MAE 434WSAE Aero EastPage 1 of 17
TABLE OF CONTENTS
TABLE OF CONTENTS 1
LISTS OF TABLES AND FIGURES 2
INTRODUCTION 3
METHODS 4
RESULTS 8
DISCUSSION 10
PROPOSED WORK SCHEDULE 12
SUMMARY 12
APPENDICES 14
REFERENCE 17
Abstract
The SAE Aero East Design Competition is a rigorous contest of remote-control airplanes with a specific mission. This year competitors’ planes are required to carry a payload of passengers, represented by tennis balls, and luggage around a course and perform basic flight maneuvers like takeoffs, landings, and turns. Unfortunately, there are limitations as well. To achieve the most points, we must design an airplane capable of carrying the highest number of tennis balls possible with a limited 1,000-watt electric power plant to get the plane off the ground. So, our design challenge is to produce an efficient airframe that is capable of holding the weight during flight, that is aerodynamic enough to keep drag down due to its limited power supply, and that is light enough to actually get off the ground. Our specific design elements are the fuselage, wings, tail, landing gear, and payload box (also referred to as the luggage compartment). The fuselage must be sturdy enough to hold our payload and other components to the airframe but must have low drag so our electric motor can move the airplane. The wings must provide the airplane with an optimal amount of lift so as to raise the plane and its payload, as well as be strong enough to handle aerodynamic loads. The tail must have a vertical stabilizer to provide robust control of our airplane. The landing gear must be heavy-duty enough to handle a hard landing, but light and aerodynamic at the same time. The payload box must be small enough to fit snugly into the fuselage but not too small to fit the amount of weight necessary. The design will be challenging in all aspects but will be very rewarding given success.
Introduction
The SAE Aero East senior design project is focused on a competition that judges three categories of model airplane. The categories are micro, advanced, and regular, with the Old Dominion University (ODU) team participating in the regular category. The regular category centers on the use of wood and some metals in the construction of its planes, while the other two categories allow the employment of composite materials[1].This will be the fourth year that Old Dominion University has competed in the event, placing sixth out of 35 teams competing last year.
In previous years, the regular category of this competition has concentrated on designing and building a model airplane that was capable of lifting the greatest amount possible given restrictions on the overall dimensions of the plane and its power. This year, the rules changed to designing and building a model airplane that can carry “passengers” and their “luggage,” so that the aim is high weight as well as high volume. The passengers in this case are simulated by tennis balls and the luggage is simulated by weights in a metal box labeled as a cargo bay[1]. The overall dimension restriction has been lifted to accommodate the extra space required for the passengers, while the power constraint remains, and new strictures have been applied. One of those restrictions is that there must be a minimum of a half-pound of luggage per passenger[1]. Some other limitations include that the passengers must all be in the same geometric plane (no double deck planes) and that the luggage must be in a compartment that is separate from the passengers[1]. In addition, the weight of the airplane after everything has been included (passengers, luggage, and the structure of the airplane itself) must be no more than 55 pounds[1].
The project has been broken down into several subsections by the group leaders and faculty adviser. Most of the subsections relate back to the physical components of the plane such as fuselage, wing, tail, landing gear, and payload box, with some extra testing and analysis on specific components such as the spar caps on the wing and the microlite covering.
Methodology
The standard process of designing an airplane is an iterative one. Typical objectives may be to maximize payload, range, speed, or any one of a host of other aspects. The goals selected determine the particulars of the plane’s design, and in all cases compromises must be reached and agreed upon before it is finalized. To meet the restrictions and requirements of this competition, the airplane will need to be designed with a specific internal volume to accommodate the passengers and with a minimum lifting capacity to accommodate the luggage weight. The airplane must be stable in its flight regime and must meet several other design specifications from the contest rules.
While it would be ideal to build from the plane design of the previous year, significant changes to the contest rules made that impractical. So, the design began by ascertaining the maximum payload (both passengers and luggage) that was feasible for an airplane to carry within the contest restrictions. Simple calculations revealed that a maximum of 60 tennis balls and accompanying luggage could be carried without going over the stipulated weight ceiling. The volume required to hold 60 tennis balls was then computed, and experiments were carried out to determine the most efficient way to carry the passengers while adhering to the contest rules. The rules were very specific concerning the manner in which the passengers had to be accommodated in the fuselage, including provisions to allow for touch counting of them.
Given a goal of a certain number of passengers to carry, the fuselage volume must be calculated to carry those passengers while decreasing area to reduce drag. A minimum wing area must also be computed in order to carry the combined weight of the passengers, luggage, and airframe. Careful study will result in a minimal airframe weight, which will maximize the carrying capacity with the least amount of drag. While a new airplane design is essential for this competition year, some portions may be recycled from the previous plane. To know what components would work within the new design specifications, tests needed to be done. This testing included examining a new type of fuselage, some variations on the old wing design, and the microlite skin covering. A new luggage compartment design was also necessary due to the expected change in the dimensions of the fuselage.
The new fuselage design is based on the semi-monocoque structure. This is where the skin of the airplane is considered to be an important structural member, functioning more than just for streamlining the airplane. Each station or bulkhead frame can easily be contoured to desired specifications, even to the extent that the fuselage can provide some lift. The frames are connected by horizontal members called stringers, and longerons add strength and aid in defining the overall shape. Because the materials used for this project are various types of wood instead of metal, the covering does not add nearly as much structural integrity. This is acceptable due to the light weight of the airframe and the relative magnitude of the expected loads. Because the structural integrity is based mostly on the frames and stringers/longerons, a model was built to determine its limits.
The wing for this competition year uses the same airfoil as previous years, the Eppler 423, but the spacing of the ribs has been changed. In order to determine the new rib spacing, two models were built. The first test wing, referred to as the single-spaced wing, is a replica of the previous wing, while the second one, referred to as the double-spaced wing, is a replica of the previous wing but it only uses half the number of ribs. The testing was set up so that deflection on both wings could be measured while the wings were loaded in two different ways: for pure bending and for pure torsion. It was also decided that both wings should be tested for bending deflection and torsion deflection with the microlite on and with it off. Mounting blocks had to be created to keep the wings from moving at the base while the tests were carried out. The blocks were then clamped to a table upside down to simulate the force of lift on the underside of the wings. They were then loaded with sandbags along the spar caps, which are at the quarter chord line, of each wing. The weight of each sandbag was carefully measured to simulate the fact that the force along a wing decreases as it nears the tip. Dial gauges were strategically placed on the ribs to measure deflection as the wing was being loaded so that exact numbers could be recorded once the loading was finished.
The landing gear on an airplane, even a remote-controlled one, is a very important component. The airplane must be able to survive a rough landing and not collapse on itself. That is why having some damping system designed to absorb the shock from landing is imperative. The landing gear must also be strong enough to support the weight of the airplane. In addition to all these considerations, mass and drag of the landing gearmust be carefully managed to keep them as minimal as possible. For strength, the trailing arm and wheels will be made from aluminum. The wheels however are not solid aluminum; they have been notched to make a four-spoke design. This will reduce their weight significantly. For simplicity and ease of replacement, we will use rubber o-rings for the tires on the wheels that will fit into a groove. The main gear will attach in the front by a tab that will be bolted to the fuselage. The main wheels are attached to a bracket with an axle in the middle. This bracket is attached to the tab at a pivot point allowing the entire bracket to swing and push up on a compressible plastic tube that is attached to the bottom of the fuselage opposite tab.The plastic tube will absorb a large portion of the shock induced by a heavy landing but will also retain structural strength in order to support the weight of the aircraft once it has settled on the ground. The front landing gear will utilize a shaft that will stick up into the fuselage and will have components attachedit towhich will give us steering capability. Attached to the shaft is a spring to absorb bumps in the runway that may cause our airplane to veer off course during takeoff and landing. For the wheel we will use a simple roller-skate wheel. It will be strong enough to support the airplane and the shocks it will receive upon landing, but light enough to keep our structural weight down.
The main concern when designing the payload box (luggage compartment) was the half-pound-per-passenger rule. Because there were no competition rules dictating the size and shape of the luggage compartment, the main size restrictions came from our airplane design. Our team eventually settled on housing the luggage under the passengers, in the area where the wings and fuselage combined. This gave a working space of approximately 7 inches by 7 inches by 3 inches, which was filled to capacity to allow for the most weight and the least amount of freedom of movement. The material the luggage compartment will be constructed from is a mild steel. This is because steel is the only high-density material allowed by the competition rules. The reason behind using a high-density material is that the luggage compartment itself can be counted toward the total luggage weight. This decreases the space needed to carry the same amount of weight. The design for the 2017 luggage compartment was based largely on the 2016 luggage compartment. The luggage weight consists of the steel box, a threaded rod going forward and aft, additional steel plates to fill the box, and two hex nuts to adjust the location of the additional plates. Unlike the 2016 luggage compartment, the 2017 compartment will be tack welded together using the GTAW (tig) method, instead of rivets. Some group members have worked at Newport News Shipbuilding, which contributed to the analysis of the luggage compartment. Considering the job experience and the fact that the luggage compartment should not be carrying any loads, ½-inch tacks with a leg size of ⅛-inch approximately 3 inches apart should be sufficient to hold the luggage compartment together in all considered conditions.
The spar analysis was performed in order to determine the dimensions that would best satisfy the desired safety factor with a given theoretical loading at conditions considered to be maximum operating capacity.The model was constructed in MSC Nastran/Patran and allows for internal stresses and deflections to be found at a large amount of locations. The beam was modeled with the CBAR function in the first trial. It was modeled with the CQUAD function in the second trial.The first model defined a uniform material at each section, whereas multiple materials existed at a single section for the CQUAD model.This was done to ensure accuracy due to the lack of differing materials in the first trial, which are present in the actual spar. The middle portion of the spar has spruce caps and a balsa web.The web was represented as spruce in this portion in the first model when it is physically balsa. The first trial is therefore not an entirely accurate representation of reality.
During the testing, the group discovered that the airplane used for the previous contest year was significantly overbuilt. One of the steps taken in response was to determine what portion of the expected loads would be carried by the microlite skin of the plane, a factor previously assumed to be insignificant. To resolve this, a test was designed that would decide which would fail first: the covering material itself or the adhesion between the covering and the balsa surface of the airplane structure. A rig was built and test specimen created to analyze the microlite and the impact of different orientations of wood grains relative to load direction.
Results
Once the group recognized that the overall structure of the 2016 airplane was overbuilt, additional testing was undertaken and resulted in the removal of excess material and the use of less members overall. The model of the new fuselage was also overbuilt, as it performed as well in its tests as the previous fuselage. Knowing this will allow for a reduction of weight and potentially increased room for more cargo.
The single-spaced wing performed excellently, deflecting little when loaded fully, but the additional ribs added too much weight. Because of this, the double-spaced wing was given more attention and tested until failure at 36.575 pounds. This fell somewhat short of the expected and necessary lifting force for 60 tennis balls, accompanying luggage, and structural mass. As a result, the ribs will need to be slightly closer together than was utilized by the double-spaced wing.
Testing has not yet been done on the new landing gear design, but its performance has been extensively modeled. The modeling includes various analyses that give a general overview of its capabilities.This has given us enough basic information to move forward in other areas for which we might have needed some general landing gear properties.
As of this moment, the luggage compartment has been 3D modeled using AutoDesk Inventor 2016, and technical drawings have been created. The drawings will be submitted to the machine shop at ODU’s Kaufman Hall, where the individual pieces will be fabricated, and a qualified member from the SAE Aero East Design team will tack weld the box together in accordance with the design.
There are no accurate results for the spar cap analysis yet because it is an iterative process and many models will need to be analyzed before solid conclusions can be drawn. One model has been finished and a second is presently being worked.These have given a general idea of what the stress distribution on a spar cap is. That is enough to allow the project to move forward on other fronts that might have needed the spar cap information.
From a test of the microlite skin, it was found that the covering material failed due to excessive creeping before the adhesive ultimately gave out. A further test was performed whose purpose was to develop a stress-strain curve, the slope of which would be the Modulus of Elasticity of the covering material.This last test also provided the most precise results for the ultimate tensile strength for the material.