Performance

Aircraft data (update all tables and graphs after having fixed all dimensions)

The plane performance is studied with all the available information. For this calculations, all dimensions must be stated first and all formulas are referenced to the first dimensions so when a single value is changed, like the cruise speed, the wingspan of the horizontal stabilizer or the fuselage shape, all the graphs and charts change in accordance.

Airfoils
Two airfoils are chosen for the wing and the horizontal stabilizer. The wing airfoil chosen is the Eppler E374, copying the airfoil of Reaper-like glider model studied before in similar aircrafts. These type of airfoils use the Eppler code, designed by Professor Richard Eppler based in some methods used in computational fluid dynamics for low Reynolds numbers.

“Professor Richard Eppler is a pioneer in the field of computational aerodynamics. He wrote his first codes using punched paper strips, the high speed main memory at these times was a magnetic drum, which could hold several bytes.

Eppler developed a very fast and elegant design method, based on conformal mapping, which is the heart of his computer code. Because an airfoil also has to operate outside of its design point(s), a fast integral boundary layer method and (for the analysis of given airfoils) an accurate third order panel method (parabolic velocity variation) was added. Furthermore the code offers possibilities to modify the geometry, to calculate drag polars, and various plotting options. Due to its early roots, the computer code has been developed as a batch code. Textual and graphical output is directed to files, which makes the FORTRAN 77 code easily portable and system independent. On the other hand, the input files are quite cryptic and hard to handle for beginners. The elaborate description of theory and code [Eppler, R. and Somers, D.: A Computer Program for the Design and Analysis of Low-Speed Airfoils, NASA TM-80210, 1980] even contains an (now outdated) version of the FORTRAN-IV program.

The strength of the code is the design part and the fast analysis part, which makes it very well suited for the design task. The results of the integral boundary layer method agree astonishingly well with experiments, if the Reynolds numbers are above 500'000. The design module can be used to design very smooth airfoils shapes, including the leading edge region, which is often difficult with other codes. On the other hand, the design method is quite abstract and difficult to handle for beginners.

The boundary layer analysis is performed using the calculated, inviscid (without friction) velocity distributions as input; there is no direct coupling between boundary layer flow and the external flow field. Transition prediction is performed by testing the boundary layer parameters against a set of empirically derived transition relations, which work quite well for attached flow in a wide range of Reynolds numbers.

In the low Reynolds number regime the results are usually not very accurate if a laminar separation bubble or larger separated flow regions occur. This is a result of the integral boundary layer method, which simply cannot model separation (this would require some sort of coupling between boundary layer analysis and the calculation of the external flow). The code has a option to perform a displacement iteration in order to take the displacement effects of the boundary layer into account, but there is no direct interaction, as, for example, in Xfoil. Recent (2007) additions to the code however, are an improved model of laminar separation bubbles and turbulent separation. The code itself is available for a fee directly from Prof. Richard Eppler in Germany or from his US distributor Dan Somers.”

http://www.mh-aerotools.de/airfoils/methods.htm
also: http://www.pdas.com/eppler.html

Max thickness 10.9% at 34.3% chord. Max camber 2% at 38.9% chord

http://airfoiltools.com/airfoil/details?airfoil=e374-il
Performance graphs to be updated with Re number and high N critic

The horizontal stabilizer has a symmetric airfoil and one of the simplest and most used in slow planes, the NACA 0012, with 12% thickness at the 30% of the chord. The plane doesn’t need too much lift in the tail, forces will be balanced by the angle of attack of the airfoil.

http://airfoiltools.com/airfoil/details?airfoil=n0012-il
change Re

The NACA 0012 is used in the vertical stabilizer too for the same reasons.

CAD modelling with Siemens NX 9

It is time to create the first model of the airplane with all the considerations taken before.
The surface of the fuselage is going to be created with two main sketches, horizontal (plan XY) and vertical (plan YZ). To find the intersection of these parallel plans with the sketches of the fuselage, vertical and horizontal straight lines are needed for creating intersections that will be start points.  Then they will be united with curves drawn along parallel plans along the longitudinal axis Y so it will be formed by several sections, each section formed by 4 curves surfaces or fuselage plates. As it is symmetrical along axis X, just one curve is needed on each side and section, the symmetry will be applied later.

The two sketches are united by 7 section curves

Front view of these sections

All sketches and section curves conform the fuselage divided in 6 main parts (24 in total)

Wing and tail are added (still without proper airfoils). Shark fin design is aesthetic, as it could be the wing with sweep angle and winglets, but harder to build.

Side view of design 0.2

It can be appreciated that even with a high wing, the downwash may affect the tail. The fuselage may also affect the vertical stabilizer.

 Interior of the fuselage with main components placement along longitudinal axis.

Isometric view of design 0.3

Main landing gear, propeller, camera and gimbal are built. The ailerons take form. The landing gear still doesn’t protect the camera in case of collision.

Landing gear detail without bearings. Rigid rods transmit forces to fuselage and wing. This camera has its own WiFi antenna

Camera and gimbal detail. It is a 3 axis gimbal with 3 brushless motors. Pitch control covers from -130° to 90° and rolls from -45° to 45°.

Fuselage side cut

From the front to the rear, the gimbal is embedded in the fuselage upwards to protect more the camera from possible crashes. It will be glued or screwed to the main structure, with a big enough hole allowing the free movement of the gimbal system. Then the battery is placed just below the wing and as low as possible. The battery is going to feed the motor through the ESC and the servos through the BEC system of the ESC, which controls that a small amount of the battery is given with proper voltage to the servos’ motors. The battery and ESC must be placed next to each other, but the ESC must be placed a bit to the rear too to reach not only the servos of the wing but those in the tail too. Between the servos and the ESC, a gyro system circuit board is supposed to control the surfaces to compensate turbulences and unexpected wing forces that disturb the plane’s flight, so extension cables might be needed to reach the servos. The further back these components are placed, the better, because there is already a lot of weight from the camera in the front.

This scheme of the Ranger Ex Platform gives an idea of a more detailed study of the components relative position:

http://www.getfpv.com/planes/fpv-planes/volantex-ranger-ex-757-3-pnp.html

The following designs of the plane have all improvements of its parts, either for making it more realistic, efficient or easier to manufacture.

The landing gear is put a bit forward to protect the camera. The single wheel is added to the back of the plane to complete de landing gear. The propeller and motor models are improved, as well as the gimbal and the union between main fuselage and the tail. Other bodies changed from solids to surfaces for a first fluid simulation.

Side view of design 0.4

The airfoil E374 conforms the detailed wing structure. The coordinates for this airfoil are taken from airfoiltools.com and plotted as a spline in NX, then extruded. The ailerons are formed almost at the tip of the wing with a rib of separation from the edge. There are two thin rods that connect the ailerons to the wing and allow their independent rotation around it.

One-piece E374 wing with ailerons and rounded edges

Aileron hinge detail

The horizontal stabilizer uses a symmetric airfoil, the NACA 0012.

Just as the wing is made, the horizontal stabilizer is built

The vertical stabilizer changes to a more common shape which is stronger, easier for manufacturing and controlling the yaw movement. For the moment it still has a symmetric cut but not a formalized airfoil, just straight lines and arcs at the front. The thin rod inside joins the rudder surface to the stabilizer, allowing it to freely rotate around it.

 Vertical stabilizer side view

The next design has a formalized landing gear structure. The main landing gear is designed according to existing wheels and mounting, as well as the tail connection, all of them available at local stores.
A new propeller is used now, with much more aerodynamic and realistic blades modelled in 4 quarters out of splines, tangent on the front and rear plans.
The wings are cut in two, the same way they will be manufactured and mounted on the plane, one attached to each side of the top fuselage. Their tips are flattened to follow the shape of the tip rib and the holes for the rod are correctly made to sew all the surfaces together.
The fuselage part joining the front and the tail is removed and replaced by a long and strong rod.

Isometric view of design 1.1

Formalized main landing gear

Formalized tail landing gear

10x6 inches propeller blades

All remaining solids are turn to surfaces with some remodeling. The motor is pulled back, out of the fuselage, and installed in a mount. The main wheels bearings are improved.

Motor mounting

Landing gear detail

There is one issue with the propeller and it is the fuselage that blocks the inlet air to the bottom part of the propeller, so that part will have its thrust reduced. A simple solution is to open holes in the foam that work as air inlet, taken from the uniform current directly to the propeller blades. The rigid structure of the back of the fuselage will be weaker but he motor will push more.

Although the structure has aerodynamic efficient curves, the available machines can’t cut that way, they just can approximate the smoothness with several straight lines.

    

                 

The fuselage must be optimized for manufacturing. The hot wire 4-axis cutting machine cuts the foam block in straight lines because the wire is tense and such is the way it is modelled now. This design includes an FPV camera too, apart from the camera mount below the fuselage, and it will be located at the foremost part of the fuselage to create a cockpit environment. The plastic cover protects the camera in case of crash and reduces the drag with the air flow.

 

             Example of a nose with a big camera and a plastic case

http://www.hobbyking.com/hobbyking/store/__38072__HobbyKing_174_8482_Go_Discover_FPV_Plane_EPO_1600mm_PNF_.html

Again, from scratch, two main sketches are made, horizontal and vertical. The plant shape must be wide not only to hold the components such as the battery, 50 mm wide, but to have wide walls of foam that give strength to the plane. At the nose of the plane, a hemispherical shape is needed to place the cover of the FPV camera. It will have big enough dimensions to hold, in case of need, one of the biggest cameras considered and a pan/tilt mechanism.

XY sketch

The top line of the vertical sketch is the most critical. Its shape has big repercussions on the total drag of the fuselage. The last version of the fuselage had 635 mm of length. Taking out the peak of the nose and the rounded back that is not properly manufactured in the simulation, the length is 600 mm, easily divided in parts of 200 mm. Once the configuration is fixed for 3 blocks with the same width, the angles of the straight lines are set with the help of the shadow from the previous fuselage to be as similar as possible and a reference arc in red which gives an approximate shape of the smoother lines that help the air flow stay close to the plane and not to become detached. If these vertices are too sharp the plane will need a more powerful motor.
The bottom lines are made tangent to the camera cover too and adjusted to join the middle rod of the fuselage. Being straight help the fixation with the landing gear and the gimbal.

YZ sketch. Red arc is tangent to the air flow and camera cover.

Then the transversal sketches join the two sketches through the intersections of those with the plans, creating the side shape of the blocks that the wire will cut.

XZ sketches

Thanks to the sketches the surfaces are created one by one, simpler than before, and made symmetrical. The rest of the fuselage is formed by the union rod and a small part at the rear that will hold the tail elements.

Isometric view of the fuselage design 1.4

Adding the main parts completes the configuration.

Isometric view of design 1.4 with main parts.