Hi guys,

Its been a while since I've shared anything with this community, apart from occasional transactions over on the sale threads. Am thankful to all the folks who kept checking on me to find out if am okay during my absence, am short of words to describe my gratitude.

Now cutting the chase, and coming to the intended topic of this thread, as you guys know I've been a fixed-wing guy for quite a while now, but due to the absence of flying spaces in and around the city and the sheer lack of free time to travel 40kms outside the city to fly, I've been drawn towards multirotor. The recent developments in the quadcopter FPV scene was another driving factor. so consequently I bought myself a whoop and went to town, however, I soon felt that the fast speed, low video quality setups that FPV guys use and absolutely love, isn't my cup of tea....somehow I was missing the relaxed no-nonsense flights of my Cessna. But then I got a chance to fly a DJI Phantom 3 and got hooked instantly, the locked-in feeling, the video quality, the point where you wanna go kinda flight dynamics was an instant catch, and I discovered a new hobby, Aerial Photography.

However the problem with aerial photography is, if you are looking for a decent setup that can shoot pictures that get you an instant Right Swipe on tinder, you need to cough out at least 2 month's salary (or maybe more), plus getting DJI stuff in India is another pain in the rear, specially the batteries. So I decided the get my act together and design a quad myself (I know, why to reinvent the wheel?.....but DIY ka keeda nahi jaata, kya karun )

All the best.
Guess you are leaning towards autonomous flight modes using GPS.
INAV is a good choice. Cheap hardware and better results.

PURPOSE

The subsequent posts in this thread would be dedicated to analyzing the current technology, designing and building a better quadcopter/X8 frame that is open-source, at least partially 3D printable, light, strong enough to carry heavy gimbals and camera, versatile, and is suited for DIY builders who have a limited budget and only basic tools. ALSO, to list properties and guidelines that can be used to improve ANY type of copter frame you want to build.

This is intended to be an educational blog, so I request input from everyone on the forum to pitch in their two cents on the designs and the requirements.

This thread is not focussed on the electronics part of the build, simply because am electronically challenged and lack the knowledge to make decisions, so if anyone wants to collaborate on that part, please feel free to drop by.

Project Goals

Design a frame that is 3D printable with open-source plans.
Developing a design that requires minimum usage of exotic tools like CNC, however, am not accounting 3D printing as 'exotic' because many folks on the forum are providing that service for cheap.
Investigating ways to make a lighter frame to increase flight times and/or payloads.
Finding out materials and ways for building a stronger frame to minimize damage during crashes. Because, let's face it, most of us crash.
Design a frame optimized for photography and filmmaking but can also be used for a variety of purposes like mapping, etc.
Build a prototype, test and refine it, then work on production methods that minimize the cost.
Personally, I want to support Pixhawk, APM and DJI Naza with their peripherals.

Quote from: sim_tcr on July 29, 2019, 12:04:41 PM
All the best.
Guess you are leaning towards autonomous flight modes using GPS.
INAV is a good choice. Cheap hardware and better results.

Yes, definately GPS flight modes are my first preferrence.....Ive heard great things about Inav, but somehow Ive been unable to run Inav on any of my boards (Naze32, CC3D, Matek-f405) so far, hence am unknown of its capabilities. However since its cheap, and capable, I'll definately do more research on it, than's for pointing that out

Project Background

I'm not very pleased with the frames currently available (In India, not accounting for the treasure on aliexpress). There are three major types of frames, true X-Frames, H-Frames and Deadcat style frames.

They're heavy, they aren't very stiff, due to plastic arms or CF boom connectors, and many don't do well in crashes.

Some are easily repaired, while others are not. Sometimes all you need is just a new arm, while at times you have to buy a whole new frame kit just to get the parts you need. Plus the spares ain't easily available.

I constantly witness sandwitch type frame designs that add weight without doinig anything to improve the torsional stiffness or minimize crash damage. Meanwhile, mainstream engineering techniques commonly used in bridges, towers, and full-size commercial aircraft, are ignored.

I've spent the last few years building my own airframes from scratch (fixed wings) as well as modifying commercially available frames. I've crashed and smashed quite a few of them. I've tested materials. I've tested adhesives. I've built test stands. I've asked questions and I've learned from some really smart people who have a lot more experience than I do in the process. I feel like this is the next step.

Quadcopter Frame Types

I want this thread to be educational and appealing for the beginners, as well as informative for the pros who've been doing this longer than I had been around in the scene. I'd try to include the math behind my design decisions (as much as possible, I sometimes go with my guy feelings too ) and expect to be corrected when wrong.

Quadcopter frame properties:
Assuming that you use the same material, same adhesives/fasteners, build frames with an identical wheelbase (motor to motor diagonal distance) while maintaining a square layout, a true X frame si the lightest, but offers the least amount of surface area for stacking the electronics, while an H-frame is the heaviest and offers the maximum surface area for mounting the electronics.

Generally, in an X frame, electronics are installed in layers stacked vertically thus having minimum effect on the CG and pitch characteristics of the quad, while in an H-frame, its spread out in front and at the back of the CG affecting the CG adversely and causing sluggish behavior on the pitch axis.

However a hybrid frame is somewhere in the middle, more on that in the detailed post

X Frame

This is probably the most popular and common frame type in the DIY world and its the easiest to build. All other things being equal, it will be the lightest and the most agile frame, FPV racing guys can tell you more about its agility.

The problem is, if using a gimbal, you'll be hanging the camera underneath the center and it has to hang low enough so you don't see propellers when the quad tilts on the pitch axis and the camera is pointing forward. If you plan to point your camera down most of the time, like folks in mapping applications do, or it doesn't matter if you see propellers in your video, then an X Frame might be the best for you.
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Almost all DJI systems, from Flamewheel F450 to Phantom series to the industrial systems like Matrice series or WIND series are essentially true X quads, with the exception of Mavic and Spark.

However, if you don't want to see propellers in your video, then you'll need some fairly long landing gear so your camera doesn't hit the ground on takeoff and landing. The problem is, the landing gear needs to be fairly strong to support the weight of everything else, so it becomes heavy. The weight of your landing gear can easily cancel any weight savings you get from the X design.

This frame will have the longest arms, compared to the other two frames and longer arms are more prone to bending, and they act as levers in a crash. The longer the lever, the more force it exerts on the end where it connects to the body. Even if you use CF tubes as arms, the connector joining the tube to the main frame becomes the breaking point.

However, for smaller agile FPV quads, that do not carry a gimbal of heavy equipment, this style of frames are the best, because they offer the lightest build while maintaining almost identical flight characteristics in roll and pitch axes, allowing the pilots to perform crazy tricks like rolls, trifecta, pinwheel etc.

you can check out some of the tricks these FPV guys use here: https://thedroneracingleague.com/trick-wiki/

H Frame

The H frame is the heaviest (not counting landing gear) because of the size of the body. Yes, you can use carbon fiber plates but even if you have access to a CNC router and cut out a bunch of holes in each plate, it will still be heavier than the X tubes.

The major problem you have with an H frame is lack of torsional stiffness. I'll get to it in detail later but basically its the front of the quad trying to rotate against the rear of the quad, especially when you give roll input. For understanding, this better, take a rectangular box, for example, a pencil box, and hold one end in each hand along the longer side. Now turn your hands in the opposite directions, you'll notice the box twist and crinkle because it has no torsional stiffness. Now try the same thing with a solid piece of wood of similar dimensions. It is torsionally stiff, but its also a lot heavier than a straw.

H frames are notoriously bad when it comes to torsional stiffness. Usually, you have to add trusses in the hollow section to make it stiff, but the weight gained in the process isn't worth it.

The arms on an H frame will be the shortest and that is actually its strong point as it offers less leverage at the time of a crash, thus resulting in less damage.

You might recall from geometry, the length of the H-arm will be the length of the X arm * 1.4142 / 2 (the square root of 2, divided by 2). For example, if the arms on your X frame are 300mm long, they would only be 212mm (300 * 1.4142 / 2 = 212.13) on an H with the same motor spacing.

Obviously, the H frame is optimized for front-mounted cameras. Because the camera doesn't hang down underneath the frame, you don't need large, heavy landing gear. Sometimes you can get away with no landing gear at all!

One very good example of this style of frame is the ZMR250 frame and HFF_Quadcopter by Flite Test.

H frames tend to have a less stable roll vs an X frame while being more stable in pitch, whereas an X frame is similar in both roll and pitch. This asymmetrical roll rate causes problem in flying, especially if you are carrying a gimbal and want to execute razor-sharp transitions between shots, it takes time to master.  Adverse roll from yaw is a little more pronounced on H frames as well.

Another issue is the amplification of vibrations, although having shorter arms, the arms are stiff themselves in anH-frame, but the central body is large and acts as a resonator, thus amplifying the vibrations traveling to the FC, which causes issues in stable hover as well.

also, when flying forward at an angle, say 40 degrees, the cross-sectional area of an H frame is much more than an X-frame, causing a large amount of drag, thus consuming more current for the same airspeed. this in my understanding is one of the main reasons why major drone companies tend to go for an X-frame design.

Hybrid H Frame
Now that we've seen the pr
os and cons of the X and H, lets compromise and have it all. Setting the arms at 60-66 degrees allows the body to be shortened by around 60%.

Right away you have a body that's 60% lighter than the H, assuming they're both made from the same materials. Also, torsional stiffness increases linearly as length decreases. A body that's 60% shorter is 60% stiffer!

The arms of the Hybrid H will be longer than the H but still shorter than the X. Mathematically, we can use h/SIN(a) where h=the length of the H frame arm, and a=the arm angle. So, for our 212mm H arms, the X arms would be 300mm (212 / SIN(45) = 299. and the Hybrid-H arms would only be 245mm (212 / SIN(60) = 244.7). Math is fun, right?

So now we have a lighter, stiffer "H" that's still optimized for front-mounted cameras, win-win for all.

In commercially available quads, 3DR Solo is one of the best examples of Hybrid X design. The arms are at 66 degrees as shown in the image, which is a bit steeper angle than 60 degrees but it makes the arms slightly shorter and stiffer, while increasing the length of the body which is a plus point here because the battery resides inside the body, so a longer body means a larger battery.

At the same time, the SOLO uses a monocoque shell design, which saves weight. The motors are spaced in a square, with the same distance left to right as the front to rear. This results in a symmetrical roll rate along both X and Y axes, another win for the design.

The advantage of having motors arranged in a square layout is having symmetrical roll rates, and less yaw induced roll as per my knowledge.

DJI Mavic, however, takes a slightly different route. The quad has an incredibly stable hover and great flying characteristics, but its an asymmetrical hybrid-H frame. Front arms are at a 64-degree angle to the frame while the rear arms are just 53 degrees. One obvious advantage that this asymmetry gives is longer front arms, thus wider front profile, which gives more clearance the camera so that the props stay out of the frame. The motors as shown in the picture are in a sort of trapezium layout, what advantage/disadvantage this has is something am still trying to figure out. Would update as my understanding grows. One thing am sure about is this quad definitely uses some electronic mixing to compensate for the asymmetry in the frame to give the smooth flight characteristics it has.

Deadcat Frame

The dead cat style is typically favored by larger quadcopter designs aimed towards carrying a heavy gimbal and huge battery. Its purpose is to remove the propellers from the sight of the on-board camera and move the center of lift, back so that a heavier, bigger battery can be slung in the back of the craft. Also known as "Spider" style, this configuration is basically an 'X' quad with a standard 90° angle between the back booms but the front booms spread much further. The reason for doing this is that when flying FPV or filming with a front-mounted camera a 90° angle between the front booms will mean your props appear in the shot all the time.

Stiffness and weight wise it's almost similar to the Hybrid H-frame, but symmetrical in only one axis due to the placement of motors at different angles. TBS discovery is one of the best example of this style of frame.

One major point of interest is the roll behavior of this style frame in X and Y axis, because of the motors being farther apart in the roll axis, and closer together in the pitch axis, this frame tends to have a faster roll rate about the roll axis and a much slower roll rate about the pitch axis. this also limits the maximum angle the quad can be tilted in the pitch axis, thus limiting forward flying speeds.

To compensate this behavior, different PID gains can be set up in the FC, but that causes overloading of the motors. To avoid overloading/overheating one or more of the Multirotor motors, it is best to have the static center of pressure and the center of gravity at the same point in x,y,z axes, which is only achieved when the motors are in the square configuration

Subscribed !!

Monocoque Frames

Finally, the last bit in the puzzle is monocoque frames. this type of frame uses the skin or outer shell as its main structural component. The interior is hollow so it can be filled with a payload. Instead of the more traditional approach of 'sandwich' type frame constructions, there are no separate arms, plates, brackets, and spacers all bolted together, instead, the arms and body are part of a single piece shell (or a detachable top and bottom half).

The entire frame is either injection-molded plastic or Fiberglass/Carbon Fiber construction made around a plug mold. For smaller batches, vacuum forming can also be sued as a viable production method. This construction ideology provides an extremely lightweight, smooth, and aerodynamic shape with compound curves to improve the aerodynamics. It's hard to beat the weight and strength of a well-engineered monocoque frame.

DJI's Phantom series is a very good example of this construction ideology. Their frame consists of a two-part shell, that houses all the electronics, and is bolted together to provide a very neat and streamlined main body of the quadcopter. If observed closely you will find strategically placed gussets, ribs and relief points throughout the shell that help distribute the load, and increases the overall stiffness of the shell. The smooth streamlined shape of the frame further enhances the aerodynamic capabilities but facilitating a laminar airflow over the shell, increasing efficiency of the system

I'll explain the reason for the greater stiffness or torsional resistance of this type of frame construction, further in this thread.

One major disadvantage of this type of construction is, if you crash hard enough and break an arm, you need to replace the entire frame, which sometimes becomes expensive.