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-   -   70% Astro Hog scratchbuild - Finished! (https://www.wattflyer.com/forums/showthread.php?t=53575)

aramid 02-04-2010 04:02 AM

70% Astro Hog scratchbuild - Finished!
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EDIT: Project is completed; details in post #24, photos in post #26.

That's 70% of the original 72" Astro Hog, not some insane IMAA-legal behemoth.

The plane I learned to fly on was a glow-powered Sig Astro Hog. Over the last ten years, this plane has acted as my primary trainer, aerobatic trainer, and more recently, relaxing toy. Obviously, I have a bit of a soft spot for the thing, and I decided I'd like to build an electric one. A few months ago, forum member Andrew provided me with a PDF of plans for a 36" Astro Hog, half the size of the original model. This was very tempting, and I looked it over for a while trying to decide the best approach to building.

In the end, I decided it was just too small - all my batteries are too heavy, and I didn't really want to buy new ones just for one small plane, especially since I'm aiming to up-size my fleet. I decided to have a copy place print the plans at 125% - that should give me a 45" span, 60% of the original plane, and perfect for the ~200 watt motors and 2Ah batteries I use in the rest of my planes.

Unfortunately, due to human or technological error, what I got instead was 140% prints - 50", 70% of the original plane. That sounds like a small difference, but it's actually 20% more wing area with a similar or greater increase in weight. I didn't notice until I had already started building, or I would have corrected it; as it turns out I'm making a bigger plane than I meant to.

So far, I've got the wing nearly finished (sorry for the cell phone picture, but that's all I had - I promise better shots in the future). All I have left is some sheeting on the wingtips, plus some rails to hold aileron servos.


When I still thought it was a 45" model, I was estimating 28 ounces and planning on a 3530-14 motor from HeadsUp; with the extra size that's not going to be enough power. I've given up trying to guess a weight and decided not to pick a new power system until the plane is finished, but I've looked around to see what my choices are. I'd like to see close to 150 watts per pound, so I suspect I'll be buying some 4s packs to finish it off. That's not a big deal, since I'm sure I'll need them for something else before long.

I'd like to hear your thoughts, comments, and suggestions. I'm sure plenty of you have flown a Hog in the past!

balsa1 02-04-2010 04:38 AM

Hi yes I have one I built in the early to mid 80's in fact I have it down geting ready to recover and I have been thought to e-power Ive been at model building for over 50 years and so have a large fleet to repower. Had a stroke about 5 years ago and just now getting back to modeling a lot has changed but not the reliablel Hog. will wathch your build with great interest maybe pickup some pointers on rework for mine good flying Les

Marc 02-04-2010 05:59 AM

Okay, arimid, I'm going to follow this thread. I'd like to see a picture of the completed wing before you cover it so I can see how the ailerons are actuated. From the size of the holes it looks like you are planning to use 9-gram servos. Chellie would be proud if you did. :)

I hope that the 3530-14 motor you planned to use would not have been salvaged from your Sig Wonder. Mine is ready to fly for a second season. How about a Turnigy SK 35-42, 1250Kv rigged 3S2P with your existing lipos.

aramid 02-04-2010 12:46 PM

I will indeed be using 9-gram servos, but there's no support structure in place at all right now. The holes I think you're looking at are for wiring; the servos themselves will be outboard in the wings, centered on each aileron with a direct linkage rather than a torque tube.

And of course I wasn't planning on parting out my Wonder - I picked up a second motor for the Hog. Since I won't be using it on this project after all, I'll have to find something else to put the 3530 on. I've already considered a 3542, and it's a definite possibility. Hadn't thought about paralleling my current batteries; I'll definitely keep that in mind. We'll have to see where my weight ends up, though. I use Zippy batteries, so buying a few 4s packs isn't an expensive proposition if that's what it takes to get the results I want.

pd1 02-04-2010 08:11 PM

I think the 70% sized Astro Hog is the perfect size.
While I do like the smaller planes, this one will be easier to see with old eyes.
I have a 70% Taurus, I'll guess your final weight around 34 ounces.
Looking forward to watching your progress.


aramid 02-06-2010 01:52 AM

4 Attachment(s)
Progress continues slowly, but now there are much better pictures of my lack of progress!

I didn't have a lot of time to work on it today, but I did get the wingtips sheeted. They still need some sanding to clean up the edges and smooth things out. Those dark lines where the sheeting meets aren't nearly as bad as they look; a lot of the visibility is from the permanent marker I use to plan my cuts, so that will also improve with sanding.


I expected the tip sheeting to be incredibly difficult to get right, but it actually didn't take all that much effort. Since I hadn't planned ahead and left part of the outer rib exposed to rest the sheeting on, I had to build a little shelf to hold the tip sheets flush with the rest of the wing. There are also a few small stringers in place to prevent the sheet from collapsing:


Here you can see that my need for sanding isn't limited to the tips; the cap strips and trailing edge sheet need smoothed into the rest of the structure, especially since the ribs are very soft and I crushed them slightly when clamping the trailing edge in place:


The orange wires in some of the above photos are what I'll be using to pull the aileron servo leads through. The square holes are where they'll enter the fuselage; Marc, I think these are what you were looking at when you guessed 9-gram servos, although maybe you guessed correctly based on the lack of a mounting position for a full-size servo.


In the future, I won't be posting such trivial amounts of progress. I only posted tonight to get some better pictures up (and maybe because I was slightly bored). Anyway, I'll keep you updated as things progress.

Marc 02-07-2010 01:23 AM

Looks good! Just be sure the little orange wire doesn't pull off the aileron leads or you'll do surgery on your beautiful covering. Looks like you'll have built-up ailerons.

aramid 02-09-2010 11:58 PM

I've gotten the ailerons more or less put together, so the wing is now complete aside from a bit of sanding. I started on the fuselage today, and I've been struggling with downthrust. The plans show none at all, but I'm not sure I trust that suggestion. My only real reference point is my Wonder, which has 4 down, and I'm fairly sure that's not quite enough for that plane. Then again, it's an unconventional design with a high wing and very short tail moment, and might respond differently.

My first inclination (based on the Wonder) was to put in 5 down, but that looks like a lot when I lay the parts out. Does anyone have any advice for downthrust on a conventionally-proportioned airplane? For reference, the airfoil is nearly symmetrical, both wing and tail are mounted with zero incidence, and the wing sits about three inches below the thrust line. Actually, since it's a low-wing plane, do I really need the downthrust at all? I suspect it's far more important with a high-wing design.

I'm not going to bother with right-thrust on the firewall, though I might put some in with washers under the motor mount.

PaperAirplane 02-10-2010 12:32 AM


Sky Sharkster 02-10-2010 01:24 AM

Thrust Adjustments?
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Hello Aramid,
For what it's worth, my 1/2 sized Astro Hog, built from SIG plans modified for electric, flys great with no downthrust, but I did end up putting about 2 degrees of right-thrust in (one washer).
The model has no tendency to climb under power, it basically "Goes where you point it".

Marc 02-10-2010 01:46 AM

Well, aramid, I thought we put the issue of downthrust to rest some time ago. But since you brought it up again, I'll add my two bits. ;)

Our Wonders have an atypical amount of downthrust and perfectly symetrical airfoils. I have noticed that when I fly inverted I need to apply quite a bit of down elevator to maintain level flight. You have probably experienced the same thing with your Wonder. I attribute this characteristic to excessive downthrust. So my question is: "If downthrust keeps the nose from rising, why do I need to apply so much down elevator when flying inverted with what amounts to upthrust?". :rolleyes:

Two or three degrees should be sufficient in my opinion.

aramid 02-10-2010 04:44 AM

After I posted that question, I stopped by a friend's house to help him with his House of Balsa Spacewalker. While I was there, I measured his downthrust at 2. Based on that and Sharkster's plane, I know my 5 was every bit as excessive as it looked. I'm not sure what the small differences are between the Hog and the Spacewalker to make them fly well with different thrust angles, but I may go ahead and play this one by the plans with a flat firewall (I think the Spacewalker has a less symmetrical airfoil; that may be the difference). I definitely won't go past 2.

I'll have to think about adding that right-thrust to the firewall, though - I'm planning to mount the motor with standoffs and I'd rather keep the thrust on-axis to the mount.

Originally Posted by Marc (Post 691202)
If downthrust keeps the nose from rising, why do I need to apply so much down elevator when flying inverted with what amounts to upthrust?

The plane is trimmed to hold a slight positive angle of attack in upright flight (otherwise a symmetrical airfoil would produce no lift). Bear in mind that this won't necessarily be trim in the control surfaces, but when the fuselage and everything are considered, there's a slight aerodynamic climbing tendency in the plane. When you flip the plane over, you're still trimmed to be slightly nose-high, only now it's pointing you towards the ground instead of away from it. You're basically holding down to counteract this trim. Want to test and make sure it's not downthrust? Cut the throttle inverted - I'll bet you still have to hold that down elevator in there!

Thanks for the quick responses!

aramid 02-13-2010 12:01 AM

9 Attachment(s)
I've made quite a bit more progress since I last posted pictures. First, the tail pieces are cut and shaped. As you can see, I decided not to go with the shapes drawn on the plans, as they were a bit too Cub-like for my tastes, and very different from the original Astro Hog. My horizontal stabilizer is almost exactly the same shape as the original, while the vertical is more unique.

Both parts of the stabilizer will probably get lightening holes and a 1/16" strip at the 1/4-chord location both top and bottom, to create a bit of an airfoil effect.

Next, I got started on the fuselage. Here's what the goal looks like:

Now, there are a few issues with this. First, I despise wing-mounted landing gear. Second, the firewall needs moved. Third, airplane wings are not held on by rubber bands. Fourth, I need an access hatch for my electronics. Fifth, the horizontal stabilizer needs to be on the centerline, not the bottom of the tail. All five of these issues will be corrected. Additionally, I'm using the recommended material sizes or thinner, even though my plane is bigger than the plans were written for. I'm adding some reinforcements in key places to deal with this, as well as counting on the lack of vibration to keep things intact.

Here, you can see the fuselage neatly held in a jig (my grandpa made the jig from plans in one of the magazines many years ago):

The fuselage is being built upside-down in the jig, and the rounded turtledeck will be added once the rest of the fuselage is complete and all the control rods have been run. Note the additional diagonal bracing to make up for the thin material.

Here's the wing saddle area, with a plywood former in front for wing dowels, and hardwood blocks in back to hold bolts. The sides of the fuselage in this area are doubled with 1/32" aircraft ply, and the wing seats have yet another strip of balsa to add stiffness and spread wing loads.

Finally, here's the firewall:

The firewall is 1/8" aircraft ply, sandwiched in place between triangle stock. The blind nuts are spaced for a typical 35mm diameter outrunner mount, and standoffs will be used to position the motor correctly (I can't decide a motor length until the plane is done and weighed). The middle-sized hole is for motor wires, and the biggest opening is a cooling channel.

And, just for fun, here's this plane's grandpa, overseeing the proceedings:

capn chaos 02-13-2010 01:03 AM

Good looking plane and definitely like your thinking in the details. Workmanship is 1st class!:D Havent done the math but I take it the WS is in the 50"+/- range. Any target weights yet.
Ill be standing by in the wings watching...gotta build one of those fuselage jigs:ws:
Capn Chaos

aramid 02-14-2010 04:48 AM

Yep, wingspan is right at 50". Unfortunately, I have no idea what to expect for the final weight. I'll be content with anything under three pounds, although I'd like to see closer to two.

Thanks for the comment!

ENUT 02-14-2010 02:25 PM


aramid 02-21-2010 05:27 AM

Fuselage Progress!
9 Attachment(s)
Another week, another progress post. The fuselage is out of the jig and starting to take its final shape:


A look at the nose shows the ridiculously thick sheeting making up the lower half of the cowl. The upper half will be attached to the main hatch for easy access to the motor.


Inside the nose, it's not quite as pretty; I was forced to cut wedges out of the sheeting to make the sharp curve without any splitting, so now there's some lightweight filler smoothing things out on the inside (no, there's no good reason for the inside to be smooth, but if you're going to do something you might as well do it thoroughly):


Moving back behind the firewall, you can see the 1/4" aircraft ply landing gear mount; the gear will be mounted with nylon bolts, so if the undercarriage decides to part ways it won't be taking any wood with it:


Further back, just above the wing saddle you can barely make out the lightweight push-rod sleeves for the rudder and elevator servos. HXT900s will be installed upside-down near the top of the fuse, keeping them well out of the way but allowing very easy access with the wing removed:


Back further still, we have the rest of the tail. The push-rod sleeves can be seen here; each one runs down a side of the fuselage before crossing over to exit at the tail. These are very carefully routed to have minimal bends; they are extremely smooth and won't add any stress to the servos:


Here's what the sheeted underside looks like:


The large opening behind the wing is the cooling air exhaust; it's probably overkill but looks decent and can't hurt, especially considering the motor is fully cowled and there's no air outlet for it up front:


Finally, here's a first taste of the assembled project:


The current weight for everything you see here, plus the ailerons, vertical stabilizer, and aluminum landing gear, is exactly 14 oz. I'm getting curious about the final weight again.

Dereck 02-22-2010 03:01 PM

This one cool model!

Your air outlet should do fine. I've used similar on my own design higher powered models and it ensures you get the hot air out without adding a lot of drag to the airframe.

Years ago, I 'designed' a large scale model of the Smog Hog - Astro Hog's high winged predecessor, at 53" span. It was a blast to fly on rudder/ele/throttle, though somewhat smelly thanks to the PAW 19 diesel - electricity on our present level hadn't been invented nor marketed then. It used to confuse regular RC fliers who couldn't figure out four point rolls and Cuban eights from a model with no ailerons, but was great fun to fly.

Yours should be a blast, good to look at - and easier to haul around! If you keep it as light as possible, you'll have good performance - a buddy of mine over in Delaware has flown his 'Pollutionless Pig' for years on 'ancient' nimh batteries and it really looks the part in flight. It's a slightly smaller Hog, with a lighter, open frame structure.

Over on some other little e-flight forum, there's a tale of a similar sized 'Taurus' that's flying well on 3S at 2lb 11oz,



aramid 03-09-2010 07:06 PM

Balsa work nearly done!
9 Attachment(s)
Well, I'm about a week and a half late on my weekly update, but at least there's progress to report. The woodworking is probably 90% complete; all that's left is to attach the tail, finish sand, and cover!

From the front, the wingtips look a little strange; I would prefer more curvature but it's nearly impossible to sheet a compound curve and I really didn't want shaped-block tips. The landing gear in this picture aren't going to be used on the final assembly, as they're nowhere near wide enough:


Here you can just make out the access hatch for the electronics; it runs from the firewall back to the front of the canopy:


Here's a look at the cowl. It's pinned into the firewall and bolted down at the front, allowing for very clear access to the motor for installation and maintenance, but keeping a safe, secure hold the rest of the time. The lower sheeting will be cut out a bit more for clearance, and the top will either be thinned or rounded for a cleaner look:


Further back, this is the view when the access hatch is removed. I'll be building a battery shelf in here once I've got final weight and balance worked out:


Now with the wing removed and the fuselage inverted, you can see the tail servos tucked up in the top of the plane, out of the way but easily accessible:


And, because Marc was interested, the wing servo pockets. They should be pretty easy to cover up to, and should be plenty strong for 9-gram servos:


That's probably about the last you'll see of this plane until it's covered. I'll try to get a shot or two once the tail is mounted and faired in, but there's not going to be much more to show for a few weeks.

Dereck 03-09-2010 11:03 PM

Prefer your fuselage mounted UC too. Much easier to locate in the correct position and keep it allied to the fuselage too. I used to use wing mounted wire gear in my slimer days, but wandered off when I caught electricity on account of I didn't like those big batteries between UC legs way out on the wings. The older designs tended to their wing mount torsion bar legs being way too far back in the wings - if you dropped in a little heavy, as some of us do on rare occasions ;), the wheels are pushed up towards the wing LE, and the rebound throws the now no longer flying model upwards with a fair amount of force. I put my torsion bar rigs just aft of the LE, so the landing forces went up into the wing and didn't distort the UC too much.

It's much easier to fit a fuselage gear - far less pieces of wood and fitting needed!

Looking at this design, I wonder why we can't have such good looking models nowadays?



Marc 03-10-2010 02:41 AM

What a great looking plane, aramid! Just the right dihedral. Looks like it will balance perfectly. I think it wants to fly right off the table. I especially like the large hatch on top. I know you'll give it an eye-catching covering. Have you decided on a motor yet?

aramid 03-10-2010 04:28 AM

Thanks, guys!

Dereck, my biggest problem with wing-mounted landing gear is that they make both the wings and the fuselage more difficult to store and transport, because neither section can sit cleanly on the ground by itself. I definitely agree that they can end up too far back and upset the ground handling, though. Take a look at post #13, and you can catch a glimpse of the dramatic forward rake the plans wanted on the gear to prevent this.

Marc, I can't wait to fly it. I'm hoping the plane and the weather are ready about the same time so I don't have to wait too long for one or the other. I'm currently guessing a bit over two pounds final weight, so my motor might be a 3536, 1100 Kv, either a Heads Up or a Turnigy. I might parallel my 2200 mAh packs, but more likely I'd pick up a couple ~3000 mAh ones for convenience.

The hatch is a nice detail, but there's a lot about this build I'm liking. Scratch-building is hard work, but I love being able to fine-tune every last detail to get a perfect match to my equipment. When I built my Wonder, I followed the plans pretty closely, so there are some small details I might prefer done differently. Everything else I've built or helped build has been either an ARF or a laser-cut kit, and those make modifications tricky. I always feel like I'm hacking something together when I try to modify a kit with so much prefabrication. With this plane, though, I have no such issues. Every last detail is exactly as I want it, and that makes me happy.

It's just a shame it's such hard work.

Dereck 03-10-2010 03:50 PM

Hi Aramid
Some of those older plans used the same position main gear mounting for trike and taildragger gear, just turning the legs around dependant on use. Not that bright...

In the interests of simplicity, I always use a taildragger. Tnt Landing gear is a great for custom UCs if I can't find a ready-made that works, and the best mounting I've found involves two 3" lengths of hardware store sourced alloy 'L' section, around 1/2" to 1" on each leg. They're epoxied to the fuselage insides, the gear bolts to them with nothing but a thin ply sheet bottom between mount and UC.

Your big hatch is a great idea. Such a widget makes life much easier. I design most of my models myself and always fit a really long hatch in - having had the odd electrocuted kit that wouldn't take a long top hatch, I have no urge to have to poke around inside a model to access batteries.

Reckon my best 'idea' was to glue a strip of 1/8 x 1/4" spruce inside the inner fuselage face to stiffen up the sides - they tend to get floppy without the top deck. The 1/4" face goes to the side, right up under the inside top. I learned that real fast with my original big electric conversion, a Sig Four Star 40, that had a huge long hatc, as I didn't really know how it was all going to work.

I tend to get really bugged with other designs - though Sig are an exception - to where I used to joke that the best part of a kit or plan was the blank expanse on the back of the plans. Much easier to start from scratch and get it right, I reckon.

It's not really 'hard' work - who ever put their back out cutting little bits of balsa wood? You have built a great model, all about it suggests it will work as well as the 'real thing' it was scaled down from and you will have the only one at your site. You improved on your 'Wonder' and I'll bet your next project will improve on this model too.

Okay, I admit to being an 'Old Fart in Training' but 'I built it all myself' still outweighs 'Look what I bought this week'

Good luck with flying her


aramid 05-11-2010 04:59 AM

Finished Covering!
5 Attachment(s)
Well, it's been two months, but I've been chipping away at things slowly and I'm happy to report that I am nearly complete! All that remains is mounting the canopy, building a battery shelf, and doing the radio installation. I'm well past "ARF" status now:


I haven't done the final weigh-in yet, but my latest estimate is 2.5 pounds. Power is therefore a Turnigy 3536-1100 spinning a 10x7 prop on 3s 3000mAh Zippy packs. Last time I tested this motor/prop combo I pulled 32 amps, although with these new, bigger batteries I imagine I'll see a bit more power. That puts me at 135+ watts per pound with a pitch speed of 65 MPH or so. Wing loading is an agreeable 14 oz/sq. ft.

If the weather cooperates, I hope to have a flight report within the next week; two weeks at the worst.

Any thoughts on whether I should go for a spinner or stick with the basic prop nut?

I'd also love to hear some control throw suggestions.

Dereck 05-15-2010 11:49 PM

That is truly a model of beauty. If there's one stand-out, you have done a great job on the cowling - it looks good, and it avoids looking like there should be a dummy glow jug poking out of it.

Love your 'paint scheme' - smart and simplistic on top, vastly different bottom for easy 'which way up' at a distance.



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