9
$\begingroup$

On cars, we have 'shock absorber' suspension that decouples the passengers and loads from the road vibrations, bumps etc.

On planes do we have any form of suspension that decouples the passengers & loads from the turbulence, and vibrations induced by 'bumps' in the air?

I imagine even if it doesn't use a spring damper system, whatever attaches the cabin to the outer shell of the plane will have some finite stiffness. If so, has damping of this spring been considered to avoid oscillations?

$\endgroup$
  • $\begingroup$ I think it should be on the chassis if the wheel can be lifted up to replace the tire, but I understand the question is about something that is active also in the air. $\endgroup$ – h22 Jul 15 at 6:49
  • 20
    $\begingroup$ Springs? Even better - all aircraft are riding on air suspension, which, as any automotive writer will tell you, is the smoothest ride you can get! $\endgroup$ – Therac Jul 15 at 7:08
  • $\begingroup$ Besides the cushioning in your seat? Not that I have ever seen. $\endgroup$ – CrossRoads Jul 15 at 15:19
  • 4
    $\begingroup$ I know you asked about turbulence and vibrations in the air, but the landing gear on an commercial aircraft have shock absorbers like cars. They are like struts on cars, built into the gear and use hydraulic dampers. $\endgroup$ – jww Jul 16 at 6:52
28
$\begingroup$

The cabin is rigidly attached to the fuselage, with bolts. But in a way the cabins do have suspension: from the wings riding on air.

  • Elasticity of the wing construction material acts like a spring. Load it with a vertical gust, and it will bend upwards, then spring back.
  • The wing bending experiences damping forces from the surrounding air, proportional to bending velocity.
  • The wing sweep angle spreads out the gust: not all of the wing is accelerated upwards immediately.

Modern subsonic airliners with swept wings, flying with near transsonic speeds, experience much less turbulence than the olden days DC-3 type aircraft. So far, so good.

OPs proposal to provide suspension between fuselage and cabin may not be very effective. Notice that car suspensions work best when the unsprung mass is low: independent suspension is able to follow the un-evenness in the road much more effectively than rigid axle suspension, due to the much lower unsprung mass of only the wheel+tyre.

The unsprung mass of the suspended-cabin would be the whole fuselage-plus-wing structure, engines, fuel etc. Considerably more than that of the cabin plus payload. A lot of extra complexity for not much gain.

$\endgroup$
  • 9
    $\begingroup$ The comparison between modern pressurized aircraft and DC-3 is unfair (but yet valid): the DC-3 flew at a lower flight level, where air is more turbulent. $\endgroup$ – Manu H Jul 15 at 13:23
  • 1
    $\begingroup$ Weren't wings built much stiffer in the days of the DC-3, too? That would make for a rougher ride. $\endgroup$ – a CVn Jul 15 at 19:05
  • 1
    $\begingroup$ @aCVn Lower wing loading makes for a stiffer ride indeed. $\endgroup$ – Koyovis Jul 15 at 22:37
  • $\begingroup$ You had "uspended- abin". I changed that to "suspended-cabin" since it looked like you'd just lost the first letter of each word. I was thinking, though, that you meant "unsuspended-cabin", but the more I read it, the less sure I am. I'll leave it to you to edit again if necessary. $\endgroup$ – FreeMan Jul 16 at 15:54
  • $\begingroup$ @FreeMan Thanx. $\endgroup$ – Koyovis Jul 16 at 21:59
3
$\begingroup$

No, they do not. Aircraft rely on the properties of the materials they are built from to absorb such forces. Adding springs would merely increase dead weight and lower profitability, while providing little added relief from temporary bumps. Besides, anyone who has ridden in anything with shock absorbers knows that the bumps are not eliminated completely.

$\endgroup$
  • $\begingroup$ Correct me if I'm wrong, but aircraft turbulence normally has a much longer period than road potholes. So it's more like going over a giant speedbump, and the cases where you wish you might wish you had "suspension" would require that suspension to travel most of the height of the cabin to isolate it from the movement of the wings. $\endgroup$ – Peter Cordes Jul 16 at 12:57
  • $\begingroup$ @PeterCordes Longer period, yes, but an aircraft travels much faster than a car, which has the effect of reducing that period. $\endgroup$ – Juan Jimenez Jul 16 at 13:05
  • 1
    $\begingroup$ Are you contradicting yourself there, or are you just saying that it is longer than in cars but would be even longer if planes didn't fly so fast? Or did you mean to talk about longer "potholes" (in the direction of travel) but higher velocity resulting in still a fairly short period? What really matters of course is the total vertical amplitude of the bumps relative to the height of the airplane fuselage, not wavelength: if it's going up or down by a large fraction of its own height, there's not much you can do to damp the acceleration felt inside the cabin. $\endgroup$ – Peter Cordes Jul 16 at 13:11
  • 1
    $\begingroup$ @PeterCordes You were the one who mentioned period in the first place, not me. If you want to talk amplitude now, fine, but I'm not the one contradicting myself. :) $\endgroup$ – Juan Jimenez Jul 16 at 13:33
  • $\begingroup$ I just wanted to clarify whether you meant that "longer period than a car" was actually still true, or if you meant that fast-flying planes shorten the period so much that it is like a car after all. That sentence seems to be saying two opposing things. Anyway, high amplitude low frequency vs. high amplitude high frequency is an important difference which is why I was bringing up period. To damp low frequency oscillations, you need really weak coupling which is maybe harder to do without often hitting suspension-travel limits hard and getting extra jolts. $\endgroup$ – Peter Cordes Jul 17 at 6:34
3
$\begingroup$

Technically, yes, but not in the way you envision, and only in academic quantities.

Any mechanical system has some damping in it due to hysteresis in the materials, friction on the joints, and elastic bending of the structural members.

An aircraft's wings bend, providing the spring component of a shock absorber, and dissipate energy via hysteresis and friction, providing the damper (dashpot) component. The overall effect is much smaller than that of a dedicated shock absorber, but it exists.

Do note that all the above effects are generally undesirable in an aircraft and are not there by design. Elastic bending in particular can couple with the aerodynamic forces and lead to flutter, control reversal and structural divergence, for example.

$\endgroup$
  • 5
    $\begingroup$ “not there by design” – really? I would have thought the wing elasticity is quite important to avoid load concentration from cracking the lightweight structure. $\endgroup$ – leftaroundabout Jul 15 at 15:20
  • 2
    $\begingroup$ @leftaroundabout the fact that elasticity is not wholly inconvenient does not mean it is designed in; in fact I am yet to see a design or strength analysis document that specifies a designed elasticity minimum. Stress concentration in the wing structure is avoided by careful geometric design and large safety factors, not by deliberately choosing more elastic materials to absorb energy. $\endgroup$ – AEhere Jul 16 at 9:30
  • $\begingroup$ Well, this might merit asking a new question about. $\endgroup$ – leftaroundabout Jul 16 at 9:44
  • 1
    $\begingroup$ @leftaroundabout Aircraft wings are pretty flexible. See popularmechanics.com/flight/g2428/7-airplane-wing-stress-tests for example. But flexibility on its own doesn't provide any damping at all. $\endgroup$ – alephzero Jul 16 at 9:49
  • 1
    $\begingroup$ @leftaroundabout Flexibility doesn't necessarily reduce load concentrations either, unless there are several different load paths and the total load can be redistributed between them. For a statically determinate structure like an aircraft wing (which is basically a cantilever beam attached to the fuselage) that does not apply. $\endgroup$ – alephzero Jul 16 at 9:56
1
$\begingroup$

The cabin and outer shell are one structure. The structure is built up from ribs (circular pieces) and stringers (lengthwise pieces) which form a sort of framework. An outer skin is bonded (adhesives and/or rivets) to this framework; the whole things becomes one integrated structure. The floors are attached to (and contribute to) the whole structure. Interior finishing (wall/ceiling) panels are rigidly (more-or-less) attached to the inner surfaces of the ribs and stringers - the space in between is filled with insulation. There is no mechanical isolation between the exterior and interior. Any suspension effects come from the properties of the structure itself - a certain amount of flex - and the passenger seat cushions.

$\endgroup$
1
$\begingroup$

It is important for handling that the cabin is not decoupled from the rest of the plane.

Suppose you have your suspension system. The plane hits a thermal and there's a bump. Now anyone who's pushed on a corner of a car knows it doesn't just stop - it overshoots slightly and comes back again. On your hypothetical plane, the pilot will get one jolt from the thermal, but then another jolt the other way as the suspension overshoots and comes back, and then another small one coming back again, typically. Controlling this would be nigh on impossible.

Henry Bessemer had a similar idea for ships. The SS Bessemer was the result. She was a complete failure - she only sailed once, and destroyed the mooring pier at Calais because she was impossible to steer.

$\endgroup$
  • $\begingroup$ Overshoots are controlled by choosing appropriate damping. Overdamped systems never overshoot. $\endgroup$ – Koyovis Jul 17 at 1:21
  • $\begingroup$ The SS Bessemer is an interesting case though, +1. Nowadays active suspension would be possible, like a motion system underneath a simulator. I have in fact seen 6-DoF motion systems that are used as stabilising platforms on board of ships. $\endgroup$ – Koyovis Jul 17 at 2:49
  • $\begingroup$ @Koyovis But overdamped systems don't soak up the bumps very well. And even active suspension will tend to do something similar - model-predictive control is pretty good, but inertia and overshoots still happen. A stabilised platform works because it isn't a significant fraction of the overall mass of the ship; the SS Bessemer shows what happens with most of your mass running on gimbals though. It might be possible to use fly-by-wire to run it, I agree, but it's a complex control problem, and generally that's not a safe place for an airliner to be. $\endgroup$ – Graham Jul 17 at 7:40

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.