This instructable is aimed at highlighting the assembly process and steps for AMRT-25, a hybrid VTOL built at T Works. The platform was developed to carry a payload of 1kg with a range of 30kms. AMRT-25 was put through multiple tests and the T Works team was able to ascertain the following performance parameters.

MTOW: 14kg + 1kg (payload)

Stall Speed: 12 m/s

Cruise Speed: 19 m/s

Max Speed: 24 m/s

Max Range: 60 km (one way)

Max Endurance: 33 mins

Min Take Off Area: 5m x 5m

Max VTOL Flight Time: 6 mins

The steps highlighted in the construction of AMRT-25 can be applied to any other balsa/ply construction UAV airframe.

Supplies

This step will be split into two - construction supplies and machines/processes required. AMRT-25 was built at T Works' facility in Hyderabad so we were fortunate enough to have everything under one roof, however, many may not have that privilege and may have to outsource some of the work to vendors that offer those services.

CONSTRUCTION SUPPLIES/PARTS:

DLE 30cc 2 Stroke Gas Engine
Pixhawk Cube Orange
Here 3 GPS
200A Mauch PL Sensor
5A Matek BEC
5x Hitec D-625 Digital Servos
RFD900x with TxMod
1L Gasoline Tank
1x 10000mAh 22.2V Tattu LiPo
1x 7000mAh 7.4V GensAce LiPo (Hardcase)
1x 1300mAh 7.2V Orange LiFe
3m x 8AWG Sillicone Wire (Red & Black)
2m x 16AWG Sillicone Wire (Red & Black)
10m x 22AWG Servo Wire (Red, Black, White)
20x Servo Jacket and Crimps
MT30, XT30, XT60, AS150 Connectors
Heavy duty switch harness
Magnets
Tygon Tubing
10m x Monokote or other covering Film
3x (920mm x 920mm x 1.5mm) Lite Ply Sheets
20x (1000mm x 100mm x 3mm) Balsa Sheets
20x (1000mm x 100mm x 2mm) Balsa Sheets
10x (1000mm x 100mm x 1.5mm) Balsa Sheets
1kg Spool of PLA
1kg Spool of TPU
4mm x 40mm x 1000mm Aluminum 6061 Strips
10mm x 400mm x 400mm Aluminum 6061 Sheets
Thin CA Glue
Thick CA Glue and Accelerator
5min and 30min Epoxy
3x (25mm x 23mm x 1000mm) CF Tubes
3x (24mm x 22mm x 1000mm) CF Tubes
3x (12mm x 10mm x 1000mm) CF Tubes
2x (22mm x 20mm x 1000mm) CF Tubes
2x (10mm x 8mm x 1000mm) CF Tubes
3x (8mm x 6mm x 1000mm) CF Tubes
4x (3mm x 1000mm) CF Rods
Servo Arms, Pushrods, Clevices, Control Horns, Linkage Stoppers Etc.
Heat Shrink Assortment.
4x KDE 5215XF 430kV Motors
4x KDE 19" Propellers
4x KDE 85A ESC
Blue Locktite

Machines/Processes/Tools:

3D Printer with bed size of 250mm x 250mm x 300mm (Prusa mk3 or similar)
Lasercutter with bed size 4'x4' or larger
CNC router with bed size of 3'x3' or larger
Dremel
Bench Vices
Z Bend Tool
Pliers
Drill machine and Drill Press
Basic Hand Tools
Basic Power Tools
Covering Iron
Soldering Station
Reflow Station
Crimping tool
Spirit Level

Step 1: Laser Cutting the Parts

All the files that need to be laser cut have been attached below. They have also been labelled with what material they should be cut. Laser cutting ply is not very difficult since it comes in large sheets, balsa however comes in sheets of 100mm x 1000mm hence cuts will be split over multiple sheets.

In order to cut the balsa parts, multiple sheets will have to be stuck together with paper tape such that the long edges touch each other and there isn't much of a gap between them. Fill the entire bed such balsa sheets are taped along the long edge to each other. Now tape the corners and edges of the big sheet to the bed of the laser so that there is no lifting of the material off of the bed due to warped sheets. The same will be done with ply sheets to avoid improper cuts and lifting of the material off of the bed.

The parts on the design files must be rearranged on the cutting area to cut 2x of each on ply and 1x on balsa.

Step 2: Prepping the Laser Cut Parts

Once the parts have been cut, you will notice that some parts have been cut with jigsaw ends. This has been done in order to fit the parts on the dimensions of the sheet that it has been cut from. All these pieces have been carefully chosen to be parts of a ply-balsa-ply laminate such the break/cut in the piece is at a place that will be reinforced by the ply laminate at the top and bottom.

Some parts that do not have these jigsaw cuts and breaks in them are also parts of this laminate sandwich so that they can derive strength from this laminate construction. In the second part of the video linked in the step above, you will be able to see us putting together these laminate parts. Its is important to align the parts correctly so that they are able to fit within the slots they had been designed for. Use drops of thin CA glue to tack them in place. Once we were happy with the alignment and fit, we proceeded to seal the part by applying thin CA along all the edges.

Step 3: Internal Structure Construction

In order to put together the internal structure, we kept referring to the CAD, however, most of the parts fit together only with the ones they have to. They will not fit together well with other parts and hence the assembly is pretty intuitive. Some parts may also have a very tight fit and may require some sanding.

Once the parts have been slotted together, use a very small amount of thin CA glue to tack them together. Slot rest of the parts and verify the alignment and ensure that there is no twist or warpage. Once we were satisfied with the fit and assembly we applied more thin CA glue to stick the parts in place.

For the wing internal structures, we used CF sleeves which also act as spars for the individual sections of the wings. Once the CF tubes are slotted in and the structure is checked for warps and twists, the CF tubes were glued in place.

In the places where we found the surface contact of the intermeshing parts to be very less, we used thick CA and accelerator or balsa saw dust and thin CA to increase the contact. In some place like the wing joineries and firewall, we cut square pieces of balsa and used them as gussets.

Step 4: Sheet Covering the Internal Structure

Once the internal structure was constructed, we proceeded to add balsa sheet covering to the skeleton. Adding sheet covering serves two purposes - it increases the strength of the structure be sharing the stresses of the internal structure and it also serves as a smooth skin to allow application of monokote.

Most surfaces were sheet covered with 2mm balsa with the grain of the wood running the direction perpendicular to the axis bending was expected. Some surfaces which had very sharp bends, and 2mm balsa was unable to contour to, we used 1.5mm balsa. In some cases we used a wet tissue and the heat gun to manipulate the balsa to bend the way we wanted it to. By applying moisture to balsa, the cells absorb the moisture and expand, bending the balsa in the opposite direction and application of hot air with the heat gun does the opposite.

In order to add sheet covering, we first rough cut out a piece of balsa that would fit the required portion that we are trying to cover. After placing the rough cut over the portion, we may need to trim the excess in order to achieve a good fit. After trimming the piece use a drop of CA glue and secure the edges. Once the edges are secure, hold pressure on the balsa and tack it in place with drops of CA glue to make it take the shape of the contour. In the case the peice is protruding out and not properly stuck to the surface, pry it off gently or use a skew knife and free it from the surface and re glue it back. Incase the piece is too small and there are gaps between the sheet and the boundary that it was intended to cover, cut small pieces that can fill the gap and glue it in place. Incase the gap is very small, fill it with balsa saw dust and drop 1-2 drops of thin CA. This should fill the gap and can be sanded down to give a smooth surface.

Step 5: Monokote the Structure

After the entire structure has been sheet covered and all the wiring has been laid out on the inside of the parts such as the wings (wing tip LED's, servo extensions, VTOL power cables, etc), the structure can be monokote covered. Monokoting the structure gives a waterproof and smooth surface along with structural rigidity.

In order to monokote the surface, some sanding and surface prep is required so as to ensure there are no sharp protrusions that can puncture the monokote while shrinking it or random bumps that will prevent it from making good contact with the surface. Once these have been taken care of, we must plan how to monokote a certain part. There are some thumb rules to follow here -

Do the top and bottom surfaces separately.
Cut 1" excess so that you can still cover the surface after shrinking and cut off the excess as necessary.
Do surfaces with a lot of curvature on a separate piece of monkote.
Secure the corners and edges first.
After securing the edges, use a hot air gun to shrink the remainder of the monokote.
It is very difficult to maintain straight edges while shrinking so ensure that the edges and corners are stuck well.

The video in this step should provide as a good guide for learning how to monokote.

Step 6: 3D Printed Tail Assembly and Ailerons

The tail for AMRT-25 is 3D printed with PLA. The main tail is printed as an 8 piece tail with 2 8mm main CF tube spars and slots for the servos. The control surface is two piece per surface and has a 2mm CF rod as the hinge.

To assemble the tail, we first take the front and rear central sections and slide them onto the 25mm tail tube and align them and glue them together using thick CA and accelerator. Be careful to wipe off the excess accelerator as it degrades the PLA and leaves white stains which are very difficult to remove. Once the center section has dried, mount the servos in place by screwing them in place with the supplied hardware and route the wires through the designed slots to the rear of the center mount. Ensure that the servos are centered and mount the servo arms.

Now cut 4x 250mm of 8mm x 6mm CF tube and slide them into the center section as forward and rearward spars. Slot in the remainder pieces of the tail section and verify if the length of the spars needs to be trimmed. If required, trim the spars. Once done, apply thick CA on the spars and interfaces between the parts and slide them together. Apply accelerator if required and wipe off the excess on the PLA.

Similarly slide the control surface parts onto the 2mm CF rod and apply thick CA at the interfaces between the two parts and accelerator if required. Do not glue the control surface to the CF rod. Do the same for the part ailerons.

For the V Tail control surfaces place the control surfaces in the slots and slide the CF rod through the tip section till the mid section and apply a drop of thin CA at the tip to hold it in place. With a dremel trim off the excess.

For the aileron, slot the CF rod through the guides and the control surface in the correct orientation and apply a drop of thin CA at the ends and trim off the excess CF rod. Ensure there is free movement of the control surface and then glue the guides on the ends and in the center with thick CA and accelerator to the trailing edge of the outer wing sections.

Step 7: Gluing the Tail Tube to the Fuse and V Tail

It is imperative that the V Tail is aligned properly with respect to the fuselage so that the aircraft flies correctly. Once the hatches on the fuselage have been cut out, slide the 25mm tail tube inside the tail end of fuselage and with 5 min epoxy, glue it in place. Before the glue sets, slide the tail cone over the tube and align it with the fuselage and apply epoxy at the interfacing walls. Use paper tape to hold it in place. After the epoxy has hardened, let it cure overnight.

Now slide the V Tail from the other end and pass the servo extension wires through the tail tube into the fuselage. Using a scale measure the vertical distance from the ground/horizontal reference plane to the leading edge tips and trailing edge tips of the V Tail and ensure the distance is the same. Rotate the V tail on the 25mm tube as required to achieve the correct orientation. Make markings on the tube and V tail for alignment and slide the tail off. Now apply 30min epoxy on the 25mm tube and slide the V Tail over it and using the markings made previously, align the tail and verify by measuring the vertical distance from the ground/horizontal reference plane to the leading edge tips and trailing edge tips of the V Tail and ensuring they are the same. Support the V tail until the epoxy hardens and let it cure overnight.

Step 8: Mounting the DLE30 Engine

Due to the effect of the p-factor of the rotating propeller disk, we need to induce a slight down and right thrust to the thrust line of the engine i.e. axis of the propeller. Upon calculating for 5deg for down and right thrust, it was found that we need to add 3 washers to the top left stand off and one washer each to the top right and bottom left stand offs to induce this down and right thrust angle.

The holes for the stand off's have already been laser cut. Use a round file and clean the holes incase the bolt doesn't go in easily. Add the washers behind the standoff and run the bolts through the holes after dipping their ends in blue locktite (semi-permanent) and tighten them. Once the engine has been mounted, route the tygon tube and ignition cap and RPM sensor cable through the central hole in the firewall. Mount the ignition cap and plug in the tygon tube to the fuel inlet. Inside the fuse, connect the RPM sensor cable to the ignition module.

Step 9: Machining the VTOL Clamps and Bending the Landing Gear

The VTOL clamps were machined at our in house CNC router. The gcode and STL files have been attached for your convenience. The following setup was used while cutting these clamps -

Feed rate = 15 mm/s

Speed = 24000 rpm

Step/Increment = 1mm

Bit Type = 3mm HS End Mill

Once the individual half clamps were cut, they were post processed by sanding them and getting rid of the sharp corners and burrs. These halves were then marked for the through holes running on the sides. After marking them, the top and bottom halves were taped together and holes were drilled through and through with the help of a drill press and bench vice to hold them in place.

We were lucky that we were able to find aluminum strips of dimensions 4mm x 40mm x 1000mm. Incase you aren't able to find strips of this dimension you may need to cut them to this dimension using a scroll saw or CNC router. After the strips have been obtained, use a bench vice and bend the landing gear to shape as in the drawing attached with this step. We outsourced this step to a press brake vendor. With a press brake we obtained sharper bends and hence better results.

Step 10: Mounting the VTOL Booms

Once the VTOL clamps have been processed, use the appropriate 3mm bolts with locknuts and mount the VTOL motors using the laser cut motor mount plates and VTOL clamps such that the motors are facing downwards. Tighten the bolts such that the motors cannot rotate freely on the booms and then mount the booms to the center wing section. In order for the wires to pass through from the tube to the center wing section, cut a hole in the center of the VTOL booms using a dremel and apply thin CA to the edges to prevent fraying and delamination of the CF.

Once the wires have been run through and the motor mounts have been fitted, using a spirit level, verify if all the motors are correctly aligned and tighten them fully. You can mount the propellers and check with visually for confirmation.

Step 11: Mount the Landing Gears

Using a template of the landing gear bay, drill the holes on the landing gears. Once the landing gear is drilled, align it with the landing gear plates and use M5 bolts and locknuts and tighten it in place.

Step 12: Mounting the Peripherals in the Fuselage

The fuselage will house the following peripherals -

1) Ignition Module - In the nose next to the throttle servo.

2) Mauch PL Sensor - In the nose just ahead of the wing.

3) 1L Gas Fuel Tank - Right under the wing at the CG.

4) Pixhawk Cube - In the first tail hatch.

5) RFD900 Module - In the last tail hatch.

6) VTOL Battery and Primary Flight Battery - In the nose hatch/battery compartment.

7) Ignition Battery and Switch Harness - In the nose next to the throttle servo.

8) Throttle Servo - In the nose on the throttle servo plate.

Please look at the pictures in this section and mount the peripherals accordingly.

The ignition module, VTOL batteries, Primary battery, Ignition battery and fuel tank are all padded with foam and mounted to their respective locations using velcro battery straps. The pixhawk cube is mounted to the 3D printed vibration isolation mount with the supplied double sided tapes. The throttle servo is screwed in place with the supplied hardware.

Step 13: Mounting the Peripherals in the Outer Wings

The outer wings house aileron servos, wing tip lights and an ASI on the right wing. The wing tip led strips are glued to the tips and the wires soldered and routed through the wing section. The aileron servos were mounted on removable plates and wires soldered along with the LED's wires to a single multiplex connector. The ASI was also mounted inside a 3D printed enclosure that houses it along with the pitot. The wires from the ASI are soldered to a single multiplex connector that extends till the first rib of the right wing.

Step 14: Soldering the Extensions From the Center Wing to Fuselage

Following the wiring diagram above, a wiring harness was soldered making the connections with appropriate lengths. Based on how you plan on routing the wires, excess wiring may have to be cut. Use 3D printed or readily available square cable tie base mounts and tidy up the wiring at regular intervals. In case you do not have much experience with soldering, watch the linked video and practise first so as to avoid cold solder joints. Use heat shrink on all exposed solders. You may even want to use hot glue as and when required.

Step 15: Setting Up the Control Surfaces

In order for the servos to control the control surfaces and throttle, the servo has to be interfaced with the surface using a 2mm threaded pushrod, threaded clevices, control horns and linkage stoppers/z-bends where required.

First using the supplied hardware, mount the control horn such that the push rod holes are aligned with the hinge axis laterally. The picture above should give you an idea. You may want to use epoxy or CA glue incase the the screws are unable to go through and through.

Once the control horn is mounted, thread a clevice half way on the pushrod and clip it to the hole on the control horn that was just mounted to the control surface. With the servos arm oriented perpendicular to the servo, mark out the position of the hole on the servo arm's projection on the pushrod. Use the z-bend tool and create a z bend with the z centered on the marking just previously made.

Insert the z-bend into the servo arm and adjust the clevice such the control surface is zeroed/centered when the servo arm is centered.

For the throttle servo, use a z bend on the carburetor end and a 2mm linkage stopper on the servo end. Cut the pushrod to an appropriate length with little excess such that adjustments can be made. Make the adjustments so that full travel of the carburetor is achieved with the servos full travel and tighten the linkage stopper. The video linked in this step should guide you on how to do that.

Step 16: Setting Up the Fuel Tank

The fuel tank used in the AMRT-25 was a 1L gasoline compatible fuel tank. Ensure that the tubing used gasoline compatible as silicon tubing degrades on exposure to gasoline. The fuel tank has 3 ports. The port in the front of the tank is the 'fuel out' or gas line and has a tube extending to the back of the tank with a weighted clunk filter attached to it. To this port attach a tygon tube of an appropriate length to go to the carburetor with some slack. The port on the top with a tube going into the tank is the fuel inlet and will be used to fill fuel into the tank, run this tube outside the fuselage from the side by drilling hole and label it fuel inlet. Ensure that the level of tube is higher than the level of the tank. The third port that has no tube extending into the tank is the overflow port. Run a tube to the outside of the fuselage by drilling a hole on the side and label it overflow. The overflow will ensure the tank is not vacuum locked and if excess fuel is filled, it can flow out. The picture above shows the setup of the fuel tank in AMRT-25 and its relative position in the fuselage.

Step 17: Assembling the Aircraft

Wing Assembly:

1) Slide the 22mm and 10mm CF tubes that were cut to length earlier and slide them into the sleeve of the midwing on both left and right sides.

2) Slide the outer wings inwards until theres is a gap of about 50mm and connect the multiplex/servo connectors to the appropriate connectors outer wings.

3) Ensure that the wires and connectors are pushed inside the wings and slide the outer wings inwards and press down on the latching mechanism to secure the wing in place.

Wing to Fuselage Assembly:

1) This step requires at least two members to carry out. Ensure you have adequate help.

2) Taking the help of one person, place the leading edge of the wing over the fuselage without latching in the notches and holding the wing at a 45 deg angle. First plug in the multiplex connectors (labelling them ahead of time will ensure there are no mistakes and confusion) and then plug in the VTOL power cables.

3) Neatly tuck the VTOL power wires to right side of the fuse and the peripheral wires to the left side of the fuse to avoid interference to the signals due to the large current being carried in the power cables.

4) Place the wing over the fuselage flat and engage the notches into place and slide the wing so that it locks.

5) Insert the wing locking bolts and tighten them. Ensure they aren't overtightened.

Step 18: Preparing for Flight

Before every flight, ensure that all wires are properly connected and secured. Check all nuts and bolts that are subject to high vibration such as those on the engine mounts, landing gear etc. for loosening. Ensure that there are no wires or tubes hanging outside the fuselage that aren't supposed to be.

Once all these items have been checked, follow the procedure below to turn on the aircraft.

1) Turn on the transmitter and GCS.

2) Plug in the primary flight pack and turn on the aircraft.

3) Connect the GCS to the aircraft and wait for parameters to load.

4) Switch the aircraft to manual flight mode and test all controls and check travel directions.

5) Plug in the VTOL battery and wait for the voltage information to show up the GCS.

6) Run a motor test to ensure all motors are spinning freely and in the correct directions.

7) Prime the engine with the choke on. Once you see fuel flowing into the carb, flick the propeller twice more.

8) Turn on the ignition and proceed to flick the propeller. Once the engine sputters, turn off the choke and attempt to start the engine.

9) Once the engine has turned on, run it on high idle for upto 2 mins for the engine to warm up.

10) With someone holding the aircraft, check the throttle response. If satisfactory, the aircraft is ready for flight.

Introduction: AMRT-25: Hybrid VTOL