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Why do engine/aircraft manufacturers discourage LOP?

Peter wrote:

The simple answer is: think of some good discussion topics and start threads on them

Ok, I’ll bite.

I’ve been thinking about why it is that engine (and aircraft) manufacturers discourage LOP ops as both their own and independent data shows that it is perfectly ok.

One reason I could think of is that many aircraft have very limited engine instrumentation. Perhaps not high-end IFR tourers, but most others. I haven’t kept track of how many individual aircraft I’ve flown, but it must be dozens (ranging from Piper Cubs to 300 hp IFR tourers). Of all those only one had EGT indications on all cylinders and only a few more had any CHT gauges at all.

We know that different cylinders reach peak EGT at different fuel flows. (The whole point of GAMI injectors is to fix this.)

Particularly at high cruise power settings, like 75%, wouldn’t you run a real risk of putting a cylinder in the danger zone if you lean to max EGT — or LOP — using a single EGT indicator and having no CHT indicators?

Last Edited by Airborne_Again at 14 Jul 08:42
ESKC (Uppsala/Sundbro), Sweden

On the rich side of peak, the power curve (HP developed vs fuel) is fairly flat.

See the Red Box in action: Red Box interactive

If all six cylinders would get the same amount of fuel, would be perfectly balanced as far as induction is concerned, then the engine would run smoothly LOP or ROP. When LOP the power curve drops off rapidly. If one or more cylinders are more LOP than the others, they will produce less HP and the peak power pulse will be delayed because a lean mixture burns slower than a stoichiometric mixture (think peak EGT).
In an IO520 intake air travels from rear to front, the fuel injections sprays continuously, even when the corresponding intake valve is closed (3/4 of the time). This means there is an accumulation of fuel-laden air moving forward and this makes the middle and front cylinders run richer than they need to be. Gamijectors compensate for this design error.

EBKT

My theory is there a disconnect between maximum range aerodynamic design and optimum engine running. These Lyco/Contis are happiest at 65-75%, which translates to full throttle operation at around FL80. Max range LOP speed in a typical design is 1.5 to 1.7x clean stall speed, or around KIAS80 in an Arrow. This might be around 35% power which is likely to lead to leaded valves, or general valve unhappiness.

Oxford (EGTK), United Kingdom

RobertL18C wrote:

My theory is there a disconnect between maximum range aerodynamic design and optimum engine running.

Indeed. If you read Stick and Rudder it is quite clear about where ‘most miles per gallon’ lies and it is much slower (in any aircraft’s range of speeds) than one would ever reasonably cruise.

EGLM & EGTN

Flying slow (near best glide or best range) is totally independent of ROP or LOP. One is airframe/airfoil related, the other is engine efficiency related.

EBKT

Do they really discourage it?

Perhaps they are embarrassed with the complexity that needs to be understood in order operate safely, and it is complex from an ab intio pilots perspective.

I.e. perhaps they would rather a pilot body or instruction to fill that role, and not be so prescriptive.

IMHO that approach is probably better, in that manufacturer should not be so prescriptive in how to fly.

Last Edited by Ted at 14 Jul 09:59
Ted
United Kingdom

dirkdj wrote:

the other is engine efficiency related.

Good luck with LOP at 35%!

Oxford (EGTK), United Kingdom

Ted wrote:

Do they really discourage it?

Quote from the POH of a 2006 Cessna 172S:

The 172S engine manufacturer, Textron Lycoming, has not approved operation of the engine at fuel flow rates (mixture settings) less than necessary to reach peak EGT in the leanest cylinder (the first cylinder to reach peak EGT.)

Quote from the engine manual (dated 2005):

Maximum power cruise (approximately 75% power) – Never lean beyond 150°F on the rich side of peak EGT unless aircraft operators manual shows otherwise.
Best economy cruise (approcimately 75% power and below) – Operate at peak EGT.

The engine in question is a Lycoming IO-360-L2A.

Last Edited by Airborne_Again at 14 Jul 10:17
ESKC (Uppsala/Sundbro), Sweden

Airborne_Again wrote:

Quote from the POH of a 2006 Cessna 172S:

The 172S engine manufacturer, Textron Lycoming, has not approved operation of the engine at fuel flow rates (mixture settings) less than necessary to reach peak EGT in the leanest cylinder (the first cylinder to reach peak EGT.)

Quote from the engine manual (dated 2005):

Maximum power cruise (approximately 75% power) – Never lean beyond 150°F on the rich side of peak EGT unless aircraft operators manual shows otherwise.
Best economy cruise (approcimately 75% power and below) – Operate at peak EGT.

Your post at least illustrates the complexity from manufacturers point of view. BTW i have never read the 172S model POH, though I gave many hours of instruction in P models an earlier.

Regarding the former, it sounds like the lawyers writing the manual, i.e. we have not specifically tested it so why should we stick our neck out and approve it. They have not disapproved it though. It also depends how you read the guidance in the handbook (excluding the limitations section), and given this probably came with fuel injection and a single point EGT probe, perhaps not a reasonable starting point. I also recall the earlier handbook says you should also land main wheel first in case you needed to know… of course you won’t do any damage if you ignore this up to a certain point!

Regarding the engine manual the limitation at maximum cruise power doesn’t sound unreasonable. AFAIK Engine damage can occur at around 75% or above with improver mixture settings especially allowing for reasonable tolerances in operation.

It underscores that this needs to be addressed as a flight training issue, it is more than just an engineering problem.

Last Edited by Ted at 14 Jul 11:40
Ted
United Kingdom

Maybe I should rephrase my question.

Do you run a risk (even remotely) of damaging the engine if operating at peak EGT or LOP on an aircraft which has a single EGT indicator because the cylinders are not peaking at the same time? At 65%? 75%?

ESKC (Uppsala/Sundbro), Sweden
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