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P51 Mustang with air intake highlighted

Specifically talking about the p51, why do they have intakes? Don't propeller-driven planes not require any sort of intake because the propeller is what propels it by pushing air behind it?

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    $\begingroup$ @MichaelHall FWIW, my take was that the core point was that the OP simply didn't know what the intake is. The question about prop-driven aircraft needing inlets is still there, in the question body. As always, if an edit isn't helpful then you, the OP or anyone else is free to roll it back or edit again. $\endgroup$
    – Pondlife
    Mar 9, 2022 at 15:58
  • $\begingroup$ @MichaelHall Looks good to me! $\endgroup$
    – Pondlife
    Mar 10, 2022 at 4:34
  • $\begingroup$ @MichaelHall much better $\endgroup$ Mar 10, 2022 at 18:52

5 Answers 5

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Propellers are driven by engines, and engines burn fuel, and burning requires oxygen from the air. The intake supplies air to the engine. Burning creates heat, and air intakes also direct cooler air onto the engine to keep it from overheating.

In the case of the P-51, the duct you see is actually to feed the radiator. As the P-51's engine was water cooled, the radiator could be mounted in the belly of the airplane. The duct used the Meredith Effect to produce thrust through the expansion of air due to heating.

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    $\begingroup$ Isn't the big scoop on the p51 purely for the radiator though? $\endgroup$
    – Sanchises
    Mar 8, 2022 at 20:07
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    $\begingroup$ Yes it is @Sanchises, I was answering in general but I've edited to expand for this exact case. $\endgroup$
    – GdD
    Mar 8, 2022 at 20:10
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    $\begingroup$ The carb air intake is also clearly visible in the question photo, just behind and below the propeller spinner. It is smaller and less prominent than the radiator air intake on the belly. $\endgroup$ Mar 9, 2022 at 18:36
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The design of the P-51 means the engine radiator (a component of the cooling system) is situated behind and below the pilot, as in this diagram (from here):

diagram of P-51 cooling system

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    $\begingroup$ Good answer to @pondlife’s edit, but this doesn’t address the air breathing nature of internal combustion engines that was implied in the OP’s unedited first version. $\endgroup$ Mar 9, 2022 at 15:06
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don't propeller-driven planes not require any sort of intake

No, propeller-driven planes (piston or turboprop) require air intakes for a variety of purposes:

  • Combustion air
  • Engine cooling (whether air-cooled or liquid cooled)
  • Oil cooling
  • Cabin ventilation and heating
  • Fuel tank venting

And perhaps more.

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The p51 has an inline engine which makes cooling with just air flow infeasible since the pistons are not all exposed to air flow equally. If you look at pictures of other WW2 era planes like the F4U Corsair or F6F Hellcat have radial engines where all the pistons are exposed to the airflow allowing air cooling. These pistons form circle at the front exposing them all to airflow. Because of the cooling problems the p51 was water cooled using an inlet scoop underneath the plane. Supermarine Spitfires also had inline engines and have a cooling intake underneath as well.

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    $\begingroup$ “Inline engine” means specifically that there's only a single bank of cylinders. Both the Merlin / V-1650 and the V-1710 are V engines with two banks. (Not that this changes much about the point regarding infeasibility of air cooling.) $\endgroup$ Mar 10, 2022 at 23:49
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That’s the intake duct for the radiator for the liquid cooled engine. Similar to the radiator in your car, the radiator for the engine in the airplane is cooled by ram air from the intake passing through it and exiting a variable geometry ramp aft of it. Some of the air also bleeds off that ram air scoop and is fed through the oil cooler just below the radiator.

The smaller intake scoop just under the propeller spinner is the supercharger inlet that the engine aspires through.

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  • $\begingroup$ I'd say Carlo Felicione is right, what is in lower part of fuselage, below wing, is an air intake for the radiator. P-51 cooling was arranged as to obtain thrust from heat, some speak about 50 kg thrust added. The Cranfield repository includes many NACA, NASA and other aviation experimental studies. Paul Lamar, in rotaryeng.net published a booklet: 'Cooling your Wankel', describes extensively how the Mustang P-51 cooling works. Blessings + $\endgroup$
    – Urquiola
    Mar 16, 2022 at 0:11

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