To place the components first it is important to know how much do they weight. The heavier components are the battery (600 g.) and the camera (335 g). The weights are taken with the most restrictive component specifications (weight and dimensions) so in case the components are changed to other dimensions, they will just leave free space in the fuselage and reduce maximum take-off weight. If the plane in the end is too light the easiest option is to reduce wingspan. According to this, the design of the wing, fuselage, tail, motor and landing gear begins.
Plant sketch of the plane with the position of the main electronic components
Wing
The wing, to begin with, is slender (aspect ratio = 12.5) for improving gliding flight. Similar to other planes, it has a wingspan of 2500 mm and a main chord of 200 mm. Choosing later a proper airfoil will state its thickness. For manufacturing purposes it is straight and rectangular, without dihedral angle and no sweep angle because it won’t be affected by compressibility effects. The only control surfaces it will have are 400 mm ailerons in each tip, not too wide because they are far from the center of gravity, so a little movement will create enough momentum.
Referred to the vertical position, the wing is on the top of the fuselage. This way it leaves free space below for the electronic components, has better aerodynamic efficiency (free extrados and less fuselage interference) and reduces ground effect that may difficult take-off and especially landing.
The battery will be placed below the wing, to increase stability, so when the wing lifts the plane, the center of gravity will be as near as possible to the center of lift and below. If the wing suffers from slipping by the airstream it tends to auto-stabilize, however it might suffer from Dutch roll (negative dihedral may be needed to reduce this effect).
In case of crash, which is very likely to happen, the wing won’t be as much affected as the fuselage. In this case it is preferable that the foam from the fuselage absorbs the impact with the ground or with a wall/tree because the wing surface will be weaker (plastic film) or more fragile (wood board). In addition the foam is cheaper, easier to get and can be repaired as a puzzle with some glue.
The horizontal position along the fuselage centerline is set between ¼ and ½ of the length, measured from the nose. This is caused by the second heavier element, the camera and the gimbal. This group has extra constraints: it must be at the foremost part and below the fuselage for evident reasons of field of view. The fuselage shouldn’t appear on the screen and the lens should cover the field from the horizon to at least 60 degrees downwards.
Now the distance from the camera to the wing is crucial. The closer they are, the less weight will be compensated in the tail, but if they are too close they create such a low momentum that the tail will make the plane pitch up.
Motor
The motor has a pusher propeller with two blades, it pushes the plane structure forward from behind the wing. This configuration makes a more complicated fuselage shape but it is the most efficient for this mission. The reasons that have led to this configuration are as follows:
The motor can’t be placed in the front as a tractor because it will result in weight problems, risk of breaking propellers when frontal crash and it will cover the view of the camera.
It can’t be placed in the rear because of the weakness of the tail (vibrations may harm the structure due to fatigue), the closeness to the ground, the long distance from the battery and the weight balancing too. The negative stability would be hard to control too, because the center of gravity would be much further from the motor, contrary to tractor motors that auto-stabilize. Furthermore, the shaft works better if it has a compression force.
Front motor and rear motor
So the best option is to place it in the middle, as close as possible to the center of gravity and lift (close to the battery and the wing). Vertically it is placed at the same height as the wing, to give the propellers more space to rotate freely and to create less momentum among the wing line. Setting it to the centerline of the fuselage could be possible but very difficult, the fuselage might be split into a twin-boom fuselage which is harder to manufacture.
A problem to deal with is the turbulences the airstream from the motor creates backwards. If the motor is placed before the wing it might give it extra lift explained with the Coandă effect, as seen in planes like the Boeing YC-14 or the Antonov An-72, but placing it behind is more spread.
The motor behind the wing may be affected by its trail or even improve the air circulation but being at the top height decreases the interaction with the fuselage and tail, as well as avoiding the ground in a collision.
Boeing YC-14 and Antonov An-72
http://www.theaviationzone.com/images/russian/an72/bin/an72_07.jpg
The motor behind the wing may be affected by its trail or even improve the air circulation but being at the top height decreases the interaction with the fuselage and tail, as well as avoiding the ground in a collision.
Fuselage
The fuselage is constrained by the payload and the wing and motor configurations, as it works like a structural union between parts, and it should have the lowest wetted area possible to reduce drag. Having pointed shapes is not very important for this mission as flight speed won’t be over 0.2 Mach. Although it helps reduce drag, the nose cone needs to be rounded to absorb crashes and wide to hold the camera.
Around the motor, the fuselage must be strong to stand its vibrations and forces pushing the structure, but not too wide because the motor needs fresh air to pass through its propeller. In a more detailed design the fuselage might need air intakes like reversed fish gills or just holes, not just for air intake but for cooling the motor (keeping it below 60 °C if possible).
The shape doesn’t need to be cylindrical because it won’t be pressurized. The sections have elliptical cuts and between them the side line is as straight as possible, so manufacture will be easier. The most demanding section in terms of structural integrity is the middle of the fuselage, next to the motor cowling. It is as narrow as possible to let air flow through the propeller and it has abrupt curves. Manufacturing simulation may cause this part to change and improve.
Fuselage side sketch
Tail
The horizontal stabilizer is rectangular too, for the same reasons as the wing. It has a wingspan on 500 mm and a main chord of 120 mm. The elevator surface covers almost all the wingspan and is wide to compensate a possible downwash and turbulence effect caused by the wing trail. The configuration is fixed by the rest of the plane, because the rear part gets the turbulences the front parts create. To avoid the wing downwash and propeller stream the horizontal stabilizer is placed as down as possible and as far as possible, to give pitch momentum too. The vertical stabilizer is large as it will get some of the turbulent flow so it also has a long and wide rudder. Further fluid analysis will tell if the vertical stabilizer can keep its original configuration (A-1), change to a B-1, with two small vertical stabilizers in each tip of the horizontal stabilizer, or a V tail (or butterfly tail). In these cases the rudders will need two servos and a bit more complex control.
Landing gear
A fixed landing gear is considered because the flight speed won’t create such a high drag on the wheels to impede normal operation.
The chosen type is the conventional mainly because of the camera placement, it wouldn’t allow a front wheel in the nose, or the main landing gear would be difficult to place back if there is a high wing.
The main two wheels are placed as close to the camera as possible to create a protection between the nose and them, so in a typical crash the camera would be covered from impact. Although the camera will be covered, in a bad landing the plane might flip over if the landing gear brakes too hard, the wind is not favorable or the pilot is unexperienced.
Also the impact is transmitted to the foam and the wings, partially absorbing the force. Crash test will be made with finite element method simulation and stress analysis.
Other similar planes are found to have a very similar
fuselage shape. The examples are:
Volantex
RC Ranger EX
Volantex RC FPVRaptor V2
Skywalker 2014 edition
3DRobotics aero
ICON A5 (manned aircraft)
