Editing Push-pull configuration

{{Short description|Combination of forward- and aft-mounted propellers on aircraft}}
{{Other uses of|push-pull|Push-pull (disambiguation)}}
[[File:Cessna_Skymaster_O-2_5.jpg|thumb|right|A [[Cessna O-2 Skymaster]], a [[twin boom]] push-pull design]]

An [[aircraft]] constructed with a '''push-pull configuration''' has a combination of forward-mounted [[Tractor configuration|tractor]] (pull) [[Propeller (aircraft)|propeller]]s, and backward-mounted ([[pusher configuration|pusher]]) propellers.

==Historical==
The earliest known examples of "push-pull" engined-layout aircraft was the [[Short S.27#Short Tandem Twin|Short Tandem Twin]].

An early pre-World War I example of a "push-pull" aircraft was the [[Caproni Ca.1 (1914)|Caproni Ca.1 of 1914]] which had two wing-mounted tractor propellers and one centre-mounted pusher propeller. Around 450 of these and their successor, the [[Caproni Ca.3|Ca.3]] were built. One of the first to employ two engines on a common axis (tandem push-pull) was the one-off, ill-fated [[Siemens-Schuckert DDr.I]] [[fighter aircraft|fighter]] of 1917.

German World War I designs included the only Fokker twin-engined design of the period, the [[Fokker K.I]] from 1915; followed by the unusual [[Siemens-Schuckert DDr.I]] [[triplane]] fighter design of late 1917, and concluding with the laterally-offset "push-pull" [[Gotha G.VI]] bomber prototype of 1918. 

[[Claudius Dornier]] embraced the concept, many of his [[flying boat]]s using variations of the tandem "push-pull" engine layout, including the 1922 [[Dornier Do J|Dornier Wal]], the 1938 [[Dornier Do 26]], and the massive 1929 [[Dornier Do X]], which had twelve engines driving six tractors and six pushers. A number of Farmans and Fokkers also had push-pull engine installations, such as the [[Farman F.121 Jabiru]] and [[Fokker F.32]].

==Configuration==
Push-pull designs have the engines mounted above the wing as Dornier flying boats or more commonly on a shorter [[fuselage]] than conventional one, as for [[Rutan Defiant]] or [[Rutan Voyager|Voyager]] canard designs. Twin boomers such as the [[Cessna Skymaster]] and [[Adam A500]] have the aircraft's tail suspended via [[twin boom]]s behind the pusher propeller.  In contrast, both the [[World War II]]-era [[Dornier Do 335]] and the early 1960s-designed French [[Moynet Jupiter|Moynet M 360 Jupiter]] experimental private plane had their pusher propeller behind the tail.<ref>{{cite web |url=http://www.century-of-flight.net/Aviation%20history/photo_albums/timeline/ww2/Dornier%20Do%20335%20Pfeil.htm |url-status=dead |archive-url=https://web.archive.org/web/20071111141212/http://www.century-of-flight.net/Aviation%20history/photo_albums/timeline/ww2/Dornier%20Do%20335%20Pfeil.htm |archive-date=2007-11-11 |title=Dornier Do 335 Pfeil}}</ref><ref>{{Cite web|url=http://aviadejavu.ru/Site/Crafts/Craft30765.htm|title = Matra-Moynet Jupiter}}</ref>

==Design benefits==
While pure pushers decreased in popularity during the [[World War I|First World War]], the push-pull configuration has continued to be used.  The advantage it provides is the ability to mount two propellers on the aircraft's centreline, thereby avoiding the increased drag that comes with twin wing-mounted engines.  It is also easier to fly if one of the two engines fails, as the thrust provided by the remaining engine stays in the centerline. In contrast, a conventional twin-engine aircraft will [[Yaw angle|yaw]] in the direction of the failed engine and become uncontrollable below a certain airspeed, known as [[V speeds#VMC|V<sub>MC</sub>]].

==Design problems==
The rear engine operates in the disturbed air from the forward engine, which may reduce its efficiency to 85% of the forward engine.<ref>https://www.rcgroups.com/forums/showthread.php?1055847-stability-changes-under-power/page2 quote:usually at least a couple percent less, typically quite a bit worse than that, and in some cases more than 15% less efficient-Al Bower</ref><ref>Long P. Yip, Nasa Technical Paper 2382, March 1985, ''Wind-Tunnel Investigation of a Full-Scale Canard-Configured General Aviation Airplane''</ref> In addition the rear engine can interfere with the aircraft's rotation during takeoff if installed in the tail, or they require additional compromise to be made to ensure clearance. This is why they are more common on seaplanes, where this is not a concern.

==Piloting==
[[Aviator|Pilots]] in the United States who obtain a [[Pilot certification in the United States|multi-engine rating]] in an aircraft with this push-pull, or "centerline thrust," configuration are restricted to flying centerline-thrust aircraft; pilots who obtain a multi-engine rating in conventional twin-engine aircraft do not have a similar limitation with regard to centerline-thrust aircraft.  The limitation can be removed by further testing in a conventional multi-engine aircraft.<ref>[http://fsims.faa.gov/wdocs/8900.1/v05%20airman%20cert/chapter%2001/05_001_004rev1.htm VOLUME 5 AIRMAN CERTIFICATIONCHAPTER 1 DIRECTION, GUIDANCE, AND PROCEDURES FOR TITLE 14 CFR PARTS 121/135 AND GENERAL AVIATIONSection 4 Considerations for the Practical Test]</ref>

==Military application==
[[Image:Dornier Pfeil2.jpg|thumb|right|The only surviving Do 335 at the [[Steven F. Udvar-Hazy Center]] near [[Washington, DC]]]]

Despite its advantages push-pull configurations are rare in military aircraft. In addition to the problems noted for civil aircraft, the increased risk to the pilot in the case of a crash or the need to parachute from the aircraft also pose problems. During a crash the rear engine may crush the pilot and if bailing out, the pilot is in danger of hitting the propeller. Examples of past military applications include the aforementioned Siemens-Schuckert DDr.I twin-engined triplane and the Gotha G.VI, with its engines mounted on the front and rear ends of two separate fuselages. More successful was the Italian [[Caproni Ca.3]] trimotor, with two tractor engines and one pusher. Between the wars, most push-pull aircraft were flying boats, of which the [[Dornier Wal]] was probably the most numerous, while a number of heavy bombers, such as the [[Farman F.220]] used engines mounted in push-pull pairs under the wings. Near the end of [[World War II]], the German [[Dornier Do 335]] push-pull twin-engined, ''Zerstörer''-candidate heavy fighter featured explosive charges to jettison the rear propeller and dorsal tailfin, a manually-jettisonable main canopy, as well as an [[ejection seat]]. One of the last military aircraft to use the configuration was the American [[Cessna O-2 Skymaster|Cessna O-2]], which was used for [[forward air control]] during the [[Vietnam War]].

==Images==
<gallery>
Image:Dornier Seastar D-ICKS.jpg|[[Dornier Seawings Seastar]]
Image:Rutan-Defiant-N57KS.jpg|[[Rutan Defiant|Rutan Model 40 Defiant]]
Image:VoyagerAircraftAtNASM-common.jpg|[[Rutan Voyager|Rutan Model 76 Voyager]]
Image:Adam500-N504AX-051111-02.jpg|[[Adam A500]]
</gallery>

==See also==
* [[Tractor configuration]]
* [[Pusher configuration]]

==References==
{{reflist}}

==External links==
{{Commons category|Aircraft with push-pull engines}}

* [http://1000aircraftphotos.com/Contributions/KleinBernhard/7698.htm Star Kraft SK-700 - 2 x 350hp] 1000aircraftphotos.com

[[Category:Aircraft configurations]]