Questions from the Cockpit: Baffled by baffling?

Jessie, a private pilot and new aircraft owner from Colorado, asks: How does engine baffling work?

In a way much more baffling than most pilots realize! Most of us —including me until I researched the question — look at the rat’s maze of aluminum sheets and rubberesque material (technically called baffle seals) that run around the upper third or so of our engines, and assume that it’s a complex system of sluices to guide airflow. 

But nothing could be farther from the truth. It’s waaaaaay more complex than that.

The modern general aviation air-cooled engine isn’t cooled by flowing air like its ancient open-cowl grandfathers were, a system called velocity cooling. Instead, it uses pressure cooling.

The baffling divides the engine compartment into two chambers, like the human heart, creating a region of high pressure in the upper engine compartment and a second chamber of low pressure below and behind the engine. The air flows from the high pressure region in the upper compartment down over the cylinders to the low pressure region in the lower compartment. Plates called intercylinder baffles serve like elevator shafts to keep the descending air moving over the cylinder’s cooling fins.

So cooling isn’t spinner to exhaust pipe. It’s top to bottom. This is also why simply going faster doesn’t necessarily cool the engine more.

OK… so how do cowl flaps, if you have them, work? Opening the flaps creates a partial vacuum in the lower chamber, lowering the pressure farther, which in turn increases the rush of air over the engine from the high pressure region on the top.

It’s all very aerodynamic, don’t you think? 

But this is why a few holes in your baffling can cause overheating of your engine. Just the way a few holes in a helium balloon will ruin the party, holes in the baffling will depressurize the top compartment, robbing the bottom compartment of fresh cooling air.

In addition to holes, old, funky, crumbly baffle seals leak like sieves, of course. I’m told that the older ones are black and that newer ones are more commonly orange.

The high-pressure zone can also be deflated by baffling that gets bent the wrong direction when the cowl is secured (the flappy parts should always be pointing in toward the center of the upper compartment).

Given all the trouble with these seals, why not just run the rigid baffles right up to the inner wall of the cowl and be done with it?

Because GA engines rock on their shock mounts when they are running, and flexible seals snuggled up tight to the edge to the cowling can maintain a pressure seal as the engine moves, while solid ones would tear the cowl to sheds.

How much pressure are we talking about here? Engine guru Mike Busch, of Savvy Aviation, says the difference is “remarkably small,” with only about a one quarter psi difference between the chambers. That’s all you need, apparently, and more just increases drag. But this small margin of difference makes even the smallest leaks in the system a big deal.

Not so baffling once you see how it works. Except for the name. Why do we call them baffles? Is it because most of us don’t really understand how they work? 

As with many aviation words, baffle is French in origin, first popping up in the late 16^th century, meaning to cheat or deceive. Ah, that explains it. We’re cheating the gremlins of overheating by baffling the little SOBs!

Or maybe not.

If we dig deeper into the dictionary we find that “baffle,” as a verb, has two meanings. One is to bewilder or perplex; but the other meaning is to re-route — especially in relation to fluids, gases, sounds, and even light. To baffle the flow is to change its path. To perplex it. Make sense?

A device used to restrain the flow of a fluid eventually became known as a baffle, starting around 1881, when it replaced the older slang for the baffle-plates in stoves: Baffler. And that’s a good thing, because I don’t know about the rest of you, but I sure as heck don’t want no baffler leadin’ my plane around by the nose. 

I get baffled easily enough, all by myself, as it is.