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Archive for the ‘RC Airplanes’ Category

Funky Radio Follow-Up

Posted by Tom Benedict on 23/08/2013

I finally had a break in the weather that let me fly my Bixler and Raptor with the funky radio setup. I spent most of the time dialing in the ratio of flaps and aileron spoilers for the crow mix, but finally hit on something that at least minimized ballooning when I hit the brakes. Once that was done, it was relatively easy to fly with the radio set up that way.

Something I don’t think I mentioned in my previous post is that while I was messing with aileron and flap mixes, I added a mix that lets me switch in full trailing edge ailerons. This ties the ailerons and flaps together so they move as one. It gives the aileron input a little more authority, and allows for faster rolls. On the Bixler, at least, it made a noticeable difference. I wouldn’t call the plane aerobatic, but it made it a lot snappier.

Unfortunately I haven’t had a chance to try that on the Raptor. After my first flight I brought it in to land so I could adjust the ratio for crow, and wound up cartwheeling it. The field I’m flying in is overrun with fireweed, a particularly nasty invasive plant that is almost impossible to eradicate, is poisonous to cows (makes them go blind), and likes to catch at airplane bits during landings. Most of the plane survived, but I snapped off a wingtip. That ended the day’s flying. Weak wingtips is a known bug with the Raptor, with the very simple fix of epoxying it back on. So no big deal. But it means I haven’t thoroughly tested the setup on this plane.

All in all, I’m happy with how this worked out. Again, I wouldn’t do this with a traditional power plane that expects to have its motor running most of the time. But for a motor glider which uses its prop strictly as a launch device, it’s not a bad way to go.

– Tom


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Dealing with a Funky RC Radio

Posted by Tom Benedict on 18/08/2013

In an earlier post I described a modification to my Turnigy 9XR radio that gave me two sticks that spring to center on both axes. This is necessary to set a plane up for 4-axis flying, and for that purpose it works great.

But what to do with my other planes?

At the moment I have four airplanes in my hangar: the Le Fish – the one that prompted this change, a Zagi 5C flying wing, a Bixler 2 foamie, and a Raptor 2000 Advance.

The Zagi 5C is a two-axis slope wing. It’s strictly a “bank ‘n yank” glider, and uses only the right stick on the transmitter. Changes to the left stick didn’t really affect how the Zagi flies, so this required no changes to this plane’s setup.

The other two – the Bixler and the Raptor – are motor gliders.

Birds of a Feather

Despite the obvious differences – foamie vs. built-up, pusher vs. puller, x-tail vs. v-tail, etc. – from a control standpoint they’re essentially the same plane. Each is a motor glider. Each has six control surfaces – two on the tail, two ailerons, and two flaps. And I have both set up identically. What works on one typically works on the other. Each needs to be tuned individually, of course. The tail on the Raptor has far more authority than that of the Bixler, and the Bixler’s flaps are more effective than those on the Raptor, for example. But if I could find something that worked on one, it should work for the other.

With that in mind I grabbed my sandbox plane, the Bixler, and got to work. Here’s what I hit on:

I left the right stick alone. Pull back, you go up. Push forward, you go down. Push left and right, and the plane banks. Likewise I left the rudder alone on the left stick. Left = left, right = right. The real change (of course!) was how the left stick’s up/down inputs worked.

Traditionally the throttle stick on a transmitter is treated as a 0-100% kind of control. Push the stick all the way down, you’re at zero throttle. Push it all the way up, you’re at 100% throttle. Having a stick that springs to center implied that the center position should act as the zero point for whatever I did. Let go of the stick, stuff should stop happening.

Throttle was easy: If I push the stick up from center, the prop should spin faster. Push it all the way up, I should be at 100% throttle. Let go, the stick springs to center and the prop stops spinning. So far so good. But what to do with the other direction?

Then it hit me: brakes!

This really isn’t a new idea. Many thermal gliders are set up so that the throttle stick applies brakes. All the way forward is zero brakes. All the way down is 100% brakes. Depending on how the plane is set up, “brakes” may mean spoilers, flaps deploying up to act as spoilers, flaps deploying down to act as air brakes, or a mix of flaps an ailerons known as crow or butterfly. Regardless of how it’s set up, the idea is the same: pull back, hit the brakes.

This can throw power plane pilots who transition to gliders, but if you think about it it’s the same idea as a throttle. Push the stick all the way up to go fast. Pull it all the way down to go slow. What I did with my stick is just an extension of this in which the upper half is throttle and the lower half is brakes.

I don’t have spoilers on either plane, so I used crow. The idea with crow (or butterfly, as it’s sometimes called) is to deploy the flaps down and the ailerons up. Careful balance between the amount of down flaps versus up ailerons will keep the plane from ballooning when the brakes are deployed. Why ailerons up and flaps down? Either one changes the camber of the wing. Moving a control surface up reduces camber (adding reflex), and moving a control surface down increases camber. Highly cambered airfoils stall before less cambered airfoils. By moving the ailerons up, it reflexes the tips of the wings, reducing the chance of a wingtip stall. Swap the two around, and the plane is a lot more likely to suffer from tip stall at slow speeds.

While I was at it, I set up the three position switch to provide two preset flap positions: One is reflexed about 2mm, the other cambered by about 3mm. The first position lets the plane fly a little faster while searching for thermals. The second slows it down to help me stay in them. The third position, of course, returns the wing to neutral flaps.

For the record, I wouldn’t try this on a traditional power plane. They rely on a running engine to fly well, and typically you want to operate flaps and throttle simultaneously. The only reason I could do this is that all of my planes, to one degree or another, are gliders.

I finally had a chance to try this in the air, though the conditions were far from ideal. By the end of the session, though, I had things working the way I wanted. The only real problem I ran into was during launch. I’m used to setting about 75% throttle using my lower lip on the stick, and tossing the plane out into the air. With this setup I had to hold the throttle with my lip until I could transfer to my left hand. Once I had both hands on the controls, though, everything worked great. After a little experimenting with throws, I had the Bixler flying just the way I wanted. The only job left was to migrate all these changes to the Raptor.

Unfortunately we’ve had a whole succession of tropical storms rolling through. Nothing strong enough to cause damage, but no lack of wind. Perfect for flying kites. Terrible for testing changes to gliders. I hope to get a calm day some time in the next few weeks so I can put these through their paces. Meanwhile, my kite bag is calling to me. Wind’s up!

– Tom

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Modifying a Turnigy 9XR for 4-Axis Flying

Posted by Tom Benedict on 06/08/2013

Le Fish

When I first ordered my Le Fish kit from Leading Edge Gliders back in March, I knew I wanted to set it up for 4-axis flying. In case the idea is as new to you as it was to me, this article should explain it:

Introduction to 4-Axis Flying

I highly recommend you follow the link, but here it is in a nutshell:

Most planes are set up with two or three axes of control: elevator (aways), rudder and/or ailerons. Powered models use the fourth joystick axis for throttle. Thermal gliders often use it for variable spoilers or some other form of airbrakes (flaps, crow, etc.) A four-axis aerobat is set up so the fourth joystick axis dynamically controls the camber of the wing by controlling the flaps or flaperons. Push up on the stick, the flaps go up, and the wing reflexes. Pull down, the flaps go down, and you increase the wing’s camber.

The only catch with this is that you need the stick to move both up and down, and you need it to center automatically. The stick needs to spring to center on both axes!

And that’s the real trick with 4-axis flying: you need a radio on which both sticks spring to center on both axes. Radios typically come with one of the sticks unsprung, so there’s no way to do this right out of the box. It’s possible to modify most radios to add a spring to the throttle axis, but I took a different route.

When I got my Le Fish (and Zagi 5C and Raptor 2000 Advance) I picked up a Turnigy 9XR radio. Spare parts (like the spring lever) for the 9XR aren’t as common as for other radios, but spare assemblies (like entire joystick gimbals) are cheap and readily available. Rather than convert my left gimbal to a sprung gimbal, I simply replaced it with a new one that came that way.

My radio is a Turnigy 9XR radio set up for Mode 2. Mode 2 has the throttle on the left and the aileron/elevator stick on the right – typical for North America. This means the right stick is sprung, but the left is not. Mode 1 radios – popular in Europe – are set up the opposite way, with the sprung stick on the left and the non-sprung stick on the right. Left and right gimbals may not be interchangeable, but gimbals from mode 1 and mode 2 radios are.

Turnigy 9XR Innards

Once you get the back cover off the 9XR, the gimbals are attached using four screws and are connectorized, so there’s no soldering involved in swapping them out. I picked up a left-hand gimbal for a Mode 1 radio and installed it in place of my un-sprung left-hand stick. Voila! Now I have two sticks that spring to center. (And I have a spare gimbal in case I want to swap back or build a customized KAP controller!)

The setup for four-axis flying is fairly straightforward. Rudder and elevator go straight in, and the flaperons are a 1:1 mix of aileron and flaps. I also added a 20% snap flap I can toggle using one of the radio’s switches. With snap flaps enabled, pulling back on the elevator stick moves both flaperons down to camber the wing slightly. This is typical for aerobatic gliders.

Unfortunately I haven’t been able to take out my Le Fish with the new setup. The only times I’ve flown it, I disabled the input for the flaps until I had the center-sprung gimbal. But the next time we hit the slopes, the Le Fish is coming with me.

– Tom

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Finishing the Le Fish

Posted by Tom Benedict on 22/07/2013

Now that I have a slope I can fly from, I’m at full-throttle trying to finish my Le Fish. Unfortunately, despite having cameras and loving photography in general, I did a lousy job of documenting my progress as I went. Stopping to take pictures always gets in the way of the build!

I finished my Le Fish over the weekend. For now I have a 4xAAA KAP battery pack powering the thing. I’d like to switch to a 2S LiPoly with 6V UBEC, but for now it’s in and it’s flying. The all-up weight came in at 24.4oz, so right at the bottom end of what’s considered a midweight build (25-30oz). Since we can sometimes get a fair bit of wind at our local flying spots, I may need to add a ballast tube at the CG point so I can dial it up to the 35-40oz of a traditional build weight when conditions warrant.

Here are the details:


I built out the wing as a mix between the kit’s contents (no instructions on this plane) and Steve Lange’s original build. The wing cores that came with this kit were made from 1.9oz EPP foam, not the lighter foam used in the ultralight kit. I glued the wing cores together with 3M Super 77, similar to how my Zagi went together. The wing is reinforced top and bottom with 6mm carbon fiber tube, joined by external aluminum ferrules at the wing root. I glued these in using white Gorilla Glue. (Note to the reader: I later read on Steve Lange’s Swiss Fish Build that he used white Gorilla Glue throughout, and skipped a lot of the other adhesives. DOH!) I did keep one aspect of his Swiss Fish build, though: Rather than use the two basswood spars that came with the kit for reinforcing the subtrailing edge, I used a single 8mmx1mm carbon fiber ribbon (part number T733L6 from This gives the wing a very solid base against flexing. Steve Lange used one of these in place of the two 6mm tube spars when he built his Swiss Fish. I wish I’d gone that route as well, but c’est la vie.

I covered the entire wing with 1.7mil CP “New Stuff” laminate from Aloft Hobbies. I then built up the leading edge D-box with an additional layer of 3mil film from the same source. This is straight out of Steve’s Swiss Fish Build. I sanded a bevel on the control surfaces so they’d get a full 45 degrees of swing up and down, covered them in 1.7mil laminate, and hinged them with 3mil laminate top and bottom. This made for a very solid, if slightly heavy wing.


The tail group is all balsa. Steve Lange built a new tail group out of Depron for his Swiss Fish, but since I’m not actively trying to shave weight, I kept the balsa tail that came with the kit. The tail was sanded to shape and covered in 1.7mil laminate. Hinges were made out of the same material, just like the wing. I glued the tail on using white Gorilla Glue. (See the pattern? With one exception, it’s the only adhesive I used on this plane. I love this stuff!)

In the event this tail is damaged or torn off, I plan to replace it with a Depron tail. I wound up needing a little bit of nose weight to bring the CG where I wanted it, and this would let me remove it. Besides, it’s easier to cut Depron than balsa.


I followed the recommendations for servos from the Leading Edge Gliders site, but I mixed and matched where they went. The site recommended Hitec HS-85MG for the flaperons, and standard servos for the rudder and elevator. I used Hitec HS-85MG for the rudder and elevator, and Hitec HS-325HB BB Deluxe servos (standard servos) for the flaperons. In retrospect I wish I’d picked up four of the HS-85MG servos and used them for everything. The fuselage of the Le Fish is barely wide enough to stack two standard servos side-by-side. I would’ve preferred more options in how I mounted them.

I wound up mounting the flaperon servos inside the fuselage, very similar to how Steve Lange mounted his on his Swiss Fish. The only difference is that he mounted his horizontally, with the shafts extending sideways out of the fuselage. I mounted mine vertically, but upside-down, with the shaft running perpendicular to the wing. The servo arms protrude out the side of the fuselage. I think Steve’s arrangement is cleaner, but it requires the smaller HS-85MG servos to pull off.

The rudder and elevator servos are mounted vertically just in front of the flaperon servos. One points up, the other points down. One control rod goes over the wing, one under. It’s a little awkward, but it kept everything inside the fuselage, with just the tail linkages poking out.

This arrangement put all four servos in as small a volume as possible, located just in front of the wing. This keeps the mass fairly central, so it should minimize the rotational moment of inertia in all three axes. All four servos were wrapped in blue painter’s tape and mounted using white Gorilla Glue. Fairly standard fare these days. It holds well, and by peeling off the tape later, the servos can be removed or replaced.


When I got my Turnigy 9XR radio several months back, I picked up a FrSky DJT 2.4GHz module and a stack of FrSky V8FR II receivers. An eight channel receiver is massive overkill for a four channel plane, but since I have these installed in my other three planes it keeps everything consistent between them. The receiver is mounted just forward of the tail servos using blue painter’s tape and white Gorilla Glue.

The V8FR II has two antennas. The idea is to place them away from everything else made of metal, away from each other (spatial diversity), and at ninety degrees to each other (orientation diversity). I put the horizontal antenna at the top of the fuselage, maybe half an inch below the surface. The vertical antenna is at the bottom of the fuse, poking out between the radio and the tail servos. It’s not ideal, but the active portion is completely clear of the electronics. I should get full range out of the radio with this arrangement. Considering I plan to use my LeFish for in-your-face aerobatics, range is probably the least of my worries.


I’m still on the fence about the battery. I like LiPo chemistry, but I’m already so tail-heavy, I need more weight in the nose than an appropriately sized LiPo battery and UBEC would provide. So I made the battery compartment so it could house a 4xAAA NiMH pack, plus room for ballast. I may still wind up using a 2S LiPo and UBEC, but that’s down the road. For now nothing is set in stone. Contrary to how most folks build their LeFish, I took yet another cue from Steve Lange and made a battery hatch on the port side of the fuselage. That way I can change my mind later on. I’m not happy with how I’ve hinged it, and there’s no real clasp to keep it closed, so there’s some work left to be done on it. I’ll probably take yet another clue from Steve and use magnets as latches.

What’s Left?

Aesthetically there’s still some work to be done. Right now the plane is as bare-bones as any plane could be: white foam covered with laminate, and unpainted balsa covered with laminate. The only splotches of color are some exposed carbon spars in the wing and the blue blocks of the tape-wrapped flaperon servos. It sounds like a minor thing, but this plane needs some color! Without some telling color on a plane, it’s hard to look up and instantly know if a plane is right side up, upside down, coming toward you, moving away, etc. Contrasting patterns on the upper and lower surfaces of the wings, a stripe on the vertical stabilizer, or a blocked out canopy shape – a trademark of the Le Fish design! – are all visual cues that let the pilot know which direction the plane is facing. So I plan to give the aesthetics some careful thought.

Then all that’s left is balance and maiden flight. But that’s a story for another day.

– Tom

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A Slope At Last

Posted by Tom Benedict on 14/07/2013

I’m almost done building my Le Fish glider. The Le Fish was designed by Steve Lange, an avid slope soarer and aerobaticist (if that’s even a word). He’s one of the big movers and shakers in the American VTPR or “in your face” aerobatics movement. Rather than try to describe it in words, this video explains VTPR and the Le Fish better than I could:

As I said, I’m almost done building mine. I’ve been “almost done” for over a month. Why so slow? Because up ’till now the only slope I’d really flown from successfully was an unforested cindercone near Kailua-Kona. It’s a great site. It gets fantastic wind. But the ground is cinder with almost no vegetation at all. Any landing results in damage to the aircraft. It’s like landing on chopped up razor blades. Something is going to get cut.

So when I was invited to fly somewhere on the green side of the island, I jumped at the chance. GRASS!

At the Slope

A bunch of us went out together with our Zagi 5C flying wings, and spent the afternoon flying, crashing, laughing, and running up and down the slope retrieving airplanes and getting generally exhausted. It was GREAT!

The lift wasn’t as good as the other spot I’ve flown, but according to one of the guys there, the wind was a little lower than when he’d flown there before. Even so, I had one flight that lasted well over half an hour. Plenty of time to explore the lift and figure out where the problem areas are. Unfortunately it’s a fairly narrow zone with vortices on either side. One side catches rotor off a nearby hill, and the other has some kind of vortex that I haven’t entirely figured out yet. Turn too sharply, though, and your plane goes into a spin you can’t pull out of. All four of us fell for it, and only a couple of times were we able to pull off a save.

But the best lift was right in front of where we were standing, right next to the ground. In your face aerobatics? PERFECT!

So I’m back on the path to finishing my Le Fish. The wing is built, the tail is built, the fuselage is shaped and reinforced. I’ve even plumbed half the servos. All that’s left is to cut out the radio/battery bay, mount the last servo, and run the control rods. I may not be able to pull it off in a week, but I’m determined to try! Finally, finally, I’ve got a slope I can fly this on.

Flying Zagis
– Tom

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Raptor 2000 Advance – A Maiden At Last

Posted by Tom Benedict on 05/07/2013

Despite having flown the Raptor several times, I didn’t really consider any of those a real maiden flight. The first set was hand-thrown gliding tests, just to make sure I got all the controls moving the right way, and to make sure I could land. The second set was powered flights, but the controls were so squirrelly and the plane flew so poorly, it was all I could do to bring it down in a reasonably safe manner. Even then, I wound up snapping off one of the tail feathers.

It wasn’t until yesterday that I did what I considered to be a real maiden flight: powered takeoff, long gliding flight, and a safe landing. But to get there I had to answer the remaining questions from the earlier flights: why was I rolling off to one side, and why did the plane pitch up violently under power? It turns out both had the same root cause: the removable tail.

The Raptor 2000 Advance is a V-tail motor glider. The tail is removable, and the control surfaces must be unlinked before removal. When the tail is put back on, there is nothing to indicate proper setting of the control surfaces. They have to be dialed back in every time. Yesterday and today I flew the Raptor, and each time I installed the tail to the best of my abilities. The flight characteristics were wildly different each time.

Yesterday’s flights were almost dream-like. The plane had no noticeable P-factor (the tendency for the plane to roll when power is applied), and it flew clean and level while gliding. This morning’s first flight was the exact opposite: it had a strong tendency to roll to the left, and rolled harder under power. It also pitched up, constantly trying to do loops. Under power the plane pointed straight up in the air and went ballistic at wide open throttle.

I traced the problem back to the tail. After looking at the setup, I noticed both control surfaces were angled up by about 2mm. I zeroed them to the best of my abilities, and with my heart in my throat I tossed the plane into the air a second time. That time it flew beautifully, just like yesterday.

Long-term I’d like to either make an alignment jig that lets me center the controls accurately every time, or replace the control rod ends with threaded clevises I can Loctite in place. In the short-term I’m just not removing the tail now that I have it set properly.

The one other piece I knew I needed was some form of airbrake. The field where I fly is quite small, and the Raptor loves to glide and glide and glide. I needed some way to stop it in the air and bring it down. So I added adjustable crow.

The idea with crow – or butterfly as it’s also called – is that you bring the flaps down, and at the same time you bring the ailerons up. This increases camber at the root of the wing, and reduces it at the wing tips. Overall it adds drag, which slows the plane down. But an over-cambered wing will stall before an under-cambered one, so this makes sure the wing root stalls before the wing tips. This keeps the pilot in control, and makes a stall a less violent maneuver than if the camber was reversed.

I tested the idea on my Bixler 2 first: The three position switch on my transmitter controls whether I’m using flaps, crow, or keeping the trailing edge neutral. With the switch all the way up, the flaps are centered and the ailerons work normally. One click down, and the flaps can be controlled via a knob. Two clicks down and that same knob controls the amount of crow. This lets me hit the brakes, and dial in how strong the brakes are.

And a good thing, too! On the Bixler 2, five to ten degrees of crow is enough to dump speed and bring it down in a hurry. On the Raptor 2000 Advance, I had the flaps 45 degrees down before it visibly slowed. With the plane at a nice safe altitude, I tried bringing the flaps 90 degrees down and the ailerons 30 degrees up. WHOMP! It slowed down FAST. Having the ability to dial in the brakes during approach was a must.

In the end, it worked like a charm. Every landing was smooth, and at the end of the day I went home with two undamaged airplanes, ready to fly!

Enjoy the video:

– Tom

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Care and Feeding of KAP Batteries

Posted by Tom Benedict on 23/06/2013

There are a lot of choices when it comes to the batteries used for kite aerial photography. Over the years I’ve used alkalines, nickel-metal hydrides, lithium ion, and now lithium polymer. Each has its own requirements for care and feeding. Here’s a look at some of these, and how to make sure you get the most out of them:


These are single-use batteries, and typically come in AAA, AA, C, and D cell sizes. For most KAP applications, AAA and AA are the only sizes that really apply. Alkaline chemistry provides 1.5V per cell, so a 2-cell battery pack will provide 3V, a 3-cell provides 4.5V, and a 4-cell provides 6V. Because alkalines aren’t rechargeable, I don’t like to use them for applications that will draw a lot of power. But for something like a small autoKAP rig, these can be an ideal choice. In case batteries go dead in the field, these can be found practically anywhere in the world.


NiMH batteries can be built into packs, or can be used as individual cells. I like using the individual AAA and AA cells Sanyo sells under the Eneloop label. These will fit anywhere an alkaline AAA or AA battery will fit, so you don’t have to change your battery holders in order to make this switch. NiMH chemistry provides 1.2V per cell, so a 2-cell battery pack will provide 2.4V, a 3-cell provides 3.6V, and a 4-cell provides 4.8V. Depending on the manufacturer, NiMH batteries can sometimes have a high self-drain rate, so it’s worth checking. Eneloops tend to have a very low self-drain rate, so you can install them in a rig and leave them until they need charging. For transmitter batteries, I re-charge after each session. For KAP rig batteries, I’ll run on the same set for a month before pulling them out to charge.

NiMH batteries charge at a 1C rate or lower. The “C” in 1C refers to the cell’s current capacity. A 2000mAh cell can be charged at 2000mA, or 2A, but they can be charged at lower rates as well. Some chargers will charge until a particular voltage is reached. Others will charge until the current the battery is drawing during charge drops below a certain level. An inherent characteristic of NiMH chemistry is that the cell’s temperature will rise sharply when it reaches full charge, so some chargers have a temperature sensor that will tell the charger when to stop charging. In all of these cases, to charge a NiMH AAA or AA cell, the batteries are loaded into a charger, plugged in, and left until the charger indicates full charge.


Lithium Ion

When I started flying a DSLR, I could no longer use Eneloop AA batteries for my camera. As with most cameras these days, the Canon T2i uses a Li-Ion battery pack. Li-Ion batteries have a low self-discharge rate, so they’re great for applications like digital cameras, in which the camera may sit for long periods of time without use. Li-Ion chemistry provides 3.7V per cell, so a 2-cell battery provides 7.4V, and a 3-cell provides 11.1V. (I’m not going above 12V on cell voltage, because there isn’t much on a KAP rig that requires more than that.)

So far the only Li-Ion batteries I’ve used for KAP have been camera batteries, so these have all had dedicated chargers. Just like the Eneloops, I pop the battery into the charger and plug it in. When the charger says the battery is ready, I put it back in the camera.

Lithium Polymer

These are the real bugaboo when it comes to batteries. We’ve all heard the horror stories: unbalanced batteries gone bad, batteries burning or exploding, brimstone and hellfire. While all this is true to some degree, as long as you treat Lipo batteries kindly, they can provide excellent long-term service. I first started using these when I started flying RC airplanes, and have since introduced them into my KAP bag.

Lipo batteries provide 3.7V per cell, just like Li-Ion batteries. This makes things a little awkward for 5V systems like RC radios, but it’s perfect for 12V systems like video transmitters, receivers, and monitors. Some RC gear is designed for Lipo chemistry, and can handle the 7.4V of a 2S pack. But unless your radio and servos all claim this capability, though, don’t plug a raw 2S pack into your gear. It’ll fry gear that’s designed for 5V supplies. In order to use Lipo batteries on a 5V system, some sort of voltage regulator is required. I use a 5V 3A UBEC on my KAP rig. It provides regulated 5V power to the radio and servos, while still allowing me to use the 11.1V of the battery to power my video gear.

That’s the good. Now for the ugly: Lithium Polymer chemistry is inherently unbalanced. That is to say, if you build a 3-cell pack and charge it with a 12V charger, one cell will inevitably draw more than the other two, and will charge to a different base voltage. Or… it will over-charge, self-destruct, possibly catch fire, or explode. The solution is simple: keep your batteries balanced. More on this in a sec.

The other drawback of Lipo batteries – one that’s shared with Li-Ion and to some degree NiMH – is that once they discharge below a certain level, they cannot (or should not!) be recharged. So it becomes imperative to monitor the battery level to make sure they don’t drop below that threshold. The solution to this is simple as well: monitor your battery voltage and keep your batteries charged.

There are a couple of tools that will let you do both of these jobs.

Rig Lipo

In this photo I show one of my rig batteries – a 500mAh 3S pack – with a low voltage monitor on the left, and a battery balance monitor on the right. The low voltage monitor was about 2 USD off of Ebay, and the battery balance monitor was about 6 USD off of Amazon. Both will monitor the battery voltage and let out an ear-piercing shriek if the voltage drops below 3V per cell. The low voltage monitor also gives you visual feedback with a set of LEDs. Three green LEDs mean all three cells are good. If one goes red, that cell has dropped below the threshold voltage, and the battery should be balance charged. I use a low voltage monitor on my rig when it’s in the air.

Rig Lipo - Reads 11.4V

The battery balance monitor has some nice added features: It’ll cycle through the voltages on each of the cells in the pack, as well as a total for the entire pack. Here you can see my pack can provide 11.4V. A fully-charged Lipo battery will provide closer to 12.6V. I keep one of these in my KAP bag so I can check all the cells in each of my batteries prior to use. Since these also have a low-voltage alarm, you could just fly with one of these on your rig. But I opt to keep mine on the ground and use the lighter weight low voltage alarm.

Earlier I referred to “balance charging”. Remember that Lithium Polymer chemistry is inherently unbalanced. You can get chargers that will charge the entire pack, but they will do nothing to try to maintain balance between the cells. These are dangerous to use! They assume that the battery has balance circuitry built into it. Unless your battery explicitly says it has built-in balancing circuitry, don’t assume that it does.

Lipo Charger

Since I got into Lipo chemistry for flying RC airplanes, I splurged and picked up a nice charger. Most chargers require 12V power, assuming you’ll either be using your car to run the charger, or that you have a 12V bench supply. The one I got will take either 12VDC, or 110/220AC. This is a really nice feature to look for in a charger if you don’t already have a 12V bench supply.

This charger will charge NiCd (a battery chemistry I haven’t discussed), NiMH, Li-Ion, Lipo, and LiFe (another chemistry I haven’t discussed), as well as lead-acid batteries like gel cells or SLAs. For inherently unbalanced battery chemistries like Lipo and LiFe, it offers balance charging. All this means is that it monitors each cell individually, and makes sure that all of the cells in a battery pack finish charging at the same voltage. So far it’s been able to charge my Lipo batteries to within 0.01V per cell. Not bad.

If you take the plunge and jump into Lipo chemistry for your KAP gear, I highly highly recommend doing some homework and splurging on a good balance charger. Unfortunately there are hundreds of chargers out there, so there’s a lot of homework to be done. Most of these cater to the RC aircraft market, however, so some of their features may not be as useful for KAP. For example, some chargers can charge multiple batteries simultaneously. This is great if you have a power-hungry airplane or helicopter that can drain a battery in fifteen minutes. Put four of them on the charger, and in half an hour you have an hour’s worth of battery ready to go. Neat! But for KAP this feature doesn’t really help much. Our power requirements are so low, batteries last for hours. There’s really no need to charge multiple batteries at once.

For me, I wanted something that would take AC or DC power, charge a wide range of battery chemistries, and could balance charge Lipo batteries up to 24V. And that’s exactly what I got.

Why Lipo?

Fair question! For years I didn’t have a compelling reason to switch to Lipo chemistry for KAP. Think about it: The voltages are weird, they require special monitoring and charging equipment, and there’s the risk of fire or explosion. Why go there if you don’t have to? The only reason I started using Lipo batteries for KAP was because I finally had a reason that outweighed all the negatives: I needed 12V power for video.

Even using AAA batteries, sticking with alkaline chemistry would’ve required eight batteries to get the 12V I needed to power a video transmitter. I tried it, and the resulting battery pack was ungainly, heavy, and an incredible pain to use. I even tried a battery pack that had the batteries separated into two banks of four each so my video gear would get 12V while the radio gear got 6V off the same pack by only using half the batteries. I almost fried my GentLED cable doing this, and wound up with all sorts of other problems. My last ditch effort to avoid Lipo chemistry was a 4xAAA NiMH pack powering a boost-buck regulator that output 12V for the video system. But the resulting power was so glitchy, I never got a decent video signal.

Using a 3S Lipo battery on the rig solved that so cleanly, I couldn’t go back. Eventually I ditched the 4xAAA NiMH pack I’d been using to power the radio and servos, and replaced it with a 5V UBEC, fed by the same Lipo battery that was powering the video transmitter. The weight of my rig went down, the overall complexity of the power system went down, and the video no longer had power glitches.

Once you take the leap to using Lipo batteries, most of the real heartache is behind you. One charger will charge all your Lipo batteries. One battery balance monitor will monitor all your Lipo batteries. Adding one more battery to the mix doesn’t make life any harder than you’ve already made it. So I started looking for other places I could use them.

Monitor with Lipo

My video receiver and monitor was a good application. Both required about 12V and had fairly small current requirements. One of the real marvels of Lipo batteries is that they come in all sorts of sizes and shapes, provided those shapes are all rectilinear. (Lipo, Li-Ion, and Li-Fe cells are all flat rectangles. NiMH, NiCd, and alkaline cells are all cylinders.) The monitor had a rectangular cavity on the rear right side of the case. All I had to do was measure it and find a 3S Lipo battery of that size that could provide at least 2500mAh of current. It took less than ten minutes of cruising the web to find just the battery I needed. It fit perfectly, and provided plenty of capacity to power both the receiver and the monitor. Powering my ground-side video system went from being a logistical nightmare to being a simple click-n-ship web purchase.

Why use batteries at all?

All this talk of batteries and KAP assumes that batteries are required at all. Honestly, they’re not. If you’re willing to use film cameras, the requirement for batteries goes away. A wound timer can trigger a shutter as well as any battery mechanism. One of my KAP rigs holds the camera lens-down – no pan, no tilt, no motion at all. Even rigs that move don’t strictly require batteries. Most early KAP rigs used spring-wound or rubberband-wound mechanisms to pan and trigger the camera. One of my favorites – a rig built by Timonoko – uses a propeller to turn a worm drive that pans the rig slowly using wind power alone.

If you don’t need batteries, don’t use them. If you need batteries but don’t need the benefits that bizarre chemistries like Lithium Polymer provide, skip the headaches that come with them. But if you find yourself in a corner, trying to design around something that a Lipo battery would solve, don’t worry. They’re not that nasty to work with.

– Tom

P.S. Bonus points to you if you saw that I had the charge leads plugged into my balance charger backward! When I plugged it in after taking that picture, it came up with a warning, “Polarity Reversed!” and refused to let me charge the battery until I’d plugged the cables in correctly. See what splurging on a charger does? It pampers you!

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RC Updates

Posted by Tom Benedict on 30/05/2013

Last weekend was a busy time. Here’s an update on a number of RC projects:

Landsailers – The wheels came in for my land sailers! Then I found out my stash of M8 bolts didn’t have what I needed to finish out the wheel hardware. So I’m close to starting the build on these, but not there yet. The idea is to build a pair of RC land yachts so the kids and I can bop around with them in a nearby parking lot any time the wind picks up too much to fly kites or planes. If I can find the bolts at the hardware store this week, I’ll start work on these this weekend.

Le Fish – My Le Fish came in from Leading Edge Gliders! This is a kit for a standard weight Fish, but I’m still planning to do some modest lightweight mods to it so I can fly it in a slightly wider wind range than it was intended for. There’s a lot of “set the glue and wait” on this plane, so I plan to work on this in parallel with the land yachts. Many pictures to come.

You Can Break a Zagi – These things really are practically indestructable. I put mine into a rocky slope at thirty or forty miles an hour, knocked both servos out, and generally made a mess of things. Getting it airborne again was a simple matter of shoving the servos back in and tossing it off the slope again. But this time was different. I found if you combine a chainlink fence, a lousy flying site, and an inexperienced pilot, you can break practically anything. I ripped both winglets off, and snapped the center spar in half. The winglets are repaired, the covering restored, and new spars are on order from Aloft Hobbies. I should have it flying again by Saturday.

I Finally Flew my Raptor 2000 Advance – Well… strictly speaking I flew it a couple of weeks ago. I did a series of unpowered flights to check center of gravity, balance, etc. It flew great, but cracked one tail fin off on landing. This time I powered it up. Holy CRAP! In an earlier post I wrote about testing the power train with a watt meter, only to find out it’ll draw half a kilowatt at full throttle. On a 1300mAh battery, that worked out to about two minutes of throttle time before the battery was kaput. During the power-on tests, I never got above half throttle. Even at that, it almost went ballistic. It also had a horrid tendency to torque steer, and roll off to the left. Violently. I did three flights, each with only one powered leg, followed by a glide and landing. On the third one the OTHER tail feather snapped off. I have some serious mixing to think about, and a tail to fix. (I already fixed it.)

I Didn’t Break my Bixler! – Granted, I never flew it last weekend. But hey, it’s the only plane I didn’t break! I’m taking this as a win.

Final update: I ran into Jerry the Glider Guy (no, that’s not really his last name) at a coffee shop over the weekend. If the weather is willing, he’s planning to go slope soaring over Pololu Valley this coming Sunday. If I can get my Zagi airworthy again by that time, I’m planning to go out and join in the fun. Yeah, this’ll cut into construction of the land yachts and the Fish. But having fun is what it’s all about.

– Tom

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New KAP Ground Station

Posted by Tom Benedict on 27/05/2013

Earlier I mentioned I’d picked up a 7″ Feelworld LCD monitor to rebuild my KAP video downlink. I finally got around to building the ground station to go around it.

KAP Video Ground Station

The heart of it is that 7″ Feelworld LCD. These are meant for wireless video, so they don’t switch to blue screen when the signal glitches, or assume the signal format has changed, and try to automatically re-negotiate PAL vs. NTSC. Those settings are manually done, and if you get static in your signal, you get static on your screen.

The larger size is also easy to see without wearing my glasses. I’m far-sighted, and old enough now that my near vision requires me to use glasses to read. Even using my phone without glasses can be troublesome. My older monitor was nice and compact, but difficult to see without putting on glasses. Unfortunately, once I put them on everything beyond about thirty feet goes blurry. This means I can’t see my kite with my glasses on, and couldn’t read that tiny monitor with them off. But the larger screen on this monitor is easy for me to see, with or without glasses. Problem solved.

I wanted the whole ground unit to be a single compact brick I could mount to my KAP transmitter, stick on a tripod, hand to someone, etc. It required some modifications, but they worked out well.

KAP Video Ground Station Rear

The monitor had a 75mm VESA mount on the back, to allow it to be used with VESA accessories. I made a backer plate for the monitor to house all the stuff I wanted to use with it, and popped four holes on 75mm centers so it could be bolted straight to the back of the monitor without modifying anything. The backer plate houses the 5.8GHz receiver – and doubles as a heat sink for it, and also contains the battery and the main power switch.

The battery is a 2.65mAh 3S LiPoly that was designed for use in an RC transmitter. It’s a 1C battery, so it can only source 2.65A. But the current draw on the monitor and video receiver are so low, I’m not even close to this value. It should provide plenty of juice for a full day of KAP, and charges in about 20 minutes.

I was on the fence about adding the power switch until I tried plugging in the battery and mounting it to my KAP transmitter. Bleah! Talk about awkward. The switch took less than half an hour to pop in, and makes it way more convenient to use.

KAP Video Ground Station Battery Compartment

The battery is held in place with nylon webbing stitched to industrial strength Velcro. The entire battery compartment is lined with fuzzy Velcro for cushioning, and the strap extends almost the entire length of the battery. Plenty of grip to keep things in place in the field. I added a finger tab at the end to make it easy to pull off to get the battery out for charging. (Never ever charge a LiPoly battery in the device!)

KAP Video Ground Station Bottom

In addition to the VESA mount, the monitor also included a 1/4″-20 threaded hole for mounting to a tripod, and a T-slot for mounting to some other rail system. The T-slot worked great on the mounting arm I’d already stuck on my KAP transmitter, so that’s the one I chose to use. But I left the 1/4″-20 socket exposed as well in case I want to mount it to a tripod for easy-chair KAP.

All in all the ground station has worked out great. The reception is far better than with the old system, and since the video doesn’t glitch every time there’s static in the signal, the usable range is huge – far larger than I really need for a KAP video downlink.

Which takes me to the other reason I wanted this to be a removable, stand-alone handheld system: It’s pretty trivial to get a second video transmitter and stick it in an RC airplane. Aaaaah! More fun!

– Tom

Posted in Engineering, Kite Aerial Photography, Radio, RC Airplanes | 1 Comment »

Lost in Shipping (Bummers!)

Posted by Tom Benedict on 21/05/2013

A couple of months back I ordered three gliders, two of which I’ve built and now flown. (Yes, the Raptor 2000 Advance has now flown! More on that later.) The third never got here.

I like to tell stories about excellent customer service, because all too often we only see stories about poor service. So here’s a shout-out to the folks at Leading Edge Gliders for providing what I consider excellent customer service: I emailed them this morning to ask if my Le Fish had made it into the mail. They replied, saying it had, and that they were boxing up another one to ship to me while they figured out what happened to the first one. All I can say is WOW! They didn’t even ask what happened. They just took care of it. Leading Edge Gliders rocks! (I am seriously looking forward to building this ship now!)

Another order that went missing in the last few months was one from Pololu Robotics. About the same time I placed the orders for the gliders, I placed an order for a new video downlink monitor for my KAP rig. The monitor came in safe and sound, and after I tested it to make sure all my hardware was happy, I opened it up to see what my options were for re-cabling it. As it turns out the cables are connectorized internally using JST XH connectors: a pair for power, and a trio for video/audio/ground. I ordered the three-conductor connectors off of Ebay, and picked up ten sets of power connectors from Pololu. The Ebay connectors came in a couple of weeks after ordering, but the ones from Pololu never made it. I’ve ordered from Pololu for years without incident, and they’ve consistently provided excellent customer service. I figure my number just came up on an un-tracked shipment. I ordered another ten power connector pigtails for $6.80, and called it good.

I placed one last order today. This one’s a bit of a story…

Years ago I picked up a set of plans for an RC land yacht from Performace RC Landsailers. The information on the web site is incredibly comprehensive, and a real resource for anyone wanting to design and build these things. The plans are well worth the price (about $20 US when I got them… not sure what they are now). They’re very complete, and explain the hows and whys of the design and construction of each assembly. I got the plans. But I never built the land sailer.

About a month ago I was poking around in my contacts’ photos on Flickr, and I ran across a set of land yacht photos by Andrew Newton. These are all home-built, and are nothing short of beautiful. That rekindled my interest in building one of these things. After going through all of Andrew’s photos, I pulled up a CAD window and started drawing.

The design from Performance RC Landsailers is tuned for performance. It’s got the weight and strength exactly where it needs it, and not where it doesn’t. This is great, but it makes the design a little daunting for a first time builder. One of the things I liked about Andrew’s designs was how utilitarian and functional they are. We have a saying at work: “Git r’ done!” Andrew’s designs do just that. And that’s what I wanted: a design that would git ‘r done. Everything was designed around what I have on hand: a sheet of 1/4″ Baltic birch plywood; green ripstop nylon from making my rokkaku; 6mm carbon fiber replacement spars for my Bixler’s wing; scrapbox aluminum and Delrin, spare servos, and spare RC radios.

The only thing I didn’t have was the wheels. Both Performance RC Landsailers and Andrew Newton use scooter wheels for their land yacht wheels. What could be simpler? I figured a trip to the local skate shop would set me up.

Nope. “Try one of the big stores in Kona.”

So I tried KMart. Nope. Target. Nope. Walmart. Nope! Sport’s Authority? NOPE! ANYONE?!   NOOOO!

Then I tried Ebay. Yesssss!

I have no idea why it’s so hard to find scooter and skate wheels on this island, but it is. In the end I picked up two white 68mm wheels for nose wheels and four green 100mm wheels for the rear axles. (Yes, I’m building two of these.) I have two yards of green Icarex kite cloth for sails, a whole stack of 6mm spars, and enough Delrin to make a slew of mast steps and goosenecks. With the wheels on order, I’m finally good to go. Like the Le Fish, I’m looking forward to building these things.

– Tom

P.S. The maiden flight of the Raptor! I forgot!

It was… not what I expected. Though in hindsight I should’ve. I only did power-off tests to make sure I had the CG right (I did) and that my control throws weren’t unrealistic (they weren’t). The tests consisted of throwing it from head height, and gliding it in to a soft landing. With 6′ of altitude, it consistently glided over 100′. This thing loves to soar!

What it doesn’t love to do is land. So until I can find a field big enough to learn on, it’s grounded. I think there are some open pastures on Mana Road that might fit the bill, but I need to check them out first. I can’t see flying this thing in town, though.

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