The Radial engine was once the pinnacle of internal combustion engine design, but has gone mostly extinct. What happened? The Radial first ousted the Rotary, then to a large extent the inline engines (V12's) before the Radial itself was ousted.
In this video we briefly go through the history of the radial and we also take a bit of a deep dive into radial engine design and see what made it such a great engine.

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Chapters
0:00 The "Magic" of the Radial
0:21 Humble Beginnings
0:49 The Rotary Engine (Pros)
1:34 The Rotary Engine (Cons)
2:01 The Rise of the Air-cooled Radial
3:02 Basic Radial Engine Design
3:40 Radial Firing Order
4:03 Radial Firing Interval
4:59 Radial Crankshaft
6:17 Radial Valve Operation
6:36 Radial Primary Balance
7:04 Radial Secondary Balance
10:46 Radial Disadvantages
12:13 The Fall of the Radial Airplane Engine
14:03 Post-War Radial Applications
14:19 Modern Radials

Creative Commons attribution:
"Radial engine timing" animation by Stoianovici (https://commons.wikimedia.org/wiki/Fi...) license (https://rb.gy/660n3a) zoom, cropped, arrows & text overlaid
"Radial engine large" animation by Duk (https://en.m.wikipedia.org/wiki/File:...) license (https://rb.gy/660n3a) arrows & text overlaid
"4StrokeEngine Ortho 3D" by Zephyris (https://commons.wikimedia.org/wiki/Fi...) license (https://rb.gy/660n3a) overlaid lines & arrows
Secondary unbalance force in a 7 cylinder radial by Pasimi (https://commons.wikimedia.org/wiki/Fi...) license (https://creativecommons.org/licenses/...) overlaid text

Boeing-Stearman E75 N5729N by Julian Herzog (https://en.wikipedia.org/wiki/GNU_Fre...) no changes
Salmson 9Z by Duch.seb (https://rb.gy/660n3a) overlaid text
Rotary engine animation by MichaelFrey (https://creativecommons.org/licenses/...) overlaid text & arrow
Rotary engine animation by MichaelFrey (https://rb.gy/660n3a) increased spinning speed
Old Flywheel by Andrew Eaton (https://creativecommons.org/licenses/...) overlaid cross
Rolls-Royce Merlin by JAW (https://rb.gy/660n3a) overlaid text
LeBlond 90-5F by Darkroom (https://rb.gy/660n3a) no changes
BMW 132 (radial conrods) by Kogo (https://en.wikipedia.org/wiki/GNU_Fre...) no changes
XK engine camshafts by HReuter (https://creativecommons.org/licenses/...) overlaid cross
Piston by S. Diddy (https://creativecommons.org/licenses/...) no changes
Crankshaft by Alex Kovach (https://creativecommons.org/licenses/...) no changes
Pleuel-Käfer (conrod) by Benutzer:Thomas Ihle (https://rb.gy/660n3a) no changes
Crankshaft two stroke Engine by Werntec (https://en.wikipedia.org/wiki/GNU_Fre...) no changes
Conrod4M5ms by Bryllig (https://creativecommons.org/licenses/...) stopped, paused
Elsbett-3cyl by Dan Wesson (https://rb.gy/660n3a) no changes
Republic P-47 Thunderbolt by Eric Friedebach (https://creativecommons.org/licenses/...) zoom
F4U-5NL Corsair (NX43RW) by Alan Wilson (https://creativecommons.org/licenses/...) zoomed in slightly
F6F-5 Hellcat by David Merrett (https://creativecommons.org/licenses/...) no changes
Spitfire Mk XIV by David Merrett (https://creativecommons.org/licenses/...) no changes
BMW VI by W2k2 (https://rb.gy/660n3a) no changes
"Biggest radial cutaway" by Phil Genera (https://creativecommons.org/licenses/...) zoomed in
Pratt & Whitney R-4360 at Kalamazoo Air Zoo by D. Miller (https://creativecommons.org/licenses/...) zoomed in, text overlaid
CL215 43 Grupo by Javiramos43 (https://creativecommons.org/licenses/...) no changes

Creative Commons video attribution:
Sopwith Camel fires up its engine by scribex (   • Sopwith Camel fires up its engine  ), License CC-BY
Sopwith Camel Run Old Warden 22 September 2012 by Oldwardenman (   • Sopwith Camel Run Old Warden 22 Septe...  ), License CC-BY
Sopwith Camel Run 2 Old Warden 22 September 2012 by Oldwardenman (   • Sopwith Camel Run 2 Old Warden 22 Sep...  ), License CC-BY
Gyroscopic precession demonstration by MITK12Videos (   • MIT Physics: Spinning Bike Wheel and ...  ), License CC-BY
Bombing Missions, 1944 220451-07 by footagefarm (   • D-Day Invasion, Bombing Missions, 194...  ), License CC-BY
RAF Spitfires Taking Off by footagefarm (   / esjgn1me5i  , License CC-BY
B 17 b roll by Arizona Commemorative Air Force Museum (   • B 17 b roll  ), License CC-BY
Radial at National Air & Space Museum, Dulles by iros532 (   • Radial engine at National Air & Space...  ), License CC-BY
Cessna Aircraft Wings for Doubting Thomas (   • Cessna Aircraft   Wings for Doubting ...  ), CC-BY
Whiskey 7 First start 2017 (C-47) by David Andruczyk (   • Whiskey 7 First start 2017, Part 1  ), License CC-BY