18 September, 2006

Rechristening the Spirit

I finally received my Graupner folding prop and spinner combo today, so I was free to go ahead and finish the Spirit conversion. The spinner collett was a little tighter than the one that I used to size the nose ring, so I had to shave a little with the Dremel and recover the front, but that wasn't a big deal.

Once I got the prop on, I balanced the plane with just a few ounces of lead in the battery compartment. The plane ended up tipping the scales at 73 ounces, with the engine producing 26 ounces of thrust (at roughly 140 W). That comes out to a power loading of 32 W/lb, a wing loading of 11.5 oz/sq ft and a T/W ratio of 0.36:1. It won't be a hotliner by any means, but I hope that it will provide some climbing power.

I had a local auto customizing shop (D&G Kustom) cut some vinyl lettering for me, and I rechristened the airplane the Esprit 100.

I was especially pleased with the way that the tail numbers looked when applied over the rudder and fin.

Good Design Should be Inspired

I was inspired a few days ago. I stopped into K-Mart to see if they carried a cheap car battery that I could use to recharge LiPos at the field, and on a lark I visited the sporting goods department. For $6 I found a beautiful composite arrow shaft, and I have every intention of building a plane around it.

I am taking my love of good design a step further this time and using Mark Drela's AVL program to do all of my aero sizing and loads prediction. From that, I'll size my wing structure. I may use some quick and dirty tests to find out the bending and torsional stiffness of the shaft so that I can predict the aeroelasticity of the empennage.

15 September, 2006

Batteries and Monitor

I got my first order from www.cheapbatterypacks.com several days ago. I had ordered a Thunder Power and an Apex LiPo. Both were in great shape and arrived in less than a week via USPS. I was so happy that I ordered several NiMH Rx packs and a NiMH Tx pack for my 9CAPS. That thing chews through the NiCds.

Once I saw that the flight times on the Spirit might be fairly long - which prompted me to order the NiMH flight pack - I picked up a LED battery monitor from Doug's and install it in the canopy. It monitors the Rx pack only and plugs into a spare spot on the Rx.

I hope that I will be able to see the colored lights on a low flyby. They go from green to yellow to red as the voltage in the pack goes down. Should be a good indicator.

Cleaning and Sorting

I spent some time this week trying to get my shop back in some semblance of order. Since Ernesto rolled through, I have had to spend a lot of time sucking up water with my shop vac, and I haven't been able to suck up a lot of the little junk that accumulated.

I also took my Dad's advice and got some track shelving accessories at Lowe's. I installed two 30" tracks in my garage and put some 12 and 16 inch shelf brackets on to hold planes. I was so happy with the results (the planes are held down with Bungees) that I am going to double the shelving capability this weekend.

Get all that stuff out of the work area and give me some space!

09 September, 2006

Battery Performance

Say something for buying good LiPos. I just finished running my thermal survey. I set the plane up just as it will be flown, and measured temperature at the battery and temperature at the rear end of the cooling tube. After a run (with an 8x4 prop) of 5 seconds at max power and 120 seconds at three-quarter power, no change in temperature was measured at either location.

This was followed by a 2 minute cooling period, and another run of 5 seconds at max power and 120 seconds at 5/6 power a 2F change was measured at the battery.

I then removed the thermocouples and ran alternate 2:05 run/rest cycles (using 120 seconds at 5/6 power). I stopped after completing 6 more cycles (for a total of 8). All this was done with a single Thunder Power 3S 1320 mAh pack.

With the correct propellor, a 2 minute climb should net somewhere between 500 and 1000 feet. This should translate into 3-5 minutes of gliding with no lift. With two packs, it should be possible to fly for 70 minutes on a single 3S-2P charge.

I had better bring a chair.

An update. I charged the single LiPo cell that I used for the battery tests, and apparently, only 62% of the capacity was used for those eight runs. When I redid the calculations for this new battery figure, it seems possible to get a flight time of nearly 3 hours!

Simply Electrifying

More conversion work last night, nothing today except a little bad golf and a lot of football. Last night, I spent a long time working the radio mixing for the wing. I ended up using the 2 Flap model for the 9CAPS because it was much simpler to mix everything the way that I wanted it. I have posted a quick overview of the method here.

I finished the front end, mounting a ring on the nose ahead of the engine and rounding the fuse to meet it. This would help ensure that there wasn't an odd looking transition between the fuse and the spinner.

You can see in the middle of the bay behind the motor (where the flight pack will go) that I have included a button magnet to hold down the canopy. With four of these (two on the fuse and two on the canopy), there is no additional hold down method required, but I will likely put a rubber band on the canopy just in case.

After mounting the canopy, I took a picture of the new profile of the front end. As you can see, I removed quite a bit of covering to round the nose properly. I am going to redo all of the covering in the front end to try and even things up a bit.

Finally, this morning we headed up to Waldorf to check out the outdoor supply and army surplus store. I picked up a 0.30 cal ammo box there to store LiPos for transport. For $4, a pretty cheap way to get some piece of mind. The boxes lock and are waterproof too.

08 September, 2006

Ahhhh...the Dremel

I can't figure out how anyone accomplished anything in the modeling world before the invention of the Dremel. It certainly made short work of the front of the Spirit.

Today, I set out with a plan. I wanted to fabricate a firewall that, with the use of a Graupner spinner, would roughly maintain the front lines of the Spirit. I started out by picking a cut line and taking the balsa block off the nose. I then machined out a cavity in the remaining wood that would enable the motor to spin.

After this, I fabricated the firewall using a piece of 3/16 ply bonded to a piece of 3/32 ply. I used this to give me the right spacing off the existing first bulkhead of the Spirit. I drilled this for the motor mount and used 4-40 blind nuts for the anchors.

Once I got this done, I ground the first bulkhead down so that I could bond in the firewall. I kept the edge of the bulkhead to use as a positioning guide, but machined enough away so that the bolts would clear and I wouldn't have to try and cut any more once the assembly was done.

Then I could bond the firewall in place. I put tape over the back of the blind nuts so that I wouldn't fill the threads with epoxy. I wet down the edges of the fuse with 20 minute epoxy and then added more epoxy with microballoons once the bulkhead was in place. After the cure, it was a simple matter of bolting the engine on.

Finally, I had to cut quite a bit of the cockpit away under the canopy to provide some air space for cooling air to flow around the motor and through the battery compartment. When complete, this provided a functional, if ugly, method of ramming cooling air through the forward fuse. I tried to compensate for the ugliness by painting the canopy silver.

Of course, like all good tinkerers, I just couldn't wait to see the fruits of my labor. I strapped an APC 11x4 prop to the front of the motor, dropped a 3S LiPo on the ESC, and let it rip on the table of my bandsaw. Two things were learned from this. First, that we surely have enough thrust and second, that we have plenty of cooling. Just to validate this, I will do some static runs of several minutes with thermocouples in the battery bays. Should be fun!

Tomorrow's episode will cover my new hatch hold downs and my current approach to radio mixing.

Spirit Conversion

Well, I got sick of not being able to fly my Spirit 100. Our powered club just doesn't have anyone who is interested in sailplanes, and there isn't enough room out at the field for a full sized hi-start. I have been spending a lot of time on little electric planes, and a friend convinced me to go ahead with an electric conversion of the Spirit. Much of the work was already done for me here . All I had to do was just tweak it to my desires.

I had an E-Flite 400 brushless outrunner at home (great motor), and while this motor doesn't quite provide the 50 W/lb desired for a sailplane motor, I thought that it would be ok, especially with others' experience. Besides, I didn't want a hotliner, I just wanted something that would climb a few times on a charge.

So, my initial pass at equipment looked something like this:

1. Brushless 400 motor with folding prop.
2. Thunder Power 1320 LiPos in a 3S-2P configuration running at 11.1 V and 2640 MAh
3. A separate 1000 MAh Rx pack to provide the amperage to run the servos and give endurance for long flights
4. My Futaba R149 (it needs some test flights before it gets put in the Tsubame anyway)

I did some weight and balance calculations, and it looked like I could get all this mounted for roughly the same weight I have now. The practicality of that might not be so true. I also have to find a way to provide some airflow through the motor/battery compartment for the ~2 minute power climbs.

More to come...

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In fact, I just got off the treadmill...