magnetos

[rans6andrew]

a non flying friend (he did have some lessons many years ago) was questioning me about ignition systems on my aircraft, I have two stroke and 4 stroke Rotax engines in my 2 aircraft. His concern was about the reliability (or lack of) of magnetos fitted to aircraft engines and I said that the systems fitted to Rotax engines are basically fit and forget. Not so, he said, the systems fitted to Lycomings and Continentals in GA aircraft. Every 2 or 3 seasons.........

So what does go wrong with them? I have heard stories about impulser faults and, presumeably, the points are as prone to wear, adjustment and fatigue as they were when cars had points ignition systems.

Of course the Rotax circuits don't have any points or moving parts and seem to be much better for it.

Rans6...

[Johnm]

Magnetos have the great virtue of being independent of any electrical system but that is their only virtue.

[IO540]

Mags (especially the single shaft dual mag types) are probably the weakest link (statistically) on an otherwise highly reliable Lyco/Conti engine, but they are fine if you follow the 500hr overhaul periods.

As Johnm says, they do not require external power and that is a really super safety feature.

Electronic ignition would be more accurate, of course, but aviation history is not exactly overflowing with reliable electrics and electronics - I think because very few people with a brain want to work in a company which brings out a new product every 20 years or so :)

[Pace]

They Rust Up Inside

[Genghis the Engineer]

Just about every fault I've ever picked up on a "magneto check" has been plugs, usually fouled.

G

[Aerozepplin]

plugs, usually fouled.
The analysis I've done suggests that spark plug failure or fouling is a far more serious problem than most people think. That is, that a number of power loss related forced or precuationary landings are plug related. A very good reason to learn sound engine handling.

[englishal]

Never had a problem with the mags other than straight after overhaul! After the overhaul we broke down twice as the impulse spring stuck and we couldn't start the engine on the ground. It was very annoying and the second time we were away from home and by the time we got going we were racing sunset to get home.

A little trick from our maintenance organisation - wind the prop back twice then forwards, which appears to have released the spring and got us going. NOTE: They say DO NOT try this on a Continental engine as it will shear the vac pump shaft apparently.

Yes there are some good articles by John Deakin (search Avweb) about leaning on the ground, in fact he advocates doing the run up with highly leaned engine as this will tell you far more about the state of the mags and plugs than at full rich.

[Rod1]

I have had 1 ½ mag failures in the air and just made it to a runway run by a parachuting club (Scotland) before I ran out of height.

The Rotax system is very good as it uses discharge capacitors, and providing the crank is rotating at 300rpm is independent of the conventional electrical system. The system is a fraction of the weight of a mag and does not have the 500h maintenance program, being totally maintenance free.

Rod1

[Tmbstory]

I remember the Tiger Moth magnetos and the need to give them a small tap with the control column when they needed it.

Tmb

[A and C]

Most plug fouling on Lycomings is due to the engines not being shut down in the approved way..........................in short poor trainning.

[IO540]

Plug fouling is mostly caused by taxi without leaning the engine. I have not had a fouled plug for about 5 years, since I started doing that, religiously.

Whether iridium plugs help I am not sure. I have been using them for longer than that, and after 1000hrs they show no sign of wear.

[hatzflyer]

You either had the mags in a funny place or an extremely long and wierd shaped control colum if you could hit the mags with it !

[Tmbstory]

Hatzflyer:

The control column in a Tiger Moth could be taken out of it's normal position and used for many things.

Tmb

[172driver]

The good news - only had a mag fail on me once.
The bad news - this happened in Birdsville, QLD, Australia :{

If you're unfamiliar with Birdsville, have a look at Google Earth....

A/c was a 210 and yes, the mags just had their 500 hour check done :eek:

[IO540]

That's why I usually get mags overhauled in the USA (http://www.qualityaircraftaccessories.com/)

I bought a spare magneto which lives on the shelf, so the overhaul is not time-critical.

[Johnm]

I've had one mag failure on the ground and one in the air, which was at almost exactly 100 hours post engine overhaul and the points actually snapped in flight. The only indication until power checks when safely on the ground was a slight drop in revs and a significant increase in EGT

[stickandrudderman]

P mag substitutes the original mag for electronic ignition with advance/retard and has a built in generator for it's own power supply.
By all accounts they significantly improve performance but AFAIK not for CAA approved use though.

[smarthawke]

Mags on the whole are very, very reliable. And, sorry to explode the myth but the Bendix single drive, dual mag is just as reliable as any 'normal' mag. Okay, there is potential for a big problem if the single drive fails but can anyone quote an example of this actually happening?

500hr inspection is just that - an inspection. It may show up worn points, cam or other parts. What it doesn't do is tell you how long the ignition coil is going to last. Much the same for an overhaul. Don't be fooled into thinking it's a guarantee of 500 hours trouble free future use...

Impulse coupling springs do go weak and are often the cause of intermittent starting problems - impulse disengages at cranking speed but 'clicks' when turning the engine over by hand, just to put you off the scent. Gipsy Major mag tapping is to free stuck impulse couplings.

For some reason total mag failure seems to occur more often on the ground between flights rather than when airborne.

Bendix mag inspection is 4 years calendar time - inspection preiods aren't just by the hour.

Fouled spark plugs can be avoided by correct shut down procedures. Lycoming's advice is to run the engine at around 1700rpm (if it's clear behind) for 15-20 seconds then back to 1200rpm and mixture straight to idle cut off. The lead scavenging agents in 100LL work better at higher combustion chamber temperatures which is why the higher rpm run down and leaning whilst on the ground helps keep the plugs clean.

It isn't just the lower plugs that can get fouled. With an excessive mag drop during power checks, aggressive leaning may well clear the offending plug. Try it three times and if it doesn't work, don't fly - seek help. I heard of one aircraft owner who went flying round the circuit to try and clear the plug after leaning it didn't work. That didn't work either. So he flew one plug down to his maintenance facility some 30 miles away. Not a very good idea...

Despite the good things about mags, the future is ignition systems like the Rotax 912. In fact the Rotax 912 is the engine of future GA. Be it in Rod's MCR01, my RV-12 or an IFR certified Tecnam P2006T twin.

PS No vacuum pump likes being turned backwards (same ones are fitted to Continentals as Lycomings). The carbon vanes are set to sweep (angled backwards) in the chamber and they can break and jam if turned the wrong way, causing the pump drive shaft to fracture (as it is designed to do to prevent potential engine damage).

[IO540]

Okay, there is potential for a big problem if the single drive fails but can anyone quote an example of this actually happening?

It has happened to at least one TB owner over the years. The drive doesn't actually fail (that would require the gears in the accessory drive box to get trashed, and then you can forget the engine anyway); what happens is that the single cam in the middle of the mag fails.

The single shaft dual mag engines are rare anyway.

No vacuum pump likes being turned backwards

None at all? There are, or have been, types which are rated for up to 50hrs in the reverse direction.

I have also built up an electrically powered vac pump rig (for erecting vacuum horizons) and the capacitor-start motor had no trouble running the vac pump in either direction, at 1300rpm. I can't remember which direction I ended up running it in the end but it worked equally. This was the very common 215CC pump. Certainly the myth that the vanes shear off following any reversal (like turning the prop backwards) is a myth - I ran it for hours in either direction.

[FlyingStone]

Despite the good things about mags, the future is ignition systems like the Rotax 912. In fact the Rotax 912 is the engine of future GA. Be it in Rod's MCR01, my RV-12 or an IFR certified Tecnam P2006T twin.

I strongly disagree: the future powerplant for GA are diesel engines. I agree, Rotax is good for smaller (two-seat) aircraft, but for training/touring four-seaters, diesel engines are the way to go. With diesel engines, all the problems with fuel availability goes away (virtually everywhere in the world you can get Jet A-1), no problems with fuel temperatures (I don't have any specific experience, but I believe there are problems with Mogas above 30°C), timed injection (especially with CR where you have multiple injections in one cycle), much higher compression ratios (and thus engine efficiency), very efficient at cruise power (they all run LOP at normal load) water cooling, ... Add to that, perhaps the most important factor: technology has proven itself on many different IFR certified aircraft (C172, DA40, DA42, C206, ...), while Tecnam P2006T is the first "serious" (IFR mission capable) aircraft, powered by Rotax engine and so far we can't make any judgements yet.

The problem with aircraft piston engines are - as IO540 says - no development, we still have newly developed engines with carburetors, which have catastrophic air-fuel mixture ratios between cylinders (sadly even many injected engines with factory-made injectors don't offer much improvement), and I could go on and on and on about it. I think aviation industry should look to engines used in todays cars, you see technology as: variable valve timing, electronically controlled ignition, accurate piezo-injectors, 4 valves per cylinder, etc. Looking at this, it almost seems as if we were flying aircraft made in Stone Age :)

[smarthawke]

The only snag you have with airborne diesels conquering the world is convincing the USofA....

[Rod1]

“With diesel engines, all the problems with fuel availability goes away”

From a strictly UK perspective the reverse is true. The problem with diesel engines in aviation is that most will not run on diesel, but use Jet A1. Most small airfields and strips are near a filling station and it is 95% likely you can get Mogas as it is the fuel used by almost all micros and quite a bit of light GA. Very very few will have Jet A1.

The Rotax has been certified for years and will have logged lots of time IFR. The new bread of 4 seaters using the 912 will be much cheaper to run than the old American machines (P400 MCR4s). The next gen home built 2 seaters which are on the drawing boards will give 90kn on 50 hp using industrial engines (mogas or Avgas), with similar lifting power to a Jodel.

Rod1

[Kolossi]

Not doubting diesel engines at all - I have one in my car - but when you say no temperature problems for diesel, I thought it got waxy when cold (and by cold I mean -10Degc not the considerably colder an airbourne engine's going to meet.

Not questioning what y'all say, just trying to understand it from a position of lack of knowledge :ok:

[Genghis the Engineer]

Not doubting diesel engines at all - I have one in my car - but when you say no temperature problems for diesel, I thought it got waxy when cold (and by cold I mean -10Degc not the considerably colder an airbourne engine's going to meet.

Not questioning what y'all say, just trying to understand it from a position of lack of knowledge :ok:

Depends upon the additives.

Plain, unadulterated Diesel fuel, this is true.

Automotive Diesel gets different additives added at different times of the year, and military and civil jet fuel comes in quite a variety of flavours depending upon where you're planning to go with it.

G

[FlyingStone]

Not doubting diesel engines at all - I have one in my car - but when you say no temperature problems for diesel, I thought it got waxy when cold (and by cold I mean -10Degc not the considerably colder an airbourne engine's going to meet.

Diesel engine doesn't neccessarily mean diesel fuel and that's why I say diesels are future - they run on Jet A-1, which doesn't have problems until -35°C ;) Plus the engine heats the fuel (at least in DA40D), so once you start the engine, the problem with fuel temperature is more or less gone. Just for information, I think that for Centurion 2.0 limits for Diesel fuel are 0°C for takeoff and -5°C in flight.

Rod1: If you look globally, there is a large trend to eliminate 100LL, mainly because it includes lead, but there are also economical reasons: why produce, transfer, store (remember, you must have seperate trucks & tanks) and sell 100LL to very limited number of customers, when you can sell one type of fuel for ALL aircraft? :cool: Looking more globally than UK (I don't know the situation there), but even some developed (well, that's debatable, but let's stick to the topic) countries in Europe are decreasing supply of 100LL. For example in Italy, it is recommended, but I'd say practicaly mandatory to check for 100LL availability with actual fuel supplier at the airport before making the flight and realising that the only 100LL at the airport is the one in your tanks :ugh:

[englishal]

In the interim, before diesels take over (they are much more efficient, stronger, liquid cooled etc...), I am surprised we haven't seen more FADECs attached to conventional Lycomings and Continentals.

I reckon if you had a one lever operated Lycoming with a computer to take care of power setting and mixture control, you could probably improve the efficiency a lot and possibly reduce maintence costs.

[IO540]

The heavy technology that increases efficiency of car engines works primarily at part load, where aircraft engines don't operate much. Complexity is not an asset in making avgas aero-engines work better, and diesels only make sense as a tax-dodge.

Exactly. Simple engines work fine for aircraft, if comparing efficiency.

Diesels got hit on two fronts: first by Mr Thielert's "little accounting adventure" resulting in its collapse, and second by taxation. In the UK, the financial case for an aero diesel has basically disappeared.

[A and C]

A FADEC is on the way from Lycoming, I would guess that they could reduce the fuel burn by 25-30%, (after all you can see a 20% reduction in fuel burn by fitting a LASOR ignition system).

Lycoming are of the opinion that the problems that require Deisel engines to have dampers fitted are outweighed by the advantages of a MOGAS burning FADEC controled engine.

At the moment I have yet to be convinced of the advantages of the diesel as an aircraft power plant, may be when I do the Thielert course next year I will adjust my opinion.

[IO540]

A FADEC is on the way from Lycoming, I would guess that they could reduce the fuel burn by 25-30%,

Compared with flying full rich, sure.

Otherwise they must have invented a whole new branch of physics :)

I can get the same result by flying at peak EGT.

require Deisel engines to have dampers fitted are outweighed by the advantages of a MOGAS burning FADEC controled engine.

I am sure that is true - so long as the juice is available, so OK from the US perspective. The case for burning avtur hangs 75% on availability and taxation. The other 25% is better MPG, but it is only about that much.

[Rod1]

FlyingStone

You have gone on and on about 100LL. If you reread my post you will see I do not use it!

Rod1

[A and C]

You are correct about saving 20% on the fuel burn by leaning to peak however you are still stuck with the ignition timing fixed firmly in one place.

The LASOR ignition system uses inputs from RPM and MP to move the ignition timing to the optimum position for the power setting and saves fuel in all phases of flight.

A FADEC will take this one stage further by enableing the engine to run so lean that it is right on the cusp of detonation, Lycoming are saying that with the FADEC they are working on the engine will in fact be four engines (or in your case six) conected to one crankshaft as each cylinder will be individualy managed.

In short the FADEC will be able to achive the sort of mixture control for each power stroke that you or I can achive only after we have been in the cruise for some time and it can also adjust the ignition timing, that we can't do.

Admitting this box of wiggly amps is better at controlling the engine comes hard from someone who's ultimate job would have been flight engineer on a Lockheed Constellation but that's what they call progress!

[vee-tail-1]

FADEC, LASOR ignition, ECUs, electronic injection, common rail for diesels, that is progress???
I thought we were talking magnetos here. And the one overiding quality of mags is that they are independent of the electrical system and continue to keep an engine running without electrical power.
All the gismos above require power, and are totally useless when the fan belt breaks or a short knocks out the electrical system. In fact with all that electronics on board a simple electrical failure will lose the engine and you will be looking for somewhere flat to put down your can of electronic goodies.
We have had this discussion here before, and IMO the advantages of diesel (simplicity, economy, reliability) have been trashed by Thielert and their ill concieved super complex and unreliable engines.
There are plenty of really good diesel designs out there which have not made it to the market because Thielert hogged the development money with their honeyed words. Now having soured the market they continue with their over complex turkeys. :\

[vee-tail-1]

http://www.ppdgemini.com/_PDF/Gemini_100_Spec_Sheet_1-10-09_B&W.pdf

If only some of that development money went into this (based on the highly successful Junkers WWII design)

[Jan Olieslagers]

If only some of that development money went into this...or in the Wilksch or DAIR engines... There have been diesel initiatives enough, besides Thielert. I can only suppose their makers were less good at marketing than at engineering - which is much better than the other way round. Yet here we are, without any reliable affordable diesel engine available.

[IO540]

Everything I have read about engine management suggests that any improvement on the present mags will be really minimal and probably well below 5% on SFC - once you are peak-EGT or LOP.

Electronic ignition would be nice because in theory it could last "for ever". But these old companies are not IMHO capable of designing a piece of electronics that lasts. They can only just barely make a crankshaft that's had the right heat treatment, without any of the steps having been skipped ;)

[vee-tail-1]

Favoris (http://gazaile2.nmr7.free.fr/favorite.html)

This aircraft can fly for nearly nine hours :eek: , and gets approx 70 mpg on Jet A1.

French homebuilders are using Peugeot/Citroen car engines to achieve astounding performance on very low HP. The simple reduction gear and high diesel torque allows a large diameter slow turning prop. This efficiency helps to offset the weight of the iron car diesel engine.

[vee-tail-1]

Ah but check out the cruise settings for the Favoris. Max torque is at 2400 engine rpm which gives 1280 prop rpm, for a cruise at 200 kph.
Take off engine rpm is 5000 which gives 2600 prop rpm, and t/off roll of 350m. Not an impressive rate of climb however at only 550 ft.min; I should have thought a turbo diesel might have been a better engine choice.
Also I wonder how a car engine copes with regular thrashings at 5000 rpm for take off?

[IO540]

I think car engines have a long life history in aviation of just about exactly zero.

I know this has been done to death here many times but a typical car engine runs at 10-20% power most of the time. A typical 2000cc engine (i.e. assuming top speed of 120mph; my 1987 Celica would do 130) doing 70mph is running at about 30-40% power.

Anybody trying to run them at 65% or g0d forbid 75% power finds they fall apart fast. On serious rally cars, they change engines almost as often as they change tyres (but you don't see that on the TV coverage).

OK, one can "do a Thielert" and beef up the bits which you think are most likely to fall apart, but this is still a long way from "most" engines (well, non turbo ones, anyway ;) which is why I never bought a TB21) making a 2000hr TBO without any issues.

Back to mags, in the 1970s I used to make electronic ignition systems for motorbikes. These initially used the contact points (which would last for ever, due to the elimination of arcing) and later I did a magnetic pickup. I don't think there was any real obvious failure mechanism, and one could massively over-engineer the bits which were most stressed (the capacitor, and the semiconductors) but I would not trust Lyco or Conti to get somebody competent on the job, even though the car makers have pretty well sussed this in the intervening 30+ years.

Mags are damn expensive. I have just paid $2.5k for a factory overhauled D3000 mag, and that is just the mag, not the lid with the ignition harness. That is plenty of a budget for an electronic ignition and a distributor.

[englishal]

I think variable timing on a Lycoming would be great because when the mixture is leaned to ROP through LOP it moves the position of the peak pressure pulse towards top dead centre. This puts more stress on the engine as the power pulse occurs when the conrods are virtually straight up and down, resulting in "punching" the piston assembly into the crank. If the ignition timing can be advanced it'd move this peak pressure pulse futher away from TDC resulting in more efficient running and less stress on the engine bits, with the leanest possible mixture setting. It is easier for a computer to accomplish this, along with prop setting that fiddling constantly with the mixture and prop.

[IO540]

Sure one could fine tune things.

However, I have done very careful flight tests and have not found any MPG improvement from LOP, compared with peak EGT.

LOP makes you fly slower, which obviously helps MPG, but if you do the test at a fixed IAS (i.e. a fixed thrust), fixed RPM (i.e. fixed prop efficiency) then LOP does not produce a better MPG. It possibly delivers a cooler combustion, but that is an issue only at high power settings, which is in issue (in practical IFR flight, which is all high altitude) only with turbo engines.

Sure I would like electronic ignition. I am just not going to be their beta tester #23 :)

[stickandrudderman]

I've just read this months LAA mag and there's an interesting piece in there from the chap who had to abort his record attempt to SA. He gives electronic ignition a definite thumbs up.

[IO540]

I think the reason for getting especially good MPG when at low RPM is largely due to the slow burning mixture at low power settings.

For example, at FL100, wide open throttle i.e. about 21" MP, 2200rpm, peak EGT / LOP, I get the best range of about 1350nm to zero fuel. This remains more or less constant up to FL160 or so, after which it gets worse again because one needs more power i.e. 100F ROP and more revs to get more air into the engine.

Whereas a turbo engine is never short of air and thus never runs at these low power settings like 16-21" MP.

I think this explains why TB21 owners get only about 80% of my range. Of course they do get there quicker :)

[Zulu Alpha]

Take a look at Lightspeed electronic ignition for Lycomings/Continentals.
Light Speed Engineering - Klaus Savier (http://www.lightspeedengineering.com/)

They advance and retard the ignition mainly on manifold pressure but also on rpm.

They vary a std lycoming from 45 degrees advanced to 0 degrees. Magnetos are fixed at 25 degrees ( for a Std compression Lycoming).

You really notice this difference when you pull the power and rpm back in the cruise, the aircraft hardly loses any speed. This is because it advances the ignition to extract more power from the cylinder charge.

Its so good that it comes as a factory option on Lycoming experimental engines. It has been certified by EASA for a helicopter installation.

Much cheaper than a new magneto as well. I got better starting (full strength spark at low speed) and was also able to use iridium racing plugs which are a fraction of the cost of aviation plugs.