P16121: SAE Aero Aircraft Design & Build

Subsystem Design

Table of Contents

1 Team Vision for Subsystem-Level Design Phase
2 Aerodynamic Design, Sizing, and Static Stability
3 Concept Selection and Schematics
4 Feasibility: Prototyping, Analysis, Simulation
5 Bill of Materials (BOM)
6 Risk Assessment
7 Design Review Materials
8 Plans for next phase

Team Vision for Subsystem-Level Design Phase

In this phase our primary objective is to drive towards our first aerodynamic design iteration. Most of the required subsystem design and selection was completed and discussed during the system design phase of the project. Details and justifications for these selections can be found in the System Design node. Completing an aerodynamic design iteration is a very complicated and time consuming process. The main factors of this analysis primarily consist of the lifting capacity of the aircraft as well as the requirements of stability. We will determine the tail length, as well as the airfoils and shapes of the horizontal and vertical tails, at least as a first iteration.

Simulation remains an ongoing problem. Verification of results is not yet complete because the results are clearly not reliable. XFLR5 simulation numbers have been used to size the tail and other stability aspects. This has prompted us to consider a change of airfoil from the Selig S1223 to the Eppler E423. The Eppler E423 wing concept is more readily stabilized and offers more efficient lifting, but does not reach the same lifting coefficient until a higher angle of attack. We have concepted both options and are making a second iteration in order to optimize when more appropriate aerodynamic coefficients are known.

The design of the fuselage has reached a fully conceptualized state. Detailed design of structure is set to begin as soon as the aerodynamic requirements are determined. Wing, tail boom, and empennage design are in a similar state. We have determined that it is most effective to use a tail-dragger landing gear configuration. This is a departure from our initial tricycle concept, but allows for a more effective utilization of our constrained 175 inch total vehicle length-width-height and will permit us to place the landing gear directly below the payload to provide maximum support to the largest weight on the aircraft. Electrical design is moving along well. Parts have been ordered and testing of our propeller and motor is on standby until they arrive.

Aerodynamic Design, Sizing, and Static Stability

Please click on the following links to view the detailed preliminary design documentation for sizing the wing, horizontal and vertical stabilizers, and the fuselage, as well as ensuring static longitudinal and directional stability:

Preliminary Wing Design Document

Preliminary Horizontal and Vertical Stabilizer Design, Longitudinal and Directional Static Stability Document

Longitudinal Static Stability Sizing Diagram

Preliminary Fuselage Dimensions Diagram

The following MATLAB scripts were used during the preliminary design process and will be used to expedite iteration and design optimization: (Right Click-->Save As)

MATLAB Script: Preliminary Wing Design

MATLAB Script: Preliminary Horizontal and Vertical Stabilizer Design, Longitudinal and Directional Static Stability

MATLAB Script: Updated Performance (Take-Off & Landing)

Concept Selection and Schematics

Fuselage

The layout of the fuselage structure was selected in this project phase. This structure will provide the main framework for the structure of the aircraft. Three major designs were considered; a truss, a platform and a keel. The truss is basically an aluminum frame that would surround the payload bay. The main components of the plane (wings, tail, landing gear, etc...) would attach directly or indirectly to this frame. The main benefit for this design is its high strength. Compared to the other two designs it scored similarly in cost and weight, while losing points in manufacturability and analysis. These losses come from its complex shape and requirement for welding. The truss places last in the points total for out pugh selection chart. Second place is had by the keel. The keel is essentially a long metal beam that extends the entire length of the aircraft, from nose to tail. Every major component directly attaches to this keel. As a result this design scores average in well. It is easy to manufacture, strong, light and cheap. It scores extremely well for analysis since the analysis would simply be one long beam with a number of forces acting at different points. The winning design for fuselage structure is the platform. The platform is essentially just wooden structure that the payload bay rests on top of. Major components would be attached both directly and indirectly to the platform. This design scores average points for weight, analysis and structure and scores very good for cost and manufacturability. Constructing the platform out of balsa wood will be cheap and relatively easy compared to the other two concepts. The pugh selection chart, along with concept sketches are shown below.

Fuselage Concept Selection

Fuselage Platform Assembly

Landing Gear Mounting

Four different concepts for the aircraft landing gear were considered. Each of these designs are all very similar with slight modifications. Differences include whether or not the landing gear is bolted to the bottom or side of the platform, or both the bottom and side. When considering the side mount, the bolt pattern is the other variation. Our wings will be mounted to the platform in the same area as the landing gear. We have the option to either share the same bolts and bolt location between the wing and landing gear or have them separated. After considering the same criteria as the fuselage, the winning design is a side mount without shared bolts between the landing gear and wing brackets. A pugh selection chart, along with concept sketches are shown below.

Landing Gear Concept Selection

Bottom Mount Landing Gear

Side Mount Landing Gear

Side and Bottom Mount Landing Gear

Landing Gear Configuration

A change in the landing gear configuration has been made since the last design review. Previously we had intended to use a tricycle layout, meaning that there is one wheel towards the front of the fuselage and two wheels towards the back of the fuselage. The configuration that we now intend to build is a tail dragger. The tail dragger will have two wheels towards the front of the fuselage and on wheel all the way in the rear attached to the tail section. There are a number of reasons for this switch. The first is that we will have a more desirable angle of attack at take-off meaning that we should be able to lift-off in a shorter distance. The switch will also allow us to better support the payload by placing the front landing gear more centered under the payload bay. The wheel under the tail will also be designed to absorb the impact if the tail happens to strike the ground on landing. This should help alleviate the risk of aircraft damage when landing the plane. Another major factor in the switch to the tail dragger layout is that we will be better able to meet the dimensional constraints and make other dimensions, such as wingspan, larger. The reason for this is that with the tail of the plane dragging on the ground the tip of the vertical stabilizer in the tail will no longer be the tallest point on the aircraft. In an attempt to illustrate this point, included are pictures of each of these configurations. The first is the tricycle landing gear. Notice the height of the tip of the vertical stabilizer is above the wing. In the following picture of the tail dragger it is shown that the tip of the tail sits below the wing. This vertical height factors into our overall dimensional constraint so decreasing it means that we can increase it elsewhere.

Example of Tricycle Landing Gear

Example of Tail Dragger Landing Gear

Wing Section

Three different concepts for wing section were considered. These all involve how many pieces the wing will be comprised of. The main considerations for selecting a design were repair-ability and mounting. As discussed in the problem definition stage, we are required to make the plane easily repairable. A one piece wing does not fit this criteria, if we were to catastrophically damage a section of the wing the entire wing would have to be replaced. This does not fit our requirement for repair-ability, a two piece does. If the left wing is damaged the right wing is left unaffected and therefore less time and money is required to fix the airplane. The winning wing section consists of two wings and a wingbox. This wingbox is how we are planning to mount the wings to each other and to the fuselage. This will be a heavy duty component that allows us to quickly assembly and disassemble the aircraft.

Wing Section Concept Selection

Wingbox for Three Piece Wing Design

Tail Mounting

Three different mounting locations were considered for the tail. All three concepts were a single beam connecting from the fuselage to the the tail, the variation was the location where the beam connects to the fuselage. High mount, middle mount and bottom mount were the three considered. The high mount is also referred to as the "wingbox mount" as it mounts directly to the wingbox discuss above. The top mount location was decided to be the best solution. The main factor for this location was was the aerodynamics. This beam location keeps the tail of the wing out of the wake of the wings. If tail is not kept out of the wake of the wings then downwash will cause the tail to not behave as needed to maintain stability.

Tail Mounting Concept Selection

Wing Material

Materials considered for wing construction are balsa wood (and/or bass wood), foam and 3D printed plastic. The balsa wood is the obvious choice for our application. Foam will not have enough structure for such a large wing and the 3D printed plastic will be way to expensive and complex to manufacture on a large scale. 3D printing specific sections of the wing with complex geometry, such as the control surface, is still being considered.

Wing Material Concept Selection

Feasibility: Prototyping, Analysis, Simulation

Airfoil

After initial stabilization calculations, the S1223 airfoil has proved problematic. Creating a stable aircraft with this design is more complicated than initially thought and as a result we have been unable to create a stable design. A second high lift airfoil, the Eppler E423, is being considered. With this airfoil a stable design was created for initial sizing. Both of these airfoils are shown below along with some of their important data points and a comparison between the two.

Airfoils

Airfoil Data

Airfoil Comaparison

Fuselage Platform

Initially, the feasibility of the platform fuselage concept, discussed in more detail withing the concept selection subsection, was taken into consideration. The main concern was the strength of a platform constructed of balsa wood having the required strength that would be found in the other concepts. A prototype was constructed and a couple simple strength tests were conducted. The simple platform, not made to the precision of final product, was able to support well over a 45 pound static load. The load was simply weights suspended by a bungiecord from the platform while it was simply supported by two tables. An image showing this test being conducted is shown below. Dynamic loading was also conducted with the same set-up, however this time the weights were dropped from a few inches up in an attempt to simulate extra stresses involved in landing. The platform was able to sustain a number of drops before eventually breaking. Through this feasibility testing we have determined that platform is, in-fact, strong enough for our purposes. A more in-depth strength analysis will be conducted when the detailed design is completed.

Strength Test of Platform Supporting 45lbs

Wing Cord Construction

With the complexity of the S1223 airfoil we initially intended on using we were having manufacturability concerns. The trailing edge of this airfoil is extremely narrow and potentially very fragile. Our main concern was that the balsa wood would shear where the chord becomes thin. One of the potential solutions for this was to 3D print all or a portion of the chord. The plastic material would be stronger and less likely to shear. Printing the entire length of the chord also did not seem reasonable, as was discussed in the concept selection subsection. The solution was to simply print portions of the wing, such as the trailing edge and/or control surfaces. In order to test the feasibility of this we decided to prototype 3D printed trailing edge. A 3D model for a full length chord was created. Due to size constraints of the printer, the last four inches of the chord were printed. This prototype was easily created and had the desired strength characteristics, it was significantly stronger than a comparable balsa wood chord. It was also heavier and curved during the printing process. This curve is typical and occurs because of the uneven drying that occurs. The method of printing a portion of the chord is still a possibility, however it is not desirable. Printing the control surfaces is more likely than printing a portion of the chord. Shown below is a picture of an example balsa chord and our 3D printed prototype of the trailing edge.

Balsa Wood vs 3D Printed Wing Cords

Force of Landing initial analysis

In order to ensure that the aircraft survives the forces associated with landing, we have begun analysis of a worst case landing. The analysis, but it's nature, should grossly overestimate the strength requirement of the aircraft. While refinement of this is desirable design for the worst case is not a poor strategy and will suffice for the time being.

Rough Landing Load Calculation

Landing Gear Bolt Strength Calculation

Our concept for interfacing the landing gear with the fuselage is a simple one: Bolts. We acknowledge that the most likely failure is in the structure around the bolts in our case, and we would like to keep it that way because we have much more freedom to design our supporting structure. For this reason, we have initiated a bolt shear calculation to ensure that we do not have that sort of failure. With the shear loading known, we can choose the appropriate fasteners.

Bolt Shear Calculation

Bill of Materials (BOM)

As our design progresses we have are getting a better idea of what parts will be necessary to build the aircraft. This is still a preliminary list and includes estimates of items we will need. The document is split into two sections, already purchased parts and parts to be purchased. A more detailed document that is broken up by subsystem with specific parts will be constructed in later reviews when a detailed design is created.

Included in this BOM we have added some of the parts donated by Dr. Kolodziej from previous MSD Teams. The parts are listed in the "Already Purchased" section with a cost of $0 listed. Two items have been purchased since the Systems Level Design Review. These parts are the required power limiter and arming plug. These specific parts were required by the SAE Competition Rules. Notable donated materials include two batteries and five servos. The two batteries are each 3s 5000mAh Lithium Polymer batteries. By wiring these two batteries in series we should be able to obtain the required batter power and voltage. The servos are Futaba S3004 which have 4.1 kg-cm of torque at 6.0V. This should be sufficient for our aircraft.

Raw materials, such as balsa wood, monokote, glue, etc.. are included in the BOM with cost estimates. These cost estimates were obtained from advice given to us by the RIT Aero Design Club based on previous plane they have constructed. The complete bill of materials document can be found here.

Financial Overview

3s 5000mAh Battery

Futaba S3004 Servo

Aero Design Club Battery Charger

Risk Assessment

As discussed above, the stability of the aircraft has more troublesome than initially expected. The nature of our wing profile has resulted in a very difficult stability analysis. This analysis is discussed in the feasibility subsection. Because of this complication, a risk has been updated from the Systems Level Design Review. Risk 10, poor control-ability of the aircraft during flight, has seen an increase in likelihood from 1 to 2 with the severity remaining at a 3. The likelihood has increased as a precaution. Our current design is stable, we have just increased the likelihood as a precaution. We fully intend to have a stable design, this risk assessment simply allows us to address this problem with the appropriate level of concern. The updated document is shown below.

Highlight of Change

Updated Risk Assessment

Design Review Materials

The PowerPoint for the Subsystem Design Review presentation can be found as a powerpoint or a PDF. The presentation is scheduled for 2:00PM on October 21st 2015 in GLE-4425.

Plans for next phase

We will be making a new round of design optimizations and a final decision concerning our airfoil at the start of the phase. Simultaneously we will be performing testing as outlined in this excel document.