Knurrus Maximus FPV Flying Wing Build Instructions

The original instructions can be found here(in german)

Parts

Our order for the Plane looks like this:

Order at epp-versand.de:

  • 1 x Knurrus Maximus FPV

Tab Zubehoer:

  • 1x Uhu Por Tube 40gr.
  • 1x Holmset FPV
  • 2 x Balsa Ruderhorn zur Ruderanlenkung
  • 1x Federstahldraht 1,5mm zur Ruderanlenkung
  • 1x Motorträger

Order at Eflight:

  • 1x ePower X 2808/1200 für 2-3 LiPo Imax 22A, 1190rpm/V (Brushless Motor)
  • 2x ePower EXP eco-20C 2200mAh 3S1P 11.1V (Akku)
  • 1x ePower Eco 45A Regler für 2-4S LiPos mit 3A S-BEC (Regler)
  • 1x aeronaut Mittelstück 42mm Verst. 0°, 8mm (7242/22) (Propelleraufnahme)
  • 1x aeronaut Präzisionsspinner 2-Blatt 36/4mm bis 45/6mm (Spinner/Propelleraufnahme)
  • 1x aeronaut CAMcarbon 10×6“ (7234/28) (Propeller)
  • 2x Dymond D 250 BX BB/MG Servo, 29 Ncm (031-14351) (Servos)
  • 1x robbe Filament-Klebeband 25mm x 50m mit Glasfaserverstärkung (5371) (Verstrebungsklebband, unter zubehoer → Klebstoffe)

Miscellaneous: A pack of fork heads or ball couplings (min. 4 Piece); Can be bought at Eflight.

Important

  • Symmetry: It is very important to build the plane as symmetric as possible.
  • Cutting : Always use a new blade to cut EPP. With a blunt cutter knife the material is going to frazzle.
  • Glue : EPP is quite reluctant when it comes to gluing. The best choice is UHU-POR since it has the same characteristics as the Material. If you need a stronger, but also harder adhere, use Epoxy (2 component glue). For a quick repair you can use superglue, however it is not a lasting solution on foam.
  • Paint : Nearly no paint will hold directly on EPP, so the best choice for a colored design are colored Tapes. They also provide additional rigidity and protection to the plane.
  • Cutouts : Cutouts can be made with the cutter knife and a small plier. Cut around the edges of the cutout, then carefully pull out the pieces with the plier. You can also cut through the whole material and glue part of it in again. The better method is to use something like a Dremel and a milling cutter. It is also possible to use an old soldering iron or a heated piece of wire to melt it out. However you have to be careful not to melt out to much.
  • Shelving : Never leave the finished plane in a hot car or something similar, there is a possibility that the plane deforms.

Build

The complete kit before the start

The first step is to glue together the wing.

Optional: It is recommended but not necessary to insert a rib made out of wood or carbon in between the wing halfs. This will reduce the vibrations caused by the motor and make the plane more quiet (the vibrating noise can be heard quite good in the video of the first flight). Make sure to end the rib several cm before the nose, to ensure maximum crash resistance and safety.

Apply some UHU-POR to both halves (Not too much, but there should be a layer on both sides). UHU-POR is a contact glue, so take a coffee brake and let it dry off for 15min before continuing.

Carefully align the two halves of the wing and put them together starting at the front, then press them together for 5 seconds.

The next step is to insert the stringer. Prepare by cutting the two 1.5mm carbon rods in 4 50cm pieces. Place the Depron template, that is shipped with the kit, on the wing and carefully mark the lines where the stringers go.

It is very important to be precise, especially when aligning the top and bottom cutouts. If those are of by to much, the plane will warp under load, leading to very bad an unpredictable behavior. While doing this also mark the cutouts for the ailerons. Only mark some points and then link them using a ruler. If these cutouts aren't straight, the aileron snaps around the middle position.

Make an incision of about 4mm with a sharp cutter knife. Also at this point make the marked aileron cutouts.

Press UHU-POR into the incision

and then immediately (don't let it dry!) push the rods in as following:

  1. One 2mm rod (100cm) for the curved part the middle (bend it)
  2. One 2mm rod (100cm) for straight part in the middle
  3. A 1.5mm rod (50cm) in each wing, overlapping with the 2mm rod from the first step
  4. Do the same at the bottom

Cutout a small piece at the tail of the wing in the middle, to place the motor mount. Align it straight, mark it's outlines and attach it with UHU-POR (again, letting it dry off is important).

Now we can proceed with the electronic equipment. This part might differ if you use different or additional electronic. It is best to begin by placing all the needed equipment on the wing and think of the best way to lay the cables. Important things to consider:

  1. The battery packs need to go at the front (between the stringers and the nose) because of the center of mass (we don't want to use extra weights). When choosing a high capacity it is best to split it into two smaller packs in parallel and have them parallel to the front. Don't make the cutouts for the battery packs before the first flight, we can use them to dial in the center of mass without extra weight.
  2. The wires to the ESC carry a currents up to 30A. Place the sensitive electronics, especially the PX4FMU with the sensors, as far away from the wires as possible.
  3. Keep the wires from ESC to motor as short as possible.
  4. Don't integrate the ESC into the wing (or only if it is additionally cooled).
  5. Place the servos so that they are at or in front of the center of mass, the pushrods to the ailerons stay short and that there is enough material under the servo to support it without bending.

Make the cutouts for the servos as precise and tight as possible. The better they are the less glue will be needed later.

To lay the cables it is sufficient to make an incision and push the wires inside.

Align the rudder horns of the aileron to the servos and glue them into the balsa wood with some super glue.

To attach the ailerons, place them at the final position, put some UHU-POR at the joint and spread it on the rudder as well as on the wing (only as far as the tape is wide).

Let the glue dry, then strengthen the joint with tape.

For the linkage of the servo and the aileron take 2mm carbon rods and fork heads with a 2.5mm thread. Set the servo to it's mechanical middle position, using the transmitter or the board, and attach the servo arm (perpendicular to the servo). Then lengthen the rods and screw the fork heads directly onto the carbon rods. Glue the servo into place using a little UHU-POR at the bottom. Adjust the length of the rod by screwing or unscrewing the fork heads. Once the ailerons are in place (neutral: They are aligned to the wing) secure the fork heads on the rods with some super glue.

Mount the motor with the provided screws

and assemble the propeller/spinner. Make sure the propeller blades point in the right direction. For a push setup like here, the sharper edge has to be at the back. The blades should have enough play to fold and unfold. If you are unsure, keep them on the loose side and secure the screw with nut lock. Depending on your setup you have to make some cutouts to allow the propeller to move freely. It should have enough room to be able to go a bit over the “straight out” position.

The last step is to strengthen the plane with tape. Everywhere you apply tape, you should first put on some UHU-POR and let it dry off, else the tape will peel of again very quickly. Start by putting a stripe around the nose of each side of the wing. Then protect the front part.

Part of the tapes point is to make the plane more rigid. This is achieved by applying the tape as shown below. Depending on your application (mainly the speeds you want to achieve) you can use less or even more tape than shown in the picture. (Or cover the plane in colored tapes. Just consider that the tapes weight can add up quite a bit, so only use as much as needed.)

Settings

First we set the servos. If the neutral position is not in alignment with the wing (it should be if you worked precise) set it right with the electronic trim (shifting the middle position of the servo). Afterwards set the maximum travel on each side. It does not need very big peak, 20-25 degree on each side should be good for a beginner. If you are an experienced pilot you can set the maximum that the servo and the ailerons can naturally do without bending. (max. +-45 degree).

The center of mass is between 240mm to 245mm from the tip of the plane. For the first flight you should dial it in at 240mm and than adjust it after the first flight. To set it, take a string and tie a big screw nut at the end of it. Push the string through the desired center of mass and you can balance the plane on the string. Move around the battery pack until you the wing is balanced, then fix them at this position (or use additional weights if it is not enough).

Either program the ESC to regulate power when the battery packs are empty, or calculate the approximate run time from the capacity. Never use more than 80% of capacity if possible, else you will damage your packs in the long term.

First flight

You should already have some basic skills flying a model plane. If not, train on a simulator first or ask someone who can to do the first flight. Find an open field, preferably with a slight downward slope. Double check that the ailerons move to the right direction! For the first try you should not use the motor, but throw the plane and see how it sails. If it does behave well, the center of mass should be more or less ok, and you can try a normal start. After it is airborne you should first trim the wing to fly straight. Then check the center of mass by cutting the engine and going into a step descent (almost perpendicular). If the plane wants to go on the back, the center of mass is to far back (this also means the input to the ailerons is very nervous and not really controllable). If the wing stays neutral or pulls out slightly you have the correct setting (it depends on the own preferences how much it pulls out by itself, however for a autopilot system it is probably the best to have a neutral setup). If it starts pulling out and then going down again (like a pumping motion) the center of mass is to far at the front.

Finish

After you have the correct position of the battery pack, you can make the cutouts and put them in the wing. The finished plane (so far we did not apply a tape finish)

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