Warp Factor - Fixing Warped Balsa Fins

Sometimes, you'll open a new rocket kit, examine the parts, and realize that the balsa fin stock is warped. This may be really bad - the whole thing is visibly curved - or it may be subtle. Perhaps you'll only notice it when you cut the fins out and stack them together, because they won't lie flat together.

Or maybe you'll have some balsa sheet you purchased for building from scratch or cutting spare fins, and find when you go to use it that the balsa has developed a warp.

Balsa warps very easily, and it's due to moisture. Balsa wood soaks up water like a sponge, so it doesn't take much for it to bend the wood out of shape. Sometimes the wood will warp if you've kept it lying on a flat surface, and the weather is particularly humid. The wood will soak up some of the moisture from the air, and the side facing upwards will absorb that moisture.

As the cells in the wood take on water, they swell, causing the whole piece of wood to expand on one side, and the fin will warp away from that side, curving in toward the drier side.

Sometimes people will head to an online forum to ask what to do in the case of warped fins, and people will reply with a number of suggestions. When I had a long plank of balsa stock get a little warped recently, I thought it would be a good opportunity to test things out.

Note: I talked about this on a recent episode of The Rocketry Show podcast. I conducted this test several months ago, and... well, I can't find some of the photos I took. Fortunately, I do have the most important ones!

I had purchased some really great "Very Hard Balsa" stock from Sig Manufacturing, a vendor of model aircraft supplies. If you are looking for some good balsa stock for fins, or some square or rectangular balsa or basswood dowels for strakes or other details on a rocket, or if you need a balsa block because you'd like to try making your own nose cones, I highly recommend Sig Manufacturing. The balsa I get from them is always better than some of the stuff I've found in the hobby or craft stores I used to frequent. These days, there are no hobby or craft stores nearby, so I have to order from the Internet. Sig Manufacturing has good stuff.

Storing balsa sheet can be tricky, because the wood is so susceptible to warping. You don't want to stand it up in a corner, or gravity will slowly cause it to sag and curve. It's best to lay it flat, if you can.

My work table has a long brace piece running down the middle between the four legs. I usually store my balsa sheeting in a stack on that. It would probably be a good idea for me to put some weight on it, perhaps with a nice, straight two-by-four I can lay on top of the stack, but at the moment, I just store it open like that.

During a humid summer week, a really nice piece of 3/32 inch thick balsa sitting on top of my stack got pretty badly warped. I cut the plank apart into lengths of about six inches long or so. It's harder to see warping in smaller pieces of balsa, and I wanted to make sure the results would show up in pictures, and I really wanted to put the various de-warping solutions to the test, so I decided to make the warping even worse. I painted one side of each piece of balsa with water. Then I strapped each piece to a dowel rod with two rubber bands.

This would cause the wood to bend, and as the wet side dried out, the warping would set in, and the balsa would turn out really misshapen. The results can be seen in the picture at the top of this post - very badly warped balsa, good for nothing.

I wanted to try several methods of flattening warped balsa. Three of them were water-based methods, and one was a chemical method. The methods I wanted to try were:

• Steam
• Soaking in water
• Merely spritzing with water
• Ammonia-based window cleaner

And as a control, I took one piece and merely dry pressed it. That is, I put it under heavy, flat books to see if that would be sufficient.

If you stare at something too hard, trying to tell if it's actually straight (or flat), your eyes can play tricks on you. And I needed something I could photograph to really show whether the method had worked.

My high school yearbooks turned out to be perfect for this. Not only were they perfectly flat, so I could tell if a piece of wood had been fixed, they were also heavy, so I could use them as weight for pressing the wood.

Steam

I started with steam. I'd seen someone recommend using a pair of tongs to hold warped fins over a kettle. This seemed a little inefficient to me, and you'd have to stand there at the stove the whole time. I figured a vegetable steaming basket would be a better idea.

Here are the before pictures of the wood. Look at it on one side, then flip it over to get a better idea of how badly warped it is.

I put about an inch of water in the bottom of the pot and brought it to a boil. When it was well boiling, I placed the balsa in the pot, put on the lid, and let the wood steam for fifteen minutes.

I then pulled the balsa out of the steam and placed it between two of my yearbooks, with pieces of parchment paper under and on top of the wood pieces, to protect my books from moisture.

Often, when pressing wet balsa, people will use waxed paper, but I worried the heat from the steam might melt the wax and leave a residue on the wood. I needn't have worried - the balsa didn't stay hot to the touch for long. But I often prefer to use parchment paper, or switch to parchment from whatever I'm using halfway through the drying process to expedite drying, as you'll see below.

I then piled several heavy books on top of the stack and left the wood to dry completely overnight.

The results were pretty good the next day.

As you can see from the second picture above, while the fin stock is certainly in better condition, it isn't perfect, and this shows why it's important to flip the pieces over and look at them from different sides. The top photo looks almost completely flat.

When in doubt, you can double-check the flatness of your fin stock by pressing on the top near the edges. If the wood is flat, it won't move much. If there is a slight curve on the underside, the piece will rock as you press on it.

The steam method wasn't a 100% cure. Still, this wood was in really bad shape. With most fins you get in a kit, they won't be this bad, so steam might well work just fine. Of course, as you'll see in the next couple of tests, it isn't necessary to boil water.

Water - Soaking

The next piece I decided to soak in water for several minutes, until it became saturated, then pressed it under weight.

On this piece, I lost the photos, so I decided to repeat the experiment for this post.

Here are the before photos:

I placed the balsa in a small tub of water, pressed it down to soak it completely, flipped it over, and repeated the process. I left it in the water for a few minutes to get it completely saturated. In reality, balsa is so porous you won't have to do this for long.

Wipe off excess moisture, lay on a flat book on top of a piece of wax paper. Place another sheet of wax paper on top, place another flat book on top, and stack a few more heavy books on top of that.

Leave the weight on the fin stock at least overnight, and preferably for several days. Make sure the fins are completely dry before you remove them from the press.

The results were quite good.

It's important to note that this is the second time I did this to this piece of wood. In repeating the test for this post, I used one of the original test pieces. That means that this wood was warped, flattened, warped, then flattened again, and still came out nice and flat. Not perfect, but pretty darned close for such a beaten-up piece of wood!

Water - Spritzing

The soaked fins came out pretty well. But what if I just sprayed the surface lightly with water? Would that be sufficient moisture to fix the warp?

Here are the before photos.

I misted both sides of the wood until they were wet to the touch.

The results: again, quite good. Not perfect, but with considering how much I abused these pieces, it's not surprising. Wood that warped naturally would probably have come out completely flat

In fact, either water method should work for less warped fins. The photos look pretty close to perfect. If you press on the fins, though, you will get a clearer idea. Flat fins will stay flat on the book. A slight warp will cause the wood to rock when you press on it.

OK, so with less warping than I was dealing with, it looks like water will do the trick. Did I even need to proceed with the ammonia?

Well, yes, as it turned out. After a few days, some of the warping returned to all of the water-treated pieces. They were badly bent, so I suppose that's not surprising. Much like you cannot make a folded piece of paper look like a new sheet, once a bad warp has set into the wood, that's it's new natural shape. Most likely, using water to flatten your balsa will work just fine, provided you make sure the wood is completely dried out before you use it.

If you have a badly curled or stubborn piece, however, you might want to try something more aggressive - to change the structure of the wood.

Window Cleaner

We're talking here about the blue stuff - Windex, or any other blue, ammonia-based window cleaner. It's got the right ratio of ammonia to do the trick, and you don't have to mess with pure ammonia. Plus, you probably already have some.

Here I have to confess I'm not entirely sure what is happening in the wood when you use ammonia, as I've seen a number of explanations, but it is said to break down molecular bonds or maybe cell walls or... something in the fibers of the wood. This makes the wood pliable, and shapable. When the wood dries out, it will be hard again.

How well would it work? I wanted to find out.

The before pictures:

Using window cleaner to fix fins is a bit more involved a process, and in using the technique, I discovered a few things to watch out for. Depending on how much you spray on the fins, it may take more than a day to get good results.

When using window cleaner to clean windows or a stove top, you don't use much, so you may not have noticed how noxious the fumes can be. But when using it to fix balsa stock, you'll want to open a window and get plenty of ventilation, because you will need to saturate the wood. Trust me - I found this out the hard way!

Hold the wood away from yourself with a pair of tongs or while wearing latex or nitrile exam gloves, and spray both sides until the wood is dripping wet with window cleaner.

Then, wrap the balsa in plastic wrap. In this case, you do not want to use waxed paper, because the window cleaner will soak right through it and will destroy your books.

The fins of my Estes Hi Flier XL needed to be flattened before attachment. I used the window cleaner method.

Place the fin stock on one flat book and cover with another, then stack a few more heavy books on top. Leave the stack to dry overnight.

The next day, unwrap the fins. They will probably be dry to the touch - but in fact, there is still a lot of moisture inside the wood, especially if you saturated them really well. If they still feel flexible, they are not done.

If you leave the fins out at this point, they may warp again, even worse than before. After I did this experiment, I used the technique on some slightly curled Estes Patriot fins, and when I set them down on the table the next day, I left the room to get a glass of water. When I returned fifteen minutes later, they had curled up like a pile of Fritos!

So, once you've removed the plastic wrap from the fin stock, you will want to press them some more. At this point, you may decide they are dry enough to press them under books with no wax paper or parchment, but in fact, I prefer at this point to expedite the drying process by switching to parchment paper.

Why? Because since parchment paper is not waxed, it wicks moisture away from the wood, allowing it to dry more quickly and thoroughly. You will see evidence of this the next day, when you take the books off the fins, because the parchment will be slightly rippled from the moisture passing through it.

After a day or two, feel the fins. They should feel completely dry and solid. Lay them on a wire rack and check on them frequently for the next half hour to 45 minutes. They will most likely be finished at this point, but if they were to warp, they would do so quickly. You'd still be have time to press them again before they harden.

Once you're done, you can shape, sand and fill the fins as you do normally. There will probably still be a faint ammonia smell, but the warp will be gone. The rocket will look great, and fly much better.

Of the methods I tried out, the most effective seemed to be completely soaking in water and window cleaner. Steam was less effective, and required boiling water and a steamer basket.

What about the dry pressed fins?

Well, they never, ever flattened out. I even tried it on some slightly warped kit fins, the Estes Hi Flier XL. I think I had those fins sitting under heavy books for two months, and they stayed warped. So I hit them with some window cleaner, and fixed them right up.

One other technique I tried, which turned out to be great for flattening - and strengthening fins...

Bonus: Papering Fins

A few months ago, Estes had a clearance sale on their Skill Level 2 Photon Probe model rocket, which had been discontinued.

The Photon Probe was a BT-55 based model, and even if you weren't fond of the Photon Probe itself, at a little over $3 per kit, it was well worth buying if only for the parts. In order to get a large stock of nose cones, body tubes, parachutes, motor hooks, launch lugs, etc. for scratch building, I bought 31 of them.

 While most of these would serve as spare parts for scratch builds, I figured I should build at least one Photon Probe - and make it a two-stager, while I was at it.

The problem - every single model had bad balsa. It was warped and soft. It actually seemed old. I considered making new fins from balsa stock, but decided I should try papering the fins.

Papering fins is a technique used both to conceal the wood grain in balsa fins, and to add strength to fins. It involves gluing ordinary printer paper (or sometimes using self-adhesive Avery-type printer labels) to the fins.

I don't normally like papering fins, partly because I like to sand my fins into airfoil shapes, and I find it difficult to get the results I want when adding paper skins. It's not that the technique doesn't work for airfoils - I just find it more frustrating than I want. My earliest attempts were mostly a disaster.

The occasions when I leave the fins square, however, I find papering much easier, and very effective for me. I'm still not great at it, but these fins needed fixing, and I figured it would be a good chance for me to try it again, and perhaps fix the soft, curvy balsa.

There are at least three main ways people paper fins - with wood glue or white glue, with a glue stick, or with self-adhesive Avery labels.

The method which will cure warped wood is the one using wood glue or white glue - spread the glue onto the surface of the fin, then use a finger to wipe off all but the thinnest layer of it. You then apply a piece of paper slightly larger than the fin itself to the glued surface and burnish the paper down with a marker or other smooth object, to squeeze out as much of the excess glue as possible. The process is repeated on the other side of the fin, and the fin is then placed under heavy books to dry thoroughly. After that, the overhanging edges of the paper are often treated with a bit of thin CA - cyanoacrylate or craft grade super glue. This stiffens the paper, and once the CA has dried, the excess can be sanded off with a sanding block, so the paper perfectly fits the outline of the fin.

There are a lot of great tutorials on The Rocketry Forum and other online sites, but here is a video tutorial from Apogee Components.

Wood glue and white glue go on wet. The moisture in the glue will make the wood pliable, and as it dries, it hardens, so if you press the fins under a book, they will come out flat. A glue stick might have a similar effect, though there is less moisture in glue sticks, and the glue doesn't dry as hard.

The Avery label method, while being the easiest way to paper fins, won't actually fix the balsa. The adhesive on these isn't a wet glue which dries and hardens. It's really just a tacky surface meant to adhere a label to paper - say an address label onto an envelope.

I've also had Avery labels peel off wood, even after being painted on, following a hard landing. Or bubble up after a month or so, causing a raised spot, under a paint job.

So, while it is a no-fuss method of papering, if you're trying to fix warped wood, stick with the glue method.

The Photon Probe fins turned out really well, and after building this rocket, I could see why some builders swear by papering. The fins were not only perfectly flat, but very strong!

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Repairing and Enhancing the Quest Magnum Sport Loader - Part 4: Fixing the Sim Part B

Click here for Part 1

In the last post, we removed all the Center of Gravity (CG) and mass or weight overrides on the simulation of our rocket.

Now we have to find the CG and total weight of the actual model, so we can get a more accurate simulation to use. This will help us figure out what motors we want to use the in the rocket on a given launch day, so that we can reach a maximum altitude on nice calm days, and keep the altitude lower on windier days, to lessen the chance we'll lose our awesome rocket.

An accurate simulation will also tell us how long a delay grain we need for those motors. For example, do you want to use C6-3 (3-second delay), C6-5 (5-seconds) or C6-7 (7 seconds) motors? The difference is important.

Even though I've been focusing these posts on my particular rocket - the Quest Magnum Sport Loader, which I recently had to shorten to cut off some damage - these principles can be applied to any model rocket.

We'll need the following things for this last step:

• The rocket, including the recovery system (parachute, streamer, etc.)
• A length of string, or something on which to balance the rocket, such as the back of a chair
• A digital scale
• Recovery wadding
• A tape measure
• A pencil or pieces of masking tape to mark locations on the rocket

You'll also need your OpenRocket or Rocksim simulation open on your computer.

Once we've found the true weight of the built rocket, and the true CG, we will override those elements in our simulation. Then we'll get an accurate representation not only of its caliber of stability, but we'll be able to run flight simulations with a reasonable degree of accuracy, which will aid us in picking the right motors for a particular flight.

First, we'll weigh the rocket to find the true mass or weight of the finished model. Our simulation says the rocket weighs 101 grams without motors. That is the sum of all the parts in the OpenRocket design file. OpenRocket will insert these automatically when you build a design file. It takes an assumed density for a particular material - say, balsa fins - and will calculate a mass or weight based on the part's dimensions.

In reality, the density and weight of items like balsa fins and other parts will vary. Fins, especially, will vary in density and final weight. There are dense balsa woods and soft, lightweight balsa woods, and different rocket kits will have different qualities of balsa. Even the weight of the individual fins in a single kit will vary slightly.

But even if all the parts in our design file were accurate, OpenRocket doesn't take a couple of important things in rocket construction into account - glue and paint. Those things do add mass, and will change the final weight of the built rocket.

So, to get an accurate simulation, we do need a digital scale. It doesn't have to be fancy or expensive - a decent digital kitchen scale will do. And even though I'm working with the metric system here, you can also work with the Imperial scale (pounds and ounces) if you happen to have a scale which uses that system. The main thing is that the scale be accurate and sensitive enough to detect small changes in mass.

I have two digital scales, and they both work really well. Both of them were very reasonably priced.

The scale above left is a small metric scale which can measure to an accuracy of 0.1 gram. That's very sensitive, and perfect for small rockets and parts. It has a maximum capacity of 600 grams, a little over 1.3 pound. It cost less than $10 on Amazon.com.

The scale to the right can weigh in Imperial units, detecting pounds, ounces and tenths of ounces, or in metric units, accurate down to a single gram. It has a capacity of 110 pounds and a larger plate, meaning it can be used for larger rockets - which is great when you graduate to building and flying high power rockets. And it was only about $25 on Amazon. I was surprised to find such a good scale for such a reasonable price!

Since I'm going for maximum accuracy and working with a small enough rocket, I'm going to use the small metric scale.

What we need to find is the weight of the rocket itself, without the motors. Once we add motors to our simulation, the weight in OpenRocket will change, as will the CG, as we will see below.

You do need the parachute installed in the rocket, and, though it might surprise you, you should also install the recovery wadding. Why, you might ask? Aren't we trying to find the empty weight of the rocket?

Well, yes. But when running simulations, OpenRocket also doesn't take recovery wadding into account. You will always fly with the stuff (unless you are using a rocket with an ejection baffle, a device permanently installed in the rocket which protects the recovery system from the heat of injection charges), so you should assume it's part of the simulation.

Recovery wadding doesn't weigh much, so if you forget to install it, it probably won't make a big difference. But, for accuracy's sake, it's a best practice to consider wadding a part of the recovery system, and weigh it with the rocket.

Turn on the scale, let it boot up, and when it reads "0.0," carefully place the rocket on the scale.

The Magnum Sport Loader weighs 106.6 grams. The simulation of the rocket states that it's 101 grams. That's pretty close. Does that mean that the glue and spray paint on this rocket weigh 6.6 grams? Hard to say, since I didn't weigh the individual parts of the rocket as I was building it, as I tend to do now.

At any rate, I need to change the weight of the rocket in the simulation, so we'll turn to our design in OpenRocket.

Up near the top left of the screen, in the design elements window, select Stage.

Double-click on Stage, or press the Edit button to the right. The following dialog box will pop up.

Check the Override mass box, and type in the weight of your rocket.

And now, as I write this, I learn something new: OpenRocket will not allow me to enter a mass of 106.6 grams. It rounds up to 107 grams. If I were to enter 106.4g, it would round down. So it's accurate to within a half a gram, but no closer. This will do just fine.

Now that we have an accurate weight for the rocket, it's time to find the true Center of Gravity. You can leave the Stage configuration dialog box open for now.

A quick and easy way to find the approximate CG for your rocket is to balance it on your finger. That's fine for flying out in the field - say, if you're trying a new, heavier motor and you just want to make sure the CG doesn't move too far back when you install it in the rocket. But I want to mark the CG on the rocket, so I'm going to balance mine. You can use the back of a chair or some other sturdy object with a straight thin edge. But I found it helpful to use a loop of string.

Here, I have the rocket balanced on the tube cutting jig you saw in Part 1 of this series.

Problem is, it took me a lot of adjusting the rocket back and forth by tiny increments to get it to balance like this, and it tried to roll off. Then, when I went to mark the CG, it fell off the jig.

With a loop of string, the rocket won't roll around, and once you find the CG, you can keep the string in the same spot on the rocket until you grab a pencil or piece of tape to mark the spot.

Make a simple loop in the string - here, I'm using a piece of Kevlar shock cord - and slip it over the rocket. Again, you need everything installed in the rocket except the motors. This includes the parachute and recovery wadding.

Find the spot where the rocket balances.

This is the CG of your unloaded rocket. You can mark the spot with a pencil or piece of tape. Since I didn't want to put a pencil mark on the paint, used low-tack painter's tape.

I grab the loop of Kevlar string to hold it in place on the rocket...

...and carefully line a piece of tape up with the Kevlar. The nose cone is to the left of the frame
of this picture, so the leading edge of the tape represents the CG of the unloaded rocket.

Now that we have the CG marked on the rocket airframe, how do we measure its position?

CG and CP are measured as a distance from the tip of the nose cone. You might just grab a cloth tape measure and place it at the tip of the rocket and then measure from there.

But this is a little like measuring your height by placing a tape measure on top of your head, and measuring around the curvature of your head down to the ground - it's going to make you seem taller than you actually are. If you measure along the curvature of the nose cone, you're going to get a false measurement.

You need to measure straight back. The way I do this is by placing the nose cone against a flat, vertical surface, such as a wall, and measuring from there back to the place.

Here, I have the rocket sitting on a cradle, to keep it horizontal to the table, and placed against a metal file box.

Then, I place the tip of a metal tape measure flush against the metal box and measure straight back to my CG mark on the rocket. As mentioned in a previous post, I always switch to the metric system when doing measurements like this, because it's simpler - everything is divisible by 10.

The CG of my rocket is 29cm (or 290mm) from the tip of the nose cone.

For comparison's sake, here's the measurement I got by measuring along the curvature of the nose cone with the cloth tape measure:

The blue tape is holding the tape measure in place just so I can take a picture.

With this method, I get a CG location of 29.5cm - a half centimeter difference.

Is this a big deal? Will it make much difference? Am I being too fussy here?

Well, maybe it won't make much difference how you measure the CG and CP locations on many rockets. But for the simulation, I'm trying to be as accurate as I can. And if you're measuring the rocket to check its stability margin, you want to measure as accurately as possible. Remember that the minimum margin of stability is 1 caliber - the diameter of the rocket itself. The CG needs to be forward of the CP by at least 1 caliber. If you have a rocket which is right at that 1-caliber margin, you want to make sure you get a good measurement. It might make the difference between having to add weight to the nose cone or not.

Alright, so we have our real-life CG location - 29cm, in my case. Let's put that into the sim. Go back to the Stage configuration dialog box in OpenRocket. Click the check box marked Override center of gravity, and input the actual CG location.

Now the blue and white CG symbol on the rocket design will move. In the case of the Magnum Sport Loader, it moves from here:

To here:

The bulk of the work is now completed. We just have a few more minor details to adjust, then we can run some simulations.

Click here for Part 5

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