Flag Towing with a Radio Controlled Model Airplane

I have been interested in using a model airplane as a tractor or platform for other things, almost from the instant I first got to fly one. One of the things I aspire to do is towing a banner. I have not gotten around to that yet, but I have managed to successfully tow a flag aloft and get it back intact. (The airplane, too!)

In spite of researching the idea long and hard, and attempting to avoid others’ errors, I have had a number of false starts. However, I have gotten it to the state where I feel I can offer pointers to others who may be interested in doing something similar.When I was first looking for advice and ideas I posted a query on RC Canada (read thread) and was directed by another member to this thread on RC Universe (read thread) where a member there had scanned and posted the full article from the December 1977 issue of Model Aviation Magazine, the A.M.A. publication. This explained much in great detail, but I still had to work a few things out for myself. Also, following their scheme is very labour intensive, as you have to construct the letters one by one in advance before you can spell anything meaningful.So, I copped out and went the easy route (Ha!!) and decided I’d try towing a flag instead, kind of working my way into it as it were. I have had mixed results with different flags and tethers, and have ended up flying the flag on its side, or upside down from time to time, but now have it working properly all the time.

The one thing that I felt was critical and that I did right at the outset was a release mechanism. Taking off is one thing, but landing is much more difficult, especially with something dragging behind your plane.  You do not want to attempt landing while you are dragging anything behind the plane as you are bound to get it caught on the way down, so you need to be able to release the towed object before you attempt to land. Also, The towed object will take less of a beating if it falls to earth on its own instead of being dragged through hawthorns or a cornfield on landing.

Anyway I needed a release mechanism, and it had to be cheap! Here is what I came up with:

Release MechanismRelease Mechanism with towline attached (Bungee Cord)

I felt I should enclose it so I made up a box with some coroplast (an election sign I ‘found’) and wrapped some bright orange peel-and-stick stuff I had, just to make it visible. (“Hey Mac, what’s that orange thing on the bottom of your plane?”)

A piece of 1/8” plywood, some thick balsa (I used some hard balsa from an herbal tea box) a couple of pieces of ¼” dowelling, an old Futaba S3003 servo I had laying around and a servo mounting bracket I picked up at a swap meet made the guts of it. The really sneaky part, I feel, is the release mechanism itself; one of those two piece double ended key chain gizmos from Canadian Tire (costs a couple-a-bucks) with the push button half embedded in the balsa bulkhead, positioned so it will be on the centre line of the aircraft when all is said and done. Remove the key ring and replace it with a short piece of music wire push rod with Z-bends to connect the servo to the release and it’s good. I slopped lots of epoxy on to keep the dowels in place and to provide a bit of fuel proofing on what will be the top surface. (It’s upside down in the picture)Release Mechanism with cover

Release Mechanism with Coroplast cover.Towline attached.

release_mechanism_mounted

Release Mechanism Mounted under a trainer, secured with six #64 rubber bands.

To use To use it I hook three #64 rubber bands on the dowels on each side, slide the whole thing under my ‘beater’ .46 powered high wing trainer, in front of the main gear (trike set-up) then stretch them up and over the FRONT wing hold down dowel with the release pointing to the rear. (That’s pretty important. See the picture to left).

With a six inch extension, I plug the servo into an unused channel on the receiver, (gear or flaps, channel 5 or 6) and it should be ready to use.

Now we need something to tow. I am presently towing a flag that measures roughly 3 x 5 feet. This is the third flag I’ve towed. The first one was much smaller, plastic, and got eaten by my prop on takeoff one day. (OOPS!) The second was somewhat bigger, but got lost in a forest of red cedar and juniper after the release didn’t, when it should have, and a strong wind took it away when it did let go. (Darn!) The current flag is the largest so far, and flies the best.

flag_with_dowel3/32″ Dowel inserted into the hoist end of the flag.

The flag I am now using is what is known as a hoisting flag, one that has two metal grommets to attach lanyards for raising and lowering. The grommets are in a narrow sleeve of white material that is sewn to one end of the flag, (the hoist) and as there is very little clearance between the grommets and the edge of the sleeve, I found it necessary to slit the fabric so I could insert a 3/32” dowel (Home Hardware) into the sleeve to stiffen it across the width of the flag. I tried a 1/16” dowel but it snapped, as it was just too light. This is also one of the points where I fasten the towing harness.

I found on a larger flag, such as this, I needed more than a string attached to each end of the dowel. On this flag I have five harness lines attached to the flag and dowel, coming together at a tow ring (taken from the key ring gizmo!) 24 inches in front of the flag. I initially had each of the lines tied to the ring, but soon found that when the flag fell to earth on release, everything tumbled and the lines tangled at or near the ring and it was no fun sorting it out.

flag_with_harnesThe tow harness: each line is tied around the dowel in the hoist of the flag, and terminated on a swivel clipped to tow ring.

I hit on the idea of using fishing line swivels so that if the lines tangle, I could unsnap them from the ring and sort them out easily. Well, I only had three swivels left at this point, and five lines, so I paired up four of the lines and put the top line on a swivel by itself.  I found that having the point where the lines come together an inch or so from the ring helped keep them clear as well. Looking at the photo, you can see that the tow ring is in line with a point six or seven inches down from the top of the flag, and is 24 inches in front. This helps the flag to fly upright. (See the photo of the harness above.) I have also discovered (the hard way) that the knots tend to come undone under the constant fluttering of the flag. A drop of CA glue on each knot is a good idea.

To ensure the flag flies properly, you also need to weight the lower edge. To this effect I cut another slit in the fabric sleeve on the hoist of the flag, about six inches from the bottom. Into this I slid a ¾ ounce fishing weight, and shook it down as far as it would go, that is, to the metal grommet. This, and the harness as described above, is all it takes to keep the flag flying upright and displaying properly.

flag_with_weightSlit cut in hoist, about 6 inches from the bottom into which a 3/4 ounce sinker is inserted.

So far we have talked about a release mechanism, and the flag with towing harness. We have to attach one to the other for anything to happen. That is actually quite simple.

On the part of the ‘key chain’ that gets released, I took off the ring there as well and replaced it with one of those itty bitty bungee cords you get in the sets from Canadian Tire, (see photo, below) and reworked the hook ends with a pair of long nose pliers so they were closed up and couldn’t hook on anything else. The bungee cord is not absolutely necessary, but I discovered much to my amazement after watching some in-flight video taken with an onboard camera that the bungee is constantly ‘working’, stretching out and pulling in, so it must be doing some shock absorption. Hence, I leave it in.

bungee10 inch bungee from Canadian Tire.

To the other end of the bungee, attach about 60 or 70 feet of fishing line. (I am using fifteen-pound test, but would use lighter if I had it; as in case of a snag I would really like the line to break.) Then at the other end I fastened a fishing line swivel. (This is the first I used of a package of four. Remember, I only had three to do the flag’s harness?) This swivel snaps onto the ring on the flag harness before a tow.When you release the flag and it falls to earth, the flag is easy to find, but the towline is not. I regularly get my feet tangled in it when I go out to retrieve my gear, so I fastened a red cloth (Canadian Tire shop rag!) about six feet behind the ‘key chain release’. On the ground, this helps you find the leading end of the towline easily.

So, how do we make it all work? This is really the fun part!

I start by taking the flag out on the landing strip and laying it out. To keep it from blowing around, I leave about 1/3 of it still wrapped around the meter stick (yard stick to us old fogies) that I wrap it on for storage. The flag is placed about a hundred feet or so from the downwind end of the strip, with the harness end of the flag pointing DOWNWIND. I attach the towline to the flag by its swivel (and ensure it is closed) then pay out the towline towards the downwind end of the strip, dropping the red cloth and the bungee near where I will begin the take-off run.Now I bring out my plane with the release mechanism rubber banded on to it, and activate the servo so that it is in the release position. Get down and shove the other part of the release onto the mechanism, and flip the servo to its alternate position. Give the line a good tug to ensure it is engaged and won’t fall off.

pre_takeoffStanding behind the plane as it begins its takeoff roll

Depending on what is permitted at your club field, you may have to take off while standing in one of the pilot stations. I do so as well when there are others flying at the same time, but my preferred takeoff mode for flag flying is to stand behind the plane, retreating to a pilot station as soon as it is airborne.

To take off, I get power on quickly and get airborne as soon as possible. It takes far less power to remain in the air than to get there in the first place, so when my plane picks up the weight and drag of the flag, I want it to have as much reserve power as possible, hence I get aloft as smartly as I can. The plane takes off, pulling the towline after it, and typically it will be at ten or fifteen feet (or more) when the flag suddenly comes off the ground. I have to be careful not to climb too steeply; I don’t want to waste power trying to horse my way out of a near stall! There is more than sufficient time to get off the ground, as the takeoff roll will be up to twice the length of the towline.

takeoffClimbing ou with the towline attached and the flag lying on the ground ready to be pulled alloft.

At this point additional up elevator is needed to maintain level flight. This is to be expected as the extra weight and drag are attached to the bottom of the plane. I usually put in several notches of up elevator on my trim before I take off. I can reduce that after I release the flag and am setting up to land.OK, now it is flying! The plane is somewhat sluggish, and will wallow about if throttled back too much. I find it best to always use both rudder and ailerons to control the plane as I fly some circuits with the flag behind. I keep the throttle up, ¾ or better, fly smoothly, and do not make any sharp turns lest I tangle the towline. This is where that red rag comes into use in the air; by observing it I can see where the towline is in relation to the tail feathers of the plane. I have had cases where I turned too sharply, the line went slack momentarily and I got the towline up on top of the horizontal stabilizer. (“Why won’t that plane stop climbing?”) (Note: to counter this problem, some sources advocate placing a ring on the underside of the tail that the line feeds through. I have no confidence the line, bungee, etc. would smoothly slide through and out and not tangle upon release, so I have not tried it.)

in_flight

To bring it back to earth, I throttle back sufficiently to descend over the strip (or wherever I plan to release the flag) and as the plane is passing by, I engage the switch or knob that activates the servo in question. The flag will suddenly fall free and plummet to the ground. (Test the release mechanism ahead of time, just to be sure!) The plane, freed from its additional load will zoom upward (neutralize the extra up elevator trim). At this I point just circle around and land normally.

release_the_flagOne extra point: I have found it works more reliably if I put a drop or two of oil from my van’s dipstick into the female part of the key chain/release mechanism.This sort of activity is rarely seen at a model aircraft field, and never ceases to please the first or second time it is seen. The amount of “futzing about” on the field, however, is disruptive if there are flyers waiting their turn to fly. I suggest saving it for a special occasion, or for a time when things at the field are slack, perhaps when everyone is just sitting around “bumping their gums”, swapping lies, and the strip itself is quiet.

Next, I have to get my alphabets created, and try flying a real banner that says something!

Dave Holmes MAAC 6881, published  June 1st, 2009

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