# Does a strobe light shield act like a wingtip fence [or better sort of vortex generator]?

I was told it is a shield to avoid to blind the pilot during night flights when the strobes are on. Now, knowing the primary reason is, as far as I know, the above mentioned:

1. Does anyone know any official document where the definition of this device is mentioned?
2. Does anyone know any reference where the effect on the wing tip aerodynamic is studied? Considering different speed regimes?
3. Does anyone know if there are any regulations on this small device?

As far as I know, this device can be founds on the following aircraft:

1. King Air
2. T-6 Texan
3. Embraer Legacy 600

As can be seen, it can be found on models with and without wing tip devices.

One thing that should be mentioned on aircraft fitted with winglet (not wing fence at the tip as those found on the A380), is that during the pull up manoeuvre, at take off, a mild/severe separation is generally observed on the inboard part of the winglet. Could the strobe shield fitted in the Embraer Legacy 600 mitigates this by re-energising the boundary layer. It would be easy considering the amount of cross flow at the tip.

Any official paper or informal discussion anywhere in the web would be very useful.

More pictures can be found on these links:

http://79ft.net/tag/mechanicing.html

• I think in some conditions it can be that it triggers the separation and it creates a vortex, but has it been designed to do so? I don't think so, while I imagine it would require intensive testing in low speed/ wind tunnel to verify the design, and I am not sure it is "worth", but maybe that part of the wing is more "critical" for stall recovery than I think... – GHB Feb 29 '16 at 23:14
• @GHB Typically you want the wing to stall from the root and progress towards the tip, so that the ailerons stay effective for roll control. I'm guessing that once you get past the aileron surfaces, the wing being stalled or not is less of a factor in controllability, and I'd venture to say that having a vortex generator on the very tip is going to do little for performance. – Ron Beyer Mar 1 '16 at 2:09
• @Ron Beyer, good arguments, so a vortex triggered at the tip could, potentially, by shifting inboard, help reattaching the flow over the outboard part of the aileron. I am not sure quantitatively the effect of this reattachment be and, most important how "robust" that would be, to be considered a mechanism triggered by "design" (so sought by engineers, tested and validated). For the same argument of "robust design" I am not sure what is the "value" to such a design from a performance point of view. – GHB Mar 1 '16 at 9:16
• Based on the comments posted by the others, which more or less agreed with my thinking and confirmed other hypothesis I have made, I would appreciate if anyone would post any official papers/documents to support their arguments. Here the difficult thing I have found is to find 'official' statements. Is it possible this is something for which no regulation at all can be found? – user5587564 Mar 1 '16 at 16:24
• As is said in an answer below, this is not a vortex generator-like device nor will it function as such. It acts more as a winglet. – SMS von der Tann Mar 3 '16 at 21:12

It is a long structured question, I will try to keep up with the answer. As first I would like to cite some sources.

I compared both CS23 and CS25 looking for the certification requirements on the lights. They are almost identical. If you want to check them yourself the keyword you want to use is "Lights". The paragraphs of main interest are from 1285, to 1401, three pages in total. That said.

I was told it is a shield to avoid to blind the pilot during night flights when the strobes are on.

The light on the wingtip in this case are both position lights and anti-collision lights, the strobing light you refer to.

The Position lights: are described in paragraph from 1285 to 1391. There are requirements on the shielding of them and the intensity of the lights from different radians as well as the colors of them, but none from an aerodynamic point of view.

From a design point of view the shield should define some dihedral angles:

• (b) Dihedral angle L (left) is formed by two intersecting vertical planes, the first parallel to the longitudinal axis of the aeroplane, and the other at110º to the left of the first, as viewed when looking forward along the longitudinal axis.

• (c) Dihedral angle R (right) is formed by two intersecting vertical planes, the first parallel to the longitudinal axis of the aeroplane, and the other at 110º to the right of the first, as viewed when looking forward along the longitudinal axis.

• (d) Dihedral angle A (aft) is formed by two intersecting vertical planes making angles of 70º to the right and to the left, respectively, to a vertical plane passing through the longitudinal axis, as viewed when looking aft along the longitudinal axis.

For the anti collision lights:

The aeroplane must have an anticollision light system that

• (1) Consists of one or more approved anti-collision lights located so that their light will not impair the crew’s vision or detract from the conspicuity of the position lights

• (b) Field of coverage. The system must consist of enough light to illuminate the vital areas around the aeroplane considering the physical configuration and flight characteristics of the aeroplane. The field of coverage must extend in each direction within at least 75º above and 75º below the horizontal plane of the aeroplane, except that a solid angle or angles of obstructed visibility totalling not more than 0·03 steradians is allowable within a solid angle equal to 0·15 steradians centred about the longitudinal axis in the rearward direction.

On the aerodynamics effect of vortex generator at the wingtip, I found a paper: Modification of a wing tip vortex by vortex generators. It is an experimental study without a wingtip device though. Citing from the conclusion paragraph:

The results show that substantial redistribution of wake circulation can be achieved by the merger of the tip vortex with a co-rotating vortex from the VG with the core radius of the merged vortex increased by a factor of up to five relative to the undisturbed tip vortex.

The VG was though positioned at a $\eta =\frac{y}{b} = 0.2$ inboard of the wingboard.

Summing up the comments above, and merging this interesting explanation from @Peter Kämpf on winglets, what I think (yes, it is an "opinion", but otherwise I would need to run some 3D CFD and/or a wind-tunnel test) is that:

• There would be a vortex generated in certain conditions for sure.
• To design this device in order to avoid winglet stall separation would be quite a challenging task. The device its positioned very outboard which would make it be more or less aligned with the winglet. I think that the close span-wise coupling between he device and the winglet would make it hard for the vortex to develop in such a way to re-attach the flow on the winglet.

Vortex generators are located along the wing, not at the wing tip. They are designed to make the wing stall break over a greater range of angle of attack. What you show at the wing tip will not have a similar effect as a VG.

• Just a comment on this answer. In my opinion is not fully true that VG were never used at the wing tip. See for instance the Gloster Javelin here aerospaceweb.org/question/aerodynamics/q0009.shtml. These in particular were designed to maintain ailerons effective at high AoA which is another use of VG. The big difference I see with respect to the strobe shield is that this is generally placed at the leading edge whereas those designed to maintain the flow attached at the aileron level are much more downstream. – user5587564 Mar 3 '16 at 21:32