Home Engineering Small Unmanned Fixed-Wing Aircraft Design. A Practical Approach

# Concept Design Algorithm

The basic steps of the UAV platform concept design process used here are as follows:

• 1. Decide target payload mass, MTOW, and cruise speed (at a maximum cruise ceiling, for us typically 400 ft as set by UK CAA regulations) - we aim to maximize range within this MTOW budget and speed setting - other choices of start point are possible.
• 2. Choose the overall aircraft configuration (canard, monocoque, tail booms, etc.) - here taken as a tail boom pusher as already noted.
• 3. Choose the landing speed and take-off speed (or set take-off speed to be the same as landing speed), which govern wing loadings if we assume we do not have high lift devices - typical values are in the 12-18 m/s range. Higher landing speeds give faster,

Table 11.1 Typical fixed parameters in concept design.

 Item Value Aspect ratio (span2 * 4 5 6 7/area) 9 Reserve fuel fraction 0.1 Minimum (wing) coefficient of drag at cruise speed, CDmin 0.0143 Viscous drag constant, kvisc 0.0329 Pressure and induced drag constant, kt 0.0334 Minimum drag (wing) coefficient of lift at cruise speed, CLmin 0.1485 Coefficient of parasitic drag at cruise speed 0.0375 Cl/ Cd at landing speed 5 Coefficient of parasitic drag at landing speed 0.05 Cl/ Cd at take-off speed 5 Coefficient of parasitic drag at take-off speed 0.035 Equivalent rolling coefficient of friction 0.225 Propulsive efficiency at cruise speed 0.6 Tail-plane aspect ratio (span[1] [2] {{ Choose values for various fixed parameters - many of these are aerodynamic, all arenondimensional or do not depend on the airframe size and here are based on typical aircraft, operational speeds, previous designs and simple calculations (such as Cd = CDmin +kvisc(Cl - CLmin drag)[1] + k(Cl)[1]), see, for example, Table 11.1.}} [5] [6] [7]/area) 4 Fin aspect ratio, ((twice fin height)[1]/area) assuming two fins 3 Bank angle in level turn 60° Minimum rate of climb from cruise 5m/s Percentage extra thrust desired to start ground roll 0%

Table 11.2 Typical limits on variables in concept design.

 Item Value Maximum wing CJ Cd at cruise speed 16 Maximum wing lift to total drag at cruise speed (allows for all drag elements) 8 Maximum coefficient of lift at landing speed 1.3 Minimum allowable rotation at take-off before grounding happens 17° Minimum static margin (expressed in fractions of main wing chord) 0.1

Table 11.3 Estimated secondary airframe dimensions.

 Item Value Units Fuselage depth 200 mm Fuselage width 150 mm Nose length (forward of front bulkhead) 150 mm Diameter of main undercarriage wheels 100 mm Longitudinal position of engine bulkhead -200 mm Vertical position of base of fuselage -110 mm Vertical position of tail-plane 0.0 mm Vertical position of engine 60 mm Vertical position of center of main undercarriage wheels -300 mm
• 8. Choose an engine/propeller combination and some likely dimensions for the aircraft fuselage (we set the datum on the center-line in way of the main wing spar, which is taken to lie at the quarter chord point). These dimensions can be used to sketch the aircraft; for the 15 kg Decode-1 aircraft used as the first example in this chapter, which has twin tail booms and conventional U-shaped rear control surfaces, they are as shown in Table 11.3. For other designs, they will need setting to different but appropriate values, based on prior experience or educated guesses.
• 9. Assess the likely empty weight based on the dimensions now available and the structural construction philosophy adopted, together with the components required to operate the aircraft (such as avionics, undercarriage, servos, and so on, that is, an outline list of component parts) plus the maximum fuel weight (without reserve). Ultimately MTOW includes
• • structural weight
• • avionics, systems, and servo weight
• • propulsion system weight
• • undercarriage and miscellaneous weight
• • fuel weight including reserve.

When summing these, note that the dry weight does not include payload or the main

fuel, but does include the fuel reserve.

• 10. Check if the following conditions have been met:
• • the aircraft can rotate by at least the minimum angle specified at takeoff without fouling any part of the structure,
• • the static margin is greater than the minimum required without being excessive,
• • if a nose wheel is not fitted, the pitching moment caused by the propeller thrust and wheel drag (set equal and opposite to the thrust) will not cause the aircraft to pitch nose down into the ground, given the position of the CoG,
• • there is enough weight difference between the MTOW, the structural weight, and the payload to carry some fuel!
• • the wing Cl/Cd ratio at cruise is below the maximum chosen,
• • the wing lift to total drag ratio at cruise is below the maximum chosen,
• • the Cl at landing is below the permitted maximum,
• • the installed static thrust is enough to start the aircraft rolling and to permit takeoff (assume that static thrust does not change until after the aircraft leaves the runway - in fact, it is likely to rise slightly for a well chosen propeller),
• • the installed power is sufficient to achieve the cruise speed and to carry out acceptably banked level turns and climbs,
• • the calculated aircraft weight including fuel is no more than the target MTOW (if it is less, more fuel is carried to simply increase the range),
• 11. Adjust the guessed inputs to maximize the range while meeting the constraints.

When this process is completed, the overall geometry of the balanced platform can then be used to begin the next stage of the design process. To carry out the above calculations, information is needed on a number of other aspects as described in the following sections.

• [1] smaller aircraft with greater range but which are more likely to have accidents on
• [2] landing!
• [3] Choose values for various fixed parameters - many of these are aerodynamic, all arenondimensional or do not depend on the airframe size and here are based on typical aircraft, operational speeds, previous designs and simple calculations (such as Cd = CDmin +kvisc(Cl - CLmin drag){{smaller aircraft with greater range but which are more likely to have accidents on
• [4] smaller aircraft with greater range but which are more likely to have accidents on
• [5] Place some limits on what is possible or acceptable for the design under consideration - examples are given in Table 11.2.
• [6] To start the design process, choose the total main wing area from the constraint diagramalong with likely coefficients of lift at cruise and landing (these are based on the chosenwing loading and the expected aerodynamic performance, including allowance for flapsor other landing aids if fitted), together with the fuel weight and longitudinal positions oftail-plane spar (to control static pitch stability) and payload (e.g., assumed to be forwardof the front bulkhead and hence fixed by the location of this bulkhead) - ultimately weadjust these to balance the design and also ensure that they do not go outside the limitsjust set.
• [7] Choose the installed power needed from the constraint diagram or try and estimate thelikely lift and drag of the aircraft at cruise, level turn, climb, takeoff, and landing for thegiven MTOW, so as to try and see what kind of installed power will be needed. If MTOWis fixed, this can be done without having to estimate the mass of the vehicle from itscomponents, which is a distinct advantage of starting from a fixed MTOW.
• [8] smaller aircraft with greater range but which are more likely to have accidents on

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