taxi and landing light focal point distances

[WLIU]

I will suggest that my ideal landing light points right at the numbers as I fly my stabilized approach.

I do not have my Cessna 100 Series service manual handy, but my hazy recollection is that you can fine adjust the lights but the gross aiming is fixed by the way the supporting structure is riveted in place. So if you are building the structure, you likely want to first, set the landing light to be aimed parallel to the aircraft longitudinal axis, and then guess that you will fly maybe a 4 degree approach path, at an airplane attitude of maybe 5 degrees nose up in relation to your flight path?

Digging into the trigonometry text, your line of sight from your cockpit to the numbers is 4 degrees below horizontal. Adding your pitch attitude to that says you might aim your landing light down 4 - 5 = 1 degrees. If you mount the light on your wing, add in the angle of incidence to the calculation. Build an adjustment mechanism that allows maybe 5 or 10 degrees of fine tuning. Check by flying some night approaches.

Now for the taxi light, I want it to show me the yellow stripe on the taxi way maybe 60 feet in front of me. So using some more trigonometry, you will need to look at horizontal and vertical aiming. For the vertical aiming, look at the attitude that your airplane sits at on the ground, how high above the ground the light is, and the distance in front of the airplane that you want the beam center to be. A bunch of triangles to compute and turn into angles. For the horizontal aiming, you look at the triangle that is the distance of the light from the aircraft centerline and the distance in front of the airplane that you want the beam center to be. You can eliminate this calculation by mounting the light on the front of the cowling, but your Piper and Grumman friends will warn you that lights do not last that long at that location due to vibration and maybe engine heat. Better put them out on the wing.

Best of luck,

Wes
N78PS