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Let us suppose that we have a civilian helicopter, such as an MD900, Bell 407, Robinson R44, Bell 47 or another common civilian design, hovering or flying slowly just above treetop level (50-70m).

If a person on the ground nearby was to shoot at the aircraft with a broadhead arrow from a bow or a crossbow suitable for hunting big game, would the arrow pose a significant threat to the aircraft or its occupants? Would it be possible to hit the pilot through the canopy, or cause a failure that would lead to a forced landing or a crash?

If it was known beforehand that the aircraft would be used to pursue a fugitive armed with a bow or crossbow, might the pilot reasonably refuse or fly in a different manner than if they were performing a mission such as search and rescue of an unarmed person?

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    $\begingroup$ This is possibly a question where there needs to be thought on 'do we want to answer how to shoot down aircraft on the site'. For what it's worth some napkin math suggests the arrow will have lost just enough kinetic energy to combine 'will get you attempted murder charge' and 'not actually do anything'. $\endgroup$ Jan 23, 2023 at 8:53
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    $\begingroup$ There was at least one incident in the Vietnam conflict where an American helicopter was brought down by a bolt from a large, powerful crossbow fired from the ground. $\endgroup$
    – Zeiss Ikon
    Jan 23, 2023 at 12:15
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    $\begingroup$ @ZeissIkon - "A large, bamboo crossbow device used by Viet Cong forces during the Vietnam War, 29th November 1965. Armed with an eight-foot long spear, the device was positioned near a jungle clearing, and was intended for use against low-flying or landing helicopters. (Photo by Keystone/Hulton Archive/Getty Images)" image here $\endgroup$ Jan 23, 2023 at 16:24
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    $\begingroup$ @MichaelHarvey Yeah, I'd call that more of a ballista than a crossbow $\endgroup$ Jan 23, 2023 at 22:33
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    $\begingroup$ @GrimmTheOpiner. Or the older ones, Rambo 2 and 3 $\endgroup$ Jan 25, 2023 at 16:08

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Yes it could pose some risk to the pilot and machine. As this video shows, arrows and bolts can easily penetrate acrylic plastic and thin metal skins if the range is close enough.

So an arrow or bolt could penetrate the side window/door skin of a light civilian helicopter and cause, at minimum, a threat of injury to the pilot if fired from, say, less than 100 ft away. They often fly with the side doors removed, and in that case the pilot is as vulnerable as someone sitting in the open on the ground if the shooter is accurate (and lucky, as the rotor wash would affect the flight path somewhat and throw off the shooter's aim).

Less likely but still possible, an arrow/bolt fired from close range could penetrate a fuel cell/tank that was integrated with the outer skin and cause a leak, although this wouldn't bring a machine down right away.

Or an arrow/bolt could penetrate the cowl around the engine, and by chance cut a fuel line and bring the machine down, so that's a possibility if not a very high one.

As far as the rotating machinery goes, the item that would be vulnerable to an arrow or bolt would be the tail rotor blades, so a hit on a tail rotor blade could likely have an effect similar to throwing a rock through it, and cause enough deformation damage to a blade to set up a vibration. A pilot sensing a significant vibration through the anti-torque pedals would immediately fly away and land in a safe spot nearby to inspect the tail rotor.

As as far as the main rotor head and blades go, the effect would be mostly scratched paint.

If it was me flying the machine, and I knew there was somebody with a high powered crossbow or compound bow right below me under a tree canopy, I would certainly be a bit nervous about an arrow or bolt coming through the side door and sticking me, or though the chin bubble and getting me in the legs.

If I was the shooter, I'd be aiming for the pilot for sure, and probably adjust my aim upward to account for the rotor downwash. A hit would be mostly luck though although it would certainly motivate the pilot to leave the scene at minimum if the tip of an arrow was stuck in his/her arm.

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    $\begingroup$ And further to John’s excellent points, distance is your friend. (a) That arrow is bleeding off a ton of speed and is losing penetration energy at the square of that reducing speed. (b) the linear width of the pilot is decreasing in a linear relationship with altitude, and his/her area is decreasing with the square of altitude. (c) In law enforcement, most departments would not allow the pilot unlimited discretion to pursue a suspect at low enough altitudes for it to be a problem. It is neither safe nor tactically advantageous. You are in the coffin corner for most helicopters. $\endgroup$
    – Max R
    Jan 23, 2023 at 21:29
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    $\begingroup$ Would it be possible to hit the pilot through the canopy, or cause a failure that would lead to a forced landing or a crash? The OP states the helicopter is a minimum of 50m (164 feet) in the air. That's probably 4-6 times the distance shown in the video. No rotorwash, hitting a stationary target at right angles vs. a moving target vs. the curves of the helicopter's shell. This is an interesting video, but I don't believe it establishes the credibility of your answer. $\endgroup$
    – JBH
    Jan 24, 2023 at 0:33
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    $\begingroup$ The effective range of a high powered comp bow or x bow is about 40 yds and if a machine is passing banked over enough to provide a reasonably perpendicular surface, it will still easily penetrate the 3/32" or 1/8" acrylic you typically find on side door windows with the right head. $\endgroup$
    – John K
    Jan 24, 2023 at 3:27
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    $\begingroup$ @MaxR: If we factor out flight time: Energy loss with is linearly proportional to height (in a vacuum; with air resistance there are extra loses). An arrow going upwards is trading kinetic energy (1/2 m v^2) for gravitational potential energy (m * height) at a 1:1 ratio because energy is conserved. (Again, not counting air resistance: average_drag_force * flight_distance = energy lost to drag, where drag force is a function of speed, from the definition of work = force dot distance.) $\endgroup$ Jan 24, 2023 at 5:19
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    $\begingroup$ @John K, Two points. I know bow archers that get consistent hits at 70 yds. But also, an arrow fired vertically, just as a bullet, will have significantly greater effective range than horizontal. Shorter absolute range, but greater effective range which is a function of circular error probability. The largest sources of error in long range trajectories are range range error and velocity deviation because in an arced trajectory these both equate directly to vertical error. As a trajectory approaches vertical, the range/velocity mis-estimation error approaches zero. $\endgroup$
    – Max R
    Jan 24, 2023 at 6:16
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There are several factors to consider, for instance the type of bow, type and weight of the arrow, etc. A hunting arrow from a strong compound bow has an exit velocity of about 90m/s (this is on the high end but possible for a strong and practiced archer). Gravity's acceleration is 9.8m/s2, and using a projectile height calculator that arrow shot straight up will go up to 413m before stopping. That's over-simplified as it does not take into account air resistance (which is very significant), but it still shows that at tree-top level, call it 50ft or 15m, the arrow will still be going pretty fast. I'm going to call it 35m range though, as it's unlikely someone would be shooting straight up, and at that range an arrow with that much power will go into the trunk of a tree.

As for accuracy, a good archer in decent conditions at 35m range would be able to group all arrows within the size of a small plate.

So from an energy perspective as long as the helicopter is not too high up or far away an arrow could theoretically cause damage, and a good archer could theoretically hit it. Aircraft are built light, much of the skin is just thick enough to provide protection from the elements, not armored in any way. That doesn't mean a shot is going to penetrate though, unless the arrow is relatively perpendicular to the surface the arrow will just glance off. If it does penetrate, or the door of the helicopter is open, the arrow could injure or kill someone inside. As for damaging something mechanical that would bring the helicopter down that's pretty unlikely.

The biggest problem I would see with this is the turbulence and power from the rotor's downwash. If I remember correctly the speed of a rotor downwash is the equivalent to a 30mph wind, and is a roiling mass of air, so it's going to disturb the arrow's path in ways hard to predict. You aren't going to hit what you're aiming at, you'd have to make a lot of adjustments and hope.

So, it's possible, that a good archer with a powerful bow and the right arrows to hit a hovering helicopter at treetop level, but unlikely that it would hurt anyone or cause major damage. If I was going out to look for a hostile with bow it would still be prudent to stay a bit higher and not hover for too long.

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    $\begingroup$ But how long will the arrow spend in that downwash? Sure it will play havoc with the flight path, but it won't have long to act on the arrow. $\endgroup$ Jan 24, 2023 at 3:50
  • $\begingroup$ The disturbed air starts before you get to the rotor, so probably half a second to a second. It doesn't sound like much but it will be enough to throw off your aim. $\endgroup$
    – GdD
    Jan 24, 2023 at 8:43
  • $\begingroup$ The downwash will certainly take away your bullseye but that doesn't ensure a miss. $\endgroup$ Jan 25, 2023 at 2:40
  • $\begingroup$ I'm not saying you can't hit the chopper @LorenPechtel, you probably can. As you say, it will take away a bullseye, but if you intend to hit someone inside that's what you need to hit. $\endgroup$
    – GdD
    Jan 25, 2023 at 8:49
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Many years ago I found a sniper's handbook at a bookstore (I don't remember the title). Admittedly it was for rifles, but it had a section on targeting vehicles: The tail rotor hub is vulnerable, even on most military helicopters with armoured cockpits and protected avionics/drivetrains.

Engine intakes will always be vulnerable to FOD (foreign object damage), particularly the civilian models which don't have covers over the engines (the MD902 in your first link only has a mesh screen over the intake). Getting a metal (or even stone) arrowhead into the engine wouldn't be very good for it and could potentially cause a loss of power resulting in a forced landing.

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    $\begingroup$ In turbofans, enormous centrifugal forces naturally push air contaminants away from the center. Thus anything like sand or even rain water end up going through the outer part of the fan bypass and can't get into the compressor core. Turboshaft engines (turboprops and helicopter turbines have no bypass, so they almost always use a centrifugal or inertial separator ahead of the compressor. The heavier the object that makes it into the intake, the more effective the separator is. It often looks like the engine is right behind the intake but actually follows a convoluted path to the engines. $\endgroup$
    – Max R
    Jan 25, 2023 at 6:45

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