The V125FPV by Madpco

by Madpco | March 29, 2015 | (11) Posted in Projects

Hello FT fans, Hello FT Crew


It's been about a year now since I discovered Flitetest on youtube and like many others, it has sparked a whole new level of interest in a hobby I had almost abandoned. I have been tinkering with balsa planes since age 11 and have become a pretty good builder since (im 36 now) but except for a pair of 2ch gliders built during high school years, I never really got the occasion to learn to fly 4ch full power RC planes.


The first FT video I saw was about the versa wing and my jaw dropped when I learned that exceptional plane was built in a few hours with 2.50$ of raw matts and a hot glue gun. From there on I was hooked; I drove in town, got foam board, a good High temp glue gun and started building. I built myself a FT Stroch and taught myself to fly over snow covered fields. Now I can do light aerobatic and land it in 20Kph winds.


From watching hour of Flitetest videos its hard no to be tempted by FPV so I decided that would be the next step for me. I had an Ebay Firesale, sold 2 RC cars, computer hardware, antiques... and finally got enough paypal balance to go "all in" and got the Attitude v2 w/trinity, 250mw vtx and CMOS cam; and also a nice pile of hardware to build.


This article will describe details about the plane and some of the design process behind it


Design requirements:

-Carry FPV gear with pan/tilt

-Flying wing - overall efficient, sturdy, easy to build and simply AWESOME

-Landing/takeoff capable

-No prop in front

-Full flight controller - better chance of surviving and sooths some of my nerd rage.

-yaw control

-No dead weight ballast!


Main configuration:

I originally wanted to use a Versa wing but I estimated the flight weight at around 1200gr. And though that weight in not 'unreasonable' it would produce a flight envelope way beyond my capabilities.. so I played around with dimensions to come up with 125% scale: that fit tightly on 4 foam boards and produced a nice wingspan of 48 inches and a wing load of just under 10 oz/sq.ft


I can do a follow-up article on how I estimate weights and CG using a "Torque budget" and Excel. This is essential to achieve my goal of " No dead weight ballast!"


Weight balance and motor placement:

Its no secret, the versa wing has weight issues when the motor is in the back, you cant fit enough batteries inside it to balance thing out so you have to add lead weight... yuk.. This was not an option for me so I decided to try a "prop-in-slot" configuration. I remember hearing concerns about noise/efficiency issues during a show so I did some research and came up with this article by Otto Dieffenbach that claims to have resolved , at least partly, this problem, so I decided to include his findings in my design. here is the link:

diagram by:Otto Dieffenbach


note: about motors and weight placement: I thought of using a driveshaft (just like the ones used for RC boats) to have the motor in front but the prop in the back but the weight loss would only be worth it on a larger flying wing.


Landing gear and main chassis:

Now that the motor location was determined, it seemed logical to have a tricycle gear as this would also serve as a propeller guard and is conveniently steerable with the addition of a servo. The landing gear on any aircraft has to be very strong so it could not be attached directly to the foam; this required a strong chassis that would serve as a mount for the motor and both landing gear components. it would also serve to house the receiver, the flight controller and the steering servo.


The wood used to build the chassis is a ply construction of balsa and fibreglass mesh. 5+4 plies for firewall and forward bulkhead, 2+1 plies for main struts

Also, there is a carbon fibre strip inserted vertically in the foam board behind the prop



Structural issues with the main chassis:

because the chassis fits between the two top foam boards it would have to take all the forces that would normally cancel out at the board junction. to solve compression issues the removable hatch over the receiver is built strong and fits tightly between the two vertical chassis plates and the foam is sandwiched between two beams to spread the load on the chassis.
As for the tension, it is held by the tape applied at the end over the top between the two foam boards.


Yaw control:

for added stability control and sheer fun in manual mode, I had to include yaw.. at first I wanted to have a single servo in the chassis with a pull-pull linkage but it turned out too complicated and fragile so instead i put a servo in each wingtip and came up with a spring loaded linkage to keep it all on a single channel. it turned out reliable, robust and easily serviceable.



I used the simple fiberglass mount from HK with two small servos I had lying around ( a nice tiny MG one for tilt). I mounted the pan servo through the bottom board to keep things nice and low, and routed its wire straight back up the hole under the rotating mount. I also added a friction 'circle' under the mount to reduce side to side movement. servos are connected directly in the receiver on ch 7-8 and the signal is fed by the trinity head tracker.


Components and electronics:

Motor: Turnigy propdrive 28-30 1200Kv 400w rating

Prop: airframe was designed for max 10 inch
-9x4.7 for econo-cruise, general use (about 230Watts)
-8x6 for speed 90Kph potential -- stall at low speed - not for beginners..
-10x4 for tractor pull - absolute limit for 30 amp esc

 ESC: HK YEP helicopter 30A esc with SBEC
-designed for flybarless controllers so it has high quality SBEC on board
-full features for airplanes too
-Nice features like freewheeling and safety cutoff (if prop hits solid thing)

Batteries: Turnigy nanotech 1400mah 40c-80c
-these were designed to fit small traxxas RC cars 1/16 series
-extremely potent, small form factor

Radio: Turnigy 9x -with ER9X firmware and stock 9xv2 receiver

Flight controller: Openpilot CC3D

Servos: Hextronic HXT500 and HXT900

GPS: Ublox Neo-6M By

OSD: minimOSD - with minOPOSD firmware

VTX: FatShark 250mW 5.8Ghz with LHCP antenna by ImmersionRC

12V BEC: HK Mini Dual BEC & Distribution Board
-Dual SBEC 5v and 12v (5v unused in this build)

The electrical system is pretty straightforward. Just two details to share here:

1: the minimOSD absolutely needs 12v power to its analog chip (videoMax chip)
1a: you can connect a 3s lipo directly on the 12v in but I prefered using the SBEC and a 4s configuration because I had 2s packs lying around.

2: out of the box, the minimOSD's analog side is opto-coupled with and cannot share the same ground with its digital side... that means you absolutely need two battery supplies.
2a: Anyway that configuration minimises interference noise through the connections..


Range test:

I installed the plane on the 2nd floor balcony where I can see all the way to the back of the fields and took a walk with my goggles and TX. At 650m, I still have good reception but IF I turn away so signal passes through my body, I start loosing RC link and video snows up a bit but still usable. So this is where I will set my distance limit; but I believe I could probably reach 1Km but that takes alot of faith in the 9$ receiver. lol At 650m radius I can still go over 1 Km across the fields.



That pretty much covers it, now I'm waiting for my runway to dry (I live in front of a cranberry plantation over a square mile!)


Also I'm a bit chicken to do my maiden and am building up my courage ( and practicing with my Stroch)


your feedback will be greatly appreciated as alot of this is new to me. I could use tips on configuring the different CC3D stabilisation modes as I'm not sure which would be best for taking off/landing. If I missed something , please .. and any questions you have!

Here Is my cad file its more of a "CAD napkin" but every thing is in place

V125FPV Cad File .DWG





mike.chuanshi on April 2, 2015
Wow - this looks awesome. Once I get a bit more experience I'm going to have to try this.
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Paulshort on March 31, 2015
That looks so cool. What is the arduino for? Are you going to do a build video?
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Madpco on April 1, 2015
I put the Arduino cause it helped me: Flash My 9x radio, Flash The minimOSD, Configure the GPS and I used it as a servo tester.

I'm not equipped to make vidoes at all and I'm not organized enough.. I will instead make Written/illustrated instructions
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Madpco on April 1, 2015
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Troop 17 Aviation Group on March 31, 2015
Awesome build, I'm going to have to consider doing one of these down the road.
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Madpco on March 31, 2015
That would truly be an honor..
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raydar on March 31, 2015
A beautifully engineered flying wing. Would love to see your flying video when you maiden it. Also, would be appreciative of a custom foamboard tool making article.

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Madpco on March 31, 2015
I'm rafining the desing to make a "V125" not"fpv" with a less intrusive chassis and let the foam board meet in the middle.. In reality, there only needs to be a slot to accomodate the FR gear, the receiver could be slid in from the back
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TimmyGT on April 5, 2015
Hey man really nice build, looks like you put ALOT of time into it! I'm new to fpv, got all my gear courtesy of a fat tax return...turns out kids actually are good for something lmao....but I don't have any experience with osd, how was it to setup the minimosd? I picked up an immersionrc EZOSD which is great for my bigger quads and wings, but too big for my 250 quad. Also looking forward to an update after the maiden!!
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Madpco on April 7, 2015
the minimOSD was not really designed to work with a CC3D but it works ok with minimal features after the firmware update. The MinimOSD its self is really nice, especially for the price. As I wrote in my article, some electrical configuration is necessary to get it working correctly ( 2 battery supplies) and I imagine it would be really awesome if paired with an APM flight controller of some sort. Also I believe the EZOSD is a far superior product and cannot be compared (I don't have one yet)
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The V125FPV by Madpco