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Lycoming WOT

I have always been taught with a constant speed prop and an IO engine that Wide Open Throttle and max RPM should essentially be used only during the takeoff and then reduce MAP to 25 and RPM to 25 as soon as well established in the climb. The reasoning behind this is it is less damaging to the engine. This makes sense to me, after all Rotax document this. However I can find nothing officially documented that supports this for a Lycoming IO-320. Can anyone point me in the correct direction on this.

Derby / Lezignan-Corbieres

Hello!

The only AOM that I have available right now is for a Piper Seminole (two IO-360 engines). The AOM says in the part "Normal Procedures" under chapter "Cruise climb": "At this point (i.e. after being clear of obstacles - my addition) power should be reduced to approximately 75% power for cruise climb." Looking into the part "Performance" shows that at sea level 25inHg and 2500RPM give 75 percent power. Should be not much different in the manual of your aircraft.

EDDS - Stuttgart

Any power setting in your POH is allowed. So simply check your POH.

A non turbocharged Lyco can be operated at 75% BHP during cruise, that equates to WOT unless you fly very low.

A typical Lyco is rated to operate at 100% power indefinitely.

Climb should be done at max power, always (all 3 levers fully forward).

You can cruise at 100% power too but it delivers very poor MPG.

The typical Lyco is able to cruise at peak EGT (approx the best economy point) if the power setting is 75% of max rated power or less.

The gotcha is CHT. This should be kept below 400F. That means one cannot usually climb at Vx or even Vy all the way to top of climb. One needs to climb steeply until clear of obstacles and then transition to some higher speed which keeps the CHTs reasonable. But that is done by trimming for the higher speed; the engine settings are not changed.

The exception to the above is when climbing to a high-ish altitude e.g. above 8000ft, when one does have to lean otherwise the engine would become too rich and would eventually stop. In that case there are various approaches, with the best being a constant-CHT climb.

The stuff about reducing to 2500/25 is OWT, pointless, and is generally nonsense.

Some notes here, though they are more applicable to an engine with CHT and EGT instruments.

Administrator
Shoreham EGKA, United Kingdom

The stuff about reducing to 2500/25 is OWT, pointless, and is generally nonsense.

The noise level inside and out reduces quite a bit between max RPM and 2500. And the lower the RPM, the more "pleasant" the engine sounds. Personally, with every engine in every vehicle I operate I reseve max. power for situations that really require max. power, e.g. emergencies.

EDDS - Stuttgart

A typical Lyco is rated to operate at 100% power indefinitely.

That is more of academic value. More than 75% BHP requires full rich and who in his right mind would cruise full rich?

The noise level inside and out reduces quite a bit between max RPM and 2500. And the lower the RPM, the more "pleasant" the engine sounds. Personally, with every engine in every vehicle I operate I reseve max. power for situations that really require max. power, e.g. emergencies.

For cruise, I agree, sure. I fly c. 65%.

But for climb, the engine needs to run very rich of peak, to keep CHTs down. That is achieved by adjusting the fuel servo such that when all 3 levers are fully forward, the engine is running around 150F ROP.

By reducing power to say 2500/25" during climb, you are compromising this adjustment. It also doesn't save fuel (per distance travelled in the climb, or height gained). Usually it increases the CHT, because the airspeed drops disproportionately, and engine cooling is much reduced at even a small airspeed drop.

Administrator
Shoreham EGKA, United Kingdom

Thanks for all the feedback, I was really trying to establish why this back to 25/25 is taught. POH says nothing however at the back of my mind I am sure I had seen documented the reason behind it all. I am aware of all the in and outs of CHT/EFT mixture, fuel consumption etc and I am also aware that air racers despite them denying everything experience shortened engine life times when operating at WOT. mr Lycoming rate their engines to operate at WOT with max RPM but no one in this world can believe that a Lycoming could get even close to TBO if it was operated like this continually. So somewhere in our world of numerous documents and research someone must have tried to quantify the extra wear and tear on an engine operating at 100% as opposed to 75%. In the world of physics the difference of 25% is I am sure not a linear relationship with regard to engine wear.

Derby / Lezignan-Corbieres

The rental fleets of the world are powered by O-320s that have been operated very hard for their whole lives, the old joke being that a rental 172 comes equipped with a two position throttle. I think an O-320 will last when run that way - CHT and oil temp issues that affect larger engines in going places airframes are not nearly as evident.

Air racers do get short engine life - because they are often running a (nominally 100 HP) O-200 Continental at 4000 rpm, and about 140 HP. They'll last longer than you'd think even when doing that.

So somewhere in our world of numerous documents and research someone must have tried to quantify the extra wear and tear on an engine operating at 100% as opposed to 75%. In the world of physics the difference of 25% is I am sure not a linear relationship with regard to engine wear.

That's another old question

I am not aware of any systematic testing ever having been done.

As Silvaire says, the smaller engines don't seem to mind getting thrashed. Most of the flying school ones make TBO.

The bigger engines (e.g. the 360 and 540) seem to have more trouble with temperature management. I think there is a fair consensus that running at about 65% of max rated power gives you the best result, especially if running at/near peak EGT (for best economy). The engine will almost certainly last longer at say 40% power but then you fly so slowly that you will need a lot more hours in the air. When you get to say 75%, temperature management gets harder, and also you lose the peak-EGT option so you waste another 10+% in fuel.

But you will never find any research where somebody put a load of engines on a bench and ran them for 2000hrs, at different power settings, then took them apart and measured the bits with a micrometer. The cost of the fuel would be absolutely massive, for a start; about €200000 per engine. Even initial engine certification testing involves c. 150hrs on a dyno.

Also different engines can have specific weaknesses. For example Lycos have the camshaft at the top, which is fine when it's running, but the camshaft gets poor lubing during startup. Also the cam followers suffer from disintegration of the surface. There are also some valve guide issues. Contis have different issues. A lot has been written about this aspect, but nobody has managed to define an ideal operating profile AFAIK.

Frequent flying (once every 2 weeks, or more often) is important. And always flying for at least 1 hour, to make sure the oil gets a chance to boil off all the water condensed into it when the engine last cooled down.

The thing which seems definitely wrong is reducing the throttle opening or the prop RPM during climb. There is no reason to do that - other than perhaps noise abatement and then perhaps picking a different departure route is better.

Administrator
Shoreham EGKA, United Kingdom
14 Posts
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