Is a broadhead arrow fired from a bow or crossbow a significant threat to the safety of a civilian helicopter?

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.