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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