Design help
 

I am working on drawing and then building a Pilatus PC7 model and I have several questions regarding design. The plane will be approximately 1/5 scale which equals a length of 72 inches and wingspan of 78.

I do not want to build a simple box, instead I want to use stringers to run from the firewall to the back of the wing saddle, with one pair running back to the tail. Then I will cover with 1/16 balsa to the proper shape. The aft end of the fuse will be built the same way. I had planned on using eight 1/4 basswood squares to hold everything together. Question, is this going to be strong enough or should I increase the size of the basswood, increase the number of stringers, and/or change the wood. And what type of glue should I use to attach all of this together, CA, Epoxy, or Poly.

Since the wing is a three piece polyhedral wing, I have two choices, build a one piece wing and transport to the field that way, or build it in three pieces with the outer wing panels connected with wing tubes. Which would be the better way for strength, and if I were to use wing tubes, how deep into each wing panel should they go, and what size wing tubes. If I use the three piece wing, that would mean that there would be a flap and aileron servo in each outer wing panel, as well as retracts and one more flap servo in the center panel. Any issues that I have to consider there? The wing will be skinned foam.

I am drawing the bulkheads and plans in Autocad 2002 LT. I am not too experiences with Autocad yet, but struggling through. When I go to have the parts either CNC or laser cut, is there anything special that I need to do to get it done. Or just draw the outline and the parts can be cut from that.

I know this might be a bit early, but can anybody give a rough estimate on weight with a built up plane this size, which will have two aileron servos, three flap servos, one elevator servo, (The elevator is one piece), one rudder servo, one retract servo and one throttle servo? Sizes are to be determined, as are the type of retracts. And engine choices, I am not looking for unlimited vertical, I would like it powerful enough, but not overdone. I have a Saito 100, but nothing else large, would a .91 or 1.08 2-stroke be the ticket?

Thanks in advance for the help.

Doug

RE: Design help
 

Set up a few layers in Acad. and draw the corrisponding parts on the right layer. Such as: have a text layer, and put all your text on it. Have a 'hidden layer' and put all your 'hidden' lines on it (set it to 'hidden' linetype) put all your dims on another layer. etc..

this might sound a little long winded, but it sets up a good practice, to have teh various design elements set up in a way, that you can turn on and off details that you might not need. The CNC cuttter will require just the outlineof tehparts. He won't need (nor care) for teh design notes, extra diomensions you're thingking about, the hidden lines (that can screw up the CNC file) etc.. you can also freeze layers, to lock parts in place, and you can erace parts of the design, without 'accidentally' eracing others..

RE: Design help
 

Having read your paragraph, I seem to be doing things a bit differently than others, as I tend to use more of the capabilites of Autocad than others. I also do not "design" a model A/C , pre se, but instead plagiarize design details from other similar looking or similar sized ones. Although I am sure your have yet more construction ideas. I can only tell you what I have run up upon so far, which seems to work for others. Then, you can draw conclusions from that.

Quite often the fuselage (for your sized A/C) is built like a box section. Your description does not visualize with me, but more often four 1/4" square strips of balsa making up the corners to the box, with 1/4" squares running vertical at a 3-4" spacing, plus diagonals at the cabin area, and at the Hoz-stab. Some semi-circular pieces are then glued to the exteriors of the 1/4" to make shape. The skin is from 3/32" and on occasion 1/8" sheet balsa. Your thinner sides may tend to make for wiggling or twisting in flight. The eight pieces of spruce specified for the fuselage frame may be strong, but it is the 1/16" which will make for the flexing. Also for consideration may be to make the fuselage box frame from 5/16" or 3/8" square balsa. The 3/32" sheeting will bend sufficiently, but the 1/8" does not like to and is used when a heavy amount of sanding to shape is anticipated.

I am not too sure on your idea as to using the tube in the wing. I have included this onto my plans, but only on occasion and then for aerobatic types, because they have lesser dihedral angle. About 90% of the A/C I draw up use standard plywood dihedral braces glued to spars. Perhaps others out there may have better input, as the folks I draw for like to go on the cheapest way possible, however considering the size of the wing and added complications, it may be best to construct as one item, then suffer the handling problem. I also think maybe your choise of engine may be a bit too much, creating a Hot-Rod instead of a scale aircraft.

At this point, I have drawn up around 7000 of the little drawings for laser cutting or CNC cutting of parts, and about 6 dozen sheets of model A/C plans for my customers. All but a couple of which are now beeing reproduced commercially for some kit. My methods are not always the same as for others, but my former Autocad instructors agree the method I have chosen works best for the style of projects engaged.

Number one, is I do not do "A Plan for a model A/C", but instead create maybe 30-60 little drawings. These then are brought into one drawing to become the plan. As you gather experience, you will find there is zero reason to create a big plan with a bunch of layers. The normal plotter (or printer) within Autocad is pen color based, as has been the system for twenty years. Most generally, I create a drawing for the fuselage side view, the top view, maybe a front view, then a wing (or wing half) and a Hoz-stab. These are all created as their own drawing files. You also from there develop several detail drawings, cross sections, etc. to explain how to build the A/C.

Then, a whole new drawing is created, usually called "PLAN" or PLAN1, PLAN2. All of the above noted component drawings are simply XREF'd into this new holding tank of a drawing. Lemme tell you, it will become readily apparent once you get the border on as to placement of views. In the XREF system, you just snap onto a view, and drag it to here or there. You do not have to play with layers, and otherwise be real carefull as to what lines and text get chosen then. If you layed on text, hidden layers, or special layers, tehy all then move as one object, not sixty. The ALL on ONE drawing system is stink-o and obsolete for most all Autocad users since the 1990's. As you develop the lines and drawings for parts, they too can be added into the main drawing. As the individual drawings are XREF'ed in, they establish theirown layers, and you have only one drawing to contend with instead of a whole bunch of loose lines and text. As a quick example, just open up Autocad and make a drawing with a big X in the center, save and exit. Open up a whole new drawing, and XREF attach in that drawing with the X and place it. Move it too. Now exit. Go to the X drawing and add in some text or mabe a circle. Then go back to that second drawing and you will see the revisions show up immediately. You can place numerous XREF'ed drawing like this, and move anyplace as you need them.

As you mentioned a desire to go to an outside source to manufacture your parts, the XREF system lends itself handy there too. Just create a new drawing file labeled as 1-8.dwg or 3-32.dwg, and then XREF in all of the little rib and fuselage parts into these. As you change the contour of parts, every single drawing that Rib2.dwg is shown in will automatically update to the latest lines. With the old fashioned INSERT method, you have to manually update the parts, and remember the names to file which require the update too. Once you have a need to place the 3-32.dwg file onto a disk and mail off, you can simply BIND all of the parts to the drawing, or a copy of the drawing. Make a mistake with the prototype, maybe you were off as to clearances some place and need to create some slightly altered parts, the system still works. It was altering the parts in the background for you already. Once you gather some experience in this, it will take less that fifteen seconds to verify all drawing have been automatically updated, and ready to BIND.

And one last thing. TEXT. Most job cutting shops do not have the equipment to understand raster (TrueType) fonts. Therefore you need to stick with the old fashioned linework based text on the parts. You can use the fancy TrueType fonts on your views and detail drawings, but not on any part. The machines just do not understand fonts other than the .SHP or .SHX types. AND, as a safety measure, the whole drawing has to be revertable to Autocad R-12 format. If it will go there, and not omit any lines or text, then the file can be put onto a disk and handed off. Much of this is laid out in the below noted web site too for you.


Wm.

RE: Design help
 

Thanks for the replies so far. So with the suggestions, it would be better for build a box type fuse and sand it to fit. I can do that as well. I will then go with one pair of spruce 3/8 square stock for the length of the fuse, along the thrust line. This will also be the support for the horizontal stab. Then I will use 1/4 balsa on the sides and sand to shape.

Here is an autocad drawing of the firewall with three views, one is the firewall with the balsa sheeting, the second is the diagram for the cutting, (should this be 3/8" ply?), and the third is what the final shape should be.

Thanks for the help so far.

RE: Design help
 

My thought from experience is that your design may be OK, it is the construction of which is doubtfull. How do you plan to hold all the parts in perfect alignment while the spruce spar gets glued into place?

You can build in two halves, however once each half of the fuselage is let loose from the board, they spring straight. Then, how do you join the two halves, and ensure the alignment is going to be true before gluing on the side panels?

See what you are getting into?

That is why so many older designs work with the tried and true box frame fuselage. You just build two frame halves, then join them together. Bit by bit adding one more brace or cross brace, keeping in good alignment. Then, when finished, bend in the sides to a taper in the stern. You have to consider what may happen after you lift the half off of the flat board. As the A/C got rounder, they simply added on semi-circular bits to the correct appearnaces before adding the skin. Will it stay in perfect alignment or not?

Brian Taylor likes to build a central H shaped box laid down. The center pan of the H is for mounting servos and abtteries. He then adds sides, bending them to a point at the stern.

You need to take a look for a few more examples of the construction method you are envisioning. See how they did it, and then adapt your intents. There are a Kaboodle of plans out there showing different methods of fuselage construction, one of which may come close to what you are representing.


Wm.

RE: Design help
 

I see your point, I will have to rethink this and see how I can make an easy but semi-scale plane. Thanks for the insight.

Doug