This is part 3 on my journey of converting this aircraft to Electric power.  To see the previous post on the topic, click here.  

As I have mentioned previously, I had picked up a nice Hacker motor from a flying club member and it seemed like it might work out as a power plant for this aircraft.  I needed to get that mounted and also figure out what sort of ESC might be appropriate as well as selecting an appropriate battery and then figure out placement and mounting for all of that.  So here was my process.

First I researched the airplane, including expected weight, wing area and ground clearance.  I also read a bit on any balance issues folks reported to see if I was likely to need nose weight or tail weight and the weight of the suggested power plants.  Armed with that I logged into ECalc and filled in the blanks with the wing area, weight and the motor type I had on hand.  I then started plugging in appropriate speed controllers, props and battery combinations to see what I was going to need to get a good power to weight ratio.  Something in the 1.2:1 or higher range seemed like a good goal for a fighter aircraft like the P47!

I like to keep the voltage high and amperage low in an RC aircraft power system.  Higher amperage is hard to accommodate as most of our common connectors are not rated for it.  If you overtax them, they get hot and waste power and eventually this causes a catastrophic failure.  For similar reasons, our LiPo batteries will fail quicker when you run demand high current rates from them, plus they cost and weigh more as you try to accomplish this with batteries that have a higher “C” rating. 

A little rant about LiPo C ratings…   Feel free to skip the next two paragraphs if you like.

In general, C ratings are useful only in that a higher C rating within a manufacturer battery line indicates the ability to supply more current without damage to the battery than the batteries with lower C ratings.  Beyond that they are deceptive at best and almost useless for comparison between manufacturers.  Feel free to disagree but I am convinced that this is the state of things at this time and don’t expect it to change anytime soon so don’t put to much faith in C ratings.

So, while my large aircraft could (according to the manufacturer C rating) run on 20C cells, typically 40C cells get pretty hot and therefore fail sooner rather than later.  The 50-60C packs simply perform better, don’t get as hot and outlast the lower C ratings… at least in the brands I most commonly run.  I don’t bother to purchase from companies who advertise 100C and better as they are pure fiction in my opinion.  A continuous draw at the advertised C rating is almost always a recipe for battery destruction in very short order and in the case of larger capacity packs is likely going to result in batteries dying from internally generated heat and wires/connectors or something similar failing anyway as the system quite literally “melts down”!  It’s a joke to say you have a 4000mah 100C pack (that equates to 400A draw) and then putting a connector on it that is rated to handle, maybe 100A for short periods before it literally “melts down”!   The cells in these packs might handle 150A draw with perfect ventilation but I doubt it.  OK, rant over.

Back to sizing the system I came to the conclusion that running an 8S system with a 16×10 prop would be a good starting point with a predicted 1.3:1 thrust ratio.  Partly this choice was based on the power I wanted and partly due to available speed controllers.  Spektrum had just released a line of Avian speed controllers and one of them was capable of 8S (which I estimated was close to the biggest packs I could likely get into the airplane) and was rated at 80A continuous with 100A peaks which was as much as I really wanted to run through the EC5 connectors anyway!  It was also far less expensive than the Castle Creations options in this same range and had Telemetry built in that I was going to want.  I can do that with Castle Creations and most other speed controllers as well, but with even more expense.  This setup also allowed me to prop up a bit for additional power if I later decided I needed to, which is always a nice option to have.  The tradeoff if I did was that a 16×10 would likely give me 7 minutes or more in the air with a 5000mah pack while a 17×10 or 18×10 would start to cut into that as they would draw more amps.

In order to take advantage of the telemetry capabilities I wanted to access from the Avian speed controller I also would have to acquire a new receiver.  I settled on a new AR6610T.  That wouldn’t be a terrible expense as I had a project that needed a simple receiver like what was currently installed in the P47 so one way or another I was going to be buying one soon anyway.

Decisions made, I placed some orders and started looking to mount the motor.   This came down to simply finding the right size spacers and then a hole for the wires to get through the firewall and some judicious trimming of the dummy engine.  

The spacer length I needed turned out to be the same as some readily available standoffs for a DLE 30, which simplified things a bit.  

Then to allow the prop to clear the cowl I added a couple washers behind the prop collet assembly.

Accommodating an already existing 6S 5000mah pack plus a 2 cell pack of equal size and rating in series (which the speed controller accommodates simply as it already has a series harness attached!) was a bit tricky as the tray exposed under that hatch was simply too shallow for this setup and I didn’t want to buy several batteries for which I would have no other use.  Here’s the original space.

So, I did some cutting and regluing to create a “ramp” that allowed for easy insertion of some of the packs I had.  I had already determined that having the batteries up against the back of the firewall was appropriate to get the balance where I wanted it and while it was a bit tricky to cut out, the existing structure, thusly relocated, works well for the new battery compartment “floor”.

I needed some 2S “booster” packs (as I think of them) to add to my 6S packs in order to run an 8S system so I started looking for something appropriate.  I ended up with some APower branded 5000mah, 60C rated packs I picked up from RCBatteriesusa based in Arizona.  I had read an article about the company recently and decided to give them a try.  They are not the cheapest place to buy packs, but not at the very high end either and offer a good warranty as well as being reputed to have top notch service.  The folks there were as good as advertised in taking care of what I wanted, including the fitting of my requested EC-5 connector even though that was not an option on the web site when I first started shopping.  At no additional cost I might add.  After unpacking, examination and a few cycles they appear to be high quality.

That about covers the power system… I think.  Next time I’ll try to run down my experiences and impressions of the first dozen or so flights.

For Part 1 of this article, click here.

It was the wee hours of the morning before I really made the firm commitment in my mind that the P47 had to make the conversion to Electric.  At the next opportunity I started the process.

First order of business was to pull all the gas accoutrements from the airplane.  First, I pulled the prop and cowl and started stripping out the gas motor mount, ignition and the supporting switch, battery etc…  I knew a guy who was looking for a  DLE-20 so I made him a deal on the engine to help finance the needed items to complete the conversion.  I had the motor but needed a speed controller and maybe some additional lipo packs.

As soon as all that was out of the way it was time to explore the “hidden hatch”.  For reasons that are beyond my comprehension, Top Flite built a hatch into the top of the P47 (and other models in this line of ARF warbirds).  They framed it out in ply, put magnets in place along with pins to hold it in place… and then put a solid balsa sheet skin over the entire nose (including the hatch) and covered it with no markings or indication that it existed.  Nor is it documented in any way in the manual!

With no guidance it became quite the task to figure out how to outline and cut the hatch free.  What I eventually settled on was to take a pair of forceps with an extremely sharp needle locked in it’s jaws and reach down into the wing saddle and force it up between the two plywood plates at each end and side of the hatch until it emerged through the covering.  By doing this repeatedly, working my way around the hatch perimeter I outlined the hatch on the covering.  Then while holding my breath, holding my head at just the right angle and sending up prayers to the modeling gods (a more fickle lot I have yet to meet) I took an Xacto knife and inserted it into one of the pin holes and slid it by feel between the plywood plates.  Working my way around the perimeter of the hatch, I managed to slice my way through the vast majority of the sheeting.  With the judicious use of my razor saw, I got the hatch loose with only one problem… I managed to cut through the pins at the aft most part of the hatch! 

Don’t worry about the wasp decal… I was planning on ordering replacement graphics anyway!

As you can see it wasn’t a perfect cut but it came out well enough!

Luckily, I had on hand an appropriate sized dowel and replacement of the pins proved to be a fairly minor task.  I would later go on to add a hatch latch on the leading edge of the hatch… or at least on the new one I had to build after I found out the hard way that those magnets just aren’t strong enough!

Next time I’ll discuss the power system choices I made and how that is working out.

To see the next post on this topic click here.

One of the first things I’m always concerned about with an RC airplane is just how easy it is to get flying once I get to the field.  I really don’t want to be fiddling with 15 bolts and nuts that require special tools to attach, nor do I want to have to crawl around on the ground to assemble the airplane, etc…  At least not if I have a choice.

With the H9 Carbon Cub I felt like it was going to just be to “fiddly” to assemble as designed from H9.  The wing attachment method with screws that would have to be put in from the bottom of the wing, up in a hole and then adding in the wing strut bolt attachments, the battery placement and mounting method which was going to have to be done reaching under the wing and into the cockpit area and then a screw inserted to hold it in place??  Did they really think I was going to do that for every flight?  I found it frustrating to do with no windows in place and the cowl off!  I certainly didn’t want to try this reaching in only through the door on the side of the cockpit.

All of this screamed out for some modifications so here is what I went with.  First, I decided that I wanted to reverse the wing mount tabs.  IE. I decided to permanently mount them in the wing and make the body end be the area where any attachment task was done.  To accomplish this I needed to remove the tabs from the body.  This was fairly quick and easy with the proper application of a ball peen hammer.  Here is one still mounted in the fuselage.

They have a shoulder on the inside of the fuselage so you need to push them into the plane body if you want to remove them.   A few raps and they pushed into the body and were easily removed.

A tiny bit of wood came off with them but not enough to cause any concern.  I then used the phenolic tab material as a pattern to make replacement pieces out of similar thickness aluminum.  Since I want them to extend a bit further into the fuselage so I can attach them on that end, each of mine are about 1/2″ longer than the originals and don’t have a tab.  I did drill the hole so I could attach them to the wing (I just intend to only do this once)!

Once all cut out and the hole drilled in each I did some sanding to round the corner and cut some slots into them so I would have a way to pull them together to keep the wings from sliding on the wing tube and opening up a gap between the wing root and the fuselage. 

Even with this mod, I didn’t want to have to reach up into the cockpit through the door and put screws in, rubber bands on or attach a tie wrap or whatever.  Between this and the need to swap out batteries I decided I wanted to have a removable top window.

This meant I needed to build some sort of frame to mount the window in and create a hatch assembly.  This sounds easy until you realize that the top of the cub is a curved surface AND the opening narrows as it goes toward the rear of the airplane!  So first step was to make some custom shaped window frames for each side.  I began by tracing the shape onto some light card stock (sometimes having a greeting card crafter across the shop comes in handy!).  I then proceeded to attach the template to some wood and then cut and sand to shape.

I then used some of my finer TLAR engineering skills (That Looks About Right) to make a rounded cutout in the bottom of these pieces in order to provide clearance around the wing tube.  After that I cut a couple of appropriate length 1/4″ square sticks to use as the front and back.  Each of these is custom of course due the aforementioned narrowing of the opening.

Once that was all completed and some careful fitting done, the frame was assembled (I laid all the pieces in the place in the top of the fuse and used CA to tack glue them together then removed them from the airplane and finished the gluing process). 

After a test fit of the full (one piece) window, I carefully marked and cut off the material for the top window.  Before attaching it, I colored the frame with a black permanent marker and then glued and clamped the window material to the frame with canopy glue.

This was allowed to dry for a few hours and then the clamps came off and I added a bit more glue around the inside edges to make sure it was not coming off anytime soon.  From there I started working on ways to mount the frame to the top of the airplane.  First I made a tab out of a couple pieces of popsicle stick and glued it to the front bottom edge of the frame at an angle so it would catch the front of the fuselage.  This prevents the front from lifting.  You can see this here.

I had to just do a bit of adding a layer of popsicle stick and then sanding a bit off to make it have a close enough fit.  More black marker to cover up the wood grain took care of hiding it.

The final stage involved another stick of wood on the back edge of the frame to add some “meat” to the frame (my original 1/4″ frame I felt was just to small) and allow for the attachment of a “handle/antenna” for easy removal of the top hatch when needed, along with a block of wood to house a magnet to hold down the trailing edge.  Adding some black pin striping around the edges to give it a truly finished look was the final step in building the top window/hatch.

After all this was said and done I decided with this easy access I could use a turnbuckle to hold the wing panels together.  Its very quick and easy to put in place and because of some fitting issues at the wing root of the port wing, I needed something to put a fair amount of pressure on to hold the wings where they belong.

Here are a couple shots showing the final product.  I feel like it came out pretty decent and I wouldn’t have been happy with the original setup.

Next time, I will discuss the bottom air vent I decide to add and a couple of other minor adaptations I made.

A couple months ago my flying buddy Gary picked up a Hangar 9 Carbon Cub 15cc ARF that someone else had begun assembly on.  After he had done a bit more work on it he decided it wasn’t for him so I acquired it in turn.  I have always thought it was a beautiful airplane and wanted one so while I’m not sure how good a flying craft it will be, I decided to give it a go.

First, I had to decide what I would power it with.  For me that was an easy decision.  I doubt I will ever have another glow engine… expensive to run and messy.  Other than having good memories that pop up when I smell hot glow fuel, there’s just no good reason I can think of to put one on an airplane these days.  I may do another gas powered RC airplane one day, but not glow.  It doesn’t hurt that I happen to have a slightly used Power 60 and 100A Castle ICE sitting in a drawer.  Decision made, I started going through the manual and deciding what still needed doing to get it flying.

In addition I needed to go over the airplane and see what the previous owners had done that might need some attention.  I also started researching the airplane online and noting what I might want to add or modify.  So far I’m still not quite to full flight status!  Like most of my builds, this one has been interrupted, delayed waiting on some parts or just to give me time to decide how I wanted to do things, and has taken about 10 times as long as the manual suggests!   I’ll start to add some articles soon detailing the changes/issues and challenges I’ve had getting the Cub ready to fly.

It seems like the work on the Balsa USA Cub never ends.  Some of it is just that the BUSA kit seems to be more complicated than most and the way the designer/manufacturer chose to do things sometimes makes no sense to me.  Also, since I got this as a partially constructed kit, I’m having to go back over everything to see what parts the previous builder changed, skipped or just didn’t follow directions well!?  Of course the fact that I’m “converting” it to a super cub is not helping the process along either!

One of the things I have spent significant time on concerns the wing attachment methods.  The original plans call for a couple pegs through the wing roots and two bolts as well to hold each panel in place.  In addition the struts are load bearing.  They are bolted to the ply plate that makes up the fuselage floor and screwed into hard points on the wing.  When I first tried to assemble them I quickly found the whole process to be frustrating and overly difficult.  Getting the wings in place and holding them there while installing the bolts was difficult and that was without any windows in place!  I can’t imagine what a PITA this would be once the plane was all covered and the windows all in.  I quickly realized something had to change.  In addition, it seemed entirely possible to cause some damage to the plane while installing the wing struts as the entire weight of the wings at that point is hanging on the wing roots and the structure between doesn’t seem all that strong.  Thus the need for load bearing struts!

So I first added a wing tube to each wing and fashioned some bracing inside the top of the cockpit area to give it some strength.  Now there is something a bit more substantial to hold the wings while the struts are being installed at the flying site.   You can see the aluminum tube in the wing center joint and inside the wing panels below.  There is a carbon fiber tube inside.

The strut attachment method seems a bit ridiculous as well with no concession to ease of installation or transport.  The vertical wires that connect to the wing mid-point don’t seem to be movable or easily removable so transporting the struts looked to be a pretty interesting prospect.  Add to that the directions for attaching the struts to the wing hard points uses wood screws… which to me is just a uniquely bad idea.  How many times reassembling this bird before a screw is over tightened, stripping the wood and weakening this critical attachment point??  I understand these kits are designed to be very scale like and that most builders are going to modify all of this to make it even more so.  Maybe they don’t intend to fly the plane all that often… but for me, if it isn’t reasonably simple to assemble when I get to the field it will likely get little flight time and become a hangar queen.   I’ll take slightly less scale like and more functional and easy to get in the air over scale in this case.  BUSA might as well just say “figure out a method that will handle the stresses and that you find acceptable to assemble” , and leave it at that.  The directions they do provide seem to me to be a poor attempt at best.

After a lot of fits and starts and coming up with several plans and then rejecting them I came across some struts and connecting hardware from a 1/4 scale clipped wing cub at a swap meet.   The struts were far to short but I cut off the ends in hopes of using the attachments.

I don’t know why he had them but the gentleman had 2 or three sets so I picked up a set for myself.  I looked at them twice with great regret that they were for a clipped wing cub but then figured if nothing else I could use much of the hardware.  Once I looked closely at the hardware the wheels started spinning and I realized the hardware alone was more than worth the price.  So I created some Frankenstein struts.

Below is a snapshot of the mid-wing attachment point.  Using the threaded “eyelets” with threads tapped into the hardwood blocks in combination with the hollow aluminum tubes with built in attachment points, a 4-40 bolt and nylon insert nut makes for a secure attachment point.  The tubes and wires were trimmed and epoxied together after adding some grooves to the wire to insure the glue gets a good grip.

On the other end of the wire, I used some nylon landing gear wire straps and #2 screws to create an attachment point that is both strong and allows for an easy pivot point for storage.  This shows them pivoted down against the struts for storage.

At the outer attachment point, the main strut connections are bolted to the hard point with 4-40 bolts and blind nuts.  The ends of the wood struts were trimmed to fit inside the aluminum tube ends as well and attached with glue and screws to the wood strut ends.  This took extensive trimming and measuring to get the correct length and support the wings in the correct position.  Each is somewhat custom!  The nice thing is the ends of the struts that I recycled have a threaded rod at each end for fine adjustment.

I attached the outer aluminum ends with expanding gorilla glue and some #2 screws to “pin” them in place.  Now I have a nice pivoting attachment point that I don’t intend to disassemble often as the struts can be pivoted down to sit flat on the wing for storage.

Each strut, once adjusted on final assembly, should take only 1 bolt at the attachment point on the bottom of the body and 2 more at the mid-strut attach point in order to easily assemble or disassemble the aircraft and still provide plenty of needed support.

Combined with the wing tubes, which require a single bolt on each side to attach, the entire assembly process shouldn’t take more than about 5-10 minutes and I expect it to be both strong and fairly straight forward to accomplish.

There is still a long list of projects to get this plane ready to fly, some small like hinging of the wing surfaces, and some large like getting the cabin windows, windshield and door assemblies all finished.  More updates soon.