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h34race
11-09-2006, 10:21 AM
Ok here is a nother I read it someplace and EVERYONE says your nuts that never happened,on the P 47 when it was under trials for acptance the plane would fly to 20,000ft maybe a little higher was it 30,000ft? and it would sputter carry on and shut off get lower and it would come to life again and sort of run,back on the ground thay found fowled spark plugs??.The bear F8 has I similar set up engine wise it think,the F8 went up there and had NO issues??P47 did a call too P&W and guess what thay said OOOOOOOO were sorry the pressurized spark plug loom for your plane is on back order we didn't think you were flying high enough yet too need it,Is this a true thing about the sparkplug wires this is the only time I have read about it anywere,any inlighten ment would be nice thanks all

Propellerhead
11-09-2006, 11:50 AM
Sounds plausible.

I don't know all of the science behind this, but the R-2800 and other radials either have high-tension or low-tension ignition systems.

The radial can use a couple of coils on the nosecase somewhere and transmit the high-tension spark to the plugs. High altitude degrades the strength of the spark (don't know why), hence the pressurized ignition harness.

Or the engine uses low-tension spark and you have a coil at each cylinder without the need for a pressurized ignition harness.

If you look at the cylinder heads of most later model radials you'll see a small ignition coil on each one between the rocker covers and the lead to the spark plug (R-2800 CB series, R-3350s, R-4360s).

I'm sure others can chime in on pros and cons. But I imagine reliability would be a biggie.

Time to consult Mr. White's R-2800 book again...

440_Magnum
11-09-2006, 12:03 PM
High altitude degrades the strength of the spark (don't know why), hence the pressurized ignition harness.


Air is a good insulator, and it gets better the higher you raise the air pressure. The other side of the coin: as you lower the pressure (go higher in altitude) the less benefit of "air insulation" you have on the ignition wiring, and the greater the probability that the spark will arc-over between the top of the plug and a cylinder head, or punch through the insulation of the plug wire and arc to a part of the engine, etc. Its not that the spark "gets weaker" it just goes to the wrong place.

The same thing happens *inside* an engine. The higher the compression ratio (including boost pressure) the harder it is to make the spark jump across the spark plug gap. That's why high compression and highly boosted engines need such powerful ignition systems. Its also why sometimes a car ignition system with a weak coil can make a nice spark from the coil wire to the block when you're testing it, but won't fire the plugs inside the cylinder. Doesn't happen so much these days with distributorless ignitions and urethane-potted coils.

W J Pearce
11-09-2006, 12:21 PM
I have looked a bit and can not find anything. This does not mean that it did not happen. One thing though, the P-47 and F8F were developed at different times. The F6F and the F4U were in a similar stage of testing with the P-47 and both used the R-2800.

Other models of the P-47, say the M and N, would be going through development with the F8F. But I would imagine by 1944 parts would not be on back order.

RandyGoss75
11-09-2006, 12:46 PM
Corky Meyer related a story in a recent issue of Flight Journal about the F6F Hellcats quitting at altitude (something over 30,000 feet) until they modified the harness like they had done earlier on the P-47's.

Sorry I don't remember all the specifics, but I did read the article this summer.

That might be what you were referring to? All the best, RG

Propellerhead
11-09-2006, 01:56 PM
Its not that the spark "gets weaker" it just goes to the wrong place.
Thanks Magnum, I knew somebody would know why a high-tension, unpressurized ignition would fail (get confused) at high altitude. I could remember the difference in ignition systems, but not what was going on.

2101
11-09-2006, 02:06 PM
Corsairs also had a pressurized harness for high altitude.And that is correct about not enough air to insulate.Pressurized ignitions take pressurized air from the blower section.It is real easy to spot.The ignition harness is cast and the distributors are large and cylindrical and squatty.This is the high tension system. Low tension has two cone shaped distributors an either side of mag and uses a round tubular harness that leads to a twin coil pack mounted on top of each cylinder

440_Magnum
11-09-2006, 03:32 PM
Thanks Magnum, I knew somebody would know why a high-tension, unpressurized ignition would fail (get confused) at high altitude. I could remember the difference in ignition systems, but not what was going on.

I *knew* that knowledge from my real job would come in handy someday! :D

I used to design and build high-voltage spark discharge systems with pressurized gas arc switches in a previous life. We would regulate the switch pressure to fine tune the self-discharge voltage- more pressure, higher voltage. I'm just glad to have gotten through that era with all my fingers and toes still attached...

51fixer
11-09-2006, 08:31 PM
2101 wrote;
-Corsairs also had a pressurized harness for high altitude.And that is correct about not enough air to insulate.Pressurized ignitions take pressurized air from the blower section.It is real easy to spot.The ignition harness is cast and the distributors are large and cylindrical and squatty.This is the high tension system. Low tension has two cone shaped distributors an either side of mag and uses a round tubular harness that leads to a twin coil pack mounted on top of each cylinder-

I took care of a FG-1D Corsair in the mid-ninties with a stock engine and Bendix mags. It had the updraft carb so you brought the hot dogs and marshmellows when you started it.
There were 2 mag systems used on the early R-2800's. A GE and the Bendix. GE mags have a combination mag and distributor and look like a flying saucer on each side of the upper nosecase. The Bendix set up has a dual mag in a single housing located on the top center of the nose case with 2 seperate distributors on each upper side of the nosecase. The distributor has an airpump built into it to pressurise the harness. It is not taken from the blower section of the engine or you would be using the same airfuel mixture being ignited in the cylinders.
The R-2800 had 2 basic configurations produced in WWII. I believe that most P-47s were the earlier series powerplant. The F8F used the later design. Although both were R-2800s most everything was different and little was interchangeable. Prop shaft was 60 spline vs 50 on the earlier. The nosecase, and cylinders for example did even look similar.
Engines had a many differeny configurations by assembling with different nosecases and blower sections to the same basic power section.
Rich

deepsky
11-09-2006, 10:29 PM
Boone T. Guyton, the chief test pilot for the F4U program wrote alittle about this problem in his book: Whistling Death, the test pilots story of the F4U corsair. An awsome book if you ever get the chance to read it. :thumbsup:

Stevo
11-10-2006, 08:28 AM
Like Randy mentioned there is an Artical in the Summer 2006 Hellcat Special Issue (http://www.rcstore.com/RS/general/detail.asp?catnum=F6F06&catego=BO) of Flight Journal by Corky Meyer that describes a simular problem in testing the F6F.