Why does the Cessna 421 have a reduction gear on the prop?
Marty emailed me tonight asking:
Why do they gear aircraft engines to slow down the prop? In a C-421, why not let the prop spin at 2400rpm in cruise instead of gearing it to about 1800?
Hi Marty,
You’ve asked a good question and one that I asked myself when I first started flying Cessna 404s (which have the same engine and geared prop as the C-421)
I looked up my notes from Flight Safety and here is what I wrote (exactly):
“geared – quieter – turn at lower RPM and a little more power”
Hmmm, not much help. A little more digging into my Cessna 404 Flight Safety Manual and I learn:
“All Cessnas, except the 421 and 404 series, have nongeared propellers mounted directly to the engine crankshaft. The 404 and 421 series include a reduction gear driven by the crankshaft to provide a 2.3 ratio between engine and propeller rpm.”
And my last source (and this is the best) is from “Aircraft Systems for Pilots” by Dale de Remer and it reads:
“The increased brake horsepower delivered by a high-horsepower engine may be the result of increased crankshaft RPM. It is therefore necessary to provide reduction gears to limit the propeller rotation speed to a value at which efficient operation of the propeller is obtained. Whenever the speed of the blade tips approaches the speed of sound, the efficiency of the propeller decreases rapidly.”
So Marty it sounds like that the McCauley 3 blade constant speed props that are found on the Cessna 404 and 421 have a ideal RPM range and the 375 horsepower engines (Teledyne Contiential tubro-charged GTSIO-520-M) might provide a little too much power at full RPM to the prop. To fix this they insert a reduction gear between the crankshaft and prop so that the prop turns a little slower and falls within the propeller’s efficient operating speed.
I hope this helps and thanks for your question.
Fly Safe.
avi on Dec 03, 2009
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?
Tim Henderson on Dec 05, 2009
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.
avi on Dec 05, 2009
Why even make more rpm when u have to reduce it and add more weight to the aircraft?
avi on Dec 07, 2009
oops repeated post didnt show up the first time
joe murcko on Dec 18, 2009
Its a balance between engine powerband and prop speed efficentcy that is why the reduction is nessary
ray on Jan 14, 2010
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.