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I am trying to design a landing gear system and I am stuck at whether I should use a spring as a kind of shock absorber in my landing gear. Currently, the scope of my project is only concerning about "hard landing"--which is just about the drone falling straight down.

From what I thought, there would be a point force acting along and landing gear's length and to the joint between the landing gear and the internal structure of my drone. I first thought a spring could cancel out the force into the shortening of the spring, then I realized that there is still a force transfer happening even with a spring. Then I thought that there are two ways a hard landing could damage my drone--the force of impact, or the changing of momentum. Both items could induce a shear force at my joint. So my questions are: Is it force or change of momentum that damages my drone during a hard landing, and how could adding a spring at the landing gear mitigate the damage?

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  • $\begingroup$ I opine the damage is mainly caused by the impact as in a collision. You can soften the impact by adding a damper or shock absorber in the landing gear. I don't think spring is a good choice though, for it is useless if it is either too stiff or too soft, and for the vibration it causes. $\endgroup$
    – r13
    Commented Mar 9, 2022 at 1:51
  • $\begingroup$ Do you want an emergency system which destroys itself to save the drone, or do you want a shock-absorber system such as automobiles use to smooth bumps? Are you landing vertically or gliding laterally? These and many more parameters will drive your choice. $\endgroup$ Commented Mar 9, 2022 at 13:27
  • $\begingroup$ I was going to suggest using a deployable air bag to cushion the landing and potentially cause a lot of drag to slow descent. I also know some small military drones use parachutes for landing since they do not have any landing gear. $\endgroup$
    – Forward Ed
    Commented Mar 9, 2022 at 18:22
  • $\begingroup$ @ForwardEd I think the biggest barrier to that for DIY is finding a suitable explosive. Although a CO2 canister might do it but you'd have to deploy well before you hit the ground with that. $\endgroup$
    – DKNguyen
    Commented Mar 9, 2022 at 21:52

2 Answers 2

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A change in momentum is called an impulse which is equal to force applied over time. It's not the change in momentum since the change in momentum to reach zero speed from a given speed is the same whether or not it happens over a longer or shorter period of time. You can use more force over less time or less force over more time.

But what this does change is deceleration. If you decelerate by quickly (i.e. achieve the same change in speed over a shorter amount of time), that requires more force and this higher force can cause exceed the structure's strength.

Springs in this cause are supposed to stretch out the deceleration over a longer period of time to decrease the force, but they also store the energy so won't be terribly helpful without a mechanism to allow the springs to dissipate that energy (it will send your robot bouncing back up only to fall again). That's why big machines have things like hydraulic dampers in addition to the springs. Obviously not of an acceptable on a drone.

I'd actually forget about springs. I'd use one of the following approaches instead:

  • landing gear that collapses and breaks in a controlled manner. This spreads out the deceleration over time and the act of breaking dissipates energy. Like crumple zones in a car. You could do this with more rigid materials but that might take too much engineering effort since they could end up being too strong. Easy collapsible materials that are also cheap (i.e. replaceable) to use include things such as foam and corrugated cardboard (but mind the direction of the corrugations since cardboard can be very rigid in one direction). Balsa is another material that is middle of the road in that it could be made quite strong but quite snappable but is more costly and would take longer to work with. You can also notch beams to adjust when and where the break occurs.

  • landing gear that has shear pins in it. The shear pins break to dissipate the energy and are easier to replace than all the landing gear like the first approach. Depending on the how the mechanism is built you could potentially have a sequence of shear pins (such as telescopic tube where the inner tube is held by a row of shear pins that are progressively weaker which would give a gradual extended collapse). Many weak shear pins that break in a sequence is better than one stronger one that breaks because it spreads the deceleration force out over time. You could make the first pin strongest since that carries the operating loads but if it breaks you know something is wrong and you want the others to break too to absorb the energy.

Just using plastic screws in your landing gear is a start. Depending on the weight you may have to use unconventional shear pin materials such as toothpicks that snap easily.

  • collapsible landing gear held by a friction mechanism (like maybe a brass/nylon tipped setscrew or sandpaper clamp). The friction mechanism spreads the deceleration over time while dissipating energy. A telescopic tube, for example where the inner tube or rod is held by a tuned set screw or friction collar. Could also be combined with a weak shear pin at the very end just as a last ditch safety if even the friction clutch doesn't dissipate enough energy.
  • If you want to get real fancy and make a hydraulic-type damper, what these do is have a reservoir of oil and when you press the piston it forces the oil through a small orifice into another chamber and this dissipates the energy. A bit fancy but a DIY version would probably involve a syringe with some liquid inside like baby oil (mineral oil) or maybe even vaseline but that might be too thick. The main advantage of the vaseline is it is very viscous so is easy to control and keep in place. You have an orifice leading to the outside that you seal off (perhaps even with a bit of tape if it's vaseline since it isn't very fluid). If you press the plunger hard enough the seal pops off and the liquid is forced through the orifice into the outside rather than into another chamber.
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  • $\begingroup$ Could you elaborate what "force that exceed structural strength" means? Is it shear force? Is it compression? $\endgroup$
    – Faito Dayo
    Commented Mar 9, 2022 at 2:36
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    $\begingroup$ @FaitoDayo This is the second time you've asked this and I am somewhat dismayed that I again have to repeat that this is not generally answerable which should be obvious. Bending applies tensile, shear, compression, and buckling. Whichever one destroys the structure is which ever one was most underbuilt for the stress that was applied which is entirely up to the structure. Weakest link. If you make one aspect stronger, then the next weakest one will fail. Ideally, all failure modes would happen at the exact same time which means no part is overedesigned. $\endgroup$
    – DKNguyen
    Commented Mar 9, 2022 at 3:27
  • $\begingroup$ I like this answer. Perhaps add as options construction foam or corrugated cardboard as energy absorbing elements. Consider how bike helmets are constructed. The foam or cardboard would be replaced after a hard landing. $\endgroup$
    – Eric S
    Commented Mar 9, 2022 at 16:49
  • $\begingroup$ @EricS Added... $\endgroup$
    – DKNguyen
    Commented Mar 9, 2022 at 17:47
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It is the deceleration that will cause the damage.

You can work out the deceleration rate as follows:

  • $v$ = velocity
  • $s$ = spring compression

If $v = 20 \ \mathrm{m/s}$ and the spring length is 15 mm (0.015 m) then from $s = \frac 1 2 \frac {v^2} a$ we get $a = \frac 1 2 \frac {v^2} s = \frac 1 2 \frac {20^2} {0.015} = 13,333 \ \mathrm {m/s^2} $.

Since the acceleration due to gravity, G, is 9.81 m/s2 (round up to 10) your aircraft would have a deceleration rate of 1,333 G.

If there is no spring and no flexing then the deceleration rate is infinite.

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  • $\begingroup$ But how exactly the deceleration damages my internal structures? Is it buckling? Is it shear? $\endgroup$
    – Faito Dayo
    Commented Mar 8, 2022 at 23:34
  • $\begingroup$ @FaitoDayo That can't be answered generally. $\endgroup$
    – DKNguyen
    Commented Mar 8, 2022 at 23:34

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