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DTFB Skymaster (build)

rcspaceflight

creator of virtual planes
#1
This idea got kicked around a bit earlier this year. http://forum.flitetest.com/showthread.php?7212-Skymaster&highlight=skymaster

I was going to just work on it and keep quiet about it, but I think it's going to be awesome and I have to share. I also have a few questions and concerns, so I want input.

For those that don't know, this is a Cessna Skymaster:


This is the 3 view I'm using:
Cessna 337 Skymaster [LIMITED to 500px].jpg

So far I just started on designing it using Sketchup. This is actually the first plane that I'm designing using Sketchup. I've designed a few planes, but all by hand. I've made a few planes for RC Desk Pilot using Sketchup, but setting out to make plans to print out and cut out is a first.

Sketchup pics:

This is the parallel view, so it looks weird. But the point is to show how tight the top power pod is to the wings. I can't exactly make the plane smaller due to this.
ScreenHunter_366 Apr. 27 20.57.jpg

Same angle, but perspective.
ScreenHunter_367 Apr. 27 20.57.jpg

I don't have the wing notches set right yet, they're just for the concept currently.
ScreenHunter_368 Apr. 27 20.57.jpg

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Questions/Concerns:

I want this plane to be a work horse. Something that has a lot of room in the fuselage. Can be a twin engine with two power pods, FT Elements instead of power pods, a single engine tracker, a singe engine pusher, or an FPV pusher plane. You know, something that looks scale, and yet is very versatile.

Currently the wingspan is 54". That is definitely a workhorse wingspan. It sounds big to me, but nothing ridiculous. I think it has to stay that big though. The rear power pod currently just had enough room between it and the wings to make it removable. Also, to keep the scale look you'd want an 8" or 9" prop. Which is the most common sizes. So I think the 54" wingspan has to stay even though I'd prefer it smaller.

I currently have a support piece on the flat part of the bottom of the fuselage, a middle support to hold the power pods, and a windshield support. Do you think that's enough to keep it strong? I think just using poster board on the rest to close up the fuselage would be easiest. Foam board seems unnecessary. But I'm also wondering if I should keep the bottom open like the FT Old Speedster.

...

I decided I should at least make the bottom section bigger. I think it should be strong enough now. Especially with the wings glued in place. Yes, removable wings would be a great feature, but how do I make the twin booms removable?!?

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#2
I would not worry as much about making the fuse/power pod sturdy, as I would about making the wing and booms sturdy.
making the booms tough, without making them to heavy is the tricky part.

at least with the skymaster the nose is a good distance in front of the wing. and due to the large size of your wing, you can likely get away with a larger battery, thus helping to balance out the plane. I feel like with my p-38 if id had a larger battery in the nose it would have helped. better to have a heavy battery, then to add dead weight to the nose to balance it out I think.

I think what is the biggest trick is getting the length of the booms right. I feel my p38 may have had trouble because the prop wash was effecting the elevator. at different throttle levels it is more/less effective. and with that plane I did try to keep it to scale for the most part. I wish I knew for certain how the prop being closer to the elevator worked....but when I think about some of the trainers that have the motor on the wing, they often have long tails (thinking like the dynam Skyhawk)

I wish I could give actual advice, but I cant say for sure why my p38 didn't fly as I had hoped. I mean, it is flyable and it was my 1st attempt at both a pusher and a twin boom...

one thing I realized with the p-38 is to make the airframe and then place electronics in general location just to check cg. much easier to move then around if needed before gluing them in place! kind of an obvious thing, but I did not do this on my first few builds and that caused some head aches.
 

rcspaceflight

creator of virtual planes
#3
This is definitely going to be a process. After version one is built, I'm sure I'm going to find a bunch of things to change.

As far as the booms, I'm planning on laminating three sheets of foam together. I'm going to have to go the Gorilla Glue route, which will make this plane go away from the FT building skills, but I think it's going to be necessary.


I already found something that needs to be changed.
ScreenHunter_374 Apr. 27 22.00.jpg

I didn't model the tabs for the power pods, but I know that with where I currently have them the tabs and going to be in the way of each other. But I really should bring the power pods in anyways. You don't want the motors to stick out past the fuselage. They should be flush or close to flush. So it's a change, not really an issue.

Also the width of the nose is currently 6". I didn't want to, but I'm going to have to tapper in the nose. Not as much as the tail to keep it scale, but I may have to go a little beyond scale. I don't want a huge air damn for the pusher version.


I think it'll be boxed up well enough. I'm just not sure where the limit is. To fly right, this plane is going to have to be heavy so it's not a kite. I may have to re-watch the Monster episode, or was it the building a Monster episode, to know what problems they had and to get an idea of what that limit is. Not that a 54" wingspan is a Monster, but it's getting there.
 
#4
laminating with gorilla glue is a good idea. I used hot glue on my p38, but you have such a short work time and you cant get the pieces perfectly tight all the way around.

as was mentioned to me, you might want to try 2 layers for the side with a layer on top...thus sides will have vertical strength with the top providing horizontal strength. if all 3 are vertical, it will bend on you if you land hard or cartwheel on a landing. I had 2 layers plus pop sticks and it still wanted to bend some.

your fuse seems complicated to me, but that may just be because ive never made a power pod. I would just have the motor mount glued to the under side of the wing and call it good :)
 

rcspaceflight

creator of virtual planes
#5
I'm not positive on the wings.

ScreenHunter_374 Apr. 30 18.44.jpg

I modeled the wings (center part and the left part) and it ended up tapered. I think I'm going to make the spar bend up so the top of the wings are perfectly flat, and the taper goes up. Which would be closer to scale, not that it would be that noticeable. The height of the airfoil at the center is 1 1/8". Which should be plenty of lift without too much drag. So I think the wings are fine. Just... unsure.

ScreenHunter_374 Apr. 30 18.54.jpg

I want this plane to be a really good workhorse. Something that can handle a lot of weight. So maybe I want an increase in height of the airfoil, but I don't want it ridiculous.

ScreenHunter_374 Apr. 30 19.07.jpg

I estimate the wing area to be 415 square inches. So 850 grams is a wing cube loading of about 6. Which is still light and could certainly handle a lot more weight. 1100 grams is a wing cube loading of 8, which still isn't that heavy and isn't pushing the limits. 1500grams is a wing cube loading of about 11, which is getting closer to the limits of an airframe, but 1800grams is probably the max flying weight. Probably. A wing cube loading of 13 is certainly flyable, but is still pushing it.
 
#6
you are way ahead of me, being able to calculate wing loading and such. Im still going by looks. yours looks good :)

I want to see how you booms and tail come out.

don't forget to make progress on the EDF old fogey!
 

rcspaceflight

creator of virtual planes
#7
don't forget to make progress on the EDF old fogey!
That's why I added a list of projects in my signature. I keep forgetting about them. lol. I haven't touched the DLG in over a month.

It was the EDF Old Fogey build where I learned that Gorilla Glue doesn't stick to packing tape. I think the route I'm going isn't the best route. I was removing the paper from the DTFB and replacing it with packing tape so it's water proof and weighs the same. I think I'm just going to start over, leave the paper on, and use hot glue. The build will go a lot quicker that way.
 
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#8
That's why I added a list of projects in my signature. I keep forgetting about them. lol. I haven't touched the DLG in over a month.

It was the EDF Old Fogey build where I learned that Gorilla Glue doesn't stick to packing tape. I think the route I'm going isn't the best route. I was removing the paper from the DTFB and replacing it with packing tape so it's water proof and weighs the same. I think I'm just going to start over, leave the paper on, and use hot glue. The build will go a lot quicker that way.
yeh, only time I remove the paper is to do a armin wing, where you remove the paper from the inside of the arc. Id go nuts if I couldn't use hot glue, it is so fast. Burn my fingers good sometimes tho.
I found I can use hot glue on the blue dow foam when gluing DTFB to it...I just apply the glue to the DTFB and give it a couple seconds to cool before applying to the dow foam. seems to work pretty good and faster/less mess than gorilla glue.
 

Ron B

Posted a thousand or more times
#9
I love the skymaster and went out to our little local airport and there were 3 of them that they are restoring that were used by the military and I think he called them B 02 I am going to have to keep an eye on the restoration of these.
 

rcspaceflight

creator of virtual planes
#10
I love the skymaster and went out to our little local airport and there were 3 of them that they are restoring that were used by the military and I think he called them B 02 I am going to have to keep an eye on the restoration of these.
I saw a few military Skymasters at AirVenture last year when I didn't know what they were. Actually, I still don't know what they where used for with the military. *looks it up* It was built to replace the Birddog? Weird.
 

rcspaceflight

creator of virtual planes
#11
I still have a lot of details left, but the plans are getting close. I have tabs to create and cut out, servo placement, a support piece for the fuselage to add, figuring out the elevator servo, the best way to mount the wing, and figuring out how to have the fuselage the most practical for electronics. I want to fully take advantage of the room inside of the fuselage, but I may have to get rid of the power pods for that. *whispers* (the power pods kind of get in the way of this design) shhhh

Wings have a slight dihedral, and undercambered wing tips as taught by David.
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I plan on cutting a slot/slit into the two pieces on each side of the rudder and gluing in bamboo skewers to brace the rudder. Otherwise the 1" height of the boom right before the rudder is going to break and going to break quickly. I haven't decided if it's worth waiting for Gorilla Glue to set for the booms/rudder, or just leave the paper on the DTFB and use hot glue. Probably the latter as an experiment.
ScreenHunter_377 Apr. 30 20.46.jpg
 

rcspaceflight

creator of virtual planes
#12
I was about to say that I lucked out that the fuselage fits onto one sheet of foam board, but I didn't add the windshield yet. Looks like it's going to have to be glued on with tabs on both sides, not hinged on like the FT Old Fogey.
ScreenHunter_377 May. 04 01.25.jpg

The elevator and rudders / booms fit onto a sheet pretty well. Hopefully I can use up the rest of that sheet without too much waste.
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I still have more work to do to the plans. Even when I'm done I'll still have to figure out how to make the Sketchup file into printable plans.
 

rcspaceflight

creator of virtual planes
#13
I don't think I'm going to end up using the sheets of DTFB very efficiently.
ScreenHunter_377 May. 04 01.48.jpg

I still have to unfold the wings, but I think I'm going to have to make the wings three separate pieces. And each piece is probably going to be about one sheet, making the total 6 sheets. I guess that's not horrendous.

The wings will probably be taped and glued together, and then mostly rely on a solid spar (made from four separate pieces) to align it.

I hope I have enough tabs to make it easy to align everything together without being an unnecessary amount of tabs and slots to cut out.
 

rcspaceflight

creator of virtual planes
#14
I pretty much have the plans all done, except I didn't plan out where to put the servos. I think that will be easier and more obvious once the first proto type is built.
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I'm having issues figuring out how to make the plans printable and to scale. http://www.flitetest.com/articles/creating-plans-from-sketchup-model-free I can't seem to get the Sketchup extension to work/install. I did find out that I can actually export a jpeg from Sketchup at an extremely high level by changing the "Options".
ScreenHunter_377 May. 06 01.19.jpg

The problem with that is that I'd have to force it into scale somehow instead of it automatically being scale. But adding a "ruler" into the Sketchup file should make it easier to do.

Hopefully I'll get it all sorted out and printed out soon so I can actually start building.
 

rcspaceflight

creator of virtual planes
#17
You should contact JasonEricAnderson for help with getting the plans from SketchUp to PDF.
I have the bad habit of going on my laptop right after work and being too tired to think straight. I have tomorrow off and hopefully I'll figure out a decent way of making the plans then.

The real problem is that I don't have a printer...
 

rcspaceflight

creator of virtual planes
#19
Plans?

I figured out Flattery and exported into the .svg format. I found some online converters to make it into a PDF, but that completely ruins the scale so I just used Internet Explorer to save it as a PNG photo.

Does it really have to be a PDF?

View attachment Skymaster Plans.zip

I admit that the plans are a little "dirty". I used power pods that I had made before and scaled them down to the correct size, but they may have been a little off and I know that the thickness is off.

I also got rid of all of the duplicate pieces.

I probably shouldn't be sharing the plans already because I'm not positive that they're scaled right. Plus there are probably some minor issues that need to be fixed.

Edit: according to MS Paint, the scaling of the PNG is correct. So printing the PNG using Paint with the setting "Adjust to 100%" should work. ... And my brother doesn't think his printer works so I get to go to the library and print off 24 sheets.
 
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rcspaceflight

creator of virtual planes
#20
Oops. I forgot to create two slots in the wings for the two vertical support pieces inside of the fuselage. Those two pieces hold the skewers for the power pods (and strengthen the fuse). Or maybe I made the support pieces to lay flat under the wings? I don't remember, but tabs would make the build easier and the plane stronger.