How to design a Quadcopter

Frame size

Quadcopter frame size (a.k.a. wheelbase) is the diagonal motor to motor distance measured in millimetres. It dictates the size of propeller you can run on it, therefore the frame size often are referred to the maximum propeller size it can support, instead of its wheelbase. For example, when people say “a five-inch frame,” it means the frame is sized for 5″ props.

As the motors are mounted on the very end of the arms, the further away they are from the centre of rotation, the larger the moment of inertia would be. It introduces a tendency to resist angular acceleration and deceleration and makes your quad feel more sluggish and less responsive. Therefore you want to use the biggest propellers a frame can support to maximize its performance. If you run 4-inch props on a 5″ frame, it would not perform as well as on a 4″ frame.

Here is a table showing roughly what maximum prop size different mini quad frame sizes can support:

A typical frames are made up of various parts: 4 arms, 2-3 top/bottom plates, hold together by standoffs and bolts.

The arm layout refers to how the arms of the FPV drone frame extend from the body. It might look like some of the motor layouts are similar (they are all rectangular after all), but it does have an effect on control feel and flight performance. There are some other reasons too when choosing one particular motor layout than another.

H Frames

On an “H” frame, the front and back arms are perpendicular to the middle plate, forming an “H” shape. This leads to a long and roomy body section for installing your electronics comfortably, makes it very easy to build. HD camera and battery can both sit on the top plate, the weight of the quad is more spread out. For this reason, it might feel less agile than X frames due to higher moment of inertia on the pitch axis.

In recently years the hobby has mostly moved to the X frame designs.

X Frames

On a “True-X” frame all the arms intersect at the centre forming an “X” shape, with equal distance between all the motors to the center of the frame. True-X frames tend to have a more neutral handling and balanced behavior on pitch and roll as the motors are contributing equally.

Some argue it has little to no difference to an H frame in terms of flight characteristics, since the mass distribution is the same, while others claim there is a difference to how the thrusts are applied on the frame due to the way the arms are connected to the body (think about leverage). Also the ways vibrations are transferred to the FC through the arms could be different.

Stretch X Frames

Similar to the “True X” frame, but with the front and back arms stretched farther apart than the side arms. The idea is to reduce air disturbance that the rear propellers get from the front propellers, and it might also improve handling at high speeds and in tight turns.

Deadcat

Deadcat geometry refers to a motor layout that is asymmetric, meaning the motors are not symmetrical front and back. This layout is designed to keep the propellers out of the camera’s view, making it an attractive option for capturing cinematic footage.

However, due to the asymmetrical motor layout, pilots may experience a slight amount of yaw mixed in with roll movements. While the mixer in your flight controller can usually compensate for this, the result may not be perfect all the time. The impact of this issue largely depends on your flying style and rates. For smooth cinematic flying, this downside is generally negligible. However, for pilots who prefer high rates and aggressive freestyle maneuvers (such as doing acrobatic moves), this mixing can be more noticeable and problematic.

Another downside of deadcat frames is their crash durability. Due to their design, deadcat frames have a higher chance for the camera to take a hit during a crash. This is an important factor to consider when deciding whether a deadcat frame is the right choice for your drone setup.

Box Frames

A box frame is pretty much an enclosed X Frame. This basically creates a tougher frame that is less likely to have broken arms. However the extra material creates more drag and weight. Not the best choice for performance but definitely helps improve strength of the arms.

Plus Frame

Instead of flying forward like an “X”, a Plus Frame flies forward like a “+” sign. There may be some benefits with regards to motor turbulence though, because the side motors on a plus frame will always be spinning in clean air. One of the downsides is the front motor and propeller can easily get in the view of the camera. It’s not common frame type apart from the fact that it looks unique and is eye-catching..

Unibody Design

Some frames have all the arms and bottom plate cut from a single piece of carbon fiber sheet, this is often referred to as a unibody design.

A frame can be designed to have separate, replaceable arms, but it involves extra hardware such as bolts, nuts, and an extra bottom plate.

Unibody is much lighter and makes frame assembly easier, but if you break one arm, you would have to replace the whole bottom plate and move all your motors across to a different frame, it’s a lot of work. In comparison, with separate arms you can simply replace the broken arm.

As for rigidity, I generally find frames with replaceable arms stiffer than a unibody, because the arms are usually sandwiched between two separate plates. But of course, arm stiffness depends on carbon fibre width and thickness too.

Durability

Carbon fibre quality: Cheap frame from unknown brands will most likely use low quality carbon fibre and it’s probably not going to hold up well to crashes. Unfortunately, there’s no easy way to tell by just looking at pictures of the frame. The best you can do is to check out reviews and base your judgement on the frame’s price tag and the brand’s reputation.

Arm thickness: In general, a thicker arm is less likely to break because more material provides a wider area to distribute stress over in a crash.

Stress points: avoid arm design that involves hard angles as it’s much easier to break in a crash, curves and round cutouts are much better for strength.

Weaves direction: there are weaves in carbon fibre sheet, and the cutting direction actually makes a different to the strength. The carbon fibre plates are stronger when cut along the weaves and weaker when it’s cut at 45-degrees to the weaves.

Carbon Fibre Thickness

Thicker carbon fiber means better strength, rigidity and sturdiness, but it also gets heavier.

Durability matters most on the arms, as they take most of the strain in a crash. This makes it common to see top, centre and bottom plates cut from thinner 3mm or even 2mm CF.

Arms in 5″ frames commonly used 5mm arms (thickness), but advancements in motors have made thicker 6mm more common, to withstand the greater forces of higher speed impacts. For 5″ frame you should be looking for at least 5mm thick arms, for light weight 3″ and 4″ you may go down to 4mm or even 3mm. For 2″ just 2.5mm (or even 2mm) is possible. Any thinner than this will break too easily and probably too flexible too for decent flight characteristics.

Filing Carbon Fibre Edges

If the carbon fibre in your new frame don’t come with chamfered edges, then you could consider filing those rough and sharp edges yourself. It’s not only for the better look and feel, but also for the following practical reasons:

• Sharp edges can cut through wires, LiPo straps, etc if they are rubbing against it over time
• Chamfered the edges, and then applying Cyanoacrylate glue (CA) can prevent the carbon fibre from delamination in a hard crash

In this tutorial I will show you how to prepare a carbon fibre frame by filing the edges. It’s best to do this under running water to avoid breathing in CF dust, the ultra fine particles are harmful to your lungs.

Flying Styles

Frames are designed for specific application and flying styles, usually stated in the product description. The flying styles are:

• Freestyle
• Racing
• Long range
• Cinematic

With that being said, there’s