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



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 . 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

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

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.

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.........?

Have fun......................................................... ........H

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.

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

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

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.