Aerodynamic and Structural Design of a Small Nonplanar Wing UAV
Summary (7 min read)
1.2 Thesis outline
- Chapter II Background and Literature Review.
2.1 Unmanned Air Vehicles
- The gross mass of small air vehicles and other flying objects vs chord Reynolds number is shown in Fig. 4 .
2.2 Nonplanar wing concepts
- After this general discussion about nonplanar concepts the next section reviews some specific concepts of the past, their advantages and disadvantages and some important theoretical concepts associated with them that will be revised in later chapters.
- Induced drag is proportional to the square of the lift being generated.
- If that lift is split evenly between the 2 wings, then each wing should have only one-fourth of the drag of the original wing  .
- Therefore the total induced drag for the biplane system should be one half of the induced drag of a monoplane with same span.
- But mutual interference effects prevent taking advantage of the full reduction and so a good design can yield on the order of a 30%.
- The main important concepts of the biplane configuration and of biplane theory will be presented next in order to ease the transition to the next chapters of the thesis.
22.214.171.124 Biplane: configuration and theory.
- The procedure was very tedious and therefore was rarely incorporated into use by the designers of that day.
- Walter Diehl , then, published 2 more practical reports on biplane theory in which he combined experimental and theoretical data by Fuchs and Hopf  to obtain a series of curves from which the lift curves of the individual wings could be found.
2.2.2 Winglets and wing tip devices
- As for the biplane, the main concepts for winglet theory are summarized below.
126.96.36.199 New winglet concepts
- Finally in 1994, Gage  produced a complexity evolutionary algorithm, which was allowed to build wings in many individual elements with arbitrary dihedral and optimal twist.
- Fig. 11 shows front views of the population of candidate designs as the system evolves along with the best individual from a given generation.
- The system discovers winglets and then adds a horizontal extension to the winglet, forming a Clike shape.
- Further studies showed that the optimal loading on the horizontal extension was downward, reducing root bending moment and providing a positive pitching moment when incorporated on an aft-swept wing.
- This led to some interesting studies with Boeing on the application of the concept to a blended wing body.
2.2.3 Closed system
- The next section deals with the most famous one, the joined wing configuration and with the Houck Configuration, since the nonplanar configuration under study is very similar to this wing configuration.
188.8.131.52 Houck configuration
- In an effort to understand the aerodynamic behavior, this configuration has been the subject of some experimental studies.
3.1 Functional requirements
- To design a small, lightweight, affordable man-portable/backpackable and hand launched aircraft capable of providing reconnaissance, surveillance and remote monitoring day and night imagery.
- Such a small reconnaissance plane can loiter about a ground target over a hill or behind a building, without being detected and without exposing human personnel to harm.
3.3 Specifications 1. Portability
- The aircraft and transmission equipment must both be man/backpack portable so that they can be transported and used easily anywhere that surveillance is necessary.
- In order to reduce the deployment time, the aircraft must be also quickly assembled.
2. Low-profile visibility, low-observability
- Due to its low target altitude, to avoid the possibility of being detected by enemies, the feature of small size of the UAV must be pursued throughout the design process.
- To achieve the same goal, the proposed aircraft must be also able to perform quiet operations.
- Therefore the feature of light weight must be pursued during the design process.
- The mission goal of the proposed UAV is to provide reconnaissance and surveillance, loitering around the target.
- An optimization of the loiter time is therefore desirable in order to extend the time this UAV performs its main functional requirement.
- The mission profile requires this UAV to be handled and driven by soldiers, not professional pilots and most of the time during a battle, therefore it must be either easily remotely controlled from the Ground Central Unit or fly completely autonomous missions using GPS waypoint navigation.
- The UAV also will immediately return to its launch point by selecting the "home" command.
7. Houck Configuration
- 14 Using these successful designs of previous Small UAVs and other less famous hand launched UAVs, an historical statistical database for Small UAVs is created here and the primary conceptual design characteristics for this class of aircraft are tabulated and reported in Table 1 .
- Fig. 22 shows some picture of these aircraft designs.
4.2 Constraint equations
- The maximum lift coefficients for a wing configuration without the use of high-lift devices like flaps and slats at a Reynolds number around 400,000 is fixed by the current technology and a typical value is 1.3.
- The stall velocity historically can be estimated around 9 m/s.
- Finally, since the proposed UAV is hand launched, an equivalent takeoff distance was considered: it represents, in this case, the equivalent distance that an aircraft should cover in order to reach the required velocity to takeoff if the required velocity to takeoff is the stall velocity and the acceleration is the acceleration that a soldier can give to the aircraft running with a weight of about 4 kg in his hand.
- A sensitivity analysis is later performed in order to take into consideration the impact of any errors due to the assumptions made.
- Fig. 16 confirms the assumptions made were sufficiently accurate, since all the UAV designs considered are located in the design space, with the exception of the 2 bungee cord launched UAVs.
- For this class of aircraft as well as the stall speed.
- This observation will drive the choice of an airfoil that satisfies this constraint in a more advanced phase of the process, Even a slight increase in stall speed for this class of aircraft represents the possibility of carrying a bigger payload.
- Refining the assumptions made at the beginning on the basis of bungee cord launched UAVs: Even though a bungee cord launched UAV provides many advantages, like a bigger design space and the possibility to carry more payload, the disadvantages due to the presence of additional equipment to transport and deploy and due to the additional weight of the aircraft are still greater than the advantages.
- An important aid that can drive the choice of the design point can arrive from the study of the design points of comparative aircraft able to perform the same mission.
4.4.3 Wing geometry sizing
- Assuming an initially rectangular wing, the aspect ratio can be defined as simply the span divided by the chord.
- A wing with a high aspect ratio has tips farther apart than an equal area wing with a low aspect ratio.
- Therefore, the amount of the wing affected by the tip vortex, due to the 3D effects that tend to lower the pressure difference between the upper and lower surface, is less for a high-aspect-ratio wing than for a low-aspect-ratio wing.
- On the other hand a higher aspect ratio will generally increase the structural weight of the wing and therefore of the airplane due to the increased stresses the wing undergoes to.
- Historical values and statistical trends can help in this decision.
- Considering the graph in Fig. 24 but assuming also that the use of composites and fibres, and the potential use of the Houck Configuration, will probably decrease the overall structural weight of the wing, an A.R. of 8 was chosen.
- The resulting span and chords can be easily calculated:.
- However internal combustion engines have been used by early designers especially by those from the model airplane community.
- Below 25%, the endurance potential declines more and more deeply.
- Therefore if the gross weight is 3.0 kg, the battery weight should be around 0.75 kg at least.
184.108.40.206 Electric Motor
- Due to the potential advantages in performance and weight, the choice of an outrunner brushless motor was preferred for the proposed aircraft.
220.127.116.11 Selecting motor/battery combination
- The capacity of 8000 mAh of the selected battery meets the requirement of 7964.5 mAh of total capacity necessary to perform the mission.
- It is interesting to see how the endurance of the proposed aircraft varies if the mission profile changes, keeping constant the total battery capacity.
- For this purpose a graph was drawn in which on the x-axis there are the minutes at maximum power and on the y axis there are the minutes at cruise power.
- The red dot represents the selected mission profile.
- Due the use of a twin boom, and due the hand-launched method, a small diameter propeller is preferred.
4.4.7 Weight build-up
- There are many levels of weights analysis.
- In this case using actual weight values performed the weight estimation of the aircraft.
- For parts that could not be weighed without first being built, such as the fuselage, wing, and tail assembly, estimates of the weight were based on data from past aircrafts and from material samples.
- The main weight is estimated to come from the airframe then the second biggest weight come from the battery.
- A battery weight in the neighbourhood of at least 25% of the gross weight or more should be considered [4.4.4.] ; below 25%, the endurance potential declines more and more deeply.
- It can be noticed that as it was anticipated in section 18.104.22.168, a battery weight in the neighbourhood of at least 25% of the gross weight or more should be considered; below 25%, the endurance potential declines more and more deeply.
- Therefore if the gross weight is 3.0 kg, the battery weight should be around 0.75 kg at least.
- The causes of this difference lay probably in the fact that most of the comparative UAVs database presented the payload weight as the sum of the actual camera plus the avionic and cables needed.
- The reasons derive on one side from the bigger battery used able to accomplish the one hour target of endurance, and on the other side the use for the airframe of cheaper and weightier material in comparison with the very expensive and light material used by big companies like Aerovironment.
- Finally this represents only a rough estimation that must be revised in the next steps of the design process.
22.214.171.124 Second Iteration
- On the basis of the characteristics listed in table #, it was possible to draw the 3 possible aircraft layouts that are shown in the next section.
5.1 Aircraft loads
- Before the structure of the wing can be designed, the authors need to determine the loads that will be imposed on the aircraft's wing.
- This section deals with the general issue of aircraft loads and how they are predicted in the early stages of the design process.
5.1.1 Maneuver Loads
- The negative load factor is assumed to be The next section introduces a very useful tool that clarifies the manoeuvre loading.
5.1.2 V-n Diagram
- Using the dive speed plus the structural limits and equation  for the lift line, the V-n diagram was constructed.
- This diagram describes that at load factors higher than 3 or at speeds higher than 25 m/s, structural failure could occur.
- At speeds outside the curves on the left side flight is not possible because the airplane would be stalled.
5.1.3 Gust Loads
- Using the aforementioned load factors the gust diagram can be constructed as seen in Fig. 41 .
5.1.4 Wing structure
- Testing of wing structures had been previously conducted at UD  , and it was shown that foam core carbon fibre composites have superior strength to weight ratios compared to balsa ribbed wings that utilize a carbon fibre tube spar, due to failure of the balsa ribs at lower loading, especially when normalized by the structural weight of each type of wing.
- The results for this testing are shown in The semi span wing structures are exposed to torsion and bending stress as explained in the next section and a possible wing box is shown in Fig. 42 .
5.1.5 Load distribution
- If the surface is relatively rough the friction coefficient will be higher.
Using the Equation listed below
- Once the laminar and turbulent flat plate skin friction coefficients have been calculated, an average coefficient can be calculated, since at this point of the design it's difficult to make an estimate of the percentage laminar flow that can be attained: C f = 0.0040.
- In order to consider the drag due to the small separation pressure drag and in order to take into account the approximations made using the theoretical equations, a safety coefficient of 1.5 is used.
Considering for example
- The loads calculated are represented in Fig. 45 where only the wing semi span is considered.
- The foam core and fibreglass were considered to not be bearing any of the stress, but merely transferring the stress to the carbon fibre spars, such that structurally, the wing could be modelled based on the properties of the spars.
5.2 Nonplanar configuration vs. standard configurations
- Using the Finite Element software ADINA to predict stress and displacement, eight structural models were created, whose geometric, Fig. 50 and physical characteristics, Table 9 , correspond to the eight wing configurations that have been tested in the Low Speed Wind Tunnel (LSWT)  of the University of Dayton.
- In the same fashion, changing only the geometric characteristics of the wing, it is possible to create the structural model of the other 7 nonplanar wing models tested in the UD Low Speed Wind Tunnel.
- From Fig. 60 it can be inferred that increasing the gap and therefore the size of the endplates at the tips of the biplane configuration has a beneficial effect on the structure, reducing the Maximum Effective Stress at the root.
- Furthermore increasing the stagger of the nonplanar configuration has a deteriorating effect on the structural performance of the wing under load.
5.3 Comparison with a biplane
- In an effort to explore the structural benefits of the use of endplates, eight models of standard biplane not joined at the tips with flow guides were created using the same Finite Element software and the results, obtained under the same assumptions, were compared, in Table 11 and Fig. 61 , to those reported in Table 10 and Fig. 60 Gap.
- As expected, the Displacement and the Maximum Effective Stress of the biplane are constant with gap and stagger because the 2 wings are not joined and therefore behave independently from one another.
- The price to pay for this reduction in stress is the increase in weight of the structure due to the addition of two endplates at the tips.
- This increase in weight can be easily quantified considering the physical characteristics of the configuration under investigation reported in Table 9 , where the material used was the same structural material used for the wind tunnel models, stainless steel.
- Even though for the final design of the Small UAV another material will be chosen, the assumption of a material provides an important estimate of the gain or loss in weight this configuration can provide:.
6.1.1 Results obtained using AVL software and Wind Tunnel Testing
- In order to better understand the flowfield around the nonplanar configuration under study, a combination of vortex lattice method, integrated force measurement and stream wise PIV were used.
- A parametric study was performed using a Vortex Lattice code (AVL) on six parameters of the nonplanar configuration: stagger, gap, dihedral, decalage, sweep and overhang.
- The effect of the other parameters was observed to be either negative or negligible.
- The stagger, from the results of AVL, has a big impact on the lift coefficient.
- Above one chord length gap, the rate of change of lift coefficient decreases with increasing gap.
- The experimental results were obtained from force balance measurement.
Gap (normalized chord x/c)
- They show some differences from the results obtained from AVL, due to the underlying assumptions of this code: incompressible, inviscid, irrotational and steady flow.
- From a design point of view, it is interesting to find the configuration with the highest aerodynamic performance that meets the requirements of the Small UAV under design.
- Therefore the figures below report the behaviour of the lift to drag ratio when varying stagger and gap.
- The only viscous coefficient considered from AVL is Cd o whose value must be entered by the user.
- In this case a historically representative value of 0.02 was used.
6.2 Decision Analysis
- The configuration with the best score is used in the following steps of the design process.
- The configurations analyzed are the configurations compared through this research, that is, the Biplane Joined at the tips, non planar configuration with 0C stagger and 1C gap, the biplane configuration with 0C stagger and 1C gap (introduced in section 5. criterion was selected as the most important.
- From sections 5.3 and 5.4, it appears evident that the use of the box configuration helps reduce the weight of the structure in comparison to the monoplane and biplane configurations.
- Due to the expensive payload carried by the aircraft, due also to the important data stored during the mission by the aircraft and due to the same high technology product this UAV represents, it is desirable to design a reliable aircraft capable of accomplishing several missions without failures and without being caught by the enemy.
- The same rank was therefore given to all three configurations for these criteria.
- Due to this small gap, that can potentially decrease to 0 or even be in favour of the monoplane configuration if some of the assumptions fall, the final best choice for the design configuration remains the use of a conventional monoplane configuration, unless some other advantages are found to sustain the position of the wing box configuration.
Conclusions and future work
- This research demonstrates that the nonplanar configuration under investigation can meet the requirements of a Small Reconnaissance Surveillance and Target Acquisition UAV listed in Chapter III.
- The conceptual design for the required mission profile of a small UAV was therefore completed in Chapter IV.
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Cites result from "Aerodynamic and Structural Design o..."
...It has been used for studies of joined-wing aircraft with reliable results(18,19) and other design studies have found results computed to be accurate when compared to measured values for stability and control too(20)....
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