I remember as a child in the 50-60s, that two pennies formed a suitable tip weight for slab wing planes with Cox Bee engines. But when I started again working with kids since the mid 1990s, that seemed marginal. I changed to using 3 pennies, and found that ok. I only just today found the reason: before 1982, two pennies total weight was 6.22 grams. Since 1982, two pennies weighs 5.0 grams!