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My Oversized FT Explorer, Long Version

#1
If you like a longwinded story, read on. If not, skip to at least chapter 4, the summary. (and it probably could have used some proofreading). I don't know... I wrote this. Love it or leave it but please try to be nice.

Chapter 1: The Great Friend

After I built my FT Tiny Trainer from a speedbuild kit, I showed it to a friend of mine who works in printing. He's creative and crafty. One Christmas Party he brought a "meat house", which is like a gingerbread house except made out of meat and mashed potatoes for snow. Another time he modified a chocolate fountain into a Christmas Tree of ribs, with flowing barbecue sauce. His pregnancy anouncement was a 20x scale game of "Ker Plunk".

I, on the other hand, am more of software guy than a hardware guy. I never quite got the knack of scissors, and even when I try my hardest, I color outside the lines.

So when I built this FT Explorer (and it worked really well!) I was happy to show it off to him. And when I did, he tells me, "You know, I have a machine that cuts foam."

To which I replied, "Flitetest's plans are available for free on-line."

As I continued to show off my plane, I paused at the bevel cuts for the control surfaces. These were... well... less than perfect. In some places I cut too deep through the paper, while other places were cut shallow and wide. For a digital guy like me, bevel cuts are my nemesis, and despite all my effort, these were not very good. At which point he says, "Don't worry, my machine will cut those for you."

I said, "I'll have a set of plans to you on Monday." And indeed, I found the plans for the FT Explorer and emailed them to him that night.

Chapter 2: The Build

A few weeks later, he dropped off a load of foam pieces, and I couldn't have been happier. There were a few pieces of the trainer wing missing, but other than that, everything was beautiful. It was definitely a higher quality foam than the stuff from FliteTest... I estimated it was about 1.5x heavier (Looking back, I think it's closer to 2x as heavy). But it also seemed sturdier. Didn't seem like anything a more powerful motor couldn't overcome.

Oh, and the bevel cuts were perfect! Even the leading edge of the wing, where there are two 45° cuts opposite each other, were done for me. Perfectly angled and perfectly straight. I started building straight away.

I noticed minor things during the build. Despite the foam being the same thickness, folds were more difficult. The paper was thicker, so I had to pay more attention to get a nice 90° bend, but also, the bends were leaving a small gap. This, unfortunately, required a bit of extra glue. It wasn't really enough to worry about, but it caught my eye.

See, when I'd sent the plans, I knew it was a pusher and might be great for some FPV, but I hadn't really looked at the details. And pictures don't really show size. It hadn't even occurred to me that this wasn't one of their mini builds. I was expecting something about the size of the tiny trainer, but clearly this was going to be much larger. I looked it up. A 54" wingspan. Oh my. My wife was already complaining about all the space my new hobbies were taking up.

Anyhow, the fuselage and nose came together great. When I built the wing, I really should have known something was up. A more experienced builder would have said, "This is too big." But I am not a more experienced builder. I'm 5'11", so quite a bit over the 54" wingspan in the plans. However, I measured, and if you ignore the tapered sections, wingspan where it's "full chord" was almost exactly 54". A more experienced builder would have said, "This wing is too big." I said, "Huh, I didn't know you measured wingspan that way."

Chapter 3: The Motor

At some point during the build process, I knew I needed to order a motor and battery. I knew the plane was going to be heavy, but I wasn't exactly sure how heavy. I read lots on motor sizing. It seems like every one of them starts with the gross or "All Up Weight" of the aircraft, which includes the weight of the motor (which is what I'm trying to pick!), and the battery (which kind of depends on the motor, I thought). A lot of them made clear that really it's best to start with the prop, and the FT store was clear that you need to order a 9060 prop for the FT Explorer because the 10" proper with their power pack won't fit. Little did they know...

I didn't take differential equations, so I was never really able to solve the problem of "what motor, prop, and battery" should I use. And I ended up taking a stab in the dark. There was a deal on an EMax GT2218/09 1100KV motor, and it looked like that would drive a 9060, so I went for it.

I also didn't want to make my own control horns or firewalls, so while those were on their way to me on the slow boat from China, I placed an order from flitetest for the various wood pieces. I probably tacked on a bunch of other things I didn't really need, because that's how I roll.

When everything arrived, I had a problem. The firewall was too small. I checked and doublechecked, and I'd ordered the right part. The balsa parts, assembled together with the motor mounted, had about a gap of about a foam board's thickness on both sides.

And, after all those clues, this was the first time I bothered to compare the foam pieces to the scale on the designs. Sure enough, when I compared the "1 inch = 1 inch" on the design to my ruler, "1 inch = 1 1/3 inches".

Chapter 4: The Summary

So, like my plane, I'd been a little dense.

I now had an FT Explorer that:
a) Was made of foam that was 1.5x-2.0x as heavy as it was supposed to be.
b) Had a non-negligible amount of extra glue to fill in gaps, because cuts for bends were 33% wider than the foam board thickness.
c) Was a full 1/3 larger than it was supposed to be.
d) Had a powerplant that was just slightly better planned than a random guess.

So I was pretty sure I needed more power, as well as a bunch of weight to counter the balance issues I was sure to have. So instead of ordering an appropriate battery for the motor/esc/prop, I decided I'd try it with one of my 1550mAh 4S batteries. Even with the 4S at the front, it was still tail heavy, so I also mounted my cheap Chinesium ActionCam to the nose.

I also read about some other folks--folks with normal sized Explorers built with normal weight foam--who were having problems with wings folding. So I strengthened the wing with a carbon fiber flat. And honestly, I think that's the one thing on this build I trust.

I set up my transmitter, receiver, and all the electronics. I ran some basic tests, and the thing was able to push itself across the floor at a relatively low throttle, which I took as an early success. I decided it was going to be a bit too terrifying to hand launch, so I built a set of landing gears out of those flags you find in your yard that mark underground pipes (I'd been told they work well).

Only thing left was to maiden this thing. Would it fly at all? Would the ESC and motor survive?

Chapter 5: The Maiden

The maiden went .. poorly. I do have video from the camera, but it's not worth watching, because it's clear blue sky until it's dirt. There's some funny banter, but I need to edit it down before it's ready for public consumption.

The first part to fail was the landing gears. Turns out free yard flag wire probably only works well for smaller models. For this model, it went forward about 3ft and then the struts collapsed on themselves and the nose went into the dirt. Fortunately, my friend was a good sport and willing to try hand launching.

The Wright brother's first flight went 852 ft. I've always thought this was a disappointingly unimpressive number, and wondered if it really classifies as a "first flight". This Explorer's first flight went about 150ft if I'm being generous, so I owe Wilbur and Orville an apology. It was definitely flight. It was even controlled flight, as I managed to land it safely.

Second and third launches were nose dives due to a less than enthusiastic toss by my friend. We had to have a brief discussion about the physics of the situation.

Chapter 6: The Fourth Flight

The fourth flight was actually decent. Full throttle plus a good hard toss, and it was in the air. And nearly full back on the elevator to stay in a gentle climb. At full elevator, it would flip vertical and stall, but at anything less than about 90% deflection, it would dive.

Similarly with the throttle. At full throttle (and nearly full elevator), it was climbing. When I dropped throttle at all, it seemed to dive. Aileron control also seemed excessive, as there were a few times where I wondered if I was inverted.

All of this was happening in a vert short amount of time, plus I'm inexperienced, so my recollection is imperfect. What I do know is that I was able to turn around it around. I think I even completed a full circle once. I know I accidentally did a flip when I thought it was stable and dropped throttle to "test the glide slope", and then had to immediately put it back due to the nosedive.

I recovered from that a little, but never really had control again. I ended up crashing into a fence at the perimeter of my flying area, which sent the nosecone, batteries, and electronics flying. It looked like total devastation from where we were sitting, but as we gathered parts, I came home with everything except one single servo extension cable. With hot glue, tape, and some spare foam board, the Explorer is in good enough shape to make another attempt. Nose and fuselage have some creases and rips, but they've been repaired.

She'll fly (and crash) again. My goal is to figure out what will lessen the crashes.

Chapter 7: My Thoughts and Questions

Yeah yeah yeah... build with the right materials, at the right scale, etc. That certainly would have helped. But lessons:

a) I probably should have paid better attention to CG. I had it set pretty well when everything was in place, and I checked it again before the second launch, but after the two failed throws, I don't think I checked it again. And after reviewing the video, the camera was now pointed up which would have shifted weight backward. Honestly, CG was close to begin with, and I suspect after the camera was displaced, it was probably tail heavy. I should probably add more weight to the nose.

b) A better maiden plan would have been appropriate. My plan on the first launch was to fly for a bit, try to keep it straight and level, and then land. It went poorly, so I should have repeated that test until successful.

c) I think it would benefit from more power. AUW with the current motor but no battery is about 1275g, plus another 180g for my 1550 4S battery, and another 150g or so to balance it out (and a bit more if I'm to put the tail as far back as recommended). Total AUW in its current configuration was 1.6kg (3.5lbs) with an EMax GT2218/09 1100KV motor, 9060 prop, and a 30A ESC. It would definitely handle an 11" prop, and probably a 12". I have 4S batteries aplenty. Can anybody recommend a power plant?
 
#2
Well, I'm not really new to the hobby, but I know relatively little about motor/prop/battery combos. I do know that the bigger the battery, the smaller the prop must be. If you increase the number of cells, it increases the number of rpms. This in turn means you need a smaller prop, less you want to burn up a motor. I would suggest finding a motor with low rpm number and high torque. This will give you a nice weight tolerance.

Someone more knowledgeable please correct me if I'm wrong :black_eyed:

Cheers,
Dark
 
#3
i think i have used similar foamboard to build before, i recommend a much beefier power plant for that sort of size plane, perhaps a ntm 35-36 or similar size that will swing a 10-11 inch prop on a 3 or 4 cell. when i have built with this my planes come out nearly twice as heavy, but still fly well with a little extra power.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
Simple rule for fixed wing is 100 to 200 Watts per pound!

100 Watt for a slow lumbering design and 200 for a sport design or spirited performance.

Weigh the plane, select a motor of between 100 and 200 Watts per pound of aircraft weight, pick the maximum prop that your motor can use for the selected battery voltage, select battery voltage based upon maximum current you want to draw. I try to keep ESC to around 30A max and if 3S motor of required power draws more then my current requirement at 3S I up the voltage to 4S and search for a motor of the right power which will give the power and not exceed the current I want. The again select prop etc.

It works for me!

Have fun!
 

foamtest

Toothpick glider kid
#5
I did some research on the motor you have and it claims that it can only handle a 3s but here is the spec sheet I found if you're interested.
GT motor specs.PNG
I circled the motor you have in red, I hope this sheet helps in some way!