I have read that propeller cuffs were used on a number of radial- engined airplanes to force more cooling air into the engine. The P-47, P-61, B-29 and C-46 with Curtiss Electric propellers come to mind. I have also seen them on Merlin-Powered Mustangs, except the XP-51G and the P-51K. None of the Mustang references that I have seen makes any mention of the purpose served by the cuffs on the Mustang. Does anyone know?
I believe that the main reason for having cuffs on the prop blades is that the inner part of the prop moves more slowly than the outer parts and therefore produces less thrust. The cuffs modify the angle of incidence of these areas to produce extra thrust. The fact that they force more cooling air into a radial engine is a secondary benefit. There may have been a similar benefit on the P-51. The cuffs would have increased the speed of the boundary layer along the nose, thereby improving the airflow into the carburettor scoop.
The shape of the blade near the hub is, due to the manufacturing process, optimized for strength and ability to connect with the hub. The cuff is a way to provide a better aerodynamic shape and still maintain the structural integrity of the blade. The result would be a bit more thrust.
Since the inner portion of the blade produces far less thrust anyway, I wonder if it is a bit like present-day winglets- very hard to measure the difference very well.
Sort of like the laminar flow airfoil on the -51. My understanding was that it was great when pristine and very clean, but efficiency dropped off when dirt, bugs, etc. got on it. Fortunately, the airfoil was pretty good even when not very laminar. My understanding is that it was quite qood at higher Mach numbers.
...a bit of all of the above, actually. The twist in the blade accounts for the differential in loading along the span, i.e.; variation in dynamic pressure and thus the need to increase angle of attack closer to the hub where the incident velocity is lower.
But the as stated, the potion nearer the hub also needs to be structurally adequate, so the airfoil is distorted in both camber and thickness nearer to the hub. Adding the cuff restores the airfoil thus aiding the propeller efficiency, but it also has to be done in a manner which maintains/restores the most optimum disk loading for that set of blades - it doesn't really produce "more thrust" per se, but it transmits more of the design theoretical thrust/applied horsepower...which also helps reduce fuel required.
If one recalls, P-51s also have/had cuffed props, but my guess would be that they were removed in the field to both reduce weight and maintenance...like removing the tail hooks from Marine Corsairs once based ashore.
...also - the flow behind the prop is turbulent, so any attachment of a boundry layer would occur some function of radii downstream, if at all...if you'd ever like to experience that first hand, step out of a DC-3 sometime...
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