Piasecki H-21 Explained

The Piasecki H-21 Workhorse/Shawnee is an American helicopter, the fourth of a line of tandem rotor helicopters designed and built by Piasecki Helicopter (later Boeing Vertol). Commonly called "the flying banana", it was a multi-mission helicopter, capable of being fitted with wheels, skis or floats.

The H-21 was originally developed by Piasecki as an Arctic rescue helicopter. The H-21 had cold-weather features permitting operation at temperatures as low as and could be routinely maintained in severe cold weather environments.

Design and development

Piasecki Helicopter designed and successfully sold to the United States Navy a series of tandem rotor helicopters, starting with the HRP-1 of 1944. The HRP-1 was nicknamed the "flying banana" because of the upward angle of the aft fuselage, which ensured that the large rotors could not strike the fuselage in any flight attitude. The name was later applied to other Piasecki helicopters of similar design, including the H-21.

In 1949, Piasecki proposed the YH-21 Workhorse, which was an improved, all-metal derivative of the HRP-1, to the United States Air Force (USAF). Using two tandem, fully articulated three-bladed counter-rotating rotors, the H-21 was powered by one nine-cylinder Curtis-Wright R-1820-103 Cyclone supercharged air-cooled radial engine.

After the first flight of the YH-21 on 11 April 1952, the USAF ordered 32 H-21A SAR models and 163 of the more powerful H-21B assault transport variant.^[1] The H-21B was equipped with an uprated version of the Wright 103 engine, developing 1,425 shaft horsepower (1,063 kW) and featured rotor blades extended by 6 inches (152 mm). With its improved capabilities, the H-21B could carry 22 fully equipped infantrymen or 12 stretchers, plus space for two medical attendants, as a medevac helicopter. With its Arctic winter capabilities, the H-21A and H-21B were put into service by the USAF and the Royal Canadian Air Force (RCAF) to maintain and service Distant Early Warning Line (DEW) radar installations stretching from the Aleutian Islands and Alaska across the Canadian Arctic to Greenland and Iceland.

In 1952, some H-21As were evaluated by USMC helicopter squadron HMX-1 for air assault.^[2] In 1957, an H-21B was loaned to the United States Marine Corps (USMC) to evaluate the helicopter as an airborne tug to tow disabled landing ships and amphibious landing vehicles to the beach. During the evaluation, the H-21B towed an LST at and a simulated tracked amphibious vehicle from the water to the beach.^[3] The uprated 1425 hp Wright engine used in the H-21B was also used in subsequent variants sold to both the U.S. Army (as the H-21C Shawnee) and the military forces of several other nations. In 1962, the H-21 was renamed the CH-21 in U.S. Army service.

In 1959 Vertol Aircraft, the new name for Piasecki Helicopters, came up with a concept for heavy lift over short distances where between two and six H-21Bs would be linked by beams to lift heavy loads. It was considered to be unsafe, because if one helicopter had mechanical problems during the lift it could unbalance the structure and cause all helicopters to crash.^[4]

Operational history

French service in the Algerian War

In 1956, seeking a way to use helicopters for ground-attack in the Algerian War, the French Air Force and French Army Aviation (Aviation Légère de l'Armée de Terre) experimented with arming the Sikorsky S-55, then being superseded in service by the more capable Piasecki H-21 and Sikorsky H-34 helicopters. Some French Air Force and Army aviation H-21C helicopters were subsequently armed with fixed, forward-firing rockets and machine guns. A few even had racks for bombs but tests found that the H-21C lacked the maneuverability and performance needed for ground-attack. The H-21C was far more successful as a troop transport, and most H-21Cs in service were eventually fitted with flexible door-mounted guns such as the .50 cal. (12.7 mm) M2 Browning machine gun or the (ex-German) MG 151/20 20 mm aircraft autocannon, for defensive use when landing assault forces under fire.^{[5] [6] [7]}

Though the H-21 had been removed from ground-attack, official U.S. Army evaluations at the time indicated that the type was actually more likely to survive hits by ground fire than was the Sikorsky CH-34; this was assumed to be a consequence of the location and construction of the CH-34's fuel tanks. By the close of the Algerian War, troop-carrying H-21C helicopters were being used in concert with H-34 ground-attack helicopters in large counterinsurgency operations.^{[5] [6] [7]}

U.S. Army operations

The H-21C saw extensive service with the U.S. Army, primarily for use in transporting troops and supplies. On 24 August 1954, with the assistance of inflight refueling provided by a U.S. Army U-1A Otter, a H-21C known as Amblin' Annie became the first helicopter to cross the United States non-stop.^{[8] [9]} Experiments were made by the Army in arming the H-21C as a gunship; some Shawnees were armed with flex guns under the nose, while others were fitted with door guns.

One experimental version was tested with a Boeing B-29 Superfortress .50 cal. remote turret mounted beneath the nose. The H-21C (later designated CH-21C) was first deployed to South Vietnam in December 1961 with the Army's 8th and 57th Transportation Companies, in support of Army of the Republic of Vietnam troops. In Army service, the CH-21C Shawnee could be armed with 7.62 mm (.308 in) or 12.7 mm (.50 in) flexible door guns. Relatively slow, the CH-21's unprotected control cables and fuel lines proved vulnerable to the Vietcong, which were increasingly well supplied with automatic small arms and heavy (12.7 mm) anti-aircraft machine guns.

The H-21, which was designed for cold weather operations, performed poorly in the hot weather of Vietnam. Despite being capable of carrying 20 passengers, it could lift only nine when operating in Vietnam. Pilots reported that engines that were rated for 600 hours of flying time were lasting only 200 hours or less in Vietnam.^[10] The shooting down of a CH-21 Shawnee near the Laotian-Vietnamese border with the death of four aviators in July 1962 were some of the U.S. Army's earliest casualties in the Vietnam War.^[11] Despite these events, the Shawnee continued in service as the U.S. Army's helicopter workhorse in Vietnam until 1964 when it was replaced with the Bell UH-1 Huey. In 1965, the Boeing CH-47 Chinook was deployed to Vietnam and later that year, most CH-21 helicopters were withdrawn from active inventory in the U.S. Army and Air Force.

Variants

XH-21

USAF designation of the first H-21 prototype.

YH-21 Work Horse

USAF Search And Rescue (SAR) version of the HRP-2 for service test, eighteen built

H-21A Work Horse (Model 42)

Same as YH-21 with detailed changes and powered by one 1250hp Wright R-1820-102, re-designated CH-21A in 1962, 32 built for USAF, 6 for the Royal Canadian Air Force

H-21B Work Horse (Model 42)

Same as H-21A but with uprated Wright engine (1425hp) and seats for 20 troops, autopilot as standard and limited armour protection and external fuel tanks, became CH-21B in 1962, 163 built for U.S. forces. 10 built for Japanese Self-Defense Forces; 10 H-21B built for the French Navy.

SH-21B Work Horse

Rescue conversion of the H-21B, became HH-21B in 1962.

H-21C Shawnee (Model 43)

US Army version of the H-21B, became CH-21C in 1962, 334 built for U.S. forces. 32 built under license by Weser Flugzeugbau for the West German Army. 98 built for the French Air Force and French Army Aviation (ALAT).

XH-21D Shawnee (Model 71).

Two H-21Cs re-engined with two General Electric T58 turboshaft engines in place of the Wright R-1820. Not placed into production.

CH-21A

H-21A redesignated in 1962.

CH-21B

H-21B redesignated in 1962.

CH-21C

H-21C redesignated in 1962.

HH-21B

SH-21B redesignated in 1962.

Model 42A

Conversion by Vertol Aircraft (Canada) of eight Royal Canadian Air Force H-21s for civilian use. Equipped to carry 19 passengers or 2,820lb (1,279kg) of internal cargo or a 5,000lb (2,268kg) slung load.

Model 44A

Commercial 19-passenger transport version of the H-21B. 11 total (Swedish military designation: Hkp 1), 2 for the Swedish Air Force, 9 for the Swedish Navy. 2 used for test/evaluation purposes by Japan Self-Defense Forces.

Model 44B

Commercial 15-passenger/freighter version of the H-21B.

Model 44C

Commercial eight-passenger executive version of the H-21B.

CH-127

Vertol Canada Model 44

Piasecki HkP1

Piasecki model 44 for the Swedish Navy

Operators

Military operators

• Royal Canadian Air Force^[12]

• Congo Democratic Air Force^[13]

• French Air Force^[14]
• French Army^[15]

• French Navy^[16]

• Japan Air Self-Defense Force^[17]
• Japan Ground Self-Defense Force^[17]

• Swedish Navy^[18]

United States

• United States Air Force^[19]
• United States Army^[20]

• German Army^[21]

Civil operators

United States

• New York Airways^[22]

Aircraft on display

Canada

• 642 – On display at the Musée de la Défense aérienne of CFB Bagotville, Quebec^[23]
• 641 – On display at the Heritage Air Park of the Comox Air Force Museum CFB Comox, British Columbia^[24]

France

• FR94 – H-21C on static display at the Musée de l'Aviation Légere de l'Armée de Terre et de l'Hélicoptère in Dax, Landes.^[25]
• FR106 is on display at the "Ailes Anciennes" Museum at Toulouse-Blagnac Airport.^[26]

Germany

• 83+07 – H-21C on static display at the Hubschraubermuseum Bückeburg in Bückeburg, Lower Saxony.^{[27] [28]}
• 83+08 – H-21C on static display at the Militärhistorisches Museum Flugplatz Berlin-Gatow in Berlin, Berlin.^[29]
• 83+11 – H-21C on static display at the Flugausstellung Hermeskeil in Hermeskeil, Rhineland-Palatinate.^[30]
• 83+17 – XH-21D on static display at the Auto & Technik Museum Sinsheim in Sinsheim, Baden-Württemberg.^[31]

Japan

• JG-0001 – Model 44A on static display at the Bihoro Aviation Park in Bihoro, Hokkaido.^[32]
• JG-0002 – Model 44A on static display at the Tokorozawa Aviation Museum in Tokorozawa, Saitama.^{[33] [34] [32]}
• 02-4756 – H-21B on static display at the JASDF Air Park in Hamamatsu, Shizuoka.^{[35] [36]}

Russia

• N74056 – Model 44A on static display at Central Air Force Museum in Monino, Moscow.^[37]

Sweden

• 01001 – HKP 1 on static display at the Swedish Air Force Museum in Linköping, Östergötland.^[38]
• 01009 – HKP 1 on static display at the Gotland Museum of Defence in Tingstäde, Gotland.^[39]

United States

Airworthy

• 54-4001 – CH-21B airworthy at the Classic Rotors Museum in Ramona, California. This is the last H-21 still flown.^{[40] [41]}

Static Display

• 51-15857 – CH-21B on static display at the National Museum of the United States Air Force at Wright-Patterson AFB in Dayton, Ohio. It was obtained from Eglin Air Force Base in January 1965.^[42]
• 51-15859 – CH-21B on static display at Battleship Memorial Park in Mobile, Alabama.^{[43] [44]}
• 51-15886 – CH-21C on static display at the Aerospace Museum of California at McClellan Airport (former McClellan AFB) in McClellan, California.^[45]
• 51-15892 – CH-21B on static display at the Quonset Air Museum at the former Naval Air Station Quonset Point in North Kingstown, Rhode Island.^[46]
• 52-8676 – CH-21B on static display at the Strategic Air Command & Aerospace Museum near Offutt AFB in Ashland, Nebraska.^[47]
• 52-8685 – CH-21B on static display at the Museum of Aviation at Robins AFB near Warner Robins, Georgia.^[48]
• 52-8688 – CH-21B on static display at the Travis Air Force Base Heritage Center near Fairfield, California.^[49]
• 52-8691/52-8706 – CH-21B on static display at Kirtland Air Force Base in Albuquerque, New Mexico. This airframe is a composite of two different airframes and is painted as 53-4343.^[50]
• 52-8696 – CH-21B on static display at Joint Base Elmendorf-Richardson near Anchorage, Alaska.^[51]
• 53-4323 – HH-21B on static display at Berryman War Memorial Park in Bridgeport, Washington.^[52]
• 53-4324 – CH-21B on static display at the Vintage Flying Museum in Fort Worth, Texas.^[53] This airframe was previously on display at the Pate Museum of Transportation in Cresson, Texas.^[54]
• 53-4326 – CH-21B on static display at the March Field Air Museum at March Air Reserve Base (former March AFB) in Riverside, California.^[55]
• 53-4347 – CH-21B on static display at the Pueblo Weisbrod Aircraft Museum in Pueblo, Colorado.^{[56] [57]}
• 53-4354 – CH-21C on static display at the Arkansas Air & Military Museum in Fayetteville, Arkansas.^[58]
• 53-4362 – SH-21B on static display at the Alaska Museum of Transportation and Industry in Wasilla, Alaska.^[59]
• 53-4366 – CH-21B on static display at the Museum of Flight in Seattle, Washington.^[60]
• 53-4367 – CH-21B on static display at the Mid-Atlantic Air Museum, Reading, Pennsylvania.^[61]
• 53-4369 – CH-21B on static display at the United States Army Aviation Museum near Fort Novosel in Daleville, Alabama.^[62]
• 53-4389 – CH-21B on static display at the Pima Air & Space Museum adjacent to Davis-Monthan AFB in Tucson, Arizona.^[63]
• 54-4404 – CH-21B on static display at the Alaska Aviation Heritage Museum in Anchorage, Alaska.^[64]
• 55-4140 – CH-21C on static display at the American Helicopter Museum & Education Center in West Chester, Pennsylvania.^[65] This airframe was previously on display at the Intrepid Sea, Air & Space Museum in New York, New York.^[66]
• 55-4218 – CH-21C on static display at the Evergreen Aviation & Space Museum in McMinnville, Oregon.^[67] This airframe was previously on display at the Wings Over the Rockies Air and Space Museum at the former Lowry AFB in Denver, Colorado. It is painted as 53-4379.^[68]
• 56-2040 – CH-21C on static display at the United States Army Aviation Museum at Fort Novosel, near Daleville, Alabama.^[69]
• 56-2077 – CH-21C on static display at the U.S. Army Transportation Museum ay Fort Eustis near Newport News, Virginia.^[70]

• 56-2142 – CH-21C on static display at the Hill Aerospace Museum at Hill AFB near Ogden, Utah. This airframe is painted as 54-4002.^[71]
• 56-2159 – CH-21C on static display at the Pima Air & Space Museum adjacent to Davis-Monthan AFB in Tucson, Arizona.^[72]

Stored or under restoration

• 52-8623 – CH-21B in storage at the Air Force Flight Test Museum at Edwards AFB in Edwards, California.^[73]
• 52-8683 – H-21B in storage with Basler Turbo Conversions in Oshkosh, Wisconsin.^[74]
• 53-4329 – CH-21B under restoration at the Museum of Flight Restoration Center at Paine Field in Everett, Washington.^[75]
• 54-4003 – CH-21B in storage at the American Helicopter Museum & Education Center in West Chester, Pennsylvania. This airframe was previously on display at the Florence Air & Missile Museum in Florence, South Carolina.^[76]
• c/n 438 – Model 44B in storage in unrestored condition at the Gillespie Field Annex of the San Diego Air & Space Museum in El Cajon, California.^[77]

References

• Arkell, Basil. "Helicopters in Contrast: (2) Flying the American Vertol 44". Flight, Vol. 71, No. 2526. pp. 844–846.
• Bridgman, Leonard. Jane's All the World's Aircraft 1952–53. London: Sampson Low, Marston & Company, Ltd., 1952.
• Duke, R.A., Helicopter Operations in Algeria [Trans. French], Dept. of the Army (1959).
• Harding, Stephen. U.S. Army Aircraft Since 1947. Shrewsbury, UK:Airlife Publishing, 1990. .
• Swanborough, F. G. and Bowers, Peter M. United States Military Aircraft since 1909. London: Putnam, 1963.
• "The World's Air Forces". Flight, Vol. 67, No. 2416, 13 May 1955. pp. 615–668.
• "World Helicopter Market". Flight, Vol. 94, No. 3096, 11 July 1968. pp. 48–60.
• United States Air Force Museum (1975 edition)

External links

• H-21 US Army Aviation history fact sheet

Notes and References

1. Bridgman 1952, p. 30.
2. https://books.google.com/books?id=ytwDAAAAMBAJ&dq=1954+Popular+Mechanics+January&pg=PA101 "Here Comes the Leathernecks."
3. https://books.google.com/books?id=DOEDAAAAMBAJ&dq=true&pg=PA106 "Flying Salvage Craft Works on Land Or Sea."
4. https://books.google.com/books?id=vtsDAAAAMBAJ&dq=true&pg=PA100 "Workhorses of the Air."
5. France, Operations Research Group, Report of the Operations Research Mission on H-21 Helicopter (1959)
6. Riley, David, French Helicopter Operations in Algeria, Marine Corps Gazette, February 1958, pp. 21-26
7. Shrader, Charles R. The First Helicopter War: Logistics and Mobility in Algeria, 1954-1962 Westport, CT: Praeger Publishers (1999)
8. The New Propwash, Fox Valley Sport Aviation Assoc., Vol.2, Issue 12, p. 5
9. Web site: Boeing: Historical Snapshot: CH-21 Shawnee/Vertol 44 Helicopter.
10. News: Army Copter Pilots in Vietnam Say H-21's Are Not Adequate . Halberstam . David . The New York Times . 19 January 1963 . 3 . 3 May 2019 . subscription.
11. Harding 1990, pp. 261–262.
12. Web site: World Air Forces 1955 pg. 621 . flightglobal.com . 4 August 2018.
13. Web site: Trade Registers . Armstrade.sipri.org . 3 February 2019 . 14 April 2010 . https://web.archive.org/web/20100414022558/http://armstrade.sipri.org/armstrade/page/trade_register.php . dead .
14. Arkell Flight 21 June 1957, p. 844.
15. Web site: World Helicopter Market 1967 pg. 60 . flightglobal.com . 4 August 2016.
16. Web site: aéronautique navale H-21 . helis.com. 5 January 2013.
17. Web site: World Helicopter Market 1967 pg. 62 . Flightglobal Insight . 2018 . 4 August 2018.
18. Web site: World Helicopter Market 1967 pg. 65 . flightglobal.com . 4 August 2016.
19. Swanborough and Bowers 1963, pp. 398–399.
20. Flight 13 May 1955, p. 668.
21. Web site: World Helicopter Market 1967 pg. 61 . flightglobal.com . 4 August 2016.
22. Web site: Helicopters in Civil Operations pg. 388. flightglobal.com . 21 March 1958. 10 February 2015.
23. Web site: Harold A. Skaarup, author of Shelldrake. silverhawkauthor.com. 6 January 2019. 6 January 2019. https://web.archive.org/web/20190106153330/http://silverhawkauthor.com/aircraft-preserved-in-canada-6-warplanes-in-quebec_389.html. dead.
24. Web site: Guide to the Aircraft of the CAFM Heritage Air Park. 2022-09-03.
25. Web site: Airframe Dossier – Piasecki-Vertol H-21C-PH Shawnee, s/n FR94 AdT, c/n FR94. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
26. Web site: Ailes Anciennes Toulouse. aatlse.org. 23 November 2016.
27. Web site: Vertol V-43 (H-21C). Das Hubschraubermuseum Bückeburg. 21 October 2016.
28. Web site: Airframe Dossier – PiaseckiH-21, s/n 83+07 Heeresfliegertruppe, c/n WG7. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
29. Web site: Transporthubschauber Vertol (Piasecki) V-43A (H-21C) (Luftfahrzeugkennung der Bundeswehr: 83+08). Militärhistorische Museum der Bundeswehr Flugplatz Berlin Gatow. 20 October 2016. de. 2 January 2016.
30. Web site: Flugzeuge. Flugausstellung. 21 October 2016. dead. https://web.archive.org/web/20151231084758/http://www.flugausstellung.de/flugzeuge.html. 31 December 2015.
31. Web site: Airframe Dossier – PiaseckiH-21, s/n 83+17 Luftwaffe, c/n WG17. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
32. Web site: Thompson. Paul. JGSDF – Where Are They Now?. J-Hangar Space. J-HangarSpace. 21 October 2016.
33. Web site: Experience Exhibits. Tokorozawa Aviation Museum. 21 October 2016. dead. https://web.archive.org/web/20160314120633/http://tam-web.jsf.or.jp/contxe/modules/tinyd3/index.php?id=2. 14 March 2016.
34. Web site: Airframe Dossier – Piasecki 44A, s/n 50002 JASDF, c/n 44-20. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
35. Web site: Airframe Dossier – Piasecki-Vertol CH-21B Work Horse, s/n 02-4756 JASDF, c/n B148. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
36. Web site: Thompson. Paul. JASDF – Where Are They Now?. J-HangarSpace. 21 October 2016.
37. Web site: Airframe Dossier – Piasecki 44C, c/n 44-417, c/r N74056. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
38. Web site: Hkp 1 . 21 April 2010 . flygvapenmuseum.se . Statens Försvarshistoriska museeer . Linköping . sv . 21 October 2016 . https://web.archive.org/web/20100813031129/http://www.flygvapenmuseum.se/sv/Foremal--Fakta/Foremal/Flygplan/Utstallda/Flygplan-teknikutveckling/Hkp-1 . 13 August 2010.
39. Web site: Helikopter – Gotlands Försvarsmuseum . 18 October 2017 . digitaltmuseum.org . Tingstäde . Gotlands Forsvarsmuseum . sv . 27 July 2020. https://archive.today/20200727143433/https://digitaltmuseum.org/021027327417/helikopter?aq=topic:%22Helikopter%22&i=11. 27 July 2020. live.
40. Web site: Vertol H-21 Shawnee/Work Horse. Classic Rotors. 20 October 2016. 3 March 2016. https://web.archive.org/web/20160303184235/http://www.rotors.org/h21/21.htm. dead.
41. Web site: FAA REGISTRY [N64606]]. Federal Aviation Administration. U.S. Department of Transportation. 20 October 2016. 29 February 2012. https://web.archive.org/web/20120229091551/http://registry.faa.gov/aircraftinquiry/NNum_Results.aspx?NNumbertxt=N64606. dead.
42. Web site: Vertol CH-21B Workhorse. National Museum of the US Air Force. 20 October 2016. 29 May 2015.
43. Web site: AIRCRAFT COLLECTION. Battleship Memorial Park. USS ALABAMA. 20 October 2016. dead. https://web.archive.org/web/20160728133844/http://www.ussalabama.com/uss-aircraft-collection. 28 July 2016.
44. Web site: Airframe Dossier – Piasecki-Vertol CH-21B Work Horse, s/n 51-15859 USAF, c/n B.6. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
45. Web site: Airframe Dossier – Piasecki CH-21C Shawnee, s/n 51-15886 US, c/r N48082. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
46. Web site: Carroll. Sean. [Untitled]]. 21 October 2016.
47. Web site: CH-21B "Work Horse". Strategic Air Command & Aerospace Museum. 20 October 2016.
48. Web site: CH-21B "Workhorse". Museum of Aviation. 20 October 2016.
49. Web site: Airframe Dossier – Piasecki-Vertol H-21B Work Horse, s/n 52-8688 USAF, c/r N6795. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
50. Web site: Airframe Dossier – Piasecki UH-21B Work Horse, s/n 52-8691 USAF. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
51. Web site: Heritage Park Tour Briefing. Matanuska-Susitna Borough School District. 21 October 2016. May 2007.
52. Web site: Airframe Dossier – PiaseckiH-21, s/n 53-4323 USAF, c/n B.73, c/r N6793. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
53. Web site: Piasecki CH-21B Workhorse. Vintage Flying Museum. 20 October 2016.
54. Web site: Airframe Dossier – Piasecki CH-21B Work Horse, s/n 53-4324 USAF, c/n B74. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
55. Web site: H-21 Workhorse. March Field Air Museum. 20 October 2016. 21 October 2016. https://web.archive.org/web/20161021131424/http://www.marchfield.org/aircraft/helicopter/h-21b-workhorse-piasecki/. dead.
56. Web site: Actual Aircraft Aircraft on Display in Pueblo. Pueblo Weisbrod Aircraft Museum. 20 October 2016. dead. https://web.archive.org/web/20161225043629/http://www.pwam.org/aircraftdisplay1.html. 25 December 2016.
57. Web site: Aircraft 53-4347 Data. Airport-Data.com. 20 October 2016.
58. Web site: Airframe Dossier – PiaseckiH-21, s/n 53-4354 USAF. Aerial Visuals. AerialVisuals.ca. 8 November 2016.
59. Web site: Airframe Dossier – PiaseckiH-21, s/n 53-4362. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
60. Web site: Piasecki H-21B (CH-21B) Workhorse. The Museum of Flight. 20 October 2016.
61. Web site: Aircraft of the Mid-Atlantic Air Museum. Mid-Atlantic Air Museum. 20 October 2016. 7 June 2012. https://web.archive.org/web/20120607004015/http://www.maam.org/aircraft.htm. dead.
62. Web site: Airframe Dossier – Piasecki HH-21B Work Horse, s/n 53-4369 US, c/r N109RB. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
63. Web site: Airframe Dossier – Piasecki-Vertol CH-21B Work Horse, s/n 53-4389 USAF, c/n B.139. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
64. Web site: Airframe Dossier – Piasecki H-21B Work Horse, s/n 54-4004 USAF. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
65. Web site: Piasecki H-21 / CH 21B Workhorse. American Helicopter Museum & Education Center. 21 October 2016. dead. https://web.archive.org/web/20161021070924/http://americanhelicopter.museum/aircraft/piasecki-h-21-ch-21b-workhorse. 21 October 2016.
66. Web site: Airframe Dossier – Piasecki CH-21C Shawnee, s/n 55-4140 US, c/n C.94. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
67. Web site: Helicopters. Evergreen Museum Campus. Evergreen Museum. 20 October 2016. 31 May 2017. https://web.archive.org/web/20170531024716/http://www.evergreenmuseum.org/helicopters. dead.
68. Web site: Airframe Dossier – PiaseckiH-21, s/n 55-4218 US Army. Aerial Visuals. AerialVisuals.ca. 20 October 2016.
69. Web site: Airframe Dossier – Piasecki CH-21C Shawnee, s/n 56-2040 US, c/n C.202. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
70. Web site: Airframe Dossier – Piasecki CH-21C Shawnee, s/n 56-2077 US, c/n C.239. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
71. Web site: CH-21C "Workhorse". Hill Air Force Base. 20 October 2016. 27 September 2007. 19 October 2016. https://web.archive.org/web/20161019071405/http://www.hill.af.mil/About-Us/Fact-Sheets/Display/Article/397285/ch-21c-workhorse. dead.
72. Web site: WORKHORSE. Pima Air & Space Museum. Pimaair.org. 20 October 2016. 21 October 2016. https://web.archive.org/web/20161021064504/http://www.pimaair.org/aircraft-by-name/item/piasecki-vertol-ch-21c-workhorse. dead.
73. Web site: Aircraft Inventory. Flight Test Historical Foundation. 21 October 2016. https://web.archive.org/web/20161019025925/http://afftcmuseum.org/exhibits/museum-aircraft-exhibits/. 19 October 2016. dead.
74. Web site: Airframe Dossier – Piasecki H-21B Work Horse, s/n 52-8683 USAF, c/n B.45, c/r N57968. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
75. Web site: Airframe Dossier – PiaseckiH-21, s/n 53-4329 USAF, c/r N6794. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
76. Web site: Airframe Dossier – Piasecki-Vertol H-21B Work Horse, s/n FR09 AdT, c/n B.155. Aerial Visuals. AerialVisuals.ca. 21 October 2016.
77. Web site: Airframe Dossier – Piasecki 44B, c/r N74058. Aerial Visuals. AerialVisuals.ca. 17 November 2016.

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