FX-79 Buffalo QuadPlane Conversion


It’s winter and I’m cold and bored. :grinning: So it’s time for another fun building project. My decision to leave my Ranger EX as a normal APM Plane left me with the 31" (800mm) quad booms. In reverse design fashion, I seem to have found a home for them in the Zeta FX–79 Buffalo flying wing.

The 2000mm wingspan with large prop in the rear makes it a difficult, unsafe plane to hand launch. Converting it to a quadplane eliminates these concerns. The design has a flat bottom airfoil with easily removable wings. The FX-79 dimensions, weight, and forward flying power system are similar to the Ranger EX but has a favorable mounting position for the quad booms that can make it a dedicated conversion while also retaining some travel convenience.

Plane Parts:
Zeta FX–79 Buffalo flying wing
HS-65HB Servo or equivalent 11g servo
Zeta FX-79 Buffalo Aluminum Motor Mount (Optional)
Turnigy SK3 3542-1250kv motor
Turnigy AE-65A Brushless ESC

APM Parts:
Pixhawk v2.4.8 on eBay
M8N GPS on eBay
Mauch Power Module - from Craft&Theory
Mauch Company in China - has distribution to many continents
APM Flight Controller Damping Platform

QuadPlane Parts:
SunnySky X3508 700kv motors - BuddyRC $28.50
TM 12x5.5" Carbon Fiber CW CCW Propeller - RC Timer (4 pairs) $25.
NFS ESC 45A Multi-Rotor ESC SimonK Firmware (OPTO) - RC Timer $12.86
4mm Male to 3.5mm Female Adapters from HK - $1.85/set
HXT4mm w/12AWG Silicon Wire 10cm (Battery Side)
HXT4mm w/12AWG Silicon Wire 10cm (ESC side)

Metric Hardware and 42mm Cross Hole Motor Mounts:
NTM Prop Drive 35 Series Accessory Pack - $1.89/ea
M10410 - M3x25mm Flat Head Socket Cap Screw – 0.5 Pitch - Black - $0.23/ea
M50350 - M3x.5 Nylock Nut - $0.04/ea

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Since I heard of cracking issues on the stock plastic motor mount, I decided to order the CNC aluminum motor mount. The quality of the aluminum motor mount was excellent and no frame modifications are needed! Most of the cracked plastic mount issues occurred on landings so it was likely due to pilot abuse. Further, since the QP conversion eliminates the normal landing field requirements, the CNC aluminum mount is not required for this conversion.

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How you solve ailerons and cuad arms separation? I made two wood spacers and think to replace with 3dprint ones, you glue them?

Hi Cala,

The ailerons are on the removable wing half, the quad booms will be glued (using Foam Tac) to the main body portion as the bottom is flat. I do not intend to use flaps. By keeping the quad booms on the main body, the wiring is easier. It will be a dedicated QuadPlane conversion like the Mozzie/mini Talon.


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Nice, my Fx61 have the ailerons side to side, this model looks better for conversion

I used some HS-65HB 11g servos that I had on-hand for my elevons. To increase the strength of the control horn glue joint to the elevon, I added some thin plywood on top so it spreads over a larger area.

The wing halves are quickly secured to the center section by tightening two screws. I haven’t added the second one yet. I layed the quad boom close to where I plan to glue it. There is a foam bump for the CG that I intend to remove so I can move the boom out next to the edge of the plywood brace. The booms and the vertical stabilizers stay with the center body.

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I moved the servo extension connection to the removable wing side so it is not under the quad boom. The foam CG bump was also removed and I marked the placement on the plywood piece that is also glued to the removable wing side.

The 800mm long quad boom centered nicely on the carbon tube joiner. There is good clearance for the 12" prop on the wing leading edge but a small wedge will be to be cut away from the aft end of the vertical stab for the prop tip to clear.

Although the removable wing side has a flat bottom airfoil, the center section gets thicker as you approach the center. This created an offset for the prop that required a section of balsa aileron stock to eliminate. Beacon Foam Tac works for all these materials (balsa, foam, carbon) so I am now ready to glue my quad booms.

Nice, where you find cuad boom end cups?
That Skywalker behind is waiting for conversion too :wink:

Hi Cala,

I found some nice 15mm end caps on eBay here back when I did the Bix3 QP conversion. They add a nice touch to the carbon booms. I’ll post my boom assembly here which was originally done for the Ranger EX.

As for my old Skywalker 1680 in the background, it is now my long-range test bed for the URLS with OrangeRx 433MHz 1watt modules and my 1.3GHz video link from RMRC. It has been a test bed for many projects in the past 5 years and an APM plane that won’t hurt too much if I lose it.

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Good luck with your conversion. I retired my FX-79 a few years ago, because it had a really nasty tendency to nose dive at higher speeds. This gave me too many brown pants moments. Recovery required full up elevator which caused the wings to almost touch at the tips from g-force. I never managed to find the cause, because all testing provoked further nosedives and sooner or later this would have let to overstress.

Hi Sebastian,

Thanks for the heads-up. I haven’t seen any bad tendencies on the FX-79 from the various on-line reviews and builds. The Flite Test crew even had it flying with a stall-proof setup. Normally, this type of wing doesn’t want to fly fast so it makes for a good survey cruiser that can hold some big batteries and a camera. It should make for a better quadplane than some of the other large foam FPV planes currently available. We’ll see how it handles loaded up.

A long time ago, say 15-20 years, I had the original 6’ (2M) Super Zagi that I made from a foam core. Of course, this was back in the days of brushed motors and NiCds. You can see my old wing collection here.

My plan is to use the same quad booms from my RangerEX QuadPlane project. I used SunnySky X3508 700kv motors with 12" RC Timer t-style props on a 4s 8AH Lipo pack. I used this power system on several Tarot Multirotor projects. SunnySky x3805 700kv on 4s with 12” props (times 4) provides 6.4kg (14lbs) thrust

QuadPlane Parts:
SunnySky X3508 700kv motors - BuddyRC $28.50
TM 12x5.5" Carbon Fiber CW CCW Propeller - RC Timer (4 pairs) $25.
NFS ESC 45A Multi-Rotor ESC SimonK Firmware (OPTO) - RC Timer $12.86

Metric Hardware and 42mm Cross Hole Motor Mounts
NTM Prop Drive 35 Series Accessory Pack - $1.89/ea
M10410 - M3x25mm Flat Head Socket Cap Screw – 0.5 Pitch - Black - $0.23/ea
M50350 - M3x.5 Nylock Nut - $0.04/ea

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Hi Greg,
Nice job and thank you for your informative text.
just wondering why you placed the ESC’s for the lift motors in the fuselage and not closer to the lift motors?
Ned Clark

Hi Ned,

That’s a good question. I haven’t actually placed the ESCs yet but my thoughts were leaning to put them inside because there is a ton of room with plenty of air flow. The motors can handle wet ground or weeds but the ESCs are in danger if they get wet. I sometimes take off from wild field clearings. For a dedicated quadplane conversion, placing the ESCs out of site seemed a more elegant solution.

On my mini Talon conversion, I placed the quad ESCs inside the fuselage with success. Typically, the quad portion of the flight is quite short so they really don’t have time to get hot assuming you have made a capable choice in the amperage.

I glued my quad booms in place using Foam Tac. I discovered that if I placed the boom center along the elevon servo cable connection to the outer wing that my rear 12" props now cleared the vertical stabilizer. The slight aft rear weight simply means more headroom for batteries and cameras up front.

Lastly, for fun, I compared the size difference between my FX-79 and Z-84.

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Here are a few pics showing the alignment and size of the quad booms. The green tape marks the center point of the boom along the elevon servo cable.

An analysis of the air flow reveals that a quick and easy mod will improve it. By cutting channels into the aft end of the top hatch, similar to the stock air intake channels in the nose, the air exit is greatly improved.


Thanks Greg,
I guess there is a trade off with weight also but as you say the heat issue should not present a problem given the vertical flight duration and its certainly nice to have all the ESC’s in the main fuselage.
Did you use ecalc for the quad maths?
I look forward to more instalments as you move forward!
Many thanks

Ned Clark
Ascend UAV
Mobile [+61] 0419399201


I have used eCalc before but in this case my quad power system is stolen from using it many years on my Tarot 650 and Tarot 680 Pro copters. A good example for using eCalc would be on Sam’s Mozzie where he made use of high Kv motors and small props.

This is my now “old” Tarot 680 Pro with the same power system.

While watching football yesterday and thinking about Ned’s comments, I decided to take a look at mounting the quad ESCs. It turned out to be so simple that I completed it.

I slightly enlarged the opening below the carbon tube so that an ESC could pass through it. It made for an easy plug-in-test setup so that I could test the rotation with a spare R/C system. I often use my old Spektrum radio for this and then use my FrSky Taranis for the actual flying radio system. I’m using an H-frame and labeled the ESC control wires 5-8 appropriately. Since my ESC to motor connections used the triple 3.5mm shells, it was easy to reverse the motor direction by simply unplugging the connector and turning one end 180 degrees.

The ESC wires were covered on the fuselage bottom with white duct tape. It’s strong and sticky so it won’t come off. Notice that I also covered the fuselage center with the same tape. I’ll give the boom bottom a finished look later in the project.

The ESC power wires were joined together with 4mm Male to 3.5mm Female Adapters from HK. The 4mm bullets will mate into my final 4mm Bullet-connector harness. Note the intended position for the Forward Flight ESC so the air flow comes from the bottom of the image and passes through all the ESCs (and FF motor) on its way out.

Lastly, it looks like the top hatch is so big that the Pixhawk will easily mount on the plywood tray (with a vibration dampener) and still have plenty of room on top for wires without hitting the hatch. It looks like more than an inch gap.

Hey Greg,
Certainly elegant!
I really appreciate your hard work, and whilst watching football too!
I’m guessing you are going to realign the Pixhawk orientation in the software?
Well done and thank you!