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Light Speed Plus P-Mag
- Thread starter whd721
- Start date
Some general issues to think about
My disclosure: I didn't end up going with Light Speed + P-Mag, but I did consider it. (I ended up going with two P-Mags.)
Some general issues for you to think about:
If going with two electronic ignitions of any kind (two Light Speeds, two P-Mags, one of each, whatever), you are now in the realm of an electrically-dependent engine (yes, even P-Mags depend on ship's power at low RPM's, as well as for backup if the internal generator fails). Now, there's nothing wrong with going electrically dependent IF your electrical system architecture ensures that no single failure scenario can take out both ignition systems. The overall system design must ensure that a simultaneous failure of both ignitions is more-or-less astronomically improbable (or at least less probable than two mags failing simultaneously! that probability is less than astronomical...). This can actually be ensured in a variety of ways, the best of which is probably to run two complete electrical systems that are independent of each other (becoming more and more common these days anyway, as aux electrical systems replace vacuum systems for cockpit instrument power redundancy). Still, many folks shy away from that, and that's why you see many instances of one electronic ignition and one traditional mag in RV's. Not unreasonable, there are trade-offs both ways. I won't get into the relative merits of electronic ignition vs mags here. Plenty of info on the topic is already available.
But anyway, so the first thing you need to decide is whether or not you're comfortable with the prospect of running two electronic ignitions, and are willing and able to design your electrical system architecture accordingly.
If the answer is no, then stick with traditional mags, or one traditional mag and one electronic ignition.
If the answer is yes, then continue reading.
So you've decided to go with two electronic ignition systems. Now the question is what type, or what types if two different types.
One advantage in favor of using two different types is simply the fact that they're different from each other. Dissimilar redundancy. This makes it less likely that the two systems will have the same failure modes, and therefore less likely that they will both fail simultaneously under some particular set of conditions or as a result of some yet unknown design flaw.
One disadvantage of using two different designs is that they'll have different installation requirements, different operating procedures, different companies you'll be dealing with for support, etc. Just more different things for you to juggle, as a builder, maintainer, and pilot. Although maybe not drastically so, since all ignition systems are pretty straight forward, but still.
Now for me personally, all things being equal, I probably would have gone with one of each for dissimilar redundancy. But all things are rarely equal. After studying both products, I chose to go with two P-Mags because I was satisfied in my assessment of that product, and I was just not satisfied with certain aspects of the Light Speed product. Now, I don't want to get drawn into a discussion here about brand A vs. brand B, though I'm sure others will chime in with a full spectrum of opinions on both sides... But I will just make the point that any product or products you choose to install in your airplane, especially a safety-critical component like an ignition system, should be selected with due diligence and scrutiny to your own satisfaction.
My disclosure: I didn't end up going with Light Speed + P-Mag, but I did consider it. (I ended up going with two P-Mags.)
Some general issues for you to think about:
If going with two electronic ignitions of any kind (two Light Speeds, two P-Mags, one of each, whatever), you are now in the realm of an electrically-dependent engine (yes, even P-Mags depend on ship's power at low RPM's, as well as for backup if the internal generator fails). Now, there's nothing wrong with going electrically dependent IF your electrical system architecture ensures that no single failure scenario can take out both ignition systems. The overall system design must ensure that a simultaneous failure of both ignitions is more-or-less astronomically improbable (or at least less probable than two mags failing simultaneously! that probability is less than astronomical...). This can actually be ensured in a variety of ways, the best of which is probably to run two complete electrical systems that are independent of each other (becoming more and more common these days anyway, as aux electrical systems replace vacuum systems for cockpit instrument power redundancy). Still, many folks shy away from that, and that's why you see many instances of one electronic ignition and one traditional mag in RV's. Not unreasonable, there are trade-offs both ways. I won't get into the relative merits of electronic ignition vs mags here. Plenty of info on the topic is already available.
But anyway, so the first thing you need to decide is whether or not you're comfortable with the prospect of running two electronic ignitions, and are willing and able to design your electrical system architecture accordingly.
If the answer is no, then stick with traditional mags, or one traditional mag and one electronic ignition.
If the answer is yes, then continue reading.
So you've decided to go with two electronic ignition systems. Now the question is what type, or what types if two different types.
One advantage in favor of using two different types is simply the fact that they're different from each other. Dissimilar redundancy. This makes it less likely that the two systems will have the same failure modes, and therefore less likely that they will both fail simultaneously under some particular set of conditions or as a result of some yet unknown design flaw.
One disadvantage of using two different designs is that they'll have different installation requirements, different operating procedures, different companies you'll be dealing with for support, etc. Just more different things for you to juggle, as a builder, maintainer, and pilot. Although maybe not drastically so, since all ignition systems are pretty straight forward, but still.
Now for me personally, all things being equal, I probably would have gone with one of each for dissimilar redundancy. But all things are rarely equal. After studying both products, I chose to go with two P-Mags because I was satisfied in my assessment of that product, and I was just not satisfied with certain aspects of the Light Speed product. Now, I don't want to get drawn into a discussion here about brand A vs. brand B, though I'm sure others will chime in with a full spectrum of opinions on both sides... But I will just make the point that any product or products you choose to install in your airplane, especially a safety-critical component like an ignition system, should be selected with due diligence and scrutiny to your own satisfaction.
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Two P-mags
I can't answer your question directly but I don't agree that having two different electronic ignitions has any advatages over two of the same. Would there be any advantage of having one slick mag and one bendix on the same engine? There would be disadvantages, such as knowledge, parts to stock to name a few. Two mags of the same type won't necessarily fail at the same time.
I chose two P-mags as I didn't want to maintain an "understanding" of a two different electronic ignition systems.
I don't believe the risks of two indepedant P-mags failing on the same flight are measureable.
The E-mag, which preceeded the P-mag, was dependant on ship's power and therefore the argument could be made for running one standard mag with that, but that is a different discussion.
YMMV
Bevan
I can't answer your question directly but I don't agree that having two different electronic ignitions has any advatages over two of the same. Would there be any advantage of having one slick mag and one bendix on the same engine? There would be disadvantages, such as knowledge, parts to stock to name a few. Two mags of the same type won't necessarily fail at the same time.
I chose two P-mags as I didn't want to maintain an "understanding" of a two different electronic ignition systems.
I don't believe the risks of two indepedant P-mags failing on the same flight are measureable.
The E-mag, which preceeded the P-mag, was dependant on ship's power and therefore the argument could be made for running one standard mag with that, but that is a different discussion.
YMMV
Bevan
Dissimilar redundancy
Bevan,
The disadvantages that you listed are real, and you'll note that I described them as well. But the reliability advantage of dissimilar redundancy is also quite real, and is a factor to consider in system design. I'm not sure on what basis you're categorically denying that fact.
All systems can fail. Subsystem redundancy reduces the probability of total system failure, but that only holds true to the extent that failure modes in the redundant subsystems are physically and statistically independent of each other. Independent power sources are one measure toward that end. And dissimilar redundancy in subsystems themselves further reduces the probability of both subsystems suffering from identical design flaws, identical manufacturing defects, and identical internal failure modes that are more likely to precipitate a failure under a set of conditions affecting both subsystems simultaneously.
But please don't take my word for it. Dissimilar redundancy is a well known concept in design methodology for high-reliability systems. If you're interested in learning about it, there's plenty of literature out there, go do some reading. If you're not interested, disregard.
-Roee
Bevan,
The disadvantages that you listed are real, and you'll note that I described them as well. But the reliability advantage of dissimilar redundancy is also quite real, and is a factor to consider in system design. I'm not sure on what basis you're categorically denying that fact.
All systems can fail. Subsystem redundancy reduces the probability of total system failure, but that only holds true to the extent that failure modes in the redundant subsystems are physically and statistically independent of each other. Independent power sources are one measure toward that end. And dissimilar redundancy in subsystems themselves further reduces the probability of both subsystems suffering from identical design flaws, identical manufacturing defects, and identical internal failure modes that are more likely to precipitate a failure under a set of conditions affecting both subsystems simultaneously.
But please don't take my word for it. Dissimilar redundancy is a well known concept in design methodology for high-reliability systems. If you're interested in learning about it, there's plenty of literature out there, go do some reading. If you're not interested, disregard.
-Roee
Thanks for the inputs, a good discussion.
I need to refine my situation. I intend to build a simple, light VFR only aircraft.
The engine I am considering is a Lyc O-233 and has only one mag position. However, it is drilled to accept the Light Speed Mini sensor.
Hence, Two P-Mags or Two Slick Mags are not an option.
My options are;
1. A Light Speed + a second Light speed
2. A Light Speed + a P-Mag
3. A Light speed + a Slick Mag
Option 1. Is dependent on the electrical system. resolution is not simple.
Option 2, and 3. will both still run if the battery falls off. Simple and good.
Option 3. seems to be the default choice.
However, I keep wondering why Option 2. is not a good choice?
Comments?
I need to refine my situation. I intend to build a simple, light VFR only aircraft.
The engine I am considering is a Lyc O-233 and has only one mag position. However, it is drilled to accept the Light Speed Mini sensor.
Hence, Two P-Mags or Two Slick Mags are not an option.
My options are;
1. A Light Speed + a second Light speed
2. A Light Speed + a P-Mag
3. A Light speed + a Slick Mag
Option 1. Is dependent on the electrical system. resolution is not simple.
Option 2, and 3. will both still run if the battery falls off. Simple and good.
Option 3. seems to be the default choice.
However, I keep wondering why Option 2. is not a good choice?
Comments?
Not entirely true, there's some subtleties here. Without the battery, the P-Mag should still run at cruise RPM. But at low RPM it may drop out, whereas a mag won't.
Also, if you have two electronic ignitions connected to the same electrical system, then an over-voltage condition could toast them both.
Otherwise, yes, a P-Mag has many advantages over a mag in performance, reliability, and maintenance.
N941WR
Legacy Member
While you are correct in that the P-mag will not run below a given RPM, that RPM is typically 800. Most aircraft will not see an RPM reading below that number until they are on the runway and slowed way down.
With either a CS or FP prop, when the engine is at idle and you are gliding in to land, the RPM will be above that magic number, thus the P-mag is still powering itself.
You will find out the P-mag no longer has power when you try to power up to taxi off the runway and that is a good problem to have. Otherwise, in flight during all flight configurations 114 P-mags are self-powered.
Regarding the original question, contact Brad at E-mag Ignitions, makers of the P-mag. They are testing the single drive dual P-mag right now and by the time you are ready for it, it might be available and you can drop the LSI altogether.
N941WR
Legacy Member
Since you are building a -9A, you might want to consult Van's regarding using the 233.
The -9A's tend to be more aft weight biased than the -9's and with the O-290 I had originally put in my -9 I almost had an aft CG issue.
Unless there is a specific reason why you want the O-233, I would recommend you use the O-235. Also, for around the same cost of a new O-233, you can buy an O-320.
The -9A's tend to be more aft weight biased than the -9's and with the O-290 I had originally put in my -9 I almost had an aft CG issue.
Unless there is a specific reason why you want the O-233, I would recommend you use the O-235. Also, for around the same cost of a new O-233, you can buy an O-320.
Ron Lee
Well Known Member
Another option with a larger engine is a P mag with a Slick mag
Ron Lee
Well Known Member
I am curious why you are considering the O-233. From the Vans website, using the 160 HP and 118 HP performance numbers, your choice would have you give up the following compared to a 160 HP engine (solo RV-9 numbers):
1) 24 MPH top speed
2) 22 MPH cruise speed
3) 800 FPM climb
4) 7000' service ceiling
Just my opinion but resale would be hard. I would not buy it and would advice anyone asking me to not buy it.
It is your choice, but I would hate to see you make a decision that you eventually regret...even if you do not see it now.
1) 24 MPH top speed
2) 22 MPH cruise speed
3) 800 FPM climb
4) 7000' service ceiling
Just my opinion but resale would be hard. I would not buy it and would advice anyone asking me to not buy it.
It is your choice, but I would hate to see you make a decision that you eventually regret...even if you do not see it now.
AK4x4
Well Known Member
O-233 info
I would expect an O-233 powered 9 to meet or slighty exceed the performance of the 118hp O-235 prototype. The method Lycoming uses to compute horsepower has changed over time. As a result todays 115hp (O-233) engine is a slightly more powerful engine than the 118hp O-235 of yesteryear. That being said Ditto Bill's coments on cg issues. I think a light O-320 will make you happy and fit your mission well. Good luck, Russ
I would expect an O-233 powered 9 to meet or slighty exceed the performance of the 118hp O-235 prototype. The method Lycoming uses to compute horsepower has changed over time. As a result todays 115hp (O-233) engine is a slightly more powerful engine than the 118hp O-235 of yesteryear. That being said Ditto Bill's coments on cg issues. I think a light O-320 will make you happy and fit your mission well. Good luck, Russ
