TexMechsRobot
Posted a thousand or more times
I think I've seen that before and that definitely helps. I'll be around 11.3-12 oz/sqft for wing loading which is very very light. I'll probably be fine with the amount of power this setup produces.
2017/2018 Balsa Build Along Dare/Brodak DC-3
- Thread starter TexMechsRobot
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TexMechsRobot
Posted a thousand or more times
I made some good progress on the nacelles. I need to do a final sanding and then finalize the motor mounts.
Covering this is going to test my skill for sure!
Covering this is going to test my skill for sure!
TexMechsRobot
Posted a thousand or more times
I'm starting to get into the servo installation and connections and noticed something that I hadn't seen before. The design calls for one rudder servo to drive both the rudder and tail wheel separately using pull-pull for both. I have no experience with pull-pull so I did a little digging and have a plan but want to see what you guys think of it. Pardon the pictures but I don't have a way to draw great diagrams today!
The basics as I know it.
If you mount the servo in the center of the plane and the rotation for the pull-pull surface is also in the center of the plane and you run the cables exactly parallel, then the setup will have no slack and perform perfectly.
If the rotation of the surface is in front of the control horn, then the cables must converge towards the middle of the servo. this will create slack on the side that is not being pulled and the air rushing past will cause a recentering of the surface when the servo is no longer pulling.
Below is the diagram (viewed from the top) I plan to implement based on the above assumptions. On the right is the servo with the servo arm, in the middle is the control horn for the tail wheel, and on the left is the control horn for the rudder. The orange dots are the centers of rotation for the tail wheel and rudder.
Thoughts?
The basics as I know it.
If you mount the servo in the center of the plane and the rotation for the pull-pull surface is also in the center of the plane and you run the cables exactly parallel, then the setup will have no slack and perform perfectly.
If the rotation of the surface is in front of the control horn, then the cables must converge towards the middle of the servo. this will create slack on the side that is not being pulled and the air rushing past will cause a recentering of the surface when the servo is no longer pulling.
Below is the diagram (viewed from the top) I plan to implement based on the above assumptions. On the right is the servo with the servo arm, in the middle is the control horn for the tail wheel, and on the left is the control horn for the rudder. The orange dots are the centers of rotation for the tail wheel and rudder.
Thoughts?
I've done pull-pull on a couple bigger planes, and for me it was easiest to simply make sure all connections are lined up. At the servo the two wires connect directly inline with the screw for the control arm. At the rudder both connections are inline directly with the centerline of the hinge gap.
For me, this simply makes it easy to setup and to trouble-shoot, and both wires remained taught enough during travel. Also note that you can cross the wires for the pull-pull as well, with no issues other than the two wires rubbing against each other. When I did that I ran the wires through pushrod sleeves to guide them and prevent chafing.
For me, this simply makes it easy to setup and to trouble-shoot, and both wires remained taught enough during travel. Also note that you can cross the wires for the pull-pull as well, with no issues other than the two wires rubbing against each other. When I did that I ran the wires through pushrod sleeves to guide them and prevent chafing.
TexMechsRobot
Posted a thousand or more times
I'd need to modify the rudder control horn. It basically calls for a toothpick about 1/2" behind the center of rotation. I could probably fashion a more standard control horn that puts the connection point on the rotation. Then I could run everything parallel and not worry about weird geometries. It sounds like I should hook it all up and test it out before I start covering and still have access to the internals.
This is the rudder servo on my 1/4 scale Citabria, using a Dubro pull-pull wire system. The servo arm came with the servo and is capable of WAY more throw than I need for the plane - you can see how it's connected on the 2nd hole out on each side. This system uses wire and crimp connectors, along with all of the other hardware you need to connect to the servo arm and at the tail. I've seen various diagrams of how to connect the hardware based on the servo arm style, but to me this is the simplest way, by using a straight servo arm. I'd be interested in seeing a picture of the plans or instructions that come with your DC-3.
TexMechsRobot
Posted a thousand or more times
I've got a few pictures for you. The first few are of the plans and then the last few are of my build. I settled on a pull-pull with both the rudder and the tail wheel driven from the same servo just like the plans call for. I'm also going to run the cables from the same holes on the servo arm and everything is crossed in the fuselage. The distance between the holes on the servo arm is the same as the distance between the holes on the tail wheel arm and the rudder arm so I shouldn't have any slack in the mechanism.
I used kevlar thread and just used knots and CA at the servo (keeping the part connected to the arm itself free from glue) and the servo arm connectors at the tail. Because of this, I don't think I saved much weight at all but the plans called for tying the thread to the brass arm and I could see this slicing through over time. I will use that method on the rudder since I can inspect it often and change it out when needed.
I used kevlar thread and just used knots and CA at the servo (keeping the part connected to the arm itself free from glue) and the servo arm connectors at the tail. Because of this, I don't think I saved much weight at all but the plans called for tying the thread to the brass arm and I could see this slicing through over time. I will use that method on the rudder since I can inspect it often and change it out when needed.
roversgonemad
Member
Pull-pull works best when you have the control width the as the controlled width, if that makes sense . They can fight a little if they aren't even. It's best if it's a parallelogram.
TexMechsRobot
Posted a thousand or more times
All widths are the same. The hourglass shape has the same physics as the parallelogram concerning tension on the cables as long as the widths of the control arms are the same.
roversgonemad
Member
Sounds like it will work great. Tritle does really nice designs. I've got three of his kits in the queue right now. I am going to build the Dare/Brodak Cessna 195 for the build along. I still have a 60" Super Cub and the AgCat short kits in the box. Thinking a ways ahead, but I think I'll order the highlander short kit next.
That looks like a very straight-forward and well laid out design for the pull-pull.
roversgonemad
Member
Not to hijack, but a fellow on RCG pm'd me after I commented in the highlander thread, and I just got a highlander short kit for $75 shipped!! I'm stoked!
Hmmm, what magic is this? I would like a 25 year old blonde with a trust fund. Now I sit back and wait?
roversgonemad
Member
Fingers crossed!
TexMechsRobot
Posted a thousand or more times
Stringers added! It's starting to look like a plane now 
I now have a lot of little things to do before I can cover it up. I need to:
1. do a final sanding on the nacelles and the fuselage
2. build the canopy as a magnetic hatch for top access to the battery
3. solder extensions for the servos, ESC's, and batteries
4. build and glue on the fairings for the wing
5. finalize the main gear installation
6. locate and cut the hinge slots
There are probably a few things I'm missing there but I think I'll be ready for covering after all of that is complete. I might be able to get started on covering this weekend if all goes well.
I now have a lot of little things to do before I can cover it up. I need to:
1. do a final sanding on the nacelles and the fuselage
2. build the canopy as a magnetic hatch for top access to the battery
3. solder extensions for the servos, ESC's, and batteries
4. build and glue on the fairings for the wing
5. finalize the main gear installation
6. locate and cut the hinge slots
There are probably a few things I'm missing there but I think I'll be ready for covering after all of that is complete. I might be able to get started on covering this weekend if all goes well.
roversgonemad
Member
Beautiful! I love the look of the stick and tissue structures. This is a neat plane. I may have to add this one to the list.
TexMechsRobot
Posted a thousand or more times
I think this one is really nice looking. I'm just nervous about the landing gear. I'm going with the design but I kind of feel like they're under-engineered. I have a habit of over engineering though and so I want to go with the design and see if I'm wrong! I think the worst thing would be a landing that breaks the mounting wood requiring some surgery and rebuilding. Based on the weather here, I won't actually get to test it for quite a few months. It stays rainy and windy until mid-April to May-ish. I'll have to hang this one in the garage until better flying weather comes.
