F4H/F-4 Phantom Ram Air Inlet Mysteries

28 January 2016: Corrections and additions.

 At the base of the leading edge of the vertical fin of the F4H/F-4 Phantom, there is a ram air inlet.

In the illustration of the original inlet above, two ducts lead downward from it, but didn't show where they went after that. Presumably, it is for cooling air for something, probably the aft fuselage and also to insure that fuel vapors don't build up there. An early Phantom illustration is of no help (the horizontal line goes from the fuselage fuel tanks to the fuel vent at the aft end of the fuselage).

I had thought that duct might have gone down to the louvers just above the afterburner on each side of the fuselage, although the air vented from them was more likely from the engine compartment.

The best guess now is that the air was piped down to the interior of what appears to be double-walled side panels aft of the engine tailpipes and exited through a large slot on either side of the aft end of the tailhook.

In a McAir inboard profile, it's referred to as a "blast cone cooling air duct". The pilots were told that the inlet was for "aft compartment cooling",

Still unanswered is why the configuration of the inlet changed over time.

For the F-4A/B/C and probably the D, it was a simple inlet almost flush with the top of the fuselage as shown in the first illustration with what appears to be a very shallow boundary-layer-removal slot at the bottom of the opening that dumps the slow moving air into the aft fuselage. (This is F-4A BuNo 145310 currently being refurbished for flight.)

Craig Kaston Photo

Then with the J/E/F, a horizontal splitter was added in front of the opening. Below the horizontal splitter, a curved wedge was provided to move the slow-moving air in the fuselage boundary layer outboard of the fin.

But wait, there's more: On the K/M, the Spey-powered Phantoms, the lower side of the inlet curves inboard so that the horizontal splitter extends slightly outboard of it for a short distance aft of the inlet.

Iain Ogilvie

That little exterior curve does more closely match the shape of the internal ducting. (Note that the internal louvers have disappeared with the introduction of the horizontal splitter.)

Bill Spidle

So: Why the ongoing "improvements"? There couldn't have been a great deal of benefit because it appears that the Bs were never retrofitted with the later inlet, not even on the Bee Line when they became Ns. It's possible that with the introduction of the Spey, the cooling being provided was marginal, necessitating a tweak in the inlet to reduce duct losses.