Wikipedia:Reference desk/Archives/Miscellaneous/2022 September 19

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September 19

Minimum runway for an aircraft to possibly get airborne

In Top Gun: Maverick, Maverick gets the F-14 airborne on a "short taxiway". In moviemistakes.com this is listed as a "factual error" because "It needs about 800m, about half a mile." But there is a difference between the usual runway length specified for an aircraft to start safely, and a minimum runway length so that it may possibly succeed with raising itself into the air. Has this minimum distance ever been analyzed for any aircraft? What may have been the minimum for an F-14? --KnightMove (talk) 15:32, 19 September 2022 (UTC)

@KnightMove: I have no idea whether the topic was ever studied, but the minimum runway length may depend on a geographic location and on weather. If you're at (or close to) the sea level, the air pressure is higher than at some plateau or in mountains. This makes air a bit more dense, which may result in a bigger lifting force, hence reduce the runway length necessary – and a similar effect may result from a barometric high. Additional, and much stronger, effect can be expected from a wind. Sometimes, when strong enough, it can even rise motion-less planes from a ground, which makes a runway length ZERO. --CiaPan (talk) 16:47, 19 September 2022 (UTC)

I saw a video discussing it and the issue was that the jet itself could do it, but the force of sudden acceleration would make it difficult for the humans to take off safely. Also, it was pointed out that flaps would be down for added lift, but they weren't during takeoff in the film and then they were when it was flying. That is a production issue. The "on the runway" scenes were filmed in a stationary museum jet that doesn't have operational flaps. The flying jet was either CGI or a real jet with normal flaps. 97.82.165.112 (talk) 21:57, 19 September 2022 (UTC)

Thank you very much. You don't by chance have a link to the video, or more info where it was screened?

The air scenes were all CGI, as no airworthy F-14 remain outside of Iran. To protect secret technology and to avoid anything being smuggled to Iran or other enemies, US authorities were keen to dismantle the interior of all remaining F-14. --KnightMove (talk) 03:13, 20 September 2022 (UTC)

How long is a "short" runway? The ground roll distance for an F-14B, flaps up, under favourable conditions (hard dry runway, no excess weight, sea level, 59 °F, head wind of 40 knots) is given as 1,455 feet (443 m).^[1]  --Lambiam 11:17, 20 September 2022 (UTC)

Take-off weight is also a factor. I remember a story about a large airplane having to make an emergency landing at an airport with a short runway, and the only way to get it out, once repaired, was to strip it of all of its seats and other non-essential equipment in order to make it light enough to use the short runway to take off and reach a conventional airport. Googling this, it seems to have happened a number of times (see for example here [2] and this forum discussion [3]). Xuxl (talk) 13:21, 20 September 2022 (UTC)

According to some sources, the F-14 engines (variant unspecified) with afterburners could deliver a thrust of 90,000 lbf, in SI units 400 KN. With a takeoff weight of 58,000 pounds (26,000 kg), ignoring the relatively low friction, that should give an acceleration of 15.2 m/s². Expressed as a g-force of 1.55 g₀ this is quite moderate and easily tolerated by untrained humans. Under the favourable conditions mentioned before, the takeoff speed is given as 152 knots, which I assume should be taken relative to the headwind. Even without headwind, this speed will be reached in 200 m. Assuming a 40 kn headwind, this decreases to 110 m. With flaps down, the cited takeoff speed is just 132 kn, giving under the most favourable condistions a roll distance of a mere 75 m. Perhaps considering the fictional stunt a "factual error" is itself an error.  --Lambiam 09:29, 21 September 2022 (UTC)

Jet engines are inefficient at low speed. PrimeHunter (talk) 13:37, 21 September 2022 (UTC)

As the pdf linked above shows, the F-14 normally took off under 'military power' - i.e. maximum thrust without using afterburner. I believe this was because in an event of an engine failure, the resultant asymmetric thrust would make the aircraft uncontrollable. Later twin-engined fighters, like the F-18, had their engines closer to the centreline, and could use afterburner on takeoff. AndyTheGrump (talk) 13:47, 21 September 2022 (UTC)

@AndyTheGrump: So... were the F-14 takeoffs in Top Gun with afterburners just shot for the movie? Or is it different for carrier takeoffs? --KnightMove (talk) 10:25, 22 September 2022 (UTC)

Ah - our article Grumman F-14 Tomcat answers it: "Thus the launch of an F-14B or F-14D with afterburner was rare, while the F-14A required full afterburner unless very lightly loaded." --KnightMove (talk) 10:50, 22 September 2022 (UTC)