reducing throttle after take-off - IO-520

[Bede]

For those of you that fly C185 & C206 with the IO-520, why does everyone reduce the throttle after take-off? I understand the prop, but I have never got a good justification why the throttle also needs to be reduced. It's not in the POH.

[porcsord]

The AFM in looking at says:

Normal Climb:

2. Power -- 25"Hg or FULL THROTTLE (which ever is less)

And

Maximum Performance Climb

2. Power -- Full Throttle and 2700 RPM

Why don't the Boeings always takeoff at full rated power, and climb at full rated power? Engine longevity, fuel consumption, etc etc. same same

[Bede]

Why don't the Boeings always takeoff at full rated power, and climb at full rated power? Engine longevity, fuel consumption, etc etc. same same

See that's what I always thought too but they make a turbo version of the exact same engine that climbs out at 38" every day, so leaving at full throttle at 28" isn't going to make a difference.

[porcsord]

But, is there more maintenance or wear on the engine than a naturally aspirated version - disclaimer: I don't know, just speculating.

[Big Pistons Forever]

The Continental Operators Manual for the IO 520 has charts which show the MP vs RPM limits. It allows 29in at RPM as low as 1800 RPM. Personally I have always climbed at the POH recommended cruise climb of 25 in and 2550 RPM. which I go to as soon as I am safely off the water.

If I need more climb performance to get over obstacles than full throttle and 2700 RPM until clear and then cruise climb

If I am lighter and noise is going to be an issue I will do the takeoff at 2700 RPM and full throttle instead of 2850.

[ahramin]

Hi Bede,

I fly a 180 and a 210 with IO-520s, we do not reduce throttle after takeoff, only RPM. The 210's engine has 2100 hours and 18 years since last overhaul, no issues so far. The 180's engine is new but it flies lots year round so we're planning on getting 3000+ hours out of it.

[sunk]

so do you leave the MP at full throttle for the trip and just roll back the rpm? Or do you reduce MP in cruise?

[Bede]

so do you leave the MP at full throttle for the trip and just roll back the rpm? Or do you reduce MP in cruise?

I know guys that fly like that, albeit not on an IO-520.

[goingnowherefast]

Why don't the Boeings always takeoff at full rated power, and climb at full rated power? Engine longevity, fuel consumption, etc etc. same same

See that's what I always thought too but they make a turbo version of the exact same engine that climbs out at 38" every day, so leaving at full throttle at 28" isn't going to make a difference.

Is it the exact same engine? I bet the pistons are different for a different compression ratio. I'd hazard a guess and say a lot of the engine internals are slightly different. What's the published TBO for both engines? I'd bet the turbocharged engine has a slightly lower TBO. Sure, one could run their non-turbo engine really hard and not break anything, but it's life will be closer to that of the turbocharged engine.

Also consider what the 38" means to a turbo engine. We already know the cylinder compression ratio is lower. We also know there's back pressure in the exhaust system from driving the turbocharger turbine. That means lesser exhaust scavenging. The 38" in the turbo engine is the same equivalent work being done by the pistons as a much lower manifold pressure on a non-turbo engine. I don't know the actual answer, but I'll guess it's pretty close to 25-29".

I'll put it this way. If one wants to make 270hp (approximate climb power), the turbo engine needs a noticeably higher manifold pressure to make that power. This is due to exhaust back pressure from the turbo itself, and lower piston compression ratio.

[Bede]

Why don't the Boeings always takeoff at full rated power, and climb at full rated power? Engine longevity, fuel consumption, etc etc. same same

See that's what I always thought too but they make a turbo version of the exact same engine that climbs out at 38" every day, so leaving at full throttle at 28" isn't going to make a difference.

Is it the exact same engine? I bet the pistons are different for a different compression ratio. I'd hazard a guess and say a lot of the engine internals are slightly different. What's the published TBO for both engines? I'd bet the turbocharged engine has a slightly lower TBO. Sure, one could run their non-turbo engine really hard and not break anything, but it's life will be closer to that of the turbocharged engine.

Also consider what the 38" means to a turbo engine. We already know the cylinder compression ratio is lower. We also know there's back pressure in the exhaust system from driving the turbocharger turbine. That means lesser exhaust scavenging. The 38" in the turbo engine is the same equivalent work being done by the pistons as a much lower manifold pressure on a non-turbo engine. I don't know the actual answer, but I'll guess it's pretty close to 25-29".

I'll put it this way. If one wants to make 270hp (approximate climb power), the turbo engine needs a noticeably higher manifold pressure to make that power. This is due to exhaust back pressure from the turbo itself, and lower piston compression ratio.

Excellent answer. Thank you.

[ahramin]

so do you leave the MP at full throttle for the trip and just roll back the rpm? Or do you reduce MP in cruise?

Full throttle from takeoff to top of descent. Small power reduction after takeoff by reducing the rpm, large power reduction in cruise by reducing rpm and fuel flow.

[ahramin]

It occurs to me that I should give some additional information. During climb I keep the mixture very rich, only leaning enough to keep the EGTs about where they are after takeoff. At all times the CHTs stay below 400. A little hot for a continental but the floatplane is draggy and the 210 doesn't have the best cooling with a 520. We accept the small extra wear and tear for good climb performance.

In cruise I'm running lean-of peak. Very cool CHTs.

The turbocharging comments reminded me of the Stihl guy who won at Reno last time I was there. He was racing his Lancair around the course at something like 70" of MP and then flying the same plane around the country the rest of the year with no worries about his engine. Air cooled Lycoming and Continental engines are very robust as long as you don't overheat them and as long as you don't get them into detonation territory (which is almost impossible with a 520).

[TeePeeCreeper]

It’s not rocket science.

25/23 respectively squared and cross checked by FF. Is the engine GAMI’ed?

TPC

[goldeneagle]

See that's what I always thought too but they make a turbo version of the exact same engine that climbs out at 38" every day, so leaving at full throttle at 28" isn't going to make a difference.

C-421 has the GTSIO-520. It is in not 'the same engine' as the 185 or such, but the core is essentially the same. I have a fair amount of time flying the 421. Manuals call for a power reduction once airborne. The fuel control unit puts extra fuel into the cylinders for extra cooling at full throttle during takeoff on that airplane, so your 'dollars per minute' fuel flow guage winds up to the top. Once airborne with gear up and over trees, throttles come back to climb settings, mixture back to set a fuel flow. When the throttle comes back, fuel control unit no longer injects extra fuel for cooling, and this is the reason all takeoffs should be full throttle on that airplane, even if you have a 2 mile runway. The extra fuel is to provide cooling at low speed before there is enough airflow for air cooling.

Dunno if the other variants have the same system, there's a lot of stuff on the GTSIO variant that's not on the other variants.

[ahramin]

See that's what I always thought too but they make a turbo version of the exact same engine that climbs out at 38" every day, so leaving at full throttle at 28" isn't going to make a difference.

C-421 has the GTSIO-520. It is in not 'the same engine' as the 185 or such, but the core is essentially the same. I have a fair amount of time flying the 421. Manuals call for a power reduction once airborne. The fuel control unit puts extra fuel into the cylinders for extra cooling at full throttle during takeoff on that airplane, so your 'dollars per minute' fuel flow guage winds up to the top. Once airborne with gear up and over trees, throttles come back to climb settings, mixture back to set a fuel flow. When the throttle comes back, fuel control unit no longer injects extra fuel for cooling, and this is the reason all takeoffs should be full throttle on that airplane, even if you have a 2 mile runway. The extra fuel is to provide cooling at low speed before there is enough airflow for air cooling.

Dunno if the other variants have the same system, there's a lot of stuff on the GTSIO variant that's not on the other variants.

Further reinforcing the idea that the MP is not what is wearing out the engine, the CHTs are. As long as your CHTs are 380 or below, reducing MP is not going to increase longevity.

[goingnowherefast]

MP creates cylinder pressure, which pushes the rings harder into the cylinder walls. An engine that spends it's time cruising around at 60% power will last longer than one that flies around at 75% (everything else being equal).

That's a long term wear thing though. 3 minutes for takeoff is inconsequential (unless you're doing circuits for the whole life or the turbo engine). A lot of engines, and especially turbo engines have full throttle (and full rich) fuel enrichment features.

Takeoff is high power, low cooling air flow. So get it over with ASAP. Takeoff manifold pressure and RPM also creates the highest combustion temperature. Pistons basically don't have direct cooling. They're cooled by the intake stroke with cool air. That air isn't exactly cold, because it was compressed by a hot turbo. Some engines have piston oil squirters to put oil on the bottom of the piston, but that only does so much too. The extra fuel at takeoff power helps reduce the combustion temperature and thus reduce piston heat.

So, unless one plans to take off at cruise power, use proper takeoff power. The takeoff roll at cruise power would probably take so long that the cylinders would get cooked from the lack of appropriate airflow. It would also be dangerous due to the lack of performance.

[rigpiggy]

The Navajo has a similar setup the last inch of travel increases the fuel flow during takeoff. After 400' power back to 38" 2400 rpm, and 30 gph. In cruise 2200/32" /16gph