Vertical take off is a big advantage, but why it is only limited to low weight?

For example, fixed wing like An225 can lift more than hundred tonnes of payload, while the biggest helicopter is 25 tonnes. Why there is no effort to build a helicopter can lift 50 - 100 tonnes. What limits them? Economic or technical problem? I guess it is a technical problem because even the heaviest helicopter is just an experiment, but what is it?

  • $\begingroup$ To be fair, the biggest helicopter lifted over 44 tons. On the other hand, An-225 lifted over 250 t :) $\endgroup$ – Zeus Apr 19 '16 at 2:14
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    $\begingroup$ If we want to stretch the definition of aircraft a bit, the first stage of the Saturn V lifted well over 3000 tons, vertically. Which if you think about it does tend to shed light on the relative efficiency of vertical takeoff. $\endgroup$ – jamesqf Apr 19 '16 at 18:02

Essentially it comes down to supersonic rotor tips

With a plane, in theory you can make it pretty much as big as you like - as long as you have strong/light enough materials, and can keep adding power, an aeroplane design scales pretty well. Bigger wing = more lift. As long as you can make the wing bigger without it breaking, and as long as you can add enough power to overcome the extra drag, there aren't many fixed limits

With a helicopter, we're limited by the rotor tips: once they go supersonic, they cause a lot of problems.

So how does a helicopter produce lift? By using rotors to push air down within a kind of circle. To add more lift we can do (essentially) three things.

  1. Make the rotor spin faster, so it pushes more air down in the circle it already uses. Obviously this makes the tips spin faster, so we can only do it to a certain extent. We've already hit this limit.

  2. Make the rotor blades longer, so they push a bigger circle of air. Again, though, due to the nature of a circular blade, the outside of a blade is moving faster than the inside. For a certain rotor speed, there's a fixed limit to how large the blades can be. Again, we've already hit this limit

  3. Add more blades, so there are more blades producing lift. This works to an extent (hence why smaller helicopters may have two rotor blades, but larger ones have 4, 5 or more. Again, though, this doesn't scale indefinitely - each rotor interferes with the next, you can't just keep adding more

There are other slight modifications we can make, such as the airfoil of the rotor, but they don't add significant gains

So, basically, we've hit the limit of what we can lift with a single rotor, The only real way to add more lift now is to add more rotors: doing that would be far less efficient than simply using an aeroplane.

Which brings me to the final point - helicopters are very inefficient and pretty slow... We simply don't need, except in a few niche circumstances, to carry more weight with them.

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    $\begingroup$ +1 for the final point. To perhaps be a little more clear, helicopters require thrust/weight ratios > 1. Airplanes do not. Even if we could put enough rotors and enough blades on it, the engines would have to produce a lot more power than those of an airplane carrying the same payload. Beyond a point, we'd have to start installing vertically-mounted GE90s or some such thing on them. It should also be noted that this limitation applies to all vertical takeoff (except lighter-than-air craft,) not just helicopters. $\endgroup$ – reirab Feb 15 '15 at 19:12
  • $\begingroup$ Aircraft require thrust to weight considerations, it's just not exactly proportionate in the same way $\endgroup$ – Jon Story Feb 15 '15 at 19:54
  • $\begingroup$ Yes, but they're usually closer to 0.25, not > 1.0. Much less engine power (and, therefore, much less fuel) for the same lift. $\endgroup$ – reirab Feb 15 '15 at 20:00
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    $\begingroup$ True, fuel burn becomes an issue too - I may add that into the answer $\endgroup$ – Jon Story Feb 15 '15 at 20:10
  • $\begingroup$ From physical point of view, isnt that power and force can be traded? E.g. a longer lever will have higher force for the same power? $\endgroup$ – user2174870 Feb 18 '15 at 16:21

Jon's answer is correct: "helicopters are very inefficient and pretty slow..."

but he misses one important point: helicopters are also incredibly fragile and delicate. Even a regular single-rotor helicopter flying is a miracle. It's been called: "10,000 spare parts flying in close formation."

You could add more rotors to gain more lift, the Chinook has two rotors which is part of why it can carry so much. The rear rotor adds complexity but also removes the need for a tail rotor, which is why it isn't quite 20,000 spare parts flying in close formation. But even Chinooks are fragile compared to C-130s.

Adding engines adds redundancy in a plane. And remember that if you are in a plane and you lose an engine (or all engines) you can still fly or glide to a safe landing.

If you are in a rotorcraft and you lose your engine (either main rotor or tail) or really any single part of those 10,000 parts, then all you can do is pray and try an auto-rotation landing. It gets worse the more rotors you add, not better. This is one of the limitations of rotorcraft and why you for the most part only see multiple rotors (4-, 6-, 8-) used in unmanned drones except for some very experimental vehicles.

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    $\begingroup$ A big advantage for the Chinook is that you can have a load center anywhere along the midline rather than only exactly below the rotor center. Very useful for a military transport with underslung cargo $\endgroup$ – NobodySpecial Mar 27 '15 at 3:25
  • $\begingroup$ "You could add more rotors to gain more lift" and yet the heaviest-lift chopper to go into production was a single rotor design. $\endgroup$ – Peter Green May 21 '18 at 0:11

For a helicopter to take off vertically: $$Lift \gt Weight.$$

The weight grows with length to the third power ($l^3$, weight is proportional to volume) while the lift only grows with $l^2$, because it's proportional to the rotor blades planform area.

More lift means higher lift coefficient and more rotorblade planform area. The maximum cirumferential speed at the blade tips is limited (there's the constraint that the tips can't move at supersonic speeds). One would need stiff longer and trapezoidal blades and there is a structural limit to the possible torque on the blade root.

Aircraft that cruise at high subsonic speeds exprience the same airspeed at any portion along the wings, unlike a helicopter that has a $v=r \omega$ relationship for the speed along the rotor blade. $v$ is constrained to subsonic speed.

  • $\begingroup$ Without more detail, this would be better as a comment. $\endgroup$ – fooot Feb 15 '15 at 19:15
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    $\begingroup$ Well, for anything to take off at all, lift must be greater than weight. Airplanes must deal with the same square-cube law in order to generate enough lift, resulting in larger aircraft having disproportionately larger wings. The main difference is that airplanes don't have to use thrust to directly generate their lift, allowing for much lower thrust/weight ratios and, thus, much better fuel economy for the same load. $\endgroup$ – reirab Feb 16 '15 at 4:27
  • $\begingroup$ You can always throw up a stone, no lift involved. Subsonic aircraft experience the same high subsonic speed on any portion of their wing, unlike a helicopter for which the high subsonic speed is present at the rotor blade tips. Going inward from the tips, the speed decreases. $\endgroup$ – user7241 Feb 16 '15 at 17:04

The power required to maintain altitude at forward flight is lower than the power required to hover. As horizontal velocity ($V_x$) increases, the induced velocity ($V_i$) at the disc decreases, and induced power ($P_i$) decreases roughly equal to $\frac{1}{V_i}$. However, as $V_x$ increases, parasitic power ($P_p$) increases proportional to $V^3$. Profile power ($P_0$ - the power needed to maintain rotor speed) remains roughly constant. If you chart all of these together with Required Power on the y-axis and forward velocity on the x-axis, you end up with something that looks like this:

Power vs forward speed

What this means is that there's an optimum forward speed at which your power is minimized. This is important, because the max rate of climb (and lifting power) is determined by your available power $$P_{av} = P_{tot} - P_i - P_p - P_0$$. If you minimize the power needed to keep the aircraft flying, you have more net power for lifting other things (and for heavy helicopters, they may not be able to take off vertically at all). For this reason, the max rate of climb will always be at some forward speed, and by definition, the max lifting capacity will also be at some forward velocity. A practical demonstration of this principle is that large cargo lifters like the sky crane and the chinook always take off with some forward velocity component. This is why.

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    $\begingroup$ good answer, do you have a source for the image? $\endgroup$ – ROIMaison Apr 19 '16 at 15:09

In addition to the power-to-weight ratio - already aptly described above - I bluntly strike at the economic reasons and cite that bean-counters look at the cost-per-takeoff and shake their head.

  • $\begingroup$ Yes. Even assuming that you could solve all the technical problems of a really heavy-lift helicopter, where's the economic demand for them? What can you do enough of cheaper with a big helicopter to pay off the development & operating costs? Even where there is demand for heavy lift, it seems that it can be met better with airships: bloomberg.com/news/articles/2013-06-14/… $\endgroup$ – jamesqf Mar 25 '15 at 19:05
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    $\begingroup$ This seems to echo the current top-voted answer's final statement: "We simply don't need, except in a few niche circumstances, to carry more weight with [helicopters]." We tend to prefer answers that add something substantial that has not already been written in another answer to the question; can you edit your answer so that it does so? $\endgroup$ – a CVn Mar 25 '15 at 19:08

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