Does this situation change with 3 or 4 bladed props? (Why is it so damned hard to find 3-bladed hubs?

From : Don Stackhouse

Steve, the most of the rules remain the same, only the number of players changes. For example, if a 2 bladed prop causes a 2-per-rev vibration, a 3 bladed prop (or as folks in the propeller industry call it, a "3-way prop") will cause a 3-per-rev, and a 4-way will cause a 4-per-rev. The forces on the individual blades still behave the same. For most aerodynamic purposes, each blade really doesn't realize that there are other blades in it's neighborhood, it just reacts to the local flowfield around it. Generally speaking, the wake of one blade doesn't interfere with another blade directly in normal operation, with the possible exception of extreme reverse thrust situations during "Beta mode" on landing rollout. As long as the air flow through the prop is positive, the helical wakes of the individual blades all nest neatly between each other, like a screw with multiple threads. The blades react to the changes in the overall inflow, but are relatively oblivious to the local flowfields of the other blades.

Mechanical vibrations are a different story. Since the blades are mechanically coupled to each other by the hub, their vibrational behaviors interact with each other in many complex ways, and more blades make the interactions more complex. 2-ways and 3-ways aren't too bad, but starting with 4-ways things can rapidly get ugly. The primary bad actor in this case is something called "reactionless modes".

On most normal types of vibration, some of the forces are fed into the propshaft and back into the airframe. This removes some of the vibrational energy from the prop's vibrating system, which helps limit the amplitude of those vibrations. It also makes the aircraft shake, which will warn the operators that the prop is vibrating, hopefully resulting in some corrective actions. Now imagine a 4-way prop, with two opposite blades flapping one way while the other two are flapping the other direction. The vibrational forces cancel each other's loads on the hub, so there is no net "reaction" force transmitted from the hub to the airframe; the vibrations are therefore "reactionless". This means that the blades could be busy beating each other to death, and nobody in the airframe would know anything about it, right up until the point that one of those blades yells "ENOUGH!", and quits the group to become a solo act. At that point everyone on and around the airframe will know about it!

The normal procedure when designing the vibrational characteristics of a prop is to try to make the blades stiff enough to force all the natural frequencies up above the max operating rpm range of the prop. The problem with reactionless modes is that they typically occur at frequencies far below the others. When you stiffen the blades to drive the other frequencies above the operating rpm range of the prop, it often pulls those low-frequency reactionless modes smack into the middle of the operating range. Keeping the other frequencies high enough while still keeping the reactionless frequencies below the operating rpm range can become a very delicate balancing act!

4-way props typically have one reactionless mode frequency, 5-ways have two, 6-ways have 3, and so on. The more blades, particularly with 4 or more, the more difficult the vibrational design of the prop.

Although the mechanical vibration characteristics of ducted fans with large numbers of closely-spaced blades follow pretty much the same rules, the aerodynamics are a bit different. The blades are so close to each other that their individual wakes begin to interact and significantly change the nature of their local flowfields. The resulting behavior is covered by what's called "Cascade Theory". In addition, the duct around the outside tends to straighten the inflow, eliminating some of the airspeed and angle of attack variations that cause vibration in props without ducts. OTOH, if a portion of the duct lip stalls at high inflow angles, the resulting blade stresses can be extreme. Likewise, just as with pusher props, any structures in front of the blades (such as fuselages, pylons, wing and tail panels for pushers, supporting pylons and convoluted ducts for ducted fans) can distort the inflow and cause severe vibratory stresses. Any time the blade sees a significant cyclic change in its flowfield as it rotates, you have the potential for vibration and efficiency loss.

As you can see, there are all sorts of ways to get yourself in trouble with prop design! Frankly, I'm amazed that we in the modeling world don't have more trouble with them, especially considering that almost none of them are vibrationally tailored for their particular applications, and how little regard many modelers have for such fundamental things as inspecting for nicks in blades. I think the main reasons for our apparent luck with them is due to some very high structural safety margins typically used in blade designs, the limited number of props with 4 or more blades, and because of the predominance of wooden props. Wood has some truly extraordinary fatigue-resistance properties, which allows it to forgive many of our vibration-related "sins of neglect". Other materials such as nylon are not nearly as tolerant, which is the main reason for the higher failure rates in those types of props.

Don Stackhouse
DJ Aerotech