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Old Dec 09, 2007, 09:54 AM
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Minimum v4 - the two-blader

The conversion of the three-bladed Minimum v2 to the two-bladed v4 took me longer than I thought because I started out on the wrong foot. My own fault. When I made the blade shims for the fisrt prototype, I built them with a leading edge thickness of 0 mm and a trailing edge thickness of 3 mm instead of the 1mm / 3 mm I had used before, without noticing what I had done. And then, when I started the flight tests of the v4, I found out that I didn't have enough lift. And did I search for that missing lift! Juggeled around with the c.g. and motor position, but nothing really seemed to work. The problem resolved itself when, after one of my numerous crashes, one of the shims went missing, and I had to build a new pair. Got it right this time, and from then on it was relatively easy. During this test phase I found out that v4 can do loops and even the rolls aren't too barrel-like. But there's not going to be an immediate repeat performance.

The plans have been ready for some time and so was the gyro built using these plans, but only yesterday the weather conditions allowed a test flight of the final version. Went well, so here are the plans - as usual in two versions, *.jpg for looking at and *.pdf for printing out.

Technical data:
DC rotor, two-bladed, 2" Aerobalsa rotor blades
rotor diameter: 1120 mm / 44"
AUW: 490 g / 17.3 oz.
RC functions: roll, pitch and throttle
battery: 3S/910 mAh LiPo
motor: Plettenberg Freestyle 24, 75 g, turning a 9x4.7 APC Slow prop at 8650 /min while drawing 12.5 A
servos: 28 g / 1 oz., metal gears

The pivot hinge and control arm are identical to v2, look at post #4 for building instructions.

The angles of the airframe and the length of the rotor mast have changed (see Airframe drawing). This is the new bill of materials:
- one piece of cf tube, O.D. 8 mm, I.D. 6 mm, length 170 mm
- one pieces of cf tube, O.D. 6 mm, I.D. 4 mm, lenght 120 mm
- one piece of cf tube, O.D. 5 mm, I.D. 3 mm, length 300 mm
- one piece of cf tube, O.D. 5 mm, I.D. 3 mm, length 230 mm
- one piece of cf tube, 6 x 6 mm, I.D. 4 mm, length 80 mm
- one piece of aluminium or brass tube, O.D.6 mm, I.D. 5 mm, length 35 mm
- one wheel collar, I.D. 6 mm
- two wheel collars, I.D. 8 mm
The building insructions remain the same as for v2.

The battery tray is new (see MotorMount&FrontWheel drawing). Cut the actual tray from 3 mm ply and cut a 5 mm wide slot into the rear end of the tray. This slot fits over the rotor mast support strut. Glue an 8 mm wide strip of balsa to the underside of the tray, at least as thick as the rim of the wheel collars you put on the airframe - 2 mm in my case. Glue two 5 mm x 10 mm wide strips of beech wood or similar to each side of the above mentioned balsa strip. The whole unit should now fit over the front part of the airframe, between the wheel collar and the support strut, with the tray sticking out in front. Use a rubber band to hold the battery and tray in place.

The front wheel unit is identical to v2, except for the thickness of the two cf parts holding the wheel. During one of my crashes those two parts broke, while the 4 mm cf rod - which is intended to break in such a case - remained intact. That's why the cf parts now have a thickness of 1.5 mm instead of 1 mm. Building instructions are unchanged.

The shape of the flanges of the motor mount and the wedge have changed a bit to fit the new mast rake, otherwise it's identical to v2, as are the instructions.

The only change in the tail unit is the length of the cf tube used: 250 mm instead of 245 mm. Building instructuions are unchanged.

The two-blader needed a new pilot, specially trained for flying these kind of gyros. Instructions remain the same.

The rotor is new, of course (see drawing rotorhub). Cut the flapping hinge out of 0.5 mm gf using the pdf-printout as a template. Mark the holes for the pivot hinge (8 mm) and rotor blades (3 mm) and drill them. Do not drill the 4 holes that hold the hub together. Cut the upper rotor hub out of some strong 2.5 mm plywood, again using the printout as a template. Mark the position of the central hole (8 mm) and the four outer holes (3 mm) and drill them. Sand the straight sides of the hub where the flapping hinge goes so that the sides are leaning inwards at the upper edge by about 30. Now cut out the bottom rotor hub and drill it's central hole.

Take a short piece of 8 mm O.D. rod and slip the bottom rotor hub, the flapping hinge and the top rotor hub on this rod. Align everything and use the holes in the top rotor hub as a template for drilling holes in the flapping hinge and the bottom rotor hub. Put a 3 mm screw in each newly drilled hole to hold the parts in place. Put marks on the three parts so you can reassemble them in the same position. Dismantle, bolt the upper and lower hub together and sand the lower hub to the shape of the upper hub, keeping the straight sides vertical this time. Take the two ball bearings and carefully epoxy them into the central holes of the upper and lower rotor hubs. Dismantle again. Make some counterbores in the top rotor hub for countersunk M3 screws and then put the flapping hinge between the hub parts and bolt together using those screws. This way you'll get a flat upper hub surface, which will protect the rear ends of your blades in crashes.

Cut two blade holders out of 2.5 mm plywood and two shims out of 3 mm balsa. These parts have the identical dimensions. Drill the holes for the blade-holding screws and sand down the balsa shims until the leading edge is 1 mm high while the trailing edge remains untouched (see cross section in drawing rotorhub).

Take two 2" Aerobalsa rotor blades of 540 mm length. Cover the inner end of the blades with pieces of 0.6 mm plywood as shown in the plan and drill 4 mm holes at the appropiate places. Epoxy small pieces of brass tube (O.D. 4 mm, I.D. 3 mm, length about 7 mm) into the 4 mm holes and sand them down until they are flush with the surface of the plywood. Cut off the inner ends of the blades according to the plan. Paint the blades with filler several times and sand them smooth after each painting. Cover the blades with some elf-sicking film or paint them in bright colors.

Put Nylon screws into the holes of the rotor blades, slip the shims on these screws and stick the screws through the holes in the flapping hinge. Then put the blade holders at the bottom of the flapping hinge and bolt everything together.

For instructions on how to install your RC equipment, read the first two paragraphs of post #9 of this thread, up to the part when I talk about the motor mount. Use some double sided adhesive tape to stick your motor to the mount provisionally, in the position suggested in the plans. Use a piece of velcro to stick the back of the pilot to the support strut and hold the feet in place with a rubber band slung around the front wheel unit. When you've installed everything except the rotor, adjust the hang angle to something like 5 by moving the battery forward or backwards. Now have a look at the video in this thread:
Adjust your motor offset to the right until you've got the prop wash compensated, then drill the appropiate holes, cut off the unneeded parts of the mount and fix your motor.

Now mount the rotor and use a delta mixer in your Tx to control the rotorhead. Set up the throws of your servos to give you a maximum roll motion of the rotor of 8. Do the same for pitch. If your Tx allows this, mix throttle onto the roll signal so that the rotor tilts a further 4 to the right (seen from behind) at full speed. This value is dependent on your motor and prop, you may have to adjust this to your own needs.

Finally read post #12 and have a good time flying the Minimum v4.


Edit: Here's the link to a video of the last prototype of the v4:
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Last edited by JochenK; Dec 14, 2007 at 03:25 AM.
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