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        Build Log 500 g/17.5 oz. dc head pusher - the Minimum

#1 JochenK Jul 15, 2006 10:14 AM

500 g/17.5 oz. dc head pusher - the Minimum
4 Attachment(s)
Hi everyone,

here's another pusher for you, and no, it's not another Micromum but a pusher conversion of the Micromum's predecessor, the Minimum. Easy to do, just had to put the motor behind the mast and tuck the tail unit under the rotor.

Technical data:
DC rotor, three-bladed, 2" Aerobalsa rotor blades
rotor diameter: 930 mm / 36.6"
AUW: 500 g / 17.5 oz.
RC functions: roll (aileron), nick (elevator) 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: 20 g / 0.7 oz., metal gears

If you've got your RC setup right, this one's relatively easy to fly - for a gyrocopter that is. Like a plane with ailerons and elevator, provided you don't forget to apply power while turning. When going full speed and banking it about 60° to 70°, you can fly continous circles with a diameter of 10 to 15 m / 30' to 50' without the gyro loosing any height. Dead stick landingss are no problem, just turn off the power and watch it glide in at an angle of about 60°. Pull a bit of elevator at the last moment - not the very last moment, mind you - and you're safely home - if you landed straight into the wind.

Plans will be following shortly.


#2 dougmontgomery Jul 15, 2006 01:38 PM

Jochen That looks great. Is that the modified delta hinge? Giving me more ideas- Time to start drawing!!!!

#3 JochenK Jul 15, 2006 04:18 PM


this is still the old delta hinge from here:

Wanted to get this gyro to fly in a known configuration before I do any experimenting.


#4 JochenK Jul 16, 2006 02:51 AM

5 Attachment(s)
Now here's the first installment of the plans. All the plans come in two versions, a jpg-version for looking at and a pdf-version for printing. If you print the pdf-files without any size adjustment, they should print to scale. Some printouts will have bits missing because of the model's size, but what you need will be printed.

Let's start off with the pivot hinge. I've desrcibed the piciple of using a drive shaft - wheel axle combination from a model car before, but this time I've picked a part that seems to be available in the US as well as in Europe. Go to http://www.cenracing.com/cars/ff/fftuningparts.html
and look for item no. FFS004, the universial swing shaft for Buggy/Rally. As the name of this item is a bit long, I'll refer to it as the CV joint from now on.

You have to do two things to convert the CV joint to a pivot joint: cut off the drive shaft part 30 mm from the ball in the actual joint and sand off the black paint from the 5 mm O.D. part of the wheel axle. This is where the ball bearings go, and without removing the paint it's a rather tight fit. If you have the possibilty to cut a M4 thread, cut the original thread as far as it will go, if not, never mind (first picture).

Now make yourself a control arm as shown in the upper left corner of the airframe drawing. I've used my favourite cf-balsa-cf sandwich combination for this part, but if you don't have access to such an exotic material use 3 mm PVC or something similar instead. Epoxy the control arm to the CV joint (second picture).

Slip an aluminium or brass or plastic tube of 5 mm lenght with an I.D. of 5 mm over the wheel axle, follwed by the two ball bearings of the rotor. I've used ball bearings with an I.D. of 5 mm, an O.D. of 8 mm and a thickness of 2.5 mm. My ball bearings have a little rim to hold them in place, but that is not really necessary. If you have read my previuos descripton of this kind of head, you'll notice that I've switched from a needle bearing to two ball bearings. Well, I've had my needle bearing lock up two times in mid-flight and what happened then was rather spectacular. No need to have this experience again. On top of the upper ball bearing goes a 5 mm thrust bearing and then a self-locking nut (third picture). If you haven't cut the M4 thread to the end you may have to insert a washer or two between the thrust bearing and the nut.

To be continued.


#5 JochenK Jul 17, 2006 12:29 PM

3 Attachment(s)
We move on to the central airframe (drawing airframe).
For this you'll need
- two pieces of cf tube, O.D. 8 mm, I.D. 6 mm, with lengths of 35 mm and 120 mm
- two pieces of cg tube, O.D. 6 mm, I.D. 4 mm, with lenghts of 25 mm and 70 mm
- one piece of cf tube, O.D. 5 mm, I.D. 3.5 mm, length 290 mm
- one piece of cf tube, O.D. 4 mm, I.D. 2.5 mm, length 230 mm
- one piece of aluminium or brass tube, O.D.6 mm, I.D. 5 mm, length 20 mm
- one wheel collar, I.D. 6 mm
- two wheel collars, I.D. 8 mm

Epoxy the 6 mm cf tubes into the front end of the 8 mm cf tubes. Now comes the only tricky part in building the Minimum: you have to drill a 5 mm hole and a 4 mm hole at angles of 82° and 70° into the reinforced 8 mm cf tubes. For the location of the holes have a look at the plan. Before you drill the holes put some duct tape or similar around the cf tubes. First, predrill the holes for the rotor mast with a fresh 4 mm wood drill, you know these drills with a pointy bit in the middle and a flat outer edge. Then enlarge the holes to 5 mm, using normal drills. Finally, drill the holes for the support strut, using a 4 mm wood drill again. Remove the duct tape. If you do it this way, you'll minmize the likelyhood of the cf tubes splitting up during the drilling. Now print out the airframe plan and loosely assemble the cf parts on the plan (first picture). Use a round file to sand off the top of the 4 mm support strut so that it fits onto the 5 mm rotor mast. You may have to enlarge the slanting holes to make everyting fit, but that doesn't really matter. Once everything is aligned, take the airframe parts apart, apply expoy at the relevant positions and reassemble. Put some 1.5 mm balsa pieces under the rotor mast and some 2 mm balsa pieces under the support strut and hold everything down while the epoxy is curing. Then slip the aluminium tube over the top of the rotor mast and epoxy in place. This tube is to keep the end of the rotor mast from splitting under strain.

The wheel collars are used to keep the pivot joint, the motor spar and the tail boom in their positions. For the 6 mm wheel collar holding the pivot joint I suggest a version with two grub screws opposite each other. If you can't find one, make one yourself. Drill a 2.5 mm hole opposite the original one and cut a M3 thread into it. Slip this wheel collar over the top of the rotor mast and epoxy it in place, about 5 mm from the top of the mast. Now, using the wheel collar holes as a guidance, drill 2.5 mm holes through the cf of the mast and extend the wheel collar's M3 threads right through the cf.
If you follow this procedure, you'll take a minimum of material out of the wall of the mast and leave it as strong as possible. Epoxy one 8 mm wheel collar over the end of the motor support and repeat the thread-cutting procedure. Put the last wheel collar aside for now.

To be continued.


#6 JochenK Jul 18, 2006 09:45 AM

7 Attachment(s)
Next we come to the bottom plate (drawing bottomplate). My own gyro has a one-piece bottom plate - again made out of my favourite sandwich conbination. of course. But I strongly recommenf building the two-part solution shown in the plans. The rear part of the bottom plate is made of 1.5 mm to 2 mm cf or gf and is epoxied and taped to the main airframe - a pretty unbreakable connection. The front part of the bottom plate is made of lighter material such as 3 mm beech plywood or similar and is just bolted to the rear part with 3 mm nylon screws. In case of a crash the nylon screws will sheer off or the front bottom plate will break, but the airframe and the rear bottom plate will stay intact. And it's much easier to make a new front bottom plate than to rebuild the airframe (remember the slanting holes?).

Now cut out the rear bottom plate using the plan as a template. Drill the holes for the rotor mast and support strut and enlarge them slightly till the airframe fits int the holes. Drill the 3 mm holes for the nylon screws and drill a cluster of 4 mm holes around the mast and strut positions. Use epoxy to glue the bottom plate to the airframe (first picture) and when that is done, use cf rovings or similar to tape all the joints marked in the airframe plan. Use the holes in the bottom plate to tape the lower 8 mm cf tube tightly to the plate, and don't forget to tape the rear end of the bottom plate (second and third picture).

While the epoxy is curing, cut out the front bottom plate. Drill 3 mm holes for the nylon screws and for the screws holding the wheel collars of the landing gear. Make a cut-out around the support strut. Now slip some thin heat shrink tube over the 3 mm cf rod of the landing gear to protect the cf rod in the wheel collars. Slip two 4 mm wheel collars over the heat shrink tube. Using a washer at the top and bottom of the front bottom plate put two 8 mm long M3 screws through their holes and tighten them lightly into the wheel collars. Now you have a bit of adjustment to do. If the M3 screws can move up and down in their holes, shorten them until a) the screws sit tight in their holes and b) the axle of the landing gear is still held in place by the pressure of the screws on the heat shrink tube. Put your favourite wheels on the landing gear axle.

Bolt the front bottom plate the rear bottom plate with nylon screws and epoxy the last remaing 8 mm wheel collar over the end of the tail boom support and repeat the drillng and thread-cutting procedure (fourth and fifth picture).

To be continued.


#7 JochenK Jul 19, 2006 10:20 AM

6 Attachment(s)
For the tail uinit (drawings rudderfin and elevatorfin) you need a cf tube, O.D. 6 mm, I.D. 4 mm with a length of 300 mm. And you need to build a holder for the rudder. Take any sort of rod with 6 mm diameter and a minimum lenght of 150 mm and glue it flush to the side of a 6 mm piece of balsa. Cover this with a bit of non-sticking household wrap and put it in a vice, the rod on the upper side (first picture). Take two strips of 50 mm wide gf tape and laminate them symmetrically over the rod. Put another layer of household wrap on top and smooth down the tape on the sides of the balsa. Use two flat rods and some clamps on the sides of the balsa to hold the gf tape in place. Cut the cured holder into the shape shown in the plan. The holder may look rather flexible, but once you've glued it to the end of the tail boom it's strong enough to hold the rudder. I've used silicon for that connection, this way you can take it apart again. Now drill three 3 mm holes for nylon screws into the rudder holder as shown in the plan (second picture). Cut the rudder fin out of 6 mm Depron (or two pieces of 3 mm Depron) and put it in the rudder holder. Make 3 mm holes in the Depron and fix the rudder with nylon screws. Dismantle again.

Print the plan of the elevator fin out two times, cut out a right and a left side, glue the cut-outs together and use as a template for cutting the elevator fin out of 3 mm Depron. Epoxy a 320 mm long flat cf rod (3 mm x 0.3 mm) to the leading edge of the elevator fin for stability. Glue the elevator fin to the lower rudder fin and use two cf rods with 1 mm diameter and a length of 195 mm to stabilize the tail unit. The places where the cf rods go into the Depron are marked in the plans by little dots. Glue the top of the rudder fin onto the elevator fin and bolt the tail unit into the rudder holder. Stick the tail boom into the tail boom support, align everything and fix with the grub screw of the wheel collar.

To be continued.


#8 JochenK Jul 20, 2006 10:35 AM

5 Attachment(s)
What's left now is the rotor (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 3 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 three 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 (first picture). Then put the flapping hinge between the hub parts and bolt together using normal M3 screws.

Cut three blade holders out of 2.5 mm plywood and three 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 three 2" Aerobalsa rotor blades of 450 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 self-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 (second and third picture).

A comment concerning the last two pictures. The blades you see there are old ones and the roots of the blades are reinforced with gf instead of plywood. My good experience with the Micromum made me change this design, but the new blades and the new pictures aren't ready yet.

To be continued.


#9 JochenK Jul 21, 2006 05:53 AM

5 Attachment(s)
Now we're going to install the RC equipment. Make a 40 mm wide strip of 2.5 mm plywood or similar that fits into the space between the rotor mast, the support strut and the tail boom support. If you are using bigger servos, make this strip a little wider. Epoxy in place. Fill up the remaing space up to the
motor support with 3 mm Depron. Mount the servos on the plywood, their bottoms parallel with the rotor mast and the servo arms at the top (first picture). Put the pivot hinge into the top of the rotor mast and fix it with the grub screws of the wheel collar. Out of 2 mm cf rods make yourself two push rods with ball links at the top and servo arm links at the bottom. Adjust the length of the push rods so that - when the servos are in neutral position - the upper part of the pivot joint is in line with the rotor mast. Use self-locking nuts to fix the ball links to the control arm (second picture).

Use some velcro strips to fix the Rx and the ESC to the Depron part above the servos. Try to mantain a little distance between those two devices to minimize possible interference. What kind of motor mount you need depends mainly on the motor you are going to use. The only thing that is a must is that it ends in a wooden rod with an O.D. of 6 mm and a free length of about 12 mm. This rod goes into the cf tube of the motor support and is fixed with the wheel collar's grub screw. The third picture shows you how I did it. This picture shows you another thing, the thrust vector of my motor is pointing to the right of the gyro, we'll come to that later.

Use a rubber band to fix the battery to the front end of the bottom plate. Then lift the gyro by the top of the rotor axle - or use a bit of string to lift it - and move the battery until the rotor mast is slanting 8° to 10° backwards (hanging angle, fourth picture).

Cut out the pilot from 6 mm Depron and use velcro strips at its back, bottom and the soles of its shoes to fix it to the support strut and the bottom plate (fifth picture).

To be able to fly this gyro you need to program a mixer in your Tx. The basic mixer you need is of the delta type, which distributes the aileron and elevator signal to the two servos. But as the gyro has a tendency - due to motor torque and coning - to turn to the left, a mixer with inputs for aileron, elevator and motor is your best choice. First program your Tx so that full aileron causes the pivot hinge to tilt about 15° to the left and right and full elevator causes the hinge about 8° forward and backward. Then program the Tx that at full speed of the motor the rotor gets tilted a further 6° to the right while staying neutral when the motor is off. Use about 50% of exponential on aileron and elevator.

Finally you have to adjust the thrust vector of your motor. As the rudder is situated directly behind the motor, the wash of the prop is hitting the right side of the rudder very hard, causing the gyro to turn right while taxiing on the ground (and while flying, of course). Use some quiet spot with a flat surface and try taxiing at full speed. If the gyro turns right, put some shims under the left side (seen from behind) of the motor mount until the gyro runs straight. I had to usea 6.5 mm shim to get right.

When all this is done, put the rotor on the pivot hinge and secure it with a self-locking nut. You're ready to fly.

To be continued.


#10 dougmontgomery Jul 21, 2006 06:55 AM

Jochen- On picture 5, Hang angle is -8, what are your ideas about how this correlates with the gyros angle of attack in level flight? Do you want questions in your build thread? Doug

#11 JochenK Jul 21, 2006 07:12 AM


the tail boom of the gyro is more or less horizontal in level flight, which means that the top of the rotor mast is then tilted 8° backwards.

I've chosen -8° as a minimum (small "m") hang angle, because I can tilt the rotor disk 8° backwards when pulling full elevator. This means that the thrust vector of the rotor will then move 8° forwards and just touch the cg. A hang angle of -10° should add a bit of stability in this situation.


#12 JochenK Jul 21, 2006 07:16 AM

1 Attachment(s)
Flying the Minimum.

For the maiden flight you should choose a day with a little bit of wind, just enough to keep the rotor spinning when you've given it a flick. Check that yuur RC equipment is working correctly. Start up the rotor and do a fast walk into the wind with the gyro held at nearly 90°. At this point the speed of the rotor is not a function of your walking speed, but a funtion of time. Walk on till you hear this swishing sound of the blades. Turn on the motor and set it to half speed. Walk a bit faster and start turning the gyro nearly horizontal. Now you should feel the lift of the rotor. When everything seems alright, set the motor to full speed, do a few fast steps and gently shove the gyro horizontally into the wind. You're airborne - and I hope it stays that way. At full speed the gyro will now start to climb, be prepared to throttle back till it flies level.

There are some differences to flying a plane, that you have to watch out for. When you want to climb or descend, you use the throttle do to this, not the elevator. As the gyro is not inherently stable, you have to end all manoeuvres you're doing by actively putting the gyro upright again. In turns the Minimum behaves like a plane with ailerons and elevator. When you want to turn, bank the gyro and then pull a bit of elevator to get it round the turn. The main difference to a plane is, that you have to apply far more power in turns to keep the flight path level, otherwise the gyro will rapidly loose height. And the response of the gyro is far more sluggish than that of a plane. When you're cruising along and the gyro starts going somewhere you dön't want to go, use short, relatively hard corrections instead of small, long ones.

It's important to land the gyro straight into the wind, otherwise it is likely to turn over and you'll have to replace one or two of the Nylon screws holding the blades. When you want to do a dead-stick landing, simply turn the Minimum into the wind and turn the motor off. The gyro will come in at an angle of about 60° and you just have to use elevator to pull it out of this dive at the right moment.

Sooner or later you're going to crash the Minimum. Most of the time nothing much happens - I'm still on my first set of blades, by the way. Things that come in handy in such a case are some Nylon screws for the bades, some styro ca for the tail unit and a 3 mm cf rod to replace a broken landing gear.

Have fun!


#13 pegasuszero Jul 30, 2006 01:49 AM

Thanks Jochen,

I'm ready to start building this DC head pusher.
Your plan is very easy to see...good!

Thank you.

#14 JochenK Aug 18, 2006 06:19 AM


just got back from my holidays. Good to hear that you are buliding the Minimum. How is it coming along?


#15 JochenK Sep 14, 2006 09:45 AM


The Minimum has a problem which I didn't notice during the development and the sort of normal flying that followed. But now that I'm pushing the limits a bit, I found that in extreme conditions with full aileron and full up elevator - a condition that occurs when you are flying thight circles wit a diameter of less than 30' with the gyro banked nearly 90° - the prop tends to strike the lower blade holders. As a solution I suggest that you either build the rotor mast 5 mm longer (295 mm instead of 290 mm) or trim off 5 mm off your 9" prop. My Minimum conveniently did this for me.


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