[VSKYLABS Test-Pilot Notes] issued 17th February 2026
(The Self-Balancing Physics of Passive Variable-Pitch).
Wait. This is not going to be a heavy aerodynamics lecture. This is a PRACTICAL look at something you can actually fly in X-Plane 12, and something that exists in the real world! It’s about real physics, real engineering ideas, and how they translate into the simulation. The most exciting part is that it happens to involve one of the most fascinating VSKYLABS aircraft ever developed for X-Plane: The VSKYLABS Rutan Long-EZ.
So instead of diving into academic summaries, let’s talk about what the Aeromatic propeller really is, and what were the reasons for its design.
*screenshot: The VSKYLABS Rutan LongEZ equipped with Aeromatic propeller physics in X-Plane.
A fixed-pitch propeller is always a compromise. If you choose a low pitch prop, the airplane accelerates and climbs well but cruise speed suffers. Choose a higher pitch prop for better cruise, and takeoff performance becomes less impressive...or even an issue. Due to propeller aerodynamic efficiency, a single given fixed blade angle simply cannot be perfect for every phase of flight. That’s why variable-pitch propellers became popular in the 1930's, following the advances in aircraft design at the time, that pushed maximum flying speeds thresholds rapidly (the evolution of the racing aircraft - will save this for another time...).
Variable-pitch propellers solved much of the 'compromising' problem, but at a cost: Hydraulics, governors, oil systems, extra weight, extra complexity. For small aircraft, ultralights (and even today’s lightweight platforms), that complexity was/is often too much, and in some cases - not practical.
The Aeromatic Propeller was one of the earliest passive variable-pitch propellers, developed in the 1930s. 'Passive' means there are no hydraulic lines, no electric motors, no cockpit controls to move ('twist') the blades. In the passive-variable pitch propeller, the propeller changes its pitch by itself, using only the natural forces already acting on it during flight. It is a mechanical balancing act between aerodynamic forces and centrifugal forces.
In short, each blade is mounted on a pivot. It is not rigidly fixed at one angle. Instead, it can rotate around a pivot axis to change its pitch (refer to the screenshot above). The hub flange holds the blade at a certain angle relative to that pivot axis, and as the blade pivots, it moves along an arc, forward toward low pitch, or backward toward high pitch. The range of movement is limited by mechanical stops that define the minimum and maximum pitch for a given Aeromatic propeller.
Now imagine the airplane at takeoff. The blades are spinning, air is flowing over them, and thrust is being produced. The aerodynamic thrust force acts at the blade’s center of pressure and tends to rotate the blade toward low pitch. In simple words, the airflow 'wants' to flatten the blade. Torque has some influence as well, but it’s relatively small compared to the thrust effect, in the Aeromatic propeller mechanism.
At the same time, centrifugal forces are acting strongly on the spinning blades. Centrifugal Twisting Moment tends to reduce pitch regardless of position. But there is another centrifugal effect: the displacement force, which behaves differently depending on where the blade sits relative to the plane of rotation. If the blade is forward of that plane, centrifugal force tends to increase pitch. If it is rearward, it tends to reduce pitch.
This is where the counterweights comes in.
The Aeromatic propeller includes counterweight arms positioned in front of the plane of rotation. As the propeller spins, centrifugal force acts on these weights and pulls them toward the plane of rotation. Because the weights are mechanically linked to the blade, this motion increases blade pitch. In other words, while aerodynamic forces are trying to drive the blade toward low pitch, the centrifugal forces acting on the blade and counterweights are trying to drive it toward high pitch.
The magic of the Aeromatic prop is that these opposing forces balance each other automatically.
At takeoff, aerodynamic pitch-decreasing forces are strongest. The blades move toward low pitch, allowing the engine to reach full takeoff RPM and power. As the airplane accelerates and forward speed increases, the balance shifts:
The centrifugal forces become more dominant, and blade pitch increases. This prevents the engine from over-speeding and allows efficient power use throughout takeoff, climb and cruise, where the airspeed vs power demand varies.
In flight, the Aeromatic tends to 'govern' engine RPM at a given throttle setting. Over a wide range of airspeeds, RPM stays relatively stable because blade pitch continuously adjusts to match conditions. The propeller load curve follows the engine manufacturer’s calculated load relationship quite accurately, meaning that throttle position, manifold pressure and RPM maintains a predictable, stable relationship. For this reason, the Aeromatic prop design can fit a variety of aircraft.
And here’s the beauty of it - no extra cockpit controls are required. No prop lever. No constant-speed governor. The system works on its own. Besides the aerodynamic benefits...this is a very efficient configuration from the simulated aircraft developer perspective; no cockpit levers, no interaction...it is all in the physics!
So, in plain language, the Aeromatic propeller is a self-balancing system. Airflow tries to flatten the blades. Spinning mass and counterweights try to increase pitch. The final blade angle is simply the point where those forces 'agree' or equalize. And it happens continuously throughout the entire flight envelope regime.
The overall configuration is a lightweight and mechanically-simple way to get some of the benefits of a variable-pitch propeller without the hydraulics and controls complexity that comes with it. In the 50's-60's, the Aeromatic propellers were certified on many aircraft including Piper, Stinson, Ercoupe, T-craft, Bellanca, Swift, Aeronca, Cessna, Fairchild and more. In the present (2026) it seems that many aircraft home-builders and Experimental/LSA owners have increased interest in this simple configuration, to improve aircraft performance and safety margins.
The VSKYLABS / X-Plane / LongEZ Connection:
The Aeromatic propeller is a remarkably elegant piece of engineering, and amazingly (but not surprisingly) X-Plane Flight Simulator can simulate passive variable-pitch propellers out-of-the-box! (if you know what you're doing...).
VSKYLABS has always tried to push the aircraft designs into each and every possible 'corner' of X-Plane physics and flight dynamics models. When developed the VSL Rutan LongEZ, back in 2017/2018, during deep study of the aircraft, I came across the 'Canard Zone' forums, where pilots had an interesting discussions of the topic, specifically about the LongEZ. This was the inspiration that triggered the implementation of the Aeromatic propeller mechanism in the VSKYLABS Rutan LongEZ aircraft.
If you happens to fly the VSKYLABS Rutan LongEZ (none-ER variant), know that it is powered by a real-physics Aeromatic propeller!
Huss
VSKYLABS.
