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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

Francis L.
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    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. – Zeiss Ikon Feb 13 '20 at 19:32
  • actually, I found it, it was storch III build by Lippisch – Francis L. Feb 13 '20 at 21:02
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    For reference, I believe the Wright Brothers' first plane was about 12HP and a 20mph max. – BruceWayne Feb 13 '20 at 21:50
  • The only Storch I can find was 240hp. It was the stall speed of 31mph that was exceptional, as well as landing in 100' and taking off in 250'. The bit about rescuing Mussolini was interesting. Fieseler Fi 156 Storch – Mazura Feb 14 '20 at 03:22
  • https://youtu.be/s__sKI0GSRw storch 3 is referenced within first five minutes – Francis L. Feb 14 '20 at 08:41
  • The Lippisch Storch series are a currently offered tailless type ultralight, not connected in any way with the Feisler STOL offering of the 1930s and 1940s. – Zeiss Ikon Feb 14 '20 at 12:14
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    @ZeissIkon "Onan industrial engine" A childish part of me is sad that the Onan Industrial Engine company didn't build Wankel engines. – Graham Feb 14 '20 at 18:17
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    @Graham I'm so with you on this one! And, if they would have, they absolutely should have been used on Fokkers. – Jpe61 Feb 14 '20 at 18:49
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    @jpe61 Old classic.. http://www.circlecity.co.uk/text_jokes/spitfire-pilot.php – Nyos Feb 14 '20 at 19:29
  • Matthew Bacon Sellers II built a quadruplane and flew it from ca 1909 to 1912, progressively reducing the engine power to see how low he could get it.He managed to fly on around 5 hp. 120 mph on 8 hp is not credible. But is man-powered flight allowed? – Guy Inchbald Feb 14 '20 at 19:48
  • @jpe61 Legend! :) – Graham Feb 14 '20 at 20:46

6 Answers6

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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)

Eugene Styer
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    was he in ground effect during his flight? – niels nielsen Feb 14 '20 at 00:44
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    At 5 ft altitude, almost certainly. Looked for one that wasn't in ground effect, but can't find one quickly. – Eugene Styer Feb 14 '20 at 01:02
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    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, – Criggie Feb 14 '20 at 04:06
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    @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. – Zeiss Ikon Feb 14 '20 at 12:11
  • See Ground-effect vehicle and Ground effect (aerodynamics) on Wikipedia – sondra.kinsey Feb 16 '20 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.

quiet flyer
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  • "are barely able to climb"-- in the absence of an updraft, of course. – quiet flyer Feb 14 '20 at 04:47
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    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). – Zeiss Ikon Feb 14 '20 at 12:08
  • yes, "and paramotors" would be a good addition to last sentence in answer – quiet flyer Feb 14 '20 at 14:40
  • But clearly, human-powered aircraft are the best contenders for answers to the question. – quiet flyer Feb 14 '20 at 17:03
  • Oh, yes. The lowest powered successful powered ultralight or paramotor had close to twenty times the long-run power of a top tier bicyclist. – Zeiss Ikon Feb 14 '20 at 17:04
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    The No Gliders was always there. You can click the "edited yesterday" link below the question to see the revision history. – JPhi1618 Feb 14 '20 at 18:37
  • I'm sure it was, I just didn't see it, sorry. Should edit to remove "was that added..." – quiet flyer Feb 14 '20 at 18:39
  • Gliders use 'gravitational power'. You multiply the weight in newtons by the sink speed in m/s and get the gravitational power in watt... – xxavier Feb 16 '20 at 13:28
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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)

LordOfThePigs
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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.

jcaron
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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.

Robin Bennett
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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

Federico
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Nathan
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