How far would a helicopter blade go if one broke off?

Posted 12 May 2017. 

This is an interesting question and one for which I have to go back to “instructor mode” and resort to my experience and knowledge of the UH-60.

The main rotor of the UH-60 rotates at 258 RPM, and each blade weighs about 220 lbs. Typically, when we did main rotor track and balances we would not exceed 5 pounds of balance weights per main rotor spindle (I am digging way back, here… so bear with me). This being said, the slightest change – even losing less than a half-pound in the main rotor tip cap – is quite noticeable. Therefore, to answer your question, more information is needed as to where on the blade and under what circumstances the separation occurred.

Over the years, I have heard some very odd and verified stories about things like this happening. My only personal experience with any kind of main rotor blade issue was when we lopped off the top of the intermediate gearbox cover and some of the driveshaft cover during a poorly-executed roll-on landing “around Barstow, on the edge of the desert” and had too much aft cyclic. Two blades made contact, and while we didn’t lose any of the blades, the vibration was akin to sitting in an out-of-balance wash machine.

Along with that, a very good friend was assigned to Ft. Rucker as an instructor pilot in the early 2000s when the Blackhawk he was in clipped an unlit pole. I have heard a couple versions of the same story, but they all follow the same basic pattern: on approach to landing, 40 feet from the ground, and they lost 80% of one of the blades. He managed to get it safely on the ground with little injury to the crew and repairable damage to the helicopter.

The UH-60 is a very resilient bird. For others, I would venture to say that the prospect of losing one entire blade would result in pretty much the same result: gravity and rotational forces become the deciding factors. To determine the distance travelled, sans blade, would depend on altitude and severity of the loss, but it would not be pretty.

[Edit]

Serves me right for not allowing the caffeine to fully root itself into my brain.

To answer the question of how far the blade would go would to have to get more information. In the example above, I was picking up pieces of the driveshaft/gearbox covers about 300 ft away, and these being light and about as aerodynamic as Wile E. Coyote, their dispersal was less than what parts of the blade (had we lost any) would have experienced.

Again, this goes back to a couple of necessary pieces of information:

What would be the altitude and attitude ( both roll and pitch)?

What would be the main rotor RPM?

I tend to run from math – after all, I think avoiding math classes was what brought me to Quora in the first place – so I would leave those answers up to those more capable and detail oriented to answer.

In my brief research, most of the information found involves in-flight failures, with the closest to the original concept of on the ground being at an altitude of 10 feet at slow speed. In this case, the blades were 100 and 150 feet from the point of impact. Again, the overall design of helicopters offers as much controllability and survivability, as in the case of this Blackhawk, but the manner in which the failure occurs plays a big role in the distance traveled of the pieces.

Going back to personal experience, I can honestly say that tools and maintenance equipment left on the rotor head, can and will travel upwards of about 100 feet. In these cases, no one was injured and no damage was sustained to any adjacent aircraft, but the point is still valid – rotational forces are complex (and dangerous when disregarded) and beyond my level of ability.