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What manned airplanes have achieved flight with the least powerful engines (no gliders!), and what was their top speed? Im sure I've heard of an airplane with an 8hp engine capable of exceeding 120 MPH airspeed

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    $\begingroup$ The one you're probably remembering was the original Rutan Quickie -- it had an 18 HP Onan industrial engine, and top speed of just about 120 mph. With 75 mpg at cruise. $\endgroup$ – Zeiss Ikon Feb 13 at 19:32
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    $\begingroup$ For reference, I believe the Wright Brothers' first plane was about 12HP and a 20mph max. $\endgroup$ – BruceWayne Feb 13 at 21:50
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    $\begingroup$ @ZeissIkon "Onan industrial engine" A childish part of me is sad that the Onan Industrial Engine company didn't build Wankel engines. $\endgroup$ – Graham Feb 14 at 18:17
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    $\begingroup$ @Graham I'm so with you on this one! And, if they would have, they absolutely should have been used on Fokkers. $\endgroup$ – Jpe61 Feb 14 at 18:49
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    $\begingroup$ @jpe61 Old classic.. circlecity.co.uk/text_jokes/spitfire-pilot.php $\endgroup$ – Nyos Feb 14 at 19:29
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There are a number of human-powered aircraft (list here). For the Gossamer Albatross, we have

In still air, the required power was on the order of 300 W (0.40 hp), though even mild turbulence made this figure rise rapidly.

As far as top speed:

Allen completed the 22.2 mi (35.7 km) crossing in 2 hours and 49 minutes, achieving a top speed of 18 mph (29 km/h) and an average altitude of 5 ft (1.5 m)

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    $\begingroup$ was he in ground effect during his flight? $\endgroup$ – niels nielsen Feb 14 at 0:44
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    $\begingroup$ At 5 ft altitude, almost certainly. Looked for one that wasn't in ground effect, but can't find one quickly. $\endgroup$ – Eugene Styer Feb 14 at 1:02
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    $\begingroup$ Seems reasonable - a top-notch pro cyclist can average ~300 watts for a 6 hour race, and average 400-500 watts for a climb of under 30 minutes. Us mere humans might be half that. 745 W is one horsepower so ballpark 1/3 HP to 1/2 HP. Its hard to measure accurately biomechanical output because of the many variables, $\endgroup$ – Criggie Feb 14 at 4:06
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    $\begingroup$ @EugeneStyer The original MacReady prize required the Gossamer Condor to get out of ground effect to win -- IIRC, they had to climb to 30 m at least once while negotiating the figure-8 flight course. They didn't stay there long, because it was just about all the pilot/engine could do to get that high for a few seconds in dead calm dawn conditions. $\endgroup$ – Zeiss Ikon Feb 14 at 12:11
  • $\begingroup$ See Ground-effect vehicle and Ground effect (aerodynamics) on Wikipedia $\endgroup$ – sondra.kinsey Feb 16 at 21:37
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Sailplanes employ zero horsepower by conventional reckoning, though an alternative definition can be proposed using the component of the aircraft's weight that acts parallel to the airspeed vector as the thrust-like force. Anyway, the world records for sailplane flight appear to include 2191 km as greatest free distance along a course involving three or fewer turnpoints, and 22657 m as maximum absolute altitude. Source: fai.org/page/igc-records

OK, I thought the original question said "aircraft"; I now see "airplane". Does "airplane" always exclude "glider"? Originally, the "plane" in "airplane" referred to the "planing" action of the wing surface, and had nothing to do with the presence or absence of a motor, though there may be no specific examples of the word historically being used in reference to gliders.

Didn't see the "no gliders!" when I created this answer-- sorry! Some self-launching powered hang gliders and powered paragliders/ paramotors intended for soaring flight must be close contenders for the answer to your question, as some of them have rather weak engines and are barely able to climb in the absence of an updraft despite overall light weight.

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  • $\begingroup$ "are barely able to climb"-- in the absence of an updraft, of course. $\endgroup$ – quiet flyer Feb 14 at 4:47
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    $\begingroup$ Some of the earliest powered hang gliders, and again paramotors, had as little as 6 hp -- this level of underpower has fallen out of favor due to safety considerations. Just takes too darned long to get to safe altitude (i.e. high enough to recover from an upset). $\endgroup$ – Zeiss Ikon Feb 14 at 12:08
  • $\begingroup$ yes, "and paramotors" would be a good addition to last sentence in answer $\endgroup$ – quiet flyer Feb 14 at 14:40
  • $\begingroup$ But clearly, human-powered aircraft are the best contenders for answers to the question. $\endgroup$ – quiet flyer Feb 14 at 17:03
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    $\begingroup$ The No Gliders was always there. You can click the "edited yesterday" link below the question to see the revision history. $\endgroup$ – JPhi1618 Feb 14 at 18:37
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Even though at 4 x 13kW its max rated power (70hp) is higher than the other answers, Solar Impulse flew almost 5 thousand miles non-stop during close to 5 days. It did not typically fly anywhere close to its max rated power in order to conserve energy that it would have to spend during the night.

Wikipedia lists the specs as:

Maximum speed: 140 km/h (87 mph)

Cruise speed: 90 km/h (56 mph) 60 km/h (37 mph) at night to save power

Service ceiling: 8,500 m (27,900 ft) with a maximum altitude of 12,000 metres (39,000 ft)

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This is of course much higher than the human-powered Gossamer Albatross in Eugene Styer's answer, but the Wright Flyer, "the first powered, heavier-than-air machine to achieve controlled, sustained flight with a pilot aboard", used a

12 horsepower gasoline engine

which is a bit under 9 kW.

The longest of its 4 flights was 260 m, and it was apparently so light that "a heavy gust picked up the Flyer and tumbled it end over end, damaging it beyond any hope of quick repair".

Later versions used more powerful engines.

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The lightest remote control airplane I could find is 0.225g (0.01oz).
Wing span 71 mm.
Length 70 mm. (about 3")
I estimate it used a few hundredths of a watt.

https://www.rcgroups.com/forums/showpost.php?p=11048403&postcount=1

That was back in 2008 though, so they've probably done better since then.

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    $\begingroup$ "Manned airplane" -- this isn't that. $\endgroup$ – Zeiss Ikon Feb 14 at 13:56
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    $\begingroup$ @ZeissIkon just gotta find a really small person. en.wikipedia.org/wiki/Downsizing_(film) $\endgroup$ – Carl Witthoft Feb 14 at 14:22
  • $\begingroup$ Yeeeeah-no. Maybe on the SF&F stack, or Worldbuilding. $\endgroup$ – Zeiss Ikon Feb 14 at 14:33
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    $\begingroup$ Oops, I noticed that the title didn't specify 'manned' but missed it in the text. $\endgroup$ – Robin Bennett Feb 14 at 14:41
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    $\begingroup$ @CarlWitthoft "Honey, I shrunk the pilot?" $\endgroup$ – reirab Feb 14 at 16:08
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The twin engined Columbian Cri cri might be a contender: each of the single cylinder piston engines were about 15hp. Cruising speed is 190km/h. Wikipedia article

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