“I am leaning towards making a 2×2 long motor-stick helicopter. And I was wondering how I can optimize this kind of helicopter based on last year’s 4×2?”

“What is the explanation for why my 2×2 short motor-stick helicopter is so unstable? And in general why is this configuration so unstable?”

Analyzing Stability in Multi-Rotor Design

Rotorcraft stability is a complex problem influenced by multiple factors, requiring careful engineering analysis. As we consider leveraging the two-blade bonus this year (2026), several critical design elements and stability physics must be thoroughly examined.

Rotational Inertia and Blade Count

A primary contributor to stability is rotational inertia. (also known as moment of inertia, a property of a rotating object that measures its resistance to changes in its rotational motion.) A two-blade rotor inherently possesses less rotational inertia than a four-blade rotor of the same general design. Furthermore, a two-blade rotor, especially when rotating slowly, provides less continuous leverage or resistance (damping) around the non-blade axis at any given moment compared to a rotor with more blades. BTW, for the best inherent ease of balancing and stability, a three-blade propeller or rotor is generally considered the optimal compromise.

To maintain or compensate for the stability lost when transitioning from a four-blade to a two-blade rotor, we should consider two key adjustments: adding mass (a little weight) to each blade and moderately increasing the rotational speed. However, increasing speed is counter-productive to the “fly slower” optimization goal because it adversely increases parasite drag, and lower the best efficiency a rotor can achieve. (See my new post, “Optimal Speed for Science Olympiad Flight“, for more.)

Differential Lift and Aspect Ratio Constraints

Another key component of stability, as discussed in other posts on my website, is the differential lift generated between the two rotors. (In short, the top rotor should generate more lift.) To achieve equivalent lift and stability with a 2×2 system without excessively increasing rotational speed on the top rotor, we may wish to increase the top rotor’s blade area. (Check Lift equation. I will write another post on this topic soon.) Since the SO rules limit the maximum diameter, the only practical way to increase the blade area is by increasing the top blade’s width. However, blade width cannot be increased indefinitely before encountering another aerodynamic limitation: a poor aspect ratio. Like many other intricate optimization problems, this entire design process is a careful juggling and balancing act.

The Impact of Motor Stick Length

When transitioning to the 2×2 configuration, designers may also seek better efficiency by choosing a larger rotor diameter, which, in turn, dictates the use of a short motor stick (MS). Unfortunately, this design choice to limit the length of the MS negatively impacts overall stability. The effect of this can be readily visualized by trying to balance a short stick versus a long stick, inverted on your hand. The longer stick is fundamentally easier to stabilize because of its greater leverage.

The dimensions of the SO’s measurement box essentially guaranteed a lack of fundamental stability for this short motor stick configuration. Having confirmed these stability deficiencies through numerous experiments by several teams, a constructive recommendation for future rule changes would be: either slightly relax the two shorter dimensions or implement a bonus for helicopters utilizing this constrained short motor stick configuration, similar to the existing bonus for two-blade rotors.

Key Takeaway from Competition Models

The success of the 4×2 rotor configuration in many 2025 winning helicopters is primarily attributed to the 4-blade top rotor’s ability to provide the necessary power and stability. This configuration allow both rotors to operating at a lower Revolutions Per Second (RPS), and at an angle of attack (AOA) that is close to their best.

That said, the 2×2 rotor system can be fine-tuned to its best RPS and AOA, too. In addition to this year’s bonus, the 2×2 design is favored for its lower mechanical complexity and potential for higher aerodynamic efficiency due to reduced blade wake interference, especially when adapting for a larger blade diameter. However, its lower overall rotational inertia demands more vigorous tuning of various settings to maintain stability.

In short, both configurations present distinct engineering trade-offs. As the ideal design remains actively debated, students are encouraged to focus on optimization and rigorous testing of their choice, or choices. Good luck to all!

Do you have a different take on this challenge? Leave a comment below or send me an email. All feedback and critiques are welcome!

Cheers!

-AeroMartin 12/12/2025