{"id":21844852,"date":"2026-09-12T13:36:16","date_gmt":"2026-09-12T11:36:16","guid":{"rendered":"https:\/\/migflug.com\/afterburner\/northrop-x-21a-laminar-flow-control-wing-slots\/"},"modified":"2026-09-12T13:36:50","modified_gmt":"2026-09-12T11:36:50","slug":"northrop-x-21a-laminar-flow-control-wing-slots","status":"publish","type":"post","link":"https:\/\/migflug.com\/afterburner\/northrop-x-21a-laminar-flow-control-wing-slots\/","title":{"rendered":"Northrop X-21A: The Plane With 800,000 Slots in Its Wings"},"content":{"rendered":"<style>.et_pb_title_container h1.entry-title { padding-top: 40px !important; }<\/style>\r\n\r\n<p>Drag is the tax every aircraft pays for existing. On a transport aircraft in cruise, about half of that drag is nothing more than friction between the skin and the air.<\/p>\r\n\r\n<p>In the early 1960s Northrop and the United States Air Force went after that tax directly, with an aeroplane whose wings were cut with eight hundred thousand slots and plumbed to a pair of turbines whose only job was to suck air through them.<\/p>\r\n\r\n<p>It worked. That is the part people forget.<\/p>\r\n\r\n<div style=\"background:#f5f5f5;padding:20px 24px;margin:26px 0;border-left:4px solid #5C91FF\">\r\n<p style=\"margin:0 0 10px;font-weight:700;font-size:17px;color:#222\">Quick Facts<\/p>\r\n<p style=\"margin:4px 0;font-size:15px;line-height:1.7\"><strong>Aircraft:<\/strong> Northrop X-21A, two built, serials 55-0408 and 55-0410<br>\r\n<strong>Converted from:<\/strong> Douglas WB-66D Destroyer airframes<br>\r\n<strong>First flight:<\/strong> 18 April 1963, Edwards Air Force Base, NASA test pilot Jack Wells at the controls<br>\r\n<strong>Purpose:<\/strong> flight research into suction-type laminar-flow control<br>\r\n<strong>The wing:<\/strong> entirely new, span increased, sweep reduced from 35 to 30 degrees, with roughly 800,000 spanwise suction slots<br>\r\n<strong>Engines:<\/strong> two General Electric J79-GE-13 turbojets without afterburners, about 9,400 lbf each, moved from underwing pylons to the rear fuselage sides<br>\r\n<strong>Suction system:<\/strong> bleed air from the J79s drove two underwing &ldquo;bleed-burn&rdquo; turbines that pulled the boundary layer through the slots<br>\r\n<strong>Crew:<\/strong> five &mdash; a pilot and two flight engineers up front, two more test engineers in a bay under the wing<br>\r\n<strong>Span:<\/strong> 93 ft 6 in (28.51 m). <strong>Length:<\/strong> 75 ft 3 in (22.94 m). <strong>Gross weight:<\/strong> 83,000 lb (37,727 kg)<br>\r\n<strong>Result:<\/strong> up to 95 percent of the area intended for laminarisation achieved. Programme ended 1965<br>\r\n<strong>Fate:<\/strong> both airframes left to rot at Edwards; neither has been recovered or restored<\/p>\r\n<\/div>\r\n\r\n<h2 style=\"padding-top:22px\">Why anyone would drill 800,000 holes in a wing<\/h2>\r\n\r\n<p>Air flowing over a wing does one of two things. It slides in smooth layers, which is laminar flow, or it tumbles, which is turbulent. Turbulent air grips the surface far harder than laminar air does, and that grip is viscous drag.<\/p>\r\n\r\n<p>Get 80 percent of a wing flowing laminar and total drag falls by something like a quarter. No other single aerodynamic idea offers a payoff of that size. NASA people have called it the holy grail of aerodynamics without much embarrassment.<\/p>\r\n\r\n<p>The trouble is that laminar flow will not stay laminar on its own. Past a certain point along the wing the boundary layer trips into turbulence, and no amount of careful shaping stops it. So you cheat: you drill the surface and suck the thickening boundary layer away before it can trip.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/lockheed-p-38e-swordfish-laminar-flow-testbed.jpg\" alt=\"Lockheed P-38E Swordfish laminar flow wing testbed in flight\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">The idea long predates the X-21. This modified Lockheed P-38E, nicknamed Swordfish, flew laminar-flow wing research in the 1940s with an extended nose fairing. Photo: U.S. Air Force, public domain.<\/figcaption><\/figure>\r\n\r\n\r\n<h2 style=\"padding-top:22px\">A bomber turned inside out<\/h2>\r\n\r\n<p>Northrop needed an airframe big enough to carry the plumbing and a research crew. The Air Force handed over two Douglas WB-66D Destroyers &mdash; the weather-reconnaissance version of a light bomber that was already going out of fashion.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/douglas-rb-66c-destroyer.jpg\" alt=\"A U.S. Air Force Douglas RB-66C Destroyer taking off\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">A Douglas RB-66C Destroyer, a sister variant of the WB-66D airframes that became the X-21As. Note the original underwing engine pods, which Northrop deleted. This particular aircraft wears later Southeast Asia camouflage. Photo: U.S. Air Force, public domain.<\/figcaption><\/figure>\r\n\r\n\r\n<p>What came back was barely the same aeroplane. The underwing Allison J71s were gone, replaced by two non-afterburning General Electric J79s hung on the rear fuselage. That cleared the wing of everything.<\/p>\r\n\r\n<p>Then the wing itself was replaced. Longer span, more area, sweep relaxed from 35 to 30 degrees, and the whole upper surface cut with spanwise slots &mdash; around 800,000 of them. Bleed air from the J79s spun two turbines in underwing fairings, and those turbines did the sucking.<\/p>\r\n\r\n<p>The aircraft that lifted off at Edwards on 18 April 1963 with Jack Wells flying was, in effect, a flying vacuum cleaner with a bomber&rsquo;s fuselage attached.<\/p>\r\n\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/8v2HtiuaALc\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n\r\n<p style=\"font-size:13px;color:#777;text-align:center;font-style:italic;margin-top:-10px\">Period film from the X-21A laminar flow control demonstration programme.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">Everything that went wrong<\/h2>\r\n\r\n<p>The problems were not the exotic ones. They were the boring ones, which is worse.<\/p>\r\n\r\n<p>First, the wings were not smooth enough. Laminar flow is destroyed by surface steps measured in thousandths of an inch, and the joints where the wing panels spliced together were bigger than that. Putty was used to fair them out. The putty chipped off in flight, and the chips tripped the flow.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #5C91FF;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><em>&ldquo;In spite of a concerted effort to design and build the slotted LFC wings for the two airplanes to the close tolerances required, the resulting hardware was not good enough.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Albert L. Braslow<\/strong> &mdash; NASA aerodynamicist and technical consultant to the Air Force on the X-21 programme, writing in A History of Suction-Type Laminar-Flow Control (NASA, 1999)<\/div><\/div>\r\n\r\n\r\n<p>Second, and more fundamental, came spanwise contamination. On a swept wing there is a line where the airflow splits to go over and under. If the boundary layer there turns turbulent for any reason &mdash; a fuselage disturbance, a smear of insects &mdash; the turbulence runs outboard along that line and poisons the flow across most of the span.<\/p>\r\n\r\n<p>British tests with a swept suction wing mounted vertically on a Lancaster had hinted at this. Nobody had grasped how serious it was until the X-21 flew.<\/p>\r\n\r\n\r\n<figure class=\"wp-block-image size-large\" style=\"margin:0 0 24px;width:100%\"><img decoding=\"async\" class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" width=\"1024\" src=\"https:\/\/migflug.com\/afterburner\/wp-content\/uploads\/sites\/4\/2026\/09\/northrop-x-21a-edwards-afb.jpg\" alt=\"Northrop X-21A serial 50408 on the ramp at Edwards Air Force Base\" style=\"width:100%;height:auto;max-width:100%;display:block\"><figcaption style=\"font-size:13px;color:#777;text-align:center;margin-top:6px;font-style:italic\">X-21A 55-0408 on public display, carrying NORTHROP LFC markings and a red-and-white research scheme. Both aircraft spent their entire lives at Edwards. Photo: NASA, public domain.<\/figcaption><\/figure>\r\n\r\n\r\n<p>Third, ice. Flying in or near cirrus, the X-21 simply lost laminar flow &mdash; and got it straight back on leaving the cloud. Northrop&rsquo;s G.R. Hall built a theory for when ice crystals would break the flow, but nobody knew how much cirrus a real airliner would meet in a year, so nobody could say what the penalty would be in service.<\/p>\r\n\r\n<p>And rain, and bugs, and dust, all doing the same thing in slightly different ways.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">It worked, and they cancelled it anyway<\/h2>\r\n\r\n<p>By October 1965 the programme had never reached its main objective, which was to accumulate service experience comparable to an operational aircraft. Every hour had gone on fixing the wing instead. The Air Force&rsquo;s advisers said a major wing modification was needed before any useful maintenance data could be gathered.<\/p>\r\n\r\n<p>It was never done. Money and attention were going to Vietnam.<\/p>\r\n\r\n<p>By then, though, the X-21 was achieving laminar flow across up to 95 percent of the wing area it was designed to laminarise. On a large aeroplane. In real air. That is an extraordinary number, and it is the number nobody remembered.<\/p>\r\n\r\n\r\n<div style=\"background:#f8f9fa;border-left:4px solid #1565c0;padding:20px 22px;margin:18px 0 24px;font-size:16px;line-height:1.7\"><em>&ldquo;Unfortunately, top management in government and industry remembered the difficulties and time required to reach this point more than they did the accomplishment.&rdquo;<\/em><div style=\"margin-top:10px;font-size:14px;color:#555\"><strong>Albert L. Braslow<\/strong> &mdash; NASA aerodynamicist, on how the X-21 programme was received<\/div><\/div>\r\n\r\n\r\n<p>Both X-21As were parked at Edwards and left there. They were used as photographic targets, and then simply as scenery. The remains are still on the base. Nobody has moved to restore either one.<\/p>\r\n\r\n<h2 style=\"padding-top:22px\">The idea refused to die<\/h2>\r\n\r\n<p>Military funding for suction laminar-flow control stopped with the X-21 and did not come back for over a decade. The 1973 oil shock restarted it: when fuel costs money, a 25 percent drag reduction stops looking like a laboratory curiosity.<\/p>\r\n\r\n<p>NASA flew suction gloves on a Jetstar and later on an <a href=\"https:\/\/migflug.com\/afterburner\/f-16xl-cranked-arrow-wing-f-15e-nasa\/\">F-16XL<\/a>, chasing laminar flow at supersonic speeds. Airbus and its European partners have flown laminar-flow wing sections on a modified A340 flight-test aircraft. Every few years the concept surfaces again in a new engine-and-airframe study.<\/p>\r\n\r\n<p>All of it stands on data the X-21 paid for &mdash; on spanwise contamination, on surface tolerance, on ice crystals. The programme was written off as a failure. What it actually proved was that the aerodynamics were sound and the maintenance was the problem, which is a completely different finding.<\/p>\r\n\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/9h8MPCZRIHA\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n\r\n<p style=\"font-size:13px;color:#777;text-align:center;font-style:italic;margin-top:-10px\">NASA&rsquo;s later supersonic laminar-flow control research, a direct descendant of the X-21 work.<\/p>\r\n\r\n\r\n<div style=\"position:relative;padding-bottom:56.25%;height:0;overflow:hidden;margin:24px 0\"><iframe class=\"skip-lazy\" data-no-lazy=\"1\" loading=\"eager\" src=\"https:\/\/www.youtube.com\/embed\/h4zVM4jpV1U\" style=\"position:absolute;top:0;left:0;width:100%;height:100%;border:0\" allowfullscreen><\/iframe><\/div>\r\n\r\n<p style=\"font-size:13px;color:#777;text-align:center;font-style:italic;margin-top:-10px\">Where laminar-flow aircraft stand today, and why the idea keeps coming back.<\/p>\r\n\r\n<p><em>Sources: Albert L. Braslow, A History of Suction-Type Laminar-Flow Control with Emphasis on Flight Research, NASA Monographs in Aerospace History No. 13 (1999); Jenkins, Landis and Miller, American X-Vehicles: An Inventory, NASA SP-2003-4531; Joseph R. Chambers, Innovation in Flight; Jim Winchester, X-Planes and Prototypes; NASA Armstrong Flight Research Center fact sheets.<\/em><\/p>\r\n\r\n<div style=\"background:#f0f4ff;border-left:4px solid #5C91FF;padding:16px 20px;margin:32px 0 8px\">\r\n<p style=\"margin:0 0 8px;font-weight:600;color:#333\">Related Posts<\/p>\r\n<p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/northrop-x-4-bantam-tailless-research-jet\/\">The Tailless Jet That Failed Its Way Into History<\/a><\/p>\r\n<p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/northrop-yb-49-flying-wing-b2-spirit\/\">The Northrop YB-49: The Flying Wing That Died So the B-2 Could Live<\/a><\/p>\r\n<p style=\"margin:4px 0\"><a href=\"https:\/\/migflug.com\/afterburner\/f-16xl-cranked-arrow-wing-f-15e-nasa\/\">F-16XL: The Cranked-Arrow Falcon That Lost to the Strike Eagle<\/a><\/p>\r\n<\/div>","protected":false},"excerpt":{"rendered":"<p>In 1963 Northrop rebuilt two Douglas WB-66D bombers with wings cut by 800,000 suction slots, chasing the biggest single prize in aerodynamics. It worked. It was cancelled anyway.<\/p>\n","protected":false},"author":26,"featured_media":21844000,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"editor_notices":[],"footnotes":""},"categories":[666,664],"tags":[],"class_list":["post-21844852","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-history-and-legends","category-military-aviation"],"yoast_head":"<!-- This site is optimized with the Yoast SEO plugin v28.4 - https:\/\/yoast.com\/product\/yoast-seo-wordpress\/ -->\n<title>Northrop X-21A: The Plane With 800,000 Slots in Its Wings | MiGFlug<\/title>\n<meta name=\"description\" content=\"Northrop rebuilt two WB-66D bombers with 800,000 suction slots to chase laminar flow. First flight 18 April 1963. 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