Dumod Infinite II
- Thread starter Tench745
- Start date
Tench745
Master member
And just like that it was August (or will be by the time anyone reads this) Like usual I've been letting various projects sit and simmer in my brain for a while while I tackle something else. The Dumod has been simmering since January and my brain was ready to start making some progress again.
The motor/retract pods I drew up last time came together fine. They've been glued into the wing and the wiring routed through the center section. They are pretty flimsy torsionally, so I'll probably need to add some bracing to correct for that. The nacelles may actually stiffen them up enough by themselves, but if they don't, a repair will be harder.
With the motors and pods mounted I was able to solder up the battery connections and plug everything into the receiver.
Previously, I had built the retaining dowels etc for the leading edge of the wing. In this update I've installed the retaining mechanism for the trailing edge of the wing. It consists of two 1/16" plywood plates on the wing side, and a 1/8" plywood plate with two spruce blocks glued atop it on the fuselage side. These were through-drilled and tapped for a pair of #10-24 nylon bolts. Initially I was going to use only one bolt in the center to save weight and complexity, but I like the idea of putting any wing loads into the fuselage as close to the fuselage sides as I can for a more direct load-path. The bolts are way long, and will be cut down at a later date.
The tail surfaces have been covered with fiberglass and WBPU and the outer V-stabs have been glued in place. I'm holding off on attaching the tail to the fuselage because I'm sure I've forgotten something which will be easier to deal with without the tail on.
The Beech 18 swung 99" diameter Hamilton Standard props. Here's what that looks like.
You'll notice that the prop tip only just clears the fuselage because the nose tapers away at that point. Now recall that the Dumod stretched the Beech fuselage forward more than 6 feet. At 1/12 scale a 99" prop comes out to about 8.25" diameter. Here is what an 8" prop looks like mounted to the motor. There is less than 1/8" clearance there. (1.5" in full scale.) Now I realize why the Dumod switched to 3-blade props; it let them drop the diameter enough to get the fuselage clearance. I may have to do the same.
Side note, I used a label maker for the servo wires and love how tidy it looks.
I had run the numbers for an 8" prop through web-o-calc and prop-calc previously and figured I needed an 8x8 2 blade. Now I need to do it again to figure out if I can run a commercially available 3-blade or a smaller 2-blade and still make my performance numbers. Maybe I'll have to cut my 8x8s down a little. We'll see. Anyone with prop suggestions etc, I'm open to feedback.
And finally, a beauty shot with the tail set in place temporarily.
Things I know I have to do still:
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays
-Line ailerons with balsa, then add control horns and hinges.
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Figure out a battery hatch if necessary
-Paint
-Practice (haven't flown anything in months)
-Preflight inspections/checks
-Maiden!
The motor/retract pods I drew up last time came together fine. They've been glued into the wing and the wiring routed through the center section. They are pretty flimsy torsionally, so I'll probably need to add some bracing to correct for that. The nacelles may actually stiffen them up enough by themselves, but if they don't, a repair will be harder.
With the motors and pods mounted I was able to solder up the battery connections and plug everything into the receiver.
Previously, I had built the retaining dowels etc for the leading edge of the wing. In this update I've installed the retaining mechanism for the trailing edge of the wing. It consists of two 1/16" plywood plates on the wing side, and a 1/8" plywood plate with two spruce blocks glued atop it on the fuselage side. These were through-drilled and tapped for a pair of #10-24 nylon bolts. Initially I was going to use only one bolt in the center to save weight and complexity, but I like the idea of putting any wing loads into the fuselage as close to the fuselage sides as I can for a more direct load-path. The bolts are way long, and will be cut down at a later date.
The tail surfaces have been covered with fiberglass and WBPU and the outer V-stabs have been glued in place. I'm holding off on attaching the tail to the fuselage because I'm sure I've forgotten something which will be easier to deal with without the tail on.
The Beech 18 swung 99" diameter Hamilton Standard props. Here's what that looks like.
You'll notice that the prop tip only just clears the fuselage because the nose tapers away at that point. Now recall that the Dumod stretched the Beech fuselage forward more than 6 feet. At 1/12 scale a 99" prop comes out to about 8.25" diameter. Here is what an 8" prop looks like mounted to the motor. There is less than 1/8" clearance there. (1.5" in full scale.) Now I realize why the Dumod switched to 3-blade props; it let them drop the diameter enough to get the fuselage clearance. I may have to do the same.
Side note, I used a label maker for the servo wires and love how tidy it looks.I had run the numbers for an 8" prop through web-o-calc and prop-calc previously and figured I needed an 8x8 2 blade. Now I need to do it again to figure out if I can run a commercially available 3-blade or a smaller 2-blade and still make my performance numbers. Maybe I'll have to cut my 8x8s down a little. We'll see. Anyone with prop suggestions etc, I'm open to feedback.
And finally, a beauty shot with the tail set in place temporarily.
Things I know I have to do still:
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays
-Line ailerons with balsa, then add control horns and hinges.
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Figure out a battery hatch if necessary
-Paint
-Practice (haven't flown anything in months)
-Preflight inspections/checks
-Maiden!
Tench745
Master member
The CAD file was brought into service to determine where and how to mount a 90° retract to put the nose wheel where it needs to be in both the up and down positions. I had to play with it and try a number of locations before I got something that would get the nose wheel in the right place when extended and still let it retract fully into the nose.
With that figured out I could then bend the gear wire and cut it to proper length. Somewhere in the middle of all that I realized that would leave a giant hole in the nose of the aircraft most of the time, so I laid up some fiberglass using the nose as a mould in case I want to make some gear doors in the future.
Cutting the nacelles to fit the motor pods was tedious. I split the first one horizontally and attacked it with a Dremel until I could get each half to fit snugly around the motor pod. With the first nacelle done, I realized that I could remove most of the material with my hot-wire cutter far quicker and cleaner than I had on the first before doing a final fitting with the Dremel.
I'm not entirely happy with the fit and finish on the nacelles yet and they still need cutouts for the landing gear, so they will be revisited. I care less about gear doors on the nacelles, so I probably won't do any glass work with the nacelles like I did the nose. I'm wishing a little bit that I had made solid patterns for the nacelles so I could vacu-form them, saving myself weight and a lot of hassle fitting them.
The motor pods are plenty strong so far as loads from the motor and gear are concerned. There's plenty of meat to transfer thrust to the wing and landing loads. However, there is nothing to keep them from twisting along the motor's axis, nor to transfer loads from the wing through the giant hole I had to cut in the leading edge to install the pod. A pair of 1/8" ply braces were cut and CA'd in place. The aft brace functions as a spar carry-through and the forward brace helps stiffen the wing's leading edge. I may still need some diagonal bracing to resist twisting from motor torque.
Speaking of spars, they were finally epoxied in place. I wanted to do this before gluing balsa in the aileron bays because I wanted the wings to be as straight as they can be. I know the left wing had a bit of up-sweep towards the wingtip before installing the spars. The ailerons were then cut free and the bays lined with balsa. This was a mistake.
I've been trying to figure out for quite some time how I wanted to do the ailerons. I always wanted to try making a pocket in the wing with some thin ply and thought this would be a good subject to do that with. But... I'm a stickler for trying to do things "scale" and upon researching Beech 18 ailerons, I found out that they are held on with a piano-hinge from the top skin of the wing to a peak atop the aileron. Beech 18 aileron:
I decided I would try that. I'm not sure yet what I will use for the hinge; I have some Blenderm tape I'll try if I don't come up with any better ideas. (Suggestions?) Sure, I could have done something simpler, but where's the fun in that?
The aileron was trimmed smaller than the opening and edged with balsa. The sides are 1/16" balsa, and the LE is 1/4". Now we come back to the mistake I mentioned earlier. Hinging the aileron from the top peak like on the full scale means that when the aileron deflects downward the leading edge actually moves forward. To fix this I had to cut out the balsa I added to the aileron bay and cut the bay deeper to allow for this movement. Two strips of 1/32" ply were then cut to be the top of the aileron pocket which the aileron will now hinge to. A recess was sanded into each wing to fit those ply strips which were then epoxied in place with lots of clothespin clamps. You can never own too many clamps.
A smaller piece of ply will finish out the bottom portion of the aileron pocket once I figure out how big it can be and not interfere with the aileron LE or control horn.
An update on my list:
New, Old,Completed
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons andadd balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges.
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Figure out a battery hatch if necessary
-Paint
-Practice (haven't flown anything in months)
-Preflight inspections/checks
-Maiden!
With that figured out I could then bend the gear wire and cut it to proper length. Somewhere in the middle of all that I realized that would leave a giant hole in the nose of the aircraft most of the time, so I laid up some fiberglass using the nose as a mould in case I want to make some gear doors in the future.
Cutting the nacelles to fit the motor pods was tedious. I split the first one horizontally and attacked it with a Dremel until I could get each half to fit snugly around the motor pod. With the first nacelle done, I realized that I could remove most of the material with my hot-wire cutter far quicker and cleaner than I had on the first before doing a final fitting with the Dremel.
I'm not entirely happy with the fit and finish on the nacelles yet and they still need cutouts for the landing gear, so they will be revisited. I care less about gear doors on the nacelles, so I probably won't do any glass work with the nacelles like I did the nose. I'm wishing a little bit that I had made solid patterns for the nacelles so I could vacu-form them, saving myself weight and a lot of hassle fitting them.
The motor pods are plenty strong so far as loads from the motor and gear are concerned. There's plenty of meat to transfer thrust to the wing and landing loads. However, there is nothing to keep them from twisting along the motor's axis, nor to transfer loads from the wing through the giant hole I had to cut in the leading edge to install the pod. A pair of 1/8" ply braces were cut and CA'd in place. The aft brace functions as a spar carry-through and the forward brace helps stiffen the wing's leading edge. I may still need some diagonal bracing to resist twisting from motor torque.
Speaking of spars, they were finally epoxied in place. I wanted to do this before gluing balsa in the aileron bays because I wanted the wings to be as straight as they can be. I know the left wing had a bit of up-sweep towards the wingtip before installing the spars. The ailerons were then cut free and the bays lined with balsa. This was a mistake.
I've been trying to figure out for quite some time how I wanted to do the ailerons. I always wanted to try making a pocket in the wing with some thin ply and thought this would be a good subject to do that with. But... I'm a stickler for trying to do things "scale" and upon researching Beech 18 ailerons, I found out that they are held on with a piano-hinge from the top skin of the wing to a peak atop the aileron. Beech 18 aileron:
I decided I would try that. I'm not sure yet what I will use for the hinge; I have some Blenderm tape I'll try if I don't come up with any better ideas. (Suggestions?) Sure, I could have done something simpler, but where's the fun in that?
The aileron was trimmed smaller than the opening and edged with balsa. The sides are 1/16" balsa, and the LE is 1/4". Now we come back to the mistake I mentioned earlier. Hinging the aileron from the top peak like on the full scale means that when the aileron deflects downward the leading edge actually moves forward. To fix this I had to cut out the balsa I added to the aileron bay and cut the bay deeper to allow for this movement. Two strips of 1/32" ply were then cut to be the top of the aileron pocket which the aileron will now hinge to. A recess was sanded into each wing to fit those ply strips which were then epoxied in place with lots of clothespin clamps. You can never own too many clamps.
A smaller piece of ply will finish out the bottom portion of the aileron pocket once I figure out how big it can be and not interfere with the aileron LE or control horn.
An update on my list:
New, Old,
-Mount the nose gear retract
-Nose gear doors?
-Decide how the nose will mount and mount it.
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and
-Install aileron servos
-Run aileron servo wiring
-Cut nacelles for gear
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Figure out a battery hatch if necessary
-Paint
-Practice (haven't flown anything in months)
-Preflight inspections/checks
-Maiden!
bracesport
Legendary member
Tench745
Master member
I had intended to use WBPU and glass just for a cosmetic finish, but I suppose I could switch to epoxy. I'd have to source some kevlar. I'm open to other options as well.
Tench745
Master member
@bracesport what weave and weight of kevlar would you recommend? I’ve done fiberglass work before, but never touched kevlar. Is there a good source for small quantities?
bracesport
Legendary member
you would have to sniff around in your neck of the woods - but search for composite material suppliers or even a local fibreglass company - mine was a kevlar tape and a tad heavier than it needed to be, but if you recess it first then glass over it any thickness can work - for glass I use a 'super' light cloth that essentially goes invisible when used.
Tench745
Master member
I'm not entirely happy with how the nacelles are turning out and being one not to make things easy on myself, I'm thinking I may vacu-form the main body of the nacelles to better fit the Parkflyer Plastics cowls I bought. They're symmetrical except for the cutouts for the wings, so I think I can make one set of plugs to pull both a left and right nacelle from, then trim to fit. I'm still debating this one. I think plastic nacelles would be lighter, easier to replace if damaged, easier to work on the motor or retracts, and easier to fit to the wing. I would have to make a plug, get some more plastic, and make sure I can pull something that size on my vac-forming box.
Now onto the actual work I've done.
I laid out and routed servo pockets and channels for the aileron servos and wires. I made up a y-harness and fed everything into place. The channels were filled with some strips of foam epoxied in place, then cut proud with a razor blade and sanded smooth. The servos were hot-glued in place. I then went over the whole wing, filling any imperfections and low spots with some HobbyLite Filler. It's just a little finer grain and sands nicer than drywall spackle.
After a lot of CAD work to make sure I was doing everything right, I cut out the underside of the nose for the nose gear retracts. Then I used my templates of the nose sections to make a 1/8" plywood plate to hold the retract. There's approximately a 12° shim under the plate to give the nose gear the correct rake angle. The retract I have is a replacement part for a Horizon Hobby jet of some sort and is designed to let the nose gear swive for steering but because I did not buy the strut and everything this one has no steering arm. I had intended to just let the nose gear caster freely, but I need some way to orient it correctly so it can retract into the gear well. Whether this is a steering linkage or something else remains to be seen. Taxi testing will tell me how much more work this system will need.
Speaking of taxi testing, if I want this bird to stand on it's own two (three) feet, I need a way to hold the nose on. For now I've decided to just epoxy it on, and if I need to add a hatch or something I can cut it away as necessary. With the nose finally glued in place I can sand the fuselage and nose to final shape, matching each other. Mating the two makes it obvious that I did not get the taper on the side of the nose consistent left to right; there is a lot more material exposed on the right side. I'll put a template on it again, correct, and shape the fuselage to match.
And here's a final beauty shot of it all mocked up. This was actually taken before the nose or retract were glued, but it makes for a good summary picture.
An update on my list:
New, Old,Completed
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel steering/centering
-Taxi testing
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges.
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-New nacelles?
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, (in progress) and glass wings
-Battery tray/hold-downs
-Figure out a battery hatch if necessary
-Paint
-Practice (Flew my McB Spirit twice last weekend)
-Preflight inspections/checks
-Maiden!
Now onto the actual work I've done.
I laid out and routed servo pockets and channels for the aileron servos and wires. I made up a y-harness and fed everything into place. The channels were filled with some strips of foam epoxied in place, then cut proud with a razor blade and sanded smooth. The servos were hot-glued in place. I then went over the whole wing, filling any imperfections and low spots with some HobbyLite Filler. It's just a little finer grain and sands nicer than drywall spackle.
After a lot of CAD work to make sure I was doing everything right, I cut out the underside of the nose for the nose gear retracts. Then I used my templates of the nose sections to make a 1/8" plywood plate to hold the retract. There's approximately a 12° shim under the plate to give the nose gear the correct rake angle. The retract I have is a replacement part for a Horizon Hobby jet of some sort and is designed to let the nose gear swive for steering but because I did not buy the strut and everything this one has no steering arm. I had intended to just let the nose gear caster freely, but I need some way to orient it correctly so it can retract into the gear well. Whether this is a steering linkage or something else remains to be seen. Taxi testing will tell me how much more work this system will need.
Speaking of taxi testing, if I want this bird to stand on it's own two (three) feet, I need a way to hold the nose on. For now I've decided to just epoxy it on, and if I need to add a hatch or something I can cut it away as necessary. With the nose finally glued in place I can sand the fuselage and nose to final shape, matching each other. Mating the two makes it obvious that I did not get the taper on the side of the nose consistent left to right; there is a lot more material exposed on the right side. I'll put a template on it again, correct, and shape the fuselage to match.
And here's a final beauty shot of it all mocked up. This was actually taken before the nose or retract were glued, but it makes for a good summary picture.
An update on my list:
New, Old,
-
-
-Nose gear doors?
-Nose wheel steering/centering
-Taxi testing
-
-Decide how the nose will mount and mount it.
-
-
-
-
-
-
-
-Cut nacelles for gear
-New nacelles?
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, (in progress) and glass wings
-Battery tray/hold-downs
-Figure out a battery hatch if necessary
-Paint
-Practice (Flew my McB Spirit twice last weekend)
-Preflight inspections/checks
-Maiden!
Tench745
Master member
I managed to get a few minutes of taxi testing in this morning. The nose gear worked well but needed a lot of rudder (differential thrust) to reverse the direction of the turn. I'm happy enough with this that I can move forward on other things and tweak the nose gear over time.
I need to figure out a way to center the nose gear before it retracts into the fuselage. I was researching ways others have done this, but I came up with very little on Google. Most of the examples I saw used the linkage from a steering servo and/or a centering spring.
I'm thinking a very light wire spring may be enough to center the nose gear, but may be too much to let the gear caster freely. I will have to experiment. My other thought is to have some sort of arm/cam on the landing gear wire which will nestle in a saddle of some sort to align the gear as it comes up.
While I was looking at nose gear retracts I realized that I never put any kind of spring loop in my gear wire to take landing loads.
If I need to add a loop I will have to remake all three gear wires. I'd be interested in hearing from anyone who has experience with straight-wire landing gear vs looped. Maybe @willsonman, @FAI-F1D, or @wilmracer have some relevant experience with gear wire and retracts?
Before the taxi testing I finished re-shaping the fuselage, chamfering the upper fuselage sides to match the nose. Not much material was removed, but I think it looks significantly better now; far less boxy. It was, however, enough material that I didn't feel like sanding it all away. So I laid out two strips of aluminum tape just inside of the lines I wanted to cut to. The tape served as a guide for my hot-wire bow and let me quickly remove a lot of material with little mess. After the tape was removed I could sneak up on my lines with the sanding block. The photo series below illustrates the process.
Right now I'm filling and sanding things to final shape, then the fuselage will get glassed; possibly tonight.
Overall weight is coming in at about 40-41oz with battery so far.
New, Old,Completed, eliminated
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel centering
-Taxi testing
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges.
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-New nacelles?
-Sand and glass fuselage (In Progress)
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, (in progress) and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
I need to figure out a way to center the nose gear before it retracts into the fuselage. I was researching ways others have done this, but I came up with very little on Google. Most of the examples I saw used the linkage from a steering servo and/or a centering spring.
I'm thinking a very light wire spring may be enough to center the nose gear, but may be too much to let the gear caster freely. I will have to experiment. My other thought is to have some sort of arm/cam on the landing gear wire which will nestle in a saddle of some sort to align the gear as it comes up.
While I was looking at nose gear retracts I realized that I never put any kind of spring loop in my gear wire to take landing loads.
If I need to add a loop I will have to remake all three gear wires. I'd be interested in hearing from anyone who has experience with straight-wire landing gear vs looped. Maybe @willsonman, @FAI-F1D, or @wilmracer have some relevant experience with gear wire and retracts?
Before the taxi testing I finished re-shaping the fuselage, chamfering the upper fuselage sides to match the nose. Not much material was removed, but I think it looks significantly better now; far less boxy. It was, however, enough material that I didn't feel like sanding it all away. So I laid out two strips of aluminum tape just inside of the lines I wanted to cut to. The tape served as a guide for my hot-wire bow and let me quickly remove a lot of material with little mess. After the tape was removed I could sneak up on my lines with the sanding block. The photo series below illustrates the process.
Right now I'm filling and sanding things to final shape, then the fuselage will get glassed; possibly tonight.
Overall weight is coming in at about 40-41oz with battery so far.
New, Old,
-Mount the nose gear retract
-
-Nose wheel centering
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-New nacelles?
-Sand and glass fuselage (In Progress)
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, (in progress) and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
PsyBorg
Wake up! Time to fly!
@Tench745 I recently saw a video on gear operation and noticed the nose gear used a pull pull wire system to steer. As the gear retracted the wires got really slack but just before entering the fuse there were / was "V" bar that spread the wires as it came up putting tension back on the gear wire and keeping the nose wheel straight as it entered the housing.
Just dont ask my frazzled mind to remember exactly where I saw it.
Just dont ask my frazzled mind to remember exactly where I saw it.
In any situation where you are not using an oleo ( linearly sprung) strut you should always use a coiled spring strut. The reason is that this will dissipate the shocks from rolling or otherwise from ripping apart your retract unit. A simple straight wire will likely bend on landing, or damage the retract assembly... or entirely rip out the retract.
As far as keeping the wheel straight, there are simple solutions and complex ones but what matters is what will work for your application. In my experience the "side arm" control has always been pretty bullet proof.
Here is a link to an example:
https://hobbyking.com/en_us/servole...450&indexName=hbk_live_magento_en_us_products
The idea is that the control rod for the steering remains fixed on the side of the bracket at the same pivot point as the trunion. When the gear retracts, that point maintains the steering angle of the strut as it retracts and will not allow any movement of the strut while retracted. So, linking to your rudder servo channel will not interfere or bind while retracted.
As far as keeping the wheel straight, there are simple solutions and complex ones but what matters is what will work for your application. In my experience the "side arm" control has always been pretty bullet proof.
Here is a link to an example:
https://hobbyking.com/en_us/servole...450&indexName=hbk_live_magento_en_us_products
The idea is that the control rod for the steering remains fixed on the side of the bracket at the same pivot point as the trunion. When the gear retracts, that point maintains the steering angle of the strut as it retracts and will not allow any movement of the strut while retracted. So, linking to your rudder servo channel will not interfere or bind while retracted.
Tench745
Master member
Not much has happened visually since the last update.
I remade the landing gear with a single spring coil in each wire. I thought this would be fairly quick, but adding the coil added a lot of time; about 5 hours to remake all the gear wires.
The wing and fuselage have been glassed and the weave filled with a mixture of talc and WBPU. Today was spent sanding off all the runs and drips. I kept forgetting to shape the wingtip leading edges and had to go back today to do that and re-apply glass to them.
Today also saw the dorsal and ventral fins cut, shaped, and epoxied in place on the tail. It's worth noting that I had completely forgotten that there was a ventral fin until I went back to my reference pictures. The new fins will be glassed soon. The central tail needs to be shaped to match the dorsal fin. I think the best choice will be to install the H-stab. and V-stabs. then glass the fins on. I'm deciding whether to fill and sand the H-stab. assembly before of after installing it on the plane.
New, Old,Completed, eliminated
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel centering
-Taxi testing
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges.
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-New nacelles?
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together.
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
I remade the landing gear with a single spring coil in each wire. I thought this would be fairly quick, but adding the coil added a lot of time; about 5 hours to remake all the gear wires.
The wing and fuselage have been glassed and the weave filled with a mixture of talc and WBPU. Today was spent sanding off all the runs and drips. I kept forgetting to shape the wingtip leading edges and had to go back today to do that and re-apply glass to them.
Today also saw the dorsal and ventral fins cut, shaped, and epoxied in place on the tail. It's worth noting that I had completely forgotten that there was a ventral fin until I went back to my reference pictures. The new fins will be glassed soon. The central tail needs to be shaped to match the dorsal fin. I think the best choice will be to install the H-stab. and V-stabs. then glass the fins on. I'm deciding whether to fill and sand the H-stab. assembly before of after installing it on the plane.
New, Old,
-Mount the nose gear retract
-
-Nose wheel centering
-Solder stop-washers to the inside of all landing gear wires.
-Decide how the nose will mount and mount it.
-Install spars
-Make braces and a spar carry-through for the motor mounts.
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-New nacelles?
-Install horizontal tail
-
-
-
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
Tench745
Master member
A quick(ish) update. I've been working up at my friend's marina past few weekends helping him catch up on canvas jobs. While I was in a sewing mood I restitched my truck's tonneau cover. I also took a little time to finish tack-welding the tail surfaces for my Corben Jr Ace. While working on those projects I let solutions for Dumod problems stew in my mind; now it's time to stop stewing and start doing.
I didn't get any in-progress pictures of it, but the H-stab and triple tail were epoxied in place, faired, glassed, filled, and sanded.
I don't want the added weight and complexity of a steering servo and I'm happy with the way the free-castering nose wheel behaves thus far. More taxi testing is in order. The solution I've come up with for centering the nose wheel as it retracts is pretty simple; the accompanying pictures probably explain it best.
I had some extra 6-horn control horns from my large Spirit of St Louis build which fit very nicely over the nose gear wire. I nipped all but two adjacent horns off and epoxied the resulting V shape to the nose gear where it will hit the fuselage and align the gear before the wheel can hit anything. I will trim down the areas of contact enough that the V can recess fully and then I'll add a little 1/64 ply for abrasion resistance. This should work fine as long as the nose gear doesn't rotate much past 90°.
I'm still undecided on how I want to proceed with the nacelles. I'm not happy with how they fit the wing and plastic cowlings, so I'm debating making a plug and forming new ones, or futzing with the existing nacelles enough to get them to a stage I'm happy with. Making a plug seems like a lot of extra work but may be cleaner and easier in the long run.
If I did make a new plug I would have to decide what I wanted to mould the new nacelles in. Laying them up in fiberglass would almost certainly be the heaviest solution, but pretty durable. Vacforming some sheet styrene in a top and bottom half would require a two-part plug. The plastic cowlings I have weigh 12grams. The foam nacelles weigh 20 without filler or fiberglass. I'm not sure there would be a weight savings in going with vac-formed nacelles, and that would be a lot of work for no appreciable benefit.
Heat-shrinking a PETG bottle around a single plug would be relatively quick and easy, but almost definitely heavier than the vac-formed version.
I'm tempted to just make the plug and try all three to see what the comparative weights look like but I'd also like to get this plane moving forward so I can get some paint sprayed before it's too cold out.
It's looking like the plane will balance with the battery about over the main spar, but that'll change when I finally get the nacelles figured out. It's close enough that I think I can safely cut a battery hatch in the cockpit area and have all the access I need. That will probably happen tonight as soon as the epoxy on my gear is cured.
New, Old,Completed, eliminated
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel centering
-Taxi testing (continued)
-Solder stop-washers to the inside of all landing gear wires
-Decide how the nose will mount and mount it
-Install spars
-Make braces and a spar carry-through for the motor mounts
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-Cut nacelles for gear
-New nacelles?
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
I didn't get any in-progress pictures of it, but the H-stab and triple tail were epoxied in place, faired, glassed, filled, and sanded.
I don't want the added weight and complexity of a steering servo and I'm happy with the way the free-castering nose wheel behaves thus far. More taxi testing is in order. The solution I've come up with for centering the nose wheel as it retracts is pretty simple; the accompanying pictures probably explain it best.
I had some extra 6-horn control horns from my large Spirit of St Louis build which fit very nicely over the nose gear wire. I nipped all but two adjacent horns off and epoxied the resulting V shape to the nose gear where it will hit the fuselage and align the gear before the wheel can hit anything. I will trim down the areas of contact enough that the V can recess fully and then I'll add a little 1/64 ply for abrasion resistance. This should work fine as long as the nose gear doesn't rotate much past 90°.
I'm still undecided on how I want to proceed with the nacelles. I'm not happy with how they fit the wing and plastic cowlings, so I'm debating making a plug and forming new ones, or futzing with the existing nacelles enough to get them to a stage I'm happy with. Making a plug seems like a lot of extra work but may be cleaner and easier in the long run.
If I did make a new plug I would have to decide what I wanted to mould the new nacelles in. Laying them up in fiberglass would almost certainly be the heaviest solution, but pretty durable. Vacforming some sheet styrene in a top and bottom half would require a two-part plug. The plastic cowlings I have weigh 12grams. The foam nacelles weigh 20 without filler or fiberglass. I'm not sure there would be a weight savings in going with vac-formed nacelles, and that would be a lot of work for no appreciable benefit.
Heat-shrinking a PETG bottle around a single plug would be relatively quick and easy, but almost definitely heavier than the vac-formed version.
I'm tempted to just make the plug and try all three to see what the comparative weights look like but I'd also like to get this plane moving forward so I can get some paint sprayed before it's too cold out.
It's looking like the plane will balance with the battery about over the main spar, but that'll change when I finally get the nacelles figured out. It's close enough that I think I can safely cut a battery hatch in the cockpit area and have all the access I need. That will probably happen tonight as soon as the epoxy on my gear is cured.
New, Old,
-
-
-
-
-Taxi testing (continued)
-
-
-
-
-
-
-
-
-
-Cut nacelles for gear
-New nacelles?
-
-Install horizontal tail
-
-
-
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
Tench745
Master member
It's been almost a month since the last update here so I guess I should say something. I decided not to build new nacelles; glassed foam seems to be the lightest option and I'd already done most of the work. I did realize that when originally fitting the nacelles to the wing, I had them positioned too far forward. In order to move them aft I had to re-cut the openings for the wings. This was actually a good thing as I had never been happy with the fit out the old cutouts and I now had a chance to improve it. Things look much better now. The nacelles have been epoxied onto the wing and glassed, but the glass still needs a coat of WBPU and talc to fill the weave. After that I'll cut openings for the landing gear and mount the plastic cowlings.
I also elected to hold off on cutting a battery hatch. I felt like using the wing cutout would be sufficient for loading up batteries so long as I can figure out somewhere to put a battery/arming switch I can access once the wing is mounted. I don't like the idea of powering up the whole plane and then trying to wrangle wires and bolt a wing into place. This will have to be sussed out before I can head into painting, which is very close. If I'm adding a battery hatch, a battery tray should happen at the same time. If I'm not adding a hatch, the tray could happen either before or after paint.
New, Old,Completed, eliminated
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel centering
-Taxi testing (continued)
-Solder stop-washers to the inside of all landing gear wires
-Decide how the nose will mount and mount it
-Install spars
-Make braces and a spar carry-through for the motor mounts
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-New nacelles?
-Install nacelles
-Cut nacelles for gear
-Glass and sand nacelles
-Install cowlings
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
I also elected to hold off on cutting a battery hatch. I felt like using the wing cutout would be sufficient for loading up batteries so long as I can figure out somewhere to put a battery/arming switch I can access once the wing is mounted. I don't like the idea of powering up the whole plane and then trying to wrangle wires and bolt a wing into place. This will have to be sussed out before I can head into painting, which is very close. If I'm adding a battery hatch, a battery tray should happen at the same time. If I'm not adding a hatch, the tray could happen either before or after paint.
New, Old,
-
-
-
-
-Taxi testing (continued)
-
-
-
-
-
-
-
-
-
-
-
-Cut nacelles for gear
-
-Install cowlings
-
-Install horizontal tail
-
-
-
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
Tench745
Master member
I'm a little excited, I found a few more pictures of the Commuter Airlines Dumod fleet. There are some good detail pictures here including one of the nose, showing just how much damage rain and ice slung from the prop tips can do to paint.
Not much more to say about the model itself. Nearing paint.
Not much more to say about the model itself. Nearing paint.
Last edited:
Tench745
Master member
Since my last photo-update I sprayed a light coat of primer over most of the aircraft and mostly fitted the nacelles. About that time my dad asked for some pictures of my models to show a student, so I shot these beauty-shots of the Dumod. A lot of clearancing was necessary to make the cowls fit over my chosen motors, but I think it turned out pretty good. After the cowling is glued in place I may run some sandpaper around the motor to get the cutout in the cowl a little rounder.
I decided against a battery hatch because I couldn't figure out a good place to locate it and retain sufficient strength in the fuselage.
Instead, I recessed a 1/16" ply plate in the underside of the wing and added a small on-off switch to the power lead from the BEC.
Now I'm working on mounting the cowlings to the nacelles.
I made a 1/16" plywood ring that fits pretty tightly inside the cowling. The ring is drilled for three rare-earth magnets and the leading edge of the nacelle gets recesses for three corresponding magnets.
To mount the cowlings and make sure they're square to the motor I made a square of foam with a hole that fits tightly over the prop shaft This foam square gives me a relatively flat reference plane and acts as a clamping block. There is another foam spacer between the square and the cowling which gives the correct amount of clearance between prop and cowl.
To actually mount the cowl I sandwich a piece of cling-wrap between the ply ring and nacelle, smear epoxy around the perimeter of the ring, and then slide the nacelle into place. The clamping block and spacer then go on and get secured with a prop nut and washer. The cowl is then butted tightly to the spacer block and we wait for the epoxy to cure.
Next up; aileron control horns, paint and a battery tray!
New, Old,Completed, eliminated
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel centering
-Taxi testing (continued)
-Solder stop-washers to the inside of all landing gear wires
-Decide how the nose will mount and mount it
-Install spars
-Make braces and a spar carry-through for the motor mounts
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-New nacelles?
-Install nacelles
-Cut nacelles for gear
-Glass and sand nacelles
-Install cowlings
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
I decided against a battery hatch because I couldn't figure out a good place to locate it and retain sufficient strength in the fuselage.
Instead, I recessed a 1/16" ply plate in the underside of the wing and added a small on-off switch to the power lead from the BEC.
Now I'm working on mounting the cowlings to the nacelles.
I made a 1/16" plywood ring that fits pretty tightly inside the cowling. The ring is drilled for three rare-earth magnets and the leading edge of the nacelle gets recesses for three corresponding magnets.
To mount the cowlings and make sure they're square to the motor I made a square of foam with a hole that fits tightly over the prop shaft This foam square gives me a relatively flat reference plane and acts as a clamping block. There is another foam spacer between the square and the cowling which gives the correct amount of clearance between prop and cowl.
To actually mount the cowl I sandwich a piece of cling-wrap between the ply ring and nacelle, smear epoxy around the perimeter of the ring, and then slide the nacelle into place. The clamping block and spacer then go on and get secured with a prop nut and washer. The cowl is then butted tightly to the spacer block and we wait for the epoxy to cure.
Next up; aileron control horns, paint and a battery tray!
New, Old,
-
-
-
-
-Taxi testing (continued)
-
-
-
-
-
-
-
-
-
-
-
-
-
-Install cowlings
-
-Install horizontal tail
-
-
-
-Battery tray/hold-downs
-
-Paint
-Practice
-Preflight inspections/checks
-Maiden!
Tench745
Master member
Short update: I finally got around to cutting out the vinyl for this and I am SO happy with how it came out.
I believe the only thing left to do on this build is the ailerons; paint and attach them, install control horns, and rig pushrods.
New, Old,Completed, eliminated
-Taxi testing (continued)
-Paint and install ailerons
-Practice
-Preflight inspections/checks
-Maiden!
-Bend up landing gear wire for the nose wheel
-Mount the nose gear retract
-Nose gear doors?
-Nose wheel centering
-Solder stop-washers to the inside of all landing gear wires
-Decide how the nose will mount and mount it
-Install spars
-Make braces and a spar carry-through for the motor mounts
-Cut out ailerons and add balsa to aileron bays (in process)
-Line ailerons with balsa, then add control horns and hinges
-Install aileron servos
-Run aileron servo wiring
-Cut out nacelles to fit around new motor pods
-New nacelles?
-Install nacelles
-Cut nacelles for gear
-Glass and sand nacelles
-Install cowlings
-Sand and glass fuselage
-Install horizontal tail
-Make dorsal strake, install central tail and glass together
-Shape wingtips
-Fill, sand, and glass wings
-Battery tray/hold-downs
-Locate and size a battery hatch
-Paint
I believe the only thing left to do on this build is the ailerons; paint and attach them, install control horns, and rig pushrods.
New, Old,
-Taxi testing (continued)
-Paint and install ailerons
-Practice
-Preflight inspections/checks
-Maiden!
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-Install horizontal tail
-
-
-
-Battery tray/hold-downs
-
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