Dual Impulse mags?

Question for you Lycoming gurus. My O-320 had Slick mags installed by Aerosport Power in 2006. Funny thing is, both mags have impulse couplings.

I always thought you should only have the impulse on the left side. Is there any advantage or disadvantage to having dual impulse couplings?

Cold weather

In very cold weather the impulse coupling sometimes does not engage. In Minnesota lots of folks have them on both mags to assure a start on a cold day

Gary Specketer

Enjoy

KatieB said:

Question for you Lycoming gurus. My O-320 had Slick mags installed by Aerosport Power in 2006. Funny thing is, both mags have impulse couplings.

I always thought you should only have the impulse on the left side. Is there any advantage or disadvantage to having dual impulse couplings?

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You won the lottery. With 2 toggle switches you have twice the spark for starting. People have paid lots of extra $ to re equip the RH side for dual impluse starting.

The only downside is the cost(behind you now), perhaps a little extra time when you time and sync the mags.

With a key start ignition you wont have any advantage to ease of starting, but with dual toggle switches, you can crank with both mags hot.

Cool! That seals the question of whether or not to use a key switch.

Depends on the configuration of the key switch. Some have multiple options depending on which lugs you use on the back. You might be able to retain the key switch and have both hot while cranking. You can also use a pushbutton for the starter which will allow 2 hot mags and a key.

+1
Standard key switches have jumpers on the back to choose L,R, or both to be on during start. Easy to configure.
For many years I had dual impulse mags on a TCM engine (182). I always thought that was standard.
Another plus for dual impulse couplings: on preflight very carefully move the prop to TDC. If the impulses snap at distinctly different times/prop locations, one of the timings has drifted and needs to be checked.

Impulse

Impulse couplings are high maintenance items on the Bendix mags. Don't know about the Slicks but likely similar.
Impulse couplings have been know to fail, usually destroying much of the engine, depending on how quickly you identify the situation and how quickly you get the engine shut down.

had dual impulse couplings on my 0-320 and that sucker always fired right up. There are instructions with the standard ACS key switch that show how to wire it for dual impulse (or vice versa) (IIRC the jumper is simply not installed)

KatieB said:

Question for you Lycoming gurus. My O-320 had Slick mags installed by Aerosport Power in 2006. Funny thing is, both mags have impulse couplings.

I always thought you should only have the impulse on the left side. Is there any advantage or disadvantage to having dual impulse couplings?

Click to expand...

Many Fuel Injected engines have dual impulse mags. Many carb equipped engines have only one impulse mag typically on the left side. Others have told you of the advantages and possible disadvantages.

I like the key idea just because of the extra security required to start the airplane, especially with a non-locking canopy.

But I can save weight and $$ by going with a pair of toggles and a start push-button. The plane had a push button start originally.

For those who went with the push button start, do you miss having the security of the key?

I have dual impulse Slicks and love them.

I use toggle switches and pushbutton start. I have the start switch wired through the "back" side of the avionics master so that the "start" circuit is dissabled with the avionics master on.

I have a locking canopy. If someone wants to steal the airplane, it doesn't take much to cut the "P" leads and be off.

For "my" feelings, the simplicity of the pushbutton start far outweighs the fail mode of "key" switches. YMMV.

Have had dual impulse mags on toggle switches and pushbutton start for years on my C85-powered "flying" airplane. Love this setup.

Pushbutton start is disabled by putting a lockout collar on the circuit breaker providing power to the start solenoid.

In our project aircraft, I went with the same setup, even though I have only one impulse mag on the left side of the O-360. The start button is under a flip-up cover which can be wired shut, in addition to the circuit breaker being collared (even a simple zip tie around the collar of the CB has proven a very effective way of ensuring the starter isn't accidentally engaged). The mag switches are mounted in "space shuttle" switch guards from Perihelion Designs. These guards facilitate the use of a long shank padlock or cable lock through them to prevent movement of the switch toggle levers. I don't think I'll ever need more security than this, but time will tell.

Marine key switch

KatieB said:

I like the key idea just because of the extra security required to start the airplane, especially with a non-locking canopy.

For those who went with the push button start, do you miss having the security of the key?

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I went with a marine key switch and 2 toggles. The switch is off-run-(start) and the "run" position powers the aprs transmitter.

Quite honestly Katie I'd be worried about the liability if kids (teens) got in there and then had an accident. With no locks of any kind, the parents would claim it was an "attractive nuisance". I'd try hard to get some sort of canopy lock, even if it is no deterrent to a real thief.

KatieB said:

...For those who went with the push button start, do you miss having the security of the key?

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A push button starter does not preclude use of a conventional keyed ignition switch - both the RV and Rocket had both.

But I am with Mel. The keyed switch does not offer much security from theft and as far as switching devices go, they are far more complex and failure prone than other methods.

The only thing I miss about the key is the inability to remove the key and place it on the glareshield as verification that the ignition is off. Its a habit I've had since my early training and I still feel funny about leaving an airplane without completing that ritual.

BobTurner said:

Quite honestly Katie I'd be worried about the liability if kids (teens) got in there and then had an accident. With no locks of any kind, the parents would claim it was an "attractive nuisance". I'd try hard to get some sort of canopy lock, even if it is no deterrent to a real thief.

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That's pretty twisted. But yes, I agree in this age of stupidity and terrorism, a lock is a good idea.

On the other hand, if someone wants to steal my avionics, I'd rather have them unlatch the canopy/frame/skirt that took me hundreds of hours to hand-craft than break it to get around a lock and take my stuff anyway.

I'd probably favor wiring it so that the mags or starter can be disabled if I have to leave it on the ramp somewhere overnight. (or at an airshow)

The security provided by the key switches is really only for honest people. If someone wants the airplane or avionics I'd rather they not damage the aircraft. The key switches can be prone to failures, giving one a false sense of security that the mags are off, when in fact they can fail in such a way that they are not. Also, some engines like the Rotax, and some electronic ignitions, do not recommend the use of the keyed switches.
There are options to adding secruity to prevent an accident as has been mentioned above. I wire the START switch through the strobe circuit when possible. The strobes have to be on for the starter to work. Personally, I think it is a good safety practice to have them on prior to start anyway.

Vic

Toobuilder said:

The only thing I miss about the key is the inability to remove the key and place it on the glareshield as verification that the ignition is off. Its a habit I've had since my early training and I still feel funny about leaving an airplane without completing that ritual.

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I feel the same way. It's a rule in my shop to not touch the prop until you personally have verified that the mags are off. That really is the only benefit of a key switch, IMHO. The key sitting on the glare shield means the mags are off.

It's certainly not going to keep a thief from stealing it, especially with impulse couplings, and especially with a carb. It is easy to cut the p-leads and hand prop the engine. At he same time, with a traditional key switch, the wires are labeled on the back, so cutting the p leads and arcing from bat to start is very easy, especially because most key switches are removable by hand anyway with pout needing any tools.

As for impulse couplings, some engines handle them better than ours, apparently. Some say the IO-540 doesn't like impulse couplings, which is why Van's engine comes without. We have a C152 with an O-235 with dual impulse couplings.

One strange thing I have seen recently is impulse couplings set at different timings to fire the coupling. One fired at TDC (which I am used to) and one fired about 15 degrees past. This was on Bendix mags and they were timed correctly.

Jesse said:

One strange thing I have seen recently is impulse couplings set at different timings to fire the coupling. One fired at TDC (which I am used to) and one fired about 15 degrees past. This was on Bendix mags and they were timed correctly.

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The difference in start timing you are seeing is called "Lag Angle". The manufactures use different "Lag Angles" for different applications.

The commonly used surplus Slick Mag that a lot of people retrofit onto VW aircraft conversion engines has a 15 deg "Lag Angle". The engine is normally times at 26~28 deg. This leaves you with the engine firing at ~11-13 deg BTDC and if the mixture it right and the prop not "slung" fast enough you get a nasty kick back.

Details

It is good to know the Lag Angle before you attempt to hand start any airplane.

Please be aware......

Just because the key is "OFF", the key is "out of the ignition switch", the key is "laying on the top of the panel", doesn't guaranty that the mags are grounded.

Unless you routinely check the "off" position, you can't be sure that the mags are off. The normal "run-up" mag check doesn't check the off position, and this is where most switch failures occur.

I personally have known of at least 2 failures that resulted in the engine firing with the key off, out of the switch, and laying on the top of the panel.

Weasel said:

The difference in start timing you are seeing is called "Lag Angle". The manufactures use different "Lag Angles" for different applications.

The commonly used surplus Slick Mag that a lot of people retrofit onto VW aircraft conversion engines has a 15 deg "Lag Angle". The engine is normally times at 26~28 deg. This leaves you with the engine firing at ~11-13 deg BTDC and if the mixture it right and the prop not "slung" fast enough you get a nasty kick back.

Details

It is good to know the Lag Angle before you attempt to hand start any airplane.

Click to expand...

Right, but both mags are timed at 25BTDC and one clicks at TDC and the other about 15ATDC.

I think many of the small continentals have 2 - my C90 does on my Jodel. I thought all airplanes were like that until I started building my RV. So there is nothing wrong with that. Obviously the starter switch should be set up accordingly (duh)

Mel said:

Just because the key is "OFF", the key is "out of the ignition switch", the key is "laying on the top of the panel", doesn't guaranty that the mags are grounded.

I personally have known of at least 2 failures that resulted in the engine firing with the key off, out of the switch, and laying on the top of the panel.

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Make it 3. Mag P lead was broken but resting against the terminal. Worked sometimes, but one time not.

Sell both magnetos and replace them with P-mags. That will save you three pounds and if the P lead is broken and the master is off, they won't fire. Also they do the shower of sparks thing for starting.

Sorry, I just had to.

N941WR said:

Sell both magnetos and replace them with P-mags. That will save you three pounds and if the P lead is broken and the master is off, they won't fire. Also they do the shower of sparks thing for starting.

Sorry, I just had to.

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I'd love to... but there's this thing called $$! At best I might sell one of them, but at core value I'd still come far short of the $1300 or whatever to get a P-mag.

After spending 5 years tinkering with this thing on chump change, all the big dollar items come at the end. Time to sell my car...

KatieB said:

I'd love to... but ...
After spending 5 years tinkering with this thing on chump change, all the big dollar items come at the end. Time to sell my car...

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I hear you loud and clear! You know you are close when you start placing two to three orders a week with Aircraft Spruce for $5 each plus shipping.

KatieB said:

At best I might sell one of them, but at core value I'd still come far short of the $1300 or whatever to get a P-mag.

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Most folks I know consider dual impulse very desirable. Additionally, you can keep a spare on the shelf and it will fit either side.

This may never effect you but . . .

Something else to think about. About 2 years ago I ran into an issue in flight where I felt there was a very slight stumble in the engine. I'm talking about flying along at 2350 happy as can be with my RV Grin fully engaged then all of the sudden like a hiccup that lasted 1/100th of a second what was that.

Then the fun starts, check fuel selector, fuel pressure, engine gauges, mags, etc. Well EVERYTHING is running great, not sure what that was but wondering, Will it do it again? Some time later. . bam - again a hiccup. Then it's "okay, it's time to get on the ground and figure this out".

I'll try to cut my story short because it lasts for 2 years!!! I have had 3 very well respected mechanics look at it over the past 2 years and have done the following to the plane because we couldn't replicate the issue at will.

First round we checked everything from plugs, to mags, to wires, timing, carb, fuel flow, fuel lines, fuel vents, induction, you name it....

- 1st so called fix- We thought the alternate air door that is magnetic was loose and the turbulence may have opened it just slightly providing a rush of air into the carb for a split second. So we adjusted it to be tighter. Flight test for 30 minutes and seemed to fixed the problem. several flights later (2 to 3 weeks later) bam same issue, turn back and find the issue.

- 2nd so called fix - Okay, let get serious now, Alternate air? come on! I'm sure it's has something to do with spark. Clean the plugs again and retest. This time spoke to several folks on this forum that told me about the Champion OHMs issue so I took out my $90 fine wire plugs and replaced them with Tempest. Once again tested the wire, the mags seemed to be testing good. Test flight showed to fine but guess what.... 2 weeks later while heading to a Saturday breakfast bam it hiccup again

- Back to the drawing board - It's gotta be the mags, although they only have 230 hours on them, take them out send them to Magneto Services, take out a second mortgage on the house to pay for all this and guess what.... same thing happened on a flight to Florida because we couldn't replicate the issue during our test flights.

- Now this is just getting silly because by now I've had several conversation with some really smart people about this and at least 2 very respected mechanics are stumped. Well, maybe it's the carb float ... don't think it is but just maybe. I hate being unsure about my engine so at this point I have an attitude of "I don't care what it costs, I want this fixed for good so I can fly with confidence".
Okay let's buy a new carb and eliminate that as a problem. Great !!! let's throw some more money at it. While we're at it let's change the wires and leads so the electric and fuel possibilities are eliminated. We also checked fuel flow again and vents, etc.

Well I'm going to stop listing all of the things that we ended up fixing/replacing because there is more but after 2 years one of my friends who is also a mechanic for over 20 years said, lets disconnect the ignition switch and see what happens. I thought, that's odd... what the heck does that have to do with it. I have an ACS keyed ignition switch with a OFF/L/R/Both placements. So he disconnected the P-leads and I went flying. We did this for over a month, over and over again, and decided the issue all along was in fact the contacts in the ignition switch. We concluded that during flight for some reason at any given time (maybe the vibration) the contacts inside the switch would lose contact and it was like turning off the ignition switch in flight and turning it back on at lightning speeds. I spent well over $3000 chasing down this issue which turned out to be a $16 dollar fix, essentially changing the contacts inside the switch with a $16 dollar kit.

Now I'm flying with my RV Grin ON again for the past 4 months and no engine hiccup and feel great about it. I'm an optimist so I look it as I have new parts on my engine and getting better "gas mileage" because my wallet is much lighter

Interesting

It's interesting in that the contacts in the switch are not actually making contact when it is in the BOTH position. They are OPEN. You may want to carefully check your P-leads to make sure they don't have an intermittent short that you have "fixed" just by moving the wires.

BTW, I had the same intermittent "miss" that you describe, and I finally chased it down to a cracked insulator on a Champion Fine wire spark plug.

Vic

Holy cow, that's a great story!

We've used ACS key switches on our LSAs for over 10 years. I know of one personally that failed on the ground, but none that caused issues in flight. I do like them in normal operation when they work right. I don't like the price, weight or the prospect of failures, so they key is officially ruled out.

I'll keep the dual impulse mags for now. I thought it was unusual, but I'm glad there are so many people who use them and like them.

So for those who have torn down their own Slick mags, how many of those specialty tools do you really need to inspect one?

INCOMPARABLE THREAD DRIFT CONTINUES!

Mel said:

Just because the key is "OFF", the key is "out of the ignition switch", the key is "laying on the top of the panel", doesn't guaranty that the mags are grounded.

Unless you routinely check the "off" position, you can't be sure that the mags are off. The normal "run-up" mag check doesn't check the off position, and this is where most switch failures occur.

I personally have known of at least 2 failures that resulted in the engine firing with the key off, out of the switch, and laying on the top of the panel.

Click to expand...

....and to add to Mel's sage advice....I've adopted the shutdown check; switch off the engine with the keyed switch. This at least confirms, at that moment, the mags get grounded. Key out, on dash, Then pull mixture & throttle, fuel selector 'off', chocks 'in'..........that's about all I can think of to safeguard things FWF....but I am all ears!

This is quite a drifty thread, but it's ALL GOOD!

vic syracuse said:

It's interesting in that the contacts in the switch are not actually making contact when it is in the BOTH position. They are OPEN. You may want to carefully check your P-leads to make sure they don't have an intermittent short that you have "fixed" just by moving the wires.

BTW, I had the same intermittent "miss" that you describe, and I finally chased it down to a cracked insulator on a Champion Fine wire spark plug.

Vic

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Thanks Vic, We spoke about this during Osh last year when I was still chasing this issue. But since changing out the contacts (kit) it's been doing awesome I'm so happy