how soon after take off.....

[Islander2]

Why would 25"/2500 put you in the "detonation region" when FT/maxRPM wouldn't?

My engine, a Continental IO-550, has full throttle mixture enrichment. I believe this to be a feature of many other aero engines, especially the 6-cylinder variants. A reduction from FT to 25" near to SL causes a significant increase in CHTs which, if the engine is poorly cooled as it is on my A36 Bonanza, could move the engine into the detonation region.

Likewise, a reduction in RPM from max to 2500 causes the CHTs to rise, other things being equal. This is not so significant as the loss of full throttle mixture enrichment and, for me, is SOP for noise considerations, but it does reduce the detonation margin.

[Chilli Monster]

I'm probably missing something here but, if you reduce the power, but leave the mixture alone - how does that cause the CHT's to rise?

[Miserlou]

When I flew the Cardinal, the climb power was set as soon as landing back was not an option or between 400 and 800'. This included leaning the mixture.

I believe the failure at change of power statement to be at least rooted in fact though may refer more to jet engines due to the air flow and pressure changes.

I do know of a couple of instances of 'failure at thrust/power reduction'.

[DubTrub]

Originally Posted by HAL-26

..as soon as I feel comfortable about the rate of climb, its back 100 rpm or so, to give the old ladies a bit of a rest!

Not always a good idea, the full throttle setting ensures maximum fuel flow for cylinder cooling in the slow-flight high-power condition.

[FullyFlapped]

in the likes of a cessna T210 if i remember rightly SOP only allows for 5 minutes at max power setting before resetting throttle/prop for a climb power setting (26/26 160lbs ff ... i think )

Mine (TSIO-520) has a 5 minute limit at max grunt (310HP), then it's back to 30/25, 126 pph for normal climb, or 35/26, 162 pph for max perf climb.

FF

[S-Works]

In my Cessna I have a Rolls IO360K. Fly it fire walled and start to lean as I climb, top of the climb I set cruise and don't touch anything again until ready for the descent. I pull the throttle back an inch at a time and turn height into speed. Mixture gets fed back in slowly at the bottom of the descent.

[IO540]

That 92l per hour figure on the digital flowmeter when everything goes forward really un-nerves me

That is correct for the 100% power setting - this is an intentional setting on the fuel servo. The factory minimum spec is 83L litres/hour.

The over-rich mixture is required to keep the CHT down to a reasonable level, for an engine which is otherwise unable to dissipate the heat that's generated. This is a feature, AFAIK, of all common air-cooled Lyco/Conti engines, and is necessary if running at the certified 100% power setting.

Nobody should fly in cruise at 100% power, but your TB20 should pull about 165kt IAS at 1500ft if you left it like that for a few minutes.

Once in cruise, you can set 75% or lower power, and lean to peak EGT, or LOP.

The max rpm setting is necessary for 100% power. One isn't going to get the spec'd 100% power (itself required to achieve book values for takeoff performance) unless going to max revs.

The C4D5D safe operating area chart (you have to buy the Lyco engine manual to see this but I have a copy online) has a little nick in one corner which requires a min rpm when above a certain MP. This, apparently, is due to a stress limitation on the crank, just behind the prop flange.

The max rpm setting is also useful for reasonable altitudes. At say FL180 you will want every bit of air you can get into the engine, and the more revs, the harder it sucks.

I am not sure whether the OP posted his two-liner to start off this debate

[bookworm]

My engine, a Continental IO-550, has full throttle mixture enrichment.

Yes good point, I'd forgotten about that.

The over-rich mixture is required to keep the CHT down to a reasonable level, for an engine which is otherwise unable to dissipate the heat that's generated.

I rather like a line that came from one of Peter Garrison's articles in Flying many years ago. He commented that as a fluid for engine cooling, fuel is cheaper than perfume or wine, but represented poor value compared to air. Experience with the CHT instrumentation appears to confirm that an extra 10-20 knots of airspeed is equivalent to a great deal of extra fuel flow. Of course if you really need the optimal rate of climb, you'd better fly at Vy and firewall the mixtures. But that's not a common requirement.

Many of these trade-offs are dependent on the aircraft, engine, cowling etc. I don't think there's a one-size-fits-all answer.

[chevvron]

With Rotax 2-strokes, I find pulling back the throttle not below 500ft agl on climbout 'til you see a drop in rpm, then edging it slightly forward again, does nothing to r.o.c. but does wonders for fuel consumption.

[IO540]

Experience with the CHT instrumentation appears to confirm that an extra 10-20 knots of airspeed is equivalent to a great deal of extra fuel flow. Of course if you really need the optimal rate of climb, you'd better fly at Vy and firewall the mixtures. But that's not a common requirement

I agree.

If I was doing a really long flight, the sort where you check the Shadin FOB against the plog budget at every waypoint, then I would climb at 100% power to 500ft, then set 60% power and cruise climb at that setting, leaning for peak EGT as I climb, at about 300fpm; about 120kt.

One should notify ATC if IFR and unable to hold 500fpm

I have planned a direct flight to Corfu (LGKR) and will do it one day. It will need a ~20kt average tailwind though to achieve the legal FAA IFR reserve...

More commonly, one wants to climb at max fpm, to get above the cloud and ice. This does want all 3 fully forward, until power drops off to about 75%, and then (this happens at v. roughly 8,000ft) lean for about 100F ROP and adjust the pitch for an airspeed which keeps the CHT below 400F.

[Islander2]

I'm probably missing something here but, if you reduce the power, but leave the mixture alone - how does that cause the CHTs to rise?

Reduction in MP from FT of course does mean lower power, but it also means loss of full throttle enrichment and therefore a reduction in the excess fuel available for cooling ... in consequence, CHTs rise significantly.

Reduction in RPM from max also reduces power, of course, but the effective timing of the combustion event is advanced, with peak ICPs at maybe 14 deg ATDC (2500 RPM) compared with, say, 17 deg ATDC (2700 RPM) ... ICPs and CHTs both rise as a consequence.

Both of these phenomena are readily demonstrable with an EDM-700 or equivalent.

[englishal]

My method is:

Fixed pitch, firewalled to top of climb
Simple complex Curise climb once at 500' (25/25)
Turbo charged complex (eg Seneca), cruise climb at ~31" MP once at 500'
FADEC - reduce to 90% power at 500' and transition to cruise climb

But it does tend to depend on the situation, i.e. what climb gradient is needed.

[bookworm]

IO540, I think you paint a picture of a choice between two extremes. I have no difficulty achieving a reasonable RoC, airspeed, set of CHTs and more modest fuel flow by leaning for about 200 degF RoP in a cruise climb. The CHTs are actually considerably lower than in a peak EGT cruise.

[Islander2]

My method is:
Simple complex Curise climb once at 500' (25/25)

Which is as taught by most instructors over generations, but the vast majority of them had no idea what that power reduction does to detonation margin. With the advent of affordable multi-cylinder engine analysers, the evidence is now there for all to see.

Our naturally-aspirated engines generally have quite a large detonation margin. However, pick one of the more detonation-prone engines in an installation that's marginal on cooling air (and the Continental IO-550/A36 combination is a good example), then add-in ambient temperature>>ISA, slavish abide by the POH to climb initially at Vy (or worse still, Vx) ... and you have all the ingredients for some damaging detonation if you follow the common practice of reducing to 25/25 shortly after a low altitude take-off.

In many 6-cylinder piston aeroplanes, it really is quite bad advice.

[G-KEST]

In the interests of both noise abatement and fuel burn I reduce power in our Lycoming IO-360 powered Skybolt to 2500 RPM/25" MP at around 25 feet or less after takeoff. 25/25@25........!!!! If the engine did fail, and it has not in the 10 years of operating the aeroplane, then I have sufficient room to land ahead either on the paved runway or, if that is short, within the airfield boundary in order to ease the RFFS access and response time. Safety first - however I never taught that technique to anyone while instructing for some 39 years.

Cheers,

Trapper 69

[englishal]

If the engine did fail, and it has not in the 10 years of operating the aeroplane, then I have sufficient room to land ahead either on the paved runway

What a bl@@dy good idea

[IO540]

I have no difficulty achieving a reasonable RoC, airspeed, set of CHTs and more modest fuel flow by leaning for about 200 degF RoP in a cruise climb

Yes, this is what I referred to when I wrote

One can even apply the Deakin constant-EGT method where one leans to keep a specific cylinder at a constant EGT all the way up; that gives you a constant CHT all the way up and fuel savings.

I have used this; the CHT is about 400-420F, whereas climbing with all 3 fully forward gives me a significantly lower CHT; nearer 370F. Both are OK but IMHO the "clever" one is hardly worthwhile for usual GA messing about. I would use the constant-EGT method for an airways flight when one is climbing nonstop (hopefully) to say FL100-150.

[mm_flynn]

Interesting,
As an io-550 A36 driver with what appears exceptional cooling. I shove full throttle at the start of the take off roll and start pulling it back a bit when I start my decent. (cruise CHT 300 at 15 gph (US) 2500, 350 at 15.5 2200), dial a couple 100 rpm off at 500 feet for the neighbours and go to LOP when I get all cleaned up or my first level off point. Coming back from Venice, did the whole climb to FL140 as a LOP cruise climb (380 is where the alarms are set - 120 kts and about 500 fpm) in ISO +20 weather and had lower temperatures, better mpg but 10 minutes longer in the climb than a ROP climb.

[IO540]

I shove full throttle at the start of the take off roll and start pulling it back a bit when I start my decent

I concur and do the same, but we are talking of airways-level flights, with a non-turbo engine.

[Cusco]

In our Hershey Bar Arrer(IO 360) , I pull back to 25/25 at 500ft and further to 24/24 when I reach cruise altitude.Don't lean till I reach cruise altitude, using EGT gauge as my guide..........that's about as sophisticated as it gets.

I too have the 'don't thrash it for more than five minutes ' aphorism in the back of my head , but have no idea where it came from: it's certainly not written down anywhere.............. mebbee an old instructor drummed it into me once long ago.

Safe (and economical) flying.

Cusco