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

[franc11HD]

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?

Thanks.

 

[rb1957]

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

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

 

[franc11HD]

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.

 

[rb1957]

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 ?

 

[franc11HD]

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.

Thanks.

 

[rb1957]

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 ?

 

[franc11HD]

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.

 

[rb1957]

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 ?

 

[WKTaylor]

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

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