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.
Replies
I love it!
This is just beautiful!
What glue do you use for the balsa to balsa and then for the balsa to carbon, and finally for the carbon to plastic pads?
What's the final weight of this frame?
At what weight does it start to flex in z axis? X,y axis?
Used EC-2216 for everything (conservative on first builds). But, the balsa to balsa isn't a critical bond. Hobby glue is fine. It wouldn't surprise me if hobby glue could also be used for the carbon to balsa given the length of the bond. Figure that each arm of the quad is 22" x .25" wide or 5.5 sq inches. So you don't need a 2000 psi adhesive. If you can make a 10cm beam using regular adhesive and test it, that would be great.
The final weight of this frame was 247 grams:
- 50 grams of carbon (.22 grams per inch x 2 (top and bottom) x 57" long booms x 2 booms)
- 46 grams of balsa (go to the store with your gram scale)
- 65 grams of carbon spars
- 21 grams of adhesive on the motor masts (just plain sloppy on my part; should have toweled it on)
- 2 grams carbon spacers
- 8 grams motor mounts
- 4 grams adhesive on plates & joints
- 1 gram motor mount bolts
- 20 grams electronics platform
- 22 grams EP stiffener and glue
- 4 grams EP to mast spacers
- 1 gram adhesive EP to motor mast
- 1 gram adhesive on EP
The joined motor masts came in at 1.55g/inch (including the spars). I'll be satisfied when I'm at 1 gram/inch (i.e., I'll move onto the camera part and quit setting world records; well at least until I see how much fun you are having setting the heavy lift records; that might be too much video hilarity to miss out on watching multi-copters trying to fly unsecured payloads).
So with 1 balsa, the adhesive and balsa weight drops in half with little decrease in performance. That would put the system at 1.27 grams per inch. For a quad, this drops to 1 grams per inch.
Thought you might be interested in this photo of load capability. The solid steel block sitting in the middle weighs 6.86 kg (15 lbs). The ship with no weight is perfectly flat. With that load put in the center, the EP only drops 12mm over its 1500mm span for a less than 1% bend. So it's just a matter of how deep of a piece of balsa one uses. This same frame in a quad config (putting the supports at the inner motors), only sags 1 mm under the same load.
In x/y, the stiffness was unexpected. One would think that by the time the counter forces transferred out to the last motor that there would be some slop, some movement. It's like trying to move in shear a piece of 3/4" plywood. Nothing. Overkill.
But I wouldn't downsize the spars much as the motors can be torqued around the mast axis. I'm not worried about torque as the motor naturally puts out its own anti-axial torque as the rotor spins up. There is enough torque to always bring the motor quickly back to it's up configuration.
Thx for the info, i'm absorbing it. What is special about this 2216 epoxy versus other epoxy glues?
Would there be a way to build a beam composed of two I beams , perpendicular to each other ( the cross section of the resulting beam would be a square with its medians)? Because I believe this would combine the rigidity of I beams in all three axis with minimal material (being mostly empty). This structure would be an approximation of the bone structure of birds.
If we inspire ourselves from the bone structure of birds, it is indeed an outer thin and light shell with inside lots of bone shards acting like bycicle wheels inner wires. Would be great if we had the tools to build this in carbon fibers...
I love it when you compare to nature. No better way.
2216 Epoxy - Don't really know why this product is so superior to others chemically. I just know it works and everything else I've tried has failed. A good place to start is to look at Airbus specs and see what they qualify for composites.
Bird Bone - Some thoughts.
- originally, i thought it important to keep the cross section under the props small because of the downward prop wash that pushes the copter back to the ground. but this doesn't prove to be as important as expected. Maybe 5 grams/sq inch ish.
- the weight of the mast to stabilize z is 1.8x the weight of the spars that stabilize in x/y and the spars create mush more stiffness than the mast ever could. hence for octs, spars are lighter and stiffer.
- but for a quad, that may not true. the EP provides some stiffness since it is bonded to the beam so you only have to stiffen the outer section and so it might have a different trade.
- but the spars also provide some axial twist resistance. so would a bird mast. they are easy enough to build. it is worth a try and test. and then again, a helix tow running clockwise and counter clockwise would also solve that problem.
There is a weave system that we use in aerospace. It would be capable of replicating the bird bone. wouldn't that be something.
you bet , that would be some innovation ! Imagine the headlines : "The first multicopter frame inspired by bird's skeleton material !
Below is a photo of the finished frame with the two continuous motor balsa/carbon masts in an X held in x/y space by structural carbon tube spars. The cost key is key. The balsa cost about $10, the carbon cords about $20, the spars about $30, the glass motor mounts about $5 and the electronics platform about $25. Total cost of about $90. This would be about 1/3rd the price of using adequately sized carbon tubes.
This is the front view of the Electronics Platform (EP) bonded to the forward beam, which is bonded to the fore part of each motor mast. The camera will be on the small forward rectangular platform of the EP, so the support under that platform had to be stiff. In the top view photo, that support is under the grayer horizontal part of the EP. I want to understand first hand the vibration problem of a direct camera mount before investing in isolated suspended gimbals. While there are many that simply don't want to believe this despite showing test sample after test sample, if you make your ship super stiff, vibrations are quite low. This will allow me to afix a GoPro and share the video. From this perch, the FOV is 90 degrees (the same as GoPro in narrow view.
The material is 1/8th inch thick single ply Nomex/Carbon floor panel. It weighs 0.54 grams per square inch. The 1/4" thick material is 8 times stiffer and only weighs 0.60 grams/sq-in.
Also shown are the standoffs for the GPS/Mag (photo left) and telemetry radio (right).
The photo below shows two spars, two of the 0.040" x .5" x .75" feet on the spars, the universal motor mount, a close up of the beam, and cured 2216 adhesive. The beam balsa part is 3/8" wide by 3/4" tall. It is hollow with 3/4" long x 1/8" square balsa spaced 4" apart holds the two 1/8" thick by 3/4" tall balsa walls apart. The 0.31" wide carbon x 0.032" thick carbon forming the cord caps the top and bottom of the slightly wider balsa web. In this configuration is a double balsa web needed? Maybe not. Note that the spar/feet not only hold the motors in x/y, but keep the motors pointed up and are able to resist motor torque.
This frame style weighs about the same as the more efficient layout of the weird octa used on the World Record duration copter. But this copter is stiffer in x, y, and z. It also has added structure for the camera. One of my first tests will be to see if stiffness is not only key for vibration reduction but also for ship efficiency.
This may or may not be the frame used for the M90 medal attempt. I'm also working on one other build that is expensive and hard to manufacture (but if it works out, nothing will come close to it). What I like about this build is the low cost and accessibility of materials. This is appealing.
A word of caution. If you crash a lot, stick with carbon tubes that are reinforced at the exposed ends.