hmm
I just tried to imagin what I (being commander of an lonbow unit) would do when I had to defend a city which is been attack by tanks...
I would look for some primitive but feasable workarounds, e.g.:
1) Buring holes and covering the trap (like stoneage men done hunting) when the tank falls into the hole I would burn a smoky fire to force the units to leave their tank and then still disorientated they could be taken out with my bows
2) I would use moltow cocktails (assuming that a commando of my longbow archers woud definitly be able to steal some fuel from the attacking army even though I don't have the technology for fuel yet)
3) I would try to steal some explosives and use them against the attackers
So for sure being realistic I would loose way more men then them but still I think IT IS realistic that longbow men could take out a couple of tanks, and sometimes even the whole attackers
Against choppers my commander would need a bit more wit but maybe the following could work:
1) Duck & Cover ;-) and maybe just by luck when a chopper coming down to low looking for me somen could bring up something like a "rope-trap" eg throwing a rope tied to a building into the rotors...
2) burning lots of fires to take out the sensors (smoke against optic, heat against infrared)
So in general if you assume that primitive units do adapt their tactics despite their primitive weapons a lucky strike is very realistic and could happen.
In real life, for example:
-A terrorist fisherboat nearly took out a modern amercian destroyer about 5 years ago or so (USS Cole (sp?)
- More or less daily criminals only armed with knifes take out policemen armed with mordern pistols or even machine guns
- When I was still serving in the German army as a commander of a "modern armor" we did had real losses (total loss of equipment, and even serious injuries) just by accidents - it was just a training mission without any real ammunition, but in unknown rough terrain these things happen quite often - and I think a good tactial commander could force these things to happen even though he only has a shouvel and some primitive bows
For example (happened to mee) you can make a modern tank useless by just chopping trees (if the barrier is big enough and hidden from infrared and optical sensors, the tank could be totally stuck.... and don't ask how my eyes looked like for the next 2 weeks because I was still on the optics when we hit the barrier with 30 miles/h

)