Tristar
Pattern Altitude
Although most A&Ps probably know this but I thought it was kinda neat.
During the beginnings of flight training, we're all taught preflighting techniques but never really stop to think about some of the metal pieces sticking out or maybe holes that were either unnoticed or just "not in the preflight checklist." It usualy goes something like, sump the wings, walk around to the nose, check oil, make sure theres no bird nests in the cowl air inlets, then check the gear for a three fingers of metal and proper tire inflation then move on. But if you push the rewind button and go back to the air inlets in the nose, have you ever stopped to wonder what that air is doing and what its going to do next? Even more directly to my subject, have you ever wondered what that angled piece of metal was attatched to the bottom front edge of a standard 152's cowl was? You know, the part in front of the landing gear?
Don't worry, if you are/were like me and just took it for granted that it "must deflect bugs or something" and continued with the preflight, don't feel alone. Little did I know, it played an important part in engine cooling.
To digress slightly, most pilots get the general idea that the holes in the front are for engine cooling. It goes in through the front around something in the engine so it stays cool and doesnt blow up and then dissapears out the plane never to be seen again. Cessna's 152 manual describes it slightly better, "Ram air for the engine cooling enteres through two intake openings in the front of the engine cowling. The cooling air is directed around the cylinders and other areas of the engine by baffling, and is then exhausted through an opening at the bottom aft edge of the cowling." (Baffling is usualy known as the "orange" or "black" rubbery stuff surrounding the outer edges of the engine interior to block the air.) By highlighting Cessna's first two words "Ram air," means that somewhere in the engine there will be a pressure increase despite it being exited out the back. "So what," you say, "whats that got to do with that lip under the cowl?" Well among other factors, this pressure increases not only because of the airplane moving through the air but also the fact that heat expands and creates a larger volume of air equaling an even greater pressure. So, aircraft manufacturers had to create a way of pulling the pressure out of the engine. Cessna, among other companies, installed an angled and slightly curved downward metal addition to the bottom of the cowling. When air was deflected downward in the slipstreem, it created a low pressure area. If I understand correctly, this acts as a suction to the now hot and high pressure air created by passage of the ram air around the engine components. Thus, pulling the air outwards into your airspace's slipstream. Pretty cool huh? So although, it doesn't act very well as a bug deflector, its a great engine cooling tool which tends to be slightly more important.
For those of you that are A&P's I do have a question. Radiators are set up sort of the same way in some aircraft, meaning they have different pressures on different sides of the radiator to "pull" the air inwards. They can have a diverging system so that it can slow, expand, build pressure and is sucked through the radiator. (The P-51 gained about 15 kts thrust by this technique which gave it a no drag penalty factor). I was wondering, although minor in measure, if the lips on the cowl can do the same. After all they are also slowed, expanded by heat, thus creating a pressure build up, and sucked out by the pressure differential created by the cowl's lip. I doubt you'd get the same thrust as the P-51 example but maybe the theory is still there?
If anything is incorrect in my explanation, I'd like to know. Its the simple things that spark my interest sometimes, yet those are also the facts that seem to be the hardest for google to find.
Fly safe,
Tristan PP-ASEL
During the beginnings of flight training, we're all taught preflighting techniques but never really stop to think about some of the metal pieces sticking out or maybe holes that were either unnoticed or just "not in the preflight checklist." It usualy goes something like, sump the wings, walk around to the nose, check oil, make sure theres no bird nests in the cowl air inlets, then check the gear for a three fingers of metal and proper tire inflation then move on. But if you push the rewind button and go back to the air inlets in the nose, have you ever stopped to wonder what that air is doing and what its going to do next? Even more directly to my subject, have you ever wondered what that angled piece of metal was attatched to the bottom front edge of a standard 152's cowl was? You know, the part in front of the landing gear?
Don't worry, if you are/were like me and just took it for granted that it "must deflect bugs or something" and continued with the preflight, don't feel alone. Little did I know, it played an important part in engine cooling.
To digress slightly, most pilots get the general idea that the holes in the front are for engine cooling. It goes in through the front around something in the engine so it stays cool and doesnt blow up and then dissapears out the plane never to be seen again. Cessna's 152 manual describes it slightly better, "Ram air for the engine cooling enteres through two intake openings in the front of the engine cowling. The cooling air is directed around the cylinders and other areas of the engine by baffling, and is then exhausted through an opening at the bottom aft edge of the cowling." (Baffling is usualy known as the "orange" or "black" rubbery stuff surrounding the outer edges of the engine interior to block the air.) By highlighting Cessna's first two words "Ram air," means that somewhere in the engine there will be a pressure increase despite it being exited out the back. "So what," you say, "whats that got to do with that lip under the cowl?" Well among other factors, this pressure increases not only because of the airplane moving through the air but also the fact that heat expands and creates a larger volume of air equaling an even greater pressure. So, aircraft manufacturers had to create a way of pulling the pressure out of the engine. Cessna, among other companies, installed an angled and slightly curved downward metal addition to the bottom of the cowling. When air was deflected downward in the slipstreem, it created a low pressure area. If I understand correctly, this acts as a suction to the now hot and high pressure air created by passage of the ram air around the engine components. Thus, pulling the air outwards into your airspace's slipstream. Pretty cool huh? So although, it doesn't act very well as a bug deflector, its a great engine cooling tool which tends to be slightly more important.
For those of you that are A&P's I do have a question. Radiators are set up sort of the same way in some aircraft, meaning they have different pressures on different sides of the radiator to "pull" the air inwards. They can have a diverging system so that it can slow, expand, build pressure and is sucked through the radiator. (The P-51 gained about 15 kts thrust by this technique which gave it a no drag penalty factor). I was wondering, although minor in measure, if the lips on the cowl can do the same. After all they are also slowed, expanded by heat, thus creating a pressure build up, and sucked out by the pressure differential created by the cowl's lip. I doubt you'd get the same thrust as the P-51 example but maybe the theory is still there?
If anything is incorrect in my explanation, I'd like to know. Its the simple things that spark my interest sometimes, yet those are also the facts that seem to be the hardest for google to find.
Fly safe,
Tristan PP-ASEL
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