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Thread: taxi and landing light focal point distances

  1. #1

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    taxi and landing light focal point distances

    I am calculating the installation angles for a taxi light and a landing light. For that I need to know how far ahead of the vehicle I need to focus these lights. I assume the focal point for the taxi light should be closer than that of the landing light, but have no idea what should be the numerical values of these two distances. I think the landing light focus distance should depend on the approach speed, for that my airplane has a 60 knot approach speed. Please help.

  2. #2

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    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
    Last edited by WLIU; 01-13-2013 at 08:31 AM.

  3. #3

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    Quote Originally Posted by WLIU View Post
    I will suggest that my ideal landing light points right at the numbers as I fly my stabilized approach.

    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.
    Wes, are you suggesting to aim the landing light 1 degree down relative to the longitudinal axis of the airplane in the above example? I think the current answer is to aim the landing light 5 degree down relative to the longitudinal axis of the airplane, so the light will be parallel to the glide path and reach the numbers.

    I appreciate your many replies which help me a lot. By the way, you use a 4 degree glide slope instead of the conventional 3 degree in the above example. Any particular reason behind it?

  4. #4

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    Quote Originally Posted by wantobe View Post
    I am calculating the installation angles for a taxi light and a landing light.
    If it's a homebuilt, adjustments will be through trial and error. Aiming the taxi light will obviously be easier than the landing light. I would just find a comfortable spot for the taxi light, then aim the landing light ~50' in front of that, for starters.

  5. #5

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    I fly a taildragger. The airplane has both landing and taxi lights. The landing light is a narrow beam HID light and is directed parallel to the ground with the airplane in a 3 point attitude. That is perfect for final approach, IMO, and gives very good visibility down the runway as you flare and touch down. The taxi light is a wide angle HID and is pointed where it hits the ground about 50' in front of the airplane with the airplane in a 3 point attitude. It provides a lot of light along the periphery of the landing area and gives a good view for taxiing.

    The HID lights give an amazing amount of light compared to the halogen lights they replaced.
    Last edited by Kyle Boatright; 01-13-2013 at 07:51 PM.

  6. #6

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    The last post is great info for one type of airplane. What airplane are you talking about. If the original poster has the same type of airplane, he may now know exactly where to start. If he is building a different airplane, it may sit on the ground in a different attitude.

    On the topic of 4 degree approaches - If you are not doing the ILS, you will find that the typical final approach following the pilot handbook numbers is steeper. Typically, flying a 3 degree ILS is a power approach. Power off, different airplanes glide more or less steeply. I say that as a guy who flys airplanes with glide characteristics that resemble a falling manhole cover. My power off approaches are VERY steep. Cessnas are sailplanes in comparison. So factor that into your landing light mounting, or don't. Maybe you will always fly a power approach at night.

    Best of luck,

    Wes
    N78PS

  7. #7

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    Quote Originally Posted by WLIU View Post
    The last post is great info for one type of airplane. What airplane are you talking about. If the original poster has the same type of airplane, he may now know exactly where to start. If he is building a different airplane, it may sit on the ground in a different attitude.

    On the topic of 4 degree approaches - If you are not doing the ILS, you will find that the typical final approach following the pilot handbook numbers is steeper. Typically, flying a 3 degree ILS is a power approach. Power off, different airplanes glide more or less steeply. I say that as a guy who flys airplanes with glide characteristics that resemble a falling manhole cover. My power off approaches are VERY steep. Cessnas are sailplanes in comparison. So factor that into your landing light mounting, or don't. Maybe you will always fly a power approach at night.

    Best of luck,

    Wes
    N78PS

    The last post was intentionally generic. The intent was to point out what I want to see when I'm on final, in the flare, and on the ground and how to select and aim lights to meet those needs. The OP should consider what he wants to see in a particular phase of flight and install a lighting system that meets those needs. Mine was a simple example of how I accomplished that.

  8. #8

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    Quote Originally Posted by martymayes View Post
    I would just find a comfortable spot for the taxi light, then aim the landing light ~50' in front of that, for starters.
    If we aim the landing light further than the taxi light, then it will not aim at the numbers during the stable final approach. As an example, Cessna 172's lights are about 3 feet above ground, if we choose taxi light focus point to be 50 feet ahead of the airplane, and landing light focus point 100 feet ahead of the airplane, then the landing light beam will be about 1.7 degree down relative to the longitudinal axis and the taxi light beam will be about 3.4 degree down relative to the longitudinal axis. It will take more than 5 degree down (3 degrees for the glide slope and at least 2 degrees for the airplane pitch angle) for the landing light to reach the numbers during approach. Here I assume the landing light beam as a single line without an angular spread in vertical plane.

    Should we expect the landing light to reach the numbers during final approach?

    Another question is how does the new LED light compare with the HID light? Anybody tried the LED?

  9. #9

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    Help me out and provide the calculation supports the assertion "It will take more than 5 degree down".

    Thanks,

    Wes
    N78PS

  10. #10
    Jim Rosenow's Avatar
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    Quote Originally Posted by wantobe View Post
    Another question is how does the new LED light compare with the HID light? Anybody tried the LED?
    We put a PAR46 LED on the 172 last annual. It was an amazing difference from the standard light. Much better from the cockpit on final and everyone on the airport noticed it was easier to acquire as traffic. Went from 16 amps to 2.6 if memory serves correctly. Around $250 from Spruce.

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