Model Shock Absorbers

[Mosey]

I need to make a pair of shock absorbers for my model airplane, and wonder if there are any ideas or plans out there. They do not need to be hydraulic, only sprung. They are for a large Pilatus Porter weighing about 7 lbs. and are about 1/4 -5/16" OD.

Ideas?

 

[Entropy455]

Shock absorbers come in three types.

Friction - where the resistive force of the shock is constant across wide changes in velocity (F = k). Examples of these are old-school torsional suspension dampers, used on turn of the 19th century automobiles. They used strips of leather sandwiched between metal plates. The friction would resist motion, thus provide damping of a spring-mass system (suspension).

Hydraulic - where the resistive force of the shock increases proportional to increases in velocity. Examples of these are modern automobile hydraulic shocks. Essentially it’s a hydraulic ram, but the piston is ported to permit fluid to pass across it. If you double the rate in which you compress a particular hydraulics hock, you double the required force (F = k*v).

Gas - where the resistive force of the shock increases proportional to increases in velocity squared. Examples of these are modern automobile gas shocks. Essentially it’s still a hydraulic ram, however the working fluid is a compressed gas. If you double the rate in which you compress a particular gas shock, you more than double the required force (F = k*v^2).

There are also gas-hydraulic hybrid shocks, where the force is (F = k*v^c) where 1<c<2.

What does this mean in simple terms? When driving a car, you want a smooth ride. To obtain a smooth ride, you want the shocks to be &#8220;highly effective&#8221;during high-impulse events (bumps, potholes, etc). Otherwise you want the contribution of the shocks to be minimal for normal driving down the road. This is why friction-type shock absorbers are practically extinct, and the automotive and aerospace industry have moved almost exclusively towards gas and hydraulic shocks.

If I were building functional shock for a model airplane, I would construct a small hydraulic shock, and fill it with thick oil to minimize leaking past the seals. Drill the piston for a metering hole, to obtain the desired value of k (i.e. how hard and fast you must comress or expand the shock for a givenforce). Remember that very high rates of compression on hydraulic shocks will cause very high compressive forces. Remember also that when the oil is cold, the k value increases beacuse the oil is thicker.

 

[Mosey]

Interesting dissertation, but not really what I am looking for.
These shocks only come into use on roll-out for taking off, and upon landing. It is for landing only that I need simple ability to absorb compression from the weight of the craft hitting the ground once. Can anyone provide a simpler suggestion? All I want is a suggestion about how to build something for holding an internal mechanical spring in a tube.

 

[rkepler]

Interesting dissertation, but not really what I am looking for.
These shocks only come into use on roll-out for taking off, and upon landing. It is for landing only that I need simple ability to absorb compression from the weight of the craft hitting the ground once. Can anyone provide a simpler suggestion? All I want is a suggestion about how to build something for holding an internal mechanical spring in a tube.

Why not go to a model shop and find something from an RC car? Some nice outer spring hydraulic shocks there and made for a similar application.

 

[Entropy455]

A shock absorber is usually used with a spring &#8211; which is why I didn&#8217;t understand your question. .

For your spring mount, I would use a greased round-bar inside of a tube, providing a degree-of-freedom for spring compression &#8211; similar to an automotive strut. Or you can use a greased tube within a tube, to keep the spring unexposed.

Nonetheless, I think you would be better off making an actual hydraulic shock to go with your spring. This way your airplane will not bounce like a ball when it&#8217;s trying to land (which it will, without a shock absorber installed)

 

[Entropy455]

http://www.rcecho.com/RC-Model-Car-Dustproof-Aluminum-112mm-R-C-Hobby-Oil-Shock-Damper-2pcs-OS345.html

Like this:

rkepler has a good idea. I'd just purchase one already made. The RC world offers them up in any and all sizes, and pretty low cost also. . . .

9 bucks is hard to beat for one of these. . . .

 

[Jasonb]

Interesting dissertation, but not really what I am looking for.
These shocks only come into use on roll-out for taking off, and upon landing. It is for landing only that I need simple ability to absorb compression from the weight of the craft hitting the ground once. Can anyone provide a simpler suggestion? All I want is a suggestion about how to build something for holding an internal mechanical spring in a tube.

The problem is if you justy have the spring when it has been compressed by landing it will have stored energy and you need something to damp this energy so it is released slowly.

If not the plane will end up going down the runway like a pogo stick

Make a tube with screwed caps, a piston with a couple of holes in it to allow oil to flow and put a spring at in the tube. On landing the piston will be pushed up into the tube with high force displacing the oil and compressing the spring. On rebound there will be less force available so the oil will take longer to pass through the holes in the piston therefore damping the springs return.

You will have to play with oil weight and hole size/number to get the movement you want.

J

 

[rcfreak177]

Hi Mosey,

I have made a couple of hundred sets of retractable landing gear for RC Jets in the past.

I used a swing-arm type set-up which also had a compression spring contained within a piston then fit into a tube.

This spring fit inside a long piston with approx 1/2" protruding. That was the area that would compress. This piston then fit inside the mentioned tube.

Hope this makes sense.

The method for the dampening funny enough was as simple as a thick grease between the tube and the piston, the grease had to be cleaned out and refreshed from time to time. There was very little excess grease due to the tight tolerance between the two. (approx 0.002" clearance from memory)

The landing gear also had brakes. They operated buy energising a rubber seal causing it to rub internally on a brake drum, all operated by air.

I still have some left floating around the shed somewhere, I will find the units and send some photo's to you.

Cheers.
Baz.

 

[rcfreak177]

Here we go,

The first picture shows the compression spring, piston and outer tube. The threaded holes are for dowels to screw in to avoid the piston flying out of the main tube and also for a stop under full compression.

The next photo is the swing-arm assy, The nice shiny one is one that I made and the other is one that I used to replicate from. This works much the same as a motorcycle. There are extra parts for the linkage that are not shown.

I made a couple of hundred units for a supplier of model aircraft here in Australia, I believe the models weighed in at around 20kg.
I hope this helps.

Baz.

 

[Mosey]

Lots of great suggestions here, thanks.

Short of building an entire landing gear with articulation and full shocks, I am trying to make a quick improvement in what I have.
The pictures below show the situation. I think I can make a new rod with external spring in place of the one attached to the large strut with a nut on top.

Later, I will make a set to be like that on the full-scale plane (Pilatus Porter).
Parden the mud.

 

[PeterB]

This is what you want

More here

 

[Mosey]

I could cut or make new large (5/16") diameter struts and put a piston, and spring, inside with grease as dampering. This is approx. how the full-scale plane is built. Now, can anyone come up with a cross section through the shocks?
A piston with a rubber piston on it, a spring, close fit inside the tube, and some fluid to damp oscillations.

Drawings, anyone?

 

[peatrich]

I fly 50cc powered rc planes and have never used shock absorbers as the landing gear and the tyres absorb the load. The wire, or dural undercarriage is very forgiving and inflated rubber tyres, rather than foam rubber tyres will help even the heavily constructed high wing loaded model warplanes land safely, although many people use retractable struts for them that have shocks for realism. Forgive me for being blunt but are you trying to engineer out the need to land more smoothly? Flaring out just before you touch down can be tricky to master, especially in blustery conditions and a cross wind but it saves the fuselage from damage. Landing, as they say, is a controlled crash!
Kind regards
Peter