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

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

(OP)

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.

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.

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

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

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

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

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

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.

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

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

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

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

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

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

SAE AIR 1780 Aircraft Flotation Analysis

SAE AIR 1821 Aircraft Ground Flotation Analysis Methods

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