6 vs 4 Cylinder Engines

middies10

middies10

Well-Known Member
Over many years, I have always heard people discussing the need to "baby" 6 cylinder engines in aircraft. Now that I am going to begin operating a 6, nobody seems to have a great explanation as to why its necessary to make smooth, slow power adjustments. It's not that I don't do this already, but I would like to have an answer for students that may be going into something like a SR22 or a BE58.

Thanks!

Don't be afraid to get real technical with your answers!
 
middies10 said:
Over many years, I have always heard people discussing the need to "baby" 6 cylinder engines in aircraft. Now that I am going to begin operating a 6, nobody seems to have a great explanation as to why its necessary to make smooth, slow power adjustments. It's not that I don't do this already, but I would like to have an answer for students that may be going into something like a SR22 or a BE58.
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6 cylinder engines, particularly turbos, are very easy to overspeed. It is easily to let manifold pressure get too high. It is easy to let them get too hot. So, there's a bit more involved as compared to an O-360 for example. Most that I have flown need a very rich mixture for cooling on climb when they don't have much airflow, so you are essentially dumping gas into them to keep them from overheating. Shock cooling is bad, so you need to plan descents around still having high power settings. They also tend to be expensive, so good not to burn them up :)

Smooth power application is advisable for ANY engine, but more so when they are more complex, since you want to see what the EGT, RPM, manifold pressure, CHT and fuel flow are doing. Hard to digest all of those quickly. Make sure what the engine is telling you matches what you should see from the book, otherwise, they fly the same. Most of the work flying something like a 206 is managing the engine.
 
Usually its suggested that rapid power changes can induce thermal "shocking" resulting in cracking and excessive wear due to small warps during uneven cooling.

Because larger engines have more thermal mass and more restricted airflow, they are supposedly more susceptible to uneven cooling during rapid power changes.

The truth of this is highly debated, most evidence of problems is circumstantial, and "best practices" are mostly propagated by "this is how I was taught to do it" (things like 1" per minute, etc etc).

Here's some reading: http://www.avweb.com/news/pelican/182107-1.html
 
rframe said:
The truth of this is highly debated, most evidence of problems is circumstantial, and "best practices" are mostly propagated by "this is how I was taught to do it" (things like 1" per minute, etc etc).
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The better reason to do things slowly is that there is no good reason to be making rapid power changes. It makes more work for you, makes the plane harder to fly, and makes the ride less comfortable for passengers.

With an O-540 in descent, we go to 2200 RPM for the first 1000', then down to 1900 RPM. Seems to work well, CHT/oil temp move quite slowly, and we haven't been burning up engines...
 
drunkenbeagle said:
The better reason to do things slowly is that there is no good reason to be making rapid power changes. It makes more work for you, makes the plane harder to fly, and makes the ride less comfortable for passengers.

With an O-540 in descent, we go to 2200 RPM for the first 1000', then down to 1900 RPM. Seems to work well, CHT/oil temp move quite slowly, and we haven't been burning up engines...
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rframe said:
Usually its suggested that rapid power changes can induce thermal "shocking" resulting in cracking and excessive wear due to small warps during uneven cooling.

Because larger engines have more thermal mass and more restricted airflow, they are supposedly more susceptible to uneven cooling during rapid power changes.

The truth of this is highly debated, most evidence of problems is circumstantial, and "best practices" are mostly propagated by "this is how I was taught to do it" (things like 1" per minute, etc etc).

Here's some reading: http://www.avweb.com/news/pelican/182107-1.html
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I freakin' love John Deakin. I just thought I'd throw that out there.

Properly managed, you don't let your power plants get too hot to begin with. People running around at 450F are asking for it, TCM or Lycoming, flat pack or not. Too hot. Ours crept up in Fresno the other day to around 380-390 on climbout...we shoved the nose down and climbed at 130 KIAS in lieu of overdoing it at Vy. Your milage may vary.

WOTLOPSOP was, in fact, SOP for me in the normally aspirated 206 I flew (minus the LOP part, it was operated ROP to appease the people paying for the airplanes—they had not yet Seen the Light). I'd keep whatever cruise manifold (usually whatever full throttle would buy at 10,000') was during descent, and then reduce power as required in level flight entering the pattern. We had an excellent engine analyzer that gave us discrete outputs from each cylinder of CHT and EGT. The CHTs didn't move very fast (if at all).

Note, they didn't say anything about this in the company beyond "run it rich of peak, and 'reduce power gradually' during descent." [ OK ]

We do something quite similar in the Twin Bonanza Dad and I fly - I'll keep cruise manifold pressure all the way down, and zoom. If it's rough, I'll slow down in level flight first, add some drag, then descend. (It's a little different because the 'Bo has GO-480s. Prop-driving-engine is, in fact, destructive on that engine-prop combination, but direct drive powerplants don't normally suffer from such things.)
 
I'll look for my Flight Express training material on the IO-520 and 550 in the 210 and Baron. Good stuff from an operator that puts a lot of hours on these things. IIRC, they were FAR more concerned with mixture control rather than the throttle.

As to why bix sixes and turbo sixes appear to be more sensitive, couldn't tell ya. My guess with turbo sixes is crappy out dated turbos. Cars go up and down in RPMs, get thrashed and put away wet with no issues.
 
UAL747400 said:
I'll look for my Flight Express training material on the IO-520 and 550 in the 210 and Baron. Good stuff from an operator that puts a lot of hours on these things. IIRC, they were FAR more concerned with mixture control rather than the throttle.

As to why bix sixes and turbo sixes appear to be more sensitive, couldn't tell ya. My guess with turbo sixes is crappy out dated turbos. Cars go up and down in RPMs, get thrashed and put away wet with no issues.
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Car engines tend to be liquid cooled with the exception of some older Porches. Airplane engines are rated at 100% duty cycle for their entire TBO. Car engines very very much are not. Go try to run your BMW at redline and maximum load that will retain that RPM for 2000 hours straight. I bet you don't make 200.
 
Eh, rotaries, VW engines, and chevy small blocks do just fine running at max power. A stable engine is also a happy engine. Piston engines usually fail during power changes. Car engines are never at a set power.

Regarding turbos, I was mostly referring to them spitting out oil if they're stage cooled.
 
z987k said:
Car engines tend to be liquid cooled with the exception of some older Porches. Airplane engines are rated at 100% duty cycle for their entire TBO. Car engines very very much are not. Go try to run your BMW at redline and maximum load that will retain that RPM for 2000 hours straight. I bet you don't make 200.
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Due to this fact, car engines hold a stable temperature throughout all phases of operation. This means there is a constant even clearance between all the moving parts in the engine. On top of this, car engines are usually built to tighter clearance limits than air cooled aircraft engines for this very reason as air cooled engines have a lot of different rates of thermal expansion and contraction between all the moving parts.

The best reason I have heard for making sure to let the engine cool slowly and evenly rather than chop and drop resulting in shock cooling is that the cylinder being aluminum and having all the external cooling fins will cool much faster than the piston. This will result in the cylinder contracting around the piston creating less clearance.
 
middies10 Continentals have crankshaft counterweights that can shift with rapid application of power. You do NOT want that to happen. That is the primary reason why they always say to smoothly apply and reduce power.

It's really the Continentals and any (4 or 6 cyl) turbo'd piston engine you have to be careful with. Things like proper leaning, MAX continuous vs takeoff power settings, referencing the book for appropriate EGTs, TITs or fuel flows, stage cooling, oil temp on takeoff, watching CHTs on the climb and descent are pretty important.

Now, naturally aspirated Lycomings like your 360s & 540s... You can just hammer the crap out of those and they just come back begging for more. The harder you work them the happier they will be. It's everything attached to the engine that wears out quicker. :D
 
IO540s are bulletproof, assuming you don't get an out of the blue crankshaft breakage. Big bore continentals will crack cylinders if you look at them funny. TIO540s can be a bit finicky but my experience with them is pretty limited. Practically any flat aircraft engine with 6 Cylinders is going to have dynamic dampers (counterweights). The key with those is to avoid rapid RPM changes as that can bruise the bushings that the weights ride in and cause them to no longer absorb the frequencies they are designed to absorb. Or thats what all the manufacturer literature says.
 
Apples and oranges really. Pulling 160 horses on an engine the size of an io360 while only cranking it at only 2350 rpm (red line)doesn't equate to doing 4,500 rpm all day in the Pontiac.

The problem with cylinders is with the cylinders themslves, I don't think you could find a group of people more ginger on machinery than big bore pilots, yet still.

Read books like

image.jpg

The stuff you pick up from inexperienced pilots, instructors, even owners can be flat wrong.
Owners can be the toughest because two coincidences can make a "certainty".
Ymmv, go in peace.
 
In our organization, which at one point had a fleet of around 60 206's (now decreasing due to moving to more turbines), with a mixture of both normally aspirated and turbo, we're pretty hard on the engines, and have had very little problems. We practice lots of aborted takeoffs during training, and do LOTS of short field landings, which necessitates the need for short bursts of power, increasing and decreasing on final for stabilized approaches and accurate landings.

We're very conservative in our maintenance since we fly over lots of inhospitable terrain with SE piston planes, but we've never had any of the supposed problems with the rapid power changes. Now we are very careful about CHT's and shock cooling, but I've seen some Navajo pilots that almost seem like they're scared to touch the throttle on descent and I've always wondered why. I've changed dozens of cylinders but I've never seen a cracked one in our ops - it's always due to worn exhaust valve guides around 800-1000 hours.
 
I have about 1000 hours operating non-turbo'd 6's in a training environment (A36 & BE58). Needless to say these engines WERE NOT babied. We did quite a bit to them, lots of "shock cooling", low power to full power back to low power, emer decents, air shutdows and restarts, etc. they were champs.

We had 8-10 instructors IIRC, so roughly 8000-10,000 hours per year on those planes, no issues I think the only one that crashed over the years ran out of gas, had nothing to do with the engine.

On a side note, I remember the engine being an IO-540, but looking it up now, I must have remembered incorrectly. It has been a while. Were A36's and BE58's ever equipped w/ IO-540's? Or could they be installed? For some reason that is what is stuck in my head, not sure why.
 
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wheelsup said:
I have about 1000 hours operating non-turbo'd 6's in a training environment (A36 & BE58). Needless to say these engines WERE NOT babied. We did quite a bit to them, lots of "shock cooling", low power to full power back to low power, emer decents, air shutdows and restarts, etc. they were champs.

We had 8-10 instructors IIRC, so roughly 8000-10,000 hours per year on those planes, no issues I think the only one that crashed over the years ran out of gas, had nothing to do with the engine.

On a side note, I remember the engine being an IO-540, but looking it up now, I must have remembered incorrectly. It has been a while. Were A36's and BE58's ever equipped w/ IO-540's? Or could they be installed? For some reason that is what is stuck in my head, not sure why.
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I used to work at a Raytheon Service Center and we saw Beech of every kind, both old and new. IIRC the only Beechcraft we ever saw with a Lycoming was a Duke. Doesn't mean there isn't some STC or some obscure Baron Or Bonanza out there with one though.
 
wheelsup said:
I have about 1000 hours operating non-turbo'd 6's in a training environment (A36 & BE58). Needless to say these engines WERE NOT babied. We did quite a bit to them, lots of "shock cooling", low power to full power back to low power, emer decents, air shutdows and restarts, etc. they were champs.
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Short version,

Make smooth power changes in flight (any competent aviator should be doing this anyway).

Don't let your CHTs get above 400 in the first place (I don't care that the manufacturers say that 450 is OK).

Avoid EGTs from 100 ROP - 50 LOP.

Reduce power as required to make a normal 500 FPM constant airspeed descent, and "shock cooling" will never be a problem (if it ever existed at all).
 
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