Wingtips and lights questions

[MidGenerationAL]

We are just now prepping the holes to start riveting the main wing skins (plans build - we're not quite to the quick build starting point) so we're way remote from actually needing wingtip lights. But I'm working on the wingtips now, so need to have a size/shape profile for whatever lights I'm going to use.

My main question is about US/FAA regulations for nav/position lights for flying at night. What is the difference here? For any lights that come in certified and non-certified versions, I can't imagine paying the extra cost for certification when the lights are otherwise identical. I'd really like to use Whelen microburst wingtip lights but these don't come in a certified / night flying version. Is this an actual safety issue? Or an inflexible regulation that must adhered to? Or doesn't matter at all? I will have a belly beacon and a tail light if that matters. From a practical standpoint I am not worried about the aircraft being inadequately lit.

Re, wingtips: pic attached is a rendering of where I'm at now. Trying to do the Hoerner thing. There seems to be "diversity of opinion" whether the underside surface should be convex or concave - I've modeled it both ways and like the aesthetics of concave if no other reason. The undercut angle is a skosche more than 30 degrees.
My plan is to use 3d printing to make molds and then lay fiberglass to make the actual wingtips.
This design adds 12.5 inches to the upper surface of the wing.
Plan to mount landing lights in the cutout bay - basically leading edge landing lights without having to cut the actual wing leading edges.
Would appreciate any feedback on this design
This is a Bearhawk "A" with the older style wing profile, if that matters.

One other random question: is there a good way to run the wiring to locate the tail light on top of the rudder? I'm envisioning the wires getting damaged or interfering with rudder movement.

[jaredyates]

I prefer to run the aft facing white light at the wingtip instead of the rudder. Where I am, folks aren't concerned about TSO or any certification for lights. Maybe check with your local DAR or other builders.

[MidGenerationAL]

I prefer to run the aft facing white light at the wingtip instead of the rudder. Where I am, folks aren't concerned about TSO or any certification for lights. Maybe check with your local DAR or other builders.

One of the reasons I like the microburst wingtip lights - they have aft facing lights built in

[whee]

My DAR didn’t care about certified or not; I don’t think that many do. You still have to meet the requirements in the reg but you don’t need certified lights. You need nav, strobe, and an aft facing light (either a tail light or aft facing lights in the tip lights). Personally think it’s silly to put a tail light in. Just one more thing to modify on the rudder, more wires to run, and you have to watch the wire at the rudder hinge for failure and more weight in the wrong place with a long moment arm.

Looks like your Hoerner rendering has the “undercut” angle measured from vertical; should be measured from horizontal and be between 30 and 45 degrees. The edge where the top surface meets the angled surface should be “sharp.” For a fiberglass tip I’d say a 1/4” think edge will be about as sharp as it can practically be. The fillet you have there looks pretty significant but it might be about right for a mold.

There are a few good actual studies on the Hoerner tip that are fun to read. I’d suggest reading the actual studies and not just forums or other people’s interpretations of the studies. I don’t have the links or I’d post them. Sorry. My favorite is the study the US Air Force did in 1949. Should come up quick on google.

[MidGenerationAL]

My DAR didn’t care about certified or not; I don’t think that many do. You still have to meet the requirements in the reg but you don’t need certified lights. You need nav, strobe, and an aft facing light (either a tail light or aft facing lights in the tip lights). Personally think it’s silly to put a tail light in. Just one more thing to modify on the rudder, more wires to run, and you have to watch the wire at the rudder hinge for failure and more weight in the wrong place with a long moment arm.

Looks like your Hoerner rendering has the “undercut” angle measured from vertical; should be measured from horizontal and be between 30 and 45 degrees. The edge where the top surface meets the angled surface should be “sharp.” For a fiberglass tip I’d say a 1/4” think edge will be about as sharp as it can practically be. The fillet you have there looks pretty significant but it might be about right for a mold.

There are a few good actual studies on the Hoerner tip that are fun to read. I’d suggest reading the actual studies and not just forums or other people’s interpretations of the studies. I don’t have the links or I’d post them. Sorry. My favorite is the study the US Air Force did in 1949. Should come up quick on google.

I was able to access the original study - didn't write down the reference but I think it was the 1949 one.
I can make the upper edge fillet sharper
Thanks for clarifying where the angle is measured from. I can easily increase the angle by adding more upper surface but I will run out of room for the light bay if I steepen the undercut too much. The landing lights I have in mind are "more than adequate" so I could probably find something smaller.
Which raises the question whether there are downsides to adding too much on the upper side (?)

[whee]

The potential down side is that the tips get longer. Bob suggested keeping the tips 16” or less. I think adding 4.5” to your top surface might get you where you want to be.

When doing this fun experimenting I try to keep in mind that everything is a compromise. The compromises I make may not be the ones you would make.

[rodsmith]

Al, I built my Hoerner tips following Harry Riblett's directions, outlined in his book. Not saying that is the only or best way. Harry's key points were that from the leading inside edge, the planform is circular with a radius equal to the distance from the leading edge to the main spar. That makes a Bearhawk Hoerner tip 13.25" wide. It maintains that width from the main spar to the trailing edge with a square corner. The top of the tip matches the upper surface of the airfoil. the bottom contour of the tip is formed from the main spar to the trailing edge by taking a straight edge from the sharp outside edge to the bottom of the airfoil at the inside of the tip. This results in a concave surface decreasing in angle from the main spar to zero at the trailing edge. From the leading edge the bottom surface transitions from the airfoil shape to the profile at the main spar. Like Jon said, the minimum practical thickness at the edge is 1/4" in foam. My finished tips with 4 layers of glass have a 3/8th radius at the outside edge. At the main spar the angle of the tip is 35 deg. I made 3 hot wire cuts to form the tips. The top airfoil surface, The angled underside cut, and the circular planform from the leading edge to the main spar. From the leading edge to the main spar, I hand formed the changing profile with a rasp and sand paper. My tips are 15 1/2" wide total, 1" is overlapping the aluminum so guess you can say I have about 1" of wing extension before the Hoerner tip starts. If doing them over, I would have made them 17" wide with 1" overlap to use Bob's maximum allowed wing extension. l formed the light mounts separately and glued them to the wingtips before glassing. My lights are the Whelen precursor to the Microburst so maybe the same planform. Of all the modifications I made to my plane, I am most proud of these tips. They are not perfect, there is visible waviness in them despite weeks of filling and sanding. Hot wiring and shaping foam was a total new skill to develop for me. Start to finish, except painting it was a three month project. I will never know what performance enhancement they might provide, but I absolutely love the look of them. My only comment on other "Hoerner" tips, is I think it is deceptive to call tips with a convex under surface profile Hoerners.

DSCF1126.jpg DSCF1128.jpg DSCF2789.jpg DSCF2786.jpg

[Bcone1381]

Hey Rod, thanks for the details on your wingtips. I made a foam hot wire saw earlier this week and when I get back I’m planning to build some wingtips for experimentation. I’ll use your data along with Hoerners text to form them. Thanks so much. Very timely.

[MidGenerationAL]

Al, I built my Hoerner tips following Harry Riblett's directions, outlined in his book. Not saying that is the only or best way. Harry's key points were that from the leading inside edge, the planform is circular with a radius equal to the distance from the leading edge to the main spar. That makes a Bearhawk Hoerner tip 13.25" wide. It maintains that width from the main spar to the trailing edge with a square corner. The top of the tip matches the upper surface of the airfoil. the bottom contour of the tip is formed from the main spar to the trailing edge by taking a straight edge from the sharp outside edge to the bottom of the airfoil at the inside of the tip. This results in a concave surface decreasing in angle from the main spar to zero at the trailing edge. From the leading edge the bottom surface transitions from the airfoil shape to the profile at the main spar. Like Jon said, the minimum practical thickness at the edge is 1/4" in foam. My finished tips with 4 layers of glass have a 3/8th radius at the outside edge. At the main spar the angle of the tip is 35 deg. I made 3 hot wire cuts to form the tips. The top airfoil surface, The angled underside cut, and the circular planform from the leading edge to the main spar. From the leading edge to the main spar, I hand formed the changing profile with a rasp and sand paper. My tips are 15 1/2" wide total, 1" is overlapping the aluminum so guess you can say I have about 1" of wing extension before the Hoerner tip starts. If doing them over, I would have made them 17" wide with 1" overlap to use Bob's maximum allowed wing extension. l formed the light mounts separately and glued them to the wingtips before glassing. My lights are the Whelen precursor to the Microburst so maybe the same planform. Of all the modifications I made to my plane, I am most proud of these tips. They are not perfect, there is visible waviness in them despite weeks of filling and sanding. Hot wiring and shaping foam was a total new skill to develop for me. Start to finish, except painting it was a three month project. I will never know what performance enhancement they might provide, but I absolutely love the look of them. My only comment on other "Hoerner" tips, is I think it is deceptive to call tips with a convex under surface profile Hoerners.

DSCF1126.jpg DSCF1128.jpg DSCF2789.jpg DSCF2786.jpg

That is fantastic looking work. I have done some plug making by shaping filling and sanding and can imagine how much work went in to that.
Whenever you make something, you're aware of small defects that nobody else will ever notice.

This keeps me with in the 16" maximum upper width and the 3/8" radius, a bit more than 30 degrees angle at the leading edge, and still enough room for the landing lights I want to use.
I do not know how to make the scoop deeper toward the trailing edge in my modeling program but my brother may be able to help me.

wingV5render.jpg

[alaskabearhawk]

FWIW, I wanted a beacon on top of the rudder and a tail/strobe light. I used the Whelen microburst system and a Whelen Orion 360 tail beacon. All are LED and very light compared to traditional lighting. Power demands are small, so you can use thin wire. I don’t remember what my calculations were on wire thickness based on the load and distance. The wires exit the bottom of the rudder and I used Molex connectors in case I needed to remove the rudder. For me, the wiring is a non-issue at the rudder. Aircraft manufacturers and home builders have been doing this for decades with no problems.

I think you are on the right track with incorporating the lights as you have in your model. It will be interesting to see what you finally come up with.

[MidGenerationAL]

Latest updates:
I was able to get a deeper cutout but can't go a lot deeper than this because it will exceed the depth of my 3d printer (a bit over 15 inches and 16 is the max anyway)
Using this cut the wingtip tapers toward the trailing edge. I can also make it a straight cut (ie, the upper surface ends in a squared edge when seen from above)
Added the wingtip light mounting spot
Threw in a representation of the landing/taxi lights. Planning to use Flyleds combo landing/taxi lights - designed for Vans planes - one of the lights points down so you can see when taxiing a taildragger. I didn't bother to model their lights exactly.

I also printed out my 3d model of the wing profile - will cut it out and compare to the actual wing. It would be rather a pain to make two of these only to find they don't fit.

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