I’m sure this is a question of mere physics which I am too lazy
to muddle through. In the movies, people shoot at helicopters all the time and sometimes make them crash. Would a bullet be likely to hit a moving rotor or is the bullet travelling much faster than the rotor is rotating?
Would the bullet likely pass through between revolutions untouched?
This is keeping me awake at night…please help.
The rotors would seem nearly stationary to a passing bullet. The odds of hitting the rotor would be pretty close to the percentage of rotor arc area that they actually cover. (Did that make sense?)
In other words: actual rotor area/area of whole rotor disk.
The fraction would be pretty close to the odds of hitting the rotor if you shot though the disk.
But in most movies, they don’t shoot the rotor blades out - they shoot holes in the hydraulic system which rapidly loses pressure, everything siezes up, and then the helicopter crashes
Russell
scotth
it would be that, provided the total rotor area was the total for all four blades
I’m not sure the blades would really be nearly stationary to a passing bullet. Let’s assume the bullet is almost supersonic - about 300m/s
If the rotor blades are, say, 5 metres long, the circumference swept out by the tips will be about 30metres. As soon as they are rotating at more than about 600 rpm, the tip speed will match the bullet. Hardly stationary.
Russell
Even on helicopters that don’t have any hydraulic system. That is about the way it goes though. I guess it is more dramatic. Hydraulics can “plausibly” be used as a device that if it is damaged the craft is still under control for a period until all the fluid is going which puts the pilot in a race against time to land or do something even more spectactular with the craft.
It would be more interesting (to me) if they would occasionally blow the engine out and demonstrate the ability of a helicoptor to land without power. They probably don’t use it, because most of the writers are unaware that it can be done, AND if they do, they figure the public wouldn’t believe it in a script.
Pitiful.
Military helicopters have rotor blades that can withstand bullet penetration. I don’t know about an R-22 or a 300CB, as they were not designed to go into combat situations. Those two don’t have hydraulic systems, BTW; but JetRangers and up do.
A good way to bring a helicopter down is to damage its tail rotor. In Somalia the bad guys fired RPGs that exploded near the tail rotors of the Black Hawks. Hit the engine, and the pilot can still fly away. He’ll destroy the engine to save the ship (and its occupants). Make the tail rotor inoperable and the helicopter is coming down right now. This is because the tail rotor counteracts torque. If it’s in-op, the pilot must chop the power or the heli will start spinning around. (You can experiment with adding a small amount of power to extend your glide, but you’re still going to come down quickly.)
I was assuming a rifle bullet that would be well over supersonic. Also, your numbers are pretty far off on rotor tip speed. 200m/s would be a very high tip speed.
Very small personal helicopters maintain the highest rotor RPM (due to their short blade length) and the highest I can find is 450 RPM. That is on a heli with rotor diameter of only 6.7 meters. The maximum tip speed on this bird would be 157 m/s.
A couple of years ago, I was involved in a test at China Lake to determine what would happen if various types of foreign ammunition hit a helo rotor while it was in flight. To be able to hit a rotor blade with anything resembling a degree of success required using lasers being reflected off of small refelectors on the blades to a timing circuit set to trigger the gun after factoring in blade speed, area of the blade targeted, cartridge action time, projectile flight time based on velocity, etc.
Years and years ago, static tests had been done and showed that the rotor would survive. While in flight however, things changed. The “surviveable” blades basically disintegrated.
An article about the test can be found here.
If we’re assuming that the round is fired perpendicular to the area swept out by the rotors, what’s the relevance of the velocity of the round?
If the round is moving slowly enough, the area that was swept by the rotor during the time it took for the round to cross the thickness of the rotor arc would become large enough to significantly alter the results.
i.e. It would significantly increase the odds of the rotor striking the bullet with its leading edge.
All helichoppers (military and civilian) have two especially vulnerable areas. The rear rotors (yes a few chopper ain’t got 'em), and the transmission.
If the transmission (or it’s pressure) is lost the blades will lock in postion and that violates the manufacturer’s warantee.
Most helicoters that go down lose one of these components or suffer a control hit (less likely in a modern bird) or in the body of either of the two pilots.
Smoke in the cabin is also very bad in a helicopter.
scotth, you’re right of course, I’d neglected to consider that the round has length too.
D’oh!
It’s one of those technological ironies whereby you could probably do more damage just by throwing a stupid rock into the blades.
We had to replace a main rotor on a Bell Long Ranger several years back when we hit a pine cone trying to squeeze into a very tight LZ. Yep, a pine cone.
The rotor didn’t shatter but it did damage it to the point of producing unsavory vibration.
So how good of a shot was Robert DeNiro’s in Midnight Run when he shot out the tail rotor of a hovering helicopter…while standing a few hundred feet away…with his 9mm? 
Correct. By firing into the rotor at the same level, you stand a fairly good chance of hitting a blade somewhere. But if you are trying to hit a specific area on a blade that is the size of a half dollar, you need a little more precision. Also, in that particular test, we were shooting up into the blade at about a 30 degree angle, basically to shoot into a blade as it passed over. The idea was to duplicate ground fire on a flying helo.
In the tenth of a second it takes for a blade to make 1/2 revolution, the projectile can travel about 270 feet. A variance of even 10 fps in bullet speed is a foot of distance. That whittles away the margin of error to near zero.
Of course the blades’ speed is lower near the hub.
I think the ratio of main to tail revs is somewhere around 1 to 8 or so. You’d stand a much better chance of hitting a blade from the perpendicular if you go for the tail. That sucker’s moving.
Of course, with the much shorter blade length, the tip speed probably isn’t much if any higher than on the main rotor.
But if it’s going around 8 times as often, good luck getting a bullet through w/o hitting a blade.
The odds of getting through should be about the same.
Figure the odds by looking at the ratio of actual rotor area to the area of the rotor arc.