A large part of the problem isn’t just that the blade gets damaged in an isolated way, it is that it causes severe balance issues on a rapidly rotating control surface. Helicopters are not inherently stable aircraft to begin with so it isn’t good to introduce more instability through damage. You can induce violent shaking in machines like lawnmowers with just a small amount of blade damage. It is much more serious on when the much longer and more fragile blades aren’t for cutting but for stable flight itself and you are sitting directly below them.
As a general rule of thumb, helicopter rotors can’t withstand hitting things much heavier than bugs.
Helicopter vs bag. The bag won.
Three pound bird vs Marine helicopter. Ended very badly for all involved.
I suspect that will be a fruitless search.
Back in my USAF days I participated in a search and rescue exercise wherein a USN SH-3 Sikorsky SH-3 Sea King - Wikipedia was supposed to pick me up from a jungle clearing. I was playing the role of the downed survivor and they were the rescue force.
The hilltop clearing wasn’t real big and the pilot inadvertently backed his tail rotor into the tipmost tops of some trees. He buzz-sawed through a bunch of twigs and a few roughly 1/2" branches.
And came within a smidgen of destroying his tail rotor and dropping a 10 ton helicopter on my head from 50 ft.
They aborted the mission immediately, landed nearby, and discovered the aircraft was unflyable until the tail rotor was replaced. The tail rotor, being much shorter than the main rotor, is relatively the stouter of the two rotors. Had he engaged the same treetops with his main rotor there’s a good chance none of you would ever have met me. Or had he backed any further into the trees before realizing his error and pulling forward and up.
Aaah yes, the good old days.
Bottom line for the OP: No helicopter can safely chop anything other than maybe a very small bird or bush. and even then only briefly.
To give some illustration of this, here is the rotorhead from a Aerospatiale SA321 ‘Super Frelon’ (five blade military transport and anti-sub helicopter). See all of the linkages, bearings, snap rings, hydraulic lines and hoses, et cetera? Most of those are single failure points, or at best, a couple of failures away from catastrophic loss of control, after which the rotor has a tendency to either become uncontrolled (by design) or spontaneously dismantle itself into component pieces or smaller. (Not all helicopter are this complex or poorly protected–here is the rotorhead on a UH-60 ‘Blackhawk’, where at least most of the delicate bits are protected and the mechanism itself is somewhat simplified, but there is still a lot going on there.) Most modern helicopters with more than two blades also use an elastomeric “flexbearing” which has a limited lifespan, requires almost completely tearing down the rotorhead to replace, and if worn or damaged can cause the rotorhead to basically eat itself in a delightful salad of flying metal lightly tossed with hot hydraulic fluid.
And this is just the rotorhead, which has to be driven by an engine, or in the case of most military helicopters, multiple engines, which are connected through a transmission that feeds into the powershaft. Never mind all of the controls, and unlike an aircraft you can’t just bleed the hydraulics and compensate or glide in; at best, at a moderate altitude you might be able to autorotate down into a “controlled crash”, which is sort of like running your car into an embankment to slow it down.
There is a reason that the English language has never produced the phrase, “Safe as a helicopter”; and this is the reason that the military special forces community, a helicopter is defined as “four million piece parts flying together in unstable aerodynamic formation, constantly shaking itself free from the the tyranny of being held together by bolts and nuts, and dedicated to rejoining the earth from which it came.” (Also, “A helicopter leaves the factory looking for a good place to crash.” More humor from rec. aviation.military here.)
That nonsense you see in Bond movies or that atrocious remake of The Italian Job remake with a helicopter blade chopping through a car or a building is not only patently absurd, it is actually impossible for a pilot to maintain stable control of the aircraft in that inclination or to that degree of precision.
God, I hate helicopters. Every time I have to ride in one I wonder what I did in a past life to deserve this.
Stranger
Helicopter blades are designed to resist tension, not compression.
I disagree. I’d much rather lose an engine in a helicopter than a fixed-wing. I love helicopters.
Much (most?) of a rotor blade’s rigidity is imparted by rotational forces.
Yeah, I deliberately picked a rotorhead that was especially complex. But even that Robinson(?) has a large number of parts that, should any fail, would render the helicopter not flightworthy and difficult to even land.
I’ve glided down in fixed wing aircraft to softer landings than some “controlled” landings of helicopters. I don’t believe in machines having some kind of independent soul or volition, but I make an exception for helicopters. A helicopter has a mind of its own, and it is the mind of a serial killer.
Stranger
Scientists have proven that helicopters cannot truly fly. They just beat the air and gravity into temporary submission through pure force with varying degrees of success.
This question has been addressed by very knowledgeable folks above. The rotor(s) do not like to cut through anything denser then air. The aircraft may be flyable, for a very short time & distance. Not only damage to the rotating parts, but the balance of these pieces is critical, as is the track of the blades in the rotor disk.
If the rotor blades strike almost anything besides small insects, at least one inspection is required, (by the manufacturer, & the FAA), to be preformed. The inspections usually required to the rotor assembly, drive shafts, and the transmission, after a bird strike are extensive and expensive. Often the engine(s) also need inspected. If any damage is found, then a deeper inspection is required. In general, both the rotor assembly and the transmission have to be disassembled and checked for damage before being reassembled.
Many expensive parts are required to be replaced regardless of any damage, or lack therein, observed. All steel parts will be Magnafluxed, and almost all of the rest will need to be replaced or will need dye penetrant inspected. Many will also need other forms of Non-Destructive Testing (NDT). This could be X-ray, temper etching, and ultrasonic inspection methods, to name a few. All of these will be done using certified, (read expensive), equipment.
This equipment is required to be re-certified on a regular basis. Yearly, biyearly, monthly, weekly, daily, and once a shift tests are required to be preformed and passed before the machine can be used to inspect the aircraft parts. This should give you an idea as to the time and $$ that a bird strike will cause the operator of the rotor-craft to endure.
BTW, They are called rotors on helicopters, and V-22 Ospreys, not propellers. Propellers are on airplanes, and boats, they also propel the Gyro-copters. A rotor provides the lift for a Gyro-copter. It has at least one of each.
IHTH, 48.
I say, somebody was a damn good shot.
I’m in the business, and report this with some authority.
Our vehicles get a mandatory blade inspection following any contact with a foreign object sparrow sized or larger. Blade inspections are also regularly scheduled maintenance procedure, and occur for each increment of 25 hours of flight.
Blades are fragile. Pulling down on the end of a blade with your hand is enough to damage one. The usual failure mode is the blade will delaminate, then come apart.
To address the OP, about the only thing a blade can hit is a June bug, and that is iffy.
I know that the outer 1 foot of an H-60 rotor blade is the tip cap, which is designed to shear off if it hits anything of real substance. This will allow the helicopter to get to somewhere to land safely, but that is about it.
Medevac Helicopter Accident: impact happens at 2:35. Main rotor blade severs overhead utility wire. Helo remains stable and performs a controlled touchdown in an adjacent clearing, but you can hear that at least one of the blades has been significantly deformed.
Sea Stallion chops its own refueling boom: During an attempt at midair refueling, the helo pilot gets a bit too aggressive with the collective, transitioning from strong up-collective to strong-down. The floppy rotor blades overshoot way below the neutral plane of rotation and sever the helo’s own refueling boom. The boom loses the fight, and the helo remains in stable flight for at least the remaining few seconds of video, but I’ll wager that at least one of the rotor blades needed to be replaced.
Years ago in grad school, the mechanical engineering department had a cross section of a helo rotor blade in display. The core was an aluminum honeycomb, and the skin was aluminum, perhaps 1/8" thick on the leading edge (not sure, this was 20+ years ago). IIRC, that display piece was from a Huey. Given that rotor tip speed is probably several hundred MPH, it wouldn’t take much of a ballistic mass to put a dent in that aluminum skin.
I would guess the robustness of a rotor blade also varies with the size of the vehicle. A Bell Jet Ranger has a MTOW of 3200 pounds; I would guess its rotor blades can’t cope with the same impacts as the blades of a CH-53 Sea Stallion, which has an MTO of 42,000 pounds.
I don’t have any knowledge on my own - but a previous reference to the Robert Mason book Chickenhawk (about a Huey pilot in Vietnam) reminded me of this passage:
Back during the first Gulf War, I read that Apache helicopter blades were supposed to be able to survive cutting through tree branches of a certain diameter. I don’t recall what the diameter was, but being in the testing business myself I’m sure it was probably either 1 inch or 1/2 inch.
This quote is from the website of pilot who is part-owner of a flight school that has a Robinson R-44.
Re: Robinsons
To check the tops of the blades, don’t pull down. (Someone mentioned that upthread.) Gently turn the blades so that they align with the longitudinal axis of the fuselage. When checking the tail rotor gearbox, push down on the tail boom so that you can see the tops of the blades. Turn the main rotor blades 180º by gently rotating the tail rotor. While you’re doing this, keep in mind that you’re dealing with a fairly fragile machine.
Re: Stranger’s post on rotor heads
Robinsons have a semi-rigid, aka ‘teetering’, rotor system. I always laughed during the preflight when I inspected the rotor head. I’d look at the teeter hinge and think, ‘That one bolt holds the rotors on. Hm… Best not to think about it.’ 
I always liked flying in helicopters. Of course, the last time I did so was damn-near 50 years ago. They used to run a commuter service from the Orange Show grounds in San Bernardino into LAX flying Sikorsky S-61Ls.
Wouldn’t this make it really, really easy to shoot down a military helicopter, if all you need to do is get something as big and tough as a cloth bag into the path of the wing?