My microflyer scratch build

My microflyer scratch build

Postby SamIam » Mon Dec 03, 2012 9:08 pm

OK, time to get my hands dirty. I've got the pyjamas and now it's time to start mopping up the floor with bozos. :lol:

My goal is to build an indoor microflyer that I can beat around in the gym. I cut my teeth on micro helis and planks just don't feel natural. Most of you have seen my Parkzone Ultra-Micro Corsair in the air (or on the floor or maybe heading into the wall as the case may be). I know it's not a good indoor flyer and really didn't expect it to be. But I was at least hoping to be able to burn a lap or two with it, and land in one piece. Anyway, rather than chew my beautiful little war bird to death learning to fly wings, I thought a scratch build would be more educational, and repairable. So hear goes...

Using Doug's Ember for reference (16.5" wingspan), I'm thinking something around an 18" wingspan in the style of a Cessna Skymaster. The Skymaster has always been a favourite of mine and it's a top wing, front wheel design so it should be a little easier on the thumbs (right?). I'm not going to try recreate a true scale skymaster, just the wing/twin tail boom concept with a (single) pusher prop. Wheels may or may not be included. So there's the inspiration.

OK, getting back to Doug's Ember, I'm shooting for a wingspan of approx 18" and an all up (no battery) weight of about 20 grams. Here's what I've got so far:

(all parts from Hobby King)
Receiver: OrangeRx R415
Motor: brushless outrunner HK-AP02-7000 (7000 Kv)
Esc: HK030
Servos (x2): HK1300

Just for fun, I'm re-using an old micro corsair prop (3 blade). I could find a more efficient prop, but the corsair prop just plain looks cool!

All the parts have been soldered directly. The servos are wired for rudder and elevator (aileron in the future?), the motor is direct drive.

Total weight of the control system (everything but the battery): 8.85 grams
Total static thrust: 18 grams (initial peak of 20).

I could do better on the thrust. A better prop and maybe a gear reduction. But this should get me in the air. I couldn't measure the current draw of the motor under load, so I hope I'm not over doing it. Should be OK unless I'm flying WOT the whole time.

So far so good. If it weren't for the air frame, it'd be flying already! If I can keep the air frame under 10 grams (ya, right) I've even got a 3D flyer.

OK, so a 10 gram frame is going to be a stretch, but then again my Corsair is listed has having a flying weight of 41.8g. Hmm... does that include battery? Let's say the corsair has a weight of 37.5g. I think I can beat that (with more wing area to boot).

So here's some pictures until the next phase.
Total parts weigh in: 8.85 grams
all_up.jpg (48.36 KiB) Viewed 12513 times
ESC: 3.5A 0.35 grams
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AP02 motor: 2.4 grams
motor_AP02.jpg (21.74 KiB) Viewed 12513 times
Last edited by SamIam on Sun Apr 07, 2013 6:53 pm, edited 1 time in total.
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Joined: Mon Feb 13, 2012 5:46 pm

Re: My microflyer scratch build

Postby SamIam » Sun Feb 17, 2013 11:05 am

No bites yet? Ok, here's some inspiration...

Possibly one of the most beautiful twin engine private pilot aircraft ever designed.
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post1.jpg (96.57 KiB) Viewed 12453 times
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Re: My microflyer scratch build

Postby SamIam » Sun Feb 17, 2013 11:55 am

The beauty of this design is in the rear prop mounted between booms. For an indoor slow flyer, that means the dangerous moving parts are protected and the risk of accident damage goes way down. That's assuming the front prop is just for show. Another advantage is the control surfaces (or at least the elevator) are directly in the prop wash, so control should be good.

The disadvantage is the extra weight this puts behind the wing, and tail heavy is never good.

So here's my first test bed, the Mark I:

This was not an attempt at 'scale', just something to prove it can fly.

The airplane was constructed from foam dinner plates. You don't get much of a continuous flat section out of a round plate, but between the centre wing section and the outer wing sections, there's enough there. I took advantage of the foam plate's natural curve around the perimeter to build in a little washout in the tips. The leading edge was curled down by pressing the edge of a metal ruler down on the bottom side and 'folding' it over a bunch of times. The rudders and elevator were made from egg package foam. I thought this would be a thinner foam, but it turns to be not much thinner than plate foam (I won't bother with egg packages again). The whole thing was glued with hot glue.

Total wing span is 48cm (19") , finished weight 25.5g (no battery). It's not clear in these pictures but I added tricycle landing gear. I wanted the landing gear to keep the prop clear, but by the time they stuck out far enough to prevent ground strikes, the gear was comically over sized. The prop is really over sized for scale.

The elevator is a single moving surface. It pivots on a carbon fibre shaft that spans from one boom to the other. The rudders are as close to a single moving surface as I could make them. There's another thin carbon fibre shaft that connects the rudders so they both move.

My construction was seriously tail heavy. I had to mount the receiver as far forward as the wires would allow, then added a boom extension in front of that to mount the battery so it would be even further forward. Even after all that it took another two pennies in the front balance out. Not good.

The first test flights were very promising. I was a little worried that the rudder might be weak, especially taxiing, but the rudder control was strong and I could burn it around on the gym floor like a race car. I even turned my rudder rates down 50%. The elevator was waaay over controlled. Rates down to 50% and that was still too sensitive. I managed to get a lap out of it. The power, weight and flying speed seemed appropriate for the space. Initially I was flying tail heavy and after a crash or two I added the pennies for balance. The extra pennies were a bit much however, and the maximum power was right on the edge. A couple more crashes later and the delicate control linkages were breaking loose and thus ended the flight.

mark1.1.jpg (21.91 KiB) Viewed 12453 times
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So what have I learned...

- this size and weight looks like it will be a very good gym flyer.
- the control surfaces can be much smaller (maybe even scale).
- hot glue isn't very precise and it's far too easy to add weight (but it's darn quick for construction).
- the tail has to be lighter, very much lighter.

For the next test bed Mark II, here's what I'm going to change:

- Move the motor as far forward as possible and use a geared shaft drive to the prop. That should really help with the tail heavy.
- Cut back the control surfaces. Get back to a more scale like fixed/moving proportions.
- Find a good foam glue that is stronger and lighter than hot glue.
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Joined: Mon Feb 13, 2012 5:46 pm

Re: My microflyer scratch build

Postby SamIam » Sun Feb 17, 2013 1:28 pm

OK, a little diversion here.

My end game is to create the 'ultimate' micro scale Cessna Skymaster. Getting a flying platform is one part of that goal, making a scale fuselage is the other part. So here's my progress on the scale side of things. It doesn't have much to do with flying ;)

My plan is simple (well, in my mind anyway). Make a super scale plug, then create a silicon mold from the plug, and make super light weight scale fuselages from the molds. If I get the mold right, I can turn out hundreds of fuselages, playing with different techniques to get it lighter.

The plug is not intended to fly!! It can be made of lead if that's what you're good at working with. Just in case anybody sees this and starts thinking it'll be too heavy to fly :o

So starting with the plug, I'm going to use a combination of plate foam (flat tail booms) and florist foam as a base. The flat tail boom I've re-enforced with a bamboo skewer so it doesn't fold in half as I work on it. The florist foam I bought from Micheals. This is the stuff decorators use to stick flowers in to make arrangements. There's a 'wet' type and a 'dry' type. The wet type is meant to be used soaked in water with live flowers, so it's extra soft (don't use it). The 'dry' type is meant for plastic flowers, so it has a little more strength. It comes in a 5 block pack and it's super cheap. The florist foam is really 'soft' and easy to work with. You could literally carve it with a butter knife and it takes very little sanding to bring it to shape. That's a big plus. On the downside, florist foam is really 'soft' and crumbly, so it won't stand for much handling as you'll see later.

Foam Plate tail boom
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Florist foam. Sold in a 5 block pack.
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Using cross sections from the plan, I cut the florist foam into blocks and glue the cross sections together. I use plain old white paper glue. Each half was done separately so that I could use a paper template down the middle as well as for each cross section. This way it's easy to line up the centre lines. Before gluing a section together I carefully sanded the block down to the edge of the paper template so I could find it easily once all the blocks are glued together. The florist foam is really easy to sand. The only hard part about this is getting nice 90 degree flat sides in each block, at exactly the right cross section thickness. This would be so much easier if I had a flat sanding table of some sort to work with, but as it is I had to eyeball each block for both thickness and angle. Some blocks I had to redo (over sanded) and I probably could have been more accurate over all. Careful that you don't over sand, it will mean more filling later. It's much more fun to sand florist foam than it is to sand filler :lol:

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Cool huh?
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Once the foam base has been shaped to size, it's time to smooth it out. I've been using an interior grade wood filler (Elmer's brand carpenter's wood filler). I really like this filler. It's water based (interior use only), so it doesn't eat away at things like, well, foam, and it seems to stick really good too. It's easy to smooth out by moistening it with more water. It can be thinned to a slurry and spread smoothly. And it sands nice and smooth. Because it's water based, you can't lay it on too thick (it will take a very long time to dry and probably shrink and crack in the process), but for this use, that's not a problem. I start by coating the entire foam plug with a layer of filler. It helps to water it down (just a smidgen). I'll squeeze some out on my cutting board, and use a wet wooden stir stick to mix it into something I can easily spread.

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Next comes the tedious part. After the filler has dried (it won't take long), it's time to sand. And sand. And sand some more. Think you're done sanding? Nope, sand again. Here's where the florist foam shows it's ugly side. It's very 'crumbly' and it doesn't take much sanding pressure to break away and then you end up with road rash. There's not much you can do about this, just sand to the final form, re-fill the holes with more filler, then re-sand until the whole plug is smooth again (and again and again). Next time I think I'll primer the whole foam plug heavily first to seal and lock the foam surface. That might save work.

Road Rash!!!
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When the shape starts getting close, I'll spray the whole thing in primer. It's easier to sand down filler than it is dried primer. So try not to primer if you think you have 'high' spots. So far I haven't found anything that eats florists foam, but the plate foam used in the tail boom was attacked where ever the foam was exposed. That's OK, just add more filler and sand it smooth again. Eventually you'll get a nice protective layer of primer and filler.

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Fill, sand, prime, fill, sand, prime, fill, sand prime. Yes it's a boring as it sounds but eventually it will start to look like something. That's when I switch to another paint colour.

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Looks pretty good until you start sanding it down again (nothing a little more filling, sanding, priming wont' fix).

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So that's where I'm at. The drawings and cross sections don't quite do justice to the nose and rear deck scoop, so I've got plastic model kits on the way to give me more of a visual guide for getting them just right. The plan is to get the form and shape of the plugs perfect, then cut/add in the details. Panel lines, rivets, window outlines, etc, etc. All of that detail will be picked up in the mold I make later.
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Re: My microflyer scratch build

Postby RCPrairieFlyyer » Mon Mar 04, 2013 3:16 pm

Wow, is that looking good. I can't wait to see it completed.
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Joined: Sun Feb 26, 2012 9:07 am

Re: My microflyer scratch build

Postby SamIam » Fri Mar 08, 2013 3:31 pm


Here's a few more updates. It's been really difficult finding time to spend on this.

My camera is really finicky. It will take the most beautiful up close macro pictures, but darned if I can get it to focus. There never seems to be the right balance of light, contrast and planet alignment to get a perfect shot every time, and the on camera display doesn't show enough detail to know if the image is razor sharp or just fuzzy. Lots of repeats :( .

I've switched to a darker shade of primer and added more detail to the tail. Note the tail light fairing and the bulge at the top of the rudder.

I've included my thumb in the picture to get a better idea of the scale here. The camera really picks up the flaws! You can see there's still some work to go here. Every layer of filler and primer gets it that much closer. I'm obsessive with the perfection, but it will really pay off when I make the moulds.

Tail fin progress
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Rudder top bulge. The corners need some sharpening up.
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The lower tail light housing.
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Once the basic form of the fin is complete, I'll scribe in panel lines and add rivets (yes rivets). I wish I could say both fins on this twin boom model are identical in every way, but they're not. There's the elevator attachment point on opposite sides (obviously), but there's also a bulge detail on the outside and a top navigation light on the right tail fin, as wall as a few odd access panels that are on opposite sides as well. So I'm going to detail in everything I can that's the same, make a mould, then make two master copies from the mould and detail each of those for their respective left and right sides, and finally make moulds of the left and right booms. I don't think it will be as much work as it sounds. Making the first generic master is the bulk of the work.

I've also done more work on the body. The basic frame is a no brainer, but the rear scoop and the nose detail are hard to get right. I've looked at dozens of pictures, model plans and airplane kits. I'm only now starting to understand the true shape of the scoop and how it flows into the rest of the body. I really wish there were a source for Cessna's very own manufacturing plans to get these details exactly right. The nose is even harder. At the end of the day, it's going to be up to my artistic interpretation and sanding skills to get it right.

Rear scoop - still a work in progress
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I don't know if I'll ever get the nose right
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It helps to have a reference model. Got my hands on a 1:48 scale "Cessna O-2 Skymaster" (the military version). I wish it were bigger,and it's not without it's own inaccuracies. But it will really help with getting the shapes, panel lines and rivets sorted out.

Helps to have a model for reference. Wish it were bigger.
mold17.jpg (47.4 KiB) Viewed 12420 times
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