A common substitute for cream of tartar in angel food cake recipes is lemon juice. This is because the acidity level in lemon juice is pretty similar to that of cream of tartar. This is good because it helps keep eggs whites fluffy after rising in the oven, benefitting the overall texture of the cake.
You don’t have to worry about the substitution ratio when swapping lemon juice for cream of tartar, so you can follow the recipe exactly without having to stop to figure out conversion rates.
Lemon juice is also great for preventing sugar crystallization which is particularly useful when making cake frostings.
Alternatively, you could remove cream of tartar from the recipe altogether.