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I know that hydrogen balloon is an example of lighter than air aircraft while today aircraft like helicopters and Boeing 747 are examples of heavier than air aircraft.

But I want to know about more differences in detail.

Especially, can we say that lighter than air aircraft are unpowered while heavier than air aircraft are powered?

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    $\begingroup$ "Lighter than air aircrafts" is wrong. As is "heavier than air aircrafts". $\endgroup$
    – Hot Licks
    Nov 14 at 21:48
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    $\begingroup$ How wrong?Please explain? $\endgroup$
    – ATJ
    Nov 15 at 4:49
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    $\begingroup$ @bvc "aircraft" can be singular or plural; no need for the "s" $\endgroup$
    – Chris H
    Nov 15 at 8:53
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    $\begingroup$ @HotLicks (and others): instead of commenting on spelling mistakes, just click Edit to fix them. $\endgroup$
    – Bianfable
    Nov 15 at 12:08
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    $\begingroup$ @HotLicks Don't be toxic, it was just a simple spelling mistake that was not made in the first part of the question and that doesn't change how one would understand it, that's exactly what edits are for. Of course not everyone has the reputation to submit less than 6 characters edits $\endgroup$
    – Tofandel
    Nov 15 at 16:20

3 Answers 3

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A lighter-than-air aircraft (also called aerostat) derives lift from buoyancy, meaning its average density is lower than (or equal to) the density of the surrounding air. A heavier-than-air aircraft must derive lift by other means. This is most commonly achieved using a fixed wing combined with sufficient forward speed or a rotary wing.

Can we say that Lighter than air aircrafts are unpowered while heavier than air aircrafts are powered?

No, both types exist as powered and unpowered types, for example:

lighter-than-air heavier-than-air
Powered airship aeroplane, helicopter
Unpowered balloon glider, gyrokite

There are also mixed types, e.g. an autogyro is a heavier-than-air aircraft with an unpowered rotary wing, but powered propeller for forward thrust.

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  • $\begingroup$ @jkztd This is the EASA definition: "‘balloon’ means a manned lighter-than-air aircraft which is not power-driven and sustains flight through the use of either a lighter-than-air gas or an airborne heater, including gas balloons, hot-air balloons, ..." $\endgroup$
    – Bianfable
    Nov 14 at 12:16
  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Aviation Meta, or in Aviation Chat. Comments continuing discussion may be removed. $\endgroup$
    – Ralph J ♦
    Nov 16 at 14:09
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The key distinction is that a lighter than air craft relies on aerostatics (aka boyancy) to stay in the air, while a heavier than air craft must rely on air movement.

This means that in still air, a lighter than air craft can in-principle stay airborne indefinitely without motive power.

In practice most lighter than air craft are unpowered balloons. It is possible to create a lighter than air craft with engines to provide propulsion and this is known as an airship. Airships have however remained relatively rare, they tend to be relatively slow, have small payloads for their size and be sensitive to wind.

A heavier than air craft on the other hand requires power to maintain altitude in still air. An unpowered heavier than air craft in still air will lose altitude. A typical glider tow apparently drops the glider off at about 1000 feet and a typical glider apparently sinks at about half a meter per second. That gives a flight time in still air of about 10 minutes.

Gliders can remain in the air for longer through exploiting rising air currents from thermals or ridge lift. However these sources of natural lift are unreliable and location dependent.

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Simple, the aircraft is lighter (less dense in average) than the air surrounding it, so it floats up.

For this definition to work, the gas inside the balloon should be seen as a part of the aircraft. It may have engines, and many did. The engines are much denser than air but the whole aircraft with these engines attached is still less dense in average.

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