18 February, 2007

World War 2 is back on...

Finally finished the rebuild of the Pax Aero WW2 Profile combat planes. This time I went with a F6F Hellcat, and took some inspiration from the February issue of MA, which featured a club profile plane with the radio gear in the wings.

After enlarging the wing chord and building for 3 days, we had a test flight last week at Helwig field. The flight went fine, save for a little engine trouble at the end. Given the 25°F temperatures, there could have been a few causes for that.

Hopefully, we can get a few of these ready for some spring combat.

28 January, 2007

Wing Skinned

I finished skinning the SGB88 wing today. I put the joiner rod in for a test fit and snapped this picture. Keep in mind that the left wing doesn't have a LE, TE or a wingtip on yet, so this is close, but not all the way to 88 inches. the left and right hand wings were built a little differently, which is why one has a joint and the other doesn't.

I used Dow Blue foam for the wing. It has a carbon fiber ply under the balsa skin, and a carbon joiner sleeve at the root, along with ply bolt blocks and a glass reinforcement near the root. Here you can see the carbon going on the wing core prior to the balsa skin. You can also see the blue painted stab in the background.

Here is a picture of my first cut at the fuselage outline and the stab/fin joiner (25% scale). The fuse is simply 1/8 ply bulkheads with 1/16 balsa skins that I will glass with 4 oz cloth.

22 January, 2007

Designing the SGB88

That carbon arrow shaft really inspired me. It inspired me to throw out all sorts of methods and techniques and try a whole bunch of new stuff. I decided to call this project the SGB88.

The first new thing that I tried was to use AVL to design the wing. I looked around and picked a planform that I liked, and then tinkered with a few combinations of sweep, panel spacing, dihedral and airfoil until I got one that had both a good L/D and power factor. I stuck with an 88" span for no good reason, other than it was somewhere between 2 and 2.5 meters.

I picked an arbitrary design load case to work towards. I reasoned that I might have to perform a 10 foot radius pullout at 35 mph (works out to 4.2g). I used this to generate the bending and shear loads for the wing. You can see a pretty load plot from AVL above, and the even more useful plot of bending moment and shear as a function of span.

From this and some additional calculations to look at load in the spar caps and deflection during this maneuver, I decided that a built up wing simply wasn't going to cut it. I had hoped to keep the total aircraft weight under 24 oz (with 8 oz for powertrain and LiPo), but I was faced with the real possibility of a wing fold in normal flight.

Given that I ended up deciding to use Drela's AG34-36 set, I made some templates and cut a set of cores for the right wing. I then epoxied carbon fiber and balsa on each skin, and joined the three sub sections together. With the inclusion of a carbon joiner sleeve (aluminum rod joiner), two servos, wiring and flaps/ailerons, the right wing weighs just under 8 ounces, but is very solid. So much for the 24 ounce airplane, but now I will try to shoot for 32.

21 January, 2007

Omei and Ascent

Over the last few months, I have added an Omei 2000 and an E-Flite Ascent to my collection. While both are ARFs, they are certainly fun to fly. I am using the Omei to check out my Futaba R149, and have had no problems thus far. It is powered by an RC Sport Evo 1480 with 40A ESC. I use a separate 1400 mAh pack for the Rx and servos, with the BEC feature disabled.

Since I have done little to the kit - apart from glassing the wing joint - I fly it like a TD ship. It floats reasonably well, and penetrates very well. Using a 2100 mAh motor pack, I was able to record a 60:31 flight (12 minutes of motor time) last December.

The Ascent has been modified from stock with an E-Flite 400 outrunner and uses a 3S 1320 Thunder Power pack for both the motor and the two SM15 servos (with an RS6UL Rx). Turning a Graupner 10x8 CAM folder, it climbs vertically, and has achieved glide:power ratios of 17:1. With some spoilers for spot landing, it could do well in an F5J competition. Given that it went together in less than 2 hours and cost $70, I'm pretty happy with that.

Reflection on my Reflection

Last summer, I had a good time flying a Great Planes Reflection Flat Out around the front yard. It was a cheap way to tinker around with 3D, and a nice way to spend some time outside on a summer day.

While my 3D flying skills never progressed much past the goof off stage, it was fun to try some of the cool moves.

07 October, 2006

Rebuilt Super Sportster 60

My most recent flight with the SS 60 was truly enjoyable. That design is one that never goes out of style, especially once you really get to know how to fly it. It is just a smooth airplane all around, and really deserves to be pulled out of the hangar quite often. Unfortunately, I found that while flying it was fun, landing was a pain on the grass field. The gear on the wings even made taxiing difficult. I shelved the plane several months ago when I noticed that the canopy was coming loose, promising that I would fix it all very soon.

Well, this rainy weekend I finally did. I got a DuBro high strength landing gear set, and cut open the forward fuse to drop in some ply mounting plates with blind nuts. While I was in there, I changed the fuel tank, put bigger wheels on, and redid the wheel pants. I also put on a new spinner and canopy. I think that it looks good, even if it does look a bit like Dracula's personal plane.

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!

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