# Is precession a problem for two-bladed propellers?

The vast majority of wind turbines are a three-bladed design. Although two blades would be more efficient, two-bladed designs are more likely to suffer from precession. Do two-bladed propellers suffer from precession? If not, why would it be a problem for wind turbines, but not aircraft props?

Wind turbines suffer not from precession per se, but from precession 'wobbling', when the gyroscopic force varies dramatically over a revolution. Normally, a 3-bladed prop would be heavier than a 2-bladed one, and will have greater gyroscopic effect (at the same RPM). However, it will be better balanced across a revolution. The closer to a solid disk, the better.

(In fact, a 3-bladed prop appears to be perfectly balanced in extreme positions. Considering blades slender rods or length $$r$$, the moment of inertia (MOI) of the propeller disk for turning across the spin axis (e.g. yawing) will be $$1/3\cdot mr^2 + 2\times 1/3\cdot m(r\cos60°)^2 = 1/2\cdot mr^2$$ in one extreme position (like −<), and $$2\times 1/3\cdot m(r\cos30°)^2 = 1/2\cdot mr^2$$ in the Y position. Compare this with variation from almost 0 to $$2\times 1/3\cdot mr^2$$ for a 2-bladed prop).

For a 2-bladed prop, this wobbling occurs at double the frequency of rotation. For wind turbines with their ~20 RPM this is < 1 Hz, a frequency that can interfere with normal yawing and even damage the structure. But aircraft props turn at much higher speed; a typical 2400 RPM will give you an 80 Hz hum. May be unpleasant, but far away from the natural frequencies of manoeuvring and of the airframe.

Also important to note that there are two effects at play here: precession (gyroscopic effect) exerts a force perpendicular to the applied rotation, but there is also simply a moment of inertia of the prop/rotor across its spin axis (as calculated above) that needs to be overcome. This latter one applies even if the rotor is stopped, but both wobble when it spins. Now, for airplanes with their fast spinning but relatively small props, the gyroscopic effect may be noticeable (see below), but just MOI is totally negligible (the mass of the blades is nothing compared to the rest). Yet for wind turbines the opposite must be true: the rotor mass/MOI is (probably) the dominant mass of the whole thing.

As for precession itself, i.e. the tendency to turn perpendicular to where you want to turn, most aircraft don't 'suffer' from it either. Aircraft simply don't turn (manoeuvre) too fast for it to be a problem, and the aerodynamic forces outweigh the gyroscopic ones by orders of magnitude. On normal propeller aircraft, only if you want to notice it, you might notice it when doing a manoeuvre.

In some circumstances, however, precession can be quite significant. This involves the cases when angular velocities are high (aerobatics), the aerodynamic forces are low or very finely balanced (hovercraft), or both (spin).

• I could probably word my question better, but you seem to have honed in on what I’m asking about, specifically the “wobble.” I think the precession effect others are referring to is a different, but related, phenomenon. – TomMcW Sep 25 '18 at 16:53
• So, the concern in the wobbling is when the frequency of the wobble is low enough to interact with other natural motions? Does the number of blades affect the frequency of the precession? – TomMcW Sep 25 '18 at 16:58
• @TomMcW, I'd say yes, although there may be higher-frequency modes (particularly aeroelastic ones) that can always cause problems. But in practice I'm not aware of any with regards to this phenomenon on airplanes. (Two-blade helicopters may be susceptible, worth checking). The wobbling occurs at a frequency with which the mass configuration repeats itself over the revolution, i.e. it grows proportionally to the blade number for a given RPM. But, in many cases greater number of blades means lower RPM. – Zeus Sep 26 '18 at 1:27
• @TomMcW, I updated the answer with MOI calculation to demonstrate how much better 3 blades are, and added the simple (non-gyroscopic) MOI effect that must be significant for wind turbines but not for airplanes. – Zeus Sep 26 '18 at 1:59

Yes, two blades propellers will produce precession effects during maneuvering, just like any other propeller will. It is barely noticeable for normal operations. It is quite noticeable during hard maneuvering or aerobatic flight and coordinated use of rudder and elevator pressure is necessary while maneuvering in these regimes to counteract this effect.

A one-bladed propeller introduces the least drag, but presents elementary balance issues. Two bladed propellers suffer from worse precession than three bladed propellers, and in a turbine that shows up with the blades are in a horizontal position and the turbine is pivoting / yawing due to a wind change. A three-bladed propeller balances some of that force and ends up being the best compromise tradeoff between So Many Blades and air resistance.

• Interesting article, but this post rather re-phrases the question than gives an answer. Yes, 3 blades are better for this phenomenon, so how do aircraft cope with 2? – Zeus Sep 24 '18 at 6:29

This question leads to a consideration of precession induced whirl-mode phenomena, irrespective of number of blades. The turbine mounting structure may be of sufficient strength, but insufficient stiffness. Many lives were lost in the early days of large turboprop airliners (Having 4 blade props) due to resonant, whirl-mode, structural failure. In the case of the Lockheed L-188 Electra, all it would take is an asymmetric gust to get it started. Due to reasons stated in previous answers, number of blades would not be of primary concern.

Yes, gyroscopic precession is a thing on two-bladed airplane propellers. I fly a C-172 and it is listed as one of the factors in the airplane's left turning tendencies.

• Gyroscopic precession is a thing of any rotating mass. The heavier the mass (the prop), and the faster it spins, the greater the force. The question is, apparently, about a different thing: the vibration of this force. Besides, precession is not part of the 'normal' left turning tendency (which is caused purely by aerodynamic forces); precession happens when there is a rotation other than about the spinning axis. For C172, it will yaw you left if you quickly pitch down, and yaw right if you pitch up. (And pitch you up if you push the left pedal, etc.) – Zeus Sep 24 '18 at 6:48

Precession is a gyroscopic effect, and –for the same angular speed– the forces involved depend on the moment of inertia of the propeller, that (for the same blade configuration and materials) is proportional to the number of blades.