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Force of wind on a landing light
http://t18.net/thorpforum/viewtopic.php?f=29&t=8368		 Page 1 of 2
Author:  Ryan Allen [ Fri May 10, 2019 1:43 pm ]
Post subject:  Force of wind on a landing light
I am thinking about mounting my landing lights to the inlet ramps of my cowl baffles. I am thinking about installing them so that the face of the light will lay flush with the baffle inlet ramp during cruise flight. I will build a hinge so that I can rotate them from a flush position to a forward facing position so they will shine out onto the runway during landing. I hope that makes sense. Anyway, my question is about how much force would be required to rotate the light into the "up" or forward facing position, which of course would be into the wind. Could a simple push pull cable do the job? Could a Ray Allen servo arm actuator be strong enough? Here is a photo of my landing light. The frontal area of the light is about 6 square inches. I just don't know the formula to use to calculate the force on the face of the light. If I knew the force acting on the light at say 150mph, I could do some tests in the garage on the push/pull cables and servo actuator.

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Author:  Unclerap [ Fri May 10, 2019 5:05 pm ]
Post subject:  Re: Force of wind on a landing light
Here's my installation done by a previous owner. Not pretty and it's coming off some day. But it works.

Rap
Image
Author:  leewwalton [ Fri May 10, 2019 6:31 pm ]
Post subject:  Re: Force of wind on a landing light
I wouldn’t overthink it Ryan, if the light is mounted solid and it rotates a tension locked push pull will hold it as long as its actuated with a solid bellcrank .. a servo with a solid arm will hold it just fine too.the idea is to rig it such that the cable doesn’t take the load. I.e. minimal cable movement or use a servo with a solid arm.
Author:  Rich Brazell [ Fri May 10, 2019 8:30 pm ]
Post subject:  Re: Force of wind on a landing light
Lee is right . Get a good nites sleep and rethink this in the morning . I tend to over complicate things , but about 1/2 way thru an idea I usually come up with a much simpler plan . I thought about putting in a retractable landing light in the wing , but instead went with the stationary in the wing lite . :P

RB O0
Author:  James Grahn [ Fri May 10, 2019 9:44 pm ]
Post subject:  Re: Force of wind on a landing light
Ok, I’ll bite. Why make it movable? The stationary ones do not affect cooling.
Cubes
Author:  Fraser MacPhee [ Fri May 10, 2019 11:49 pm ]
Post subject:  Re: Force of wind on a landing light
This is what I mounted on the flat baffling just ahead of the #2 cylinder. Works good so far - been there about a year and 1/2 and 100 hours or so. One carriage bolt and I put some thin padding between the bracket and the baffling just to keep any stress risers from causing the baffling the crack.

49 hard earned dollars plus shipping.

https://www.superbrightleds.com/moreinf ... 3542/7536/
Author:  Ryan Allen [ Sat May 11, 2019 7:53 am ]
Post subject:  Re: Force of wind on a landing light
Thanks for the input because I was wore out from just typing that post, much less trying to do some math again. I agree, I had always thought the lights at the cowl inlet didn't affect CHTs. I flew after removing my old cowl inlet lights, and I thought I saw my CHTs 10-20 degrees lower and my oil temps about 10 degrees lower with no lights installed. But I think other things were helping those numbers, like a much cooler ambient air temp on that one flight. I'll just fly with the new ones stationary, and if I just cant stand it, its good to know a cable or servo actuator should handle the rotation.
Author:  DrDrift [ Sun May 12, 2019 1:49 am ]
Post subject:  Re: Force of wind on a landing light
Everyone else is right, but to answer the question directly:

The force on the light due to drag is:

D = q * S * Cd

Where q is the dynamic pressure, calculated by q = 1/2 * rho * V^2, rho being the density of air and V being the airspeed.

S (or A) is the frontal area, which you said was 6 in^2

Cd is the drag coefficient, which varies depending on shape.

A cube has a Cd of 1.05 (no relation to Cubes who has a very low drag coefficient). A flat plate is about 1.3, per https://www.grc.nasa.gov/www/k-12/airplane/shaped.html

Using metric units (sorry):

V = 82 m/s (160kts)
Rho = 1.225 kg /m^3
S = 0.004 m^2 (6 in^2)
Cd = 1.3

Calculating...

D = 21 Newtons or 4.8 pounds.

4.8 pounds is not a lot of force, so depending on the actuator and linkage geometry, it should be quite reasonable. If the light is mounted fixed, this is roughly the force that it will impart on the surrounding structure. Note that the Cd estimate is for a free flying flat plate. Mounting the light against a wall may decrease the drag coefficient.

As a side curiosity...

Power is drag force times speed, or P = D * V

4.8 pounds of drag at 160 kts requires 2.4 HP to push. That's more than 10% of my engine, so even though it's not a lot of force on the structure, it could impact your top speed if done improperly.

There's another thread about maximizing speed and another about action camera mounting externally. Appendages aren't a big deal structurally, but a clean airplane goes fast.
Author:  James Grahn [ Sun May 12, 2019 8:52 am ]
Post subject:  Re: Force of wind on a landing light
I love this forum cuz we have real geniuses! Nice discussion.
And yes, my drag is going up as I age, but that’s why I keep my hair so short!!
Cubes
Author:  Fraser MacPhee [ Sun May 12, 2019 11:16 am ]
Post subject:  Re: Force of wind on a landing light
I really get a kick out of posts like DrDrift's.....though I must point out to the good doc that (at least according to my shoulder top bio-computer) 2.4 HP is, I think only 1.5% of a 160 HP engine.....sorry to be the math police....but nonetheless may impact the speed as an exterior appendage (parasite drag).... though if mounted in the (pressure) cowl on the baffling, me no thinky impact speedy..... :P
Author:  fytrplt [ Sun May 12, 2019 1:19 pm ]
Post subject:  Re: Force of wind on a landing light
That would be 160 hp at sea level wide open on a standard day. Let's assume 75 % power high cruise yields 120 hp, or a whopping 2% penalty. However, nestled inside the cowling, it adds nothing to the flat plate area of the plane. Turbulizing the inlet air may even aid in cooling, further negating the perceived loss. Take that Dobbin! Now off to the glue factory.
Author:  James Grahn [ Sun May 12, 2019 4:57 pm ]
Post subject:  Re: Force of wind on a landing light
As another point of interest, a seven time perfectionist builder on my field tuft tested his engine compartment. Yes, he put tufts in there with cameras. He was surprised, as well as the rest of us, that the air went nowhere close to what we imagined. The left inlet performed as expected. Air actually was exiting the right inlet!!!! That was on a Sportsman 2+2. It was much less in cruise than climb. Anyway, I have flown with and without my light in my right inlet and could not notice a difference.
Just a data point.
Cubes
Author:  DrDrift [ Sun May 12, 2019 7:13 pm ]
Post subject:  Re: Force of wind on a landing light
Fraser,

I am humbled, thanks for noticing my error. Nothing ruins an explanatory equation faster than a misplaced decimal point. Either that or I'm sporting an 18HP engine and the rest of you guys are making me feel even more inadequate.

Cubes, I agree with your sentiment about the flow tufts. I've put them on flaps and on the back window of my race car, and in both instances it wasnt close to what I was expecting.
Author:  Rich Brazell [ Tue May 14, 2019 7:25 pm ]
Post subject:  Re: Force of wind on a landing light
Naught from Naught = Naught ! ??? I applied this formula to all the items I added to my AC . ::)

RB O0
Author:  Fraser MacPhee [ Sat May 18, 2019 6:37 pm ]
Post subject:  Re: Force of wind on a landing light
Here are a couple of pics of my installation.....and Naught from Naught = Naught means Rich is now a "Triple Naught" spy!!

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