High mp & low rpm. A bad thing?

[LowNSlow]

A comment on another thread has prompted me to ask this question:

Is running an engine "over square" ie higher than "normal" manifold pressure and lower than "normal" rpm bad for the engine if there is no detonation and the cylinder head and oil temperatures are in the green arc? ie: instead of 24/24 (24" of mp and 2400rpm) say 26" mp and 2200 rpm.

My, admittedly dubious, memory seems to recall that Mr. Lindberg (he of Spirit of St. Louis transatlantic flight fame) taught this technique to P-38 ferry pilots during WW2 as too many of them were having to ditch in the Pacific whilst delivering their aircraft to far flung islands.

[Timothy]

The POH for the aircraft will give the answer for this. It is almost invariably OK to operate somewhat "over-square," but there are usually limits set. I can't remember exactly what they are for my Aztec, but something like 26"/2200.

It is more of an issue in a turbocharged engine, where it is quite easy to "overboost" if automatic wastegates are not installed.

The "don't go oversquare" mnemomic is there to stop people going mad, as a sort of rule of thumb, but it's very over cautious.

I was reading in another forum about someone who had managed to get through a twin rating and a twin IR without ever finding out what the black, blue and red levers actually did (just set the numbers I tell you, boy, and don't ask silly questions.)

The oversquare rule works well for such people :)

W

[Miserlou]

Check your engine manual for the settings and fuel consumptions.

The effect of running 'over square' is a slightly higher internal pressure but as long as the setting you use is a published setting then the difference in engine wear and life is negligible.

The real consideration is your type of flying and method of payment. If you paying per tacho hour then you'll want as low an engine speed as possible. If you pay by the clock it doesn't make any odds whatever setting you use.

[Flyin'Dutch']

Well it does make an appreciable difference to the amount of din in the cabin and the ground too!

FD

WCollins; don't you think that is a bit of an urban tale?

[Timothy]

FDWCollins; don't you think that is a bit of an urban tale? Um, which bit? If you mean the bit about the untrained pilot, it was on the PPL/IR forum, which I believe you are on, if memory serves? See below:

Could someone please explain to me, in simple terms (I only have a degree in mechanical engineering) the relationship between the black lever and the blue lever in the aircraft I am being trained on that has a variable pitch propeller.

I have asked my instructor (and several others) to do this and they do not seem capable. Their answer is that I am being too technical and should just learn and memorise the settings for various stages of flight. I think this is all they can tell me because this is all they know.

I want to know the point of having a lever that changes the manifold pressure (inlet or exhaust?), does it not just change the pitch of the propeller? I hope someone has had a similar mental block and has a simple explanation for the system.

If not can you please refer me to a publication or article that explains the subject.

W

[IO540]

Is running an engine "over square" ie higher than "normal" manifold pressure and lower than "normal" rpm bad for the engine

That is a tale probably dating from before WW2. Taken in isolation, MP and RPM are in different units (inches and rev/min) so it makes no engineering sense to say that MP must not exceed RPM/100.

Your engine operating manual (which you may have to purchase from Lycoming etc) should have full details of what you can do. For example on one particular version of the IO540 the MP should not exceed RPM/100 if the RPM is below 2300, but above 2300 there is no limit on MP; in practice you would be pushed to exceed 28" without a turbo :O

[GT]

I was told that this over-square stuff was way back from the earlier days of the big rotaries, where it did have some relevance.

What about today's turbo charged engines, e.g. 34"/2400rpm, 30"/2200rpm? Can't get much more over-square than that! Also, if you want to keep the fuel consumption down, keep the rpm down.

Regards, GT.

[Timothy]

That is a tale probably dating from before WW2. I am surprised that constant speed props were widely available before WW2. The earliest Spits had huge, fixed pitch, two bladed jobbies in 1938, replaced in 1939 with two position (coarse and fine). I don't think that the MP/Revs distinction came in until the MkII in 1940 (says he, looking at the Pilot's Notes for the Spit Mk II and the Hurricane MkII.) I doubt if private aircraft were ahead in this technology.

I apologise if I'm wrong.

Sorry, I'll write out 100 times:
I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick,I must not nit-pick ;)

W

[LowNSlow]

Thanks for all the info chaps. The question was out of curiosity as my Auster doesn't even have carb heat or a mixture control :ok:



GT I think you mean radials rather than rotaries by the way!

[Flyin'Dutch']

WCollins,

Sorry to be a bit slow in replying.

Took me a few moments to compose myself.

It puts a complete new slant on the old adage: 'there is no such thing as a stupid question'

Well spotted re PPL/IR but as you know no qualifications at all required other than the ability to type one's name and to part with £30!

:D

FD

[FlyingForFun]

Wasn't going to reply to this, since everyone else has answered it so well already - but since I suspect it was my comment that sparked off the thread, I thought I probably ought to!

My comment was:If you pay by the tacho hour, you'll be best flying at high MP and low RPM. Which is interesting, since that's not exactly an economical way to fly in terms of engine maintenance costs!(Italics added to emphase the bit which I think LowNSlow is probably refering to)

Implicit in my comment about "high MP and low RPM" was that you must stick to the numbers in your POH, and I hope everyone would do that anyway. If the numbers in your POH allow you to "over-square" then that's fine! As at least two others have already said, since the RPM and MP are in completely unlreated units it's only a coincidence that the numbers (if you divide the RPM by 100) are close enough that it makes a handy rule of thumb.

It will be generally true that the higher the MP and the lower the RPM, the more engine wear will occur. The POH max/min figure isn't some magical number below which no damage will occur, and above which everything will go Bang - it's a sensible compromise which gives you the flexibility to use many different power settings during flight, none of which will cause an unacceptable level of wear.

Personally, if I owned an aircraft with a constant speed prop (which I don't) or even flew one regularly (which I also don't), I'd be looking to fly in the middle of the acceptable range, where I'm not using loads of fuel, and not putting the maximum allowable strain on the engine.

My Europa's prop is variable pitch, but not constant speed, so once you set the pitch it behaves like a fixed-pitch prop. It is certified for all power settings at all prop settings (as long as you don't overspeed the engine, of course, which is a real danger if you cruise around at a fine pitch), and although I usually cruise at full-coarse to save fuel, I always fine the prop for take-off, climb, slow-cruise, etc because I can't see any point in straining the engine more than necessary.

FFF
--------------

[BEagle]

We often used to fly the Bulldog at 2400 rpm and 19" MAP with 'best power mixture'. Very uneconomical, but best engine response.

However, I once went in formation with 9 others down to St Mawgan from Abingdon with quite a strong westerly wind. They all went at 2400/19+BPM; however, I pulled the prop back as far as I could and pushed up the MAP to just below the (RPM/100) + 4.5 limit and leaned off the mixture without cooking the CHT. It made a huge difference to the fuel consumption and I landed with a lot more fuel than the others. But it made it much more difficult to fly in formation as the throttle response was so much poorer!

[LowNSlow]

Well spotted FFF t'was indeed your comment that sparked off my question. It's all academic for me as I haven't flown with a wobbly prop since flying a Twin Comm eons ago. The only way my current prop would vary it's pitch is if I taxi it into the hangar door :uhoh:

[IO540]

FFF

It will be generally true that the higher the MP and the lower the RPM, the more engine wear will occur

I am not at all sure the above is true. It depends on the degree in which engine wear comes from torque (and MP is basically a cheap and crude way to measure torque) or from movement. Or from other things.

There are loads of engine parts which don't see torque; e.g. the rear gear train, cams, pushrods, valves etc. The wear on these will be mostly rpm dependent. The parts that see torque are e.g. the crank and conrod bearings. As regards to cylinder bores, rings and pistons... I suppose there is an element of torque related wear there.

Higher MP and lower rpm is better for efficiency (less friction losses, less pumping losses, the more open throttle means even less pumping losses). But if you want best economy, the engine timing must be optimised for whereever it is being operated, and the sort of engines we have are pretty well pre-WW2 technology and while they are probably quite efficient at a particular operating point, one doesn't know where that is.

Also it is stated by many "experts" that on a typical flight a lot of the engine wear takes place during startup, before the oil has got around it properly. And what about oil changes?

The best economy gains come from LOP operation, and that tends to imply more MP to get the same power (because the fuel flow rate is lower). Most owners of well instrumented planes fly LOP nowadays.

Also a huge factor in engine life is how often it runs. Flying school engines tend to make it to TBO, despite the abuse they get. A typical PPL-owner-pilot plane might sit in a hangar for 3 months each winter... lots of internal rust! He will be lucky to make half TBO. I know of one TIO540 engine, 800hrs TT, which is completely knackered and rusty inside, having been sitting outdoors most of the year and only travelling to airshows in the summer.

So it's a complex picture, I am sure. Personally, my feeling is that if we are talking about operation at 2200rpm versus 2400rpm, and varying the MP to achieve the same HP in both cases, any difference to engine life will be far less than that caused by the other factors.

[bluskis]

Many of my car engines sit for extended periods, where they may be subject to rust, they don't suffer the problems an IO540 from Lycoming suffers. I have come to the conclusion that Lycomings are as much a bunch of rubbish as Continentals were before.

As for Lycoming rebuilders, they should be subjected to safety authority investigation.

It does not matter much how you manage these engines in accordance with the books, they are not fit for the purpose they are sold for.

What engines are fitted in French airplanes?

[IO540]

bluskis

Sadly I think your comment applies to a lot more than just aircraft engines. It applies to most avionics too. I believe it stems from the fact that through the ages, anyone with a plane was expected to have a big cheque book and a well developed reflex for extracting it from his pocket without asking any questions... so the manufacturers have never had much incentive to improve, and they've got the "certification cost" excuse (false in most cases) to conveniently hide behind.

Every engine shop I have ever spoken to (except the authorised Lycoming distributor) thinks that present-day Lycoming QA is c**p. I've just bought matched fuel injectors for mine; there is a very noticeable reduction in engine vibration. This is on a £40,000 engine...

[LowNSlow]

Diverting the thread slightly (hey, I started it :D), why do aircraft engines have a greater tendency to rust than car or bike engines?
The Cirrus engine in my Auster has apparently got some corrosion on the exhaust valves resulting in low compressions. This has, apparently, been caused by the aeroplane sitting around for a month every other month. I find it strange that my old BMW which sits around for the same length of time as the Auster doesn't suffer from the same problem.

Aero engine camshafts seem to suffer from corrosion but the old BMW doesn't. Why? The BMW cams are at the top of the engine and the oil runs off them same as in the Cirrus. I can only put it down to the oil used in aero engines being inferior to the multigrades used in car engines in terms of protecting the internal surfaces of the engine from corrosion. AeroShell have a new multigrade out, I'll have to check it out and see if it's suitable for the Cirrus.

[GT]

LowNSlow,

Oops, I did mean radials, didn't I! Thanks for pointing out the slip.

GT.

[KitKatPacificuk]

Going back to the Blue and black levers.

Two aircraft I fly regularly is the Cirrus (SR20 and 22) with a constant speed prop and no prop lever, so It does it for you easy.But more confusing is the Yak 52 which has the 2 levers both yellow but the RPM is in percentage and MP is in centimetres. So you might be running at 70 percent on the RPM and 5 on the MP. So who knows what square is? some Russian probably!!

[Davidt]

Go to;
www.avweb.com

Look up John Deakin's articles on engine management,
best explanation of manifold pressure and wobbly props I've come across yet.