1From conception to realization: A case study of aircraftdesign laboratory course

Amit Batra, Jai Mirpuri, Praveen Gill, Harimohan NavinkumarRajkumar Pant#

Senior Undergraduate students#Associate Professor

Aerospace Engineering Department,Indian Institute of Technology Bombay,

Mumbai 400076, India.

AbstractConceptual design contributes strongly towards development of engineering judgment, andhence is considered to be an essential part of engineering education worldwide. This paperbrings out the importance of student design projects in teaching aircraft conceptual design touniversity undergraduate students. The course contents of an existing aircraft design laboratorycourse were revamped from theoretical/computational work to a more practical, hands-onexperience for the students. The students were divided into teams and were asked to designremotely controlled model aircrafts to meet specific performance requirements, which wereevaluated against a figure of merit. The paper describes the various designs of the aircraft thatwere considered, and brings out the lessons learnt and experience gained, both from studentsand instructors point of view.

IntroductionAs a part of the curriculum of the undergraduate program in aerospace engineering at IIT-Bombay, an aircraft design laboratory course is conducted during the first semester the fourthyear. This is preceded by a theory course on Aircraft Design. For several years, the studentswere being exposed to the aircraft design procedures by making them do hand-calculations inaircraft conceptual design. They were also being encouraged to write computer codes, or usecommercially available software tools to carry out the design.

Of late, it was realized that the laboratory course was not very different from the theory course,and there was little scope for the students to study innovative concepts and gain practicalexposure. The course curriculum was reformed to introduce an element of creativity andcompetition while imparting practical, real-life experience on the intricacies of design activity.

Course organizationThis course for the undergraduate students holds five academic credits for which three hours ofinstructions are allotted in a week. For this activity the idea of team meetings and brainstormingsessions was evolved instead of classroom sessions. Various specific tasks were identified and aschedule was prepared for the whole semester in advance. In addition to this, lectures byprofessional aeromodellers and other experienced people were planned.

The students were required to complete a group project involving the design of a remotelycontrolled aircraft to meet a specified mission. The group project was aimed to achieve thefollowing learning goals for the students:

1. To provide hands-on experience related to Aircraft Design2. To be able to plan and execute a multi-disciplinary design task

23. To be able to successfully present the results of the design task verbally and in the formof a report and drawings.

4. To learn to work efficiently in a group and as a member of the group

The specific tasks to be carried out by the students were:

1. Study of the various aircraft concepts that can carry out this mission2. Carrying out initial sizing studies3. Geometrical sizing of wing, fuselage, tails, payload bay4. Deciding the aircraft layout and aerofoil cross-sections5. Design of mechanism for payload release6. Preparation of detailed drawings and instructions for the aero-modeler7. Liaison with the aero-modeler during construction and flying8. Preparation of a detailed design report

For this purpose, the class was divided into four groups each consisting of 5 or 6 students withone member as the group leader. Within a group, individual students were assigned specificresponsibilities viz., literature survey and documentation, design or liaise with the aero modeler.

Initially, all the four groups were to work independently. After four weeks, all groups were to makea presentation on their concept, and submit a report on their design. The best two out of the fourconcepts were to be chosen for further study, and then two groups were to be merged. The twogroups were then required to prepare the detailed production drawings and plans, and submitthem for construction. They were then required to liaise with the model maker, incorporate anychanges/modifications and test fly the model, before its final evaluation.

The ScheduleThe stage-wise breakdown of the task, along with the time allotted for each stage was as follows:

Tasks WEEK NUMBER

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15Problem Formulation *Literature Survey * *Conceptual Design * * *Selection of two concepts *Prepare report and productiondrawings * *

Model fabrication * * *Modifications * *Flight Testing * * *Evaluation * *

Criteria for evaluation of the studentsThe weightage for various tasks was assigned as under:

1. 20% for ingenuity of the concepts and the design philosophy employed2. 15% for the report and presentation related to the conceptual design3. 20% for clarity and detail of the drawings and construction instructions4. 20% for the performance of the end-product vis--vis the mission requirements5. 25% for the quality and technical detail in the final design report

3Resources made available to students1. Books related to model aircraft design and aerodynamics2. Powerplant system with propeller and fuel tank3. Professional help for construction of model as per drawings, and flight-testing.

The students were expected to carry out this task without seeking any direct help or guidancefrom the faculty members, staff members, or other students of the institute. Each team was todiscuss their progress and problems faced by them during the weekly meetings.

Design objectivesThe aircraft was to be designed around a given powerplant system i.e. engine, propeller and fuelweight. The mission was identified as typical of a model power-assisted glider. The details of themission and the design objectives were laid down as follows:Design an aircraft having minimum all-up weight and lowest overall cost, that remains airbornewith a specified payload till a given amount of fuel runs out, after which the payload is dropped,and the aircraft is glided back safely to the starting point.

The payload was a wooden blackboard duster (weighing 0.157 kg), and the engine used was OSMAX 15LA model engine, details of which are given in Figure 1.

Figure 1: Three-View Drawings of the OS MAX 15LA engine

The overall cost included the cost of basic material, equipments such as servo-controllers,engine, propeller and the cost of construction of the model.

Release mechanismAn important part of the design was that of the payload release mechanism. Students were told todevice mechanism using which payload drop can be initiated at desirable time during theaircrafts mission.

Phases of the activity

Literature SurveyThe students did not have any prior experience in aeromodelling; hence they started off bybrowsing through a number of websites (e.g., [1], [2] and [3]) to get an overview of aeromodelling.This provided them an insight to the different types of model aircraft such as glider, trainer,aerobatic models, and the different materials and methods used for construction of radio-

4controlled models. The design of a model aircraft requires knowledge about various issues likeweight estimation, selection of design parameters, wing design, airfoil selection and choice ofcorrect materials. Guidelines for design of such models were obtained from standard referencebooks for design of radio controlled model aircraft, such as [4] and [5]. Next, the attention wasfocused on obtaining details of specific designs of model gliders for similar mission requirements.Literature related to new model aircraft designs such as Turning Point [6] and Ole Man Mose [7]helped in drawing a few important conclusions about the design features. As mentioned above,each group had to submit full-scale drawings of their designs with all the details like, the sparlocation, number of ribs to be used, the payload drop mechanism, the location of the payload, andthe position of the servos. Hence, sample drawings of model gliders (e.g. [8]) were obtained andstudied in detail.

Some software packages available on the internet for performing certain analyses were identifiedand downloaded. These are briefly described below:

Profili 1.2 [9]: A software package containing over 18,000 airfoil sections with provisionfor adding and modifying airfoils for different requirements.

VisualFoil 4.0 [10]: A software to duplicate the function of a wind tunnel and generatesaerodynamic data.

Airfoil Web [11]: Java applet for estimation of the lift and moment coefficients of an airfoil.It also estimates the boundary layer properties and aids in calculating the friction drag ofthe airfoil.

XFOIL [12]: It is an interactive program for the design and analysis of subsonic isolatedairfoils. It consists of a collection of menu-driven routines, which perform specificfunctions.

Panknin Twist Formula Template [13]: The fundamental information required to designand build high performance swept wing and tailless sailplane is available on this web site.

Conceptual design proposals of various groupsThe first group proposed a hand launched flying wing configuration with skids instead of landing gears,citing low weight and excellent glide characteristics as the deciding factor [14]. The payload was to becarried under the fuselage and the proposed release mechanism is shown in Figure 2. The rudder andaileron were coupled to reduce the cost of one servo-controller, and reduced all-up weight.

Figure 2: Top and side view of the flying wing configuration and its release mechanism

The group also developed a spreadsheet [Table-1] to evaluate the initial sizing of the model aircraft thatthey were building. The spreadsheet made an estimate of the longitudinal stability margin of the aircraftthat can be compared to the required margin and each configuration can be evaluated.

5Table 1:Spreadsheet of initial sizinggms ounces Atmosphere SI

Weights 28 Density 1.225Propulsion 125 4.46 Viscosity 1.85E-05Battery 100 3.57 C/LServos (three) 105 3.75 Takeoff 1Payload 160 5.71 Landing 0.8

Subtotal 490 17.50 Glide 0.43Airframe 300 10.71 C_D0 0.01

0.43415435 cl @ max l/dGrand Total 790 28.21 C_D = 0.0433

L/D max = 10.03Landing Weight 630 22.50Takeoff Weight 790 28.21Climb Weight 790 28.21 W/S S-wingPerformance m/sec f/sec kmph mph Reynld no SI Ozs./ft.sq feet sq

0.3048 0.28 0.5 0.30Landing Speed 10 32.81 36.00 20.00 1.36E+05 5.00 16.59 1.36Glide Speed 12 39.37 43.20 24.00 1.59E+05 3.87 12.84 1.75Wing geometry Structural weight of wing Area 1.75 Sq. Ft. ozs./sq.ft. = 6.18AR 6 Est weight = 10.82 ouncesTaper 0.6 303.09 gmsSweep (1/4 crd. line) 15 deg 300 last guessDihedral 0 deg Sr.No C.G. Estimation

Feet Inches Part Weight Position X Position YSpan 3.24 38.91 1 Engine 125 -1 0C_root 0.68 8.11 2 Servo1 35 4 0C_tip 0.41 4.86 3 Servo2 35 4 0

feet inches 4 Servo3 35 4 0Aerodynamic Center 0.37 4.42 5 Battery 100 8 0MAC- (y) for half wing 0.74 8.92 6 Circuitry 0 0 0MAC 0.55 6.62 7 Wing 310 6 0Neutral Point(x) 0.42 5.08 8 Fin1 15 10 0

9 Fin2 15 10 0Stability Margin Desired 7.0 % 10 Skid 12 3 0Stb. Mgn. with Payload 6.7 % 11 Ld Gear1 12 2 0Stb. Mgn. w/o Payload 6.6 % 12 Ld Gear2 12 2 0

13 Skid1 12 2 014 Skid2 12 2 015 Payload 160 4.6 0

890CG with Payload 4.63 0.00 inchCG w/o Payload 4.64 0.00 inchDesired C.G. Location 4.61

The second group proposed a pencil fuselage design with a podded gondola. The payload was to be housed inthe gondola, which was expendable. The use of aileron was discarded and all moving horizontal tail and rudderwere proposed as the two control surfaces. The possibility of an expendable landing gear was suggested. Theidea was to meet the mission optimally with minimum add-ons. Figure 3 shows the sketch of the proposeddesign.

6Figure 3: Pencil fuselage with podded gondola

The remaining two groups came up with conventional designs for the model aircraft. One of the maincontributions from these designs was the suggested coupling of the release mechanism with theelevator control to save the cost and weight of one servomotor. The other group suggested a simple pullrod mechanism for payload release as shown in Figure 4.

Figure 4: Simple pull rod mechanism to drop the payload.

At the end of these presentations the panel of judges suggested changes in each of the designs. Theflying wing concept had to be abandoned because of the lack of experience in building such models. Assuch, the number of flying wing models built till date are very few. The pencil fuselage was notrecommended since it was felt that it could not withstand the weight and the vibrations of the engine.Also the change in the center of gravity caused by dropping of the payload with the gondola could causethe aircraft to go out of control. It was also decided that the models would be hand launched. Thisremoved the task of landing gear design from the process.

The four teams were then merged into two, the AIRBUS team and the BOEING team, and wererequired to come up with modifications in the designs. The aim of this phase was for the students tolearn the limitations that come up when moving from design to the manufacture phase. Owing to theinsufficient experience and lack of design tools and experience, the general consensus was to go aheadwith two separate designs having rather conventional configuration and two servo-controllers for rudderand elevator control. Also since it was a first attempt at building aero-models it was decided to stick toproven configurations, since the thrust of the project was to get flying models and not to develop fancydesigns. Figure 5 shows the sketch of one of the conventional designs that was suggested.

7Figure 5: Conventional design with a separate servo motor for release mechanism

Evolution of the detailed designAfter the first evaluation, of the conceptual design, the two teams were asked to subject their designs todetailed analysis, leading to the identification of the most critical design requirements. The studentscarried out an in-depth study of the various plans and drawings of existing and well-proven modelaircraft, to get a first hand feel of the design process, and to obtain pointers to the specific designfeatures that would lead to a better design.

Next, they selected appropriate aerofoil sections for the wings and horizontal and vertical tails of theiraircraft, and attempted to carry out a preliminary estimation of the aerodynamic characteristics andmass breakdown of the aircraft. The sizing and configuration layout of aircraft was then carried out usingthe established design procedures for model aircraft from [4] & [5]. At each of the above stages, thestudents had to apply the concepts that they were taught in the aircraft design course that they hadtaken the previous semester. The AIRBUS team decided to go in for a simple wing having dihedral wingfrom the root itself, whereas the BOEING team went for a polyhedral wing. The sample designcalculations of the AIRBUS group as follows.

Initial sizing or weight estimation forms the first step in aircraft design and it involved the following.Engine Weight = 0.15 kgServos Weight = 0.105 kg (3 x 0.035)Payload Weight = 0.157 kg

Battery pack Weight = 0.095 kgTotal Fixed Weight = 0.50 kg ~ 0.51 kg

From [4], total weight can be approximated as twice the fixed weight and henceTotal Aircraft Weight ~ 1.0 kg

The important design parameters for the given mission have the following range of values as given in[4].

Aspect Ratio = 8 to 15.Wing Loading = 20 to 40 N/m2L/D = 10 to 16.

8An iterative procedure was followed to obtain the values of the design and other geometric parameters.The iteration starts by assuming initial values of Wing Loading and Aspect Ratio and subsequentdetermination of lift coefficient (CL). This leads to the selection of airfoil and then to estimation of L/D.One or both of the design parameters are altered until acceptable values of all the design parametersare obtained. The values obtained after the final iteration were:

Initial values ofWeight = 1.0 kgWing Loading = 30 N/m2Aspect Ratio = 10Density of air = 1.225 kg/m3

ThereforeWing Area (S) = 0.33 m2Span (b) = 1.8 mChord = 0.18 m

At Cruise Airspeed Vcr = 14 m/s (assumption based on experience)CL= 2W/(Vcr2S)= 0.25

At Landing Airspeed Vland = 8 m/s (assumption based on experience)CL-land = 1.00

The airfoil section was selected based on the above values of CL required by the model. Since theremote controlled models fly at very low speeds there was a need to select airfoils that have high liftcharacteristics at low Reynolds numbers. The Selig 3021 flat bottom airfoil section was selected by theAIRBUS group from the Profili software database and the download is available at [9]. The BOEINGgroup on the other hand chose the S1223 [15]. The horizontal tail area and vertical tail area werecalculated based on the thumb rules given in [4]. Important characteristics of the airfoil for low Reynoldsnumbers (60,000 to 100,000) typical for R/C models have been studied from [5]

Horizontal Tail Area = 20 % of Wing Area = 0.064 m2Vertical Tail Area = 6 % of Wing Area = 0.02 m2

The location of the Center of Gravity (CG) of the model has a direct influence on the longitudinal stabilityof the model. The CG location was intended to give adequate stability margin and hence has beenplaced at 15 % chord from the leading edge of the wing. The relative positioning of the engine and theservos was determined by balancing them about the above said CG position. The payload waspositioned such that its CG was near the CG of the entire aircraft.

The calculations shown above resulted in a conventional type of aircraft designed by the AIRBUS group.The BOEING group also went through a similar process in designing their aircraft. Figures 6 and 7provide the details of the wing designed by BOEING group. Figures 8,9 and 10 provide the details of thewing and empennage designed by the AIRBUS group.

9Figure 6: Wire frame and rendered view of the wing designed by the Boeing group

Figure 7: Front view and top view of the tapered, polyhedral wing designed by the Boeing group

Front view ofthe polyhedral

wing

Wire frame View

Rendered View

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Figure 8: Empennage of the Airbus group design

Figure 9: Rendered and exploded view of the Airbus group wing

Figure 10: Construction details of the wing designed by the Airbus group

Design of the release mechanismThe students realized very quickly that a good and reliable release mechanism was essential forsuccessful completion of the mission; hence they decided to spend a considerable amount of theirenergy and time in its design and testing. Several radical concepts were suggested, including the use ofcontrol surface deflection beyond expected values during normal operation of the aircraft to trigger therelease sequence. Despite the fact that such a scheme ran the risk of inadvertent release of payloadduring the flight, the BOEING team decided to go ahead with using the rudder control servo-motor for

Side View

Rudder Deflectedby 15 degrees

Top View

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payload release, since it lead to a narrower fuselage (hence lower drag), and a reduction in the cost andweight of a servo-motor. The AIRBUS team decided not to take this risk, and went for the provision of aseparate servo-controller for its release mechanism.

Model fabrication, integration and initial trial runsThe students maintained a constant liaison with the model-maker during the entire fabrication process,and some changes to the layout and dimensions were made to both the models after several rounds ofdiscussions, on the basis of the model-makers previous experience. The students learnt that they hadoverlooked many fabrication related issues while carrying out their designs; for instance, the skinthickness specified in the wing-tip region of the aircraft by both the teams were far below what waspossible to fabricate. Further, the trailing edge had to be fabricated with an insert instead of joining oftwo thin sheets as was suggested in the drawings.

The initial shakedown flights were carried out in a progressive manner, first without the payload,and then with the payload. The flight characteristics and handling of both the aircraft were foundto be satisfactory. However, the release mechanisms of both the aircraft showed some problems,which were attended to and solved by the students in situ. Table 2 lists the main differences inthe two designs.

Table2: Features of the two designsAIRBUS BOEING

Dihedral wing Polyhedral wingRectangular wing Tapered wingStrut type Horizontal and Vertical tail Solid plate type Horizontal and Vertical tailSeparate Servo-controller for payload drop Payload drop mechanism coupled with rudder

servo-controller.

Final Evaluation of the projectOn the day of the final testing and evaluation both the aircraft flew very well, and fulfilled themission requirements. The payload release mechanisms of both the aircraft worked properly inthe final evaluation flight. The flight characteristics and handling of the aircraft developed by theAIRBUS Team were found to be much better, and in one flight, it remained airborne for morethan 8 minutes after encountered a favorable upwind! However, the aircraft developed by theBOEING Team was declared the winner, as it could complete the mission with lower all-upweight and with only two servomotors (compared to three in the other). After the completion ofthe mission, the weights of the components of the two aircraft were obtained and are listed inTable 3.

Table 3:Weight components of the two designs

Component AIRBUS(kg)

BOEING(kg)

Fuselage 0.195 0.190Wing 0.360 0.240Servo controller 0.105 0.070Power plant system 0.300 0.300Total Empty weight 0.960 0.800

Lessons learnt from the experimentsStudents realized the iterative nature of design process and also that it may not be possible totake up unconventional configurations, unless the appropriate design database and analysis toolsand prototype are well developed and validated. However, the successful completion of themission by both the aircraft indicated that when established procedures are followed, the resultingdesigns are likely to perform and handle well. The design of the payload release mechanism gavethe students an insight into the study of mission-critical parameters, and some room for trying outtheir creative skills and ideas. Students also learnt that even while designing an outwardly

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conventional aircraft, there is some room for trying out innovative concepts, if certain calculatedrisks are taken. The successful coupling of the payload release mechanism with the ruddercontrol was one such lesson learnt.

Many suggestions for the improvement of the undergraduate study program emerged as part ofthis course, for instance the inclusion of a chapter on low Reynolds number aerodynamics andthe need for a more practical exposure in the Flight Mechanics courses. The students alsosuggested that this laboratory activity and the aircraft design course that preceded it should beconducted a little earlier in their Undergraduate program. The experience of seeing a producttake shape from conception to reality within a time span of four months also provided them witha lot of thrill and satisfaction.

ConclusionsThis experiment provided a first hand exposure to the students to the dual processes in a designexercise, the creative process in which various design concepts are considered, and theanalytical process in which the concept with best chance of success is selected and furtherdeveloped.

This experiment developed the vocational skills of the students and instilled in them anawareness of the complex, multidisciplinary and integrated nature of aircraft design. It wasestablished that the best way of imparting design knowledge is for the students to learn design bydoing it themselves, in a structured manner. Further, the students were also made to undergo theexperience of working synergistically, by placing them in a project group with an individualresponsibility, but having to cater for the needs of the group and project as a whole. The need forcontinuing changes in engineering teaching curriculum has long been felt, emphasizing theinclusion of more intense design education to meet the requirements of industry. It can beconcluded that this experiment has been successful from an educational standpoint and wouldserve as an effective model that could be adopted by other universities.

References:

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40-2.5. Simons Martin, Model Aircraft Aerodynamics, Nexus Special Interests Ltd., 1999, ISBN 1-

85486-190-5.6. Phil Owen, Experts Forum- Phil Owens way with A/2 gliders, Aeromodeller, January 1986.

pp 42-457. Andrew Crisp, Ole Man Mose, Aeromodeller, March 1986. pp 136- 1398. Telspark, Aeromodeller, July 1986. pp 405 4079. www.kenob.supereva.it/profili.html?p10. www.hanleyinnovations.com11. http://beadec1.ea.bs.dlr.de12. http://www-aero.aae.uiuc.edu/~m-selig/profoil/041-xfoil.html13. www.halcyon.com/bsquared/twist2.xls.zip14. Nickel Karl, Wohlfahrt Michael, Tailless Aircraft in Theory and Practice, translated by Eric

M. Brown, Edward Arnold publishers, 1994.15. Selig, M S and Guglielmo J J, High-Lift Low Reynolds number airfoil design, Journal of

Aircraft, 34(1) pp. 72-78, AIAA.