Adding a rear battery bay

After determining I would need approximately 750g of battery weight to get the range out of the Sky Eye that I needed for 30 miles of flight, I loaded the plane up with roughly that amount of weight of spare 4-cell batteries I had on hand to see what kind of performance I could expect.

Well the first thing is that it is nose heavy at that weight. Not unflyable, but it needed substantial elevator trim to maintain level flight, even with all of the weight shifted as far aft in the compartment as possible. Since a nose heavy condition is far more draggy, I started cutting away at the rear fuselage to add a battery compartment on the aft-side of the wings.

When I began cutting away, I was met with a pretty nice surprise regarding the Sky Eye’s build quality:
There is actually a large carbon spar that runs the length of fuselage. I had noticed that the stiffness of the tail was quite a bit better than other long-tailed planes I have flown (cough cough Radian cough) but given that they couldnt even put bearings on the wheel, I didn’t expect a second, very large spar!

There are also foam struts between hollow cavities running all the way back to the tail. These double as nice guides for the elevator/rudder pushrods. Given that the Bixler doesnt have any such fuselage struts or spars, I know that they are not required for flight but it is nice that HobbyKing has beefed this thing up so much.

Anyhow, I carved out two of the struts to make room for a battery bay accessible by a taped on side hatch. I ran a charging harness along with a power lead up into the front so I won’t have to remove this battery very often.

The battery fills the gap that was left by the strut I removed quite well and supports the pushrod tubes. It fits in quite snugly and doesnt need any extra attachment.

After patching everything back up, adding my big batteries and re-checking CG, I was elated to see that I’m back near the tail-heavy side of things! I will shortly begin flight tests for the cruise/climb/longetivity of this configuration.

Initial Flight Tests

With the Sky Eye roughly set up and trimmed out, I was ready for some initial flight tests to get a feel for the airframe.


Being Tehachapi, it was pretty windy at about 10kts coming from the North. This is actually pretty calm for Tehachapi. Anyhow I bring this up because to my delight when I placed the model on the ground to get everything plugged in and checked, I discovered I could actually keep the wings level and the model resting squarely on it’s one wheel with just a little bit of wind. Those big ailerons are already coming in handy!

Unfortunately, as I said in my last post, the wheel binds up pretty badly when it has weight on it so taxiing was pretty ineffective. Would probably be possible with a real runway instead of the cul-de-sac I was using but this wheel is made for landing, not take-off.


I did this flight with a small 2200 mAh battery up front. To balance out the CG I also mounted the Galaxy Nexus up front with the camera poking out into the bubble pointing downwards. This was mostly to run My Tracks and get some performance data, but I also wanted to see what kind of video I could grab with this mounting position.

Flight impressions were great. It actually flies almost identically to the Bixler. I was expecting a dampened roll response due to the large wings but this has proved not to be the case. The flaps are some of the most effective ones I have seen in a model of this type.

- Speed at around half throttle sits around 35MPH after adjusting for wind. The moving speed was adversely affected by the wind since I spent more time flying upwind, so average moving speed cannot be used.
- At half throttle I ran through 1600mAh and traveled 7.8mi in 14 minutes. This translates to better than 5 miles per Ah of battery charge, meaning only a 6000mAh battery will be required to fly the mission goal.
- Power-off glide looks to be around 400fpm @ 30MPH. Pretty awesome.
- Climb rate is not as good as the Bixler. I do not have any numbers for now, nor would they be relevant due to the light weight. This is somewhat a concern for me as I intend to load the plane up quite a lot.

Overall things are still looking promising. The next set of tests will be performed at the estimated gross weight. I am figuring a 8000mAh 3S battery for now along with the electronics. Once I have loaded the plane up, I will re-test climb, sink, cruise and longetivity. I am also planning on figuring out what the maximum weight is for the stock power system. Stay tuned.

Sky Eye Construction

The Sky Eye arrived from HobbyKing in a non-descript box after a week long shipping delay from HobbyKing. Upon removing the parts from the box, I was a little surprised at how complete this plane already is:



Really all that needed to be done was gluing on the tail, installing the nose dome, installing the control surface horns and installing/configuring the electronics.

I think it is a little funny that even with only four basic building steps, I still had a lot of trouble with one of them because of botched hardware.


Specifically, the threaded piano wire supplied from the factory that you screw the plastic clevis’ onto were WAY too long. Like half an inch for every single control surface. The flap rods were so long that I had to dremel off the entire threaded bit of the piano wire to get them to fit in! So what I ended up for doing with these for the time being is glued them in place. I will likely go back and form my own piano wire once flight testing is done but for now the glue is holding ~10lbs of applied force fine and I’m not too worried about flying the plane if the flaps give way.


Overall, I like the plane. As anticipated, it is very reminiscent of the Bixler, with a few improvements. The wingspan is awesome and I can tell this will be able to lift a ton of weight. There is enough room in the electronics bay for at least four 2000mAh packs as well as my flight controller and the phone and that’s without any modifications. I will probably be removing the wooden platform that comes with the plane as it just wastes a few cubic inches of space with not much real benefit.

The plane is very tail heavy by default and took the equivalent of 4000mAh of weight right up against the firewall to get within range of the intended CG. This is great news as I will be putting a ton of stuff in here.

Control throw is great, I can get about 45 degree deflection from both ailerons and flaps and somewhere between 30 and 40 on the elevators. With the longer wingspan as much deflection as possible on the ailerons will assist in improving roll rate. Not that this is a huge priority for standard UAV applications.


Something I did not know about the model is it comes with a neat little single landing gear embedded into the bottom of the fuse. There also seems to be foam inserts in the wings for little struts to keep them level on the ground – kind of like the pogo legs found on a U-2, but the struts were not included in the kit. The main gear was pre-built, which is nice, but is not supported by a bearing so has quite a lot of binding if you put any pressure on it. At least it will save some wear and tear on the belly.


There are a few fit & finish problems that I’m not a huge fan of. For one thing, the instructions with the kit say to glue the nose dome on. I want this to be removable so that I can install/remove the smartphone through it. For now I’m taping it, I don’t know what my ultimate solution will be though.

The canopy is secured by 3 big magnets. They are very strong, which is awesome coming from a Bixler, but there is nothing that locates them. This means the whole canopy can slide to one side or another without much force.

The wings also have nothing that holds them in place. It’s a friction fit. This works for now when the plane is new but past experience tells me it will not last. I will probably end up adding a set of washers in each wing and a through-bolt to hold them in place.

Despite these issues I’m optimistic. This is a pretty nice plane at a great price. It looks like it will fit the bill.


Project Overview

This site is a design log for a project I am undertaking to design a UAV that will carry an Android smartphone onboard to assist in it’s control logic and communicate with a downlink.

When this project is done, it is my goal to have a UAV that is capable of flying completely autonomously at low level a total distance of at least 30 miles while remaining in contact with the ground controller via a custom interface that interacts with an onboard Android smartphone.

Basic features will include the a full downlink application that will display basic statistics such as altitude, ground-speed, battery charge level, and other essential flight metrics. The downlink software will run in the browser and will be able to adjust the route on-the-fly as well as take pictures and video on command and have it sent to the downlink station.

When a mission is complete, the UAV will circle over a designated mission completion point until an operator turns on a radio transmitter and lands it manually.

The purpose of this project is to improve the state of production UAV’s available to the general public. Currently most UAV’s are at best tied to a high powered RF downlink which has at best a 10 mile range. As cell networks with internet capability spread and cover the planet, the concept of using them to control UAV’s at long ranges and out of line of sight becomes one of the best solutions available to us. Unfortunately, there are currently no easy to use / install systems that utilize both smartphones and mature UAV platforms to give forth a working system.

The work on the project encompasses four major categories:

Airframe – Work on the actual flying vehicle, which will initially be designed as a RC aircraft and slowly developed into autonomy. A COTS airplane will be used (see below) so most of this work will involve electronics mounting and placement.

ArduPilot – Work in tuning and customizing a build of the ArduPilot UAV controller to properly interface with Android in the desired way.

Android Development – Work on an Android application that communicates with the ArduPilot controller over an OTG USB cable. This will presumably only be a middleware that works between the downlink and the ArduPilot.

Downlink Development – Work on a web-based downlink application that allows control and monitoring of the UAV by the above-prescribed means.


The UAV I have chosen for initial development is the HobbyKing Sky Eye. I have chosen this airplane because I am very familiar with and a huge fan of the Bixler, and this is essentially the same airframe but upscaled with a longer wingspan and a bigger compartment up front. The model is very affordable and comes mostly pre-built. While I do enjoy building, this is a desireable quality for this project if I decide I want to produce multiple UAVs.

The Bixler is renowned for its tame flying characteristics and efficiency. I have easily gotten 20 minutes of flight time from them with a cruise of around 25MPH. I have also loaded Bixlers with 3x the battery weight they were designed for. I am hoping that the Sky Eye only improves on these characteristics of a Bixler.

The UAV Controller


I chose to use the ArduPilot UAV controller running ArduPlane for this project. This was mainly done because I had one on hand from previous UAV work and I am familiar with the codebase. DIYDrones is also a great resource for support and help when working on this platform, something other platforms don’t have.

The board I am using is an older APM 2 unit. These days you can buy the controller boards pre-built and packaged from 3D Robotics, but this one has served me well in the past so I will be re-using it. It has a GPS chip, a compass and a barometer built in. I have also customized it with an extension for the telemetry port. These are the wires you see coming off of it.

 The Phone

I will be using an old Galaxy Nexus as the onboard Android smartphone for the project. This is a reasonably powerful phone with a great set of features. Since it is a Nexus, it’s easily rootable and comes loaded with the latest versions of Android (necessary for OTG USB hosting). For whatever reason, Google put a barometer in these phones, which is a boon for my purposes. This will serve as a sanity check for the on-board APM barometer and I will be able to cross-calibrate altitude with it. Being a T-Mobile/international variant, swapping SIM cards in and out to give it cell access is simple.

About Me

My name is James Betker. I’m a 27 year-old software engineer at Garmin in Diamond Bar. I currently live in Tehachapi, which is beautiful, but known for it’s constant, high winds – not so great for model aircraft.

My interests lie in computers and aircraft, plain and simple. I designed my first website when I was 10 years old and wrote my first C++ program a year later.  I began flying model aircraft my first year of college and have been hooked ever since. I received my private pilots license in 2013 and became the proud owner of a full-size airplane in 2014.

My job specialty is low-level embedded development. At Garmin I help in designing and developing software for next-generation automotive applications. On the side, I have developed a set of smartphone applications that utilized sensors built into the latest phones in clever new ways. I believe this project is just an extension of that work.

A design log for an ArduPilot UAV designed to use an onboard Android smartphone for command & control.