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
- Freestyle or Racing?
- 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.
The Ideal Frame
It should be strong against crashes
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.
It should be as light weight as possible
Quadcopter frames have significant impact on your quad’s flight performance: weight distribution, rigidity, weight, aerodynamics and so on.
Aesthetic and easy to build
A mini quad frame is more than just some sheets of carbon fibre. The frame design determines how easy it is to install components in it. Frame designers also incorporate different material and shape into the frame to make them look cooler.
As an interesting fact, we usually call our quads by the name of the frame (like, “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 a lot of factors to think about frames, we will try to cover as much as we can in 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 plastic, injection molded plastic, fibre glass, aluminium, or even PVC pipes. However carbon fibre remains to be the most popular material for mini quad frames thanks to its relatively low cost and excellent physical property:
- Light weight – a lighter racing drone means faster speed, better agility, longer flight time, and smaller impact in a 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:
- Max propeller size
- Moment of inertia of the craft
- Air resistance
- Total weight
A more powerful copter Frame sizes determine how large the propellers it can support, and a more powerful copter requires a larger frame while a less powerful copter uses a smaller frame.
Mini quad frames can be classified by the maximum propeller size that it supports. For example we can call a “210 frame” a “5 inch frame”, if it can support up to 5″ propellers.
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.
To make things easier, here is a simplified table with mini quad frame sizes and maximum propeller size they can use:
|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. Sometimes a 250mm frame might only accept 5″ but not 6″ props. It all depends on the actual design.
Although you can run smaller propellers on a frame, always try to avoid doing it for optimal performance. The quad perform the best when using the smallest possible frame that matches that prop size. For example don’t use 4″ props on a 5″ frame, just get a 4″ frame for that. :)
Frame Shape and Arm Layout
The frame shape, or arm layout is determined by how the arms are connected to the body. It does not only affect how the frame look but also has impacts on how the mini quad flies. The different frame styles are developed to suit different needs:
- hybrid X
- Stretch X
- Square (Box)
It might look like the motor layouts are very similar and are all rectangular, but each of these frame type performs differently in some way. Whether or not the different arm layouts bring in any benefits has always been a controversial subject. But based on my own personal experience they do feel differently in the air.
Early mini quads started with H style frames. They have seemingly unlimited space for components, making these frames very easy 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. For this reason, it might feel less agile than the newer X frames due to higher moment of inertia on the pitch axis. However some people prefer this behavior for smoother freestyle 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 a more centralized mass. This results in less moment of inertia and ultimately more snappy flight characteristics compared to H frames, more ideal for FPV racing.
X style frames normally are lighter in weight because of the reduction in material. But due to the smaller body, it could be a little bit more challenging to biuld on. Everything is stacked 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 stretched out forth and back. The idea is to improve cornering performance in racing and high speed flying.
By moving the rear props away from the front props can minimize air turbulence, this allows the copter to fly fast yet remains stable.
What I personally found with stretch X frame is I have to re-tune PID and rates because of the change of frame geometry, roll seems to be more sensitive on the roll axis than a standard X frame.
A stretch X frame also tends to weight more because of the additional material.
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.
Instead of flying forward like an X, a Plus Frame flies forward like a +. It’s a special type of frame because it’s not very common and popular. One of the downside is the front motor and propellers can easily get in the view of the camera.
To solve the problem with the motor being in the view of the camera, frame designers stretched the front and rear motors away from the centre. But it’s still not a popular frame design apart from the fact that it looks pretty unique and eye-catching.
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 could mean a lot of extra 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.
With light weight racing frames where you are going to crash a lot and have a higher chance of breaking an arm, having replaceable arm design is definitely a bonus. But modern frame design has brought us very durable unibody frame designs too, but I would only consider those for a freestyle quad, and uses at least 4mm thick CF.
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.
Freestyle or Racing
For racing, I would say the most important factor is weight and crash-resistance, having very thin and easy-to-replace arms are going to be really helpful.
For freestyle, the most important factor for me has to be how easy it is to build/repair, and how tough it is against crashes.
FC standoff spacing – Make sure the frame supports the FC and PDB you are planning to use, common FC’s can have mounting spacing of 30.5mm x 30.5mm, or 20x20mm. Although there are 3D-printed adapters you can get easily, it’s not always supported in some frames.
FPV Camera Mounting – There are now 3 main sizes in FPV camera, standard, mini and micro. Make sure to find out the frame support the camera sizes you are planning to use. You will also need to find out what kind of camera tilt you can achieve, a tilt angle that is too small or too big can hugely affect your FPV experience
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 just by looking at pictures and spec. The best thing to do is to look at reviews and comments. Pay attention to how well the components are protected by the frame and make sure nothing is exposed to crash damages.
HD Camera Mounting – If you plan to fly with an HD camera such as a GoPro or Runcam 3, make sure to check if the frame can support it at all! You might have to spend extra to get a 3D printed mount, or it might be included 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.
Battery Location – top or bottom?
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, it also makes the quad handle corners better in my experience
- Landing on battery can damage it (crashing is another discussion, because it’s unpredictable which side would hit the ground first)
- Pendulum effect when mounting battery on the bottom which makes control a little less precise when doing extreme movements
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… ?
There are a lot of overpriced frames out there, you should consider carefully how much a frame kit is really worth. And frame cloning is a very common problem in this hobby because how cheap and easy it is to copy a frame design.
If you are considering a clone, then you must really like the design. I don’t encourage people to buy clones, but if that’s what you can afford, I don’t blame you. Anyway, try to avoid clones whenever possible, not just for the quality, it’s more about sustainability.
It takes a company months to design, test and release a frame, it doesn’t cost the cloners anything. It’s especially so for mini quad frames, after all they are just pieces of carbon fibre, anyone with a CNC machine can do it. Therefore frame manufacturers inevitably suffer from copying and cloning. Cloners can sell their frames at half of the price or even lower, because they didn’t invest in R&D, and they are using cheaper material.
You can measure the performance of a motor or ESC pretty easily and fairly 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 and user experience.
Anyway I hope this guide has helped you understand the basics of mini quad frame!
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.