The View Up Here

Random scribblings about kites, photography, machining, and anything else

Archive for August, 2013

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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KAPing Again!

Posted by Tom Benedict on 09/08/2013

I’ve had a couple of years now of almost no KAP at all. A combination of higher gas prices, pressures at work, my own personal issues, and an overall withdrawl from photography in general kept me out of the field. When I did get out, it was to go to Hapuna Beach with my family to do some incidental KAP in a convenient location. But cripes, I’ve photographed Hapuna to the point where I don’t even want to do sunset photography there. (Which is a sad statement to make… it’s a beautiful spot.)

During that time I’ve been upgrading my gear, trying to address every little irritation or disappointment I’ve run into in the nearly seven years I’ve been doing KAP. There’s one thing I can’t stand when it comes to photography: heading out to the field, pulling out your gear, and having it not work! Or having it not do what you need to get the photograph you had envisioned. This has been a strange process since I haven’t always tested the changes I’ve made before moving on to fixing the next problem. But I kept plugging away at it, sometimes tearing things apart and rebuilding them before I’d even tested my first solution, just because I knew it wouldn’t be good enough. Eventually, though, I reached a stopping point. I knew my gear was as close to what I wanted as I could get.

I think yesterday was the real payoff. The pressures at work let off long enough for me to take a week long vacation. Much of it was spent catching up on stuff I’d put on the back-burner. I re-programmed the radio for my planes. I made fruit preserves. I cleaned house. And I finally grabbed my gear and did some real KAP for a change. I went to Pololu Valley.

It’s been years since I’ve been in Pololu. The last time I was there was for the ill-fated Nokia N8 project. Back then the wetlands were dry, the forest was struggling, and I wasn’t sure how much longer before there was no green left in the valley at all. In retrospect this also probably played into my general malaise when it came to photography. We were in drought, so many of the places I enjoyed doing photography just weren’t photogenic. But a wet winter and a recent tropical storm brought a lot of green back to the islands. It was time to return to Pololu.

Pololu Cliff Trail 2

You reach Pololu Valley by driving the coastal road north past Hawi, past Kapa`au, and around to where the highway ends at the Pololu Valley Overlook. The valley is part of the forest reserve system, and has an excellent trail that leads from the parking area down about 500′ to the valley floor. There are three switchbacks, and several really good vantage points from which to see the valley. This is one of those rare spots where tripod photography will get you as good a view as that from a kite. (But I made this photograph from a kite anyway.)

Pololu Valley Southern Cliffs

To my delight, I saw that the recent rains had replenished the valley, and much that was brown or gray was once more green. The wetlands were back!

Pololu Valley Wetlands

Pololu Valley is the northernmost of the Kohala Mountain erosion valleys. At the mouth of all of these lies a black sand (or rock) beach. Pololu is no exception. There’s a truism regarding beach sand color in Hawaii: White sand beaches are safe to swim at. Black sand beaches are not. For the most part this is true. White sand beaches are made of crushed coral, typically coral that has passed through the digestive tract of a parrot fish. This means there’s an offshore reef that breaks up the incoming waves. Black sand beaches, on the other hand, are typically made from broken or eroded lava stone – as in the case of Pololu – or from lava that poured directly into the ocean and fractured as it cooled – as in the black sand beaches in the Puna District. Black sand beaches have no offshore reef, so there’s typically nothing stopping the full force of the ocean from hitting shore.

Pololu is one of the rare exceptions. The slope of the beach is so slight, the sand itself attenuates the force of the waves before they reach shore. The strength of the waves is no greater than those at Hapuna Beach. It’s one of the safest black sand beaches to swim at. But not having a reef, there is still a strong current that can sweep an unwary swimmer out into open water. So be careful!

I don’t often swim at Pololu. Swimming in shorts just saturates the cloth with saltwater. And with a 500′ climb at the end of the visit, it’s a great way to get chafed. I didn’t swim this time, choosing to focus on KAP instead.

Two and Two on a Black Sand Beach

Black sand beaches make fun KAP subjects. The unusual color of the sand can make for difficult metering with the camera, but the view is reward enough for the effort.

I did KAP in Pololu Valley for almost three straight hours. This was the most intensive test of my new system I’ve done to date. I brought two 500mAh batteries for the KAP rig and video transmitter (both powered off the same battery), one set of AA NiMH for my RC transmitter, and one 2650mAh battery for my video receiver and monitor (again, both powered off the same battery). In three hours I drew around 300mAh out of the transmitter battery (200 on one and 100 on the other for about 100mAh per hour of operation), and about 500 out of the video system. My RC transmitter batteries charged within an hour, so they were no more than 25% depleted. I’m pretty sure I could do an eight hour KAP session with my gear and not run out of power. This is really good to know.

I ended the day out on the beach at my original launch spot, packed my gear, and hiked back up. It was a strange hike. Previous to this session, every time I’ve hiked out I haven’t really known what I’d find on the camera when I got home. I tend not to check my photos in the field unless I have reason to believe something went wrong during the session. This time, thanks to the video downlink, I knew precisely what was on the camera. It made for a spring in my step as I hiked out.

Pololu Self Portrait

– 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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