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Archive for April, 2013

A Week of Kites and Planes

Posted by Tom Benedict on 29/04/2013

World Wide KAP Week 2013 began Friday! W00t!!

Bummer it was Sunday before I could get out, though. Friday I was at work. Then the trade winds started howling. Then the rain started. By the time I got home it was not comfy weather for kites. We made dinner, we watched a movie, and we all stubbornly went for a walk around the neighborhood in the cold wind. Saturday was loads better, but between play dates with the kids and volunteering to help set up for an event, by the time I had some me-time, the sun had set. I was determined to kick anyone’s butt who tried to stop me from flying Sunday.

Sunday afternoon, I took two of the kids and drove down to Hapuna Beach. >sigh< Yeah, same old KAP spot I’ve been going to for the last year and a half. But hey, I flew! Considering I couldn’t get time off from work during World Wide KAP Week this year, I’ll take whatever I can get. All in all it was a good session: two flights with the BBKK rig, and one flight with a BEAK:

Shutter Noise

The inevitable test-shot to make sure the shutter is tripping correctly.

Hapuna Beach South

A Hapuna panorama, despite the lousy light from having too much volcanic gas in the air to see the horizon. I tried tried tried to keep my winder dry, but one of those waves finally splashed high enough to touch it. Ah well… It was dry by the time it went back in the bag.

Walk This Way

If my son turns into a private detective, I swear I’ll get him the hat and pipe that Sherlock Holmes would’ve wanted him to have. He’s on his own for finding a suitable waistcoat and trousers, however. I don’t think the swim trunks will stand him in good stead.

All of this was complicated by something really stupid I did earlier in the week: I took the Zagi 5C out for a test flight.

The Zagi is really a slope soarer. But with the exception of two really scary slopes, I haven’t found a good place to fly the thing yet. So I took it to the park and flung it into the air. Trick RC, the makers of the Zagi, make a much lighter version called the Zagi THL, or Zagi Thermal Hand Launch. The idea with the THL is to fling it into the air, catch a thermal and smile a lot. With the 5C, I flung it into the air, flew for about five seconds, walked over, picked it up, and flung it again. And again. And again, until I felt something wrench in my left shoulder. Uh oh…

But first some back-story:

Years ago I did something else that was really really stupid: I went out bodysurfing in eight foot surf without a clue. A couple of waves later I got hit in the back, and wound up dislocating both my shoulders. There’s something surreal about lying face-down in the surf with two dislocated shoulders while your wife and kids are standing less than twenty feet away. You can’t stand up because your arms don’t work. You can’t scream because your head is under water. And as I ran out of air I found myself thinking, “What an embarrassing way to go… drown right in front of my own family. Rydra is going to be PISSED!

I’d love to say that my family recognized my dilemma and rushed to my rescue. What actually happened is that another wave came along, smashed into me, and popped one of my arms back into joint as I rolled across the bottom. It was enough to let me get an elbow in the sand and lift my face out of the water. Once I was standing, I shoved the other arm back into position. Vin Diesel made this maneuver look cool beyond cool in Pitch Black. Mel Gibson made it look clever and tough in Lethal Weapon. What it actually looks like is a wet frog trying to whack its legs around until they work again. It was agonizing. I was in physical therapy for months afterward.

As I flung the Zagi into the air that last time, I felt my arm fall out of joint. Just like holding my breath under the waves, I maintained enough self-awareness to dead-stick the plane to a good landing. Then I put down the radio and shoved my arm back into place. I remember thinking, “Dammit! Not right before World Wide KAP Week! This SUCKS!”

So the wind and the rain Friday wasn’t entirely a bad thing. And the preoccupation Saturday was ok, too. Sunday I figured I’d had enough R&R to risk using a winder. I took it easy at first, but by the third flight I figured I was mended enough to be ok for the rest of World Wide KAP Week. Now I just need some time in the evenings to catch some sunset light!

– Tom

P.S. I can attest to the toughness of the Zagi. I’ve lawn-darted it enough times now to trust that it’ll survive my learning curve. But I think I’d better stick to actual slopes from now on.

Posted in Kite Aerial Photography, RC Airplanes | Leave a Comment »

Zagi 5C Build

Posted by Tom Benedict on 22/04/2013

While waiting for the new ESC for the Raptor, and while I figure out what I want to do about ventilation to keep the motor cool, I built a new plane. Friday while I was at work, my Zagi 5C flying wing arrived from Trick RC.

Trick RC makes a number of flying wings, some powered some unpowered. The 5C is an all-purpose unpowered glider that’s really better suited to slope soaring than riding thermals, but there are people who have adapted their 5C to thermalling as well. My plan was to build it as a slope soarer, but to be as careful as I could about weight along the way. You can always add weight in the form of ballast to give a plane better penetration in higher slope winds. But it’s a lot harder to take weight out.

The Zagi 5C comes with an excellent instruction manual, which is also available in PDF form on the Trick RC web site. Trick RC updates the manual as new building techniques come along, so it’s pretty current. For example, the original Zagi was reinforced with fiber strapping tape. The Zagi that arrived at my house came with flat carbon fiber spars that are glued into the wing for reinforcement. As new glues have been developed, the instructions have been changed to reflect best practice there as well. Even so, I found a couple of build logs that provided information that wasn’t in the manual that came with my plane. The build posted by Mokulele808 to the RCGroups forum is the one I based a lot of my build off of.

The only thing I really changed from Mokulele808’s build is how I covered the wings. For that I took inspiration from surfimp’s build of the Swiss Fish, also on RCGroups. surfimp, aka Steve Lange, the designer of the Le Fish aerobatic glider, used laminating film to cover the wings rather than the tape covering used in the Zagi manual, or the covering that Mokulele808 used. (As as side note, Mokulele808 mentions CP laminate covering at the tail end of the build, and said this is probably the route they’d go if they built another one. See? I really did read all the instructions before beginning!)

Friday night I glued the wings together using 3m Super 77 adhesive, as per the instructions both in the manual and in Mokulele808’s build. Super 77 never completely hardens, so it’ll flex with the foam without cracking. Once the wings had set, I installed the carbon fiber spars. This is where Mokulele808’s build branches off from the build in the manual. The build in the manual uses Super 77 to install the spars, but Mokulele808 used Gorilla Glue, a polyurethane adhesive. This provides a stiffer connection between carbon and foam, and results in a more rigid structure.

Polyurethane adhesives foam up when applied. Gorilla Glue is no exception. In both Mokulele808’s build and surfimp’s build of the Swiss Fish, they leave the spars uncovered to let the glue foam up and out of the slot. Once the glue has set, it can be trimmed back with a sharp blade. I took a different approach, and taped over the glue line with blue painter’s tape. This let me lay the wing in the wing beds to keep them from twisting as the glue cured. After an overnight cure the wing came out quite straight and amazingly stiff. The painter’s tape came off with no problems, leaving a relatively flat glue joint behind. A little sanding, and the wing was ready for the next step.

When I ordered the kit, I was under the assumption that two servos came with the plane. I was wrong, of course, and was left with the question of what to do. Because of my kite aerial photography habit of the past six years (six years?! (yes, six years!)) I have a stash of full-sized servos. I’ve really come to like the metal geared Hitec servos I started using on my DSLR KAP rig, but they’re heavy, and for this they really are overkill. Rather than weigh the plane down with these, I opted to install two Futaba S3003 servos instead. These are plastic geared, they’re relatively light, and because I’ve been swapping out my KAP servos for the Hitecs, I had several of the S3003 servos that were looking for a new home.

The Zagi 5C, like most flying wings, embeds the servos in the wing during the build. They do this by having a die-cut “cookie” of foam where the servo goes. You press the servo into the die cut slot, and the cookie pushes out the bottom of the wing. Once the servo is flush with the top of the wing, the cookie is cut flush with the bottom. Voila, two perfectly depthed servo pockets!

The kit came with fiber strapping tape, which is still used to reinforce the trailing edge of the wings and the nose, even with the carbon fiber spars. I opted to use biaxial fiber tape instead. This shows up under a couple of different names: “biax”, “crossweave”, or in the case of the brand I’m using, 3M Extreme Shipping Tape. In appearance, it’s very similar to a 3D mylar sail for a racing boat. It’s wider than the supplied tape, so I took a weight penalty in using it. But with the CP laminate wing covering rather than a tape covering, I made most of that weight penalty back.

The manual suggests cutting a recess in the electronics bay to house the excess servo wire. I really don’t like excess wire hanging around, though. It’s unnecessary weight that’s easy enough to remove if the servo leads are built to length to begin with. I do this with my KAP rigs, and did this on my Raptor as well. The servo leads on the Zagi only need to be a couple of inches long. Rather than wad the wires up at the back of the electronics bay, I cut them to length and re-terminated them. (Actually, I’m getting short on connectors so I unsoldered them from the servo, cut them to length, and soldered them back on. No need to crimp on new connectors that way.)

After I had the basic wing put together and had the trailing edge reinforced, I decided to add some color to the plane so I would be able to tell its orientation from the ground. I bordered the top of the wing in orange tape, and the bottom in black. The whole wing was then covered in 3 mil CP laminate film. To do that I wrapped the laminate film around the leading edge, and trimmed it at the trailing edge. This left me with a really clean leading edge that had no seams. I used the same color scheme on the control surfaces, so the entire wing is bordered in color. It really is easy to tell if the wing is right side up or upside down.

My zeal for lightweight building wound up biting me in the rear in the end, though. The build weight for the wing is 17 ounces, battery installed. Mine came in considerably lighter than that. Unfortunately, the geometry of the wing means that most of the wing area is behind the center of gravity. I lightweighted the covering on the wing, so it unbalanced things and made it tail heavy. In the end I had to add 36g of ballast to the nose of the plane in order to get the CG right. So much for lightweight builds! I still came in under weight, but not by much.

The real treat was when I took it out Sunday evening for its maiden flight. Before taking it out I double-checked the CG, checked the control throws (1/2″ up, 1/2″ down, just like in the book), checked the placement of the antennas, etc. When I finally tossed it out for the first time, all the care and attention paid off. Its first flight wasn’t long, maybe thirty feet or so. But it flew hands-off without any tendency to pitch up, dive, or roll. It flew beautifully.

Zagi 5C

Now I can’t wait to take it out for a real flight!

– Tom

Posted in RC Airplanes | 1 Comment »

It’s drawing Watt?!

Posted by Tom Benedict on 18/04/2013

As part of the whole spate of orders I placed when jumping into the RC airplane hobby, I picked up a wattmeter. The one I got is pretty cool. It’ll tell you (semi) instantaneous current draw, power draw, and supply voltage, as well as the peak current, peak power, and minimum voltage. It’s got two inputs, one for high power systems (think battery/ESC/motor) and one for low power systems (think battery/Rx/servos). For me, this is perfect. I can use it to characterize the servos in my planes, find out the stall torques on each of them, how much I’m likely to draw when pulling out of a dive, etc. And I can also use it to characterize the servos in my KAP rigs, estimate run time, see from current draw how balanced my rig is, etc. And I can use it to measure current draw on my video transmitter, receiver, and monitor. In essence I can finally find out how big a battery I actually need for each application, and get the run time I want out of each one.

My KAP gear works well as-is, so I started with the planes. I hooked it up to my Bixler and fired things up. The Turnigy TG50090M servos I put in the flaps draw 0.6A stalled. Put two together, and a full on airbrake landing can potentially draw up to 1.2A. Not too bad. Then I fired up the motor and ran it up to 100% throttle: 13A. At 12V (give or take), that’s 156 watts of power to run the motor at full throttle. I have a 25A ESC on the plane, so that’s not a problem. The BEC on the ESC can supply 2.5A to the servo system, so as long as I don’t hammer it too hard, I’m safe. Excellent!

Then I put it on the Raptor. I have six of those TG50090M servos in that plane, so there’s the potential for over-drawing the BEC if everything is working at once. Since this is largely a thermal glider, I don’t expect to stall all six servos simultaneously. But it’s something to be aware of.

Then I spun up the motor… CRIPES! The current meter went up to 45A before I shut it down. It’s got a 30A ESC in it that can handle 40A peak current draw. Already I was exceeding what the equipment could handle. That’s when I did the math… 45A @ 12V = 540W. It’s drawing Watt?!

I honestly didn’t expect this plane to draw half a kilowatt of power. The thrust was tremendous! But after I shut everything down as quickly as I could, I felt the ESC (warmish) and the motor (HOT!). I knew I couldn’t run this way. I played around with the endpoints on the throttle, and dialed down the upper end to about 70%. That kept the constant draw under 30A, but the motor was still running incredibly hot. Hot enough that the prop and spinner were getting warm.

That raised red flags in my head. The shaft on this motor is steel, and the collet style prop adapter is aluminum. What this means is that if the temperature goes up high enough, the prop adapter will become loose on the shaft just from differential thermal expansion. Clearly something had to be done.

I asked on the RCGroups forum, and was advised to cut cooling ducts into the plane: two in the fuselage next to the motor, and one more in the canopy to cool the ESC and battery. Exit vents underneath the wings finish the job, and should serve to remove at least some of the heat generated by the motor and ESC. A few weeks ago I put together a CAD drawing of a NACA submerged duct, based on the original NACA report. What timing! It’s easy enough to scale the drawing to the right size, trace the ducts onto the fuselage and canopy, and cut them out using a Foredom and some needle files.

So the maiden flight of the Raptor has to wait another couple of weeks while I ventilate the fuselage and wait for the new ESC to arrive: 40A constant, 55A peak, and a 3A BEC to boot.

But here’s the real funny: The Bixler 2, with its 13A draw on a 2200mAh battery, has a total run time of ten minutes at full throttle. The Raptor with its 1300mAh battery? 2.5 minutes. My hope is that with that much thrust and with the plane’s ability to catch thermals, though, their actual in-the-air flight times will be comparable. I’ve flown my Bixler 2 for over an hour on a single charge. I hope to see the Raptor perform as well or better.

– Tom

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Nighttime KAP – End in Sight?

Posted by Tom Benedict on 16/04/2013

There was a recent post in the KAP Forum that may have answered the last piece to the puzzle of the stabilized KAP rig. But let me back up first:

Some years ago I fell in love with the idea of a KAP rig that could stabilize a camera to the degree that a Cineflex mount does on full-sized aircraft. To give you an idea of what that means, a Swedish film company released a demo reel of their work that began with a frame-filling shot of the full moon, then zoomed out to show that the moon was being videoed from a helicopter that was bopping around in the wind. The moon subtends half a degree in the sky. The full HD frame is just over 1000 pixels high. In the opening shot, the moon was not moving in the frame. Put another way, the Cineflex mount was stabilizing the camera to better than 1/1000 of a degree of rotation, or better than 3.6 arc seconds.

Such stabilization simply doesn’t exist in the world of KAP. But here’s what it would mean if it did:

At the long end of the KAP focal length range, consider a camera with the equivalent of a 35mm lens on a 35mm camera. This is the focal length my A650IS has at its widest focus, and is typical of a crop-sensor camera with a 24mm lens. The horizontal field of view of this setup is 54.5 degrees. Assume the KAPer is of the extreme variety, and that they’re flying a 5DmkIII with its 5760 pixel wide detector. To first order, each pixel covers 0.009 degrees of sky. Mounted in a Cineflex mount, the camera would be stabilized to better than 1/10 of a pixel for the duration of the shot. Considering that Swedish film crew held that shot of the moon for over ten seconds, the “duration of the shot” could be a very long time for an aerial photograph. Long enough to do full night-time aerial photography without a single blurred photo.

So that’s what’s out of reach for KAP. This begs the question: what is in reach for KAP?

Most KAPers, myself included, tend toward very short exposure times. 1/640, 1/1000, 1/2000 second exposures are typical. The reasoning behind that is simple: If your camera is bopping around in the wind, the only way to avoid blurry photos is to make the exposure as short as possible. Much longer than this, and the number of blurry shots per session starts to go up. Go as long as 1/15 second, and the number of non-blurry shots per session gets down in the single digit percentages.

But that’s on an unstabilized rig. Add even the barest attemps at stability, and those numbers improve dramatically. Years ago Scott Armitage developed the GS-1, a gyro-stabilized servo that could be used in a KAP rig. For video, this meant that the horizons stayed level enough that most of the rest of the jitter could be removed by a decent deshaker program. They left enough jitter that still photography suffered, but it was still worlds better than an unstabilized rig.

A while back I started designing a toolkit for experimenting with damped pendulum suspensions. I’ve just about finalized the design for all the parts, and should start cutting metal soon. The finished design looks close to this CAD rendering I posted a while back:

Damped KAP Pendulum - Uncompressed

Or in shrunken form using short spars so all the moving bits can be seen:

Damped KAP Pendulum - Compressed

It uses a long bar to attach to the kite line so there is plenty of purchase to push against in the pan direction. This reduces recoil and oscillation in pan rotations. It uses double-row ball bearings for all pivot joints so there’s no play in any of the axes. This reduces high frequency jitter in all directions. Any or all of the pivoting joints can be damped. This reduces oscillation in the pivoting axes. And I added in a provision to take out the real bugaboo with KAP: roll around the kite line (though I haven’t proved this in the air, so it’s still theory as far as I’m concerned.)

This gets closer to the mark, but it won’t remove everything. Even if the damping on this is carefully tuned, there will be residual oscillation. It’s unavoidable in a passively damped system. But tack active damping on the end of this, and the active system only has to deal with the residuals. It doesn’t have to fight the full motion of the kite, the line, and the rig.

Which brings us back to that post in the forum. RCTimer offers some gimbals for multirotors and RC helicopters, which was part of the discussion in that thread. But they also offer the building blocks for making your own gimbals. Namely, they offer a 2-axis brushless controller board and direct-drive brushless motors for driving a gimbal. Why direct-drive motors? It removes servo backlash from the list of possible sources of high frequency jitter. And why a custom controller rolled around brushless motors? The loop rate of a typical hobby servo is 50Hz, though you can push that higher by using digital servos. The loop rate of a typical brushless gimbal controller is closer to 500-1000Hz. Basic control theory says that if you bump your loop rate by a factor of ten, you also bump the frequency of the events you can sense and correct for by a factor of ten.

The damped pendulum should take out the oscillations above a few Hz, and the brushless gimbal controller should take out the oscillations below 100Hz. Put the two together and you have a system with very little residual left. Good enough to challenge a Cineflex? Probably not. But good enough to do one-second exposures with a KAP rig at night? It might just be possible now.

Unfortunately I used up all of my hobby cash on RC airplanes recently. I won’t be able to pursue this for a while. I have everything to make the pendulum, but I can’t afford the gimbal hardware yet. I’m writing this in the hope that someone decides to take this on and open a new chapter in the history of KAP: the KAP rig that simply doesn’t wobble in the wind.

– Tom

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More New Hardware and WWKW 2013

Posted by Tom Benedict on 16/04/2013

More new stuff showed up over the weekend. The first was an 808 #16D v.2 camera. They’re the next logical progression from the gumstick camera I played with back in 2009.

Gumstick Camera - View 1

The newer cameras are leaps and bounds better than my old gumstick, which suffered from a harder-than-recommended impact with the ground shortly after I got it, may it rest in peace. The specs on the #16D are a 120 degree FOV lens, 1280×720 30fps HD video, room for a 32GB microSD card, and upgradeable firmware – something the gumstick certainly didn’t have. People have even written utilities for configuring the #16D via its USB cable. All in all it’s a very nice bit of hardware.

808 #16D

On a whim I taped it to the tail of my Bixler 2 and flew out over the Waikoloa turnoff, a few miles from town. The weather wasn’t great, the light was terrible, and the winds aloft were a lot stronger than I’d reckoned on. Also, light though this camera is, its weight on the tail was enough to throw the balance off on the plane. The first flight was seriously tail heavy, and despite trying to correct through pitch trim, it porpoised madly throughout the entire flight. The second flight was better, with the battery shoved full-forward into the nose of the plane to level things off.

This was actually what I got the camera for: creating videos from an airplane showing most of the control surfaces working. I wanted it more as a diagnostic tool than as a videographer’s camera. But I’m actually impressed by the video it generated! The quality isn’t as good as the Hero 3 I’ve been using, nor did I expect it to be. But it’s a far cry better than the video from the gumstick. In decent light, it’s quite nice. (This isn’t decent light.)

And yeah, I can see using it on kites as well. I’d love to mount it on a tripod of kite spars on the back of a kite (say a Dopero?) and make a video similar to the video from the Bixler 2, but this time from a kite. If I worked out the geometry correctly, it should actually look quite good. It might be fun to do the same sort of thing on the back of a two-line sport kite, but the resulting video might make people motion sick. Eeeeh… I’ll probably try it anyway, just to see.

Speaking of kites, the other bit of kit that showed up was the new monitor for my KAP rig. Back in December, 2011, I added a 5.8GHz video link to my main KAP rig, and tacked a small monitor onto my RC transmitter.

5.8GHz KAP Gear

This worked, but the system was plagued by glitchy video. I changed the power setup, cabling, antennas… everything but the monitor. And in the end it turned out that the monitor was the culprit. The one I originally got was intended to be hard-wired into a car as an assist for going into reverse. It was never intended to be used as part of a radio video link. Two of its “features” were that it would detect the timing of the incoming video signal and switch between NTSC and PAL mode automatically, albeit with a bright blue glitch as it swapped, and that when it lost video signal, it would go to a blank blue screen. The glitches that are inevitable in a radio video link meant that it was always swapping modes and blanking out.

Feelworld 7" LCD Monitor

The replacement screen was designed for aerial use. It has to be told whether to be in PAL or NTSC mode, and when it loses signal it shows static. No more blue screen! Even better, it’s about four times larger than the original. Not great for transportability, but fantastic for my far-sighted eyes. I no longer have to squint at the screen to see what the camera is pointed at.

I still haven’t mounted the new monitor, but I’ve come up with a plan for a mount that will let me remove the monitor when not in use. This will make it so I can zip my KAP bag shut when everything is put away. I haven’t been able to do that since mounting the original monitor back in 2011. It’ll be a good change.

And it’s just in time for WWKW2013!

I can’t take vacation time during World Wide KAP Week, so my time will be limited. I’ve been trying to plan out where I’ll go and what I’ll do, but about a week ago I had an epiphany: I keep thinking of KAP in terms of landscape photography – a form of photography that I’m quite fond of. But with a couple of exceptions, the KAP photos I’m most drawn to aren’t landscapes. They’re something else: portraits, architecture, abstracts, action… not landscapes.

So rather than drift farther afield this World Wide KAP Week in search of new vistas to photograph, I’m thinking of drifting a little closer to home. One of my favorite things to do with a camera is just walk around and see what I see. Rather than plan out what I want to do with KAP, I’m going to put a kite up, hang a camera on the line, and see what I see no matter where I am.

Well… that and play. I’ve just been itching to play with kites and cameras. Hmmm! Maybe that’ll be a good time to try the “camera on the back of a kite” trick. It would make for a cool still, done using a Hero 3.

Ok, maybe I really should make some plans… this is getting complicated.

– Tom

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Raptor is Done

Posted by Tom Benedict on 14/04/2013

I punted on the science fair project until later in the weekend, and finished my Raptor Advance 2000 instead. Yaaay! It’s done!

Raptor Advance 2000 - Completed!

The all up weight with a 1.3mAh battery installed came out right around 970g, about 20g heavier than most people were  reporting. I think some of the discrepancy is due to the servos I used, but I really don’t know what I could do to shave it by much. With reasonable placement of the battery, it balanced out quite nicely at 80mm behind the leading edge of the wing. Again, I’m taking this number on faith since a number of people have made successful maiden flights with this CG.

As I mentioned in an earlier post, I’m not ready to fly this yet. I have two other models coming in that I’d like to build first, and quite honestly I’m just not ready. From what I’ve gleaned from the RCgroups forum, maiden flights with this plane either seem to go amazingly well (skilled pilot, competent build, due caution) or really really poorly (for a variety of reasons). Considering my propensity for accidents, getting my control swings backwards, having props fall off, etc. a little more stick time on my Bixler 2 and a lot more simulation time just seems prudent.

Meanwhile I have to install pulleys and lines so I can hang my models in my room. This one is pretty, even sitting on the ground. It’s a shame to keep taking it apart and storing it in its box.

Now the race is on! Which plane comes in next? The Zagi wing, or the Le Fish aerobatic glider?

– Tom

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Science Fair and New Radio

Posted by Tom Benedict on 11/04/2013

Ok, so I have to eat crow. I thought I had a relatively simple science fair idea. I thought it would take two days, at most, to gather my data. This is how it actually went down:

Me: “Hey, can you give me a qualitative estimate of which of these foam/tape sandwiches is stiffest?”

Wife: “Yeah.” <takes samples>

Me: “Just be careful not to bend them so far that they snap!”

Wife: “Oh… Um… What if I snapped all of them?”

Me: “WHAT?!”

Yeah, this is what researchers call “scheduling setbacks”. And this is what co-authors call “crappy communication from the principal investigator”. After I recovered the tattered remains of my experimental samples, I told my wife that what I meant was that she should flex the samples, but not destructively test them. Not yet, anyway. So I’m going to start again this weekend.

Meanwhile my Turnigy 9XR radio arrived along with its FrSky DJT module and four receivers. The battery I bought for the radio hasn’t arrived yet, so I stuck one of my existing 2200mAh batteries into it and fired it up. In a word, coooooool!

But that coolness comes at a cost: it’s a lot more complicated than my good ol’ six channel 2.4GHz radio I borrowed from my KAP gear. Cripes, there are so many ways to tune this thing, I knew I’d make a mistake. Turns out I was right.

Last night I pulled the old radio out of my Bixler 2 and stuck the new one in. The whole point of getting this radio was to give me the flexibility to set up the Raptor Advance as a full-house glider, to set up the Le Fish as a 4-axis plane, and to still be able to do the minimal mixing necessary for the Zagi wing. So first things first, I cabled up the Bixler 2 as a full-house glider.

The whole idea behind a full-house setup is that you have six control surfaces, each of which is individually wired into the radio. The flaps can move in unison, but they don’t have to. The ailerons can move in opposition, but they don’t have to. Depending on how the transmitter is set up, the entire trailing edge – ailerons and flaps – can move as a single control surface (FTE), or the flaps and ailerons can move in opposite directions to act as air brakes (crow or butterfly), or the entire trailing edge can be tuned up or down to introduce camber or reflex along the entire wing, and all of this can be mixed in with the normal control inputs for ailerons and flaps. It opens up all sorts of options for tuning the entire airframe, depending on what you want it to do.

The Raptor Advance and the Bixler share the same control surfaces: rudder, elevator, ailerons, and flaps. Both have a motor and throttle. Seven inputs in all. So I set up the Bixler to use seven of its eight channels so each control surface has its own input from the transmitter. Then came the fun part: setting up the 9XR to take advantage of that.

Which… I didn’t. Not for now, anyway. Ailerons are ganged, flaps are ganged, and basically I made it work exactly like my old radio. Because let’s face it, with that much flexibility I was bound to get something wrong. Better to get the plane back into flying shape first, and then start to add the bells and whistles.

I’m glad I did. I took it out this morning and found out just how utterly I’d botched it. Any time I make changes to my Bixler 2, I’ve made a habit of throwing it out without the throttle on so I can dead-stick it back to the ground. That way when the inevitable happens, it will tap the ground at a glide rather than slam the ground under power. Let’s see if I can remember everything I did wrong. The list includes: ailerons reversed, rudder reversed, endpoints on elevator, ailerons, and flaps all wonky (I had about 10% down-travel in the elevator, and maybe 120% up-travel), and all kinds of tweaking required to center up the control surfaces. It took a while to sort out everything I’d done wrong, but by the time I was done the plane was flying great under throttle and while gliding.

There’s still some clean-up to do. On the old radio the flaps were wired in through a Y-harness, so one side had a servo reverser wired in with it to make it move correctly. That reverser is still there. I’d like to clean up the wiring inside the plane, get rid of that reverser, and make it look exactly like the Raptor. Then one more session of cleaning up all of my mistakes, and then I can start to add the full-house mixes.

It’s a learning experience. But it sure is a lot of fun!

– Tom

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Servo Leads and Science Fair

Posted by Tom Benedict on 10/04/2013

Last night and this morning I soldered the new servo leads onto the wing servos for my Raptor Advance 2000 sailplane. I still need to test them before they go in the wings since the installation is semi-permanent. But I should be able to do that tonight and do the installation tomorrow evening.

Meanwhile I decided to take on a fun project. I want to know what wing covering offers the most stiffness to weight for a foam plane. I have two foam planes coming in, a Le Fish and a Zagi, and I want to be able to make a good educated decision about wing covering for them. In the past, builds for both planes have involved spackling the foam with some sort of RTV rubber, placing strategic strips of fiber-reinforced packing tape along high stress areas, and then covering the whole thing with clear packing tape. Since then, new tapes have come out, new laminate coverings have come out, and a number of people have chosen to skip the fiber reinforced packing tape altogether in favor of carbon fiber reinforcement. This is one of those instances where you can’t necessarily trust what you read online because a build log from even one year ago may be missing a new technique that is stiffer, lighter, and offers more durability overall.

So I cut ten 1″x4″ strips of some unspecified foam that I’m going to cover on both sides with a variety of coverings. I’m pretty sure the foam is Depron, but I didn’t buy it myself so I really can’t say. Four of the strips will be covered on both sides with CP laminate of 1.7mil, 3.0mil, 5.0mil, and 10.0mil thickness. Another four will be covered with four different kinds of tape: Scotch packaging tape, Scotch extreme packaging tape (aka “biax”), Scotch clear duct tape, and Bunker hurricane tape. The last two strips will be left unmodified.

There are two parts of the test. The first is to use one of our scales from work to calculate a weight per unit area for each covering. I’ll do this by cutting squares of tape and laminate, and weighing them. In the case of the tapes, I’ll also work out how much they weigh per unit length, since tape is typically used in a single strip.

The second is to test each of the foam sandwiches for flex. The scale at work came with a 100g calibration weight. I plan to clamp (gently!) each of the strips with 3″ of cantilevered overhang, then place the 100g weight 2.5″ out from the clamp point. Then I’ll measure the overall flex using a machinist’s scale and a digital camera with a reasonably long lens to minimize parallax. No, this isn’t the most scientific test in the world. And no, I won’t get any “real” engineering values out of this, such as an effective Young’s modulus. But I’ll get enough information to say, “This one is stiffer than that one, and that other one over there is the stiffest of all.” In the end, that’s the value that’s the most useful for my application. After all, I’m just trying to choose a wing covering.

In case you haven’t made the connection between this test and the title of this article, I’ll help you out: Essentially, this is a science fair project. Except for borrowing a 20USD scale from work – well within the budget of most high school or even middle school science fair projects – I’m not using anything that a typical middle school or high school student wouldn’t have available to them. The complete assembly time will be less than a day. The time to take all of the measurements on all ten samples will also be less than a day. It’s nothing that couldn’t be done in a single weekend by a dedicated student. You’ll get to see the writeup here on this blog. From experience I can say that it’ll take me less than a week of evenings to write, edit, and post. This is not a year-long project, and it’s not rocket science.

But it is science – material science. And it’s good enough for a science fair. I doubt it would win any awards, but it’s a solid project. Even better, it has an immediate real-world application: providing me with a plan for covering the wings of two RC airplanes. That’s motivation enough for me to take it on.

So here’s my bit about science fairs: If you’re a student who’s planning to enter a science fair, keep in mind what science is all about. It’s a systematic way of investigating questions you want the answers to. Nothing more. You can look through a book of “100 Awesome Science Fair Projects!” or do a Google search for “Winning Science Fair Projects”, and you might even find one that’ll work for you. But it’s far more gratifying to use it as an opportunity to answer a question you want answered. You’ll be more motivated to finish the project, and you get the added bonus of having your question answered at the end. Besides, judges like to see self-motivated science fair projects. Trust me, they can tell.

– Tom

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Raptor Tail

Posted by Tom Benedict on 08/04/2013

Still no pictures of my new glider under construction, but they’re coming soon.

In the mean time I finished the tail on the Raptor, fitted the tail servos inside the fuselage, and ran the control linkages. I finished the linkages this morning before heading to work.

All of this has been happening when time is available. I don’t have a lot of room in my house to work on this plane (or the next two, which are on the way!) so at the end of each work session I break everything back down and stick it back in the box. This has worked fine up ’till now, but last night I ran into a snafu.

I fitted the servos, control rod guides, and rods for the tail control surfaces, but hen I had to go to bed. The untrimmed control rods were poking out the back of the fuselage by more than a foot! But until I fitted the tail and installed the control horns, I couldn’t trim the rods to length. No way was it going to fit in the box with the rods poking out the back. Nor could I leave it on the table. That’s where we eat breakfast! So I did the only reasonable thing: I stuck the fuselage in my kite bag. Hey, it fit!

But immediately after breakfast this morning I pulled it out and finished the job. The tail on this plane is removable, and once trimmed to length the control rods and fuselage fit in the original shipping box. So I’m back to being able to put everything away at the end of a work session, and my kite bag is back to being just a kite bag.

The next step is to install all the servos in the wings. The Raptor comes with servo extensions pre-installed in the wings for the ailerons, and with pull strings to thread the flap servo wires through the wing structure. Only problem: the wires on the Turnigy servos I’m using are too short for the flap wires to reach, and the combination of servo wire and servo extension is too long for the ailerons. Ugh! So I’m going to do what I’ve done with most of my KAP rigs: make custom servo wires!

Years ago I picked up a crimp tool and a couple of gauges of servo wire from Servo City. For anyone messing with servos, this is a really really good idea. It lets you make custom servo wires, build your own Y-adapters, make custom servo extensions, etc. I like my KAP rig wire runs to be neat, and to minimize weight in the air. It’s a pretty simple matter to install the servo in the KAP rig, remove the back cover, and run a custom cable length to the radio. Put the back cover back on, and it looks like the servo came that way.

I’m planning to do the same with the Raptor. The idea is to tape the servos in place and install unterminated servo wires a couple of inches longer than I think I’ll need. Once the servos are installed in the wings, I’ll cut the cables to the precise length I want and crimp on the servo connectors (also available from Servo City). That way there’s no more and no less than the wire needed, and no chance of a servo extension coming loose in the wing.

So I’m about one evening’s work away from reaching my first real stopping point on this plane. Until the radio, speed controller, and propeller arrive, there’s not much more I can do.

Actually, there is one thing: I’m installing a system of pulleys in my room so I can hang these planes from the ceiling. When I want to actually fly one (why on earth would I want to do that?!) I can uncleat the line holding that plane up, and lower it. Then I won’t have to put the plane away in its box each time I finish working on it! YAY!

– Tom


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Raptor Underway!

Posted by Tom Benedict on 05/04/2013

About half of the stuff I ordered has already arrived. The Zagi and the Le Fish are both coming by slow post, but the Raptor Advance 2000 is already here! W00t!!

Even though it’s largely pre-built, there’s a ton of work to do on this plane. It’s also my first crunchie (or stick and tissue). In a word, the kit is GORGEOUS. But it’s also terrifying. I can already tell if I have any landings similar to the early ones with my Bixler, it’ll spell the end of this wing, if not the whole plane. It may be the first of the three planes that I finish, but I expect it’ll be the last of the three that I take out for its maiden flight.

Unfortunately, for such a photogenic subject, I haven’t taken one single picture. Not one. I’m ashamed! But pictures will come. I swear!

So here’s the build:

So far the build is going well. A lot of what I’ve done up to this point has been pretty standard stuff: modify the firewall to fit the motor; start building the tail; repair the slight damage that occurred during shipping (one wing tip separated). You get the idea. Still to come this weekend is finishing the tail, installing all six servos (WOOHOO! full house glider!), re-hinging the control surfaces on the wings (too little range of motion), and finally dry-fitting everything in anticipation of the next shipment of stuff.

Still missing are the spinner, props, ESC, and radio receiver. Yeah… most of the guts of the plane. But the spinner and props should be here by Monday, and the rest should be here within another week or so. It’s not going to be a long build.

So I mentioned those TG50090M servos… These things are COOL! I got two of them to use as flap servos on my Bixler, and fell in love with them. They’re 9g metal geared servos with a surprising amount of torque. And at less than 5USD apiece, I didn’t see the need to use weaker servos for the tail on this plane. All six control surfaces are using them. I can’t help looking at them and thinking, “These would make for a cool KAP rig!” There’s plenty of power to move a compact camera around, and with the metal gear train they don’t have the slop and bounce that even my full-sized Futaba S3003 servos suffer from. At some point I’d like to get a couple of these and see how small and compact a KAP rig I can make. Maybe a Canon Powershot? Maybe an NEX? It’d be a fun project.

But for now I’m building an airplane. And within the next few weeks I’ll be busy building two more. The mini KAP rig will have to wait just a little bit. (Yeah, like maybe until I can afford the camera to go in it!)

– Tom



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