Night flying Float Plane
Night Flying on a Float Plane, A Construction Article
Well, it's getting into the building season again. You know how it goes, if it's raining, it's the building season. While we enjoy a bit milder winter than many, still can't fly when it's raining.
One of my projects this year is something with a few LEDs on it. Recently some friends were building float planes, so I decided join them and to also incorporate my LED goal. Here is a picture of the nearly completed Ocelot.
Tim’s Ocelot, float plane with LEDs. Basic kit is from Gary Morgan that lives in the south bay. http://www.morganmill.com/RC/Ocelot.html
With the advent of low cost led strings there has been an upsurge in night flying. The LEDs are pretty cheap these days and you can even get them outside the normal hobby circles as some are showing up on amazon.com. Other sources are typical of hobby vendors such as hobbypartz.com, hobbyking.com, valuehobby.com, and many more. Following are a few examples.
5mm wide strips, white(other colors exist too)
It seems the standard configuration for the strings is 3 leds in series with a current limiting resistor (let's call this a set for ease of description); this is repeated multiple times in parallel; this parallel scheme is convenient since it allows you to cut the length as needed for your install. See pic below, Cut on lines then solder connectors to the end.
5mm wide, hobby partz, 1 set
The construction of the led strings uses surface mount components on a flexible circuit so you can go around gentle curves or or even sharper corners if bent at the cut line . The conducting copper is under an insulating legend screen just like on a conventional printed circuit board and there is an obvious line of demarcation between the sets. This line of demarcation allows you to cut to length with a pair of scissors as shown in the above picture. Each set also has a couple solder pads for you to attach the connectors for hookup.
Many flavors of led strings exist, some led strings are bare LEDs on flex circuits, as shown above, others tout being splashproof ( see links below).
The splash/water proof strings carry a weight penalty, this gives you an idea. 28.3 gr/oz
5mm bare strings, hobbypartz, 2.1 gr/ft.
10mm valuehobby, splashproof, 8.1gr/ft
10mm hobbyking, waterproof, 20gr/ft
Following are schematics.
Schematic for 1 “set”
Typical string of multiple “sets”
This set configuration makes the voltage requirement to drive the string about 12v, or a 3S lipo. You can use a separate battery for the LEDs or even tap from the high current 3S battery terminals. I took a different path using the balance connector, which is good up to 4A, adequate for most installs.
On/off control can be achieved with a TX operated switch that connects to an unused channel on the receiver. Here are a few.
TX controlled switch, 10A, solid state switch
10A Battle Switch, with relay
1A Pico switch
Personally, when switching something like 10A I prefer an electromechanical switch but you can use solid state as well.
With this led hookup you could make a little dimmer circuit to vary the led brightness if you like. However, with this led configuration, dimming is about all you can do. This is because the configuration is multiple sets in parallel, which means what you do to one set, you do to all sets. Dimming is accomplished with a PWM scheme.
There are some devices that will give you a bit more than on/off control and are listed below. Basically, the level of control depends on the circuitry and wiring. Dimension Engineering has an assembly that allows a bit higher level of control than just on/off .
One of the issues you encounter with LED installs is how to connect all the strings together. While the vendors supply both male and female connectors in the package (link earlier in this article), this requires one continuous LED string throughout the entire plane. Sometimes it seems having multiple short strings is a more efficient install, as long as you can easily connect them together at one point. I tend to use multiple strings so needed an alternative gathering method; some commonly available electronics hardware makes this achievable.
As it turns out, the male plug has terminals on 0.1" spacing and there exists rather prolific connectors around this standard as shown below. The supplied LED male connectors fit perfectly in the socket and the pins on the left are added with some heat shrink to other strings.
Electronic Connectors, 0.1” spacing
Below is a little wiring harness I made up so I could gather several strings into one point. Note the white connector in the upper right hand corner which plugs directly into the battery balance connector for my 3S 1350. The female connector on the left goes in the nose for one string hookup and the two other are near the wing, so each wing half can hook up easily. Battery is right there so very convenient. I glued the harness to the inside of the airplane so it’s not moving around.
Since there is some exposed copper that needed to be insulated, I used some silicone (shoo goo, silicon bathtub caulk, etc). Another issue that needs to be addressed is that, as with any connectors, vibration can tend to work the connections apart. I used the same silicon to stick the connectors together so they don't work apart.
Regarding LED application to the plane, I've seen some fliers just stick the leds on the wing surface and this will certainly work. While this would be fine on many surfaces of the plane, a concern I have is about disrupting the airflow over the wing which could result in a loss of lift. These days with massively over powered planes this is less of a concern, but mentioned here simply as a point of awareness.
On a flat wing, like my ocelot, there is no wing camber at all. A couple of my friends put the leds on the top of the wing and then added balsa between the LEDs and leading edge, sanding in a bit of wing camber. This did result in a gentler flier.
A mechanical variation in the strings is the physical string width. I've seen some as wide as 10-12mm and as narrow as 5mm. Many times this is not of concern because you're just sticking them on a flat surface anyway. However, on my ocelot, which is constructed from 6mm flat foam, I wanted them to fit neatly on the leading edge, so I used the 5mm wide strings.
The Ocelot construction plans suggested covering with 0.75 oz glass and water based polyurethane (minwax blue can) for added strength. Since this thing is all white foam, I wanted to add some color. From old combat days, I had some rip stop nylon in a drawer which has good color. I compared the weight of the 0.75 oz glass cloth to rip stop nylon and found that the rip stop was a third the weight at 0.25oz/sq ft. So, I covered the wing and bottom of my little float plane with rip stop nylon.
Since the petromat always puts black marks on everything, I used black on the bottom and red on the wings for visibility and left the top natural white. I also used packing tape on the bottom so it would be slick and I could take off the matt or water easily without injuring the rip stop.
The LEDs are adhesive backed and stuck pretty well on the blunt leading edge. Then, to add a bit of resistance to water and airflow, I wrapped packing tape over them which added a bit of natural curve. On the wings, I then added rip stop nylon covering.
Here is a pic of the fin showing a nice LED fit.
Fins with LEDs on the edge
More LED stats on the Ocelot.
1 set (5mm) = 2”
LED current (without fin) = 0.95A => 10.5w
Each wing half 35
Total LEDs 98 => 0.95A
Fin 18” 27 => 0.26A
So, once fin is hooked up, 1.25A, 14w
Flying the Ocelot
Once in the air, the Ocelot is a nice flier. It has a good amount of power so it can take off directly from grass or even the petromat at the EBRC field, just skidding along. I set my timer for 4:30 just like all my other 3S1350 planes.
The motor needs about 3 degrees down thrust, currently I have a little more than that (one thick and one thin washer) and on rapid motor acceleration there is a noticeable nose down tendency. However with slow throttle transitions it’s ok. I think on the next series of flights, I’ll take out the thin washer. Calculations show you need 0.017” shim per degree of thrust line adjustment.
On all takeoffs, the plane seems to leap off the ground or water. Part of this seems to be due to the thrust to weight ratio which is substantial. If you’re happy with this takeoff style, you’re good to go with the stock setup.
Some Modifications We Tried
One of my friends also has a Polaris, so my float group was trying to make the Ocelot ROW closer to the Polaris which has a very smooth transition from water to air.
Fundamentally, we were trying to change the porpoising and it’s been a bit of a challenge. At this point we are unsure if this is a mismatch between the CG and the center of buoyancy (seems you would want them to be very near the same) or the lack of positive wing incidence compared to the hull in the water.
It seems the center of buoyancy is a bit too forward so there were modifications to the width of the fuse in front of the step. The Ocelot is intended to be more aerobatic than the Polaris and it is.
Initial Ocelot observations showed that the plane would skip along the water and porpoise instead of rising smoothly from the water like the Polaris. You can see this on the online videos if you slow them down. From this, we concluded that some improvements could be made to the Ocelot for ROW (Rise Off Water).
One Ocelot element is that the step is directly under the center of gravity. Since several float plane sites recommend that the step be 15 degrees off CG vertical, we moved the step back a couple of inches.
The thinking is that the CG position on the step should have some room to move around in a stable manner instead of falling off the edge which could change the wing’s angle of attack.
Another element is that the bottom of the plane has much more curvature than the Polaris, so we flattened the bottom by adding a couple foam thicknesses to the front of the hull, then sanding flat to near the step. At this point my hull (at the step) and wing are parallel up till the very front of the hull, then it curves up.
If I were building another one of these flat wing floaters, I would make the wing have a couple of degrees of positive incidence to the hull at the step.
It was some fun to actually do a build for a change from my more recent ARF planes. It did seem to take a long time by comparison and I sort of felt impatient and expected it to be done sooner. At any rate, it was very gratifying to see this one actually fly and get it tweaked in. I think I might do a full build again sooner than I did before.