Carbon tubes are the only way to go, right?
For those that like a simple build, maybe. But for those hoping to make large frames really stiff, then there are better ways. In this issue, we explore the use of laminates using low cost and easy to access materials.
The copter being the guinea pig for this discussion is as follows. It is hoped that it will break the out and back 1 hour 30 minute barrier with lots of altitude gains while carrying a camera.
Each circle is 15.625" in diameter. Each motor mast is 1.5 meters long. So it doesn't take a lot to imaging the structural issues with unsupported motors that far away from center.
The motor masts will be made from hobby store balsa wood forming the web (center part) of an "I" beam. The cord can be made as thus:
1) Make the web part of the beam (the middle vertical part) by making a balsa sandwich panel.
a) Buy four balsa 4" by 36" pieces of 1/8" thick sheets. If you can, take a scale to the store. Pick the two lightest sheets. Balsa density varies greatly.
b) Buy a two sticks of 1/8" straight square balsa. Cut the sticks into 4" lengths.
c) Putting the two sheets directly on top of each other, cut both at the same time using a straight edge at one end making an acute angle (say about 60 degrees). Flip the top piece over and place both pieces end to end. Glue the ends together making a longer piece.
d) Repeat for the other two sheets.
e) Glue the 4" sticks every 4" along the one of the sheets. Put glue on top of those sticks and place the second sheet on top of that.
f) Let the balsa sandwich panel cure.
g) Cut the sandwich panel into two 3/4" strips wide strips (wider if more strength is needed).
h) Notch the two strips where needed to form an intertwined X (like a log house).
h) Locate the X. Glue. Let the web of your motor beams is cure.
2) Make the cord part of the beam (the flat top and bottom parts) from light, high-strength carbon.
a) Buy .31" by .031" x 3 meters long carbon extruded rectangular rod.
b) Bond these to the tops and bottom of the balsa webs.
c) The result is a ship that is stiff in z, but not stiff in x/y.
3) Add spars to stiffen the ship in x/y (shown below are .280"/.244" OD/ID carbon tubes. This can be done as easily as just cutting them square and drilling them through the balsa web and bonding them in place. Or you can get complicated like I did (wanted better torque control).
a) Cut each spar to the correct length and angle leaving room for .050" end pads.
b) Cut sixteen end pads from .050" fiberglass that are .5" wide x .75" tall.
c) Bond the end pads to the spars (see photo below)
c) Bond the spars to the motor masts.
The port side spars are curing as this is written, so I'll update with a final photo tomorrow.
So what is the advantage of a beam made in this manner?
- It is lighter than a tube of equal strength. Carbon is placed only where you need it. Lighter materials can be used for lower stress central areas.
- It is hugely less expensive.
- Continuous motor masts in an X can pass through the other because the top and bottom cords that carry all the load are continuous.
- All motors on the same level and creating an Electronics Platform with more real estate on top.
Is this good for commercial use? Absolutely not. But commercial manufacturers would have the equipment to layup carbon over core. This is all ready done for props that you might be using. The carbon is laid over cork, crushed Nomex, or balsa wood that is pre shaped.to the dimensions of the prop thickness. This using lighter materials as fill where stresses are less.