The "Scrap Fan" (known as "SCREWA" in the game's files) is an unused piece of machinery found in Metal Slug 7.
The only sprites found in the game's tiles are this spinning animation, it dropping something from the back, the object being destroyed, and a can, which makes it hard to guess what exactly this is.
Presumably, it was supposed to be part of a larger vehicle. One theory is that the Scrap Fan could be part of an underwater vehicle like a submarine, since there's an unused section of Mission 5 with underwater physics (similar to Metal Slug 3 and Metal Slug 5) but with the heroes on foot rather than in scuba gear and a bubble sprite. Two sound effects specifically labeled "Screw" for Mission 5 (St5_Screw_spin and St5_Screw_vacuum) are also present in the files.
Regardless, if the heroes get caught in it, they'll pop out from the other side inside of a bouncing can (seemingly ground apart by the propeller blades). This costs a life although no death yells are heard.
- The "Scrap Fan" spinning.
- The "Scrap Fan" producing the can.
- Gussprint. "Screw Sub". The Spriters Resource. https://www.spriters-resource.com/ds_dsi/metalslug7/sheet/36431/
- Mr. Beacon00. "Metal Slug 7 under water stage?". YouTube. https://www.youtube.com/watch?v=q6GrGuMgVt4
- "Metal Slug 7 Unused Graphics". The Random Hoo Haas Hub: Metal Slug: Missing in Action. http://randomhoohaas.flyingomelette.com/msmia/7/ug.html
- Mr. Beacon00. "Metal Slug 7 Unused Object Scrap Fan". YouTube. https://www.youtube.com/watch?v=OouLD5Zeg1c