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Aug 16, 2015, 10:49 AM
Better SAFE than sorry!
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Build Log

UMX Pitts SAFE Conversion

I now own my first biplane - a UMX Pitts! Since I'm a big fan of the SAFE flight technology to tame the plane down and reduce crashing, I converted this model to SAFE using the receiver brick from the UMX Habu S 180 DF (part # SPMA3165).

Although it's pretty straight-forward to remove the control board from the micro planes and replace it with another one, a couple of details made this conversion a little more interesting.

First, the control board is mounted vertically - not horizontally - in a Habu S, so the SAFE software is calibrated with that assumption in mind. In the pics below you'll see that I cut a C-shaped piece of Styrofoam to serve as a cradle for the board so that I could firmly mount it in a vertical orientation. It took a couple of attempts to make a piece that held the board snugly - I kept cutting out too much foam in the middle and then it didn't hold the board. I also found that it worked better with that little tab at the top to help the board stay vertical. Finally, I covered the piece of foam with Kapton tape (a film-like alternative to vinyl electrical tape) just to ensure the foam didn't short something out on the board.

Second, I've been complaining about the fact that the Sport Cub S adds too much up elevator in Beginner Mode when the plane is actually level. It turns out that the Habu S board does the same thing, so I had to be more creative. Both the SCS and the Habu S will eventually straighten out the elevator when the nose is pitched up at an angle that's even a little higher than the normal tail-dragger stance on the table. So the SAFE receiver does work just like the Apprentice S receiver, but it's just that its definition of level flight is different and wants to pitch up. So to compensate for that, I installed the board on an angle, such that the board is pitched up when the plane really is level. That idea worked great! I'll have to go back to my SCS and make a similar mod to tilt the front of the board up. This way, you don't have to totally mess up the elevator trim in order to fly straight in Beginner Mode at the expense of the other flight modes. The diagram below shows you what I'm talking about. I adjusted the angle such that it still had a tiny bit of up elevator in a level attitude, but not nearly as much as it would have if the board were mounted level. The micros severely limit the amount of elevator travel you have in Beginner Mode, but with this idea there's just enough down elevator movement to give you the ability to point the nose down while in the air.

The diagrams and pics below will give you an idea of how the mod turned out. This is the first time I've opened up a micro, and while it seemed intimidating at first to work on something so fragile (compared to a bigger plane like an Apprentice S or a Carbon-Z Cub!), it's actually very easy to do. The stock board in the Pitts is held in with the white goop (silicone?) that's used in a lot of foamies, and I wasn't sure how to get it out until I found that it came out easily with just a little bit of upward pressure. It was all plug-and-play - no soldering required. There's a plug for the motor hiding under the battery compartment. I didn't have to remove the battery deck; instead I was able to cut a small slit in it to let the plug come through. The one mistake I made was to cut too much of the tape holding the upper section and the lower section of the fuse together. As you can see in the pics, there's a vertical seam just in front of the pilot, and that's as far as you need to go. I originally followed the horizontal seam all the way forward to the motor mount, and that wasn't necessary. But all you have to do to recover from a mistake like that is tape it back together!
Last edited by flightengr; Aug 23, 2015 at 07:39 AM.
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Aug 23, 2015, 07:41 AM
Better SAFE than sorry!
Thread OP
A little more clarification about how to get that board mounted in the correct position for Beginner Mode to work correctly...

The fact that the Pitts is a tail-dragger isn't an issue. Normally, you'd mount the a SAFE receiver level relative to an area inside the fuselage that's level when the plane is in a level flight position.

In order to fly correctly in Beginner Mode, you want that board to be as straight as possible from left to right. Once I got close and had the board snugly positioned in my Styrofoam mount, I set it in the Pitts. Then I put a battery in the Pitts, powered it up, and activated SAFE by momentarily throttling up past 25% and setting the TX for Beginner Mode. In Beginner Mode, the ailerons and elevator will move and stay in position as long as the board thinks the plane is not level. To get the ailerons right, I made sure the plane itself was level, and then I tilted the board left or right until the ailerons were even on both sides and flush with the surface of the wing.

When it comes to mounting the board with the correct pitch forward/back, I chose to pitch the board up a little bit so it's not sitting level in the plane on that axis. You'll notice that Beginner Mode wants to add a lot of up elevator input even if the board is level with the plane and the plane is in a level flight attitude (pick it up and hold the plane level - don't check this on the table in the tail-dragger stance). You'll notice that as the nose keeps pitching up, the elevator would eventually level off and line up with the horizontal stab. So I chose to pitch the board instead so that it thinks the plane is pitched up farther, and then I could adjust the amount of up elevator that's being added when the plane really is level. I set mine such that there's just a tiny amount of up elevator added with the plane itself in a level flight position.

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