I am Charles Linquist, a semi-retired (previous) CTO of a tech company in Silicon Valley. I wanted time to explore my hobbies and do some other, personal things that I have neglected over the years. Working part-time gives me that opportunity.

I wanted a challenge, something that would utilize my skills as a mechanical designer, an Electrical Engineer, a firmware writer, and a guy interested in general science

I have 4 quadcopters, a hexacopter and a 3D printer, so something using those seemed like a good start. Quadcopters use all the disciplines mentioned above AND can benefit from the use of 3D printed parts.  I had an idea.

People have set records with the length of time a quadcopter could hover.  I believe the current record is almost 90 minutes. Those times were set with stripped-down frames, very large propellers and slow motors. This long hover time took a Herculean effort in weight reduction.  Before you get off on the wrong track - adding batteries is not the answer.  Yes, more batteries increases the power available, but it also increases the weight. At some point, it is a losing proposition - or very, very close to one. Weight is the enemy. NASA realized this very early on.  Putting one extra pound into orbit takes 9 pounds of extra fuel. Now you know why rockets are so large!

But rather than set a record for the longest hover time, I wanted to do something different - I wanted to see how FAR I could fly a quad. That seems like a simple thing: If you are in the air for 90 minutes, and you can go 30 miles/hour then you could go 45 miles.  But wait!  Going "sideways" is much more than simply hovering.  The entire craft has to tilt in order to provide a sideways thrust vector, and when it tilts, the downward thrust vector (= lift) decreases. This means that the motors must spin faster just to hold altitude.  And the more you tilt the craft to go faster, the more lift you lose.  Since a 'quad' normally hovers at about half throttle, when it is tilted at a 45 degree angle, it has lost half its lift and must run full throttle just to stay in the air. 

The sideways speed is determined by the tilt angle, the RPM of the props and their pitch, since it has to "screw" its way forward. 

Quadcopter propellers are classified by length (generally 6-16") and pitch (generally 4.5-6").  The length is self-explanatory. The pitch is the distance the propeller will "screw" through the air with each revolution, and is basically the same idea as the pitch on a screw. 

It is generally known that a large propeller, turning slowly is more efficient at producing lift than a small propeller turning fast. To prove this "old flyer's tale" I turned to ecalc.

http://www.ecalc.ch/

This is a very helpful tool that seems to be quite accurate.  I recommend that - if you use this tool, you pay the $5 fee and get full access. The extra features that are enabled with the "full version" are worth it.

After some research I found that a very good combination would be the SunnySky V3508 motors (the number refers to the diameter - 35mm and the height - 8mm). and a 16 X 5.5" pitch propeller.  The motor has a KV rating of 380, which means that every volt fed to the motor will give an unloaded 380 RPM.  These motors are expensive - $58 each, which is about 2.5 times that of most quadcopter motors. But the SunnySky motors are known for their good bearings and high efficiency.

Using eCalc, I found that if I used this motor, I could hover for some amazing times - 40-50 minutes if my airframe was light enough. But there was a slight problem: Slow turning propellers - while they may do wonders for hovering, simply do not go fast sideways. Their slow speed doesn't screw through the air fast, so even if you tilt the craft 45 degrees, you move slowly. Even if I could stay in the air for 20 minutes at full throttle, I could only go about 13 MPH, which would allow me to travel a paltry 4.3 miles.  My goal is 18 miles! 

I have a plan to eventually reach my goal, and I will describe that in future posts.  In the meantime I realized my largest home-built quad could handle 16" propellers, so I mounted the SunnySky motors and the big props this morning. I figured that this craft would be a good test bed to get started. The picture of this test bed is below.

It definitely flies, but I have to change some of the controller parameters, since the props are so large they have a lot of inertia.

Today, I went to an un-named city park in Campbell, CA. with my 16" prop monster quadcopter.

I had previously programmed in some waypoints that would cause it to make a circle around the park. 

So, I armed the craft, and gave the throttle a little 'nudge'.  It took off just as expected and went straight up to the programmed altitude of 60'.  Then it took off at full speed in the WRONG direction!  The park is ringed by trees, and by the time I could get the remote set to MANUAL, the craft had hit the top of a tree.  It then fell about 20 feet through the branches and got stuck!

I tried to re-arm the craft in an attempt to have the motors break it free, but I saw no movement - either it wouldn't arm because it wasn't level, or else none of the motors could freely turn.  Brushless motors of the type used in aircraft have very low starting torque.

My toy was stuck WAY up in a tree that had lots of small branches.  Even if I could climb the tree (hard, because there were no branches below the 15' level), I couldn't get far enough out on a limb to grab my toy.

So,  I went to Home Depot and bought a 100' length of clotheline and a large, heavy steel hook (the type you normally screw into wood).  I wrapped the cord around the lag-screw end of the hook and tied it tightly.  I went back to the park and started throwing the hook up into the branches.  I first thought that if I broke it free, I would try to run underneath it and catch my precious toy, but decided against it.  It weighs about 5 lbs, and has lots of sharp edges.  A 40' fall would give it enough velocity (if it didn't get slowed by the branches on the way down) to do some real damage.  So I decided to let if fall to the ground if I could break it free.

I am not the best when it comes to throwing hooks into the air, and getting it that high took some real force.  Several times, I caught branches with the hook that were almost too big to break by pulling on the rope.  In that case, I would have to leave my hook in place and try another method.  Fortunately, I got the hook back every time.  After about 10 minutes of trying, I hooked on to the branch very near the quadcopter and gave a hard pull. The craft broke free and fell to the sidewalk under the tree. It hit on one arm VERY hard.

Inspection shows that I bent one of the 4 arms - only.  My construction technique of using 13mm thinwall square aluminum tubing stuffed with a 7/16" hardwood dowel proved itself.  The arm was bent and the dowel inside was broken, and there was a crack in one of the plastic brackets, but virtually nothing else was damaged!

I don't like to leave things broken for long, and I had another length of 13mm square tubing and another length of dowel.  I printed a new plastic bracket, and put the craft back together.  Now, 2 hours later, it is ready to fly again!.

So why did it take off in the wrong direction?  

I checked my setups and realized that there was no "waypoint #1 OR "takeoff point", only waypoints 2, 3, 4, & 5.  I don't know how that happened,  or exactly what effect that would have, but right now, that is the only explanation I can think of.

Weight is your enemy.  Consider the weight of EVERYTHING.

You can typically lift about 0.25 oz/Watt.  That means that one pound of aircraft will 'cost' you 64Watts. If you have a 3000 maH 3Cell battery, that is 3A X 12V = 36 Watt/hours. Most quads will weight between 2.5 and 3.5 lbs (battery included). 

So, if your quad weighs 2.6lbs (such as a Cheerson CX-20), a 3000 mAH battery will give you 36/(64*2.6) = .216 hours = 13 minutes of airtime. If you add a gimbal and a GoPro Hero3, you add approximately .5 lbs.  This will decrease your airtime to 10.8 minutes.

Of course, the aggressiveness of your flying will cause these numbers to go up or down, but you get the idea.

Adding weight will also decrease your horizontal speed.  As the craft tilts, the lift decreases.  More weight means that you can't tilt as far while maintaining altitude. Not tilting as much means your horizontal speed will decrease.

Buy or build the lightest, strongest frame you can and also buy a kitchen scale and an electronic fish scale so you know how much your individual components weigh. Even the wiring adds substantial weight.

A good rule of thumb is that you want a motor thrust that is twice the "All Up Weight" (AUW) of the craft.  Another rule of thumb is that the batteries should weigh no more than about half the AUW (or equal to the weight without batteries).  So, if you have a 500g battery, your airframe should (ideally) weigh no more than 500g and your total thrust should be at least 2000g.  

But, it is hard to find an airframe that weighs only 500g, since your motors probably weigh 100g (or so) each. Your airframe will probably weigh closer to 1Kg.  So, for maximum airtime, you would choose batteries that weigh 1Kg (maybe two 4500mAH, 3 cell batteries in parallel?) and the thrust per motor would need to be 1Kg (4Kg total).

For the above craft, the weight would be 70.4 oz, which would take 70.4 * 4 = 281 Watts, and the batteries would be 9AH * 12V = 108 Watt/hours.  That would give an airtime of 23 minutes.  Not bad!

I should point out that while I am generally talking about building your own aircraft, most of the topics below are pertinent for those who simply BUY a ready-built machine.  There are lots of options.  Hopefully, the information presented in these posts will help you decide whether to build your own aircraft or buy a RTF (Ready-To-Fly) model, and if you decide to buy a RTF, WHICH model is the most appropriate for you.

In the last post, I talked a lot about weights and flight times, but I really should have started with a few more more basic questions -

What do you want to do with your aircraft?  Are you interested in.

1. Sport Flying?
2. Nature photography?
3. Mapping/Surveying?
4. Surveillance/Spying?
5. Simply checking out places you can't normally get to?
6. Inspection of roofs, transmission towers, smokestacks, etc?
7. Photographing sports events, weddings, crime scenes,disaster areas?
8. Photography for Real-Estate (properties look better from the air)?
9. Agricultural uses (crop conditions, finding livestock, etc)?
10. Impressing your friends?
    
11. Utilizing your knowledge to explore a new area of technology?
12. Advancing the technology or adding to the general knowledge base?
13. Education?
14. Using your hobby as a catalyst or "ice-breaker" to meet others interested in science and technology?
15. Something to occupy your time?

Usually people want some combination of the above. 

I am personally interested in items 1, 2, 5, 6, 11, 12, 13 & 14.   

Regarding item 14:  Every time I fly, I find that people approach me and ask questions. Just as taking a dog to a park is a great way to meet people for dating, flying a UAV (Unmanned Aerial Vehicle) is a great way to meet other technologically-minded people.

When planning your design, you should also consider your level of expertise in building things, and the facilities you have at your disposal.  If you have a complete machine/sheet metal shop, a vacuum-forming machine, a 3D printer, an electronics lab - and know how to operate all your equipment you obviously can tackle a more complex build.

I went to a meetup with my son Kris last Sunday.  The topic was vision for autonomous vehicles.  

The topics were very interesting, and mentioned some techniques and equipment that I might be able to use in the future (Lidar Lite). Of course, the best part is meeting new people interested in the same things as me.

One thing I have given quite a bit of thought to - using the 4G cellular network as a  control/fpv system. It should be possible, with Verizon among others,  claiming sub 200mSec latency. Apple FaceTime is almost good enough to use as a fpv (First Person Video) system just as it is - but seeing what the 'copter sees doesn't do me a lot of good if I can't control it.

Using 4G would be so much better than trying to use ever-increasing transmitter power to send video signals back to the "base station".  Theoretically, the distance would be nearly unlimited.  Yes, most of my 5 aircraft have GPS, and the ability to fly autonomously, but it is better if you can see something from the air and then go over to explore it more closely.  It is also handy if an obstacle is in  the (pre-set) flight path.  If you can see it, you can probably avoid it.

Using 4G as a control method also would solve the problem of controlling the craft at long distances.  I could theoretically fly a quad around Chicago and capture the pictures (assuming some friend was in Chicago to handle the battery charging).

I suppose I could develop a modulation scheme that would take the output from the joysticks and encode them on an audio stream, and then a decoder at the other end to split out that audio signal into individual channels which would control the aircraft. That way I could use Face Time pretty much as-is. I would get my signal from the audio jack of an iPhone that was carried by my aircraft and would use the iPhone's camera for the vision, but I really worry about reliability - I have had FaceTime videos freeze. If the control system also froze, that would not be so good. 

Maybe that is when the "autonomous" features could take over for awhile and do some fairly safe maneuver, like hover.  If the signal disappeared for too long, it could just return to the takeoff point and if it got partway home and the signal was re-established, it could be under manual control again.

I guess that I would really like to buy (or help develop) an ARM-based device (or one device for each "side") that had a RTOS (Real Time Operating System) that was custom designed to do only the tasks I outlined, and to do them very well. I think there are some real possibilities, and it shouldn't be too long before someone comes up with a good solution.

I know I'm getting off on tangents, but I figured I would write when I had a thought.  At my age, that thought might not come back!

Not too many people know the necessity of propeller balancing.  An imbalance can cause airframe vibrations that make it to the accelerometers.  That is not good!  All control programs have a filtering algorithm that they pass the accelerometer input through to reduce the effect of small vibrations, but it doesn't take too much shaking before your quad can't quite figure out which way is up.  

Many of the control programs have a parameter that allows you to choose the degree of filtering. If you have a lot of vibration, then increasing the  filtering will allow the craft to at least fly.  But that added filtering really hurts the handling. It prevents the craft from realizing it is not level until it is very 'un-level', and it will jump around and be a lot harder to fly.  For ease of flying and stability, you want to use as little filtering as possible - but for that to work, you can't have significant vibration.

As a real-world example:  I had a quad that seemed to fly OK, but when it hovered, it would jump up and down about 10' !  I couldn't figure out why until I had it hover very close to me.  I saw the spinner on prop #3 vibrating back and forth about 3/16 of an inch!  After I landed, I took the craft in to my basement and tied it down.  I then revved up the motors, one by one with a "servo tester" (highly recommended tool that costs less than $5).  At certain RPMs, but ONLY at certain RPMs, motor #3 vibrated a lot. A small imbalance was triggering a resonance in the airframe.

I didn't think this amount of vibration was possible, since I balanced my props with a magnetic balancer before I mounted them.  It simply shouldn't have been vibrating - but it was.  

I took some masking tape about the size of a large postage stamp and placed it on the top of one 'side' of the prop, then spun the motor with the servo tester.  The vibration was worse! So I put the masking tape on the other side and spun it.  This time, the vibration was much less.  So I used a single-edged razor blade and scraped some plastic off the side that did not have the masking tape, and spun it again.  This time, it vibrated even less. I scraped a little more and removed about half of the masking tape. It still vibrated a bit, but I was getting closer.  So I took the masking tape off entirely and spun it, removed a bit of material and spun it again.  Eventually I got to the point where it didn't vibrate at all.  My quad flew better immediately.  

So, my recommendation is to use a good magnetic balancer to get the prop statically balanced, then put it on the motor and spin it with a servo tester (make certain your craft is well tied down!) to do a dynamic balance.

An acquaintance said that they balanced the motors themselves.  An un-balanced motor may explain why the static balancer is not good enough. I'll figure that out and report my findings.  

My first quad used KEDA KA22-15M motors.  These motors are powerful for their size and they have a KV rating (unloaded RPM/Volt) of 1050, almost perfect for a 11 X 4.5" prop (11 inches in diameter with a 4.5" pitch)  and a 3 cell (12.6V) battery.

These motors are used on two of my quads, and when I went to buy spares, I found that Hobby King no longer sold them.  I had to buy them from Strong Motors in Michigan.

I may know why they are getting hard to find - they do have some problems. The most serious is that their shaft has a groove for a snap ring which keeps the motor together.  The groove is not really wide enough for the snap ring (or 'E' clip), and will sometimes come off. I found this problem early on when I put my quad into a hard climb, and the propeller and the rotating armature flew off upwards. The quad then came crashing to the ground, of course.

I had a several of these motors and I didn't want to throw them away (or send them back) so I made a "jig" with three short pieces of a 1 X 4 and a hinge.  One  piece was clamped to the table of the drill press.  Another extended upwards at right angles and was fastened to the first piece with drywall screws.  One side of the  hinge was connected to that "vertical" piece.  The third 1 X 4 was connected to the other side of the hinge. This piece could swing toward the chuck of the drill press and was parallel to the drill press table.  I used two large tie-wraps and fastened a Dremel tool with a light-duty cut-off wheel (abrasive disk) in its chuck.  Then I put the 3mm motor shaft in the drill press and spun it.  I then moved the Dremel over to the shaft and let the abrasive disk cut a deeper, wider groove in the hardened steel shaft.  That provided a perfect solution to the inadequate shaft attachment problems. I cut some nice grooves in hardened steel shafts without having to take them to a machine shop.

Today, I went flying with one of the quads that had those motors.  It was flying great! Then I pushed the throttle all the way up.  A prop (and motor bell) went up to the sky and the quad came crashing down!  I had forgotten to rework the shaft of that motor!  Fortunately, I had a spare prop and I could re-print a broken piece with my 3D printer.  I re-grooved the shaft, printed the new piece and put everything back together. Now, an hour later, it is ready to fly again.

I spent over a week in the Palm Springs area.  I kept looking at the immense amount of unused airspace with great visibility - free of wires and trees.  I can't imagine how anyone could monitor the FAA's rule that you must be in visual contact with your craft at all times, or that you cannot fly higher than 400'.  I think the desert would be an ideal place to learn to fly and to test out new designs.  

Over the holidays, we went to the Palm Springs, CA area, and all I could think about was the clear sky - with no trees, wires, or park rangers.  What a flying paradise.

But I had none of my flying machines!  Not even one would fit in the rented Chevy Impala with my wife, two in-laws and all the luggage. So I made a pledge to build a quad that could either fold up or was just plain small.  It had to have some of the basics - but it didn't need a GPS or the ability to autonomously fly.  It did need a camera, though.  A gimbal would be nice, but not absolutely necessary.

So I built one.  I couldn't find suitable props anywhere, so I took some 8" props and cut them down to 6" and carefully balanced them.  They fly surprisingly well.

So now, I have 8 multirotors!  I'll probably build two more soon - a folding one with 9" or 10" props, and a new test bed made with 100% carbon fiber for the 'Santa Cruz Project'.

Above is a picture of all 8.  Cosest to furthest. They are:

1. An ESTES Proto X
2. Cheerson CX-10
3. My 6" 'traveling' quad with 28mm 1050KV motors spinning 6" (cut from 8") props, a KK2 controller, 20A ESCs and is powered from a 3 Cell 2200-3000mAH battery. It has a fixed (no gimbal) mount for a SJCAM (Similar to a GoPro).
4. A copter that WAS a Cheerson CX-20, but it now has a new controller, a modified circuit board, new motors and new 9"  props - the only parts that are original is the case and the 12A ESCs. The battery bay was modified to use 3 cell batteries up to 3850 mAH and the landing gear was extended to better handle a gimbal.  Although it has no gimbal now, one can be moved from one of the other craft if desired.
5. A "stretched" quad that is 100% 3D printed. It has large (36mm) 910KV motors driving 12X5.5" props. It uses a 4-cell (16V) battery pack and has 35A ESCs,  an APM controller, and has GPS, 900Mhz telemetry, voltage/current sensor, a BASECAM Gimbal (for a GoPro Hero3), and a 600mW 5.8GHz FPV transmitter with a cloverleaf antenna.
6. A "square" quad that also is 100% 3D printed. It has 28mm 1050KV motors and spins 11X4.5" props. It uses either a 4-cell (16V) or a 5-cell (20V) battery. It has an APM controller, GPS, 900MHz telemetry, voltage/current sensor, a home-built gimbal driven from a Martinez board, and a 5.8GHz 1.5 Watt FPV transmitter with cloverleaf antenna. This model has an auxilliary 4A 12V switching power supply that provides 12V to the lights, gimbal, and FPV transmitter even if a 6 cell (24V) battery is  used.  The ESCs used are 40A/12-24V types. 
7. A super-strong quad built from Home Depot aluminum stock with 3D printed 'joints'.  It has 28mm 900KV motors and spins 12X4.5" props. The ESCs are 40A types and a 4-cell (16V) battery provides the power.  It has an APM controller, GPS, 900MHz telemetry, and a Chinese gimbal with a Chinese controller (but it works well) and a 5.8GHz 2 Watt FPV transmitter with cloverleaf antenna.
8. A big, aluminum and carbon-fiber frame hexacopter with 36mm 1350KV motors.  The props are 10X4.5 and the power is from two (usually) 5000mAH 3-cell batteries in parallel. The controller is an APM with GPS and it has  900Mhz telemetry and a 600mW 5.8GHz FPV transmitter with cloverleaf antenna.  It currently has no gimbal, but has mounting holes so it can accept a gimbal from another craft. A complete gimbal for this hex is 'on the way' from China.

All except the ESTES and the CX-10 have LED strip lights to help with orientation and night flying.

This morning, I went to a meetup "Flight Time" at Cataldi Park in San Jose hosted by Ed Martinez.  The park is great for close-in flying in that it isn't that large, but it is nice, clear of trees and wires, and people, and is JUST outside the "no fly" zone around San Jose Airport.

Several people were there with their DJI stuff, and Ed brought a cute little 200mm (if I remember correctly) quad.    I believe I was the only one who brought something custom. In fact, I brought 5.  I think my huge 'hex' and the little 150 mm quad got the most attention. Although DJI stuff is nice, it doesn't excite me as much as flying something that I built myself.

This was the first time I had a chance to fly the 150 in open spaces, and it was a dream.  The little thing is fast, and I'm almost ready to try some aerobatics with it.  Considering that it was made from 'junk' parts lying around my one-room workshop, it is a really fun machine.

I have a GoPro mounted on it, but I have some more work to do on the mount.  The mount must be very small, yet isolate the camera from the motor vibration. Video from this morning's flight has too much 'Jello' effect.  

So, I made a slight change and my latest iteration is shown below:

Shown below are the top of the anti-vibration "balls".  The flight this morning was made using the same sized balls, but the original ones were stiffer.  These new ones are very soft.  If you wonder what the green stuff in the middle of the balls are, it is earplugs - straight from CVS drug store!  These earplugs are of the memory-foam type that expand when you put them in your ear.  They also make excellent vibration damping material. Jamming an earplug into the center of the anti-vibration ball makes them much more effective than the balls alone (just check the research on the net).  

And here is a side view:

I got a new gimbal today ($61 with controller from China), and I noticed that all the screws were loose. Transportation in a bag would not cause them to work loose - I'm convinced that they were not tightened in the first place.  So I tighted the screws and noticed that one tiny screw was missing.  I found an equivalent screw in my junk box and installed it.  Next I noticed that the pitch motor didn't turn freely.  Closer inspection showed that there was a loose screw inside the motor !!! (the missing screw!), and it was held in the motor by the magnetic field.   I noticed that the loose screw had cut the tiny wires (probably 34GA) of the motor coil. If only one wire had been cut, I would have attempted a repair, but 10 or 12 wires were broken. 

So, I will probably contact the seller (on Ebay) and tell him that he screwed up, and please send me a new motor. Who knows how long that process will take, so today, I ordered a new same-size gimbal motor from Hobby King's US warehouse ($17 + shipping).  I'll probably have that in 3-4 days. If and when I get a replacement from the seller for the defective motor, I'll keep it as a spare.

And I said it was more fun building your own stuff!

When you have as much "stuff" as I do, packed in (mostly) category-labeled shoeboxes, you forget everything you have.  This evening, I remembered that when I built my last gimbal (from scratch), I bought an extra motor as a spare.  That motor, while not identical to the defective motor in the gimbal, is close enough to fit.  Anyway, the gimbal motor that I ordered from Hobby King is the same type, so even that one would not be an identical replacement and would require the same modification.

So I did some measuring and drilling and Voila!, I now have a working gimbal. Now the motor coming from Hobby King will be a spare, and if I get a replacement from the Chinese guy that sent me the defective gimbal in the first place, I'll have two spares.

All good!

I believe the danger of the hobby LiPo batteries is underestimated.  The batteries in laptops and other consumer equipment is not designed for very high currents (discharge rates), but the batteries used in electric-power models are.  When you see a rating of "30C", it means that the batteries can output 30 times the amp-hour rating (which is milliamp/hour divided by 1000).  So, if you have a 3000mAH battery, with a 20C rating, that means it can output 3A X 20 = 60Amps on a short-term basis - but that is their LOAD rating.  They can output several HUNDRED amps during a short-circuit condition.

Since commercial batteries (like for laptops and phones) are not required to produce very high output currents, the cells are designed differently and they usually cannot deliver huge currents - even during short-circuits. The cells are usually small (a bit bigger than AA size) and are arranged in a series-parallel arrangement to produce the required current and voltage.  The fact that each cell is not too large means that a catastrophic failure in one cell is not likely to produce enough heat to cause serious damage.  Also, laptop and phone batteries have a fusible link that blows if a cell shorts or the temperature gets too high. This keeps the energy from the adjacent cells from feeding power INTO a shorted or overheated cell, further decreasing the risk of fire or explosion.

Hobby batteries have NONE of the protection mechanisms mentioned above. The cells are usually put in series only, which means that every cell must put out 100% of the output current.  This requires a large cell. There are no fuses, no temperature limits, no "smarts" of any kind.

A charge cycle is especially dangerous. The material inside the battery actually expands with time - which is why older batteries get that "pillow" look.  Sometimes the insulator which separates the (+) and (-) plates inside the cell breaks down and the cell shorts. At that time, the full output current capability of the cell appears across its internal resistance.  My Turnigy charger has an internal resistance tester, and I find that most cells have an internal resistance of  0.010 ohms (10 milli-ohms).  If a cell has a voltage of 4V, then 4V across 10 milliohms = 400A.  400A at 4V = 1600 Watts!!  That heats the battery up within seconds. Note that this sequence occurs even if the charger stops immediately when the short is detected!  There is NO WAY to stop this.

And let us not forget that lithium (and each cell has a lot of it), is reactive.  It burns!  The end result is a very, very nasty fire with flames shooting out of the battery for several feet.  Anything nearby WILL catch fire if it is at all flammable.

But is this likely?  YES!!!  I have had this happen TWICE.  I was not charging in an ammo can, but this morning I had the battery on the charger on my concrete garage floor.  I heard a POP, and within seconds, flames shot out of the battery by more than 3 feet!  The bottom line is NEVER, NEVER, NEVER charge a battery in anything that is flammable - or within 10 feet of anything that is flammable.  I normally charge batteries in a metal "AMMO" can, and I'm going to get a second where I store charged batteries. I'm not certain that a battery sitting by itself, not on the charger will not do the same thing.

I do know that if I charge a battery in an ammo can with a bunch of other batteries, ALL of them will be destroyed if one goes up in flames. I don't know if the lack of oxygen will help quench the flames or not - but I'm not going to test any theories.  As far as I'm concerned, the only safe approach is to charge in one ammo box and store in another.

One of my bigger quads, and my little 150mm copter had a really high-pitched squeal at certain RPMs of the motor.  At first, I thought it was the bearings, so I changed the bearings in the motor that made the most noise, but the noise persisted!

I put an oscilloscope on the motor leads and found that the frequency the ESC fed to the motor was around 3Khz.  But that is a square wave, meaning that there would be substantial harmonic power at the 3rd harmonic - 9KHz.  I think that is the squeal that I heard.  I can still hear 9Khz if it is very, very loud.  I can imagine what some youngster would think! 

So why do some motors produce noise at high audio frequencies, while some do not?  Is it the motor "bells". I put some silicone seal (a pretty good sound dampener) on the outside of the motor and did my best to balance the weight to see if that made a difference.  It did not. If it wasn't the motor bell, what was it?

So I took the motor apart and figured out what was going on - the windings were loose!!!  The copper wires in the coils themselves were moving ever so slightly back and forth in response to the driving current and making the noise.  I realized immediately that this is a VERY bad situation, since the wires will constantly bend back and forth (even if only a fraction of a thousandth of an inch) and eventually work-harden and break.  That could put your expensive toy on the ground in a hurry.

I wanted to hold the wires in place, and thought epoxy might be a good choice, but I didn't want to block the space between the windings - that is needed for cooling airflow.  I was looking for something really thin that would adhere to the wires via capillary action.  I found something - superglue  (ethyl cyanoacrilate).  It worked perfectly when dripped over the top of the windings. It wet the wires and embeded itself into them, then hardended.  The only downside is that 150C is the maximum temperature. I don't know if the insides of my motors ever get that hot, but maybe they do. 

So, after super-gluing the coils, I put the motor back together and let the glue harden for an hour or so.  Then I powered it up.

NO SQUEAL!!

So far, so good. And I'm looking for some suitable glue that has a higher temperature rating.  If anyone knows of one, I'd like to hear from you.

Next summer, my wife and I are considering hiking in the Swiss Alps.  

Over the past XMas, we went to the Palm Springs/Palm Desert  area and I was drooling over the thought of flying my toys in all that open space - but I had no aircraft with me because nothing else would fit in the car. I thought it would be great if I could take a 'copter with me on the European hike - but I can't imagine taking anything as large as a Cheerson CX-20 (or a DJI Phantom) hiking.  Besides, by the time you load up one of those quads with a camera, gimbal and all the extras, and then try to fly at a base altitude of 9,000 feet, they just don't have much 'go'.

So, while I'm waiting for new motors to arrive for the "Santa Cruz Project" (from Hobby King), I decided to rekindle my mechanical engineering skills and design a quad that could lift everything I wanted (at altitude), have good battery life, and be small enough to carry in a backpack. I have most of it planned out.  The design - as it stands, can carry an APM-style controller with GPS, Lidar Lite laser rangefinder, full 900Mhz telemetry, 2W 5.8GHz FPV video transmitter and 2-axis gimbal  capable of handling a GoPro.  It can carry up to a 5000mAH 3-cell LiPo battery and should have a flight time of about 25 minutes.  

The 1100KV motors spin 9" propellers. Calculated all-up weight is about  2 lbs (battery included).

The frame is aluminum and carbon fiber and is extremely strong, rigid and light.  The overall (flight ready) size is 12" wide X 13.5" long X  6.5" high.  The gimbal is mounted far enough forward and low enough such that that the props will only come into view if the GoPro is set to the max wide angle and the craft is pitched more than 30 degrees forward.  If the GoPro is set for a slightly less wide angle, the props will never come in to view.  

But... the design is such that it will easily fold down to a package that is 8" wide X 10" long X 5" high.  It should take about 60 seconds to fold/unfold the unit (no tools required).  And it will easily fit into a backpack!

I'm seriously considering building and selling these "Little Giant" quads.  Let me know if you are interested.  No price has been set yet, but it will be less than a DJI Phantom with camera and gimbal - and the Little Giant will certainly have higher performance, longer flight times and be much more customizable.

When I realized that the SunnySky 380KV motors were not powerful enough to power the Santa Cruz Project, I ordered one of each of 3 other motors.  That way, I could test them without spending a fortune.

One of the motors I ordered was the Turnigy AeroDrive 4240 - 530KV from Hobby King.  When I first looked at this motor, I noticed that it was big - bigger than I had imagined.

It wasn't too heavy, and it looked nice.  Best of all, it looked to have much better construction quality than most of the hobby motors I have bought.  The bearings were very tight and smooth. The windings looked good.  

So, I bolted on a 16" X 5.5" propeller and screwed it to one of the frames I have.  I connected to a 40A Turnigy PLUSH ESC, which can handle batteries up to 6-Cell.  I have a 5000mAH 4Cell battery (on bigger machines, I like to go with 4 Cell = 16.8V  batteries because for the same amount of power, the current is less and all the wiring has less loss). 

I used a "Servo Tester", an invaluable little accessory.  It is about 1" X 1", has a knob on the top and can be connected to any ESC and send the proper signals.  The knob allows you to adjust the PWM from 1ms to 2ms - full range.

So I held onto the frame tightly, and slowly turned the knob on the servo tester.  The motor started winding up.  There was very little motor "whine" - only the sound of the big 16" prop slapping the air.  I turned it up more and was astonished at how little throttle I had to give it before it REALLY started to pull.  So I connected it to my digital fish scale and put a DC current probe on the ESC input wire to measure the current.  By the time I got to 10 Amps, the motor/prop combo was pulling about 2lbs. I turned the knob more and the motor just kept winding up.  By the time I got to 30 Amps, the scale read 6lbs 5oz !!!  At 40 Amps, it was 6lbs 13oz!  4 of those could lift 27 LBS !  

40A at 16V is 640 Watts.  Since there are 745 Watts in one Horsepower, the little motor was probably putting out a little more than 3/4 HORSEPOWER!!! I admit that it could not do this forever, but I ran it at 30A for several minutes and it barely got warm. Best of all, it had a 4 ounces more pull when drawing 10A than my 28mm X 16mm NTM 800KV did (the NTM was spinning a 12" prop).

I ordered 3 more.  They should be here by next Monday.

Just to think... If I powered it with several 4-cell batteries in parallel (I would need 120 Amps),  I could probably lift 14 pounds of payload with a quad - and still fly decently.

I got the new motors in, and bolted them to my Santa Cruz 'test' chassis.  The all up weight with a 5000mAH 4-Cell LIPO is 7.4 lbs.

When I was testing it out, I had an 8 lb weight tied to it. Before I was 3/4 throttle, it was trying to lift the weight! This thing is as strong as I thought it would be.

So I took it to a local park to give it a 'spin'.  It was not easy to fly.  The PID parameters of the controller were set for something with much less power.  I suppose the experience could best be described as having a Toyota Yaris driver take a spin in a Bugatti Veyron.

The big quad was very 'touchy' and wanted to climb at the slightest provocation. Still, I'm skilled enough (now) to fly it around without crashing.  One thing that I noticed - as powerful as this craft was, the combination of the inertia of the over 7 pound weight and the inertia of the big 16" props gives it noticeable throttle lag, kinda like turbo lag on a car. .  When it is hurtling toward the ground from some high altitude, you had better hit the throttle hard before it gets below 20-30' or you will have a pile of junk very, very soon. 

But when it gets going !!!!!  

I flew for 20 minutes and had quite a bit of battery left - YAY!  I'm on the right track.

Below is a picture, along with a 12" hammer so you can judge the scale.

I have 4 camera gimbals, and I'm building a 5th.  Most of the motors have a common problem:

"Normal" brushless motors have a few turns of wire on each core.  For current-carrying reasons, the wire has to be fairly large diameter.  This makes it relatively strong.

Gimbal motors, on the other hand have hundreds of turns on each core.  In order to get that many turns, the wire has to be very fine (it looks like 32Ga or 34Ga to me).  This wire is not strong!  Even when 3 or 4 ends are combined to attach to one of the lead wires, the arrangement is weak.  Pull on the lead wires with more than a pound or two, and you will have a lead wire in one hand, and a motor in the other. Don't ask me how I know this.

So,  use a substance like GOOP (available at Home Depot) and use a tie-wrap end to force a small amount into the motors right at the point where the wires exit the motor case. If the wires go through some heat-shrink as they leave the case, put a small tie-wrap tightly around the heat shrink to hold the wires in place.  After all, your GOOP made certain that the heat shrink tubing wouldn't be pulled out of the case, but it didn't prevent the wires from being pulled through the heat shrink.

And then there are crashes.  After a crash, I have had props, batteries, cameras, arms, etc.  winding up in places where they should not have been. You should design for crashes - that is, make certain that the expensive (or hard to get) stuff is NOT the part that breaks in a mishap.  In some cases, simply pulling on a wire to re-route it can break it or the circuit board the wire is attached to.

Use tie-wraps to tie the motor leads to the frame close to the motor - assuming you trust your motor mounts.  Use another tie-wrap to tie the wires exiting the ESC to the frame near the ESC.  If something bad happens, hopefully the bullet connectors will pull apart before too much damage is done.

When planning the wiring of an aircraft, always consider what will happen if it crashes.  Simply tying everything down is not always the right approach.  It depends a lot on just what you tie the wires to.

I have attached an EXCEL file that should help those wanting to know approximately what the effect of a larger battery will have on their "airtime".  This calculator can be used several ways:

You can put in the mAH and voltage of your current battery (use 11.5V for 3-cell and 15V for 4-cell) and change your "inefficiency" number up or down until you get the airtime  you have measured on your present setup.

I call this number the "inefficiency" because as the number goes up, the efficiency goes down. So, since it represents 1/efficiency, I call it IN-efficiency..

Once the inefficiency number for your craft has been determined, figure out what your craft will weigh with the new battery, change the maH number and enter those into the appropriate cells.  The airtime number should be very close to  the new airtime you can expect with the new battery.

Keep in mind that if your motors or props are heavily loaded, the inefficiency goes up rapidly, and the approximation will no longer be valid.

I was testing a new vibration damper for the camera gimbal of my little 6" prop "fun flyer".  This little beast has been so much fun to fly because it behaves pretty much as you would expect - always.  It is also extremely fast. 

It would climb out of sight in about 12 seconds. I figure that is about 500-600 feet, giving it a climb rate of over 2500' / minute.

It would also go "sideways" faster than anything I have ever seen fly.  The park that I normally go to is simply not big enough to gauge its true speed, since it will traverse the park in a few seconds.  With a different quad, I once had a greyhound (dog) race me across that same park.  They were 'even'.  But that quad wasn't nearly as fast as little one. The craft has a KK2 controller.  KK2's are easy to set up, easy to fly, and - since they don't have a lot of extra features like GPS, autonomous flight, or even a magnetometer, generally more reliable.

Controllers like APMs will go NUTS if they loose the signal from the compass, or if you accidentally mount the compass upside down, so it reads North and South correctly, but East and West backwards. I have done this.  The craft would fly perfectly if you flew directly North or directly South. But if you tried to fly more than a few degrees off the N-S axis, it would veer one way or the other..

Back to my story - I was testing the gimbal vibration damper on my street.  I did't want to take the time to drive to the nearby park for my test.  After all, I only wanted to see whether or not there was "Jello" in the video that came from the camera. I ran up the street and down the street, then I took it up fairly high - probably about 200'.  It was hovering at a steady altitude, then I started bringing it down. Not real fast, but not real slow, either.  All of the sudden, it flipped over and started falling.  I hit full throttle, thinking that it would have a better chance of catching itself if the props had some RPM.  I heard the motors wind up - then it flipped again!  This happened about 3 times 

And then it hit!

Hard pavement. Sad. Big mess. 4 Broken props. 3 broken motors. Broken frame. Sad. Broken gimbal .1 broken gimbal motor (of 2).Broken SJCAM HD camera. Bent (and therefore useless) battery. Cracked case on the radio receiver. Sad. It would have been in better shape if a car had run over it. Maybe 2 or 3 cars. I was really sad! The only other thing that could have created more damage is if the wiring had shorted and the whole quad burned up in a fireball fueled by lithium. 

So, I salvaged one motor, one gimbal motor and the main wiring harness - and the controller (amazingly, the only damage was 12 bent connector pins (which I fixed with a needle-nosed pliers). And I resoldered two tiny wires and used a bit of epoxy and fixed the SJCAM camera. I used more of that same epoxy to fix the radio receiver case.

The rest of the metal and plastic went in the trash.

I ordered a new frame ($8.99 from HK's US west warehouse) and a new gimbal and one gimbal motor.  Fortunately, I had 3 new (or slightly used) "main" motors that I can use to rebuild the craft.

I should have a complete replacement running by next weekend.

And I STILL don't know what happened.

I wanted to get my Santa Cruz project quad in the air so on Sat I took it to the park. It flew great and got to approx 200ft. Then it spun around on its axis and came crashing to the ground. Examination showed that one of the prop nuts came loose.  Moral of this story... USE LOCKTITE.

I have a quite a few different ESCs lying around.  Most ESCs have something called a "BEC" that converts the battery voltage to a steady 5V for the use by the controller, most of the radio equipment, the GPS, and by the ESC's themselves.

Some are BECs are listed as analog, some are listed as digital, some are listed as "opto".  What's the difference?

The "Opto" designation was first used with ESC that had an opto-isolator on their control input.  An opto isolator can allow one device to control another - even if their GND potential is different.  That can be important if the primary (main power feed) wires to the ESC are long or are of too fine a gauge to handle the currents required and have a high voltage drop (> 1V).  I have not seen a problem with this in quadcopters. If you happen have that much voltage drop in your power feed lines (at full throttle), you are wasting an awful lot of power and you should rewire immediately.

The ESCs that I have purchased recently that have been called OPTO do NOT have an opto-isolator on board. To that manufacturer, (and probably most others), OPTO simply means that it doesn't have a BEC at all.  A quad with 4 of these ESCs needs some other 12V -> 5V converter somewhere simply to fly at all.

Some of the ESCs have an ANALOG BEC.  The one I'm looking at right now claims 3Amps output.  Do I believe that -NO!.  A linear voltage regulator has an input current equal to the output current even though the voltages are different.

So, if an ANALOG ESC has an input voltage of 12V and an output voltage of 5V, it has a "drop" of 12V - 5V = 7V.  If you take the output current times that drop, you get the power dissipation of the regulator in the ESC.  If the output current is 1A, the power dissipation is 7 Watts!!  That is more than those little regulators can handle.

The problem is much worse if we use a 4 cell battery (16V).  Now the voltage drop is 11V and a 1A load causes a dissipation of 11Watts.  This ESC will melt (or the regulator will most certainly fail.

If a digital or switching ESC is used, the regulator drops the voltage with an efficiency of around 85%, so if the output current is 1A, the output power is 1A X 5V = 5 Watts X (1- 0.85) = .75 Watts.  This is a level that can be sustained.  The best part is that when we increase the input voltage from 12V to 16V, the output stays the same, and the efficiency is nearly the same, so the dissipation will STILL be around 0.75W.  

I can't stand having a pile of junk lying in a box waiting for repair.  I have to drop everything and fix the broken item.

So, I fixed my little 6" (actually 6.5") prop quad (picture below)

And I'm nearly finished fixing the 16" prop "Santa Cruz Project" test bed. I may get a chance to try it out this afternoon.

Tomorrow at 8:00 AM, I'm hosting a MEETUP at Cataldi Park in San Jose.  I'm expecting at least 15 attendees (could be up to 30).

Leave a comment if you need directions.

Building a multirotor "from scratch" is a daunting prospect.  I would recommend that you first buy a kit from a a company such as Hobby King. 

But, If you are going to design and build your own machine from scratch, you will need some tools and supplies. The list can be intimidating, but you would be surprised how most of the stuff listed below can be used for a lot of other household and hobby tasks as well. Although you can get by with less than the extensive list below,having everything listed will make your job a lot easier and save time waiting for deliveries from China.

Two soldering irons (or a temperature-controlled one with interchangeable tips).  One should be high-power with a large tip (for 10-12GA power leads). The other should be 35W or so, with a fine tip, suitable for PCB work.

Solder

Two wire cutters (dikes). One should be small for cutting 18-30Ga wire, the other should be large enough to easily handle 10GA.

Small needle-nose pliers. Most of the ones you find will be too big. I like the ones that are about 4" long overall.

3mm socket head capscrews of all lengths. 25 of each size - 5mm (thread length), 8mm, 10mm, 15mm, 25mm, 30mm, 40mm.

Lots and lots of 3mm nylok hex nuts

7/32" nut-driver (works fine with 3mm nuts)

2.5mm hex wrenches - buy 10.  You will use them for nearly EVERYTHING.

Small metric hex wrench set (so that you will have all the sizes that aren't 2.5mm).

English hex wrench set with .050 as the smallest size (believe it or not, some Chinese equipment is built with English-sized setscrews).

3/8" and 1/2" aluminum angle and channel stock (Home Depot or McMaster-Carr).

Hacksaw with fine teeth.

Dremel tool with assorted abrasives/cutters.

Electric drill, preferably one that can be mounted in one of those cheap drill-press stands.

One of those cheap drill-press stands.

Small center punch

XActo knife

Metric drill set. (or a very complete English set).  Make certain that the smallest bit is no larger than 2mm (.079")

Some very limp 12GA and 20GA wire in red and black (silicone insulation)

3.5mm "bullet" connectors, male and female.

2 pin JST connectors is both genders.

MT60 connectors in both genders.

Shrink tubing in all sizes.

Heat gun (air).

Hot-melt glue gun.

Digital calipers.

1/4" Tapered reamer

Voltmeter.  A cheap one ($10) will do.

Lots of small tie-wraps

Double-sided foam tape

Adjustable square.

Some .050" or .060" aluminum sheet 

Some .063" bare fiberglass sheet (bare PCB material).

A 3D printer is not necessary but very useful

Patience!

It should be noted that I have everything listed above except the last item, and I'm still hurting for a few tools, like a sheet metal shear and a metal bending brake. I can borrow those for the time being, but I'll eventually need to buy my own. 

I'm just going to have to learn the patience part on my own.

.

When wiring up a multirotor, you often find that there is a need to splice some very heavy gauge wires together (like connecting 4 ESCs to a single wire to go to the battery). 

If you twist all those wires together to solder them, you have a big mess.  The wires are so large that you need to strip at least 1" of insulation off the wires just to get enough bare wire to make one "wrap".  And one wrap is barely enough to get them to hold together. And if you move them, they fall apart. Not to mention that the resultant connection looks like a knot and is as big as a marble.

So what is the solution?  Buy a spool of solid 28Ga or 30Ga Kynar-insulated solid wire - also known as "wire wrap wire" and strip off the insulation from a piece 5" or 6" long.  Put the ends your heavy gauge wires together - just lay them side-by-side.  Don't twist them together.  Now take the 30Ga wire and wrap it tightly around the bundle to hold everything together.  If it makes things easier, wrap your 30Ga wire several times around just one wire.  Now lay the second wire next to it and wrap both wires. Add a 3rd and wrap and  a 4th and wrap ... Solder.  

Now you have a neat, well-soldered connection.

It is not uncommon to find yourself needing to splice several small wires together and put heat-shrink tubing around them for insulation.

Many times, you need to splice some wires that are short.  If you cut your heat-shrink tubing long enough to cover the splice you may find that - when placed on the wire,  you can't get one end very far from the splice that needs to be soldered.

I find this happens when I want to shorten and connect several ESC power-feed wires together.  I cut the heat-shrink tubing 1" or so long, and slide it over the wire.  But the wire itself is only 1.5" long.  Now, when I solder the wire, the heat flows through the heavy gauge copper wire and heats the heatshrink tubing to the point where it shrinks. 

So what is the solution?  Get a pair of medical hemostats.  You might need some larger ones for the larger gauge wire, but clamp the wire with the hemostats between where you are soldering and the end of the heat-shrink tubing.  The hemostats will act as a heatsink and prevent the tubing from shrinking before you want it to.

I wanted to repeat my "wiring tip #2" again, and with a bit more information.

The technique works REALLY well!

Today, I had to make a wiring harness that connected a 12GA battery wire to (quan) 4  14GA wires (ESCs).

The technique below mentions 12GA and 14GA wires, but it will work for any wires, small or large.

Strip about 3/8" insulation off the ends of all wires and twist each one tight. I recommend that you do not 'tin' them, and do not twist the several ends together at this point.

Strip about 4" of the insulation off some 30Ga KYNAR insulated wire (solid wire commonly used for wire wrapping).

Hold the 30GA wire (the part that still has insulation) in your hand and wrap 4 turns of the bare 30GA  around the 12GA wire. Then take one of the 14GA wires and lay the stripped end next to the stripped end of the 12GA wire (but in the opposite direction - overlapping only the stripped sections.  It may help to use a medium-sized hemostat to help hold them together (but it can be done without a hemostat). Now wrap the remaining 30GA wire around both the 12G and the 14GA wires.

Strip some more 30GA wire.  Wrap 3-4 turns around the splice and add a second 14GA wire.  Now wrap the 30GA wire around all 3 wires.  Do the same for each wire you have. 

Now solder.  Because the wires were not previously tinned, they will take a lot of solder. I recommended that you do not pre-tin the wires because the solder adds bulk, and will prevent the wires from fitting tightly together, resulting in a larger joint.

I think you will find that this technique works remarkably well.

I'm finally getting back on track with the Santa Cruz project.  The frame needs to be very strong, yet very light. I have settled on the "Box - X" design, that is, the frame is a normal "X", but the arms of the "X" are fastened together with small diameter carbon fiber tubing.

By connecting the ends of the arms, the diagonals (the "X" part) can be much thinner while still maintaining overall strength and rigidity.

The craft will need a "landing gear", so I plan on extending short legs down from the end of each "X", and to add strength to those - as well as further preventing twisting of the arms, I'm adding a very thin carbon fiber "X" between all the legs.  Since these "arms"are - in effect - trusses, they can be thin while being strong.

I'm planning on the main "Box X" part (3 pieces to make the "X", and 4 to make the "Box") from 6mm round carbon-fiber tubing. The legs (approx 6" long)  will also be made 6mm carbon fiber tubing, while the "X's" that connect the legs will be made from 3mm carbon fiber rod (solid).  4 more pieces of 3mm carbon fiber rod will connect the bottom of each leg back to the center platform of the copter. These additional supports are needed to handle the 5-6 lb weight of the batteries, which will hang under the center of the craft. Carbon-fiber disks glued and screwed to the carbon fiber tubing will act as motor mounts and a few 3-D printed plastic pieces will aid in holding things in place. An acquaintance who is very familiar with (real) aircraft frames and carbon fiber fabrication and has a machine shop has tentatively agreed to provide help.  I'll need it!

At least three of the 4 batteries that power the craft are mounted vertically in "chutes" made from carbon fiber angle stock.  A nylon string (fishing line?) connected across the chute under each battery holds them in place. A 26GA nichrome wire is twisted around each nylon string and the ends of the wires brought to the battery controller. The fact that the batteries are mounted vertically explains the quite long length of the legs.

The battery controller consists of a Microchip PIC 18F2321 uC.  4 outputs are routed to 4 N-FETs acting as a level shifters.  The drain of each of the N-FETs is connect to a 60A,40V P-FET held in the "OFF" state with a 10K resistor. Each battery is connected to the Source of a separate FET. The drains of all FETs are connected in parallel and feed power to the craft's main power rail. 

4 channels of 10-bit A/D (in the 18F2321 uC) are each connected to a separate battery. When initiated by a trigger signal, the uC brings the first output high which turns on FET #1 and battery #1 powers the craft.  When battery #1 voltage drops below a set potential, the 18F2321 brings output #2 high.  This has two effects:  It turns on the FET connecting battery #2 to the main power rail, and it also feeds power to the nichrome wire wrapped around the nylon string.  Within a second or two, the heat generated by current flowing through the nichrome wire melts the  nylon string and battery #1 is no longer held in place.  It drops down the (short) chute by means of gravity and a short loop of wire allows the battery to gain a bit of momentum before it "yanks" on the 3.5mm bullet connectors that connect it (electrically) to the craft.  The disturbance from the "yank" is small enough, and of short duration, so the craft simply shudders and continues running on battery #2.  This process repeats for battery #2 and battery #3.  Battery #4, of course does not need to be disconnected!

I'm laying out the PCB for the above controller right now.

I have the basic airframe designed in AutoCad, and a friend is converting the design to SolidWorks.  SolidWorks has a FEA 'add on' that will show strengths and stresses.  I will use that to refine the airframe.

I need to join a "10 step" program - MULTIROTORS ANONYMOUS.

I am in the process of building a small (9" prop size) folding quad, but I wanted to try out some ideas. 

So I built this 10" prop foldable model as a "test bed".  The diagonal is a full 500 mm, and as you can see, it folds down to shoebox size.

It has 

6 channel reciever,

KEDA 28-15  1050KV motors

10" APC props

CC3D controller

Cheap HobbyWing 40A ESCs

3 Cell 3850 mAH battery

The frame has a few plastic parts from THINGIVERSE (3D printed by me) and arms made from 3/8" X 1/2" AL channel with 6mm square AL tubing (in the channel) for extra stiffness.  The body is simply 2 pieces of 3/8 AL channel with "expanded AL (.060") forming the top and bottom of the body.  The legs are short pieces of 6mm square AL tubing.  The whole thing (with battery) weighs 3.2lbs. 

I have had it a few feet off the ground and it appears that it wants to fly.  I'll find out in earnest tomorrow.

When building small model aircraft, it is often necessary to provide an insulation barrier between a PCB and other components or between a PCB and a metal frame. That insulation barrier must be thin, light and puncture resistant.

And sometimes you must prevent a nearby screw from cutting through heat-shrink tubing or wiring insulation

So what is the best material?

I have found that old credit cards or old gift cards or stolen hotel key-cards are absolutely excellent. They are thin, lightweight and very puncture-resistant.

They are readily obtainable and can be cut with scissors or drilled, and most (flexible) glues will stick to them as well. If you look at my aircraft, you will probably see a Mileage Plus card under the flight controller, or as protection for the ESC wires.

When I check out of a hotel, I always have two new fresh cards to use for my "builds".

I wanted to explain my concept of the frame for the Santa Cruz Project in more detail.

In the picture below, the RED frame members are 6mm round carbon fiber tubing

The BLUE frame members are 3mm round carbon fiber rods (not hollow)

The LIGHT BLUE frame members are also 3mm carbon fiber rods.  They are shown in a different color because they extend from the ends of the "legs" back up toward the center of the craft..

The square in the center of the top view is the electronics platform ( I realize now that I'm going to have to make it a bit bigger).

The yellow in the side view are representations of the 4 batteries in their "chutes" made from thin carbon-fiber angle stock.

Not shown are:

The motor mounts made from carbon fiber.

The ESCs

The wiring tie-wrapped to the arms carrying power to the motors.

The 3-D printed parts that form the joints at the junctures of the carbon fiber pieces.

A test model cobbled together from 1/8 and 1/4" wooden dowels shows that the basic design of the airframe is very strong and stiff.Carbon fiber is stronger and stiffer than hardwood.