Avia 87

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I am preparing to Build an Avia 87 for the new season of Control Line flying. I wanted an airplane that was easy to build, use small engines and was capable of flying the AMA Stunt Pattern. I picked this airplane as I had flown it before and it was a fine flying airplane. The whole point is to have a training airplane that I can build quickly and spend less time in the shop and more time flying. I started by scaling up the plans from an issue of Model Aviation which took two days. I was pleased at how close I was to the original plans when I finally acquired them. The first image is the whole design and will give you an idea what is at stake. What is interesting is that this is a really large airplane for a OS .15 sized engine. Well it is long but the wing area is 390 square inches and that is not too large. It will be a very light airplane but quite long which gives the impression of size.

The first task after the plans was to make a full kit which is different than what I usually do. I make parts and build as I go but in this case I thought that I would make all the parts first and then build my own kit. I began by cutting the fuselage to shape from a sheet of 3/8th balsa and this process was fairly simple to do.

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Construction of this kit continues with the 3/8th inch maple engine bearers which took a little longer to make than expected. I had to cut and sand these to shape from a couple of lengths of 1/2 x 1/2 maple and my saw really works hard to get through maple block. They fit good and I will go with them, so I marked them top and bottom and set them aside.

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Next up was a set of plywood doublers, the plan called for these to be made from 1/32 plywood and after scrounging around the shop I realized that I was short some 1/32 ply so I have made one of them from a sheet of 1/16th. I will glue the thicker sheet to the engine side of the fuselage and perhaps sand it down a bit more in the long run.

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After making the parts for the fuselage, I moved to making some templeates so that I could make some wing ribs and to do this I used plywood templetes. This method is called the sandwhich method and it is used when you have a tapered wing and every rib is of a different dimension. It is a little involved in making the sandwich tool but makes the right ribs for the job. Each template is plywood and they are matched to the root and tip rip profiles from the plan. I matched them up together and drlled them for two threaded rods that can hold a stack of ribs between. In the last image the ribs are sanded and carved to shape to match the root and tip rib.

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Next was to fabricate the stabilizer and elevators and this was easy as it was trace and cut.

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This was not without it's problems as I had not made a wing with an internal spar, and making all things line up for that spar was a bit more effort than expected.

I use a Aeroalignment Wing and Fuselage Fixture system to build some wings some of the time. This system consists of Jig Uprights that are fully adjustable and allow the wing to be made suspended and all aligned regardless of the nature of the building surface. These uprights make for very straight wings and are fun to work with but do take a little setup to get things going. I use these jigs for building wings that are tricky building flat. They hold the wing firmly and this makes for solid support.

I build on glass that is on the plan and each aluminum jig is ca'd to the glass at the right spot, all around the outside of the wing. later I use some magnets to hold the glass to the metal door workbench so it does not slide at all over top of the plan. Then the trailing edge is suspended in the jig holders and made straight and level with the small sight level. This leveling process makes things flat. Then I install the leading edge into the front jigs and make the process straight and level and measured for height against the trailing edge. This completed the setup process and building can begin.

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Once things are all leveled up I could begin to cut the shaped ribs to length and install them, one at a time. What mistake I made was to glue them in at this point, without the main spar installed. In the process of installing the main spar, I proceeded to damage a bunch of ribs just to get the spar in. I think on a later version I will install all the ribs on the spar and then install the ribs and spar into the leading and trailing edges. Once things are all aligned and straight I will glue it all together. Well I did end up with a partially built wing that turned out ok.

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Next was the installation of the bellcrank where things went together in a traditional manner. I did choose to flip the bellcrank inwards to put the down line on the aft position. I then removed the bellcrank and installed the leadouts with the use of brass tubes as this makes a very secure attachment that will stand the usual wear and tear. Following on was the task of sheating the center section after the pushrod to the flaps was installed. Sheating was 1/16th balsa and was not too hard to do.

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Of course, there is a period of time that passes that a pile of build sequences are omitted, and that is my fault. I had a deadline to meet to complete this airplane and taking images just slowed me down. In the meantime, the fuselage is finished in Varathane Dimond Coat, and Rustoleum paint and the wings are Monokote with a trim pattern that is also Monokote. I was fairly pleased with the design but have a few more things to learn as it apples to double layer Monokote work.

This airplane flys well and is powered with a Fox 15 which is more than enough to allow it to do the full pattern.

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Here are my line lengths,

Center of PLane to Center of handle, 55.8
Line length Eyelett to Eyelett is 53.3 but these are too short and I get a lap time of 4.1 seconds which is pretty fast for me. I am thinking of installing an 8 x 3 prop and going to 60 foot lines to get a lap time of 4.9 seconds. The airplane should be going about 56 mph at that point. The airplane is a real joy to fly and handles very well. My CG is at 1.75 inches (dry) and my lines rake will be 2.75 inches mesured between the lines that are spaced 3/4" apart. I am using .015's and this is fine for me, I could go to .012's I suppose, but all that will do is change my lines exit location by a few degrees.

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Interesting bit of information, I liked the article and the story of the youth movement in model aviation. Thanks for your efforts in translating that article in Model Aviation. I have now flown this plane yesterday and I am really happy with the design, it looks and flys like a larger model and is very responsive and certainly goes where you direct it. It will take me through beginner pattern without any problems. Thanks for sharing your story.

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This airplane has been very easy to get trimmed for stunt, I added a bit of weight in the tail to help the CG fall in the right place and to make the turn a bit brisker. I also had a bit of differential flap that made the plane fly a bit banked out of the circle so we adjusted the flaps so that the plane was level. The plane was adjusted with a 8 x 4 prop and I made the lines 64 feet in order to get the lap time down a bit but it still moves along with a lap time of 4.5 seconds. It's pretty hard to get it much slower. This plane glides well and is easy to keep gliding with a bit of line whip to a landing spot of your choosing. I am still working through engine run issues and this past weekend I had it going lean and then rich, then lean, then rich. We first thought it was fuel, but a loose Needle valve Assembly was observed and it was tightened. I have yet to run the engine again to see if the changes helped. In all I will enjoy flying this plane as I learn the pattern, it is a superb pattern trainer and looks great in the air.

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Made some changes to the plug and installed a hotter plug, and went back to a 28% castor based fuel and this seemed to help make the airplane run a bit better. I also installed a piece of silicone fuel tubing over the needle valve and this seems to help keep the needle more stable. The tightening of the needle valve spray bar seemed to help make the motor run much better and starting was easier as well. So todays progress was a bench run in preparation for a practice session on Sunday.

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As things turned out, the engine still began to run lean when elevated so the next plan was to dissassemble the engine right at the field. Added new gasket goo and tightened the heads and backplates and tried the engine again, still the same result. During the tear down, it was discovered that the engine crank had it's intake hole modified slightly larger, perhaps for it's days as a combat engine. It was assesed that this engine needed some fuel pressure to make it behave so it now needed a muffler and pressure fitting. I aquired the muffler and made the modifications for the pressure and routed now lines to the tank. Well, did this make the difference, it began to run very evenly right up to the end of the tank. However, with a full tank there is a slight burp in the pitch to inverted flight that is a tiny annoying but it tends to moderate as the tank uses up some fuel.

I am able to get upright and inverted lap times of 4.7 seconds and this is a little fast but workable. We were shooting for 5 seconds. So with that I began to fly with a bit more confidence. The airplane performes all the maneouvers required and it fun to fly but in my enthusiasm and a slighty poor run, while conducting a inverted loop I pulled out a little low, zingged the top of the plane and trimmed the prop blade off and suffered a break in the covering in the right wing. So a change of prop and a bit of tape on the covering break, I was back in business. The airplane got a bit of a needle adjustment and the 8-5 prop is a bit nicer to fly with then the 8-4 prop I completed the beginner pattern, minus the overhead eight. So now I have a good tool to use to completely learn the whole pattern.

Next up is a practice flight, that will allow me to continue the beginner pattern including the overhead eight. I am keeping the manoeuvers pretty big and open for the time being as I get used to making the right moves with each stunt. We calculated the flight time on the current tank and it is 6 min 30 seconds long so that is plenty of time. Starts are getting better and I am settling into a start sequence that is predictable.

Choke 3 times forward,
turn through forward 5 times,
turn through backward 5 times,
Attach glo driver and flip forward three times and on the third time it starts.

This sequence is good and stable and works most of the time.

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So the next issue with this airplane was a stable high quality plug as the engine would not run consistant and burn out plugs quickly. We settled on a Thunderbolt #3 Glow Plug which is a short plug. Now the airplane runs the whole tank through consistantly. My success this weekend was a completed beginner pattern and this was a high point for me. Other low points were my inverted loops as I am inconsistant at keeping them in the same place and size resulting me taking te canopy and rudder of in a very low inverted loop. So the airplane is repaired and flown again.

Progress is a result of determined application of small steps forward and a few large steps backward. Success is the result of persistance and overcomming the heartache when something gets damaged in the process. The thing for me is to keep going and respect what my coach tells me I should do. I sometimes fail to listen and appply the instructions. I am getting better and we are having FUN...

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Hey that's a good idea, I will try the 8-3 prop and see what difference it makes. So far the 8-5 does sometimes detonate and the lap times with that prop are still 4.9 seconds and I am hating doing inverted loops with the fast plane and the very fast controls I got going on. I will go shopping.

Thanks

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I accomplished 3 flights of the beginner pattern, back to back without any problems this past weekend. I am making progress and excited to be able to get this airplane through the beginner pattern properly. I have kept the 8-5 prop as it is working out well. So this past weekend was a major success for me to fly properly and as accurately as I can.

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You are correct, it was the Fox .15 schnuerle engine and this particular engine had a bit of work previously done on it to hop it up as it was intended for combat use. It was a loaner engine and I had to return it to it's owner. I have retired this plane but it sure was a good one and it will always be a great plane to help someone learn the pattern. The trick is to keep the airplane simple and use a reliable predictable motor. This plane took me through 50 flights and I have never had another plane or engine that I had run so much in my life. It was a very successful program. I was getting 15,000 rpm from that engine.