finalized" desired form factor for some time but it was expected that small changes and unexpected problems may result in deviation from this. Of particular concern for the sizing of the control surfaces and tail was the drag and projected areas which did change as driven by the requirements of mechanical design. We expect more changes may occur as we revise after this upcoming review, and so we will not engage in the effort of sizing and design when we expect there to be variation which will waste that effort. In the images below is important to note that the motor is not pictured to reduce the clutter of the images and the wheel models being used are placeholders with the desired radius.
We have designed this aircraft from the fuselage and wingbox out during the structural phase after spending most of the semester primarily concerned with aerodynamics. The main structural members were designed around the payload bay in order to ensure a direct transference of our heaviest and most concentrated load between the ground or wing and the payload bay. The fuselage and aerodynamic fairings are in fact secondary to the structure of the aircraft and experience few forces. As can be seen in the image below the payload bay area is approximately the large blue box. The red vectors indicate the desired forces while landed and while in flight.
WingThe Wing in its present form can be broken down into a non-tapered balsa section, a tapered balsa section, and a tapered foam section. The main aluminium spars extend out as far as possible to support as much of the wing as possible. As the main spar could not extend to the tip of the wing, secondary, tertiary, and quaternary spars of wood were added. The design premise is spars or more attached to each wing rib. This is to ensure that no significant moments can be put on the ribs to cause deformation. Another reason for the secondary, tertiary, and quaternary spars was the tendency of the ribs to migrate down and toward the trailing edge as the ribs decreased in size. This dictated that one spar in any particular location would not be able to connect all the ribs in the wing.
The wingbox serves several key functions. Primarily, it is the main connection between the wings, the fuselage, and the tail. Specifically it mediates the load between the main wing spars and the fuselage aluminum structure and is the point of connection for the tail boom. It is also the housing for the servos which control the elevator and rudder. The batteries, receiver, speed controller, and power limiter are stored in the fuselage but most of the wiring harness will be within this structure. It is secured by two 1/4-28 bolts to the fuselage bracket. These bolts will run the entire width of the fuselage. Removal of these bolts will allow access to the payload bay.
The B0003 and B0004 parts are the main load bearing structures within the box. Simulation with COMSOL and expected loading suggests that the parts will be structurally sound. When viewing the COMSOL images on this page please note that the scale is not normalized from image to image. The yield stress of the aluminum is assumed to be 40000 PSI for 6061-T6 aluminum. With this strength we should be able to make the parts lighter. Other example wingbox drawings are contained here and here.
FuselageThe fuselage has four key roles: Support the payload, house the batteries and other electronics, support the motor, and interface between the landing gear, the payload, and the wingbox. For this reason, it is built around a central aluminum structure which we are calling "the bracket". There are two identical copies of this critical component which connect the bolts supporting the payload bay floor to the landing gear and wing box. Our design is built around the plan of interfacing the bracket, with the use of 5 1/4-28 bolts, to the payload bay floor and a basswood inner wall. These two wooden walls and the floor structure formally define the payload bay as required by the competition rules and will support the electronics bay, motor, and the arming plug which is required to be 40-60% of the total aircraft length aft of the propeller.
Analysis of the fuselage bracket is critical because it is subject to the most dynamic loading of the structure. In flight it must support the mass of the fuselage, its contents, and the landing gear from above. On the ground it must support all those weights and transmit them to the landing gear at its bottom. While landing it must ensure that the payload does not become disturbed by the impulse associated with the sudden slowing and load transfer of landing. Analysis by COMSOL suggests that it should be up to the task.
Landing GearThe landing gear must support the aircraft while it is on the ground and provide for stability. Unfortunately, many of the features of an aircraft which lend it stability in flight act to destabilize it on the ground. The main landing gear are in the rear. These must be springy to absorb the forces of landing and prevent a strong impulse from causing damage to the other structures but they must also be strong enough to support 80% of the total weight of the aircraft. The secondary landing gear, located in the front, must only support 20% of the landed weight. It is desirable that this structure be substantially more rigid so as to prevent deformation which may result in the propeller striking the ground. The uneven force distribution of the landing gear is intended to encourage the aircraft to pitch up easily during takeoff and to provide for a smooth landing.
Analysis suggests that the secondary landing gear is substantially over designed. Options to reduce weight are being considered.
Electrical SystemThe electrical system of this aircraft will be built entirely using off the shelf components. They are conventional except for the power limiter which is required by the competition rules. The wiring schema of most model aircraft are practically identical, but we must deviate slightly from the norm. In common use the Electronic Speed Controller (ESC) will supply 5V to the radio reciever, which then sends the necessary control signals to the servos and back to the ESC. All the power sent to the servos will come through this 5V line. The power limiter requires that the signal line which leads from the receiver back to the ESC is passed through the limiter. Under normal operation the throttle signal is passed through the limiter unaltered. If the limiter is tripped by drawing more than 1000 watts for more than two seconds the throttle signal is interrupted and a throttle off signal is replicated down the 5V line to the ESC. This has several key repercussions for our electrical and control surface design, but the most important is that the power draw of the control surfaces does count towards the 1000 watt limit.
Bill of Material (BOM)The current draft of our bill of materials is contained here as a .xlsx. The document is still developing as we work through some issues of efficiency. It is apparent that the cost of two smaller sheets of balsa that may be pieced together into one piece is substantially lower than some larger pieces but doing so adds an additional joint which may have negative repercussions on the structural strength of the airframe. We are in the process of balancing the needs of cost and strength. Likewise, we will need to devote more time to determining the most efficient way to place pieces onto a cutting pattern to utilize the balsa with as little waste as possible. There is no requirement that adjacent pieces will come from adjacent woodcuttings, so we will see how this unfolds more fully when all of our parts have been designed.
At the present the design uses 6061-T6 aluminum for all the aluminum parts. We have parts in .125" plate and 0.09" sheet (these sizes are commonly available) and one in 1"x0.5" rectangular stock. We are considering a transition to 6061-T0 aluminum for parts which well be bent to prevent micro-cracking on the outside of the bends as is common when 6061-T6 is utilized. 6061-T0 is more expensive, less readily available, and has a lower ultimate tensile strength but it will not develop microfractures which makes it a substantially more reliable and predictable structural component for parts which we will be forming.
The document contained here as an .xlsx describes our plans for testing. We have yet to set firm dates because we will need to integrate with the timing of MSD II. We intend to be in progress on static thrust testing by the end of the semester. Most other tests will require completed designs and additional hardware to execute.
Design and FlowchartsTwo system level changes occurred during this phase. The decision to switch from the S1223 to the E423 airfoil was on our minds at the time of the Subsystem design review. The greatly improved manufacturability, reduced pitching moment,and improved aerodynamic efficiency at particular and relevant angles of attack resulted in our decision to switch.
There was a great deal of debate concerning the landing gear. Ultimately, it was decided to utilize a tricycle gear. More in depth analysis demonstrated that the gains to the efficiency of vertical space use were offset by the complications of needing to keep the tail boom permanently secured to the fuselage. There was also debate about the take-off and landing routine of an aircraft using the tail-dragger configuration. In order to quell debate and go with a simpler design we elected to revert back to the tricycle landing gear configuration.