Warbird Information Exchange

I'm usually a B-25 & Skyraider guy, so forgive me for all the Mustang questions lately ...

On the P-51 cowl, below the exhaust stacks, there is a rectangular panel which is sometimes a grille and sometimes a plate. What's the purpose of this and why is it sometimes solid and sometimes vented? Is it always one or the other or does the owner change out for reasons like cold weather - or whatever?

Seen it for years & always wondered what it was - thanks.

Ken

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"Take care of the little things and the big things will take care of themselves."

Filtered ground air intakes. Most rebuilds have the actual ducts and filter box deleted since the aircraft are no longer operated in real dirty areas.

John

I think the grill you are asking about is the air intake on both sides of the engine cowling. This is where the air intake for the carb comes in. There were originally two air filters in there, sort of a pleated material about 6 o8 inches long and 3 or 4 inches high. When you are using filtered, non ram air the engine sucks outside air through these filters. Some 51s have these filters, but a lot have had them removed and not reinstalled.
I have been told that it is easier to service the engine without having to remove these filters, I don't know for sure as I have never seen it done. I am sure if I had a Mustang with an engine that costs that much to overhaul, that I would want to protect it with clean air. But you see lot's of owners who have air filters for the $1000 engine in their car, but their $100,000 engine in their airplane is sucking in dirt with no filters. One engine builder I know who flies his 51 sure has the filters on his own plane.

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Bill Greenwood
Spitfire N308WK

"Cripes A' Mighty 3rd" is a noted example that did not have vented panels:

Bill,

Out of curiosity, does your Spitfire have ground air filters on it?

The filters were for ground use only. They were too restrictive to pass enough air to feed the engine at higher power settings. There is a spring loaded flapper door that opens up if for some odd reason the control flapper would not open from filter to ram air or, if the flapper for some reason closed at higher power settings.

Having the filter box installed does add another layer of "Stuff" to work through, work around or remove at inspection time. Most Mustangs out there today do not have them installed. Good idea or bad? Your call on that. As far as I know, over the last 50 odd years Mustangs have been in civilian operation, having the filters removed has never been cited as a primary factor in engine wear and tear. In fact, most warbirds in general do not have, and were not designed with ground air filters.

John

John, I am going to make this a separate topic as it seems important enough, and Spitfires do have a big air filter that can be opened in the air.

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Bill Greenwood
Spitfire N308WK

The 51 can have either the holed or solid plates installed, but the solid plate is more common away from a dusty environment. They are an alternate input source for carb air used on the ground if ice or dust is present along the taxi route.
The controls for selection are on the left console rear. You can select RAM air or Unrammed Filtered Air (if the holed plates are present). Later 51's also have a hot/cold slider handle selector.
Most civilianized 51,s use the solid plates as they are mostly operated out of hard surface environments.

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Dudley Henriques
International Fighter Pilots Fellowship

P-51D Pilots manual AN 01-60JE-1 States;
"Desert Operation
Dust filters are installed in the air intake ducts, at each side of the engine compartment. When conditions warrant, or at the direction of the Operations Officer, use "UNRAMMED FILTERED AIR" for starting, take-off, and landing."

1. For today's operation they really don't work well for operations. The filters are old and probably dirtier than the air around us from sitting around for 50-60 years. I don't know if I would trust there integrity and ingesting debris into the induction system would be bad.

2. The air box gets in the way! To change the generator or alternator with it installed requires you to drop the forward air inlet duct with the filter box attached. To drain the oil from the cuno oil filter you need to drop it. With it installed the coolant line is routed where it is in your way to pull the intake spark plugs on the A or R/H bank. If you run a V1650-9 the bumps on the crankcase will rub on it.

PE had it and it caused me grief. BE doesn't have one and it makes life easier. I have several on a shelf upstairs. Thats where I want them.

To compare the filters on a P-51D to a P-38, the 38 filter's are about 2 times larger IIRC. They are mounted in the main landing gear wheel wells. The gear retracts around them. The doors close and you seal them off so they weren't designed for operation for other than near the ground.

The Merlin also has the flame guards mounted in the induction system were the rams horn attaches to the head. The flame trap is a mesh which will reduce the amount a flame can travel if you have a back fire. There is many feet of pressurized fuel-air mixture waiting to get to a cylinder. If a backfire should happen then there is enough explosive there to create a big mess. This mesh also acts as a filter to a degree. Things such as grass and such won't travel through it. The oil and goo from the fuel residue makes it necessary to clean these after 200 hrs or so.

Rich

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Rich Palmer

Remember an Injured Youth
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