VTOL-Trainer Build Thread - Page 4 - RC Groups
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Feb 07, 2013, 12:35 AM
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Cool! Gonna strap on a plane to my quad, springs' comin' !!
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Feb 09, 2013, 03:00 PM
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Ran D. St. Clair's Avatar
I am still waiting for the carbonbird motors to be in stock.

Meanwhile, I have been playing with basic VTOL aerobatics. I don't mean the basic loops, rolls, and inverted flight that the plane can do in FF (forward flight). That much is a given, and pretty much what you would expect from any sport plane of this weight, power, and proportions. I am talking about aerobatics based on the hovering capability of the plane in combination with its aerodynamic surfaces.

Back Loop - From SFF mode (Slow Forward Flight mode) bring the plane to a full hover at altitude. Apply full upelevator and hold the plane straight in yaw. As it translates backwards the tail with catch the air and the plane will do a back flip, followed by a swooping recovery to SFF.

I also tried a hovering roll, but the nose drops and it turns into a downward spiral with an uneventful recovery. A little negative airspeed might make this work better, but I haven't tried all the possible variations yet.

Some hypothetical maneuvers I haven't tried yet...

Figure P - Enter an inside loop from full forward flight and switch to SFF as you begin the loop. The plane should perform a half loop and then flop into a hover at the top. For extra credit continue into a back loop.

Half Outside Loop - Enter an outside loop from FF while inverted, Switch to SFF as you begin the maneuver. Perform the half loop and end in a hover. Alternatively, extend the upline until your airspeed is depleted and then flop into a hover.

Flat Spin to Hover - Enter a spin in FF mode, flatten it out as much as possible with up elevator, inside rudder, and opposite ailerons. Transition to SFF while in the spin but continue to hold full inside rudder, but back off of the ailerons and up elevator as necessary. The maneuver ends in a hovering piroette.

Spin entry from a Hover - From SFF, enter a full hover and then reduce throttle while applying full up elevator, full rudder, and if necessary opposite ailerons. Try and drop directly into a flat spin without entering a spiral dive.

Note: I haven't done a spin with this plane yet, but traditionaly multi engine planes are good spinners (deadly bad spinners) due to so much mass so far from the CG.

So what other VTOL maneuvers can you think of?
Last edited by Ran D. St. Clair; Feb 09, 2013 at 03:09 PM.
Feb 09, 2013, 07:04 PM
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OK, so don't try spins to the left. Full low throttle and full left rudder shuts the motors off. Don't ask me how I know that...

The only solution I have at the moment is a few clicks of throttle up trim after arming. Unfortunately it seems to be necessary to have the motors at least start to spin before the arm/disarm is disabled, so at that point it is not possible to have all the motors go completely dead.
Feb 13, 2013, 09:50 PM
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Ran D. St. Clair's Avatar
The Carbonbird motors are finally in stock and my order has been placed. The Bixler should be here any day now. Soon, we will get this build thread started.
Feb 14, 2013, 08:43 AM
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Originally Posted by Ran D. St. Clair
The Carbonbird motors are finally in stock and my order has been placed. The Bixler should be here any day now. Soon, we will get this build thread started.
Cool; anxious for this project.
Feb 15, 2013, 08:48 PM
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The build thread begins

Some of the parts I ordered are starting to arrive, so lets get this build thread started.

Let me begin by inviting, comments, questions, and most especially corrections. I would rather be corrected than put bad information out there, so please jump in. Also, there are many right ways to do this stuff so don't hesitate to jump in with what has worked for you. I treat this sort of thing as one big learning opportunity, both for me, and for everyone who will ever read this thread, so keep the knowledge coming.

Lets begin with some of the parts I ordered, where to get them, and how much they cost. The first shipment is from HK (Hobby King). I will avoid any discussion of their business practices, quality, or even the US trade deficit and just list the parts. Not all of this stuff will necessarily make it into the final design, but it's good to have (cheap) options.

Lets start with the KK2 Flight Controller board - $29.99


A 7 channel RX for use with my Spectrum DX7 TX. Obviously any 7 channel RX will do. - $19.95


Orange satellite RX to add RF path diversity. – $11.83


4 each Turnigy Plush 18A brushless motor speed controllers - $11.90 each. I have never used these before.


Turnigy BESC Programming card for use with the motor speed controllers - $6.95. I have also never used this before either.


Turnigy HV SBEC 5A Switching Regulator – $8.48. It is surprisingly heavy at 37g. For reference, the 18A speed controllers weigh 34g each.


Li-Po Low Voltage Alarm – $1.99. For that price I bought 3 of them.


I bought another Eagletree Guardian direct from the Eagletree web site - $74.99


It may not make it into the final design, and I have already proved that it is not absolutely necessary, but I have lots of projects that can benefit from a Guardian so I bought one anyway.
Feb 18, 2013, 03:21 PM
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More Parts

There is still no sign of my 2nd HK order which includes the Bixler kit, but I did get my order from


I ordered the wrong motors and got 4 of these for $23.50:


What I should have ordered is these “reverse shaft” motors, also $23.50:


Fortunately I was able to convert the normal shaft motors to reverse shaft per the instructions on the web site. I repositioned the circlip and removed the grub screw (set screw) and pressed the shaft through using my small bench top drill press. I found it was important not to press the shaft too far in order to avoid putting a load on the bearings.

I have not used these motors before, but they come highly recommended by the folks that fly quads in my area (Jason and Kasra).They come with replacement shafts in case they get bent. They also sell replacement bearings, so they are not meant to be throw away items.

They come with collet type prop adapters which will give me about ¼” more prop clearance than my existing design, so should help reduce the possibility of prop strikes on the motor pods.

They come with 2mm long GPC type connectors already installed and the mating female connector is included. At 1175KV they are recommended for use with the GWS 9” diameter 5” pitch 3 blade props with 5mm shaft adapter, though that is the upper end of re recommended range, so will draw the most current

The GWS 9” diameter 5” pitch 3 blade props are $19.85 for a set of 8, 4 CW and 4 CCW. These props are recommended for aerial photography because they can be very well balanced for smooth operation. They do need to be balanced though, as they are consistently out of balance as they come out of the mold.


I bought these button head screws to mount the motors for $2.75 for a pack of 10. I see now that they aren’t really necessary as the motors come with flat head screws so all I have to do is countersink the motor mounts to match.


For those who want a little more power and slightly more efficiency for longer battery life, you can try this motor on 4S for $27.50 each.


At 775KV it should also swing the GWS 9” diameter 5” pitch 3 blade props using the same 18A motor speed controllers on 4S.

Speaking of which, I got 4 of these 18A BESC (Brushless Electronic Speed Controller) for $16.50 each. They come with the mating female 2mm long GPC type connectors that match the motors. They are also a little smaller than the 18A plush BESC’s I bought, and only provide 1A for the BEC (Battery Eliminator Circuit), instead of 2A for the Plush, but they are the same weight. We won’t be using the built in BEC for this project anyway.


I could have bought, but did not, this 3A SBEC (Switching Battery Eliminator Circuit) for $8.75. At 11g it is considerably lighter than the 37g, 4A SBEC I bought from HK. An SBEC is more efficient than the linear BEC included as part of the BESC and will provide more current as might be needed for the motor tilt servo’s.


I also bought these connectors for power distribution. They are $6.50 for a pack of 6 each, including both the male and female. They are rated for 12A continuous and 18A burst, but I estimate we will be drawing less than 7A per motor in a hover. The wires are 20 AWG and we will be keeping them as short as possible.


I bought 8 of these female to female cables for connection from the RX to the KK2 board for $3.75. We should only need 5 of them.


I also bought this 4Y connector for $5.75. Up until now I have been using the Castle Phoenix BESC’s. They are fine units but they only come in a 10A and 25A version. The 10A is a little small though it is working fine on my first prototype. The 25A is more than adequate but a bit expensive. The Phoenix BESC’s can be set to fixed endpoint mode using a special USB interface cable or via TX “stick programming”. The throttle range on the Turnigy and CarbonBird BESC is set via stick programming and it is convenient to connect all 4 BESC’s in parallel so they can be programmed at the same time. It remains to be seen if this 4Y connector is even necessary as the KK2 board has provisions to do this, and I also have a Turnigy programming card. I will report on how this all works out later.


Now that I have the motors I can get on with designing the motor mounts. It looks like they will be almost identical to the motor mounts that I have already made. That should simplify the design. It’s time to get busy….
Feb 23, 2013, 11:34 PM
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Motor Mounts

It has been over a month now and I have still not received my Bixler kit from HK. I am beginning to think it is lost for good.

Meanwhile, I have made progress on designing and building the motor mounts. The motor mounts are the most difficult part of this project, which is why I am hoping that someone will pick up the design and offer it commercially. They are not difficult to build if you have some basic tools, but it would still be much easier to buy them.

Lets begin with some of the basic design philosophy and specifications. They need to be simple, strong and reliable of course, and they should be as easy to make as possible. I wanted them to be as narrow as reasonably possible because the motor boom will be as wide as the motor mount, and it blocks a portion of the rotor disk and reduces the resulting lift when in hover mode.

The motor pivot point is roughly at the C.G. for the combination of the motor and the propeller. This reduces the gravity loads on the tilt servo, and also reduces the shock loads in case of a rough landing. This design can’t protect the servo if the motor or prop hit something though. The motor pivot point is also in line with the prop shaft, so motor thrust will not be translated to servo torque.

The mount has a mechanical over rotation stop in the vertical direction. This is a safety feature to try and avoid a prop-boom strike. Depending on the accuracy of the build, the stop may engage 1 or 2 degrees before the motor reaches vertical. This is by design as it provides more clearance between the prop and the boom. It is also convenient to have the motors tilted slightly forward even when in full hover mode. It just means that the aircraft hovers 1 or 2 degrees nose high which is convenient for a number of reasons. If you should use this design where you need the motor to rotate farther you can simply remove the over-rotation stop, or you can file a small depression in the side of the outer yoke to have the stop engage wherever you want.

The tilt bearings are stainless steel in a nylon bushing. They require no lubrication which might attract dust. They are not perfectly without slop, but the math says that the slop due to the bearing alone is just slightly over 1 degree. This has not been a problem in previous designs.

The tilt input is via a standard 2-56 threaded ball link, Du-Bro catalogue number 181, with a lever arm of 0.5”. This works well with a typical servo output arm of about the same radius or slightly less. It is best if the linkage uses close to the maximum available servo travel for maximum torque and minimum slop.

The motor wires are well managed throughout the tilt range with no kinks or stress points. There are provisions for a nylon cable clamp on both sides of the outer yoke.

The design is intended to be left/right symmetrical with one exception. The standard motor mount bolt pattern is not symmetrical with respect to the direction that the wires exit the motor. A universal mount would require slotted holes, which is not convenient or necessary for hand made prototypes, so the motor mount holes are simply located as required.

The motor mount is suitable for both tractor and pusher applications, however, the possibility of a propeller-boom strike is greater for a pusher due to flexibility of the propeller blades.

The mount is intended for use with a simple plywood firewall and will typically be attached with 4 each #4 self tapping screws. It weighs about 18g including bearings, over-rotation stop, ball link and wire management hardware. For reference, it is typically used with motors weighing 50g to 68g on VOL aircraft weighing about 42 oz (2.6 lb).

The mount works with the Carbonbird motors which are 28mm in diameter. It should also work well for similar Scorpion, E-flite and AXi motors of essentially the same diameter.

I did run into an unexpected snag during the build of the 4 prototypes. Of the 4 motors I received from Multiwiicopter.com, one has a reverse image mounting bolt pattern. You can see what I mean in the attached picture labeled “MM Screwe.jpg”. This is not a huge problem for me. I simply filed the mounting holes into slots with a needle file. It means, however, that I can’t give you a universal motor mount design. You can use this design as a basis, but you will need to check your specific motors to determine if they will fit. It’s much easier to drill them to fit in the first place rather than filing them to fit later.

I also note that the Multiwiicopter.com web site calls for the mounting holes to be 16.5mm by 19mm spacing. They are not. They are 16mm by 19mm. Personally I would rather they were square, like 19mm by 19mm, but this offset spacing seems to be some kind of an industry standard. The E-Flite and AXi motors I own have a similar spacing.

Now for the build instructions…

Print the file “Motor Mount V2.pdf” attached herein at 100% scale and verify that both pieces are 1” from top to bottom. Also verify the motor mount holes as needed for your specific motors. You can often use the “X” mount that comes with the motors as a template. Make sure the motor wires exit straight down as viewed on the drawing.

You will need 4 more copies to use as templates. The templates don’t need to be cut exactly on the line, but if you are using 1” wide base stock then the one long edge should be cut precisely to aid in positioning the templates.

You can use 1” strip stock from your local hardware store or you can buy 1/16” thick strip or sheet stock from McMaster Carr or most any other source. The alloy is not critical but 6061, 1001, or 3003 is most common. You don’t want any prior heat treatment because you need it to be soft for bending.

Spray a wet coat of 3M Super 77 spray adhesive on your aluminum stock and place the paper templates for efficient material use. You can shift the templates slightly while the glue is still wet. Let the glue dry so your templates don’t shift when you begin work.

I recommend you drill the holes first and then cut the pieces apart. Carefully center punch each of the holes to be drilled. This is the most critical step for the accuracy of the final product. Using sharp drills at low speed, drill each of the holes to the size specified on the drawing. Holes larger than 1/8” should be drilled to 1/8” first and then drilled to their final size. A drill press makes this much easier and more accurate.

Soft aluminum is gummy and will leave large burrs on the exit side of the drill holes. I like to use a piece of soft wood under the aluminum to avoid smashing the burrs which makes them more difficult to remove. I also use a sharp wood chisel to slice the burrs off the backside of the aluminum. You may need to do this after every hole, or every few holes, otherwise the burrs will accumulate and you won’t be able to lay the aluminum flat for further drilling.

The 5/8” hole in the center of the outer yoke is not absolutely necessary. It is only there to remove excess material and make the mounts lighter. I used a bi-metal hole saw, but you can also drill it out with the largest drill you have, or not drill it at all.

Cut the pieces out using whatever tools you have. A hack saw and a file will work if necessary. A good jig saw with the proper blade is better. A sheet metal sheer is best, or since these pieces are all fairly small I used a manually operated corner cutter. I also used a belt sander to clean up the edges and remove the burrs, but you can do the job with a file or even a sanding block.

You can make the bends in a bench vice, but a sheet metal brake is best. You will note that the drawings show 3 lines at the bends. The one in the middle is the center of the bend, and the ones on either side show where the bend begins and ends. They also show precisely where to position the brake shoe. The tip of the shoe is placed on top of the line nearest the end of the part. See the included picture for reference. Also note that a plastic square was used to insure that the parts were exactly perpendicular to the brake. Bend the inner motor mount piece slightly past 90 degrees. It will be easier to bend it back out a little if necessary later. Also, it will tend to bend out a little when the motor screws are tightened.

Use a small tray with a little acetone to loosen the paper templates, and then use an acid brush to scrub off the glue. You can use a 3M paint stripper wheel to clean up the surface of the metal and remove any remaining burrs if you are so inclined. I use mine in the drill press at high speed, but work gently as it will remove material quickly if you let it. Once the parts are clean, wash with soap and water to remove any residue. You can paint the parts with rattle can paint if you want, but it isn’t necessary.

Final assembly is a little tweaky, meaning you will have to adjust things to get them to fit just right.

Insert the McMaster Carr, 90062A015, nylon bushings from the outside in, into the two 3/8” diameter holes of the inner motor mount. They should fit snug. If they are too tight you may have to file them out a touch. If they are too loose you may need a drop of CA to lock them in place. You don’t want them to be sloppy as it will contribute to motor tilt angle slop. Don’t allow the CA to get into the bearing surface.


Mount the motor in the inner motor mount using 4 each of the 3MM button head screws. You can also use the flat head screws that came with the motor but you should really countersink the holes in that case.


Test fit the inner motor mount into the outer yoke. You want the outer yoke to pinch inward on the nylon bushings slightly, but not so much that it creates excessive drag on the nylon bushings. Make any necessary adjustments to the bend angles of the outer yoke. If you see that you also need to make adjustments to the inner motor mount bend angles then remove the motor first. You don’t want to bend the motor shaft or damage the motor. When everything looks good and the pinch pressure is about right remove the motor from the inner motor mount. You will need it out of the way to install the stainless steel shaft stubs, McMaster Carr, 91125A339, using a 3/16” long 4-40 screw. A standard mild steel screw with a Phillips button head will work fine.


Now re-mount the motor and check the pinch fit. You can pry the outer yoke open slightly if it is too tight, and you can pinch the outer yoke together slightly in a vice if necessary. Eventually, you will want to lock down all of the various screws with 242 blue removable Loctite. For now, only put Loctite on the 2 screws that hold the ¼” stainless steel standoffs. You will need to remove the motor to access the standoffs. Use a minimum of Loctite and keep it out of the bearing surface area because once these are installed there is no easy way to remove them.

Now you can mount the over rotation stop, the ball joint, and the cable clamp.

The 3/16” cable clamp, McMaster Carr, 88763T5, is mounted with a 3/8” long, 4-40 screw, with a Phillips button head, from the inside out. Use a washer and a 4-40 nylock nut on the outside. Small cable clamps like this should be available at your local hardware store, or similar. You may have to slightly file the upper rear corner of the inner motor mount to make it clear the head of the cable clamp screw.


The over-rotation stop is assembled from a ¼” nylon spacer, McMaster Carr, 94639A193, and a 3/8” long, 2-56 button head Phillips screw, and a 2-56 nut.


Mount the 2-56 threaded ball link, Du-Bro catalogue number 181, with the included washer under the ball link, not the nut. Otherwise the threaded shaft of the ball link might extend too far inward and touch the motor bell housing. Speaking of which, make sure that the motor turns freely and nothing is touching the bell housing.

Thread the motor wires though the cable clamp and make sure they have an adequate service loop when the motor is vertical. When the motor is rotated to horizontal there should be no stress on the wires.

You will need to remove the motor from the motor mount one more time when you install the assembly to the firewall. Sorry, but it’s all in the interest of making everything fit just right.

I have attached lots of pictures for reference.
Last edited by Ran D. St. Clair; Feb 23, 2013 at 11:47 PM.
Mar 05, 2013, 08:22 PM
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Motor Pods

My Bixler still hasn’t arrived from HK, but fortunately I already have the same model of Bixler that I bought for general flight training of guest flyers. I don’t want to chop it up, but I can use it to make measurements and continue building.

The two wing mounted motor pods made from 2” thick blue foam, available at your local building supply store, or if you don’t want to buy so much of it you can get a 2” x 12” x 24” piece for $5.00 plus shipping here:


Print 2 copies of the template in the attached file “Motor Pods Template.pdf”. Be sure to print it at 100% scale. Both pieces should be 9 1/8” from end to end. It is drawn in two pieces so it will fit on a standard 8 ½ x 11 piece of paper. Cut the pieces out and tape them together at the join line making sure that the datum line that runs through both pieces is straight. You will need a right and a left, so make two. As printed the template is for the inside surface of the left motor pod. You can use PDF software to flip the image, or do it old school with a straight pin and a pencil. Just poke holes at the various corners of the servo and then flip the paper over and connect the dots.

Spray a light cote of 3M Super 77 spray adhesive on the templates and lay them on the foam. Cut out the pieces with a jig saw, or whatever you have, but take care to cut them square. You will note an arrow on the template that says “Cut to here”. We are going to intentionally cut the wing opening slightly short. Later, we will test fit the pods onto the wing and use the wing itself as a template to mark and cut the trailing edge of the slot for a perfect fit.

Use your finger to smear a coat of Elmers glue in the wing saddle area and let it dry. This will toughen it up a bit.

Use the attached template “Firewalls.pdf” to make all 4 firewalls out of 3/32” aircraft plywood. The templates should measure 2” wide, use 3M Super 77, and you know the rest.

Double check the foam for square before attaching the firewall pieces with epoxy. The flat side of the firewall goes to the top on the front of the pod, and to the bottom in the rear. The little round notch goes to the inside, which is the side where the paper is attached. Tape the firewalls in place with some scotch tape and let them cure.

Use a sharp X-acto knife to cut the openings for the tilt servo’s. I am using Hitec HS-85MG servo’s but any similar metal gear servo should work. You don’t have to cut the servo pockets to full depth right now, but you need to cut them deep enough so you can tell where the servo’s will go after you remove the paper template. When you do cut the pocket to full depth later, leave a shelf on the front side for the servo mount boss. There is no shelf on the back side since the extra material is removed to allow room for the servo wires.

Remove the paper templates and carve and sand the motor pods to shape. Round all the corners as much as possible except in the area of the wing saddle, and even there a little rounding at the leading edge and trailing edge of the wing won’t hurt. We want to allow the air to flow around the motor pods as easily as possible, especially in the area of the prop disk when in a hover. Don’t be afraid to sand down the edges of the firewalls a bit, they are a little larger than necessary.

Finish digging out the servo pockets until the servo’s fit so the top of the servo arm mounting screw is flush with the outer surface of the pod. A small blade screwdriver seems to be the best tool for this. You will also need to carve out room for the lower part of the servo arm to swing through its range of motion. Poke a hole through the bottom of the motor pod into the servo pocket for the servo wire. There will be a 6mm strip of carbon fiber running down the center bottom of the pod so locate the hole to the inside of the carbon fiber strip.

Carve out the slot for the Sullivan Gold-n-Rod outer housing. I used the semi-flexible type with the blue outer sheathing and the yellow nylon inner rods. The slot is aligned with the tip of the lower servo output arm, across the top surface of the wing and is clear of the motor pod for the last 2 ½” or so. I carved the slot with a 3/16” diameter round file.

The Gold-n-Rod outer housing is 12 5/8” long and the rear end aligns with the surface of the rear fire wall. It is intended to support the yellow nylon inner rod throughout its entire length to prevent unwanted bowing.

Dry fit the servo, servo arm, clevis, push rod, etc. and make sure it all lines up nicely with plenty of clearance for the full range of servo travel. Also, check that the rear end of the pushrod housing is pointed directly at the ball joint with the motor mount test fitted in place. The top of the motor mount will be located flush with the flat edge of the firewall and is centered left to right.

Permanently glue the Gold-n-Rod outer housing in place. I used a bead of Foam-Tac and some scotch tape to temporarily hold it in its slot.

You will need 2 each 6mm x 1mm carbon fiber (CF) strips at $3.50 each available here:


Rough up one side of both CF strips with some fine sandpaper. I do this in the sink with a little water running so any loose fibers go down the drain. You may also want to wear gloves as the fibers can be very itchy. Cut one strip in half for use along the top of the two motor pods.

Groove the foam on a shallow angle about 1” away from the firewall on the top and bottom at both ends of the motor pods. Dry fit the CF strips in the slots in the top of the firewall. It is not necessary to groove the foam along the entire length of the motor pod. Cut a triangular hole in the foam where it meets the firewall under the groove. The idea is that we will fill this hole with epoxy when we glue the CF strip in place. It will form an epoxy filet and strengthen the joint between the CF strip and the firewall.

Glue the CF strips to the top of the motor pods using epoxy and tape the strips in place while the epoxy cures. Trim the CF strips to length after the epoxy is hard. If you want, you can apply some non-shrink spackle to fill in the area around the CF strips. The bottom carbon fiber strip will be applied after the motor pods are glued to the wing.

Now is the time to finish the motor pods by whatever method you prefer, however, no finish is actually necessary. I applied white Ultracote with a low temperature iron. Do not cover the wing saddle area or the bottom where the CF strip will be glued. Cover over the grooves for the motor wires and then slit the covering and press the edges down into the grove. Later, when you put the motor wires in place, you can cover them with another strip of Ultracote, or just use some tape.

Smear some Foam-tac on the inside surface of the servo pocket to toughen it up and then let it dry, then glue the servo’s in place. I used a bead of Foam-tac along the top surfaces only, that way I can peel the glue out to remove the servo’s if necessary.

Remove the motors from the motor mounts and then attach the motor mounts to the firewalls. Mark the holes and drill to 1/16”. Attach the mounts with 12 each #4 by ½” Phillips pan head self tapping screws.

Complete the linkages by whatever method you prefer. I used steel clevises instead of nylon just because I don’t like fighting with the screw threads in order to make length adjustments.

This completes the motor pods as much as they can be for now. I have attached lots of pictures.
Mar 06, 2013, 08:43 PM
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Ran D. St. Clair's Avatar


While I am waiting for parts, a few words about the design…

It starts with the Center of Gravity (CG) of the underlying airplane. The Bixler, like most conventional airplanes, has the CG located under the wing spar, at about 33% of the average wing cord. This is a very docile CG location with good stability and pitch damping characteristics.

Now imagine that you are looking at the aircraft from above, with all four rotors centered over this CG location. At this point the rotor disks fully overlap one another. Now evenly push the rotors out on the diagonals and at some point the rotors no longer overlap each other, but they still overlap the wing. Push them out farther and the front rotors clear the wing first, since there is less of the wing in front of the CG than behind. Now push them out further until the back rotors clear the wing with a somewhat arbitrary ½” to spare. For the Bixler, this requires that the rotors are 20 ½” apart from center to center.

Once we know where the rotors need to go, everything else is just the practical matter of making the structure work.

The extra ½” clearance is to minimize the impact on the rear rotors as they begin to tilt forward for forward flight. You can imagine a cylinder with a diameter equal to the rotor disk, and its long axis in line with the motor shaft. In hover mode, the cylinder misses the trailing edge of the wing by ½”, but as the motor is tilted more than about 10 degrees it will hit the wing trailing edge. Now the air entering the propeller disk doesn’t travel in only a straight line, but there is some small degree of blockage associated with the wing in front of the propeller. The more forward airspeed we develop, the more the air will flow over the wing in the normal manner of an airplane and feed into the rear rotor disk at a diagonal. In full forward flight all the rotor disks will be vertical and the airflow will be no different than any other airplane, except that the rear propeller will be operating in the prop wash from the front propeller, as well as any turbulence from the wing. This is not so bad, but it can’t be avoided.

We could push the rotor disks out even farther along the diagonals. If we did the structure would have to be heavier to be both strong and stiff enough. It would, however, give greater control authority in both pitch and roll when in a hover. Pitch and roll authority are a good thing, especially when we have large wings with dihedral sticking out to catch any gust of wind and upset the aircraft. This is one of the reasons it is important to hover with the nose into any significant wind. At that point you aren’t really hovering any more, but are in slow forward flight, and the aerodynamic control surfaces can begin to help with control authority. The aerodynamic stability of the airplane works against you, however, if you try to move through the air at any significant speed in any direction other than forward.

You can use this same concept of pushing the rotors out from the aircraft CG with a quad-copter in a Plus configuration or a tri-copter, etc. It is not absolutely necessary that you push the rotors out from the CG by the same amount, but it does simplify things. For example, you could push the tail motor out farther in a plus configuration. In this example, you might have a very powerful motor in the nose, 2 weaker motors beyond each wing tip, and one even weaker motor behind the tail. If each of the motors and associated rotors had a thrust inversely proportional to their distance from the CG then everything would balance perfectly in pitch and roll, but not necessarily in yaw.

It might seem strange, but the yaw torque produced by the rotors is not multiplied by the lever arm like the lifting force. In this particular example, the combination of the strong motor up front and the weak motor in the back might just about balance the torque of the two medium motors at the wing tips, so it all works out. Even if it all didn’t balance out perfectly, the flight controller can compensate so long as it is reasonably close. There are lots of non-symmetrical configurations that fly just fine.

A tri-copter achieves yaw control by a different mechanism and all sorts of non-symmetrical configurations are possible. You could put two large motors on pods in front of the wings, like a conventional twin aircraft, and one small motor on a long lever arm behind the tail. This tail rotor would not even have to tilt forward for forward flight. It could swivel as is typical for a tri-copter for yaw control, and the long lever arm does help give it good control authority in this case.

Another consideration is the vertical placement of the rotors. Leadfeather has suggested the idea of putting the rear rotors up high, and the front rotors down low. That way the rotors still have some horizontal separation when the aircraft is vertical and the rotors are all tilted forward. It would be possible to hover in full control while the aircraft is horizontal, vertical, or anywhere in between.

For the VTOL Trainer, the vertical rotor placement is much more a practical matter. I just wanted to get the rear rotor up as high as possible to get it up out of the grass when in hover mode. It would also be nice to be able to take off or land like an airplane without the propeller tips dragging through the grass. I doubt the rear propeller will have quite enough clearance except for a soft landing with little or no throttle, so most landings, and probably all takeoff’s will be in hover mode, or at least in slow forward flight mode.
Mar 08, 2013, 08:11 PM
Registered User
Ran D. St. Clair's Avatar

New Bixler Arrived

My Bixler kit finally arrived. No, not the first one I ordered, but the 2nd one I ordered after giving up on the first one. They now tell me that the 1st one is on its way back to HK because I never claimed it. I wonder how much that will cost me? This is the hidden cost of a really cheap price.

I ordered the Bixler V1.1 kit, for $34.44 plus $37.51 shipping:


As the web site explains, there are a number of differences with the previous Bixler V1 that I already own. Most of these will work well with the conversion to VTOL. One I hadn’t expected is that they changed the airfoil slightly. The leading edge is a bit sharper and it has a bit less undercamber. My motor pods still fit, but not as perfectly as they did on my older Bixler. I will need to make a new motor pod template for the benefit of future builders.

The old Bixler also had a covered channel for the aileron servo wires. I was planning to use that for the motor wires as well. The new Bixler has a tiny slot for the servo wires, so I guess I will have to dig out a slot for the motor wires.

The new Bixler uses two screws to bolt on the wings. I suspect the old method was more crash tolerant, but the new method will work better for my purposes as I need to remove the fuselage structure above the wing because that is where I will be mounting the flight controller.

My intent is to have all of the radio gear permanently mounted to the wing, with the exception of the rudder and elevator servo’s and the battery, which will be in the fuselage. That way the one piece wing and motor pods will be removable for transport. This method should minimize the number of electrical connections required for assembly to just 3, two if you don’t count the battery. I will be using external struts to stiffen the structure so there will be 4 more mechanical connections to make as well, but I plan to use heavy duty steel clevices so no tools should be required.

Overall I still think the Bixler is a fine platform for this project. Not perfect perhaps but an excellent starting point.
Mar 09, 2013, 12:39 AM
Registered User
Ran, Thanks for the write up and all the details. Hope to see you at the field soon.

Latest blog entry: Car Show
Mar 09, 2013, 02:41 PM
Registered User
Ran D. St. Clair's Avatar

New Motor Pod Template

Thanks for the words of encouragement Kasra. I had expected more back and forth, but I guess I type enough for everybody.

I have attached the new revised motor pod template to better fit the current version of the Bixler V1.1. The datum line is now essentially the undersurface of the wing which is now pretty much flat. This gives a very slight positive incidence relative to the horizontal stabalizer, as you might expect for a trainer, but it has no practical effect on the thrust line since the motor mounts make the thrust line variable. I will not be rebuilding my motor pods to the new templates since the differences aren't that significant. I may have to fill a few little gaps with light weight spackle, but it shouldn't be noticable.

Here you go.
Mar 09, 2013, 08:25 PM
Registered User
RCvertt's Avatar
Very nice build. Those new motor mounts look good.
Mar 09, 2013, 10:42 PM
Registered User
Ran D. St. Clair's Avatar

Getting Ready to Build the Bixler

Oddly enough, the first step for building the Bixler is to program the Transmitter. It doesn’t need to be completely programmed, but we want to identify the channels and center the servo’s so we can mount the servo arms. The screws that mount the servo arms aren’t easily accessible so we need to do all this before we glue the servos in place. Further complicating matters, The KK2 Flight controller has a specific input range and polarity it wants to see so we need to set up the TX accordingly.

I am doing this with a DX7s, but any 7 channel TX will do. You might be able to get away with a 6 channel TX, or even less, but we will talk about that later.
I am starting with a new model in airplane or “acro” mode. Name the model as you like. Select the wing type with 1 aileron servo and 2 flap servos. Use the standard tail type, meaning one elevator and one rudder servo. Set the flaps up on the flap switch with +100% travel with the switch up, -100% travel with the switch down, and 0% travel in the middle. These are just initial values to get the servo’s moving.

Power the RX by whatever means is handy and bind the TX to the RX if it isn’t already bound. Use a spare servo to map out the RX outputs. In my case they are as follows. You might find it convenient to label your RX accordingly, if it isn’t already labeled.

1. Throttle
2. Aileron
3. Elevator
4. Rudder
5. Gear
6. Right Flap
7. Left Flap

Now, on the bench, connect the RX outputs to the inputs of the KK2 board. You will need the user manual for the KK2 Board located here:


You will also need double ended female servo cables like these. I recommend you remove the polarizing ridge on one edge of the connectors, at least on the end that goes to the KK2 board.


Connect the Throttle, Aileron, Elevator, Rudder, and Gear output from the RX into the corresponding inputs of the KK2 board per the diagram on the front page of the KK2 User manual. The gear output from the RX will connect to the AUX input of the KK2. The ground pins on the KK2 are along the outside edge of the board.

When you power up the KK2 board the initial screen should look like this. “Menu Screen.JPG”

Press the “MENU” button (lower right) and then press the “DOWN” button, to highlight “Receiver Test”, and then press “ENTER”. You should see a screen that looks like this. “RX Monitor Screen.JPG”

As you move the TX sticks you should see the numbers changing for the various inputs, and you will also see helpful words like “Idle” or “Full” for throttle to indicate the direction of stick movement. In my case the aileron and elevator were reversed so I reversed them using the TX programming. Don’t worry about the polarity of the Auxiliary input for now, just make sure it moves when you flip the appropriate switch.

If you have already set up dual rates or multiple rates, make sure you are on high rates for the next step.

Use your Sub Trim and Travel adjustment (also known as endpoint adjustment) so that neutral stick reads a count of zero and full stick deflection reads 100 or -100 as the case may be. For the throttle, low stick should read zero and high stick should read 100%. You want to make sure that throttle linearity is maintained by setting the two travel limits to the same value and adjusting the sub trim as necessary. You will also note that for throttle only, readings below zero or above 100 are not possible. You want to make sure that full stick travel just barely gets you to zero or 100 as the case may be. You don’t want a lot of travel at either extreme of stick movement where the numbers aren’t changing, but you do want to make sure you get to 100 and zero with a small amount of margin to spare.

That is all the TX programming you need to do for now. Disconnect the KK2 from the RX and set the KK2 aside.

You will not want to touch the Throttle, Aileron, Elevator or Rudder sub trim or travel adjustments again. Do not use them to adjust the centering or travel of your servos. All such adjustments will need to be made via the linkages. You can and will need to adjust the sub trim and travel for the two flap servo’s but they do not affect the KK2. You will also eventually need to adjust the RX Gear output, which is also the KK2 Aux input, but that is only used as an on off switch when the signal crosses the zero threshold.

Now plug in your rudder, elevator, and two aileron servo’s into the RX and set the servo output arm in the best possible position as limited by the splines on the output shaft. You can do this one servo at a time if you want. This is not a permanent installation, we are just checking things out. While you are at it, check the direction of travel for all servo’s. You cannot change it in the TX as you normally would. You will need to identify which servo’s will need to be reversed.

In my case, only the elevator was correct. I needed to reverse the rudder and both ailerons. In the old days we would have opened up the servo’s and reversed the wires to the motor and the feedback pot, but that is no longer a reasonable thing to do. I bought two of these Dionysus in line servo reversers for $11.95 each.


I only need one reverser for both ailerons since I can reverse the aileron signal and then use a Y harness to drive both servos.

Speaking of Y harnesses, I need 3 more to split the Aileron, Elevator and Rudder output from the RX. One side will go to the KK2, and the other side will go to the servos. In the case of the rudder, it will go to the servo through the servo reverser. In the case of the ailerons, it will go through the servo reverser, then through a 4th Y harness to drive both aileron servo’s in parallel. The bottom line is that we need 4 Y harnesses, something like these for $5.95 each.


I have been warned not to use the GWS Y harness with reverse that includes a trim pot. I am told it is very noisy when used with 2.4Ghz radio systems. I am not bashing GWS and I have not personally experienced this problem, so I am just passing on the warning.

It is unfortunate and almost unnecessary that we need all these Y harnesses and servo reversers. The KK2 has 8 outputs and could easily drive all 4 motors plus these 4 servo’s, but it has a feature (bug) that prevents its use. If you take the throttle to full idle, all of the motors will go dead, which is fine, but all of the servo’s will also go dead, which is unacceptable for an aircraft that can glide.

We will also be needing various servo extensions for the aileron servo’s in the wings, and also for the tilt servo’s in the motor pods. I will come to those later when we start installing everything in the aircraft.

In the mean time, do not permanently mount the servo arms or glue the servo’s into their pockets until you install the in line servo reversers. They might change the optimum servo arm position and it is difficult to remove and reinstall the arms once the servo’s are glued In place.

That’s enough for one post. We will continue later.
Last edited by Ran D. St. Clair; Mar 09, 2013 at 10:54 PM.

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