This tutorial explains the basics and overview of mini quad frame, and how it affects flight performance. There are many considerations to bear in mind when choosing an ideal FPV racing drone frame.
Index of Content
- What is a mini quad frame
- Frame size
- Frame shape
- Unibody design
- Other considerations
- Evolution of Frames
What is a mini quad frame?
Mini Quad frame is a structure that contains the components of a mini quad, or quadcopter. Many considerations go into choosing a good frame.
First of all, the mini quad frame design determines where your components are going to go, and how easy it is to build the quad.
Secondly, quadcopter frames have significant impact on your quad’s flight performance: weight distribution, rigidity, weight, aerodynamics and so on.
Thirdly, the design and material of a frame determines how crash-resilient it is. Miniquads are designed for FPV racing and fast flying, crash is inevitable. For typical hobbyists with limited budget, this is especially important IMO. It doesn’t matter how well your quad flies or how cool it looks, if you were going to break an arm or top plate every session it’s definitely going to ruin the fun.
As an interesting fact, we call our quads by the name of the frame (like, oh yea, for example “let me show you the Blackout I just built”). Maybe not as deciding as other factors, but the name does matter too :D
There are some other things to think about too, we will try to cover as much as we can in the rest of this article.
Material For Mini Quad Frames
A mini quad frame can be made from any material you can possibly think of: Wood, 3D printed, injection molded plastic, fibre glass, aluminium, or even PVC pipes. However carbon fibre remains to be the most popular material for mini quad frames (i am guessing 99% of them are CF), thanks to its relatively low cost and excellent physical property for racing drones:
- Light weight – a lighter quadcopter means faster, better agility, more flight time, and smaller impact when you crash
- Strength – carbon fiber is known to be relatively tough and durable
- Rigidity – CF has high stiffness to weight ratio. Frame rigidity is a great thing to stability and flight performance
However there are some downsides to using carbon fiber for frames:
- Carbon fiber is electrically conductive. If you have live wires touching the frame it could cause short circuit and burn out your components
- It also blocks radio frequency signals (such as 2.4Ghz and 5.8Ghz), so make sure your antennas are not hidden inside the frame
In the rest of this article, we will focus on carbon fibre mini quad frames.
Construction of a Mini Quad Frame
A mini quad frame can be simply divided into 2 parts, the body, and the arms.
The body houses and protects your electronics components including flight controller, PDB, FPV camera, VTX etc. Typically it consists of a bottom plate, top plate and some standoffs in between to hold them together.
The arms are where to install your motors, and very often your ESC’s as well. The arms are designed for durability and they are usually made from 3mm to 4 mm carbon fiber sheets.
The Size of a frame, sometimes known as wheelbase, is the diagonal motor to motor distance in millimeters.
The frame size have influence on the following factors:
- Moment of inertia of the craft
- Air resistance
- Total weight
- Max propeller size
Because motors are mounted on the very end of the arms, the further away they are from the centre, the larger moment of inertia (tendency to resist angular acceleration). With all other things being equal, the smaller the frame, the more nimble the craft becomes.
Furthermore, the bigger the frame, the more air resistance when flying forward. The frame weight also goes up due to the increase in material.
Frame sizes determine how large the propellers it can support (partly), therefore mini quad frames can be classified by propeller size. For example we can call a “210 frame” a “5 inch frame”, if it can support up to 5″ propellers.
The general mini quad frame sizes, and the propeller size they are designed for are:
|Frame Size||Prop Size|
Please be aware this table only aims to give you an idea, and not always the case due to differences in the frame design.
Pro Tip: Regardless what the size of the frame is in “mm”, always check what prop sizes it supports. For example an 5″ frame might be larger or smaller than 210mm, or maybe a 250 frame only accept 5″ but not 6″ props. It all depends on the actual design.
Although you can run smaller propellers on a frame, we tend to avoid doing it for optimal performance. You could have a more agile quad using the smallest possible frame that matches that prop size. For example we wouldn’t run 4″ props on a 5″ frame, just get a 4″ frame for that.
Frame Shape / Arm Layout
The frame shape, or arm layout is determined by how the arms are connected to the body. It has been part of the non-stop mini quad development, more and more frame styles are invented to suit different needs. Today’s popular variations are:
- hybrid X
- Stretch X
- Square (Box)
It might look like the motor layouts are very similar (all rectangular), but each of these frame type performs differently in some way. There has always been debates about what benefits there are with the different arm layouts. Anyway I will try my best to explain the differences based on my own experience.
Early mini quads started with H style frames. They have seemingly unlimited space for components, making them easier to build.
The arms connect at the front and rear of the body on a “H” frame. It creates a long and roomy body that can house your electronics comfortably. HD camera and battery can both sit on the top plate, and the weight of the quad is more spread-out. It might feel less agile than the newer X frames due to larger moment of inertia on the pitch axis. However some people prefer this behavior for smoother free-style flying.
On an X frame all the arms meet at the centre forming an “X” shape, with equal width and length (motor layout is a square). The body is shortened as much as possible, and components are meant to be stacked up in the middle to achieve more centralized mass. This results in less moment of inertia and ultimately more snappy flight characteristics compared to H frames, which is ideal for FPV racing.
X style frames normally are lighter in weight because of the reduction in material. But due to the lack of space, building could be a little bit more challenging. You have to mount everything in the center like a tower, including your HD camera and battery. People sometimes call it the “true X frame” to differentiate from “hybrid-X frames”.
The Hybrid-X has the X frame arms, but H frame’s long body. Some argue this has little to none differences to a H frame in terms of flight characteristics, since the mass distribution are the same. But I think there is a difference to how the thrust are applied on the frame (think about leverage), and how the prop/motor vibration is transferred to the FC in different directions through the arms.
Stretch X Frames
Very similar to an X frame in appearance, but the “stretch X” is longer in length than width. The idea is to improve performance in racing and fast flying by:
- moving the rear props away from the front props to minimize air turbulence. This allows the copter to fly fast more stably and with less tilt
- providing better stability on the pitch axis while maintaining the same agility in the roll. Since in a race, pitch angle is mostly constantly tilted forward
Square (Box) Frames
A square or box frame could be based on a H or X frame, with extra material on the outside that connects the motor mounts. This basically creates a tougher frame that is less likely to have broken arms. But the increase in material blocks prop airflow, increases weight and air resistance. Not a good choice for performance aircraft, unless you are new to this hobby and worry about breaking stuff.
Other Frame Configurations
There are some other different frame designs out there, such as the deadcat, V-tail etc. But these configs are not as popular because they often require custom motor mixing and they are harder to tune.
A frame can be designed to have replaceable arms, but that means extra hardware such as screws, bolts and extra bottom plate. That’s a lot of dead weight!
You can avoid the extra weight by having an unibody design (aka monobody), where the arms are just part of the bottom plate. It’s not only lighter but also makes frame assembly easier. However if you break an arm, you would have to replace the whole bottom plate. Taking out all the electronics and motors could be very time consuming.
Modern frame design has brought us both super light weight frames with separate arms, as well as very durable unibody frame designs. As long as you go for name brands and quality frames, unibody or not isn’t really that important anymore IMO.
As for rigidity, I generally find frames with replaceable arms design stiffer than unibody. The stiffness of unibody depends on how wide the arms/body are, how big the cut-out is and how thick the plate is. I have had some bad experience with 3mm unibody, It was so bendy and I had oscillations that was really difficult to get rid of. I replaced it with 4mm and it was much better.
Carbon Fibre Thickness
Thicker carbon fiber results in better rigidity and sturdiness, but it also gets heavier. 150mm or smaller frame often use 2mm thick CF sheets, while larger frames normally uses 3mm to 4mm.
However the thickness matters the most to the arms, so you might sometimes find thinner top/bottom plates. That’s totally fine because it’s your arms that takes most of the impact in crashes. 4mm arms are the standard for durability, some serious racers use 3mm arms to save weight.
Some people prefer 3mm just because they have lighter arms and less overall moment of inertia and it contributes to the better flight performance.
Filing Carbon Fibre Edges
If your frame comes with sharp finish, it’s recommended to file the edges for the following reasons:
- file it and then apply Cyanoacrylate glue (CA) can prevent carbon fibre from delamination in a hard crash
- the sharp edge can cut through wires, LiPo straps, etc
It’s best to do this under running water to avoid breathing in CF dust.
Rigidity – We have mentioned the importance of frame stiffness more than a dozen times . A well designed frame should be strong, crash resistant, yet rigid. If there is bending or warping in the arms during flight, your quad would be prone to vibrations. As a result the quad would be hard to tune, and you might get jello in the flight footage. Flight controller also had to work harder to stabilize the copter, it could even cause overheat to your motors.
Weight is an extremely important factor to your quad’s performance. Good carbon fibre all weight similarly. The main difference comes down to how the frame is designed, how much material is used while maintaining the strength, what sort of screws/bolts/standoffs are used and so on.
Durability – It’s very difficult to predict how durable a frame is only by looking at pictures and spec. Best thing to do is to find out from reviews and those who have tried it. Also check if your components are well protected by the frame and nothing is exposed to crash damages.
FPV Camera up tilt angle – I’m finding more and more people use crazy camera up-tilt, such as 60+ degree. Many frame makers assume pilots to fly slower and allow only moderate tilt angle. Make sure the frame supports what you need.
HD Camera Option – If you plan to fly with a GoPro or Runcam 3, make sure to check if the frame is designed to carry a HD camera at all! Sometimes you might have to spend extra to 3D print the mount yourself, or they might include it in the kit.
Spare Parts and Warranty – No spare part available means buying a new frame when you break an arm or top plate. Having spare part available is extremely important. Some manufacturers even offer “lifetime” warranty which allows you can get replacement parts whenever you break it. Although these frames normally cost a bit more to begin with, it’s still something worth considering.
Protection for the motors – Extra material around the motor mount could potentially protects your motors from damaging in crash. Downside is the weight.
Chamfered carbon fibre edges – more and more frames come chamfered finished. What used to be a time consuming DIY job filing those sharp edges, you could now save a lot of work.
Should we put LiPo battery on top or bottom of the frame?
This is another controversial subject, and yet again I think it’s really down to personal preference and frame design.
Let’s say if I had the choice, I personally would prefer to have battery on top whenever possible for 2 reasons:
- center of mass is closer to the level (or plane) of the propellers, where the force is generated. This reduces the moment of inertia when rotating
- landing on battery is more likely to damage them (crash is another discussion, because it’s unpredictable which side would hit the ground first)
Anyway, it’s getting pretty common these days to mount battery on the bottom with the popular X frames due to space limitation.
Evolution of Mini Quad Frame
The world of mini quad evolves extremely quickly, products can often become redundant in just a few months. In this section I will feature some of the interesting frame designs throughout the history of mini quad, they all have some unique features to offer. Please note that this is not a comparison. :)
If you think I should cover other frames, please comment down below!
The Blackout frames were the beginning of mini quad, released back in 2014. It was expensive and hardly available back then due to popularity. Shortly after the ZMR250 made it more affordable and accessible for hobbyists world wide (many consider it as a clone of the blackout). Most frames at this time were 250 sizes and running 5″ and 6″ props.
The Lumenier QAV frames were made popular by Charpu (he was sponsored by Lumenier after all). And then there was this cool looking, but heavy Atas Defiance 265 and Robocat.
The Alien by ImpulseRC was an iconic frame, as it was one of the first hybrid-X frames and endorsed by many top pilots at the time. The Armattan F series frames were also very popular back then.
The most popular size of mini quad settled at 5″, because most people believed 5″ is the best balance between power and agility. And we began to see more and more “True-X” frames.
Later we have X frames with different camera pod designs, and 3D printing started to play a bigger part in mini quad frames.
Different materials are used in mini quad frames, such as the Ragg-E that uses super durable HDPE. On the other extreme, some frames tried to shave as much weight as possible by making skinny arms, such as the X-foot or the QAV-ULX.
Some frames released in the 2nd half of 2016, we started to see vertical standoffs were replaced with side plates structures. The Armattan Armadillo and DemonRC Fury are great examples of that.
At the beginning of 2017, aluminium (metal) brackets are used in carbon fibre frames. The new designs create a strong structure yet it looks awesome! Both of these frames, the DQuad Obsession and Armattan Chameleon have been featured in our top 5 best frames in March 2017.
Buy clone frames to save money… ?
NO!!! Try to avoid clones whenever possible.
It takes a company months to design and release a good frame, but it doesn’t cost the copycats anything. It’s especially so for mini quad frames, after all they are just pieces of carbon fibre, the entry level for this industry is very low. Therefore frame manufacturers inevitably suffer from copying and cloning. Cloners can sell their frames at half of the price, because they didn’t invest in R&D, and they are using cheap material.
You might save $$ using this cloned, low quality frame, but this is really bad for the companies that create and develop cool products. In the long run this is going to hurt the hobby and ultimately, us. If you can afford it, always buy the genuine and support true innovation.
You can measure the performance of a motor or ESC pretty easily and objectively, but it’s a little bit more complicated than that with a frame. There are a lot to consider in the design, aesthetics, features etc. Anyway I hope this post have helped you understand some of basics and make the decision for your next build!
Looking for frame ideas? Check out the frames we reviewed and built in the past: https://oscarliang.com/tag/frame/ and our 250 parts list: https://oscarliang.com/250-mini-quad-part-list-fpv/#frame.