# Wind loading due to gusts; resultant torque on a 6' parabolic dish

1. Apr 29, 2014

### mmh

Hi,

We had a company do a pointing error analysis on a proposed pedestal system, on which is mounted a two-axis (azimuth and elevation) 6 foot parabolic dish. I estimate the distance from the rotation axis to the apex of the dish to be roughly 43 inches. Among the error sources are errors due to wind loading. The following is an excerpt from the section about wind loading:

The wind loading was specified as 50 knot continuous with gust of 75 knots. For this analysis it was assumed that the worst-case gust would be from 0 to 75 knots. This gust differential resulted in a transient torque of 796 lb-ft for each axis. Knowing the drive compliance from testing of previous systems for this pedestal drive train, we are able to calculate the transient deflection of the axis due to wind load as 0.0253 degrees for each axis.

In addition to drive deflections due to wind loading, there is also structural deflections of the pedestal. These deflections include torsional error (azimuth axis), and bending error (elevation axis).

The error in the azimuth axis from torsional deflection from wind loading (75 knots resulting in 796 lb-ft of torque) is calculated to be 0.0044 degrees. The error in the elevation axis from bending deflection from wind loading (75 knots resulting in 927 lb-ft of torque) is calculated to be 0.0107 degrees.

The author said that "...the calculation was made assuming a worst-case gust was the delta force between a no wind load condition to a 75 knot wind load condition. Thus, the resultant load was that of a 75 knot wind on an 6-foot dish. We had to assume a distance from the rotating axis to the apex of the dish for this calculation."

I tried to figure out how the torque values were arrived at, and how the associated pointing errors were calculated, but had no luck. I tried modeling the wind gust as a force impinging on the apex of the dish, and then causing a torque on the center of rotation along the virtual lever arm defined by the center of rotation and the apex of the radar, but wasn't sure if that was the right approach. Does anyone have any idea how I might proceed?

2. Apr 29, 2014

### Baluncore

Welcome to PF.
A 6 foot dish is probably modelled as a simple disk with an area of about 30 square feet.

The elevation error is the angular deflection of the dish mounting structure that results from the 75 mph wind pressure applied to the centre of a disk.

The azimuth error is probably calculated based on the 75 mph wind gust applied to one side of the disk only. That will result in an azimuth torque and an elevation bending of the structure. The vector sum is the pointing error.

If you need to know the precise assumptions made then you should contact the author of the report.

3. Apr 29, 2014

### mmh

Great, thank you!

4. Apr 29, 2014

### SteamKing

Staff Emeritus
Nit pick: 75 mph ≠ 75 knots

5. Apr 29, 2014

### Baluncore

SteamKing. That is knot nit picking. It is my carelessness while working in too many systems of units.