Why the rev drop?

[old man emu]

I’m now wondering if the Jabiru engine designers slapped themselves across the head at any stage and went “that’s why they put them on opposite sides” “Bugger, I better make up a good sounding engineering reason why mine are on one side"

 

They were probably aware of the reasons for positioning spark plugs where they are, but there was probably more to it than meets the eye. What is the width of the combustion chamber - or simply the bore of the cylinder? Maybe the calculations showed that there was little or no power gain if the plugs were apart by a lot more. That calculation would impact on the engineering design of the heads, and that would ultimately impact on the manufacturing costs.

 

So maybe the engine designers had both engineering and manufacturing reasons why they did what they did. In the end, it is the final cost of production that determines engineering decisions.

[kgwilson]

Redundancy is the only real reason for dual ignition. The Jabiru placement means very easy maintenance and checking and the design of the head with placement of valves gives little choice of location. The over square engine argument is rubbish. Numerous car engines have over square engines and single ignition. Efficiency is a distant second reason. Most aero engines are way less efficient than modern car engines.

[Flightrite]

Redundancy is the only real reason for dual ignition. The Jabiru placement means very easy maintenance and checking and the design of the head with placement of valves gives little choice of location. The over square engine argument is rubbish. Numerous car engines have over square engines and single ignition. Efficiency is a distant second reason. Most aero engines are way less efficient than modern car engines.

 

Nicely put?

[derekliston]

100% still the number 1 reason would make sense. I’m wondering if anyone has a story of having to switch after take off to avert an emergency landing.

 

Second reason in the case of the spaced spark plug engines you’d think would have to be a say 5% increase in efficiency. That would add up in commercial flying.

 

I’m now wondering if the Jabiru engine designers slapped themselves across the head at any stage and went “that’s why they put them on opposite sides” “Bugger, I better make up a good sounding engineering reason why mine are on one side"

When I was a working LAME with a flying school I used to advise instructors going on a cross-country and landing away in a PA38 to take the requisite spanners to remove the spark plugs. The Lycoming 0-235 with one plug on top of the head and the other beneath and running on 100LL had a big tendency to fill the bottom plugs with lead and hence a major mag drop. If it wouldn’t clear on an engine run then the simplest thing was to swap the top and bottom plugs over and the lead would run out and mag drop cleared. Don’t have that problem on Jabiru engines!

[facthunter]

There's very little increase in efficiency or power with having two plugs . Central in a 4 valve head is an optimum position Poor location and/or combustion chamber shape will require more advance. The main advantage of a larger bore is larger valves permitted and therefore better gas flow possible.

Running on one magneto where two is fitted is for limited time. In the Douglas DC 6's the F/E had an ignition analyser and usually any engine with any ignition problems had the engine shut down in flight..

I've flown some pretty ancient stuff (pity there wasn't more). By about 1913 magnetos were well made and reliable.

Some mag failures require one mag to be earthed (rendered inactive) if certain types of failure occur ie timing slipped. if you (must) continue flight.

Most of the trouble was water condensing in the tropics. Blowing out the Mag (s) with compressed Dry Nitrogen overnight was standard practice where I went.

Like everybody, I've had a few times where the rev drop was more than permitted so I've had it rectified except for a time when a short ferry was needed with the plane lightly loaded, to a repair location. You need antiseize and the correct tension when installing spark plugs. Nev

Edited April 21, 2020 by facthunter

[derekliston]

There is one mark of Lycoming 0-320 that mechanics used to call the K mart engine, I forget which one, however it had a dual magneto which was two magnetos with a single drive. Could never see the sense in that, lose the drive and you’ve lost both!

[Flightrite]

Lyc 0320 H2AD, 'H' stands for horrible, they had the silly idea of a dual mag single drive, IO540's (some models) likewise!

[facthunter]

Bad idea The RR Merlin's the same. Has a single skew gear in the drive shared by both mags. Nev

[Geoff_H]

Sure that both mags on a single drive has less reliability. But the loss of reliability is so low that it's minor. Mooney's J series have Lycoming engines in this configuration. You can get a dual conversion kit, if one gear goes, it will probably destroy that drive and all drives will be cactus. How many Mooney's have come down owing to the mags? None that I have known, I elected to overhaul the existing one. If you want to increase the reliability of your aircraft with such little increment in reliability buy a twin.

[old man emu]

If you want to increase the reliability of your aircraft with such little increment in reliability buy a twin.

 

Then you will have four magnetos for the gremlins to pick from!

[facthunter]

They have KNOWN problems. In aviation, I try to eliminate known problems where possible. Engine accessory drives often have quills that fail rather than damage other components. Yes in plenty of circumstances more than one engine is justified on safety grounds. Not flying it properly can cancel the advantage of having extra motors. Nev

[Yenn]

The Lycoming H2ADhad a D4Rn-3000 impulse coupling dual magneto nd ran to 2700rpm. As far as I know there were no other 320 engines with that mag setup.

[Geoff_H]

My project has two engines. Each can have two spark plugs, I have chosen a single on each engine to keep the weight down.

[Geoff_H]

During my career I did a lot of failure probably studies. The surprise to me was that if a human was responsible for preventing a failure then the failure probability of the human was 0.5, limit switches were 0.0001. in other words a human is the weak link, making a wrong decision every second time.

[facthunter]

Wrong decision every second time? I know to ERR is human but no manned plane would fly anywhere if that was the case. That's random, toss a coin performance..Nev

[Geoff_H]

It's what the requirements are for failure probability. I tend to agree with it. It's about a decision required to stop a catastrophic result. It's an Australian standards thing.

[facthunter]

I cannot understand in what context you would use a comparison of a limit switch with a human brain. Surely NASA would be involved in such considerations and all aircraft manufacturers, instruments presentation and FMS logic designers. Nev

[Geoff_H]

When you are determining the safety of a system for controlling some piece of apparatus you have to look at each part of that system and them calculate the probability big failure. Should the probability be low then a physical inspection of the system is required at a frequency that is significantly shorter than the failure probability. If for example you have a control system that keeps the altitude constant and uses a human that can only see an altimeter, nothing from outside then you have to put 0.5 as the probability of his failure. If an electronic control system is used to keep the altitude then the system usually calculates a fairly low probability of failure. So an inspection/testing regime will be developed. Hopefully an inspection is calculated to be greater than 100hours.

I did a lot of work with TMR (tri modular redundant) system uses three computers checking each other and all three must agree to execute a control function. These systems of have a reliability of one failure every million operating hours. I designed these systems for gas turbines.

[Flightrite]

When you are determining the safety of a system for controlling some piece of apparatus you have to look at each part of that system and them calculate the probability big failure. Should the probability be low then a physical inspection of the system is required at a frequency that is significantly shorter than the failure probability. If for example you have a control system that keeps the altitude constant and uses a human that can only see an altimeter, nothing from outside then you have to put 0.5 as the probability of his failure. If an electronic control system is used to keep the altitude then the system usually calculates a fairly low probability of failure. So an inspection/testing regime will be developed. Hopefully an inspection is calculated to be greater than 100hours.

I did a lot of work with TMR (tri modular redundant) system uses three computers checking each other and all three must agree to execute a control function. These systems of have a reliability of one failure every million operating hours. I designed these systems for gas turbines.

 

Oh that's way to difficult to think about, life can be made simple or hard, your choice???

[Jase T]

Best thread i have read on here in a long time!!!!

[Geoff_H]

Oh that's way to difficult to think about, life can be made simple or hard, your choice???

I find it very interesting, fun and not so difficult. I have retired now but still dabble.

[Flightrite]

I find it very interesting, fun and not so difficult. I have retired now but still dabble.

Ive retired too, time to let the poor old brain cells (what's left) have a rest, gardening, chasing women etc, no brain power needed there?

[Jase T]

In the Douglas DC 6's the F/E had an ignition analyser and usually any engine with any ignition problems had the engine shut down in flight..

I've flown some pretty ancient stuff (pity there wasn't more). By about 1913 magnetos were well made and reliable.

Some mag failures require one mag to be earthed (rendered inactive) if certain types of failure occur ie timing slipped. if you (must) continue flight.

Most of the trouble was water condensing in the tropics. Blowing out the Mag (s) with compressed Dry Nitrogen overnight was standard practice where I went.

Like everybody, I've had a few times where the rev drop was more than permitted so I've had it rectified except for a time when a short ferry was needed with the plane lightly loaded, to a repair location. You need antiseize and the correct tension when installing spark plugs. Nev

 

 

I am thinking a night telling stories with you over a few bottles of wine would not be wasted time!!!!!!

[Paul davenport]

There's very little increase in efficiency or power with having two plugs . Central in a 4 valve head is an optimum position Poor location and/or combustion chamber shape will require more advance. The main advantage of a larger bore is larger valves permitted and therefore better gas flow possible.

Running on one magneto where two is fitted is for limited time. In the Douglas DC 6's the F/E had an ignition analyser and usually any engine with any ignition problems had the engine shut down in flight..

I've flown some pretty ancient stuff (pity there wasn't more). By about 1913 magnetos were well made and reliable.

Some mag failures require one mag to be earthed (rendered inactive) if certain types of failure occur ie timing slipped. if you (must) continue flight.

Most of the trouble was water condensing in the tropics. Blowing out the Mag (s) with compressed Dry Nitrogen overnight was standard practice where I went.

Like everybody, I've had a few times where the rev drop was more than permitted so I've had it rectified except for a time when a short ferry was needed with the plane lightly loaded, to a repair location. You need antiseize and the correct tension when installing spark plugs. Nev

Nev if you can remember back to 1913 how old are you

[facthunter]

I work on magnetos older than that year, and I know how well they are made. I'm prepared to fly any old plane or replica in the right environment. Not over steep wooded slopes, shark infested water or the suburbs with powerlines everywhere. or if it's shoved together any old way by a "she'll be right, attitude" aero person.

So far that's worked.. No one has been scratched or a plane damaged except when something self destructs "up there". Nev