It’s a common myth that helos like mi-24D and apache, which have 1 frontal armor, are more resistant to SPAAG fire. This is not true.
1 armor has two effects over 0 armor. It reduces small arms damage to 10% (against 0.75 HE machineguns like the browning or NSVT, the damage reduction is only around 50%), and removes the doubling of AP damage that 0 armor vehicles experience. Since AP damage is not relevant against helos and planes, the only effect of 1 armor is the small arms one.
So, your 1 armor Mi-24D will take almost no damage from infantry fire to its front. An unarmored lynx on the other hand will die quite quickly to these weapons and generally cannot “duel” infantry with its own machine gun. This gives us the intuition that the hind is overall more survivable against everything, but this is not true; hind and lynx die equally quickly to SPAAGs or AA missiles (If we ignore their difference in total HP, of course).
Even against infantry, 1 armor is not invulnerable. Although it helps massively by making sure the helo’s own gun can outdamage individual infantry squads, there’s still a weak spot. Numerous infantry squads focus firing a helicopter can stun it with their suppression output, causing it to spin and expose its vulnerable rear side, and then killing it. Armor also does not protect from crits.
As an aside, this is also why APC CVs aren’t really worth it if comparably mobile jeeps are available. You generally pay 20-30 more and lose a lot of availability for five more health points and 1 armor on all sides. The 1 armor has no effect on the practical threats to a CV, so the only gain is in 5hp. Even this health gain is marginal and mostly only useful for surviving the first shot from low-HE artillery fire or an autocannon vehicle – both of which will just keep firing until the still practically unarmored target is dead.
What does 1 armor do on planes? Nothing. 1 armor decreases small arms and AP damage; neither can target planes. It’s only added on planes with 2 armor sections, and my guess is its there for cosmetic purposes; weird to have 2 armor at the front but 0 everywhere else.
What does 2 armor do on planes? It decreases HE damage to 40%. 5 HE missiles on ASF will do 2 damage instead of 5; 9dmg heavy SAMs like Patriot will need three instead of two hits to down the plane. This is a significant boost; many ASF are likely to run out of missiles before scoring the kill on a 2-armor plane. Usually these planes are most vulnerable during evac when their 1 armor sections are exposed.