Comments on: Why does the Cessna 421 have a reduction gear on the prop?

By: ray

ray — Thu, 14 Jan 2010 16:18:01 +0000

The forward speed of the aircraft wouldn’t affect the tip speed of the props; the main reason for the gearing is to reduce the speed of the props to one that they operate most efficiently at since as Tim pointed out, The engines are running at a higher than what is normally found (on piston engine aircraft with these types of engines) rotational speed to achieve the required power output.

By: joe murcko

joe murcko — Fri, 18 Dec 2009 19:59:50 +0000

Its a balance between engine powerband and prop speed efficentcy that is why the reduction is nessary

By: avi

avi — Mon, 07 Dec 2009 21:13:43 +0000

oops repeated post didnt show up the first time

By: avi

avi — Sat, 05 Dec 2009 21:27:31 +0000

Why even make more rpm when u have to reduce it and add more weight to the aircraft?

By: Tim Henderson

Tim Henderson — Sat, 05 Dec 2009 21:13:59 +0000

The issue isn’t one of the engines producing too much power if they turned at propeller rpm, it is one of them producing too little. The engines need the rpm, about 3400 inorder to produce the 375 horsepower. The same basic engine, an IO520, without the gears makes about 325 horsepower in its most powerful configuration at about 2750 rpm with a direct drive to the prop. If you tried to turn it fast enough to make the 375 horsepower and had the big 90 inch propeller fastened to the crankshaft, the prop tips would go supersonic when their rotational speed was ‘mixed’ with the aircraft’s forward speed.

By: avi

avi — Thu, 03 Dec 2009 20:34:42 +0000

Thanks this is great. But why dont they just make the engine have less horse power so they dont need to add a reduction gear and increase weight of the airplane?