The stock skyfun platform is pretty good, but as a lot of people have pointed out, the power system is barely enough for our needs by the time you add a few goodies on.
There is lots of info on the web about motor upgrades for the Skyfun but most of the motor upgrades published on the net are all about speed. I wanted my Skyfun to have enough power to lift all the extra weight, but then to be able to settle down to a slow and efficient cruise speed to maximise the amount of time in the air. That means means going for a slow turning large prop. Finding info on this sort of upgrade is next to impossible. 

Lets start with the airframe:

I took a stock RTF Skyfun, removed all those hideous decals and then set about applying Gaffa tape to all of the vulnerable surfaces. This has already proven its worth during my initial testing as I had a high speed collision with a power poll with practically no damage. Gaffa is quite heavy, but it is incredibly strong and prevents the foam from tearing. Applying it to any part of the airframe where torn foam is possible reduces damage considerably. The tape can then be removed and easily replaced after an 'incident'..


After multiple failed canopy gluing attemps, I have also applied a thin strip of Gaffa tape around the edge of the canopy to keep the plastic on, which has proven effective. The canopy now feels very sturdy.

I'm using the stock servos, they seem fine for the task as it is not going to be a high speed machine.
RC equipment was upgraded to a 9ch JR PCM system.

 

Mounted just behind the receiver is a keyring video camera, I set it into a hole in the fuse at a 45 degree angle. This gives a great view and has the advantage of being closer to the CG for maximum stability and does not result in any part of the aircraft obscuring the picture.
I chose to mount the camera upside down so that the USB cable can be inserted into the camera (from the bottom) without needing to dismount the camera, it also allows easy access to the camera controls. The downside is that the image is upside down, but this can easily be flipped on a PC later.

 

 

 

 

I spent a lot of time on motor research and determined I needed the biggest prop I could spin at the slowest speed possible in order to gain maximum thrust and low speed efficiency. I settled on a Rimfire 400. Specs are below..

Model: RimFire 400 (GPMG4560)
Diameter: 28mm (1.1in)
Length: 30mm (1.2in)
kV 950
Constant Watts: 160w
Burst Watts: 220w
Weight: 54g (1.9oz)
Shaft: 3mm (0.12in)
Voltage: 2-3S LiPo
Airframe: Up to 1kg
ESC: 25 Amp
Prop: 8x6 to 10x4.5(slow fly)

I decided to run with a 10x4.5 slow fly prop, however a prop this big is wider than the distance between the two vertical stabilizers. This requires reversing the motor, pushing the shaft through to the opposite end and drilling some holes in the motor mount in order to attach the motor to the back of the mount. Doing so clears the back of the stabilisers by 8mm.

Next came the battery, I wanted to be able to cruise for at least 30 minutes and my rough calculations indicated this would require at least a 3000mAh battery. I settled on a DN 3300 30C pack. In my first few flights with this I have achieved 35 minutes and 37 minutes respectively without much consideration for efficiency. I think this can be extended further by reducing airspeed.

A battery pack this large would normally have to be seated well towards the back of the cockpit, this would have meant mounting the APM in front of the battery, something I really wasn't too keen to do. A battery this big in a crash would completely destroy anything in front of it (APM). My camera, receiver and APM were already consuming most of the back of the cockpit and putting a battery this large at the front of the plane shifted the CG too far forward.

I opted for belly mounting the battery, bomb style. This proved to be very useful as it allowed the CG to be easily shifted by quite large amounts without running into any obstacles. It also keeps the CG very low which has improved stability noticeably. It now flies more like a high wing trainer.

At the moment it is being held by industrial Velcro, however my next upgrade is to install a Velcro strap through the fuse for additional safety.

The only downside is that it exposes the battery, particularly on hard landings. I couldn't see an easy way around this that would maintain CG flexibility and didn't add a lot more weight, so I opted for sponge foam with gaffa tape over it along the bottom of the battery. I will review this after a few more flights to see how it goes but so far if I'm careful with landings its fine. If it looks like the battery is wearing too hard, then I will add some more rigid protection. Long term I will probably build some form of capsule underneath for at least the front of the battery to slot into. Most of my landings are in long grass so isn't an issue. 

 

Total weight including the battery is now 900 grams which is under my target of 1kg. I have yet to measure the thrust from the motor, but at full throttle it achieves a slow vertical climb so it must be slightly higher than 900 grams.

 

The next step is establishing the slowest speed I can possibly fly so I can figure out how long I can stay in the air for. I would be very happy if I could get it closer to a 45 minute flight.

 

 

 

 

Views: 3997

Comment by Greg Fletcher on July 31, 2011 at 9:01pm

If you are going for endurance this is not the best air frame. If your going for distance then run it as the jet that it is. don't try to make it somthing that it is not. At least build a structure for the battery for landing. I have a skyfun and it flys very well. If you want hang time then go with a glider type airframe. If you want fun and excitement then go with a jet.

Fighter jets don't get the best MPG.

Comment by Greg Fletcher on July 31, 2011 at 9:05pm

I upgraded the motor on mine and it 's 622 g with a 1600 mAh lipo. Has plenty of power and lands pretty fast.

Comment by Toby Mills on July 31, 2011 at 9:23pm

I would agree if the Skyfun were actually a jet, in reality it is a flying wing with a better glide ratio than many purpose designed foam gliders. On top of that it has a much wider speed envelope than the foam gliders so you can have fast or slow flight. The skyfun is capable of very very slow flight or very very fast flight. That makes it ideal for a UAS because you can fly to the target quickly, then slow down to loiter over the target for a long period of time, then return quickly. There are not many low cost airframes that can do that.

Comment by Rick Fetters on July 31, 2011 at 10:29pm

What sort of speed range are you talking about?

Also, have you measured speed vs power usage and calculated distance traveled vs power use?

nice project, btw

Comment by ionut on July 31, 2011 at 10:30pm
Toby Mills you should become a politician.
Comment by Toby Mills on July 31, 2011 at 10:38pm

Rick, the Skyfun is capable of 125Mph +, I haven't measured my top speed with this upgrade. It will be something far less than that, but I should get greater efficiency than the screemer upgrades for the Skyfun.

I have a whole series of missions planned to measure speed, power and efficiency and will report back.

The long term goal is to eventually create different modes for the APM that allow you to either travel as fast, power efficiently or time efficiently as possible on different legs of a journey.

It all ties into my previous thread here.

Ha ha, I've never been called a politician before, but I'm sure I could run the country better than some of the morons that are in charge.

Comment by Toby Mills on August 1, 2011 at 2:12am

here is a video to further the argument that the Skyfun is a better glider than jet...

http://youtu.be/PTYY6tsnA9w

Comment by Ramon L on August 1, 2011 at 9:47am
Toby in that wind even I can soar if I extend my arms.... that would not be a standard. For the 125mph+ it would need to be highly modified, but going in to your plan, you could try putting it in to a diet... gram per gram you could achieve more time.
Some tips i can suggest:
-Smaller receiver without the cover... ( Yes..PCB ugly thing only )
-Remove the ESC covering... ( YES, naked is lighter jejej )
-Look for torque in a motor.. like 700kv to 850kv
-Battery outside is not going to help at all... It only creates drag and thats is stoping you from your goal. Find a place inside or remove some foam to make room for it.
- Try to be precise with the CG
- Once you have done all the possible mods, then start with a 2350mAh battery and start measuring your results, if you achieve it with 2350mAh, then you wont need the extra weight of a 3000 mAh 30C, if 2350 is not enought, go for 2500mAH . Also if your goal is time and not speed, i dont see the need of extra C in the battery go with 25C, 25C packs are lighter ;)
Comment by Toby Mills on August 1, 2011 at 2:27pm

Thanks Ramon, not just endurance, I suppose I am looking for a better balance of many things than the stock skyfun. Greater strength (hence the additional tape), more speed if needed and significantly greater endurance.
I have built extremely light gliders before but they are very fragile. I also have a limitation on what is available here in NZ. So had to balance availability and cost. I will definately do something about the battery, I am thinking of a more aerodynamic pod. Although if I can get away with a lighter battery it might be able to go back in the canopy. It can be quite windy here so a slower motor might give me more time but it might not have the strength in higher winds. I'd be really interested in seeing what sort of flight times others are getting on their Skyfuns and what configurations they are running. I'm pretty happy with 35minutes, but think this can be increased further through a few more optimisations.

Comment by Hawhoo on May 22, 2012 at 12:50am
Toby! I have a very similar setup with a 890kv eflitte 450 motor. 9/6 prop. The motor is mounted aft of the mount just like in your setup. I started the first flight with half throttle and the plane went immediately into a very steep climb as if it was tail heavy. I checked the cg and it was perfect just like on the flights before with the stock motor. I moved the cg a bit forward and same result. Broke my motor mount in the process.
You seemed to have been successful with this setup, so did you angle the motor a bit down to compensate for the increase in thrust? Did you have problems with the mount breaking in this setup?

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