Shock Absorbers for Aircraft

[ran13]

Does anyone have a spreadsheet that would help with designing the front strut for a single engine aircraft? This is a clean sheet design so we're not limited by much. The only things we know for sure are 8" of travel and 2.5" diameter. How would I select the internal gas pressure, oil volume, and orofice size?

Thanks,

Roger

 

[GregLocock]

Is this a real project, or is it a student project?

Assuming the latter then: you need to know your loads for a start, and you probably want to build a single degree of freedom dynamic model to get the damping ratio right. The equation for flow through an orifice is well known, as is the gas pressure required to support a given load.

I would observe that in the automotive world we use different orifice sizes for jounce and rebound, I am not sure what you need. Is the aircraft's weight taken entirely by gas pressure, or is there a spring in there as well?



Cheers

Greg Locock

 

[ran13]

Well you're wrong. It is a real project. I don't think aircraft companies can afford to pay an experienced suspension person so we generally have little knowledge of how this is done every day in other companies. We have less than a dozen engineers.

I'm not sure what I need either. The weight is taken entirely by gas pressure. We haven't developed all the loads yet; like what size rut or tree stump do we want to hit. The basics are easy as you say yet that doesn't make it easy to make a reliable, serviceable, light, and cheap one.

I was hoping someone had a dynamic model. I took dynamics in the mid 70's. I wouldn't know how to make a variable damping orifice with different sizes. Can it be done without moving parts, what are the best materials, etc?

Thanks for your help,

Roger

 

[GregLocock]

Well if it is a real project I'll get specific. What's the allowable vertical landing speed of the plane? How much weight is on the strut? What is the surrounding structure rated to take? Those would be my guess as to the important parameters in to how to size the shock absorbing part.

There's a discussion elsewhere about how we size the suspension travel for a car, that may help a bit as well.

Automotive shock absorbers are pretty easy to understand - go and hit an enthusiasts shop and have a look at a take-apart shock so you can see how the valving works. basically there's a spring loaded flap that closes off one set of orifices in one direction.

I do have a 2DOF model for a vehicle over a bump, which runs in mathcad. Is that any help? I last used it 5 years ago but it should be somewhwere at home.

Finally, I'd do what we always do, go and talk to the supplier!

Is the main weight taken via gas pressure or spring? I'd guess a spring. Cheers

Greg Locock

 

[GregLocock]

Oh sorry, orifice sizes and oil volumes (ie what bore cylinder do you need).

Quick answer is that I don't know about volume, aero shocks have a very low duty cycle so the oil can run very hot. I guess you could work back to a maximum desirable oil temperature and then look at how much energy it would take to get there. We'd use 110-135 degrees C continuous for many oils, for a long life.

orifice sizes - sorry I don't have the equation for the force for a given velocity and viscosity and diameter, but the holes are small, of the order of 6 off 1 mm diameter, for a 100 kgf strut, if that is any help. If you like I could reverse engineer a proper shock to get a better handle on this, but I'm sure that the equation is somewhere on the web .




Cheers

Greg Locock

 

[GregLocock]

http://www.penskeshocks.com/7000 Series Manual.pdf

is a very useful reference, certainly good enough to get you in the right ball park. Cheers

Greg Locock

 

[harrisj]

Roger

I've been browsing through old threads and come acrooss your airplane problem. I expect you've got it all sorted now as it's 9 months old. If not, let me know. I've been working on hydro-pneumatic suspensions for land vehicles for some (many) years. There are some neat tricks for giving different damping rates for jounce / rebound. For aircraft landing gear, I guess you need low jounce / high rebound rates.

John