britbrat

Sport_Pilot, Thrust isn't speed. You are confusing thrust with delta-V -- they aren't the same. Thrust does not determine speed, discharge velocity (delta-V) does that --- regardless of how fast you are attempting to fly, or how high.

For example, the turbo-fan engines on commercial jets make much more thrust than the turbo-jets & low-bypass turbo-fans in fighters, but their discharge velocity is much lower, & hence so is top speed. Even if you put a 50,000 lb thrust turbo fan on a small airframe it wouldn't go particularly fast. However, if you put a an afterburning turbo-jet, or afterburning turbo-fan, with exactly the same 50,000 lbs total thrust, the airframe would go much faster. Afterburners are used to increase both thrust & delta-V in jet engines. An afterburner does two things -- it increases total thrust (power), to be sure, but more importantly it greatly increases the velocity of the exhaust stream.

Lets take two similarly sized jets as examples -- the Harrier & the F-16. Both generate essentially the same total thrust, but the afterburning F-16 has a discharge velocity more than twice the speed of the exhaust discharge in the Harrier. Despite having as much thrust as the F-16, and despite actually being slightly smaller & lighter, the Harrier remains firmly subsonic, while the F-16 can hit Mach 2.


Getting back to propellers -- the number of blades is irrelevant to discharge velocity, & hence speed -- it is only relevant to power transmission. More blades mean that more power can be transmitted for a given propeller diameter. However, there is a penalty associated with multi-blade props -- weight & drag. They obviously weigh more, but more importantly they cost more power through parasitic losses. Airframe builders only go to more blades when they can no longer transmit increased power through the existing prop due to constraints in blade diameter. The constraints can be limits on available clearance, or limits in blade tip velocity &/or mechanical strength.

In the case of your Bearcat example, it is up against propeller RPM limitations, so it is forced to use a 5-blade prop. It can't use a 4-blade of the same diameter as the 5-blade to transmit the available engine power (not enough blade area) without an unacceptable rise in RPM.

Multi-blades are no magic solution to speed. The Schnider trophy racers had an absolute requirement for speed & they used 2-blade props. The 2-blade fixed-pitch prop on the Supermarine S-6B float plane was quite sufficient to transmit well over 2,000 HP and push it to 407 mph. That is a FLOAT plane with floats, braces, struts & wires hanging out in the breeze going more than 400 mph. If they could have gone faster with a multi-blade prop it would have been fitted instantly -- except that the multi would have been slower due to lower overall efficiency.