The AOS 3.5 is optimized to deliver maximum performance for freestyle flying with a total flying weight under 250g with analogue or digital video systems.

The AOS 3.5 builds on the scientific approach used to design the AOS 5 and AOS 5.5. In many regions quadcopters that weight less than 250g are not subject to the same burdensome regulations as heavier drones.

 

But just because a quad cannot weigh more than 250g doesn't mean that we shouldn't strive for the speed, power and agility typically only available from heavier drones. The AOS 3.5 is designed for a flying weight just under 250g and when paired with the right components can go toe to toe with 5" freestyle quads.

 

A combination of finite element simulation and real-world flight testing gives this frame exceptional vibration and resonance performance. This means you can run less filtering and higher PID gains for a smoother and more responsive feel with greatly improved mid-throttle oscillations and cool motors.

  • Approx. 50g with all hardware

  • Designed for up to 3.5 inch props

  • 4 mm Arms

  • 2 mm Top Plate, Bottom Plate and Sandwich Plates

  • One 25.5x25.5 mm or 20x20 mm stack in the front

  • One 20x20mm stacks in the rear

  • Fits the Caddx Vista perfectly

  • Fits both 20mm (DJI camera) and 19mm (Micro sized) cameras

  • Recommended motor size 1404, 1506

  • Standoffs, frame and motor screws included

 
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WHY 3.5"

Maximum performance under 250g

3.5" Props give you the maximum performance
for freestyle flying under 250g

The AOS 3.5" was born from an analysis of quadcopter weight against prop size. The result was the identification of disc loading as a key parameter that could be compared between quads of different sizes and was a strong predictor of flight performance for freestyle flying.

By looking at the flight performance of a large number of quads it was possible to identify the ideal disc loading for a freestyle drone and thereby calculate the ideal prop size for an FPV freestyle drone of a given weight.

When that analysis is performed for 250g the ideal prop size turns out to be very close to 3.5" and several good 3.5" props are available.

 

In that moment the AOS 3.5 was born!

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THE CHALLENGE 

Learn about the challenges that led to the AOS 5.5

 

Vibrations limit performance 

As a pilot I often struggled with Jello, mid-throttle oscillations and propwash handling on my freestyle mini quads. As an aerothermal engineer I realized that the issue lay in the vibration and resonance performance of the quadcopter frame itself.

Even the cleanest motors and props produce large amounts of vibration in flight. These vibrations excite the resonant modes of the frame amplifying vibrations at certain frequencies, increasing their amplitude and energy. These vibrations pass into the gyro and create noise on the signal. This noise requires additional filtering and limits maximum PID gains through motor heating.

 

The result is a flight controller that is slower to respond to propwash and your stick inputs. The AOS 3.5 is designed to be better.

 
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THE DESIGN

Understand the design methodology behind the AOS 5.5

Harmonic analysis is the key 

I used Blackbox logging and harmonic analysis to visualize the vibrations of the AOS 3.5 during the design process. This allowed me to tune the geometry of every part of the frame, moving the frame resonances up and away from the motor frequencies and separating them as far as possible. This avoids, as far as possible, the situation where the motor frequencies match a frame resonant frequency and so amplification is minimized.

As a result, you can run less filtering. Every layer of filtering adds delay that’s mathematically related to how effectively it removes vibrations. This delay prevents the flight controller from responding rapidly enough to propwash leading to poor propwash handling. Reducing the amount of filtering is the best way to improve propwash handling. With the AOS 3.5 you’ll be in the best possible position to reduce your filtering to an absolute minimum.

Another advantage of lower noise is the ability to run higher PID gains. Higher PID gains allow the quad to track your stick movements with less delay and more precision. The result is greater confidence, particularly when flying tight lines where fast reactions are vital.

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KEY FEATURES 

See the key features of the frame

 

Top and bottom plates

Both of these plates are 2mm thick to provide an optimal balance between strength, stiffness and weight. The cut outs in the top and bottom plate of the AOS 3.5 are tuned using finite element analysis to preserve as much of the bending and torsional stiffness of the plate as possible whilst minimizing weight.

There are mountings for a 25.5x25.5mm or 20x20mm stack up front and a 20x20mm stack in the rear with space in between for the receiver, capacitor and other accessories.

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Arms

The AOS 3.5 uses the same distinctive truss design for the arms as the AOS 5. This truss design provides enormous torsional stiffness, resolving a key source of vibration in traditional frames. It also allows two particularly harmful resonant modes to be individually modified to separate them in frequency.


Truss structures are inherently very strong as they convert bending loads into compression and tension in the individual members. Carbon is much stronger in pure tension and compression than in bending. Elongated slots for the outer motor screws are just one of the many ways weight has been trimmed from every part of the frame.

 

Y-shaped motor protectors minimize the risk of damage to those expensive components. Should you ever need to replace an arm, there are only two screws to undo and neither are shared with your electronics.
 

Camera Protection

The AOS 3.5 uses 2mm Carbon Plates to hold and protect the camera. These are stiffer, lighter and cheaper to replace than aluminum or titanium.

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Motor screws

6mm M2 Cap Head motor screws are provided with the AOS 3.5 to eliminate the risk of shorting the motor windings or stripping out the threads in the motors if your motors didn't come with the correct length screws.

Manufacturing

A great design is nothing without great manufacturing. I trust CNCMadness (based on the Canada–United States border) to manufacture this frame to the highest standards.

 

I have been consistently impressed by their quality and attention to detail throughout the prototyping process. They provide high quality carbon fiber cut to exceptionally tight tolerances ensuring interlocking parts slide together without any slop.

 

Once you’ve held a frame made by these guys I hope you’ll agree that you won’t want anyone else cutting your carbon.

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