ORIGINAL: LNEWQBAN
ORIGINAL: Switch_639
i am running an os91fx 2stroke with 12x8 prop and ultra thrust rossi muffler... on the ground the engine is fine, revs 12900...
... I have hit 128 with the engine before by using a 13x8... wanted faster, thought the 14x8 would do it... but the engine doesn't like the coastal air it seems... so tried the 12x8... think its too little by what people have described... so I guess the 13x8 it is... so 128 is almost max it will reach... don't have cowling on yet, so it could help when I install it finally... but will need to check those bearings for sure...
12,900 rpm on the ground.
Asumming 10% increase in the air = 14,200 rpm = 236 rev/s
128 mph = 188 ft/s
13x8 prop can advance a max of 236 rev/s X 8 inch = 1,893 inch/s = 157 ft/s
128 mph is achieved helped by gravity, and the model is moving faster than the propeller pitch would do at blades' AOA = 0 (zero thrust).
Unless rpm's reach 188 ft/s X 12 / 8 = 282 rev/s = 16,920 rpm!!!
where do you get all these theoretical calculations
in theory yes it should get those rpm... don't think it does... I measured with GPS in plane... was about a 30 degree declined dive downwind... this was still when I had the BCM with 13x8 prop on and no issues... this engine was running in another airo... took it out and needed to fit to revolver... BCM didn't fit unless I cut out the whole bottom end of cowling...