"Flap" sounds like something (a flag, a sail) flapping, and "flop" is the dull sound of something falling down flat.

而flop听着就象什么东西直直落地发出的沉闷一响.

youdao

Look, whoever is making that flapping sound, quiet down.

谁在弄出那翅膀拍打的声音？静下来！

youdao

Then he ordered a man to be the first musician, lying on the ground and flapping its tail on its stomach to make sound.

他又令一人率先做乐工，它就躺在地上，用尾巴敲打自己的肚子，发出声音。

youdao

Along with the flapping, the robotic bird included a small sound generating system to play authentic calls and swamp noises.

ENGADGET: Alt-week 3.2.13: A mission to Mars, robosparrow and facial recognition in fertility treatment

While the project might sound a little bit gruesome, the research helped gather data to support the hypothesis that wing flapping is often used to deter rivals.

ENGADGET: Alt-week 3.2.13: A mission to Mars, robosparrow and facial recognition in fertility treatment