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Oct 06, 2013, 06:02 PM
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Build Log

Hammer 38: Tough, overpowered 3D biplane (Plans, videos, guide, pics, links)

1 – Videos, and the basic concept

It flies fairly well:

1250-watt motor, 4-cell, 14x7 prop, 9mm Depron
820-watt motor, 4-cell, 13x6.5 prop, 6mm Depron
450-watt motor, 3-cell, 13x4 prop, 6mm Depron
1200-watt motor, 4-cell, 15x6 prop, 11.3mm laminated MPF
900-watt motor, 3-cell, 13x6.5 prop, 6mm Depron
850-watt motor, 4-cell, 13x6.5 prop, 5.7mm MPF
250-watt motor, 3-cell, 13x4 prop, 6mm Depron

(Many thanks to the ace flyers in the above videos who made the Hammer look better than it really is: Brian Pham, Chris Myers, and Bob Ross.)
__________________________________________________ __________

There's nothing subtle or sophisticated about this plane. It's just a profile-fuselage flat-sheet foamy with no airfoils, almost a full roll of reinforcing tape encapsulating it, fully-exposed components, heavy batteries, stout metal-gear servos, lots of external carbon fiber guy rods, beefy landing gear, tons of wing area, huge control surfaces, a big prop, and a very long list of useable motors, many of which are definitely oversized. Its configuration is based on the most crude and primitive of design philosophies: "More is better."

This is a tough, durable, good-sized plane that you can fly as hard as you wish. It’s not a delicate lightweight. You won't have to be careful with it. Abusive piloting behavior is permitted and, in fact, encouraged. The idea here is to have a very-high-performance overpowered stunt plane that you can torture test on every flight and be confident that it won’t break up in mid-air. If you do bang it up, it's easily repairable.

Yes, it’s a lot of fun to fly with “moderate” power systems, as you can see in the above videos. You may have components in your existing inventory that you could use on this plane, and the videos are posted to show how it behaves with several different combinations. However, the intent of this build is not to have yet another moderately-powered aircraft. We all have plenty of those. The Hammer is about the application of brute force and seeing how much of it the airframe can handle. It's the blunt instrument of foamie biplanes. It's the kind of model airplane that would appeal to these guys:
Last edited by Depronicus; Jan 26, 2014 at 10:39 PM.
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Oct 06, 2013, 06:03 PM
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2 – Plane pics, radio note, table of contents

To give you some sort of an idea of what you’ll be getting into, a few representative shots of finished planes are posted below. However, please wade through all thirty-two of my wordy, verbose, overly-detailed posts before you embark on a build of this plane. There’s somewhat more to this project than you might think at first, and there are several things to consider before you get started.

Perhaps the most important of those considerations is the fact that, as rough-hewn and basic as the airframe is, it really needs to be controlled with a modern programmable computerized radio system by makers such as JR, Futaba, Spektrum, Tactic, and OrangeRx. If all you have for it is an old-school system that is not fully capable of programmable trimming (rudder-to-aileron and rudder-to-elevator mixing in particular), you should stop right here. The Hammer requires an up-to-date radio system. See posts 13 and 14 below and you’ll understand why.

(I know nothing about the newish OrangeRx T-SIX transmitter. It seems to have most of the features included with the big-name units, but at a remarkable price. All I know about the OrangeRx brand is that I’m using five of their R610 receivers and R100 satellite receivers with my Spektrum DX7 transmitter, and I’ve never had a problem, nor have I heard of any of my FBs having problems with them.) Here’s the thread on the TX, and this is one on the receivers.

Please feel free to point out any errors or omissions you may encounter as you fight your way through this seemingly endless build guide. I’m eager to edit anything that needs to be fixed, and I’ll appreciate any proofreading assistance you may be able to provide. Specifically, if you encounter a dead URL link, I’d really like to know about it.

This little project was instigated after seven or eight guys who had seen me flying the original Hammer asked me how they could get one for themselves. That made me think that perhaps an RC Groups build thread, which I had never done before, was in order. However, it soon took on a life of its own and got out of hand, and the thread is now so cumbersome and unwieldy it’s difficult to navigate through it. For those precious few of you out there who may actually build one of these birds, the following table of contents may help eliminate some tedious searching and scrolling through the 32 separate posts:

1. Videos
2. Finished plane pics
3. Foam choices
4. Plans and drawings
5. Components list
6. Motors – Power range
7. Motors – Shaft configurations
8. Motors – The agony of replacing bent shafts
9. Motors – The three basic sizes
10. Motors – Links to many that can be used
11. Motors – Preventive maintenance
12. The three-pattern firewall
13. Transmitter trim settings – Part 1
14. Transmitter trim settings – Part 2
15. Cutting foam, at last
16. Aileron connector tabs
17. Velcro battery retainers
18. Pre-painting
19. Control surface attachment
20. Control horns
21. Linkage connectors
22. Aileron servos
23. Fuselage assembly
24. Firewall installation and thrust angle setting
25. Wing assembly
26. Guy rod installation
27. Prop prep
28. Landing gear
29. Voltage protector
30. Center of gravity and thrust angle determination
31. Prop strike warning
32. Epilogue
Last edited by Depronicus; Oct 06, 2013 at 06:46 PM.
Oct 06, 2013, 06:04 PM
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3 – Which foam? 9mm Depron? 6mm Depron? Laminated MPF? 6mm MPF? 9mm EPP?

There are many possible ways to go on the basic material of construction of this plane:


Depron foam in either 9mm or 6mm sheet is stiff, light, and perhaps the easiest of all foams to work with, but we all know how brittle it is. It cracks quite easily. It’s a delicate material that has poor impact resistance and structural integrity compared to most other foams. Fortunately, there’s a simple, cheap, and very effective method of vastly increasing its strength and durability: Tape it up with Scotch HD clear shipping tape. This is definitely the best tape for this application that I’ve found. Nothing else I’ve tried is comparable to it. It’s thick, extremely strong, and the adhesive sticks to foam quite well.

I don’t know how you would calculate how much stronger tape-reinforced Depron is than bare material, but you can see for yourself by doing a quick comparison: Cut two pieces of Depron about three inches long and two inches wide. Leave one bare, but wrap the Scotch HD tape completely around the other. Take the bare piece in your hands and tear it in half. Now try to do the same to the taped piece. Go ahead. Try to tear it in half.

How much stronger is the taped Depron? Is it ten times stronger? Twenty? Fifty? Again, how do you calculate that? The point is, while adding very little weight, and at almost no cost, the application of high-quality clear packaging tape has transformed a fragile, brittle material into something that, for our purposes, might as well be aluminum plate. A properly taped-up Depron plane is really durable, and can withstand a hell of a beating. If you go even farther and apply two layers of tape to both sides of the foam, the piece becomes remarkably rigid. Double-taped Depron is a tough, stiff material of construction for model airplanes, and you’ll be doubling up the tape on the Hammer in some key areas.

In the next post, along with the full-sized plans, are two PDF files (“Tape 1” and “Tape 2”) that show you where to apply the all-important tape. Note how the tape pieces overlap each other so that there are very few un-taped gaps. I can’t over-emphasize how important it is that you tape your foam pieces per those two drawings, whether you’re using Depron or the other materials I talk about below. Going without full tape reinforcement, per the drawings, defeats the purpose of this build. If you have some sort of problem with the concept of applying lots of clear packaging tape onto sheet foam, this project is not for you.

While either 9mm or 6mm Depron are good materials for this build, as of this writing I’m leaning toward the thicker foam as perhaps the better choice. The plane in the soon-to-be-added seventh video is made of it. Much stiffer and quite a bit stronger than 6mm material, going with 9mm adds about four ounces to the all-up weight, which is inconsequential on this 38-inch-wingspan high-powered biplane. If you use it, don’t try to slack off on the tape reinforcement. Sure, it’s a little thicker, but it’s still just lame old brittle Depron. Tape it up. Plans for 9mm material are included in the next post.


I’m recommending taped-reinforced Depron as the primary material of construction because of its inherent stiffness. Rigidity matters on this plane. As you can see in the videos posted above, the Hammer 38 is capable of some pretty crazy high-energy gyrations, and subjects its control surfaces to high stresses. You really don’t want the plane to be twisting, bending, flexing, and otherwise distorting itself while you’re flying like a maniac, banging your transmitter sticks around with the throttle wide open.

However, there now exists an alternative to Depron that all foamy scratchbuilders should be familiar with: It’s the recently-introduced foam known as “MPF” (Model Plane Foam) offered online by these guys in big 24” x 48” sheets. It’s a really nice material that is definitely stronger and far less prone to cracking, while still being as easy to work with as Depron. Think of MPF as something of a cross between Depron and EPP. However, It isn’t inherently as stiff as Depron, and that’s turned out to be more of an issue than I first thought it would be.

When I got my first box of MPF I was impressed with its resistance to cracking and how nice it was to work with, but I was concerned about its rigidity, or lack thereof. Sure, it was undoubtedly stiff enough for just about all of the planes posted on this forum, but would it be okay for the Hammer, especially with a big motor and prop? I figured there was only one way to find out, so I went ahead and built one of MPF.

The verdict: While the flying characteristics of the MPF plane are pretty much the same as those of its two 6mm Depron hanger mates, there are some caveats. The MPF version definitely needs additional taping (beyond that required for Depron) especially on all of those big control surfaces, which are too flexy in MPF if not stiffened up with additional tape. The rudder should probably also have a carbon fiber strip installed along its full length, as well, and maybe the elevator, too.

The MPF plane is shown in the fifth video, with an 850-watt motor and a 13x6.5E prop. It did fine with that drive combination, but not so much when I installed a 1250-watt 50mm motor and 15x8E prop. As unlikely as it may seem, the vertical stabilizer of the MPF plane could not hold up under the heavy wash of the big motor and prop, and fluttered very badly at full power. Much reinforcement of the vertical stab and rudder initially solved that problem, but after a couple of more flights the horizontal stabilizer and elevator began fluttering even more than the rudder had.

So now I have to say that if you’re going to build a Hammer of single-layer MPF, and you want to try it with a really big motor and prop, you’ll need to reinforce the vertical and horizontal stabilizers with carbon fiber strips or tubes, and lay on a lot more tape. Beyond that, I have to think now that Depron should be used on the ailerons, rudder, and elevator. After seeing the horrendous shuddering of the big control surfaces on the Hammer’s tail, I reluctantly have to conclude that MPF is probably not suitable for use on them.

That is not the case at all, however, if you should consider laminated MPF construction, using water-based polyurethane (WBPU) varnish as the glue. The laminated MPF came out to an average thickness of 11.3mm and resulted in a plane that’s a flying brick. It’s easily the stiffest, strongest, toughest plane I’ve ever built, and all who have flown it really like the way it flies. Doubling up on the MPF foam added a little less than seven ounces to the AUW over single-layer. It’s the plane in the third video, with a 1200-watt motor and 15x6 prop. Yes, it weighs in at nearly three-and-a-half pounds, but watch the video and judge for yourself how badly you think the extra weight is affecting its performance.

A plan file for the 11.3mm version is posted below. You’ll need to laminate four sheets of MPF into two. The WBPU takes a long time to completely dry when you’re doing two-foot-by-four-foot sheets, so weight them down as shown in the photos below, and leave them for a couple of weeks, at least. Then uncover them, stand them up so they’re exposed on both sides, and let them finish drying for maybe another week or so, or even longer if possible.

The thick laminated material does require some special consideration when installing the control horns. Please refer to the photos in post 20 (including the shots of the nylon control horn installation on the 9mm Depron version) to see what needs to be done in order to use the bolt-on style of horns that I recommend because of their strength and great reliability. The beveling of the leading edges of the control surfaces needs to stop under the horns, and notches need to be cut in the trailing edges of the wings, vertical stabilizer, and horizontal stabilizer.

PLEASE NOTE: Don’t get me wrong about MPF. For the vast majority of plane designs posted on this forum, MPF is a very good choice of material. All I’m saying is that single-layer MPF construction might not be the best way to go on a 38-inch super-aerobatic biplane that’s being flung around the sky by a 1250-watt motor, a 4-cell battery, and a 15x8 prop.


9mm-thick EPP is tough and durable. Planes made of it can have long life spans. EPP is also something like 15% lighter than Depron. However, it isn’t nearly as stiff, and it’s a little difficult to work with for some of us, isn’t it? Consider the existence of this excellent thread: Leadfeather’s EPP guide.

That’s one of the very best threads on the RC Groups web site. Everything you need to know about EPP fabrication is there. However, it does speak volumes about the material that such a thread would need to exist for it. We also see other EPP help threads pop up now and then. Note that there don’t seem to be any threads like those for MPF or Depron.

Don’t even think about 1.3-pound EPP. That limp, mushy stuff is utterly unsuitable for the Hammer, but 9mm 1.9-pound EPP could be a good choice. Tape it up per the drawings and you would have a seriously tough biplane to bang around. It might not be quite as indestructible as a Zagi-type flying wing, but it’d be a rugged aircraft for sure, and the taping, along with some additional CF stiffeners, would probably make it stiff enough, as well.

I’ll never build an EPP Hammer because I dislike working with the material so much, but if your EPP fabrication skills are up to the task, there’s probably no reason to not do it. Just be sure to give extra consideration to stiffening the control surfaces to head off any possible flexing problems.

(Yes, I know what you’re thinking: A properly-stiffened 9mm EPP Hammer with 9mm Depron control surfaces could be a great combination. I agree with you.)

Two plans for 9mm Hammers are in the next post. One is a full 38-inch version that you’ll need 24” x 36” EPP sheets to make. The other is a 35-inch version that you could cut from 12” x 36” EPP sheets if you happen to have them already. RC Foam offers both sizes, and parts layout drawings for both are also in the next post.
Last edited by Depronicus; Oct 14, 2013 at 05:46 PM.
Oct 06, 2013, 06:05 PM
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4 – Plans and additional drawings

Plans are posted below for the 6mm version, 9mm version, and 11.3mm version, along with the tape guide, parts layout drawings, firewall drawing, landing gear drawing, and thrust angle guide.

I have no means of making tiled PDF files for you, but that won’t stop you from tile-printing the full-sized files. Download and install the free PDF printer PDF995.

• Open the full-sized 34” x 44” PDF drawing you want to tile, click “print”, and choose “PDF995” as your printer.
• In the middle of the dialog box, under “Page Sizing & Handling”, click “Poster”.
• Stay with the default 100% tile scale and 0.005” overlap, and check the “Cut marks” and “Labels” boxes. Portrait orientation always seems to work.
• Click “Print”.

Here’s how to tile-print using the free download Adobe Reader XI:

• Open Adobe Reader XI, click “File”, then “Open”, and select the big PDF you want to tile print, such as any of the 34” x 44” plans below.
• Click the print icon (or “File”, then “Print”).
• Under “Page Sizing & Handling”, click “Poster”.
• Stay with the default 100% tile scale and 0.005” overlap, and check the “Cut marks” and “Labels” boxes. Portrait orientation always seems to work.
• Click “Print”.

Here’s a blurb on doing it with Adobe Reader X, if that’s what you have:

Print large documents on regular paper
Last edited by Depronicus; Oct 09, 2013 at 11:19 PM.
Oct 06, 2013, 06:06 PM
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5 – Components list

You’re going to need a lot of stuff to get one of these slugs into the air, and I’ve attempted to list it all below. I hope I haven’t left anything out, and I hope I got the quantities right. In many cases you can make substitutions based on your personal preferences. It’s a somewhat daunting list. Yes, there will be some noticeable charges on your credit card statement. Your wife might make some comments about them.

Foam: See materials blurbs in post 3, above. See also the notes on the plans and the parts layout files.

Glues: Both thin and medium foam-safe CA (or maybe Foam-tac or white Gorilla Glue), contact cement, hot glue. If you’re using laminated MPF construction, you’ll need a pint of water-based polyurethane varnish to laminate four sheets into two, and you can use white Gorilla Glue to put the plane together, although I’ve found foam-safe CA to work very well on MPF.

Tape: Two rolls, Scotch Heavy-Duty Packaging Tape, product number 34-8704-6777-5; one roll, 1” Blenderm medical tape

Transmitter: Fully-programmable 2.4 GHz computerized unit such as those by JR, Futaba, Spektrum, Tactic, OrangeRx, etc., capable of control surface movement mixing, exponential input sensitivity adjustment, and dual-choice control surface travel rates.

Receiver: Six-channel minimum, satellite capability highly recommended. Spektrum AR7010 or AR6210, OrangeRx R620 with R100 satellite, or equivalent.

Metal gear servos: Four Hitec HS-82MG or equivalent (or stronger, such as HS-85MG)

Servo arms: Du-Bro #675 large

ESC: 40 to 100 amps with onboard switching BEC, depending on motor choice. I’ve had nothing but great luck with these Turnigy Plush 60-ampers and their programming card.

Motor: 450-watt to 1250-watt. See the interminable multi-post motor section below and take your pick, or pull an equivalent out of your own inventory.

X-mounts: One each, C35, C42, and C50, for use as motor mount hole pattern guides when making the three-pattern firewall

Batteries: 1800 to 2700 mAH, 3-cell and/or 4-cell (or two 3-cell in parallel, or two 2-cell in 4-cell series), 35C minimum highly recommended, 45C even better

Series and parallel battery “harnesses” or “adapters”: Heads Up Hobby has both

Voltage protector: Turnigy TURN-VP or Spektrum SPM1600

12AWG silicon-jacketed battery and ESC lead wire: 14” each of red and black

Propellers: APC 13x4E, 13x6.5E, and 14x7E (and possibly 15x6E, 15x7E, and even 15x8E)

8mm reamer or 21/64” drill bit (to enlarge APC prop holes for C42 and C50 prop adapters)

Prop balancer: Great Planes GPMQ5000

Wheels: Du-Bro 300MS or 80mm (3”) equivalent

3/32” retaining collars: Five Du-Bro #138 Dura-Collars

Carbon fiber rod, 1.8mm: Eight pieces, 750mm long (HK), or seven pieces, 1000mm long (RC Foam)

Carbon fiber wing spar tubes: Depending upon the foam you’ll be using, you’ll need either two 6mm tubes 1000mm long (for Depron or MPF), or two 8mm tubes 1000mm long (for 9mm EPP or Depron), or four 5mm tubes 750mm long (installed over-and-under and staggered with laminated 11.3mm MPF). See the corresponding plans and build guide, below

Carbon fiber strip: 1mm x 6mm, for the aileron connector tabs (and MPF rudder and elevator stiffeners)

Velcro: One roll each of 1”-wide sticky-back and ½”-wide double-sided

Aircraft plywood: One 3” x 3” piece, 1/8” thick, or two 1/16” pieces laminated together (Most common brand: Midwest). This is for the firewall. Do not substitute! Use of anything other than aircraft plywood for the firewall could be a catastrophic mistake.

Wood craft sticks: Bass sheet, medical tongue depressors, or Popsicle sticks will also work. Balsa not recommended. See the firewall post

Control horns: Four Du-Bro #107 1/2A with stock screws for 6mm-thick material. Half-inch-long 2-56 (or 2mm) screws required for 9mm material. 5/8” or 3/4” needed for 11.3mm laminated MPF. I also like Hobby King’s three-screw 16x20 horns. Their broad base and third screw makes for a solid mount. You’ll need to get the 2-56 or 2mm screws separately, though.

Control surface linkage connectors: Ten Du-Bro #605 EZ Connectors (used with 1.8mm CF rod), two Sullivan 2-56 metal clevises, two 2-56 partially-threaded rods. (Or use other linkage connection methods that you prefer, but be sure they’re seriously strong and reliable. All the linkage pieces on the Hammer are subject to very high stresses.)

Nylon tie wraps (cable ties): 4” long, one bag.

Blind mounting nuts: Four 4-40 and eight 6-32 (Du-Bro #135 and #136 ), or four 3mm and eight 4mm. See the firewall post

Socket head cap screws: 4-40 and 6-32, or 3mm and 4mm. Length depends upon motor spacing from the firewall, but ¾” long should cover any possibility.

Plastic spacers: Typically ¼” long, 5/32” ID (for spacing the motor away from the firewall if required). Nylon nuts can also be used for this.

Music wire: Two 3-foot lengths, .078” (or metric equivalent). One 3-foot length, .055” (or metric equivalent).

Brass tubing: 6”-long piece, K&S #1145 1/8” OD (or metric equivalent).

Full set of soldering equipment: 80-watt iron, 60-40 solder, flux, fine steel wire, jig, sponge. See landing gear post 26, below

Monokote: Black and white (For making the “windows” on the vertical fuselage)
Last edited by Depronicus; Oct 06, 2013 at 07:00 PM.
Oct 06, 2013, 06:07 PM
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6 - Motors, motors, motors (Part One: One thing continues to lead to another)

Over a year-and-a-half ago, when I was working out the details of the Hammer and putting the first one together, I had no idea of what it was going to turn into. I was just hoping it would have a wide performance envelope, with a nice floaty feel at low 3D speeds, along with very good high-speed flying characteristics. I was thinking that maybe it could be sort of a cross between Dave Royds’ classic “Infineon/i3D”, the floaty DW Foamies “Skua” (now discontinued), and the nifty Charger RC “Shockwave”.

It worked out pretty well, but developed in a way I never originally envisioned. The thought of it having a firewall with three X-mount hole patterns (for motor diameters of 35mm, 42mm, and 50mm) did not occur to me until much later. I had no clue that this thing would be able to handle a heavy 1200-watt 890Kv motor turning a 15x6 prop, powered by a 4-cell system with an 85-amp speed control, but as you can see in the third video, the airframe is very happy to be hurled around by that way-oversized power train.

In the posts that follow, I’ll be incessantly going on and on about the many different motors you can use on the Hammer. If I include more information than you need, please be patient with my obsessive-compulsive behavior. I’m assuming that not everyone who reads this thread will know everything there is to know about all the brushless outrunner motors that could be employed on this plane, so I won’t withhold any information. As I’ll try to do on every aspect of this plane, I’ll make a full confession. I’ll divulge all of what little I know.

The point of talking about so many different motors is that your existing battery and speed control inventory may be the deciding factor in your motor choice, so it would be nice to know what’s available that might work with what you have. For that matter, you may have one of these motors already. Also, the flying characteristics of the Hammer vary considerably depending upon what power train is installed, of course, and your motor choice might be determined by what you’re looking for. At the lower end of the power train spectrum the plane is a friendly 3D puppydog. At the upper end? Well, refer to the two photos below. They represent the different flying experiences you’ll get at the opposite ends of the range of power systems you can use on this plane:
Last edited by Depronicus; Oct 06, 2013 at 07:04 PM.
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7 - Motors, motors, motors (Part Two: Motor shaft configuration)

Brushless outrunner motors of the type that can be used on the Hammer are manufactured in two shaft-orientation configurations. For our purposes here we’ll call the two configurations “aft-shaft” and “forward-shaft”. See the photos below of the now-discontinued Turnigy aft-shaft “C” motors, and Turnigy’s newish forward-shaft “D” motors, which they’re calling the “XP” series.

Aft-shaft motors can use bolt-on propeller adapters, installed on the rotating case of the motor. The motor’s shaft extends out towards the rear of the plane from the stationary end bell, and is secured to the rotating case with a set screw, which you always need to tighten when you get a new motor. If you’re using it on a plane where the motor is mounted behind the firewall, such as all planes made by Precision Aerobatics, you turn the motor around and bolt its stationary end bell to the back side of the firewall. You then use a collet-type shaft-clamping propeller adapter.

With planes like the flat-sheet-foamy Hammer, however, the motor must be mounted in front of the firewall. A behind-the-firewall configuration on a plane like this is impractical because it would be tough to properly secure the firewall to the plane with it located way out on the nose, with very little foam in front of it. You would also need lots of open space aft of the motor to remove it.

That’s just fine, because with an in-front-of-the-firewall configuration we can use the bolt-on prop adapters, and put the motor shaft out of harm’s way, extending out behind the firewall, or we can just cut the shaft off and be done with it. This is the advantage of the aft-shaft configuration. What’s the first thing that arrives at the scene of the accident when a conventional single-motor model airplane takes a frontal impact? The propeller and the shaft it’s mounted on, of course. In a behind-the-firewall configuration, that means you’re very likely to find that the motor shaft has been bent by the impact. Big props you’ll be using on the hammer exert just that much more leverage, and make it more likely that the shaft will be ruined. You now get to have the great fun of pulling the motor out of the plane and replacing the shaft. See exactly how much of a pleasure that can be in the following post. Personally, I find that it’s more fun than I can stand.

With an aft-shaft configuration, the prop and its bolt-on adapter shaft take the same impact, and the adapter shaft is likely to be bent, but all you have to do now is simply unbolt the bent adapter and bolt on a new one. You don’t even have to take the motor out of the plane to do it. If you’re using a motor that uses an industry-standard adapter like this, or this, or this, you can lay in a couple of spares for very little money and have them ready to go when needed.

Forward-shaft motors, with the shaft extending out of the rotating case end, can also be used with in-front-of-the-firewall configurations. A collet-type shaft-clamping prop adapter is used instead of a bolt-on, and an X-mount secures the motor to the front of the firewall. Problem is, you now have the same shaft vulnerability that you have with an aft-shaft motor turned around and bolted in behind the firewall. The motor shaft takes all the impact with the forward-shaft configuration, and unless your idea of a good time is changing out motor shafts, this is not the way you want to go.

I do not recommend the forward-shaft configuration, with a collet-type clamp-on adapter, on the Hammer. There’s no good reason to use it when there are so many great aft-shaft motors available, and replacement standardized bolt-on prop adapters are so cheap. Also, some, but certainly not all, forward shaft motors come with a hole pattern for bolt-on adapters (either the standard 4-hole pattern or the newer 3-hole patterns) on their rotating ends, such as the Turnigy “XP” D-series. It’s a simple enough task to cut off the shaft and bolt on an adapter which, in the case of the XP, would be the bolt-on adapter for the Turnigy SK3 line. It’s a near-perfect fit. See the photos below.
Last edited by Depronicus; Oct 06, 2013 at 07:12 PM.
Oct 06, 2013, 06:08 PM
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8 – Motors, motors, motors (Part Three: Replacing bent motor shafts)

In order for you to fully understand what a joy it is to replace motor shafts, you need to know the procedure of how you do it. Start first by taking a look at Jackerbes’ very good motor thread. The first three posts apply to shaft removal. My photos below are pretty much a rehash of his, but more specific to the type of motor that’s used on the Hammer.

Of course, if you don’t have a bench vice like his or the one pictured below, or you don’t have one of these little jewels, you’re SOL and you get to buy a new motor anyway. There’s no way you can get the shaft out of most of these motors by tapping on it with a hammer. A few might come out fairly easily, but most put up a helluva fight.

Please, slog your way through the threads that Jackerbes posted links to in his thread. See what you’re getting into with shaft replacement. The excellent Scorpion video is particularly good and very complete, but even it doesn’t tell you what to do if you have neither the nifty little C-clip removers or the big arbor press shown.

(BTW, using pipe pieces as shown in the photos below ensures that you don’t ruin the end bell of the rotating case as you push the shaft in and out of the very tight hole with the vise or arbor press. Using a block of wood with a hole in it, or something similar, does not support the end of the case where the force is being directly applied. The end could easily be bent inward without the support of the pipe, especially if you’re unlucky enough to have a motor with as incredibly tight a shaft fit as it was on the two Turnigy XPs I tried to work with. I was able to get the 5mm shaft pushed in far enough to be flush with the end of the case, but the 4mm screw I used to try to finish pushing it out kept collapsing off to the side again and again as I tightened the vise. As far as I’m concerned, Turnigy XP shafts are non-replaceable.)

Point made, one would hope, about why you don’t want to use forward-shaft motors on this plane. Do yourself a huge favor and use aft-shaft motors. Just cut the damned shafts completely off, and use the nice bolt-on prop adapters they come with. Replacement motor accessory kits are dirt cheap, and laying in a few for spares won’t bankrupt you.
Last edited by Depronicus; Oct 06, 2013 at 07:15 PM.
Oct 06, 2013, 06:08 PM
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9 – Motors, motors, motors (Part Four: The three sizes and their accessory packages)

Photos of 35mm, 42mm, and 50mm motors, along with shots of the dimensions of the industry-standard C35, C42, and C50 X-mounts that are bolted onto them, are posted below. Save yourself some grief down the road and try to stick with motors that have mounting hardware packages that comply with these standard dimensions. The lower-priced “generic” motors all seem to conform, but watch out for the high-enders. Some of those makers seem to feel they’re too good to conform to a generic dimensional standard on their motor mounts.

One more thing: 35mm motors come with bolt-on prop adapters that have 6mm threaded prop shafts. You need to be a little careful when tightening the prop nut on a 6mm shaft. Some of the adapters are not made of the toughest grade of aluminum available, and their shafts can be popped. With the 42mm and 50mm motors it isn’t an issue because they come with bolt-on adapters that have 8mm threaded prop shafts (although I’ve seen a couple of oddball C42 adapters with 6mm shafts, such as those on the Turnigy SK3 line), and you can snug the prop nuts up without fear of breaking the shaft. You might want to keep that in mind when choosing a motor. It’s a real advantage that 42mm motors have over their 35mm power equivalents.
Last edited by Depronicus; Nov 07, 2013 at 07:33 PM.
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10 - Motors, motors, motors (Part Five: A few motors you might want to consider)

Again, pardon my obsessive-compulsive behavior, but there are a zillion motors that can be used on the Hammer, and I figure I should point out a representative sample of them. All have been selected because I’m confident they’ll work with the recommended three APC propellers that I hope you’ll use on this plane: 13x4E, 13x6.5E, and 14x7E. I do not recommend using a prop smaller than 13”. You want the full control advantage that you’ll get with the great prop wash created by the big propellers blowing over the control surfaces. The plane dances around at low speeds like it does in the videos because of the heavy prop wash effect it is enjoying.

The price range here runs from $115 down to less than $14. You’ll need to be the decision-maker as to how much you want to spend, but my experience, and those of many others I’ve heard from, is that there is no completely assured correlation between how much you spend on a motor and how it works out for you in actual service. We can’t say that you always get what you pay for with model airplane electric motors. You can be very pleasantly surprised in some cases, and bitterly disappointed in others, no matter how much, or how little, was charged to your credit card.

To try to organize things a bit, I’ve split the motors into those that would be good for 3-cell service (Groups 1 through 3), and those that are best for 4-cell use (Groups 4 through 6). Under each of those two categories the three sub-category groups are divided generally by size and power output. The size and Kv RPM ratings shown in the sub-category headings are typical for each group, with different but similar motors also included. If you know of other good motors that would fit in these categories, don’t be shy about letting us know about them, too.

One more thing, though: I’ve listed a large number of motors for 3-cell service because we all have many 3S batteries in our inventory, and building a Hammer will be an expensive-enough endeavor that it may motivate you to save money where you can. That said, I have to also say that I think of this plane as a 4-cell rig. Those extra 3.7 volts in its power train really bring it life, folks, and I recommend you go with a 4-cell setup if you can swing it.

Finally, keep in mind that the fully-exposed motor on the Hammer gets cooled off nicely, but the ambient temperature has a very significant effect on that cooling . The motor runs way colder on a cool morning than it will on a hot afternoon, and you need to keep that in mind when you select the prop you’ll be using. For example, on many of the motors below you could get away with a 14x7 prop on a cool day, but that might not be the case when it’s warm, and you would probably want to think about propping down.


Group 1: 3536, 910 to 1100kv (Approx. 100 to 115 grams, 350 to 530 watts)

(Some in this three-cell group are also okay for four-cell service, but others may overheat with a 13x4 prop on four cells, so be sure to check for heat after the first couple of minutes of flight if you try 4S power with any of these.)

Hyperion ZS3014-972kv 37mm diameter, 30mm long. They’re claiming 530 watts. Super-high quality, supposedly, but for $100 it had better be, huh? (Edit: As of August, it appears to be on sale.) It would be nice if they posted the dimensions of the X-mount, but they don’t. Let’s hope they use standard mounting dimensions, but some of the high-end manufacturers seem to have their own hole patterns, and I suspect Hyperion’s may be non-standard. This is a very short motor, and even though it’s rated at 530 watts, I doubt that it could handle a 13x6.5 prop.

Torque 2818T/900kv More detailed info on these $75 motors would be nice, but I guess the site lists enough, and they have a good reputation. About 400 watts, and quite light at 104 grams. 13x4 prop will be fine on 3-cell power.

RC Timer A3536-9 910kv Nice price for what I’m sure is a good standard motor. Note: No accessory kit included. Buy it separately: Standard C35 accessory kit

GForce G10 3536-1100Kv High Kv rating may be a problem in hot weather, but a 13x4 prop should work if it’s a cool day. Good price.

Heads Up Hobby Firepower 10 Sport 3536-1050kv Same note as above. 450 watts, 105 grams.

Turnigy SK3 3536-1050kv 110 grams, 505 watts. Again, same note as the previous two motors.

Turnigy “XP” D3536-910kv These forward-shaft motors apparently all have a bolt-on prop adapter 3-hole pattern on their rotating ends, bless Turnigy’s heart, so just cut off the shaft (which on two XPs I worked on were too tight in the case to remove and reverse-install) and use one of these SK3 bolt-on adapters, and you’re good to go in an aft-shaft configuration, but with no shaft to worry about.

NTM 35-36A 910Kv 117g, 350 watts. I have one of these and I have to say that the performance of this motor is disappointing. Every 3536 910Kv motor I’ve ever had has been stronger than this one, and I don’t recommend it. No accessory package is included, so order one or two with it if you still want to try one..

Group 2: 3542/48, 800 to 1100 Kv (Approx. 125 to 175 grams, 530 to 910 watts)

Omega 130g 870kv Not inexpensive, but if 3DHS offers it, we can expect a quality unit. Note their prop recommendations for 3-cell service. That looks like a standard C35 X-mount. 700 watts, 870kv, 130 grams: This should be an ideal 3S motor for the Hammer.

Hacker A30-14L V2 3746-800kv I’m putting this one in here because I don’t know where else it belongs. They seem to discourage using this motor with 4-cell batteries even though the kv rating is 800, which is very odd. In an earlier version of this web page, a 14x7 prop was recommended with a 3-cell battery, but there’s no prop info there now. Notice that the X-mount is not a standard C35 (so typical of high-end brands), but that’s okay, because you can bolt a standard C42 X-mount onto that 25mm x 25mm hole pattern on the motor. Hacker is definitely a high-end brand, but the general consensus in the 3D forum seems to be that they sometimes run a bit hot. Fifty-three bucks.

RC Timer A3542-6 1000kv Nice price for a good standard motor. 13x6.5 maybe, 13x4 for sure. Note: No accessory kit included. Buy it separately: Standard C35 accessory kit

Heads Up Firepower 15 Sport 3542-960kv Pretty good price for what is undoubtedly a fine motor. I’ve heard a lot of good things about Heads Up’s service.

Heads Up Firepower 25 Sport 3548-900kv Big brother to the one above. Looks great for a 13x6.5 prop, and at 900Kv a 14x7 is feasible, you would think, but maybe only on colder days. I’m quite sure that this is an EMP N-series motor, and if it is, I recommend it. I have three of the 800 Kv versions of it, and they’re really good with 4-cell batteries. This should be a fine 3S motor.

GForce G15 3542-1100kv Assuming that the X-mount has the standard C35 outer hole pattern dimension, this looks like a very good choice. Watt rating appears to be conservative. Check the heat carefully if you try a 13x6.5 with it.

Mile High 3542-1000kv 600 watts. This has to be another EMP N-series motor, and that’s just fine. It may or may not handle a 13x6.5 on hot days, although they’re saying a 13x8 is okay. Price is good.

Turnigy SK3 3542-1000kv The SK3 line replaced their now-discontinued (at least with the “Turnigy” label stuck on them) SK units. The prices have nearly doubled on some sizes, but they seem to be better than the old motors. Note the annoying non-C35-standard 3-hole prop adapters, though. Putting a couple of accessory packs in your inventory wouldn’t be a bad idea if you have SK3s in your motor drawer, especially because HK is often out of them.

Turnigy “XP” D3542-1000kv Yes, this is a forward-shaft motor, but it has the newish 3-hole bolt-on prop adapter hole pattern on the rotating end of the motor. That’s very cool, because all you have to do is cut off the shaft, bolt on one of these SK3 three-hole-pattern bolt-on adapters and you’re good to go with a shaftless motor. You may have to grind off the ends of the two set screws a little, and very slightly enlarge the three holes in the adapter, but the SK3 adapter does work. I also slightly chamfered the three holes in the motor so the screws could start easier.

Turnigy “XP” D3548-1100Kv Same as above: Chop off the shaft and put on an SK3 bolt-on adapter. 159 grams, 910 watts. I have one of these. It provides great power, and on cool mornings it only warms up slightly with a 13x6.5 prop. Flight lengths are quite decent, as well. This is a very nice 3-cell motor for the Hammer, and I highly recommend it. It’s the one used in the 900-watt 3-cell video (with two 1300 40C batteries in parallel). The prop adapter mod is very much worth doing, IMHO.

GForce G25 3548-1100kv This should be okay with a 13x6.5 prop, but monitor the heat at first, and drop to a 13x4 if it’s running hot. Good price for a 650-watt Power 25 equivalent.

SunnySky X2820 920kv Not the cheapest you could get, and the X-mount is non-standard, but the motor hole pattern will fit a C35 mount. They’re claiming something more than “generic” quality. Looks pretty good, but I know nothing about “Buddy RC”.

Group 3: 4240, 890 to 1020 Kv (Approx. 125 to 195 grams, 550 to 650 watts)

Heads Up Firepower 30 Sport 4240 900kv Not a bad price at all for this . 650 watts, 150 grams. 13x6.5 will be fine, they say, but that’s probably conservative. On 3S batteries a 14x7 is likely to be okay in cool weather. This appears to be an EMP N-series motor, which, like I’ve been saying, is a good thing.

Abell EMP 4240-1020kv 600 watts, 140 grams, C42 standard on everything, nice accessory kit, good prop recommendations. There’s nothing here to not like, except maybe the price, which could be a lot better. I have several EMP motors. They’re fine.

Electrifly RimFire .25 4240-1000kv Very expensive, but note the remarkably light weight. A 13x6.5 prop might be okay, but start with a 13x4, and monitor the heat. Rated at 650 watts, this should be a heck of a 3-cell motor for the Hammer.

Esprit Model Neo 25 4240-890kv 13x6.5 prop, maybe a 14x7, but check the heat carefully. It might also be okay on 4-cell service with a 13x4 or even 13x6.5, but be careful. Note the 150-gram weight, and it’s a little pricey for what appears to be a an EMP N-series motor (which are good units), but it’s very likely to be a strong runner. Wattage rating is super-conservative. EMP rates their 850kv version at 720 watts. Mounting hardware is undoubtedly standard C42.

OK Hobbies EMP 4240-890kv Good price. 550-watt rating is conservative, 140 grams, same prop note as above. They say this is an EMP, although that photo is certainly not an EMP, and all EMP motors come with nice standard accessory packs. I have no idea how good the vendor is.


Group 4: 3542/48, 800 Kv (Approx. 125 to 175 grams, 480 to 900 watts)

AXI Gold 2820/14 860kv This is a 3542 that weighs 150 grams. A whopping $115 and there’s no indication whether or not it even comes with a mounting kit. Yes, we know AXI has the big name and all, but at that price, I’d pass on this one.

Motrolfly DM3615-750kv Pricey, yes, and the website has little info about it, but the guys on the 3D forum absolutely swear by these 900-watt units. They feel you can’t find a higher-quality, better-performing motor. 4-cell only, 14x7 prop recommended. I have no idea what the X-mount dimensions might be, and I don’t know what it weighs, but I think it’s something like 190 grams. Hand-made in the USA by Subsonic Planes, the U.S. distributor for Motrolfly.

Torque 2814T/820kv $80. Little detail available on these, but they recommend a 12x6 prop on 4-cell service. Seems that an 820kv 143-gram motor should be able to handle a bigger prop, like a 13x6.5, with 4S power. Surely a 13x4 will be okay.

GForce G25 3548-850 kv They say a 14x7 will work on this with 4-cell power, but I’d try a 13x6.5 first and see how hot it gets. If it can handle a 14x7, it’ll be a monster for sure, and at a very good price.

Mile High 3548-790kv They’re saying 14x7, but caution is advised, I think. Price is good for an 850-watt motor, certainly. You probably can’t go wrong with this one. Note that they offer spare accessory kits for just five bucks.

EMAX GT2826/06 3548-710kv They specifically recommend a 14x7 prop on 4-cell service. Note the non-C35-standard X-mount. A standard C42 will bolt right on to the 25mm x 25mm motor hole pattern, however.

Turnigy SK3 3542-800kv See the 1000kv version, above in Group 2.

SunnySky X2820 800kv This is a 3540 that weighs 140 grams. Wattage and voltage aren’t listed, but it has to be okay for 4-cell use. A little pricey, but it seems to be what amounts to a high-end line of motors, and, sure enough, the X-mount is not C35-standard. No matter. A standard C35 X-mount will bolt right on to the 25mm x 19mm motor hole pattern.

Turnigy SK3 3548-840kv See above. This 750-watter should be really something with a 13x6.5 prop, but watch the heat very carefully. Moderate price, a little heavy, but it has to be a 4-cell powerhouse. Heat considerations may necessitate using a 13x4 prop, though.

Turnigy SK3 3548-700kv According to one of the reviews, it handles a 14x7E prop very well, puts out 670 watts, runs cool, and works with a 60-amp ESC. 177 grams, decent price.

KDA 36-12XL-770kv I have one of these, and I highly recommend it. It’s a very good 4-cell motor, and handles a 14x7 prop very well (although maybe not on a hot day) while still yielding pretty good flight durations. It’s a little hefty at 190 grams. It probably develops something like 750 watts, and the price is not bad for a quality unit. Note that the X-mount is not a standard C35 because the motor’s diameter is 37mm, so their mount has non-standard 44mm centers. You’ll need to get a standard C42 X-mount like this for it, which will fit the three-mounting-pattern firewall you’re going to make for the Hammer. The motor’s mounting hole pattern is the standard 25mm-by-25mm used on the C42 X-mount. A standard C35 bolt-on prop adapter will also fit this motor, although KDA for some reason uses little 2mm bolts in the end of the rotating case, rather than the standard C35 2.5mm bolt. See what I mean about the so-called high-end manufacturers? They go their own way.

Group 5: 4240/50, 600 to 890 Kv (145 to 210 grams, 600 to 1010 watts)

Electrify RimFire .32 4250 -800kv Expensive at ninety bucks, but certainly a very good 4-cell motor, you would think. 925 watts, 198 grams. 13x6.5 definitely, 14x7 possibly marginal.

Turnigy SK3 4240-740kv This 870-watter should be fine with a 14x7 prop, but check the heat at first. Once again, laying in a spare accessory pack might be advisable, because for some ridiculous reason the 3-screw bolt-on motor adapter has a 6mm threaded shaft rather than the C42-standard 8mm. That’s downright infuriating.

Mile High 4240-850kv Everything looks really good here. This is an EMP N-series motor, which I have a few of, though not this particular one. They’re very nice motors, and I can confidently recommend this one. Price is not bad at all. 13x6.5 prop, and you may get away with a 14x7, but be very careful. Complete spare C42 accessory kit for five bucks is a nice deal, as well, and while you’re at it, you could also pick up a C50 accessory kit.

Aeolian 4240-890kv Dealmetic is not the quickest-shipping vendor you’ll ever buy from, but they’re the only ones I can find who have Aeolian motors, which used to be carried by the now-defunct Leaders Hobby. I’ve had nothing but good luck with the brand. 550 watts, they say, which seems very low. 13x6.5 prop, for sure.

Heads Up EMP 4250 600 kv This a very interesting low-Kv motor. 700 watts, 198 grams. Note that they’re saying you could use a 15x8 prop with it. Hmmmm. How about a 14x7 with a six-cell setup?

OK Hobby EMP 4250-600kv Same as above, different color, apparently. Again, the vendor is the question.

Mile High 4250 710kv Powerful 900-watt, 195-gram motor. 14x7 for sure. Fifty bucks, but this has to be a good unit.

NTM 4238 750kv 785 watts, 169 grams, and they show a 14” prop with 4S power. Nice price. Looks pretty good, doesn’t it? Please be advised, however, that I have two other NTM motors, and both are disappointing. They just don’t put out much power. They act as if the Kv ratings are way off, and are actually slower-turning motors than they’re supposed to be. On the basis of my experience with them so far, I can’t recommend NTMs. (BTW, if you do order an NTM, don’t forget the accessory package, which is not included with NTM motors. At that price, get two or three of them, I guess.)

GForce G32 4250-800kv Decent price for a big motor. They’re conservative on the wattage rating and prop recommendation. It might be able to handle a 14x7. At 214 grams it’s a bit heavy. It sure looks to be an EMP N-series, which is perfectly fine.

EMP N4240 750kv One of my all-time favorite motors. 820 watts, 155 grams. It’s the one in the first video.. Terrific power with a 13x6.5, decent flight times, and everything stays really cool. On chilly mornings the post-flight motor heat is barely detectable, and the ESC and batteries are cold. Problem is, I got it from Leader’s Hobby, who went out of business, and I can’t find anybody else who carries it. It you see one, please let us know who is offering it, or, for just $327.00, you can get ten of them. Such a deal, huh?

Group 6: 5045, 660 to 890 Kv (Approx. 200 grams, 900 to 1410 watts)

(There used to be a lot more 50mm motors like these available. Turnigy had some nice ones in their now-discontinued aft-shaft “C” line, and Leaders Hobby offered many EMP and Aeolian varieties, but they went out of business last winter. At the moment there just isn’t much of a suitable 50mm motor selection out there. The four below are all I could find that would be okay for the Hammer, which is unfortunate because I have yet to try a 50mm motor that I didn’t like. They run cold, and yield surprisingly decent flight times.)

Turnigy SK3 5045-660kv They’re claiming 1410 watts. It’s very heavy at 280 grams. Good candidate for a 15-inch prop if you’re inclined to try one. I have the 500kv version of this on 6S power in the 48-inch Hammer, and it’s great.

Firepower 46 Sport 5045-720 Kv 1000 watts, 189 grams. 14x7 prop has to be fine for this, I’m sure, and a 15x6 or 15x7 may be usable. Everything else about it looks good, as well, and Heads Up is a good source to work with. Get a couple of C50 8mm prop adapters while you're at it.

Aeolian 5045 890Kv This 1200-watt motor is in the 11.3mm laminated MPF plane in the third video. It’s a good one: As you can see, the power is great, and it stays very cool with a 15x6 prop. I recommend it without qualifications.

OK Hobby EMP 5045-890Kv Not a bad price for an 1185-watt motor. 210 grams. I have one of these, and it’s good with a 14x7 prop. There’s no indication about the inclusion of an accessory kit, but EMP motors always seem to come with a nice one. That photo on the web page is definitely not of an EMP 5045, however. It’s just a generic photo they pasted in. Again, the biggest question mark here is the vendor.
Last edited by Depronicus; Jan 27, 2014 at 04:06 PM.
Oct 06, 2013, 06:09 PM
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11 – Motors, motors, motors (Part Six: A little preventive maintenance)

Ever heard of “thread galling”? It’s the ugly phenomenon of screw threads going bad as a bolt is screwed in or out, causing seizure of the bolt in the tapped hole, which, in our case, is the end bell of the motor. If that happens, you can pretty much kiss off the possibility of getting the screw out. If one of the bolts holding your prop adapter seizes up in the rotating end bell, you’re just SOL. Read this thread galling blurb. This is another good one. It appears that stainless bolts and aluminum tapped holes are probably the worst combination.

Fortunately, it’s quite easy to essentially eliminate the possibility of thread galling on your motors by simply putting a drop of oil in the threads of every tapped hole in the motor, as stated in the above-referenced blurb. It’s one of the first things I do when setting up a new motor. I’ve thrown out two motors over the years because of seized screws, but since I started oiling the threads, it has never happened again.

Also, oddly enough, a fair percentage of motors come out of their places of birth with shaft set screws that aren’t properly tightened. Assuming that you have the correct size of hex wrench for the socket screw in your motor, be sure to tighten it.

Another thing (although it isn’t actually a motor issue): Do you use Deans plugs like so many of us do? If so, you should probaly clean out the female plugs on your batteries with alcohol on a small piece of cloth, using the end of a paperclip to scrub it around in the socket. I’ve been shocked at how dirty some of my battery plugs have been. One was so dirty it broke its connection when wiggled and made the ESC reboot. I don’t know how I thought to clean out the plug, but when I did I was stunned at how black the bit of white cloth was when I pulled it out. Subsequent cleaning of all my other plugs, including some very new ones, yielded similar results on a few of them.

That begs the question, of course: How many “signal problems” have we had happen to us that weren’t signal losses at all, but were actually broken power connections caused by dirty Deans plugs? You really should spend some time cleaning out your battery plugs. You may be surprised at what you find, as I was.
Last edited by Depronicus; Oct 06, 2013 at 07:23 PM.
Oct 06, 2013, 06:10 PM
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12 – The firewall

You might as well start this little exercise by getting the firewall done first. It’s shown on all the plan drawings, and on its own separate drawing, in post 4. This is what it’s all about, folks. The firewall is the thing that, more than anything else, differentiates the Hammer from other planes. It’s festooned with three sizes of the standard X-mount hole patterns. It will allow you to pick and choose from a seemingly endless list of available motors that are suitable for use on this bird. Your choice of power plants will be pretty much unrestricted, and you’ll be able to swap them out easily if you wish.

It’s made of a slab of 1/8”-thick birch “aircraft grade” plywood, the most prevalent brand being Midwest. Any good Local Hobby Shop should have it, but maybe not in 1/8” thickness. If all they have is 1/16”-thick material, laminate it up with contact cement, wood glue, or even plain-vanilla Elmer’s Glue-All.

PLEASE BE ADVISED: It really needs to be made of high-quality plywood. A non-ply piece, such as basswood, will not withstand the violent forces that you’re going to inflict on it, and will eventually fracture. You don’t need a broken firewall. Order it online if necessary, but make sure you use the good grade of birch aircraft plywood. Use of anything other than aircraft plywood would probably be the biggest mistake you could make in the construction of this plane.

Two sizes of blind nuts are used: 4-40 for the C35 mount, and 6-32 for the C42 and C50. Metric equivalents are 3mm and 4mm. Use a 5/32” drill for the 4-40s and 3/16” for the 6-32s. I don’t know what the drill sizes need to be for the metric versions. Definitely glue the back side of the blind nuts with thin CA, being very careful to not get the CA in the threads, of course. After the CA is dry and completely set up, put a drop of oil in the threads of all twelve of the blind nuts as stated in the previous post, to prevent an occurrence of thread galling and subsequent seizure of the socket-head cap screws.

You definitely should have all three sizes of the X-mounts on hand for this, for use as hole patterns, per the attached photos. Getting the hole patterns right is very important, and will eliminate a lot of annoyance later on. Mile High RC is a good source for C-standard accessory kits at a fair price. Scroll down to the bottom of these pages: C35, C42, C50.

Be careful! Make absolutely sure that the X-mounts you use for the firewall hole patterns conform to the C-standard dimensions! There are a lot of oddballs out there, so take the time to measure the outside hole dimensions and check them against the dimensions shown in the drawings in post 9.

Obviously, the width of the four slots around the perimeter will depend upon the thickness of the foam you’re using. Please be sure to check the fit of the firewall in the foam fuselage pieces before you apply the reinforcing tape around the nose of the plane. You don’t want to have to trim the foam after the tape is on it. Make sure you’ve got it right by temporarily putting the fuse pieces together and checking the fit of the firewall carefully. Remember that the two layers of tape you’re going to apply later will make the foam a little thicker, so the firewall slots should not be a snug fit on the bare foam. It should be a little loose.
Last edited by Depronicus; Oct 08, 2013 at 07:54 PM.
Oct 06, 2013, 06:11 PM
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13 - Transmitter trim settings, Part 1: Dual rates and exponential

You’ll need to have your transmitter set up and ready to go when you get to the point in the build where you’ll want to start making your control surface linkage adjustments, so we might as well get it done now.

I really hope you have a modern computerized radio setup for the Hammer (JR, Futaba, Spektrum, etc.), with all the trims and adjustments available, because you’re going to need them to be able to fly it like you’ll want to. Again, I won’t assume that everybody reading this is fully up to speed on trimming out planes (3D rigs in particular) with a programmable radio, so bear with me as I talk about the most basic of principals and walk you through the process, going into way more detail than most of you need. It’s conceivable that some of those who take on this build may have only recently graduated from the trainer stage, and have very limited experience in programming trim settings on a transmitter, so I’ll try to divulge all of what little I understand about it.

We’ll talk about “dual rates”, “exponential”, “rudder-aileron mixing”, “rudder-elevator mixing”, and “differential” (which very few flyers that I know actually use). I have a Spektrum DX7 transmitter, so everything below refers to it. If you’re using something else, you’ll be pressing different buttons or perhaps scrolling with rollers to get where you need to be, but this should at least give you a pretty fair idea of what you’ll need to do.

(NOTE TO EXPERIENCED FLYERS: The rest of this post is for those who aren’t yet fully versed on programming trims on a transmitter. If you’ve got it down, save yourself some reading and just go with the following settings:

Expo: Start with 70% on ailerons and elevator, and 45% on the rudder. Adjust to taste from there.
Rudder-elevator mix: Minus 16% left and plus 13% right to start, adjust as required
Rudder-aileron mix: Minus 20% left and minus 25% right to start, adjust as required

Yes, that’s heavy mixing, but that’s what it takes to deal with the Hammer’s severe pitch-coupling and roll-coupling issues. Even those settings will probably not be enough. Be prepared to put in some outrageous mix percentages, especially for the ailerons.)

Wing type: Since this plane employs a separate servo for each of the two sets of ailerons, your first task will be to set the transmitter’s “wing type” setting. On Spektrum radios, this is mistakenly called the “flaperon” wing type, which is confusing, because that term really has nothing to do with using two servos for the ailerons. It actually refers to the use of the ailerons as flaps:

Wiki flaperon page

Airfield Models’ rant on the misuse of the term.

Pardon the nit-picking, but “dual aileron” like OrangeRX calls it would’ve been a much better term for this than “flaperons”. In any case, per Spektrum’s instructions on page 46 of the DX7 manual, plug your right aileron servo into the “AILE” port on your receiver, and plug the left one into the “AUX1” port. You may need to reverse one or both of your servos.

Dual rates: We’ll call them “low rates” and “full rates”. A transmitter with this feature makes it possible to change the maximum travel (and the exponential characteristics) of any or all of the plane’s control surfaces at the flick of a switch. You can fly the plane on low rates, with way less travel than the control surfaces are capable of, while you’re getting used to the plane and when you’re landing it. Using low rates while landing makes a smooth touchdown much easier with a plane that has enormous control surfaces like this one has.

Exponential: This is the setting that allows you to “soften” the plane’s response around the center of the sticks’ travel. The higher the exponential percentage setting, the less the control surface moves as you initially move the stick away from center. 3D planes like the Hammer, with their epic control surfaces, need considerable exponential applied in order to not be twitchy and jerky.

These have graphics that may provide some insights, perhaps:

Dual rates and exponential

Dual rates and exponential

RC Help video

Fly Easy RC video

From what I’ve seen and heard, most flyers set up their transmitters so that just one switch changes the travel rates (and exponential rates) of all the control surfaces at the same time. On a Spektrum DX-7, for instance, press the DOWN and SELECT keys simultaneously while turning on the transmitter to enter the system mode. Pressing the UP or DOWN key scrolls until “D/R switch select” appears, and pressing the INCREASE or DECREASE key eventually brings up “COM AILE”, short for “common aileron switch”. That puts all the control surfaces on one switch, and means that simply toggling the “AILERON D/R” switch, conveniently located on the DX7 on the upper-right-hand corner of the unit where you can flick it with your right index finger without having to look down at the transmitter, changes the travel rates of all the control surfaces back and forth from “low” to “full” at the same time. Consult the manual of your particular transmitter, of course, but you’re looking for “dual rate (D/R) switch select” (or something to that effect) in the table of contents.

Now set the rates of the two selections. On a DX7, press the UP and SELECT keys to enter the function list, and use the UP or DOWN keys to scroll through the list. Select “D/R and EXP” and simultaneously press the DOWN and SELECT keys to access it I flip the AILERON D/R switch forward (away from me) for full travel rates, and back (towards me) for low rates. The transmitter’s readout on the “D/R & EXP” screen shows them as positions “POS-0” and “POS-1” respectively.

Press the SELECT key to scroll through the three choices. Choose the top one, and press the INCREASE or DECREASE key to scroll through the three control surfaces. Start with “AILE” and, with the AILERON D/R switch forward (“POS-0”) for full rates, press the SELECT key to choose EXP. Use the INCREASE and DECREASE keys to set the exponential percentage at +70%. You may or may not want to change this later, but many of the best 3D pilots in the world recommend 70% expo, so we know it’s a good place to start. Now press the SELECT key again to select D/R, and set the ailerons’ travel at 100%.

Flip the AILERON D/R switch back (“POS-1”) to set the reduced rates for the ailerons. 70% expo and 40% travel seems to tame the Hammer down quite nicely.

Pressing the SELECT key takes you up to the top line again, and use the INCREASE or DECREASE keys to select ELEV, and set the elevator’s full rates at 70% expo and 100% travel. If the elevator has too much travel and bangs into the vertical stabilizer, reducing the travel here is easier than doing it mechanically on the plane. Reduced elevator rates (POS-1) of 70% expo and 45% travel seem good, but adjust to your taste, of course.

For the rudder rates, you may want to have a lot less than 70% expo on full rates. It seems to be easier to get the Hammer to hover with an exponential rudder setting of 45% or so. Full rate travel should be 100%, you may need less to keep the rudder from banging into the elevator. For reduced rates, try 70% expo and 65% travel.
Last edited by Depronicus; Oct 06, 2013 at 07:32 PM.
Oct 06, 2013, 06:11 PM
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14 - Transmitter trim settings, Part 2: Mixing

Rudder-to-Aileron and Rudder-to-Elevator mixing: This is all about roll coupling and pitch coupling, and using the capabilities of a modern programmable radio to essentially eliminate both of these unwanted flight characteristics.

“Roll coupling” is the tendency most planes have to bank into a turn when you apply some rudder. That’s fine when you’re flying a plane that has only rudder, elevator and throttle controls (“RET”) like the great Easy Star trainer, and you want it to bank over a bit so the plane actually turns. However, with a stunt ship like the Hammer, it makes it far more difficult to do basic 3D exploits like knife-edge (KE) flying and hovering when the stupid plane tries to barrel roll every time you move its enormous rudder, forcing you to apply the ailerons in the opposite direction to counter the rudder-induced roll. To make matters worse in this case, biplanes like the Hammer tend to display a lot more roll coupling than monoplanes generally do.

Fortunately, thanks to the “mixing” feature available on modern computerized transmitters, we can set them up to automatically apply the correct amount of opposite-direction aileron movement when we apply the rudder, keeping the plane flat and resulting in it only yawing in the direction we want it to, with no roll at all if we get the mix settings just right, which can take some effort to accomplish.

“Pitch coupling” is the tendency most planes have to nose down (“pitch to the belly” or “pitch to the gear”) when rudder is applied. Like roll coupling, it also makes it tough to do those basic 3D stunts, as you might imagine. Combine roll coupling with pitch coupling and it’s easy to see why it can seem so hard to become proficient at even basic 3D flying.

But, once again, lucky us. Using the mixing feature, we can set the transmitter up so that, when rudder movement is applied, the transmitter automatically applies the right amount of up-elevator needed to counteract the pitch coupling tendency. By mixing opposing aileron movement with the rudder, and also mixing up-elevator with the rudder, we can get the plane to pretty much act like a boat on a lake when we give it some rudder. That’s the goal of mixing rudder movement with aileron and elevator movement.

This video demonstrates what rudder-aileron and rudder-elevator mixing looks like on the Hammer, along with shots of the flat yaw-only turns you’re trying to get the plane to do as you adjust your mix settings. This is what you’re looking for:

Hammer 38 mixing video

Consult the manual on your particular transmitter, but here’s how you get there on a DX7:

First, shut off the two built-in mixes the DX7 has. Press the DOWN and SELECT keys simultaneously to enter the function list. Press the UP or DOWN keys to select ELEV-FLAP MIX. You aren’t using it on the Hammer, so you’ll want to have it off. Press the SELECT button to access the bottom line (SW). Press the INCREASE key to choose “FLAP0”. That’s the three-choice FLAP MIX switch at the top of the DX7. “FLAP0” means that any position on the switch other than “0” turns the elevator-flap mix off. It’s only on at the “0” setting.

Now press the UP or DOWN key to select “AILE-RUDD MIX”. You’re not going to use it either, so turn it off by pressing the SELECT button to access SW, and use the INCREASE key to once again select the FLAP0 switch. That one switch now turns off both of the unwanted built-in mixes when you move it to the”1” or “2” position. (The FLAP MIX switch can also be used for a third function. See below.)

The DX7 has six programmable mixes you can set up if you wish. We’re only going to use the first two (unless you also want to have a throttle lever kill switch, described below), so you might as well turn off mixes 3 through 6 right now. Press the INCREASE key to select “PROG.MIX3”, and press the CLEAR key to set it at INH (inhibit). (Hitting INCREASE un-inhibits and returns to the settings screen.) Pressing the UP key takes you through the rest of the six programmable mixes. Inhibit all of them except 1 and 2.

Now you can set up the all-important rudder-elevator and rudder-aileron mixes. Use the INCREASE key to select PROG.MIX1, which we’ll use for rudder-elevator to counteract pitch coupling. Use the SELECT key to move the arrow to the beginning of the top line and use the INCREASE key to select “RUDD”. Hit the SELECT key again to move to the right, and use the INCREASE key to select “ELEV”. Now we’re getting somewhere.

Hit the SELECT key again to move the arrow to RATE, and move your left transmitter stick left and right (as if you were applying rudder) to toggle the little mini-arrow between the two rate selections. This determines how much your elevator moves when you apply rudder. For now, use the INCREASE and DECREASE keys to set the upper (left-rudder) percentage at -16% and the lower (right-rudder) percentage at +13%. Why those percentages? Because that’s what they are on my second Hammer, so I guess they’ll be a good place for you to start with yours. You’ll probably need more, and maybe much more. All planes are different, so you’ll adjust your own settings as needed, of course. More on that later.

(I have never seen a good explanation about why, on a DX7 at least, the top setting, which determines how much the elevator comes up when you apply left rudder, should be a negative percentage, and the bottom one, for how much the elevator comes up when you apply right rudder, must be a positive percentage. If anybody out there knows why, please advise.)

Use the SELECT key to move the arrow to the SW (switch) line, and the INCREASE key to bring up “ON”, which means that your mixes are always on. If, for some reason, you should want to turn the mixes off, select “MIX” instead of “ON”, and the “MIX-RUDDER D/R” switch, located on the DX7 on the extreme upper right-hand corner of the transmitter, can be used to toggle back and forth between having the mix settings on (switch setting “1”) and having them off (switch setting “0”). With the Hammer, which has heavy roll coupling and pitch coupling issues, I recommend the ON setting. I can think of no reason why anybody would want to try to fly one of these pigs without rudder-elevator and rudder-aileron mixes.

Use the SELECT key to move to the bottom line, for the “OFFSET” setting. What this setting actually does is a bit murky. From the DX7 manual: “The purpose of the mixing offset is to redefine the neutral position of the slave channel.” In this case (PROG.MIX1) the slave channel is the elevator. So, what does “neutral position” actually mean? Well, this is the best explanation I’ve found yet, from “Baron” on the Extreme Flight forum:

“For a given mix, you have two values (say, for left rudder and right rudder stick command). By default (offset=0), the change between these mix values occurs at the midpoint of the stick travel. Making the offset setting something other than zero basically moves the transition point between the two mix values away from the midpoint of the stick travel. You can test for yourself by making the mix value zero in one direction, altering the offset setting, and then watching where the mix actually kicks in.”

Well, thanks for that, Baron, but what does that actually do for us? Why would we want to move the transition point? I’ve researched it a bit and not found a good explanation. Again, if anyone can shed some light on this, don’t be shy. Until then, just leave the offset at “0” and carry on.

Now hit the UP key to move to PROG.MIX2, which we’ll use for rudder-to-aileron mixing to counteract roll coupling. Use the SELECT key to move the arrow to the top line, and the INCREASE key to bring up “RUDD”. Move the arrow to the right with the SELECT key, and bring up AILE. Set the rates at -20% and -25%, using the left transmitter stick to toggle back and forth. Again, those are settings from my second Hammer. You’ll be fine-tuning yours, of course, but you’ll undoubtedly use aileron mix settings in those very high ranges on your plane. The Hammer is a roll-coupled female dog, and it requires a ton of rudder-to-aileron mixing to make the rudder actually usable.

Here are some links that may (or may not) help clarify this issue somewhat. While there are things to learn in all of them, some of these definitely need to be edited and updated:

BoneDoc on knife-edge mixing

HK’s Red Baron on KE coupling issues

HK’s Red Baron on the KE

Ins and outs of KE flight

(Yes, there are some hard-nosed individuals out there who hate the concept of computerized mixing, and feel that if you have any self-respect or sense of decency and morality at all, you do all the mixing manually, with the sticks, while in flight. Fine. Let them try to compensate for heavy roll coupling and pitch coupling while they’re attempting a knife-edge loop. Meanwhile, we normal people will follow the lead of the best 3D flyers on the planet, and use the nifty features built into modern programmable transmitters.)

Throttle stick kill switch on the transmitter: You can also use one of the six programmable mixes to set up one of the switches on your transmitter to be a throttle stick disabling switch. This is a nice safety feature you can use while you’re working on the plane with the prop installed and the system armed, or maybe when you’re carrying it on and off the field if you don’t have a switch on the plane itself and disconnecting the battery is not convenient. It could keep you from chopping yourself to ribbons if you should inadvertently nudge the throttle stick. With these big props and motors, it’s a real consideration. Obviously, they’re capable of doing some serious damage, and you need to be very careful with them.

Go to PROG.MIX3 (hit the INCREASE key to un-inhibit it) and set it for throttle-to-throttle. Set both rates of the mix at minus 100%. That tells the output to cancel the throttle input altogether. Now set the offset at minus 100% to tell the output you want the shutoff to start at a throttle stick setting of zero.

Finally, select the “FLAP0” switch to activate this mix. That one switch (FLAP MIX) now performs three functions: When in position 1 or 2 it shuts off the two built-in mixes that you won’t be using, and in position 0 it acts as a throttle stick safety switch. Pretty cool, huh?

Differential: This setting makes the ailerons move so that the aileron deflecting upward does so at a greater angle than the aileron deflecting downward. For example, when banking left, the left aileron deflects up more than the right aileron deflects down. The effect is that the greater deflection of the aileron on the inside of the turn creates more aerodynamic drag than the lesser deflection of the aileron on the outside of the turn, which helps to steer the plane into the turn. Think of it as applying a speed brake on the wingtip that’s on the inside of the turn.

That’s fine for a plane that needs help to turn, but, like all proper stunt planes, the Hammer needs no help at all in that regard. IMHO, the only thing that aileron differential does for the Hammer is foul up the way it does barrel rolls. It’ll tend to corkscrew rather than rotate around its centerline. Leave the differential setting at zero, and move on.

(To access the differential menu on a DX7, press the DOWN and SELECT keys simultaneously, then use the UP key to scroll to DIFFERENTIAL. Use the INCREASE key to apply more differential, and the ADJUST key to reduce it all the way down to zero if you so desire.)

Sub-Trim: Usually when you put an output arm on a servo, you'll find there is no notch that will position the arm at exactly 90 degrees to the linkage rod it is attached to. The primary purpose of the sub-trim feature is to enable you to position the servo arm perfectly, so that it’s at a 90-degree angle to the linkage connector rod when your transmitter sticks and trims are centered. This should be done when you install the servo, and you shouldn't have to touch the sub-trims after that.

First, make sure your transmitter’s rudder, elevator, and aileron trims are all centered. Then, to access the sub-trim menu on a DX7, press the DOWN and SELECT keys simultaneously, then use the UP key to scroll to SUB TRIM. Use the SELECT key to move the cursor to the servo you want to adjust, and use the INCREASE and ADJUST keys to move its arm to a position that is 90 degrees to the linkage connector rod.

Binding: Please note! If you’re using an OrangeRX receiver without the R100 satellite receiver, and you later add the R100, you must re-bind your transmitter to the receiver! A range check would also be in order.

Antenna placement when using remote (satellite) receivers: The antennas of primary and satellite receivers should be perpendicular (90-degree angle) to each other for best reception. Here’s a good blurb on using remote receivers:

Horizon Hobbies’ video
Last edited by Depronicus; Oct 06, 2013 at 07:37 PM.
Oct 06, 2013, 06:12 PM
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15 – Cutting (and taping) foam, at last

Refer to the parts layout PDF drawing below for the proper positioning of the paper parts patterns on those expensive foam sheets.

Suit yourself, but I find it’s handy to use a combination of both pins and masking tape to hold the paper parts patterns down on the foam as I trace their shapes. To mark the foam for the many holes required (such as the tab slots in the wings and horizontal fuselage piece, the long spar slots in the wings, and the wire routing holes in the fuselage pieces), I punch a lot of holes through the paper with pins at first, then come back with a sharp pencil and spin the tip a little in a few of the holes to make the pin holes more visible on the foam after the paper pattern is removed. While you’re at it, you might want to punch a few holes in the upper vertical fuselage piece for the side “windows”, as well, to make it easier to position the black Monokote piece you’ll be applying.

Having both a snap-off “boxcutter” utility knife and an X-acto with a #11 blade can be very handy during the foam-cutting process. Make the long cuts with the utility knife, breaking off the end as soon as it begins to dull a little. You really want your blades to be as fresh and sharp as possible.

By the way, you may notice some instances where the photos don’t precisely match the drawing. One of the planes depicted in the photos was completed over a year-and-a-half ago, and has been flown a lot since then. I’ve learned a few things, and revised the drawings accordingly. Go by the drawings if there’s a discrepancy. They’ve been updated many times since some of the photos were taken.

When you have all the parts cut out, test the fit on all of them. Sand and trim until everything drops together with ease. You don’t want any tight fits at this point. Remember that you’re going to be taping the parts at virtually every connection point, and the thickness will be increased. Tight fits of bare pieces will be too tight when taped.

Test the fit of the carbon fiber wing spar tubes as well. Ideally, they should drop into their long slots with no slop, but without having to be pushed in, either. You don’t want them distorting the wing as they’re pressed into place.

As is normal practice on any aerobatic foamy, bevel the leading edges of the ailerons, rudder, and elevator at a 45-degree angle. If you’re going with 9mm Depron or laminated 11.3mm MPF, and you’re using the through-bolted DuBro or Hobby King control horns, you’ll need to leave the area under the control horn un-beveled, per the photos below. You’ll also need to notch the trailing edges of the wings, vertical stab, and horizontal stab as well, as shown. The shots of 9mm material also apply to the thicker laminated sheets. For maximum unhindered control surface travel, you should also bevel the trailing edges of the wings, vertical stabilizer, and horizontal stabilizer at a 10-degree angle.

Take the time to lightly sand and round off the leading edges of all the parts. Yes, there are no airfoil shapes on the Hammer, but nature hates square corners, and rounding off the leading edges with fine sandpaper just makes more sense than leaving them in their as-cut sharp-edge state (although an aerodynamic case can be made for leaving the trailing edges of the control surfaces square, some say). Besides, you’re going to be wrapping the clear reinforcing tape over a high percentage of all the exposed edges, and it’ll wrap over a rounded shape a lot nicer than it will over square corners.

Once again, before you do any taping, temporarily put the three fuselage pieces together and check their fit, and the fit of your firewall. Trim both the fuse pieces and the firewall slots as required, keeping in mind that the fuselage foam will be thicker when double-taped.

Start the taping process with the wings. I really hope you’re using the Scotch HD clear shipping tape I recommended in the third post, above. Cover the long wing spar slot on one side only of both wings. Do it with one long piece of tape if you like, or in two or three sections if you prefer. Flip the wing over and insert the carbon fiber spar tubes into the slots, pressing them down onto the sticky side of the tape. You’re now ready to begin applying quite a bit of thin CA all along the full length of the tubes, on both the fore and aft sides of the long slots. This is why we put the tape on one side of the wing only at this point. You can apply the CA from the open side, and the tape on the closed side keeps it from running out, making a royal mess and gluing the wing to your work surface.

Wait a couple of hours after your first CA application and do it again. Getting the CF spar tubes completely locked onto the wings is crucial for their stiffness. When you feel good about your CA application, and you’re confident you have a good glue joint along the full length of the CF spar on both the fore and aft sides of the slot, put the wings aside for a least a day. You want the CA to fully cure in open air before you come back and seal up the long slot with tape along the full length of what is now the “open” side.

While you wait for the CA on the wing spars to cure, you can start the taping of the rest of the parts. This little task will keep you occupied and safely off the streets for a while. Try not to get too mad at me as you tear off piece after piece of the tape, cutting many into narrow strips, and stick them on. You may find that it gets a bit tedious after a while, but suck it up and do it anyway. Please go by the tape drawings to the letter. I’ve learned the hard way where these damned foamy biplanes like to break, and taping per the drawing will head that off. Double up on the tape where indicated (especially in the firewall area), and take the time and trouble to wrap it around the edges where indicated. The wrapping is a crucial part of strengthening the structure.

Note that you’ll be wrapping the tape on both pieces at the many tab and slot connections. Refer to the photos, which are a bit hard to make out, the subject being clear tape against white foam. Cut the tape into strips narrow enough as required to feed down into the slots, and do your best to have a fairly equal length of the strip on the upper and lower surfaces as you smooth the tape onto the foam.
Last edited by Depronicus; Oct 08, 2013 at 08:34 PM.

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