Struts spring formula not matching SDK calculation sheet

Hi everyone!

I am working with contact points and I needed both to tune them correctly but also to be able to compute the actual wheel weights for internal purposes.

However, while using the SDK sheet providing the new spring fix force, the sheet calculation do not match what the sim shows.

This is the .cfg file supposed to activate new formula using exponential fix = 1

Then this is what the SDK sheet gives:

So I can see for those parameters a 30% compression yields around 40900 lbs of force.

Now if I load the sim with this .cfg, here’s what I get around those 30% compression (which is the compression I expect at this wheel weight):

14cm of compression = 30% is around 0,459 ft so it matches with the sheet.
But the sim shows there a 72992 lbs force, which I don’t understand.

Am I overlooking something there?

Thanks for your help!

2 Likes

Hello Davy,

This difference is probably due to the fact that in the Excel table in cell D9 a static load 49460.22809 (lbs) is entered, which differs from the actual one in the sim (72992 lbs).

Also, according to your .cfg settings, the “balanced” static compression should be exact 0.545 ft = 16.6 cm, which is 1/2.857 = 35% of full length of amortization: 0.545 ft1 * 2.857 = 1.557 ft = 47.4 cm (you can find this value in the Excel table in cell D7).

The reason why the sim shows compression of only 30% (14 cm) is probably due to the fact that the current legacy landing gear initialization algorithm automatically calculates the load on the contact point at maximum gross weight and, accordingly, automatically calculates the spring factor for the aircraft “standing” horizontally (with zero pitch angle), which is not always the case, of course. With a different weight and balance and pitch of the aircraft, the load on the landing gear will be distributed differently, which leads to inconsistencies like this.

I think that in the foreseeable future we will add the ability to manually set spring and damper factors for contact points to make the setup more transparent, which could be more convenient in cases like yours.

3 Likes

Thanks for your answer.

Sadly, the max gross weight can be an infinite combination of where fuel and cargo is, and I cannot seem to find a way to reach correct values…

Knowing the actual spring constant and ratio chosen by the sim would already be a good help if not being able to set them ourselves indeed.