VansAirForceForums  
Home > VansAirForceForums

-POSTING RULES
-Advertise in here!
- Today's Posts | Insert Pics

Keep VAF Going
Donate methods

Point your
camera app here
to donate fast.

  #1  
Old 10-31-2013, 03:14 PM
blueflyer's Avatar
blueflyer blueflyer is offline
 
Join Date: Nov 2008
Location: Shreveport, LA
Posts: 1,044
Default P-MAG Question

I was reading the FAQs about the PMAG, and came across this one:
------------------------------------------------------------------------------------
Q: Will the P-MAG work when I slow the engine on final approach?

A: The P-MAG is designed to operate at less than typical in-flight idle speeds (1,000 rpm) . It will operate at 700 to 800 rpm, and sometimes less. If a particular craft is known to have an in-flight idle below the speed the P-MAG alternator will sustain the ignition, it should be noted so the pilot can stay above that speed if the emergency condition is ever encountered.

This minimum P-MAG speed is easy to ascertain on the ground. Your ground idle will be significantly less than your in-flight idle. When doing a run up, simply isolate the P-MAG side of the ignition (if not running dual P-MAGs), and gradually slow the engine toward idle. Note the speed where the engine quits (if at all). If this speed is comfortably less than your in-flight idle speed, you should be covered. If not, note the operating limitation so you'll be prepared in the even of a power emergency.
-----------------------------------------------------------------------------------
I know some folks have their ground idle set at 500RPM or so, but can you get your in flight RPM's below 700-800? I would hate for my mags to quit on me if I chop the power for some reason.....what are your thoughts?
Reply With Quote
  #2  
Old 10-31-2013, 03:30 PM
Bevan Bevan is offline
 
Join Date: Sep 2007
Location: BC
Posts: 1,698
Default

The FAQ answer is referring to the self generating power capabilities of the P-mag. It is my understanding that the P-mag will idle the engine slower than 7-800 rpm provided that "ship's" electric power is available as would be normally anyway. The internally generated power becomes primary power above 7-800 RPM.

Bevan
__________________
RV7A Flying since 2015
O-360-A1F6 (parallel valve) 180HP
Dual P-mags
Precision F.I. with AP purge valve
Vinyl Wrapped Exterior
Grand Rapids EFIS
Located in western Canada
Reply With Quote
  #3  
Old 10-31-2013, 03:33 PM
krw5927 krw5927 is offline
 
Join Date: Aug 2010
Location: Wichita, KS
Posts: 1,966
Default

As long as the Pmag is supplied with ships power, it will not die below those quoted RPMs. Your particular engine/prop combination may or may not be able to sustain such a low RPM, but the Pmag will still fire appropriately.

Only when the Pmag is depending on its internal generation and has a loss of ship's power (emergency scenario?) will the RPM get low enough that the internal power generated will be insufficient to sustain firing.
__________________
Kurt W.
RV9A
FLYING!!!
Reply With Quote
  #4  
Old 10-31-2013, 03:47 PM
Toobuilder's Avatar
Toobuilder Toobuilder is offline
 
Join Date: Jul 2009
Location: Mojave
Posts: 4,737
Default

My ground idle is set to ~500, and even with a C/S prop I can't see less than 1000 RPM in flight. Slowed to taxi speed, yes, but you would have to really work hard to get the prop to 7-800 RPM in flight. As discussed, the low RPM cutout is only an issue if you have lost all electrical power anyway.
__________________
WARNING! Incorrect design and/or fabrication of aircraft and/or components may result in injury or death. Information presented in this post is based on my own experience - Reader has sole responsibility for determining accuracy or suitability for use.

Michael Robinson
______________
Harmon Rocket II -SDS EFI
RV-8 - SDS CPI
1940 Taylorcraft BL-65
1984 L39C
Reply With Quote
  #5  
Old 10-31-2013, 04:22 PM
blueflyer's Avatar
blueflyer blueflyer is offline
 
Join Date: Nov 2008
Location: Shreveport, LA
Posts: 1,044
Default

got it now. Thanks for the clarification. The mouse wheel running my brain turns really slow sometimes!
Reply With Quote
  #6  
Old 10-31-2013, 09:07 PM
N941WR's Avatar
N941WR N941WR is offline
 
Join Date: Jan 2005
Location: SC
Posts: 12,887
Default

Here's the deal...

The 114 series P-mags have an internal generator. Once they are spinning fast enough to produce enough power to fire the plugs, they automatically switch over to internal power. This is typically around 800 RPM +/- and the transition is seamless. You will not notice when they switch over and there is no way to know when they are running off of internal power.

This the key thing, when running on internal power and you throttle back while on the ground and the RPM drops below the magic number (around 800 RPM, as I said above) the ignition will stop firing the plugs. The part you must understand is that while throttled back in the air, such as when on short final, your prop is still spinning faster than 800 RPM, regardless of what your ground idle speed is. Thus, you have a functional ignition that will work fine in the event you need to go around. However, once on the ground, unless you realize there is a problem, the engine will probably die on roll out. Not necessarily a bad place to have your engine die.
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
Reply With Quote
  #7  
Old 10-31-2013, 09:38 PM
Ironflight's Avatar
Ironflight Ironflight is offline
VAF Moderator / Line Boy
 
Join Date: Jan 2005
Location: Dayton, NV
Posts: 12,457
Default

Quote:
Originally Posted by N941WR View Post
Here's the deal...

The 114 series P-mags have an internal generator. Once they are spinning fast enough to produce enough power to fire the plugs, they automatically switch over to internal power. This is typically around 800 RPM +/- and the transition is seamless. You will not notice when they switch over and there is no way to know when they are running off of internal power.

This the key thing, when running on internal power and you throttle back while on the ground and the RPM drops below the magic number (around 800 RPM, as I said above) the ignition will stop firing the plugs. The part you must understand is that while throttled back in the air, such as when on short final, your prop is still spinning faster than 800 RPM, regardless of what your ground idle speed is. Thus, you have a functional ignition that will work fine in the event you need to go around. However, once on the ground, unless you realize there is a problem, the engine will probably die on roll out. Not necessarily a bad place to have your engine die.
OK Bill - I know that you know a lot more about the P-Mags than I do... but my idle on the ground is less than 800 RPM's, and the engine keeps running, even when idling before shutdown after the flight. Are you really saying that the P-Mags don't switch back to ship's power when the RPM gets too low for internal power? Or is the internal power drop-off at a lower RPM?

I'm now confused!

Paul
__________________
Paul F. Dye
Editor at Large - KITPLANES Magazine
RV-8 - N188PD - "Valkyrie"
RV-6 (By Marriage) - N164MS - "Mikey"
RV-3B - N13PL - "Tsamsiyu"
A&P, EAA Tech Counselor/Flight Advisor
Dayton Valley Airpark (A34)
http://Ironflight.com
Reply With Quote
  #8  
Old 10-31-2013, 10:33 PM
PCHunt PCHunt is offline
 
Join Date: Feb 2007
Location: San Diego, CA
Posts: 1,714
Default

Post #3 seems to indicate that the P-mag will only quit at low RPM if ship's power has failed.
__________________
Pete Hunt, [San Diego] VAF #1069
RV-6, RV-6A, T-6G
ATP, CFII, A&P

2021 Donation+, Gladly Sent
Reply With Quote
  #9  
Old 10-31-2013, 10:45 PM
Flyguytki's Avatar
Flyguytki Flyguytki is offline
 
Join Date: Jun 2007
Location: Fairbanks AK
Posts: 758
Default

I have called P-mag on this same question, With dual P Mags, with the power at idle on the ground (Rpm's around 700) when you cycle the mags, one would continue to run and the other the motor would quite. Thought it might be something wrong with the mag so I made the phone call. It was explained that the internal power generation RPM is not an exact science, they have seen the internal power kick on as low as 600 on some and as high as 800 on other, it will vary with each individual mag. This is not a concern because as said previously in the post that you cannot get the idle RPM that low in flight. With a IO-360 and a 3 blade Catto Idle RPM over the fence was around 1050 rpm.

Therefore for my normal runup, i would always do a mag check at 1600 rpm as well as right around 1000 to check both.
__________________
RV-7 N87DX Built, Flown, and Sold!
Reply With Quote
  #10  
Old 11-01-2013, 07:35 AM
N941WR's Avatar
N941WR N941WR is offline
 
Join Date: Jan 2005
Location: SC
Posts: 12,887
Default

Quote:
Originally Posted by Ironflight View Post
OK Bill - I know that you know a lot more about the P-Mags than I do... but my idle on the ground is less than 800 RPM's, and the engine keeps running, even when idling before shutdown after the flight. Are you really saying that the P-Mags don't switch back to ship's power when the RPM gets too low for internal power? Or is the internal power drop-off at a lower RPM?

I'm now confused!

Paul
Paul,

Yes, you are correct. The P-mags convert back to ship's power when the RPM's drop to a point where the internal generator can not support the ignition. This happens automatically and unfortunately the P-mag does not report its power source, internal or external. Thus, the EICommander cannot tell you which power source it is running off of. It does report the voltage but this is the voltage on "the board" and not its source.

As stated by some of the follow-on posts, it really depends on your P-mag. For some reason the exact cut off seems to vary between P-mags. That is why it is important for each individual to test their P-mags at some point after installation so they know what to expect.

Regarding runups, I perform mine at 1400 RPM and due to my installation, I test them prior to each flight using this sequence.

That will test the internal generator but it really doesn't test to see if ship's power is reaching each ignition. This is because the P-mags automatically cut over to the best power source. Meaning, I can drop ship's power and that test will tell me if the internal generator is working or not but since I start on both P-mags, it will not tell me if both are receiving ship's power. The only way to test that is to leave one switch up, turn the other off, and reduce my engine RPM below the magic number and see if the engine keeps running. Then turn on the previously off ignition, turn off the previously on ignition with the engine still at low idle RPM and see if it continues to run.

I hope that clears up your confusion.
__________________
Bill R.
RV-9 (Yes, it's a dragon tail)
O-360 w/ dual P-mags
Build the plane you want, not the plane others want you to build!
SC86 - Easley, SC
www.repucci.com/bill/baf.html
Reply With Quote
Reply


Thread Tools Search this Thread
Search this Thread:

Advanced Search
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -6. The time now is 04:52 PM.


The VAFForums come to you courtesy Delta Romeo, LLC. By viewing and participating in them you agree to build your plane using standardized methods and practices and to fly it safely and in accordance with the laws governing the country you are located in.