Contact US

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Aircraft Ground Flotation Analysis (CBR) S-77-1

Aircraft Ground Flotation Analysis (CBR) S-77-1

Aircraft Ground Flotation Analysis (CBR) S-77-1

Hi Everyone.

I was wondering if there were anybody familiar with the document S-77-1 Procedures for Development of CBR Design Curves. My question revolve around Figure 19 and equation 47 about the thickness / squareroot of footprint area vs CBR/ Equivalent Pressure.

They give the equation 47 to build the curve. The problem is that when I use the equation, I fall very far from the curve.

I thought that there might be a typo in the equation, butit is an old document and I guess they would have flagged it somehow.

The equation I have is: T/squareroot(Ac) = -0.0481-1.562*log(CBR/P)-0.6414*(log(CBR/P))^2-0.4730*(log(CBR/P))^3

Anyone have seen this equation or use it or see a different one?


RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

can you relate to an existing aircraft ... say B777 ... wt, tire area, CBR ?

another day in paradise, or is paradise one day closer ?

RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

I don't think this particular curve is aircraft specific. I find the same curve in S-77-1 or ASD-TR-70-43 which respectively gives an example for KC-135 and C-131E.
So whatever the CBR/P ratio is, T/squareroot (Ac) should be the same whatever the aircraft.

But for the sake of the example, we can use KC-135 data for MLG.

Single wheel load: 36 000 lbs
Tire inflation pressure: 155 psi
Tire contact area: 232.3 in2
footprint radius: 8.6 in.
Critical spacing diagonal: 8.14
Critical spacing width: 4.18
Critical spacing length: 6.98

I can't give you any CBR, because the goel is to find the critical CBR for a particular aicraft at different pavement thickness.

RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

CBR is a number. what's "P" that CBR/P is constant

what's T ? Ac looks like contact area.

if you're "far off the curve" then one of your numbers is "wrong" ...

maybe your Ac is too small ?

another day in paradise, or is paradise one day closer ?

RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

I understand that. What I meant with the question is that the given equation does not match the given curve which it tries to approxiamte (sorry if it was not clear at the start).

So I used the given equation for multiples values of CBR/P from 0.001 to 1.0 (on a log scale) and calculated the respective values of T/squareroot (Ac). Then I plot those values and I guet a very different curve then what is shown in the reference documents. That is why I was wondering if there was anything missing in the given equation.


RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

i didn't see your equation in the ASD ref (ridiculously easy to find !). The ASD seemed to be calculating CBR ?

another day in paradise, or is paradise one day closer ?

RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

Agreed. ASD only shows the curve. S-77-1 (as easily easy to find!) gives you an equation saying that you can use the curve or the equation to calculate CBR/P for different thicknesses.

RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

i guess all i can say is that fig7 (in ADS) doesn't match the equation

another day in paradise, or is paradise one day closer ?

RE: Aircraft Ground Flotation Analysis (CBR) S-77-1

Are You aware of the following SAE AIRs, this subject? The AIRs, and the references embedded within them, might be ale to point You in the right direction.

SAE AIR 1780 Aircraft Flotation Analysis
SAE AIR 1821 Aircraft Ground Flotation Analysis Methods

"A mile of roadway can take a car a mile. A mile of runway can start an airplane on a journey around the world"

Regards, Wil Taylor

o Trust - But Verify!
o We believe to be true what we prefer to be true. [Unknown]
o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]
o Unfortunately, in science what You 'believe' is irrelevant. ["Orion"]
o Learn the rules like a pro, so you can b

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members! Already a Member? Login


Low-Volume Rapid Injection Molding With 3D Printed Molds
Learn methods and guidelines for using stereolithography (SLA) 3D printed molds in the injection molding process to lower costs and lead time. Discover how this hybrid manufacturing process enables on-demand mold fabrication to quickly produce small batches of thermoplastic parts. Download Now
Design for Additive Manufacturing (DfAM)
Examine how the principles of DfAM upend many of the long-standing rules around manufacturability - allowing engineers and designers to place a part’s function at the center of their design considerations. Download Now
Taking Control of Engineering Documents
This ebook covers tips for creating and managing workflows, security best practices and protection of intellectual property, Cloud vs. on-premise software solutions, CAD file management, compliance, and more. Download Now

Close Box

Join Eng-Tips® Today!

Join your peers on the Internet's largest technical engineering professional community.
It's easy to join and it's free.

Here's Why Members Love Eng-Tips Forums:

Register now while it's still free!

Already a member? Close this window and log in.

Join Us             Close