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.
thornton on Jan 14, 2012
I have purchased a 1979 cessna 421 and would like to communicate with person who has same. I would like to know more about propspeeds and settings in different power configurations. I HAVE PILOTS HAND BOOK but it lacks much info.
Chris on Feb 14, 2012
I realize this an old post, but I noticed nobody really answered avi’s question.
“Why even make more rpm when u have to reduce it and add more weight to the aircraft?
”
The reason you want more horsepower is for better fuel efficiency. With more horsepower comes a higher compression ratio. With a higher compression ratio you get better long range fuel economy by being able to convert more heat energy into useful work. Low compression engines also burn hotter at the cylinder walls and that will rob you a little of fuel efficiency.
J C Saavedra on Jun 03, 2014
A very interesting subjet but at the end of the day everything is a compromize in the real world.
” Simplisity is the ultimate level of complexity ”
Leonardo da Vinci
SC on Dec 16, 2014
Hi! How much is the thrust produced by the propeller?
Justin on Aug 06, 2018
[Preface, I know this is a 9 year old thread but the questions here have not been answered properly to this day].
From Avi “Thanks this is great. Why dont they just make the engine have less horse power so they don’t need to add a reduction gear and increase weight of the airplane?” “Why even make more rpm when u have to reduce it and add more weight to the aircraft?”
Answer: They already make identical size and displacement engines that spin slower, make less horsepower, and are lighter. – Those exact same basic engines were adapted to spin 3400rpm to make more horsepower because You need more horsepower in order to provide the Takeoff, climb rate, and speed performance of the larger and heavier 404 and 421 aircraft. Keep in mind, any additional power you have available while cruising along can be used to go faster or to climb at a better rate and angle over terrain.
Apparently, the engineers did not want to completely re-design those Lycoming engines entirely to be larger and more powerful for use in only one or two airplanes, because creating a new engine from the drawing board is very expensive. – So they decided to use the same basic engines they already had and squeeze more power out of them.
Now, Engines make power by burning oxygen and fuel and converting that chemical-thermal reaction into motion and power. Simply put, you must get more Air and Fuel into them to make more power. Air Density, or Atmospheric Air Pressure is basically set, being 1 Bar or 14psi at Sea Level decreasing to half that around 20,000 feet and less going higher.
You can add more fuel to any engine, which enriches the air/fuel ratio mixture and you will make more horsepower to a point, after that point adding even more fuel eventually floods the engine and you actually lose power, because you have to much fuel vs oxygen in there. Most engines installed in aircraft already allow you to add as much fuel needed to gain the best power output. You need more Air!
Here is the different ways to add more air, so you can add more fuel AND get more Power;
#1 You can add more oxygen by pressurizing the intake air with a supercharger or turbocharger.
#2 You can also add air by making the engine larger, more displacement per cylinder.
#3 The third way to add air is by spinning the same engine faster, which draws in more air and fuel. (Example; 3400rpm “digests” about 1.2x Times the air/fuel as 2800rpm for identical engines, giving you about 20% more power from the same small package!). – That translates into the power-bump of 40-50 Horsepower, from 330hp at 2800rpm up to 375hp at 3400rpm. simply spinning more RPMs means drawing in more air and burning more fuel, which makes more power.
Now you solved the millions of dollars of design and manufacturing work and the new horsepower requirements, but there are several problems you just created by spinning the engine faster.
#1 you increased the propeller RPM beyond what its efficient speed range was of the stock/basic engine prop combo.
#2 If you need a reduction gear drive to reduce that prop speed back to an efficient range
#3 You increased internal stress and wear, and added a gear unit, both are increasing maintenance requirements.
(Some propeller math, if mach 1 is 760mph at sea level, decreasing to 700mph or below at higher altitude you want the propeller tip well below that to avoid breaking the sound barrier with the spinning tips, more explained later). Hypothetical propeller tip design speed; 600mph x 5,280ft in a mile = 3,168,000 feet per hour / Divide 60 mins in an hour = 52,800 feet per minute. / divide by 2800rpm = 18.857′ feet per revolution, then / by Pi. = [6.002′] feet in diameter.
Cool, huh? At 2800 Revolutions-Per-Minute you are allowed to use an 18.857′ foot circumference propeller, That is 6.002′ feet in diameter, which is a very common direct drive propeller size.
Now, apply the same math at 3400RPM you are limited to 15.529′ foot circumference, or 4.943′ foot in diameter propeller. Because a larger prop would be spinning very close to or exceeding the speed of sound. – (A propeller tip traveling faster than sound is very bad, due to it needing a mach1+ designed knife thin airfoil for the prop that wont make very good power below mach1, and you get damaging mach buffeting that hurts the propeller and engine, also you are dragging a sonic boom wherever you go which is excessively loud. AND you actually use exponentially more horsepower to gain very little additional thrust due to excessive prop drag. Think of a fighter jet cruising easily at airliner speeds of 600mph, vs the fighter exceeding the sound barrier where it needs it’s afterburner the entire time to operate, exponential drag).
Also going back to propeller sizes, another consideration is that small 4.9′ foot diameter propeller would need far to much blade angle pitch to absorb the horsepower. Otherwise the engine will spin faster than its redline and will also not transfer the power into the air, if it was transferring horsepower via “traction” in the air, it would “bog down” the engine and maintain safe rpm levels. But being to small of a propeller it cannot move enough air to slow the engine down to its safe operating range of 3400. Also, using very steep propeller blade angles would mean your propeller blades are “stalled” just like an airplane wing stalls, and would be generating very little thrust even though your engine is operating far beyond its redline. A propeller generates “pull” force the same way an airplane wing or helicopter rotor generates “lift!” – Think of an eggbeater, now think of a corkscrew, or a large boat propeller. You need traction in the air.
As you can see, a larger, slower turning propeller within design specs is the answer to utilizing all your added horsepower and achieving or increasing the performance of the aircraft. In order to mate a large, slow turning and efficient propeller to your 3,400rpm 375hp engine, you need a gear reduction drive.
You add some weight, maybe 30 or 40 pounds / 20 kilos. But that is like bringing an extra suitcase with you that adds 50 horsepower any time you need it to take off in a shorter distance, and climb out over a mountain range.
It also adds more maintenance requirements, it needs inspected and possibly more frequent oil changes. When the engine needs rebuilt you have to also rebuild the gear drive. Adding cost and complexity is worth it in this trade-off, because the alternative was to design and build a 375 Horsepower engine from the drawing board for millions of dollars.
I believe this is a fairly accurate post based on a decade of working on and around engines, thinking about engines, driving cars, riding motorcycles and flying airplanes. Hope this helps somebody moving forward.
Scott F on Aug 07, 2018
Wow I had this same question last night, and just happened to keep this webpage up for 24 hours on my phone to read it later and someome answers this the same night! Great! Thx Justin.
Justin on Aug 19, 2018
Hey Scott,
It came about from my own online meanderings trying to get some answers on similar topics. Seeing that there is a plethora of unanswered questions out there, and a general lack of understanding about some of these topics (especially around pilots)… I figured that I would throw out some thoughts for those that care enough to think on. – It’s rather impressive that after reviving an ancient message board it would so quickly benefit anyone. I actually came back here to check before deleting this link from my toolbar… haha!
Hugo on Dec 10, 2018
God!!! I’ve been reading all over this engine on every website. Finally the best complete answer. Thank YOU Justin!
hanker on Jan 21, 2021
It’s now January of 2021. This site continues to give the best reason for geared props.
Justin, I appreciate you knowledge. I hope you are teaching what you know. Good job and keep up the great explinations.
Daryl Carpenter on Apr 14, 2021
I concur that this is by far the best answer. Props to Justin! (pun intended).
I’m curious to know if anyone has come across any other engines that can replace the GTSIO-520-M. I know there are several single engine pistons that can be powered by different powerplants. Just curious whether any manufacturer has built a replacement for the 421 without a gear drive. This is the perfect machine for personal and business use but the reports I hear regarding how much time the 421 spends in the shop is a turnoff.
kyle goldston on Sep 16, 2022
The Cessna 414 is essentially a 421with less horsepower. The landing gear and wing are inferior. Most people can’t tell them apart visually.