The View Up Here

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

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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