Over Square?

[cjam]

G'day,
The other day I was on a check ride with an instructor in a C206 and in the cruise I set up 24 Map and 23 RPM. He was not happy about the fact that I was flying "over square" even though I told him that I had got the cruise setting out of the POH. I se2424 and the flight continued as advertised. When I got back I checked the POH and it is allowed, in fact you can set 25/22 if you want but thats only about 50% horse power.
For me 24/23 gives the best comprimise between horse power/ TAS/ fuel consumtion and a quiet ride.(from looking at the book,not experience)
I have also been told that a lower rpm for a given horse power results in greater range because the fuel consumtion drop is greater proportionally than the loss of airspeed due to less internal loading on the engine.????Is that right?
I am sure that it is ok to use these settings as it is in the manual but why is there such widespread fear of going "over square"? I have not flown turbo-charged planes so maybe thats where my knowledge is deficient.
Any thoughts from those with some time up;
a) to confirm that the power settings I am using are ok
b) to explain exactly how flying "over square" damages the engine, on what planes this occurs etc, and
c)any other comments you care to make
Cheers, cjam

[Bevan666]

Oversquare is one of the great myths of aviation. If an engine blew up 'oversquare' no turbocharged engines would ever work. The numbers are just that - numbers. There is not relationship mechanically between RPM and MP.. if you measured MP in mmhg instead of inches of mercury maybe this myth might never have started.

The things that destroy engines are excessive temperatures and pressures (which leads to detonation). When operating larger engines, care must be taken to aviod areas of high temps and pressures which occur when operating at high power settings (above 60% power). Keep the engine cool and power settings low and you can be safe to operate the engine anywhere in the permitted RPM/MP limits which will be defined in the POH. For higher settings use the specific settings which are in the POH, but be careful - for some engines (Turbo'd Lycomings - TIO-540) the recommendation for 'best power' puts the engine in the worst possible operating conditions (50 degrees rich of peak EGT - where the peak cylinder temps and pressures are).


As for reducing RPM to increase range - it works. At a lower RPM there is less frictional loss inside the engine as it is turning slower. That being said, you cannot do this at high power (>60%)settings as you are increasing the peak cylinder pressures so reducing the engines detonation margin.

Bevan..

[cjam]

"but be careful - for some engines (Turbo'd Lycomings - TIO-540) the recommendation for 'best power' puts the engine in the worst possible operating conditions (50 degrees rich of peak EGT - where the peak cylinder temps and pressures are). "

Thanks Bevan,

the company I am with lean to 50 degrees rich of peak egt all the time at all cruise power settings, can you elaborate on why thats the worst condition?

[Bevan666]

Have a look at this diagram

http://www.avweb.com/newspics/182536landmarks.jpg

As you can see from this diagram, peak cylinder temps (CHT) and pressures (ICP) are not at peak EGT, but slightly richer than peak EGT. On most engines, this is the published 'best power' setting, which is usually around 50 degrees rich of peak EGT.

What causes engine damage? - high cylinder temps and pressures. The higher the power setting the higher these temps and pressures are going to be. Now for the IO-520 which I am familiar with, above about 65% power, operating at 50 degrees rich of peak is the WORST place to operate this engine. 50-75 degrees richer is OK and so is 50-75 degrees leaner.

With turbo'ed engines, they can maintain these higher power settings for longer, and as the aeroplane climbs higher it gets less cooling for the same power setting, raising CHT's further.

I would suggest you read John Deakin's articles on engine management, which can be found here (http://www.avweb.com/news/columns/182583-1.html) . He knows more about piston engine management than I will ever know.

Bevan..

[Woodend1]

cjam,

just curious as to what your instructor gave you for best glide speed in the 206. Different speeds for different weights I presume?

[Dale Harris]

Best power for a piston engine is 125 deg rich of peak. As stated in every Lycoming manual I've ever seen. 50 degrees rich is a half way setting between best power and best economy (Peak) I don't know anyone who uses that for TIO-540's. 50 degrees is used a lot in non turbocharged engines The main difference being that they aren't operating at 65% power or more because of altitude. As for oversquare, if it's in the POH, you can use it. Oversquare works well for economy and noise. But be aware some a/c have limitations on certain combinations of RPM/MAP. Check the POH always.

[hoss]

Amen, Bevan666:ok:

cjam, your 'C-206 checkie' shouldn't have had a problem with your POH power setting, sounded fair enough to me.

Woodend1, I just had a bit of a look through my bookshelf and found my trusty old 1981 U206G manual and it has 3 maximum glide speeds 75/70/65 depending on weight. Curiously, and it's all coming back to me now the Engine Failure Immediately After Takeoff has an airspeed of 80KIAS. Another one is Engine Fire In Flight which has a target of 105KIAS.

Safe flying :)

[dogcharlietree]

Oversquare is one of the great myths of aviation.
Careful Bevan. There are some old Radial-engine aviators who surf this forum.

[currawong]

Come on Dogcharlietree, quote a POH for us.

Not being a smart ar5e - it is an interesting thread and it sounds like you have another angle on it.

POH from Mr Cessna in front of me now states -

CRUISE

"For a given throttle setting with a constant - speed propeller, select the lowest engine speed in the green arc that will give smooth engine operation"

ok for a flat engine, are there other angles?

[Woodend1]

Cheers hoss, appreciated.:ok:

[Jamair]

Said it before & I'll probably say it again - THE best thing you can do for your piston-prop aeroplane is buy & fit a JPI or equivalent all-cylinder monitor. Agree 120% with bev 666 re John Deakin also - use SCIENCE and factual DATA rather than sad old wives tales to run your aircraft.

I've been able to tell the LAME that the lower plug on No 3 LH Eng is knackered, before it started costing money; and that an exhaust valve was sticking on the No 4 before it belted a hole in the piston. Those 2 examples paid for the device; let alone the benefits available if you also fit GAMIs and choose to run LOP (how about a 20 LPH per side reduction in fuel consumption at the same cruise speed and temps as at 100 ROP?)

Another OWT - reducing power just after take off. It is a BAD idea. Piston prop aircraft have an enrichening system in their throttle setup (both carb & Injection) so at full (take off) power, extra fuel is delivered to aid in cooling. By reducing power immediately after take-off, you are INCREASING temperatures and making the engine work harder.

[Highbypasss]

Best power for a TIO-540 is definately not 50 degrees rich of peak EGT. It's 125 degrees rich of peak EGT (PA31 POH). Also, just for your info. The company that I work for specifies 30 in MAP and 2200 rpm for cruise and 37 in MAP and 2400 rpm for climb. You don't get much "oversquare" than that.

A propellor is more efficient at lower RPM's. Reason; as a blade passes through an area of air, it leaves a wake, just like a wing. The faster the next blade gets to that same area of air, the less time the air has to smooth out again and therefore the next blade passes through more turbulent air. Just like an aircraft flying very close behind another aircraft would experience rough air and therefore be aerodynamically inefficient. The more blades, the more this is true. ie. a two blade prop would have to turn 180 degrees to get to the wake of the other prop. However, a 3 blade prop only turns 120 degrees before it's blades pass through the wake of the other blades, therefore it would be more efficient at a lower RPM, even if it's on the same type of engine. The highest possible (allowable) MAP therefore gives more "oooomff" behind the prop. More power, same RPM/fuel flow.

...ummmmmm..........get it.........?:confused: :8

Have fun......................................................... ........H:cool:

[Chimbu chuckles]

It would be interesting to see the reaction of that instructor if he sat in the rhs of my Bonanza and watched me flying along at 27/22 leaned to 50 degrees lean of peak egt?

Look down the graph Bevan posted and tell me what harm I'm doing my engine...answer?

NONE the engine loves it!!!

Chuck

[splatgothebugs]

Its painfull, what is the guy/girl doing checking you on an aircraft if they come up with a stupid comment like that. They obviously don't know much about that aircraft or aircraft engines in general.

The other thing that we are all forgetting is the fact that many of these gauges are very old and you are very lucky if they are reading accuratley all the time.

I am also enclined to agree with most of you, keep the engine running cooler and look after it because one day you may need it to run very hot for a while (E/F after T/O) and its then that you will be thanking your lucky stars for looking after that engine.

As far as reducing the power as soon as you are airbourne, remember the most common time for an engine failure to occur is on the first power reduction. I would rather be at 500' plus prior to that happening. :ok:

splat

[cjam]

Hey Woodend, the instructor didn't give me any speed for glide but I had previously read the speeds and weights that Hoss posted and I decided on 70kts for that flight.
What made you ask about that?
Looking at that graph makes me wonder about the stories about running engines too lean resulting in engine failures after time. On the graph running it lean of peak reduces egt, cht , and pressure, in saying that, it must be detonation that causes the p's and t's resulting in engine failures. Is that the basic jist of it? Run it as lean as you like as long as detonation doesn't occur?
If it is then, can you hear detonation?

[Bevan666]

Looking at that graph makes me wonder about the stories about running engines too lean resulting in engine failures after time. On the graph running it lean of peak reduces egt, cht , and pressure, in saying that, it must be detonation that causes the p's and t's resulting in engine failures

Detonation occurs at peak cylinder pressures (its the excess pressure in the cylinder which is causing the fuel to burn too fast). So detonation is going to occur at peak TCO in the diagram above, which is around 50 degrees rich of peak EGT. That is why its the worst place to run your engine. Detonation will also only occur when cylinder temps are also high, or obviously your engine would kill itself all too often as you reduce power/mixture through this region. Looking at the above diagram.. leaner than peak EGT means a cooler engine, and a cooler engine is a happy engine.

BUT

With stock standard fuel injectors, the fuel distribution across cylinders is not usual uniform. Running lean of peak EGT will result in an engine that does not run smoothly, as each cylinder is not producing equal power. The diagram above shows that power (HP) drops sharply lean of peak, so this unequal distribution leads to rougher running, than running rich of peak (ROP). With newer fuel injectors (like GAMI's) the fuel distribution is almost exactly right, so your engine will run smoothly no matter what the mixture is. With this uneven fuel distribution, you may have set 100 degrees ROP, but without an EGT per cylinder, it is hard to know what all is doing, so its wise at high power settings to set 100-150 rich of peak rather than 50.


Now, splatgothebugs said

As far as reducing the power as soon as you are airbourne, remember the most common time for an engine failure to occur is on the first power reduction. I would rather be at 500' plus prior to that happening.

This is also another myth. When is the most likely time for an engine failure? Its not takeoff, its in cruise. This is becuase most time is spent in cruising flight. Looking at real NTSB data on engine failures it has been shown that an engine can fail at any time, not on the first power reduction. Also the most likely cause of an engine failure is the absence of fuel (due to selection of empty tank or fuel exhaustion). So make sure you have juice in the correct tank and you cut out around 75% of the chance of an engine failure.

Bevan..

[Spotlight]

You have to love old wives tales. Common sense tells you that a metalurgy fault will manifest itself when the engine is working hardest.
What might cause a latent weakness to propogate most, full power or not full power?

There may be something to support the idea that the engine might fail due to ancilliary gear like throttle cables etc breaking with the strain of a power reduction.

[cjam]

Cheers Bevan,
I thought that running too lean was the main cause of detonation, that in combination with the high pressures you mentioned.
What I still don't understand is that people and magazines attribute engine failures to prolonged running at too lean a mixture, yet lots of people post that running lean of peak should be good for your engine.
Is it that un-even fuel distribution that you mentioned, is that it?
I'm not saying you are wrong but it is such an important thing to get right that I want as much info as possible.
The 206 manual says "25 degrees rich of peak below 75% power and best economy at peak, operation on the lean side of peak EGT is not approved"
Can you anyone explain why the manual states that running lean of peak is not approved?
Cheers, cjam

I just did a search of the Lycoming site and found this;

"The engine power (i. e. speed and manifold pressure) and mixture settings recommended in the Pilots’ Operating Handbook (POH) for a particular aircraft model have been determined by a detonation survey. These surveys use special instrumentation to detect and record detonation as it occurs. Based on these surveys, the detonation limiting conditions are defined. Data from the surveys indicate that detonation occurs in varying degrees; it is sometimes possible to operate an engine for relatively long periods in the first minor phase of detonation without inducing damage. Textron Lycoming does not recommend or condone engine operation which even approaches conditions which might cause detonation. The laboratory quality equipment used for the detonation survey is not practical for use in an aircraft engaged in normal flight operations. Without this equipment, the pilot may not know that detonation is occurring, and it is impossible to establish the fine line between the first phase of minor detonation and the detonation magnitude which induces preignition and/or engine damage. For this reason it is imperative that power and mixture recommendations of the POH be carefully observed."
It's clear from this thread that the poor old fleet is getting different mixtures from different pilots, some good points have come out of it though, I for one am learning when I thought I knew what I was on about!

I'm off to fly shortly, this flight I will use rich of peak because thats what the manual says, looking forward to being convinced otherwise on my return.
Thanks chasps, cjam

[Dale Harris]

Lean running CAN cause detonation, detonation is not only caused by fuel. It is possible to have detonation at full rich mixture too, although very uncommon. If you do not have the instrumentation set up for the proper monitoring process, DO NOT USE THE LEAN OF PEAK METHOD. Unless your POH says you can. A single EGT probe, and single CHT probe DOES NOT cut it as proper instrumentation, especially if they are untested original instruments. Further, the manual states, in your case, running lean of peak is not approved. Probably because during testing, as stated in the exerpt from Lycoming, that paticular engine and airframe combination did not test well for detonation, or possibly was not even tested at all. Only the airframe or engine manufacturer could actually tell you this I guess. As pilot of the aircraft, you WILL NOT hear this detonation at any stage.

[Jamair]

Do what the POH says, which may include LOP operations (like the PA31) but if you don't have a JPI or similar, and GAMIs, DON'T attempt to run LOP.

The standard gauges are usually 20+ years old and only read one cylinder or exhaust pipe, which may not be the hottest one. I have demonstrated for a non-believer, running one engine LOP and the other ROP, with the same temps on both sides, but a higher MP and lower fuel flow on the LOP side.

Anyone wanting to know the science, MUST read Deakins discussions based on fully instrumented test-bed engines.

Hi Chuck.

[Bevan666]

A full index of John Deakin's articles on engine management and other topics can be found here (http://www.avweb.com/news/columns/182146-1.html) .

Especially look at the series on "Where to run my engine".

Bevan..

[Woodend1]

cjam,

I've recently started flying 206s on ATOs and was curious as to the glide speeds you used as the person who checked me out didn't really give me a definitive answer. Cheers.

[Hudson]

Dogcharlietree. Should be no problem with the old radials - we invariably flew oversquare. Example: DC3 take off 48 inches manifold pressure and 2700 rpm for take off - 30.5 MP and 2050 rpm cruise - all ops normal. On base and final DC3 15MP and 2050 rpm. GA myths are like cockroaches - hard to kill and keep coming back.

[swh]

Just need to be careful with the older engine gauges, some of the only have a single EGT probe which may not be the most critical cylinder, you might have one that is optimum, but unknowingly running one far too lean. Jamair has the right idea, probes for each cylinder..

[Bevan666]

The risk is not running 'far too lean'. Far too lean will just mean the fire going out in a cylinder and no power being produced. The vibration alone will indicate something is not right.

The risk with uneven fuel distribution is running around peak EGT at high power settings, which cause the highest peak cylinder pressures and temperatures. This is the worst place to run an engine.

To quote John Deakin. This was about the large flat sixes, such as the IO-520/550 and the IO-540, normally aspirated.

The Dangerous Red Box

Just where is that "red box" I keep talking about? Some rough numbers, good (that is to say, BAD) for most of these engines -- these are "no fly zones," DO NOT set the mixture between them:



--------------------------------------------------------------------------------

Red Box = No Fly Zone
At and below about 60% power, there is no red box. Put the mixture wherever you want it.


At about 65% power or so, 100ºF ROP to Peak.


At about 70%, 125ºF ROP to 25ºF LOP.


At about 75%, 180ºF ROP to 40ºF LOP.


At about 80%, 200ºF ROP to 60ºF LOP.



--------------------------------------------------------------------------------


All those numbers are approximate! Please don't start splitting hairs, here!

You probably don't want to run your engine between those mixture settings. If you do, you are running very high peak pressures inside the combustion chambers, and that peak pressure is occurring too close to top dead center.

There's a chance you read too fast, and missed this very important point, so let me put it another way:



--------------------------------------------------------------------------------



Outside the Box
At 65% power, use richer than 100 ROP, or leaner than peak EGT.


At 70%, use richer than 125ºF ROP, or leaner than 25ºF LOP.


At 75%, use richer than 180ºF ROP, or leaner than 40ºF LOP.


At 80%, use richer than 200ºF ROP, or leaner than 60ºF LOP.



--------------------------------------------------------------------------------


(On most of these engines, with a properly set mixture at full rich, at sea level, full power, the EGT ends up at about 250ºF ROP, with some as high as 300ºF ROP.)


Bevan..

[cjam]

Bevan,
what would you do in my situation, POH and company SOP's saying do one thing, and a few seemingly intelligent and experienced people on the net saying that that is the worst place to run my engine.
I'm going to stick to the POH and SOP's until I can learn more but it is a little disconcerting hearing that the most dangerous place to run it is exactly where the POH says to.
How do you know you don't have mild detonation occuring when you are running lean, purely due to a lean mixture being more susceptable to detonation? Do those extra gauges you were talking about let you know?
Cheers, cjam

[Bevan666]

cjam,

If you are not paying for the fuel or the maintenance, operate the engine how the SOPs/POH says.

If you are paying for the engines and if the instrumentation in the aircraft does not allow LOP operations, then enrichen the mixture to 75-100 ROP not 50 ROP (This is how I operate the aircraft I fly (A36 Bonanza - IO-520BB) as it only has the old style single EGT and does not have GAMI injectors.

If you use the guidelines above, your engine will run cooler, have greater detonation margins and last a lot longer.

I wish everyone would get away from Lean == Bad == Detonation. Lean == Cooler Engine == Happy Engine.

Detonation is caused by high cylinder temperatures and high pressures (which are found near peak EGT at high power settings). Stay away from these zones at high power settings and wrong mixture settings (around peak EGT) and you'll have more than adequate detonation margins. (Remember there are some margins built into the engine design to protect against dumb pilots, on the larger engines, this margin is not so great, as they have been designed to produce more power than smaller engines.

How do you know how cool your engine is running - the CHT guage. I endevour to keep mine below 175 degrees C (350F) and at those temps detonation should not be an issue.

I believe John Deakin also wrote an article in there about detonation.. give that a read.

Bevan..

[cjam]

sweet, thats what I'll do then, I'll run at 75 ROP instead of 50, that will keep everyone happy, hopefully the plane too.
cheers.
PS Is a Gami injector similar to a Gami leg?

[dogcharlietree]

Hudson. Yes, this is what I was saying about some of us were taught oversquare all the time on radials.
In fact when I was taught (yes I know - tin hat's on and in the trenches ;-) we had our wrist broken if we ever came close to not being oversquare.
From base position on, we could bring the MAP back a little if required.
And pertaining to another thread re pitch full fine on finals, No we never did this. Not good for the engines or pax confidence.
Procedures for missed approach were;
Throttles - 30"
Undercarriage - UP
Pitch - Full Fine
Throttles - 48"
Retract Flaps in stages.

When gear is up and 100Kts - METO power.

Ah, after that, I'm just gonna go and put my "Round Sounds" CD on full volume............Ah, I miss a proper (round) engine!

[I'm gone!]

Gday all,

Splat,
It is an old wives tale also that engines fail more often on the first power reduction. Thats just when most instructors simulate the failure. Unless you brutally yank the pitch levers back and fling a counterweight of cutting your engine in half, you can safely reduce your power when you or the boss see fit.

Lycoming and Continental both have data to prove more donks go to pieces in the cruise.

The over square myth has been sorted by previous posts.

Cheers,
I'm gone!

[Eee Tee]

Could be interesting to get the opinion of an experienced LAME?

BTW, if you can get your hands on it, "Don't Baby Your Engine" is a great read.....

[Planned Root]

Eee Tee,

Is 'Don't baby your engine' a John Deakin article?

[I'm gone!]

Gday,

My first boss( a LAME ) on his first flight with me,(after watching me set flying school power in one of his C182's) said to me " you won't hurt the old sh!ts by gettin up 'em!" and promptly put on a couple more inches!

Cheers,
I'm gone!

[AerocatS2A]

On radials and over-square. Most (all?) radials are supercharged and so they are capable of quite high MAPs. Eg we used to do aeros in our Harvard at 30" and 2000 RPM and cruise at 26"/1600.

[GW_04]

Just read John Deakin's comments.
Its all explained very well here...

http://www.avweb.com/news/columns/182146-1.html

G;)

[avi8rgeorge]

Dont use over square power settings?

What a load of bull! The instructor who told you this needs to go and have a chat with their local LAME.

Dont forget, that the stuff about engine operations in your POH is approved by the engine manufacturer before its incorporated in to the POH when the aircraft is being certified.

As for reducing power after T/O. I havent seen 1 POH that says to reduce power after T/O, unless the aircraft has a time limit on MCP listed in the POH. If you reduce power at 300ft after T/O on most light twins they wont outclimb even the most average looking hills, with a full load in summer. If the book says you can use full power for an en-route climb USE IT! If it says you can use over square power settings USE IT!

I have been using full power for all en-route climbs in all of the twins i have been flying on charters over the years. I have never had an engine overheat or misbehave in any sort of way - even in the middle of summer with the OAT showing 40 degrees plus outside. If you operate the aircraft according to the book, you will get the right performance and you wont hurt it in anyway.

I also teach my students not to reduce power after T/O, and to use over square settings on the company Partenavia and Arrow if they are required to be used and the book says they can be used.

[scrambler]

Interesting topic.
Was taught in normally asperated engine "if book figures are not available don't oversquare"

big fan of lower rpm.

work it out. 200 rpm = 12 000 extra turns per hour, 1.2 million extra every 100 hours and a staggering 20.4 million extra revalutions of the engind over a 1700 hr lifetime. Thats alot of wear and tear to save by running a lower RPM.

[Not_Another_Pot]

This old myth is still flying around:ugh:

Our POH (IO540) allows 26" (+ for OAT>ISA) and 2200 rpm and we use it as often as poss.

RTFM, it tells you all you need to know.

NAP

[Continental-520]

What I don't get, is why two aircraft with the same engine and prop can have differing power settings for certain conditions despite having the same engine and prop. Example, a Bonanza I used to fly used 23"/2400RPM for the cruise. A Baron I've been flying recently uses 24"/2300RPM for cruise. Both figures as per the respective POH and both aircraft have same engine and prop.

Why so?


520.

[Chimbu chuckles]

Conti 520...and others,

The big problem with talking power settings is people don't understand what the terms MP/RPM/Mix really mean.

24/23 or 23/24 are NOT POWER SETTINGS.

Manifold pressure is simply an indication of how much air is available for combustion....my Bonanza is sitting in a hangar at Redcliffe, the MP gauge says 29.5 odd inches...is it developing full power?

The throttle controls how much air the pistons can suck in on the intake stroke.

RPM is certainly the speed that the prop is turning but it, in a direct drive engine like most aero engines, it is also an 'indication' of how fast the pistons are travelling up and down, and therefore how much air they are sucking into the combustion chambers, and to a lesser extent how fast the engine driven fuel pump is turning.

What RPM is not is an indication of power/thrust. If I climb overhead YRED and pull the mixture control to cutoff and lower the nose enough the RPM will remain at whatever it was....but it's not producing any thrust/power.

Mixture controls the fuel side of the fuel/air ration while MP & RPM control the air side. Fuel will only ignite and do 'work' over a reasonably small range of fuel/air ratios. Too rich and it wont burn...too lean and the fire is snuffed out as well.

At a specific ratio all the fuel and all the air is burnt and the maximum work is done..cant remember the word (stociometric?) off the top of my head. Imagine that fuel/air ratio plotted on a graph of fuel/air ratios....it's the pinacle of a parabola...leaner down one side/richer down the other (you can see a depiction in JD's articles)

That point is essentially, (although not exactly because it's almost impossible in the real world to get perfect burning), 100% power but at ANY mixture setting either leaner or richer you can get any other power setting between 0 & 100%.

What has always stopped LOP in the past has been crap injectors. Balanced injectors tailored to specific cylinders balance the fuel air ratios across the cylinders and the engine then runs smooth at almost any mixture setting therefore opening up the other half of the mixture spectrum that has been almost impossible to achieve since flat engines were invented...on radials LOP was normal but EGTs were not invented so it was not called LOP.

So to 'oversquare'.

On the vast majority of piston engines there is absolutely NO limitation on MP/RPM...MCP being usually full throttle/2700rpm.

Closing the throttle is materially exactly the same as flying with a blocked air filter....how many of us do that knowingly?

The correct RPM is whatever RPM you want and that only varies with what you want to do...high/low/fast slow etc.

Changing RPM can be likened to changing gears in a manual car.

The red mixture knob/lever is the only control that can vary power from o% to 100%.

With standard ****e injectors you can only make that variation on the rich side of peak 'cause the engine will run too rough because the fuel/air ratios will be so out of whack that you have essentiall 4 or 6 single cylinder engines all running at different power outputs...no wonder the thing shakes like a dog passing a peach seed.

On the rich side of peak and at full throttle you have lots of both fuel and air so you must run very rich to control CHTs or reduce the air so that you can reduce the fuel flow to reach a new 'power' setting that doesn't cook the engine. Other than a small influence on the speed of the engine driven pump RPM is immaterial.

Bottom line is you're cooling the engine, partly, with extra fuel.

On the lean side of peak you are cooling the engine with excess air but the same range of power setting is available (at MSL/ISA-- turbonormalising fixes that nicely)

I get 75% power in my Bonanza at about 27in/2200rpm and 50 odd degrees LOP EGT (roughly 50 liters/hr)....or I could get it at about 23in/2400 and 80 odd ROP EGT and around 65 liters an hour.

The first way gives me cooler CHTs, slows down the rate of combustion thereby widening the detonation margins, lowers the peak pressures in the cylinder and saves nearly $20 bucks an hour in fuel.

If you aren't taking into account the mixture you are not talking power settings...you can get essentially any power setting at any combination of MP/RPM...what you do with the mixture is the important bit.

Oversquare being bad, like setting 25/25 right after takeoff is flying school horse****e!!!:ok:

Chuck.

[Continental-520]

That has to be by far one of the most thorough and well layed out explanations of the engine instrument indications and their meanings/relationships I've ever read. Thanks be to Chimbu chuckles for that - I'm sure I'm not the only one who managed to understand it a little better.

Perhaps you can also tell me why in some aircraft (eg some C206 that I've flown) there is a small sticky label on the panel which appears to have been there since new that reads "Do not exceed 22" MAP when below 2000RPM" or something like that...(?)


Cheers,


520.

[Dale Harris]

Probably a vibrational or detonation limitation on the airframe/engine combination. That could be as a result of testing, or a result of lack of testing. There are other airframe/engine combinations out there that have similar limitations, not always placarded. That's why it pays to RTFM.

[Chimbu chuckles]

Conti,

Once again a label like that doesn't really give enough information to decide why it's there without some knowledge of what's going on inside the combustion chamber during a combustion event.

I agree it's probably to do with detonation margins for the following reasons.

All piston engines on aircraft have fixed timing, via the magneto.

The spark is timed so that it goes off at a specific point in the compression stroke BEFORE the piston reaches TODC. This point is based on a very narrow range of RPMs so that the peak power pulse (maximum pressure in the chamber) occurs after TODC to apply the maximum rotational leverage on the piston. ie it pushes as efficiently as possible on the piston to convert as much 'work' into rotation of the crankshaft as is possible.

The fuel air mix doesn't just explode BANG and all burn instantly it takes time...albeit not much time :D

The spark might happen 16 degrees before TODC and the maximum 'push' happens 12 odd degrees past TODC. At the point where the fuel/air mix is burning the hottest and applying the maximum force the piston is moving away at an accelerating rate so pressure is quickly dropping in the combustion chamber....hence combustion peak temps, for a fraction of a second might be 3000F but it (very) quickly cools to the 1650 odd snap shot that the egt probe reads as the exhaust valve opens.

Now the RPM range that keeps this relationship right is around max RPM to gve the widest possible detonation margins on takeoff.

Any reduction of RPM means that the piston will be closer to TODC when the combustion event is at max, the combustion chamber is smaller and the piston is not moving away as quickly due to the leverages involved with the piston rods/crankshaft geometry required to change linear motion (piston up/down) to rotational (crankshaft round and round).

So LOTS more heat and LOTS more pressure.

With MP over 22in (in this hypothetical) there is lots of air avail for combustion and if ROP lots of fuel too...LOTS & LOTS of heat/pressure.

ROP means reasonable amounts of unburnt fuel gumming up the system with carbon. Carbon deposits can be heated up by the extra heat/pressure and cause pre-ignition.

Pre-ignition is when the fuel/air mix ignites prematurely (maybe 20 degrees before TODC causing more of the above dramas with pressure/heat which can, probably will, lead to detonation...which may, probably will, blow the engine to bits....quickly.

The pre-ignition/detonation could also be caused by a bit of broken helicoil poking into the combustion chamber being heated up to very high temps by the above high pressures/temps...igniting the fuel/air mix early with the same results.

But we need heat to cause this.

If LOP the extra fuel is not there...the excess air is cooling things...the lean mixture burns slower so the piston is further past TODC at max pressure...no carbon buildups to get hot...less chance of preignition from either carbon/broken bits...lower temps/press all around mean detonation is almost impossible...certainly dramatically less likely.

So once again if we are not talking about the mixture we are not armed with enough information....that limitation, and I've not seen placards like that, ceases to be a problem 1/. LOP or 2/. when armed with knowledge.

Chuck.

Edit to clarify pre-ignition/detonation.

When the mixture is ignited early by pre-ignition it burns a lot quicker than the controlled flame front resulting from normal ignition (it truly explodes rather than burning very quickly)....I guess hotspots/overheated helicoils are like having multiple sparkplugs firing off all over the cylinder walls. Clearly that makes the situation reference piston position vs TODC even worse driving pressures/temp quickly through the roof which then causes detonation where the fuel/air mixtures just explodes (as opposed to burning) as soon as it enters the combustion chamber....within seconds it becomes self perpetuating, followed sooner rather than later to departure of effected cylinder heads.

[Jamair]

Chuck - I wus just gunna say the same thing........;) :ok:......How's the 'big mutha' goin - don't need none of those fancy JPI thingies for engines what rotate instead of going backward & forward hey?

[Chimbu chuckles]

re the big mutha.


If it aint Boeing, I aint going!!:D

I have only two 'complaints' about the 767 so far.

1/. You can't slide the seat away from the control column far enough(before it moves outboard) to comfortably have your food tray on your lap...if you're a tall b-u-g-g-e-r such as we.

2/. A major design flaw in the fuel system...too much freaking fuel (72000+kg). :E Makes for some very long sectors :(

Lovely machine to handfly...not that we get much op for that. It does really nice aileron rolls at very low level down the runway (in the level 5 sim :D) A couple of the checkies managed a lovely stall turn the other day....but ripped the wings off recovering from the verticle exit dive :}

Got those wingovers down pat in the Truck yet?:E

Chuck.