If they were used, how did they solve problems such as air pressure removing the flame?
I hope this is not against the rules, but allow me to simply copy my own answer to an identical question posted before (for those too lazy to click a link)
they were quite uncommon, due to low effectiveness. The main problem with them was that the speed of the arrow would extingush the flame, unless tho bow/crossbow was intentionally strung lose, in which case it would be useless for actual combat. Among the known examples of use of such arrows is the Siege of Oran (1404) where the attackers shot incendiary crossbow bolts (and presumably arrows and other similar devices) to set fire to the port. While I know of no archeological finds of such bolts from Oran, there is at least one from 14 century Russia. Flaming spears and javelins, on the other hand, were much more probable, either thrown by hand or from a siege enngine. Normally howerver, if a siege weapon was available at all, it could be used to shot barrels of burning tar, or baskets with oil-soaked rags, which would be more effective. source:
K.Nossov, Ancient and Medieval Siege Weapons
D. Nicolle, Medieval Warfare Source Book: Christian Europe and its Neighbours