Thread Tools
This thread is privately moderated by Tsavah, who may elect to delete unwanted replies.
Apr 03, 2016, 11:07 AM
Watt Waster
Tsavah's Avatar
Thread OP

Center of Gravity - Airplanes

After looking for a few minutes I found it odd there wasn't a thread dedicated to discussing the basic details of the age old flying model builders question - where is the location of the center of gravity for this model airplane? Maybe I wasn't looking in the right area of RC Groups, or the search using "center of gravity" wasn't restrictive enough. In any case, I have noticed a lot of new builder/pilots of RC airplanes tend to ask where the center of gravity, CoG, CG, (and other abbreviations) is for a model airplane they have, or want to build. Generally speaking, the CoG for an airplane will focus on the main wing in a standard configuration and is often found near the 1/3 back from the leading edge area. The catch is there are a lot of influences in a model aircraft design that can change the location for the best center of gravity spot. Most of those influences have to do with drag, thrust, and incidence angels (main wing, and stabilizer - primarily).

My idea was to demonstrate how to glide test a lightweight foam glider toy for the best center of gravity location (balance point), which is best determined after giving the toy foam glider a gentle throw during a dead calm wind day. For most areas in the world the dead calm wind part of a day comes in the early morning, or just before sunset. Of course if you have access to an adequately large indoor area with no air conditioning, no heat being applied, no windows open, and etc., you likely have a dead calm wind place to complete many glide tests of a model airplane design. I learned how to do this many years ago before my teenage years while playing with very basic balsa wood gliders. I used the principles of glide testing to determine where the best place was for the main wing in the provided slot, or on the stick of a rubber band powered toy airplane. The principles remain the same for all sizes of model airplanes and the multitude of designs, or configurations.

In those days I would slide the main wing forward if the glide tests showed a tendency for the model to glide downward into a nose first hard landing. If the model normally would glide forward, climb a little, stall, nose downward, pick up airspeed, climb a little, stall, and nose downward again, I knew the main wing needed to be moved backward a little in the slot, or on the stick. I would continue to adjust the position of the main wing until the balsa profile toy airplane would glide forward in a constant, smooth glide path until it landed smoothly on the ground without any signs of stress, or damage to the soft wood fuselage. This may seem like a simple thing to do, but it is the starting point for determining the best location for the balance point for a common aircraft configuration. It also works if testing for CG on odd configurations of model aircraft.

Once you start using motors, or engines of some sort to provide thrust to a model airplane, the balance point known as the center of gravity location becomes a big deal. Normally the balance point for powered flying models will need to be slightly more rearward to compensate for thrust and provide additional stability during flight and landings. Most of the time that means a very small adjustment to make the model fly slightly nose heavy. If you do this with an unpowered toy glider, it will have a slightly steeper glide slope and land a little harder on the ground rather than a gentle skid to a stop. Due to this slightly nose heavy adjustment to the center of gravity, or the balance point, it is a common practice to "flair" a few moments before the model makes contact with the runway, or ground. Otherwise the model airplane would land itself during a calm, no wind condition in a very gentle manner if the main wing was properly located on the fuselage. There are reports of aircraft landing themselves after the pilot escaped, or died if the autopilot was engaged. The idea was the aircraft would slowly sink in altitude until it made gentle contact with the ground after the fuel ran out and the engine, or engines shut down. The aircraft had become a well balanced, correct CG glider.

Obviously each model airplane design will have a specific best place for the main wing location and it is also true no matter how carefully constructed, two model airplanes of the same design often don't have the same exact flight characteristics. Very hard to detect differences will influence how the model glides, making it necessary to either add a tiny amount of additional weight to the nose, or tail of the fuselage. This can be demonstrated easily by purchasing three or more of the same toy glider made by the same toy maker. The old Titan solid foam toy glider is a good example of how to demonstrate how one copy of the toy glider will need a little added weight in the nose for a gentle glide slope until ground contact, but another will need a little added weight in the tail. Since I haven't been able to find more Titan toy foam gliders, I purchased what appears to be the replacement sold as a "Sky Riders" foam toy glider.

Most of the time it is impossible to see a difference between the two, or more toy foam gliders, and since the main wings have a fixed location with no forward, or rearward adjustments possible, the only option is to add a small amount of weight were needed. This often means a small screw will be twisted into the foam at the desired location to improve the glide slope of the model. In the RC world of model airplanes with electric motors and battery packs, it means the location of the battery pack will be adjusted until the desired flight characteristics are demonstrated. That is why the vast majority of the ready to fly (RTF) foam airplane models have a specific location for the recommended battery pack that allows minor adjustments. All bets are off if you use a heavier, larger, or battery pack not recommended by the model airplane maker.

The same is true for fuel powered model airplanes that come with specific engine recommendations. If the pilot doesn't follow the recommendations of the model airplane maker, it is likely the balance point will be too far back on the main wing, or too far forward of where it needs to be. The issue with fuel powered model airplanes is fuel is a heavy liquid, and the fuel moves around inside the tank during flight maneuvers. This changes the effective location of the CoG slightly from moment to moment, which is why the fuel tank has to be firmly attached to the model airplane fuselage at the recommended location. The model airplane starts out nose heavy and after the fuel is burned off, becomes less nose heavy. If the fuel tank location makes it possible for the balance point to shift to a tail heavy condition, the pilot is going to have a difficult time landing the model without risking damage, or a disaster.

To demonstrate the problem with liquids, you would need to install a small container of water in the toy foam glider slightly forward of the best CoG location and allow the water to escape in a timed manner. If all of the water can escape before the glider makes contact with the ground, you will notice the landing is less hard, or damaging to the foam fuselage nose as it would have been if most of the water had remained in the tank. This is why fueled model airplanes normally recommend the fuel tank to be located on, or slightly forward of the best CG location for the design. The problem with the large, heavy, fuel powered model aircraft is you cannot do effective glide tests without risking damage, but you can make a careful, smaller copy as a hand glider and determine the best location for the CG. When the larger model aircraft is flown for the first time, often called the maiden flight, the CG location might need to be adjusted, but normally only slightly for best flight characteristics.

This is why it has been a common practice to first make a careful, small hand glider of a design to determine the best location for the balance point. As the model design gets bigger and heavier, the balance point (CG) will be moved as needed for the best flight characteristics, but the adjustments are normally very slight. This is easily demonstrated by making larger copies of the same design using foam, balsa wood, or whatever material is deemed necessary while preserving the same wing loading numbers. I suspect this is why I have adopted the process of making build-over plugs and frames so the last copy of a design could easily be made larger using the prior, smaller copy to preserve the contours of the design. In the experiments to follow for this thread, I will use multiple kits of the same toy foam glider and glide test them after making slight, or significant modifications to see how the CG and other characteristics change.

If I reduce the size, location, or other detail of a flight surface (main wing, stabilizer, rudder), the flight characteristics of the toy glider will change and sometimes the influence of the changes cannot be easily predicted. The best advice I can offer is to make the changes before you add the weight and influence of the RC gear so it will be more likely the soft foam toy glider won't suffer significant damage. Once the best CG is found for the design with any desired modifications, a flyable RC version can be practical and is more likely to be a successful project. After a bit of experience and attention to details about how to prevent undesirable flight characteristics, it is normal for a builder/pilot to make a checklist to follow before flight tests begin on a new design. If the new design follows the rules of flight, it is often a good glider. A good glider will easily make a good RC conversion later with a few more minor adjustments if it is a proper copy of the glider aircraft design.
Last edited by Tsavah; Apr 03, 2016 at 05:26 PM. Reason: spelling
Sign up now
to remove ads between posts
Apr 03, 2016, 03:07 PM
Watt Waster
Tsavah's Avatar
Thread OP

Center of Gravity - Toy Foam Glider

The toy foam glider for this series of experiments and demonstrations will be the "Sky Riders" version currently available for purchase in Wal-mart, Target, and a few other stores. I was hoping to find the old Titan foam glider toy by Air Hogs, but I haven't seen one of those for a few years. It really doesn't matter I will be using the Sky Riders solid foam toy glider for the current set of demonstrations since the principles are the same. You could even purchase a small balsa wood toy glider to do the same glide tests for finding the best balance point (CG), but once I start making modifications to a solid foam Sky Riders glider toy, repeating the same process with a balsa glider toy would be difficult. Most of the modifications will require cutting the foam, gluing parts together, and even reconfiguring the arrangement of the parts. In some planned experiments I will be removing some of the foam for various reasons and even adding foam parts not included in the original kit of the toy foam glider. This is not so easily done with a balsa wood glider toy, but a variation of the idea for the same purpose is possible. It boils down to comprehending the principle being tested and the purpose of the modifications.

The first principle to demonstrate is how copies of the same glider toy often don't have the same "best" location for the balance point, or center of gravity location even though there is no obvious difference between the toy gliders. It is like each toy glider has a slightly different personality (flight characteristics) no matter how equal they appear to be. Measuring the difference can be difficult due to so many minor adjustments, or influences that will play a part in the glide tests. The key point is each toy glider will perform well and are good toy airplanes with satisfactory performance most people will accept as well balanced and see no need to adjust for the "best" balance point. A glider pilot has to get very picky to notice a difference is possible with careful attention to details during the glide slope testing. The same is true with RC model airplanes and how they perform. It is all about careful observation and attention to details. How a model airplane performs during flight maneuvers is a broad area of opinion and a number of trade offs will be made until the builder/pilot decides the model is either a good design, or needs some tweaking.

One example of a model airplane design needing some tweaking would be comparing the performance characteristics of the model at slow speed vs high speed. How well the design performs during slow speed determines the stall speed so the pilot will need to always fly slightly faster during landings, but can "flair" at the last few moments before ground contact to ensure the softest possible landing. This becomes more noticeable with jet designs. Jet model wing designs normally have thin airfoils, or no detectable airfoil shape in the main wing to provide lift. Many RC jet designs are flat foam board wings with the leading edge having been sanded round (half round) to aid the airflow. Other simple jet wing designs have two layers of foam board glued together with either the top layer being less wide, or the bottom part is less wide. These flat foam board wings are normally called a KFm design after the Kline-Fogleman airfoil experiments with paper airplanes and provide a significant amount of lift like a conventional airfoil design. The primary differences being ease of construction and flight performance.

After a set of initial demonstrations using the solid foam toy gliders as they come with a few modifications to the main wing a point will come when it would be wise to replace the given wing with a KFm foam board wing and try to measure the difference in flight performance. Often the objective will be to see if the toy glider will have a longer glide slope with the original wing with, or without modifications, or with a KFm wing. The hard part will be ruling out any influence from a slight breeze. Most days don't have a dead calm wind and I don't have access to a large enough indoor area free of air conditioning, or heating ducts. A key detail is how a slight breeze can skew glide test results even if I use some sort of launcher to ensure each launch is nearly equal in many measureable aspects. A soft tail breeze can extend the glide slope before ground contact and a head breeze can shorten the distance the glider will go before ground contact unless it provides a slight up draft. If the updraft is more than slight, the glider might stay airborne for a significant distance, which is why many RC model aircraft glider designers seek a hill, or higher elevation with a constant wind blowing towards the higher point of launch. One of the best locations for glider pilots is a cleft near a large body of water like an ocean, or huge lake. If the wind is at least 3 mph and blowing towards the cleft, a lightweight glider model can be flown for as long as the battery pack will last before the receiver goes dead.

Since most of the first set of toy glider design tests will be without RC gear installed, we will have to be satisfied with launching the model from a ladder, or short platform using some sort of launching device. This is likely to be a simple PVC pipe rail system that allows the toy glider to slide off after a short distance to begin the glide slope towards the ground. I am thinking a simple and effective PVC pipe rail launcher would be like designs that use a bungee cord, but without the cord. Instead the launcher would have a negative incline and the glider model would slide off due to the influence of gravity. The first decision would be how much of an incline of the launcher would provide a proper launching speed after compensating for friction on the bottom of the main wing against the PVC rails. An ideal incline of the launcher would help the lightweight glider toy reach enough speed to start a slight climb just before the model reached the end of the rails. As modifications are made to the glider toy's design, or configuration it is likely the glider might lift off the launcher a little earlier, or later depending on how much lift was being provided by the wing, or wings. In most cases, the combination of wing lift and airspeed at the moment the model reached the end of the rails would play the most significant influences that would either increase the glide slope and distance traveled before ground contact, or reduce either.

Glide slope is the angle of decent the glider travels until it makes ground contact and as you probably know, if the angle of decent is low (measured in degrees in reference to the ground), the model will travel a longer distance before landing. Jets don't travel very far as a glider since they require thrust to stay airborne, but long winged gliders that fly ever so slowly require little, or no thrust to stay airborne for long periods of time. How this can be done with no thrust added by a motor, or engine has a lot to do with how much lift the long wings provide and available updrafts. This was learned from birds that can fly for very long periods without flapping their wings to provide thrust. The birds that can fly without flapping their wings for very long periods are normally called predators and they are hunting for prey. They flap their long wings vigorously until they reach the desire altitude and find a suitable updraft so they can fly inside the updraft area with little additional effort. This often requires the bird to fly in a large circle to stay within the updraft that often doesn't stay put, but gently moves across an area with the necessary contours and adequate wind. What birds do at low airspeed is change the configuration of their wings airfoil shape ever so slightly to maximize the lift provided by the updraft and wind.

Engineers have schemed how this could be duplicated in mechanical and "soft wing" designs for many years. Mechanical wing designs use flaps on the trailing edge of the wing at lower airspeeds and slats that slide down and forward from the wing's leading edge. The effect is to give the wing an undercamber that approaches what is observed while watching birds glide. What that means is the bottom of the wing airfoil isn't flat, but curves inward, which acts upon the air moving past the bottom of the wing and forms a vacuum. We can modify the toy glider wing so the bottom has the same shape, or curve towards the surface of the top of the wing. The curve will need to be minor, or shallow to preserve the strength of the foam, or the wing is likely to fail during flight by folding, falling apart, cracking, or some other undesirable defect. There are ways to strengthen a foam wing with more than a minor undercamber, but they normally add enough weight to off-set the benefits of the effect. Even so, we may want to learn more about the effects of undercamber when doing glide tests and see what is possible with the wings provided in the glider toy kit and what we can achieve with a homemade wing using the KFm designs, or another airfoil design. There is a lot of possibilities, so it is clear I will need a lot of toy glider kits to play with, so I predict more than six glider toy kits will be required and this will take more than a year, or two to complete.
Last edited by Tsavah; Apr 23, 2019 at 03:21 AM. Reason: spelling
Apr 03, 2016, 04:52 PM
Watt Waster
Tsavah's Avatar
Thread OP

CG, CoG, or Center of Gravity - Model Airplanes

Currently most of the stores offer the newer version of the old Titan toy foam glider by Air Hogs, which has been given the name of Sky Riders. I did discover Kmart is still offering the Titan foam glider and a much smaller foam glider given the name of Air Bandit Sky Glider. I purchased some of the Air Bandit Sky Glider kits to start some indoor glider experiments since they are only $3 each. Right off I noticed they are very boxy and have an extremely high drag. In short, they aren't very good glider toys, but the youngest glider airplane fans probably won't know this. The first thing I noticed was the rudder is very thick and has a flat leading edge and so does the main wing. The stabilizer is very thin and easy to crease, or damage in other ways. The nose of the foam toy glider is large, round, and thick. The fuselage looks like a twin jet engined Cessna, or Lear small passenger Jet I have seen in the past. I will have to see if I can find a picture of the small passenger jet and post it to this comment. For some reason I'm not finding the small passenger jet with the large round nose.

Oh well, moving on. After an hour or two of careful, light handed sanding, I reduced the thickness of the rudder, and gave it a slightly less draggy leading edge. I also sanded in a better leading edge to the main wing so it looked more like an airfoil. It still doesn't glide very well and is nose heavy. Even so, the slightest breeze reminds me how lightweight this foam is. So I took it indoors for some test glides and it is obviously way too nose heavy. As soon as I let it fly with a gentle forward push above my head with a slight upward orientation of a few degrees, it quickly noses down and bounces off the floor. I suppose the large, round nose has a purpose. If it was pointed and thinner, it would have shown signs of damage by now. My choices at this point are; reduce the size and shape of the nose, shorten the nose, add weight to the tail by either replacing the rudder with a larger, heavier one, or twisting in a small screw in the back end of the fuselage. The quickest option is a pin, nail, or small screw in the tail end of the fuselage.

I had a thin dress makers pin on hand, so I pushed it into the tail of the fuselage. It wasn't enough weight to make a difference and the lightweight foam toy glider bounced off the floor like before, nose first. OK, fine. I will try a heavier T pin, if I can find them. Maybe a very small nail instead since I am not having any trouble finding those. Wow, that nail is many times heavier, but it didn't make much difference. The fuselage behind the main wing needs to be longer and heavier for sure, but that would take a bit more work and not a quick fix. Let me see if I can find a heavier screw, or something. The small screw did the trick, and was almost twice the weight of the small nail, but at least the toy glider doesn't hit the floor nose first. The glide path is respectable and the toy foam glider lands on it's belly after a shallow glide slope. That suggests to me I need to cut off the thick foam rudder and replace it with one made from foam plate material cut from the center. The best I can get from a foam plate is a 5"x 5" square, but that will be enough.

I will glue two pieces together using a very thin coating of urethane glue (think Gorilla glue) from my old Elmer's Ultimate glue stock bought on clearance more than a year ago. Then cut the double foam plate piece into a slightly larger rudder then half a stabilizer would be. It will be necessary to remove the rudder and cut a slot into the top end of the fuselage to give the new rudder a tight friction fit. That way if I decide to change the size, shape, or other detail while making a new rudder, it won't be a big deal. I am also thinking I will want to change the stabilizer since the slot provided for it is a little too big and this toy glider could use a bigger stabilizer. At some point I may cut the fuselage just behind the main wing and do a few other airframe configurations, but that is a bit more work and I will have to engage my imagination after reviewing what others have done with larger toy gliders. I want to first do a smaller copy of their projects and then spin off from there with my own ideas to see what happens to the CG and flight characteristics of the toy glider. The key point being if you can make a good glider, it shouldn't be all that much harder to make a larger park flyer that glides and flies well as an RC version if you follow the rules of good design.
Last edited by Tsavah; May 07, 2016 at 06:58 PM. Reason: spelling ...
Apr 04, 2016, 11:50 PM
Registered User
I'm just a newbie in Fixed wing's area, this basic information is exactly I'm looking for but really hard to find. Looking forward to the experiments/demonstrations in the upcoming discussion.

Apr 10, 2016, 11:40 AM
Watt Waster
Tsavah's Avatar
Thread OP

Center of Gravity - Toy Foam Glider

Excellent. Your comments speak volumes to why I am doing this in the hope newer builder/pilots will find the information they need and be able to repeat the same experiments at a very small cost. Of course I am assuming foam toy gliders will continue to be available for purchase by those who want to repeat the experiments and for some that might not be possible. Instead, they will have to look in the stores that offer toy gliders, rubber band powered balsa toys, and other like options to repeat the demonstrations to learn what I did a very long time ago. The majority of the stores I checked are currently offering a new version of the old Titan by Air Hogs and it is called a Sky Riders, but is generally of the same size and also of the same type of soft, lightweight foam. Many of the modifications that have been done in the past to the Titan toy foam glider to make it an RC park flyer are possible with this newer version with the same restrictions to limit damages to the foam due to the higher flight weight.

I recently discovered Walmart was still offering the old Titan toy foam glider by Air Hogs if ordered online. I ordered 5 kits and picked them up yesterday. I had already purchased six kits of the newer Sky Riders version from Target earlier in the week, so now I can do a comparison show-n-tell. I plan to make a short video using the two larger foam gliders to show there are some differences between the kits, but most of them are easy to ignore, or work around. Then I visited Kmart and discovered they are currently offering the old Titan foam glider at a slightly higher price, almost $12 instead of the old price of $10. The other stores offer the Sky Riders foam toy glider kit. Kmart is also currently offering a much smaller toy foam glider given the name of Air Bandit Sky Glider by Banzai. The recommended age is 6+, so that is well within my age group and I suspect it will make a good option for the experiments at a lower cost of $3 each and I won't need adult supervision since I am a retired, old guy.

The Air Bandit Sky Glider isn't all that big at less than half the size of the other two with wingspans of around 4 feet, but looks to be large enough for basic demonstrations after a number of modifications. I also noticed it is near the same size as many of the 50mm EDF RC jet models Sky Angels came out with more than six years ago, but are now nearly impossible to find for purchase. As with the other two larger foam toy gliders, this one is made from styrene foam that is soft, feels, smells like, and has the appearance of a foam cooler plastic. Just in case this isn't obvious, all foams normally available for purchase are plastic that has trapped air in one of three common methods, which is why the plastic is so light weight. Foam is primarily trapped air bubbles, but that air is more likely to be a gas, some of which can be toxic to some degree. If you are sanding, cutting, or otherwise breaking the tiny foam bubbles containing gas and start to feel a bit strange, be aware you are having a reaction and need to improve ventilation, or wear a respirator. Toys are less likely to be a problem due to many government regulations requiring safety for kids.

These days the gas trapped by the plastic is normally safer then it was in the past and likely Co2, which is contained in many products like soda pop. Sometimes the plastic bubbles also contain additional gasses, but by government regulations they are also suppose to be rated safe and restricted since the same foams are offered in other forms of products for purchase. In this situation, the example of a foam cooler is a good comparison product. In fact, if you are determined to use the same foam as a source of material for making modifications to a toy foam glider to ensure the finished project has the same soft foam characteristics, cutting up a foam cooler for parts would be a good choice. I prefer to use a slightly better foam product, so I have on hand some white foam that is stiffer and reminds me of Depron foam formulas. It was a packing foam used for shipping chocolate since the foam is very lightweight. That way the chocolate could be frozen, or nearly so, and shipped without fear of melting with one day shipping.

A number of years ago I joined a local recycling group that was fairly new and online based so folks could offer stuff they had and wanted others to take for free and recycle instead of throwing it in the trash. As you might imagine there were all sorts of things offered and one of those things that caught my eye was foam packing sheets. They are generally around 2" thick and of a small variety of dimensions, but in only six sizes. I ended up with two small pickup truck loads and have six stacks of matching pieces. Two stacks are large enough pieces for cutting out some wings for park flyers in the small to medium size. One stack could make slightly larger models and the other stacks would be for rudders, and stabilizers. Otherwise I would have to glue pieces together, which is doable to make a fuselage, or main wing for a larger park flyer. I was impressed with the stiffness of the foam board, so naturally I took all I could get from the lady making chocolate candy.

I mention this because it is likely possible your area has an online recycling group, a free classified paper, or some other way to ask for packing foam sheets, or shapes on the cheap. It will be important to say the foam piece will need to be larger then 4"x 4", or folks will try to offer packing peanuts and foam sheets many things come wrapped in, but aren't useful in the RC airplane building part of the hobby. You want to say what you are looking for needs to be stiff foam, otherwise you may be given foam rubber, which isn't a bad thing if the foam rubber is at least 2" thick, firm, and at least a 6" x 14" piece. I personally use larger pieces of firm foam rubber to cold roll thin foam board, but the smaller size can be useful for many options like protecting LiPo battery packs. Cold rolling thin foam board is another part of RC park flyer construction I have already addressed in another thread, but it is likely to be part of this thread also at some point. Since this is getting a tad long, I will end here.
Last edited by Tsavah; Apr 14, 2016 at 10:32 PM. Reason: spelling ...
Apr 10, 2016, 02:29 PM
Watt Waster
Tsavah's Avatar
Thread OP

CG for Small Toy Gliders - Foam

After the urethane glue had set and cured for a few hours while I watched a movie, I cut out the new rudder from the center of two foam dinner plates I had glued together. The rudder is a little larger than half of the stabilizer and has the same outline. As the urethane glue continues to stiffen the rudder will become very stiff and freeze into shape. I noticed it had a slight curve, so I got out a recent purchase - a very firm foam roller for ink art and rolled over the lamination while it was on a thin foam sheet. The thin foam sheet is a foam rubber product I had received in a package of things I had ordered. It is around 1/2" thick, but worked well for the purpose since I didn't want to chance using a much thicker piece of firm foam rubber. The rudder now looks very straight with only a hint of a mild curve from root to tip. I suppose I could cold roll it some more, but it is plenty good enough for test gliding this slightly modified toy glider. I will do the same thing when it comes time to make a new stabilizer, but I will have to piece one together from more than a few foam plates due to the limited size of the parts I can harvest from the center of a foam plate. What I should do is see if I have some very thin Depron foam sheet in my storage area, but I doubt I have any this thin. My other option is to use the stabilizer that came with the toy glider and laminate a few pieces of foam plate to it.

I have tested the toy foam glider with the larger, thin rudder and it glides further, which is predictable since there is less drag from the rudder. In further I mean two, or three inches on average. After looking at the model for a while I believe I might get another two, or more inches of glide distance from it if I make new, thinner wings for it with a better airfoil. Even though I have sanded the leading edge of the wings, they are still very thick and only hint at an airfoil. Maybe another modification would be to cut some sheet foam and add a strip to each side that is two, or more inches wide. This would lower the wing loading since the result would have more surface and if I added more than three inches to the width of the wings, it would start to look like a delta wing design. As you can see from the posted pictures, the wings are swept back. If the wings were swept back more, the model would fly even more nose heavy. If the swept of the wings were closer to straight with little, or no swept back, it might make it possible to remove all, or most of the tail weight I had to add to get a gentle glide slope.

The larger, heavier rudder didn't add very much weight to the back half of the airframe, so I had to keep a small nail stuck in the foam, but a light weight option compared to the other one I used before. The small screw was much too heavy and the tail would drop after the nose pitched upward soon after release. When a model, or glider is tail heavy even a little bit, the glide slope will be a series of climb, stall, dive, climb, stall, and dives until it hits the ground. If you are lucky it will make ground contact near the point of stalling, rather than a hard dive when it is more likely the nose of the soft foam will be damaged if the model is heavy. This is especially true after RC gear has been installed in one of these toy foam gliders. As long as the toy foam glider is lightweight, there is less chance of damage even if it should nose dive into the ground rather than make a shallow glide slope to a soft belly landing. The first mistake most builder/pilots make when converting one of these soft foam glider toys to an RC park flyer is use heavy RC gear. If the model builder kept the weight down by using lightweight, small model airplane RC gear, it is likely they used a small battery pack. After flying about for 5, or more minutes successfully for a few days, the first temptation is to install a larger LiPo battery pack.

Obviously this will make the RC park flyer heavier and if crashed, it is predictable what is likely to happen to the soft foam fuselage and other parts. There are ways to transfer the compressive forces applied to soft foam to spars and fuselage tubes, or rods to cancel most of the damage. Some call this making the foam model crash proof, but that is a bit of an exaggeration. There is a bunch of math one can learn about compressive forces common to an RC aircraft model during a crash, but it is easy to put that off until another day and just realize it is all about weight, mass, and speed. How hard the surface crashed into is a factor, but most of the time RC park flyers are flown over grass and weeds with an occasional tree, light pole, car, building, and other obstacles getting in the way. A small bush is often not enough of an obstacle to be concerned with unless the RC model airplane is very heavy and you are short of replacement propellers. So the bottom line is respect the fact the foam is soft and easy to damage when it is carrying RC gear. There are ways to protect the soft foam and give it a damage resisting skin, but even that adds weight and has limits. I have discussed some of the options for protecting soft foam model airplanes in other threads, but it is likely I will get into this detail again in this thread at some point.

A new video to watch if CG is still a mystery to learn about. Check out this video -

CENTER OF GRAVITY (12 min 47 sec)

OK, now compare the first video to this one about how an aircraft is designed to be stable -

Basic Aerodynamics - CG and Stability (7 min 30 sec)

To explain a little about what these two videos are trying to show is in a common configuration, or I probably should say, the popular configuration for an aircraft, the main wing carries the full weight of the flying object and if made small enough to balance on your fingers, or a balance board (2 pencils in holes of a wooden board), you can find the CG of the non-flying model airplane on the main wing. Generally speaking, you should be able to find that balance point around 1/3 back from the leading edge of the main wings width. The other 2/3 of the main wing width will be behind the two pencils, along with the rudder and stabilizer. The balance point for the model airplane can be moved forward of the 1/3 back position on the main wing in some designs, chief of which is the straight, or squared wing design. The assumption is the model flies, or glides well to a soft belly landing. If it doesn't, the main wing needs to be moved either forward, or rearward towards the tail until it does glide properly to a soft belly landing. Obviously if the main wing needs to be moved for a proper glide slope and soft belly landing, it is too early to check for the best CG location on the main wing.

In our experiments the toy foam glider maker has already determined where the main wing will be located and provided slots cut into the fuselage. If the toy glider fails to fly properly in a good glide slope that ends with a soft belly landing without a series of climbs, stalls, and dives common to a tail heavy flight path, we will soon learn the main wing's location isn't yet ideal and want to know what other way we can improve the glide slope without moving the main wing. On the other hand, if the glide slope is steep and the flight path ends with the nose of the model airplane striking the ground first and a bit hard instead of a soft belly landing, we know something has to be changed so the model glider isn't so nose heavy. If we put the nose heavy glider on the balancing board, we would notice the balance point is further back than the common 1/3 spot from the leading edge. Keep in mind the 1/3 back from the leading edge of the main wing is a starting point for the first glide tests and not a hard rule for the best location for the CG of the toy glider, or RC model airplane. As Ed said in his video (the first one), the best CG location may be in front of the 1/3 location on the main wing, but to determine how far in front is by the glide tests for a soft belly landing. This is the old school way of finding the best CG spot that uses very little math to find the proper balance point. It is all about glide testing the model, but there are a number of assumptions made for this to work properly.

One of the assumptions is the incidence angle of the stabilizer is as good as it will get for the airframe design. Another is the incidence angle of the main wing is ideal to the thrust line for the weight of the glider and the configuration of the flying surfaces (main wing airfoil, stabilizer, and rudder, or rudders). The purpose of modifying the toy foam glider is to see if we can take a poorly formed foam airframe and get it to glide better. Part of the reason the toy glider doesn't perform well has to do with the shape of the kit components. The first thing you may notice is the leading edge of the main wing is flat. Next you may notice the rudder is small, thick, and has a flat spot on it's leading edge. Those two design problems cause a lot of drag, which slows down how easily the foam toy glider will "cut through" the air. Since the glider is very lightweight foam, the slightest breeze will confuse the results of any glide tests, so it is best to wait until there is a dead calm, or seek an indoor location that offers a dead calm testing site. For this size of glider we will need a bit more than a common garage with no obstacles to get in the way. Once the glide slope is proper, holding the toy foam glider above your head (assuming your not taller than 6 feet) and giving it a slight forward push while level to the ground is likely to glide to the garage door and hit it an inch or two from the bottom before it makes a soft belly landing on the floor. For most of us the glide tests in an empty garage with no wind influences will be plenty of space and the glider will belly land before it hits the garage door. More about these details in comments to follow.
Last edited by Tsavah; Apr 19, 2016 at 03:03 PM. Reason: spelling ...
Apr 10, 2016, 06:55 PM
Watt Waster
Tsavah's Avatar
Thread OP

CoG, CG, or Center of Gravity - Foam Model Airplanes

The next phase of experimentation with this $3 foam glider toy is to move the main wing forward a bit to see if it will glide without additional tail weight. What I did was remove a small amount of foam from the fuselage slot the wing parts fit into. I used my fine tooth hobby saw that looks like a smaller version of a miter box saw. I do have a coping saw common to cutting soft woods like pinewood derby cars and balsa, but I like the wide, flat blade this hobby saw has, even though I don't use it very often with the handle attached. The wide, flat blade with more than 38 teeth per inch will saw into this soft foam without popping out beads if you go slow and don't press it into the foam all that much. I made the slot in the foam a bit longer towards the nose of the fuselage about the width of a popsicle stick. I was guessing that would be an easy way to measure how much foam I removed and that would be just enough to balance the foam toy glider so I wouldn't need tail weight to get a good CG. I used a finger nail file, also called an emery board, to sand the slot smooth a little. I didn't want to get to aggressive, or the slot might get too large and loose for the foam wing roots that slide into the slot. Otherwise they would need some sticky tape to tighten the fit. After a few glide tests I was happy with the glide slope even though it is a tiny bit nose heavy.

I am betting if I cut out a new set of wings from some foam board I have I can make a thinner, one piece wing that fits into the fuselage slot like the original wing set does. Then I can make a few examples of KFm wings with minor differences and give them a go. Since the KFm designs are simple layers of flat foam board, it won't take very long to make them and once the urethane glue has set and cured for an hour or two, they will be very stiff. I will do like I did with the rudder and spread the glue out with a gift, or credit card until it is a very thin film, mist it with water, and press the lamination together with a flat board, 1/2" foam piece, or thick piece of glass. I normally do this on a thick piece of glass because I know the glass is flat and has no twists, or warps. If I was using wood to press the foam lamination flat until the glue had set and cured for a few hours, I would constantly have to check the wood for any warps since wood is sensitive to moisture. It will also warp if it gets too dry and the wood cells shrink too much. Glass is much more stable and worry free, but thick glass plates are often not so easy to find on the cheap. Common window glass to very thin and wouldn't hold up for very long used as a press, or flat construction surface in RC building methods. It is likely to crack, or break when weight is applied. Not the sort of thing a builder of RC park flyers wants to deal with, or get cut by.

If you sandwich a thin foam lamination between two thick sheets of glass and add some weight, say around 5 to 10 pounds, there isn't much chance the cured glue is going to shrink and the part is going to come out warped, or twisted. A very handy detail to be aware of if you are laminating thin foam to make a wing, rudder, or a stabilizer. The other thing to be aware of is urethane glue stiffens the lamination considerably after it cures, yet still provides a small amount of flex. Urethane glue is easier to work with than spray, or contact glues since you can spread it very thin and have at least 15 minutes of working, or alignment time before it starts to set unless the room temperature is very high and the air is very moist. Even then if you can stand the heat and humidity, the glue normally won't set for at least 10 minutes and won't finish curing for at least 8 hours. For what we are doing with foam board, it is hard to ask for a better arrangement and working time. I tend to let any glued parts set for around 2 hours before using them on small, lightweight gliders and four hours before using them on small, lightweight RC park flyers. I might be able to get by with less time for curing, but I am happy with the results so far and see no need to rush construction. I can do other things, or other parts of the build while the curing process continues to a useful stage.

At some point I will cut some foam to fill in the space in the long wing slot so there is no hole behind the main wing in the fuselage, or I will make wider wings that fit the slot well. For now that is a very low priority since I may want to continue the modifications with wider KFm wing options and a thinner airfoil wing. I did notice if a larger, lightweight foam glider toy hits the ground hard enough nose first, the wings would slide forward, which reduces the compressive forces on the nose of the fuselage. Years ago I wondered why the Titan toy glider had a notch cut in the wing root of each half towards the rear and I soon figured out it was so the wings would slide out of the fuselage easier in a nose first impact. If you modify the wing root by adding foam board so the notch is effectively filled in, the wings don't come out of the fuselage slot very easily anymore. An interesting, yet simple detail that helps to limit damage to the nose of the soft foam glider toy in the event of a hard, nose first impact with the ground, or an obstacle. The current project glider toy didn't have that detail in the rear of the wing root, so the wings didn't slide out of the wing fuselage slot easily on a hard impact. Something to consider when modifying the fuselage wing slot and making a new pair of wings to test.

I cannot find the Youtube video for some reason, but years ago I swear I saw a build video where a RC builder/pilot had converted a foam toy glider to an RC park flyer and his method of reducing damage to the foam fuselage was to add a small wooden dowel that ran from nose to tail on the bottom of the fuselage. He had to run the small diameter wooden dowel rod carefully so it wouldn't be in the way of the RC gear, the main wings, or the stabilizer. What he had done was insert the wooden rod with a slight curve, or bow to match the contours of the non-Titan foam glider toy. It looked like the one that was sold as a Sky Rider by Guillows, which sort of looks more like a passenger jet without engines. I think it was like the second, and fourth picture of toy foam gliders in post #2. I have some very thin and flexible bamboo rod that could be used in the same way on this small foam glider toy, but I would need to get out my wood burning pencil, or iron and carefully melt a channel into the bottom of the fuselage. At this point the toy glider is still very lightweight and I don't expect the fuselage to break, or the nose to crush in, or break off. If the toy glider gets heavier I will probably add one of the thin bamboo rods from a room divider I bought more than a few years ago so I could use the parts as lightweight stringers in large park flyer projects. I probably won't live long enough to use all of them and they will break if you apply too much force. Better than balsa stringers, but not a great deal stronger. One long stringer of around six foot cost less than a penny, which was why I spent more than $40 to buy the bamboo room divider.
Last edited by Tsavah; Apr 11, 2016 at 10:24 PM. Reason: spelling ...
Apr 10, 2016, 09:51 PM
Watt Waster
Tsavah's Avatar
Thread OP

Center of Gravity - Toy Foam Glider

If you happen to be looking for small diameter rod for lightweight park flyers to use as spars for wings under 3 feet long, you may want to get to Walmart and check out the camping department. They have true arrow bamboo skewers in a pack of 8 (or was that 12?) for less than $1 plus tax. If you buy the same skewers in the barbeque, or garden department you only get six for the same price (or maybe that was four). Hey, I'm getting old and sometimes I have trouble with minor details. I seem to be having an elder/senior moment with these skewers I saw a few days ago. Two years ago I found the same skewers sold as garden stakes on clearnance, but they were a bit longer and a tiny bit thinner. The box had at least 30 packages in it and each package had a dozen bamboo rods, most of which had no defects I could see. Arrow bamboo gets thicker, and you can probably figure out how it was named, but the thinner versions are lightweight, flexible, and strong. Once they are glued into a narrow slot in foam board with urethane glue, they are nearly as good as carbon fiber tubes, but many times cheaper. Of course the rules to how to use urethane glue properly still apply, but if you spend only a few pennies per spar, how can you still want to use expensive carbon fiber products that cost at least $5 each plus shipping? OK, I get it, the carbon fiber tubes have less weight per inch, or foot, or yard, but hey, we aren't talking about a model airplane that has to break the current speed record. At least that isn't my focus at the moment. I'm old, so what I build is lightweight slow park flyers that can do maneuvers that look more like a graceful dance rather than a break neck race to death.

As a side note - I spent $40 on a Nikon brand ballistics Wind meter that was said to be compatible with Apple products and Droid. Well, maybe it is since I have an old Droid cell phone. I may never know for sure since the advertised free application isn't. Even worse, the Nikon website doesn't offer the app to download, nor does it offer any information about the wind meter. I called the technical support folks and after a bit of delay a guy answered and I asked questions. You probably guessed he didn't have any for this product. That seems to suggest this wind meter by Nikon is a forsaken product and difficult to get working. He did send me an email like I suggested within a few minutes that was basically an advertisement for the wind meter, but again, no link to download the app. Talk about an odd way to do business. So much for measuring the wind speed before testing a foam glider, or other options. Oh wait, I bet there are wind meters made by others I can buy and they are probably cheaper. Sure enough, there are choices available and many are less than $20. Oh now, lesson learned! I will hang this wasted $40 item by Nikon in a prominent spot so every time I walk past it I will remember the disappointment it was. Go for a refund? Nah, I would rather remember the bad experience and avoid Nikon products in the future. That's just me, but your mileage may vary.
Last edited by Tsavah; Apr 11, 2016 at 06:37 PM. Reason: spelling ...
Apr 10, 2016, 09:56 PM
Watt Waster
Tsavah's Avatar
Thread OP

CG for Small Toy Gliders - Foam

I made a new wing out of Blu-Core fan fold foam (FFF) by Dow with a top layer of pink FFF by Corning and center layers of Dollar Tree ($T, DT) foam (DTF, $TF) core poster board. It took two layers of $T foam without paper skins to fill the fuselage slot for the original wing set. There is a tiny gap, but it isn't enough to be concerned with and does allow the tight fitting new wing to be adjusted a little up, or down so I can change the incident angle a little. If I continue to believe the incident angle is as good as it can get, I will use some thin dinner plate foam to fill the gap, but I will have to sand most of it way after the urethane glue has cured. The new wing is only two layers thick, so it is 1/2" thick and a little wider and longer than the original wing set, but otherwise the same. Each layer of thin foam board is around 1/4" thick, so if I decide to make another one using three or more layers of foam board, it will soon become as thick as the original wing set and a bit more drag will be noticed. The lightweight glider won't glide as far with more drag.

The new wing is a KFm type where two layers of foam board are glued together and the top layer isn't as wide as the bottom. The bottom piece is the full width of the fuselage wing slot and about an inch longer then the original wing set, but has the same profile, or outline. The top layer of pink FFF is around half as wide and matches the leading edge, so I sanded each layer to suggest an airfoil edge without getting concerned about rounding the leading edge all that much. The trailing edge of both layers is left flat, which is common for quick and easy KFm wings using sheet foam board. The average glide distance is around 1 inch further than the last modifications, which suggests the KFm style wing has a lower drag coefficient. I really need to make a launching rails set-up to be more consistent and maybe even add a rubber band to power the glider off the rails. That would help with confidence each launch was with the same applied force and suggest the results were more accurate for gaging what the last modification did to improve, or set-back the airframe design. After RC gear is installed the flight characteristics would be more predictable using a simple launcher like this, but that is assuming the glider with RC gear wouldn't gain all the much weight.

The Blu-core FFF is an old favorite of foam RC airplane model builders and you will find it was used as the primary foam board of many projects in RC Group threads. The pink FFF by Corning is also used, but not as often since it tends to be a little more brittle, which gives it less crash resistance. The two foam board types do make some good KFm style wings and a glider of this size and weight doesn't need carbon fiber products, even when RC gear is used if lightweight. The fact the new wing is a slightly larger one means it has more surface to provide more lift than the original wing set, which helps keep wing loading low. Low wing loading normally translates to a lower stall speed, but lightweight RC foam model airplanes and gliders need very calm winds, or they get buffed, or thrown around by wind a bit easier than a heavier version would. Since most RC pilots have to fly in winds greater than 2 mph, it is common to put a protective skin on the soft foam surface to make it a little heavier and to resist minor crash damages and damage from the less than the softest belly landings. If the model sports wheels for take-off and landing, a side wind (cross wind) might make both a challenge to maintain a straight orientation.

I suppose I should start calling Blu-Core FFF dinosaur foam board since folks are saying it is very hard to find these days. I noticed last year Lowe's Home Improvement warehouse stores stopped carrying it and it then became difficult to order online through them. What they have now isn't as nice, but it is like the old Blu-core, except it is green and has a lot of holes. Others who have tried the green version tell me it is brittle like the pink FFF made by Corning. Not good. I probably won't run out of Blu-core FFF anytime soon, but it also means others wanting to copy what I have done are going to have to select a different foam board. In that case, I would recommend the $TF option. Strip the paper off, which is pretty easy to do, and use it like the FFF versions. It is a little softer, but when I get into how to apply a lightweight skin to soft foam board, that won't be all the much of an issue. Of course the focus will still be how to keep the park flyer model very lightweight since I'm not suggesting building heavy to fight the wind is a good idea. No, instead wait for the calm wind days and relax while you putter about slow and easy. Besides, the battery pack will last longer. How can flying longer while relaxed be a bad thing? Now to decide what the next modification will be while keeping it simple to learn a thing or two.
Last edited by Tsavah; May 07, 2016 at 07:03 PM. Reason: spelling ...
Apr 11, 2016, 08:35 PM
Watt Waster
Tsavah's Avatar
Thread OP

CG for Small Toy Gliders - Foam

After thinking about it for a bit, I am thinking the best next topic will be the stabilizer. Basically the stabilizer does two things; helps the glider maintain a steady flight path by helping the main wing stay slightly leading edge high and provide a way to influence how high the nose of the fuselage will rise at a given airspeed. How this is done is the stabilizer's leading edge points downward in reference to the thrust line, horizon, and main wing. The thrust line is either an imaginary line drawn through the center of the fuselage from nose to tail that represents how high, or low the nose of the airplane will be while in straight and level flight, or it is drawn on the fuselage. I drew a line using the wing bottom so I could see if it had any upward incidence at the leading edge. Compared to what I see as the trust line, the leading edge is a few degrees positive, but that is normal for gliders. Normally the thrust line means the nose of the fuselage is neither high, nor low in reference to the horizon at the cruising airspeed. The fuselage appears to be even with the horizon as it cruises along in the air, but that is assuming the horizon is flat and level like a calm sea for as far as the eye can see. If the aircraft model fuselage isn't flat and level to the horizon as far as the eye can see it is either climbing, or descending. It would look awkward to see an model aircraft fuselage nose high all the time since that would make a soft landing a little more difficult without the model plopping down in an abrupt fashion. Of course it is common for park flyers to fly nose high if the idea was to fly in a "high alpha" to slow down and show off. Some RC pilots haven't mastered the standard controlled landing procedure that requires the airspeed to remain slightly above the stalling airspeed, so they land by using the high alpha approach with the engines running hard.

In a normal landing the engines are at, or near idle and provide only enough thrust to ensure the airspeed doesn't drop to, or below the stall airspeed for the airframe. A lightweight foam airplane helps to lower the stall speed, but it doesn't eliminate it. If the main wing is flat, broad, and angled high in reference to the leading edge and the horizon, the airframe may begin to rock from left to right. If the same broad flat wing design is slightly V shaped at the center line where the two wing halves join, enough air will likely pass across the wing halves to reduce, or eliminate the rocking motion at low airspeed while in a high alpha position. What is happening is the air isn't passing straight over and under the flat wings, but moving at an angle away from the fuselage towards the wing tips. The angle of this cross wind is increased away from the fuselage as the V joint becomes more pronounced and this is called dihedral. There are many dihedral configurations possible, but generally speaking the purpose is to stabilize how the airframe sinks during a landing at the lowest possible safe airspeed. The trade off is keeping the dihedral angle as shallow as possible to aid lift from the main wing instead of needing a higher airspeed for the airframe design and weight. That means it is less likely the wings will lose adequate lift and the aircraft will either drop out of the sky, or will suddenly roll to one side in agreement with the torque influence of the engine, or motor. Examples of ways to control the cross wind on a wing while in high alpha and slow airspeed can be seen on both the F-86 Sabre and the MIG-15. They are often called wing fences.

A spinning propeller, or EDF fan can influence how well a model aircraft is able to fly straight and level and that becomes evident at, or below the stall speed. Building a lightweight model foam airplane wing is a balance between sacrificing lift for more stability at a lower airspeed, or a lower stall speed by the influence of dihedral at a given airframe weight. Practicing touch and goes with models with wheels is all about getting use to how the model airplane reacts while descending at a specific airspeed. If you build the same airframe, install the same RC gear carefully to preserve the best CG, but make slight changes in the dihedral of each copy, you will soon understand how dihedral makes a difference and why it is rare to see an aircraft without any. Even most jets have some dihedral in their wing designs, even if it is very slight due to the fact the wings are very thin and narrow. Until experiments showed a wide, short wing could pull more Gs and not come apart the jet wing designs got thinner, shorter, and angled back more and more. Good examples of wide, short wings on a jet are the Russian Fox bat and the F-15 Eagle. Notice when they are landing the engines are providing a significant amount of thrust and the nose of the jet is very high, as if doing a high alpha maneuver. In the case of the F-15 Eagle there is a long flap like device that is deployed from just behind the cockpit. That odd looking fuselage flap is helping the pilot prevent over lifting the nose and bleeding off too much airspeed before the wheels have touched down. That is also why the twin engines are running hot because a lot of drag has to be kept in check until the wheels are on the ground and that requires a lot of thrust.

Also notice how the stabilizer is large, flat, and moves up and down a lot as the F-15 Eagle is landing. Most modern jets have large stabilizers and rudders to influence how the heavy airframe moves while flying near the airframes stall speed. If the rudder, or stabilizer was smaller, it would be much more difficult to land at a lower airspeed, which would stress the airframe, wheels, and other parts as the heavy jet plopped down on the hard runway. Obviously the slow flying WW1 aircraft where lightweight and mostly doped canvas and wood with a lot of main wing surface and a lot of drag to overcome by thrust. As the aircraft had better engines canvas and wood designs for airframes had to advance to cope with the stress of faster airspeeds. During WW2 the Germans and others continued to build wooden airframes due to shortages of metals like aluminum while our designs became all metal even before WW2 started. What the Germans did was advance the use of plywood and glues they had used during WW1, but they had trouble all through the world war keeping up with engine advances. Many of their last WW2 airframes had to be heavily repaired, or replaced before 20 missions had been flown, so the advanced engines kept finding a new airframe to power. The first jet engines couldn't stand up to the heat produced and major overhauls were required within 400 flight hours, or less. Again the problem was having the proper metals and minerals needed. As if that wasn't enough, they had problems like the Japanese keeping enough fuel on hand and getting it were it was needed quickly.

When building foam model airplanes we wrestle more with how best to use the soft foam in ways to ensure it can manage flight stress, landing stress, and the occasional unplanned impact. Most of the time we have access to better glues, epoxies, and other options that make the airframes able to handle the small amount of vibrations a well balanced propeller will produce. The heat from the motor, or motors, ESC, and battery pack is manageable with adequate ventilation, but foam board does have limits. To give those limits proper respect we need to build light and use lightweight RC gear while avoiding obstacles and keeping impacts with objects to a minimum. If we get into the speed demon craze, we have to adapt our construction methods much like the Germans had to do during WW2. That means if you are determined to build with soft foam board, you will have to learn how to make foam laminations, or keep looking and buying better foam types that are often stiffer and harder to glue well to wood and other materials. You will also need to use products like carbon fiber tubes, rods, flats, or strips to help transfer the compression and vibration stress on an airframe from the foam to the "crash proofing" materials. What that means is your scratch build is going to get expensive and it will often be cheaper to buy an almost ready to fly (ATF) model aircraft kit. The other option is to build lightweight and fly slow and graceful.

OK, enough talk for now, so it is a good time to do some show-n-tell with the aid of a video. Give this one a look-see. I made it from a small collection of videos about testing the original foam glider kit to a few early modifications. The idea was to see if small changes would improve the performance of the $1 toy glider the Dollar Tree store had for a while. I currently have a case on order and if the back order doesn't take too long, plan to do more modifications for comparison, some of which are likely not going to look like the original glider toy. The KFm wing was changed again to have a very gentle step, which means I shaved off about a third of it and ended up with a pink bump on the first 1/3, or so of the main wing. I also sanded in a bit more leading edge curve on the bottom half of the wing. The ink lines drawn on the foam should help show some of the changes. After drawing the ink lines I used them as a way to gauge how far back to sand away a bit more foam to get more of an airfoil shape.
Last edited by Tsavah; Apr 23, 2019 at 03:22 AM. Reason: spelling ...
Apr 12, 2016, 01:34 AM
Watt Waster
Tsavah's Avatar
Thread OP

CG for Small Toy Gliders - Foam

It is clear I will need to get into using a hotwire cutting device. I have purchased a nice foam cutter, so in the near future I will do some show-n-tell on how quickly foam can be shaped using a hot wire cutter. There are many options to pick from and some are useful in the RC airplane, or simple hand glider hobby. There are also a lot of Youtube videos for those who want to make their own, but be aware there are many kinds and sizes of wire that can be used to cut, or shape foam. I tend to use thicker wire known as Nichrome wire. Some types are stiff and can be shaped, which comes in handy when you want to scoop out some foam to provide a channel, or compartment for RC gear. Thinner options for Nichrome wire works well if you want to carve, or shape an airfoil quickly with less fuss and dust. It is possible to do many of the same task possible with a hot wire with a long, thin saw blade, as in hack saw, or cooping saw. The catch is finding one long enough, wide enough, and with a lot of small teeth per inch. If you try to use a saw blade that has few teeth per inch and aren't all that tiny, the foam tends to tear rather than cut. You may already know a hot wire also presents it's own set of problems until you learn how it can be used for the best desired results. There is much more to say, but this is meant to be an introduction, so I will end it here.

Since YouTube has a new policy that automatically deletes videos if anyone claims copyright violations, the account has been deleted. Maybe, at some future time I will remake the videos, if I live long enough. I am an old guy, so don't bet on it.

If you find the comments are shown a little too fast, pause the video as often as needed to complete reading, or to observe other details. I will continue to make additional videos as time allows and my eyes can manage the work. I am making these for those who are curious how I went from a cheap toy glider to a rubber band launched airframe, which later became a small RC park flyer. This is how anyone with the time, funds, workspace, and desire can convert soft foam toy gliders to an RC park flyer. I have noticed a lot of newer builders/pilots of toy foam gliders that take the time and trouble to make an RC park flyer tend to make the same mistakes with predictable results and wanted to offer some suggestions on how to end up with a better flyer. I tend to want to make clones of expensive RC airframes to save a few dollars if I have spare RC gear on hand. In another thread I showed some of the details of how to make a slightly larger copy of a popular solid foam toy glider named the Titan by Air Hogs. By making a slightly larger copy of the Titan I was able to use RC gear I had on hand so the parts won't stay on a shelf, or in a box collecting dust waiting for a new airframe to call home. I suspect I will do the same thing with this smaller solid foam toy glider for the same reason, but I am looking into what I have and can buy to keep this project around the same size as the 50mm EDF foam jet models by Sky Angel.
Last edited by Tsavah; Apr 23, 2019 at 03:26 AM. Reason: spelling ...
Apr 12, 2016, 02:34 PM
Watt Waster
Tsavah's Avatar
Thread OP

CG of Foam Model Airplanes

OK, since I made the decision to focus on the stabilizer, I need to make a new one. For this next experiment I will make a slightly longer, wider version with a flat, or straight trailing edge. The one provided in the glider kit has a slight V shape on both the leading and trailing edge. I will make a new one and make the V shape of the leading edge shallow compared to the one in the kit. I will save a straight, or flat leading edged stabilizer for when the main wing is made the same way since making a wing like that now would move the CG balance point too far forward and it would be necessary to add nose weight, or lengthen the front half of the lightweight foam airframe significantly. At the moment I am giving the urethane glue some time to set and cure for around an hour, or two. What I did was cut the center out of four foam dinner plates, then trace around the stabilizer in the kit. The best I could do was make a stabilizer half that included the foam that fills the slot in the fuselage. I used this first half to trace out another and with the two halves overlapping, the stabilizer is thick enough to fit the fuselage slot. The other two foam dinner plate centers were used to fill in the other part of the stabilizer so it would be two dinner plates thick from one tip to the other. I know the urethane glue will stiffen the assembly well, otherwise the new stabilizer would be a bit too flexible even though I didn't use more than a few drops of glue on each half and spread it very thin with a gift card. I placed a 1/2" thick piece of foam board on top and then some zip-lock sandbags to press all the parts together on top of a thick sheet of plate glass.

Update: installed the larger stabilizer and after a few test glides it was obvious the glider is a tiny bit tail heavy again. Soon after releasing the small, modified $3 glider it would start to nose up a little for a short distance and settle back down into a series of typical, yet shallow tail heavy glides . Since it is only slightly tail heavy with the new, larger stabilizer I have two choices; reduce the size of the rudder, or the stabilizer. I could also reduce the size of both a little bit and see if I can catch the weight of the two just right and not need to add any nose weight to get the near perfect CG point established. If I decide no further modifications need to be done because I am happy with the glider as is, it would be time to get out the balancing board and mark where the CG is on the main wing, or the fuselage. If I decide to add lightweight RC gear later, I would need to scheme the location of the parts to preserve the CG with a very small adjustment to make the RC version slightly nose heavy. I am talking making the RC version balance point around 1/16th of an inch rearward when the battery pack is installed. Clearly that is a very slight change to the "best" CG location as a lightweight glider. After flying the RC version I might decide the CG needs to move further back towards the tail, but again it would be a very slight change. If past experiences prove to repeat with this modified foam toy glider, I am very likely to not adjust it rearward more than once.

I will see if I can get another set of pictures posted soon since they will help show some of the details I am talking about. Three details in particular will be the current incident angles of the stabilizer, main wing, and the thrust line I'm working with. The more I look at the stabilizers fuselage slot angle, the more I am convinced it is too steep. I am currently mauling over the options I have to change it without getting into a lot of extra work. One option is to remove a very thin sliver of foam just behind the main wing fuselage slot and glue the foam back together. Another is to make the slot a little bigger and insert slivers of foam to reduce the slots angle towards the thrust line. I could also cut the tail of the fuselage at both ends of the stabilizer slot, glue in a small piece of foam dinner plate and sand it thinner towards the rear of the slot. If I go with that option I would need to sand the plug of foam I removed for a proper fit, which is likely to require a little sanding in the front of the slot, on the inside top. Otherwise the part I cut out will be too tall when the stabilizer is installed to match the contours of the fuselage again. I may need to make a short video of this if I go with this option so the details would be easier to see. Sometimes words just don't get the job done.

As you can see I went with the cut just behind the wing slot from the bottom towards the top, but left a small amount of fuselage foam to act like a hinge. I didn't remove very much foam, so after the fuselage was glued back together the stabilizer slot angle changed only a few degrees at best. It still looks exaggerated to me and the test flights seem to agree. If I reduce it enough towards the thrust line, I can launch it using a rubber band on a stick and it will shoot upward like a rocket, slow down, and glide back to a soft landing. If I adjust the stabilizer slot too flat to the thrust line, it may still shoot up like a rocket, and slow down to begin to glide, but it will likely hit hard on the nose since too much influence of the stabilizer had been removed. The stabilizer, CG, and weight of the airframe work together with the influence of the drag of the airframe to slow to a glide speed. Once the glide speed has settled, the glide slop and landing tell you if the stabilizer has enough influence, too little, or too much. Assuming the CG is as prefect as it can be, which is effected by the stabilizer's angle, the glide speed will either be slow, or fast, but how it lands tells the story. Generally speaking, most stabilizers are set around 2.5 degrees negative compared to the thrust line, up to four degrees. It is rare for it to be set more negative for a proper, gentle glide slope. When I had grandkids living with me, or visiting I let them play with some small, simple hand gliders I made in an attempt to teach them these principles. I have a thread about making hand gliders for kids and it is likely this one will end up having some of it's pictures posted in the other thread.
Last edited by Tsavah; Apr 15, 2016 at 02:41 PM. Reason: spelling ...
Apr 12, 2016, 02:35 PM
Watt Waster
Tsavah's Avatar
Thread OP

CG of Foam Model Airplanes

The next video in the series is below. I will need to make a few more to get anywhere near adding lightweight RC gear. Notice how flat the main wings leading edge is in the picture.

Videos were on YouTube, which has become a risky place to post videos to share.
Last edited by Tsavah; Apr 23, 2019 at 03:27 AM. Reason: spelling ...
Apr 21, 2016, 05:23 PM
Running jokes into the ground
b-29er's Avatar
So are you planning on making an rc from this, or just a better glider?
May 02, 2016, 09:43 PM
6 months to finish a rtf
pulsery2k1's Avatar

What works ?

Not sure how I stumbled on this But I've been designing and building
models over 50 years .I have an engineering degree .
but always believe to stick with what works. today most including
Boeing use the computer to design there airplanes - years ago they
use to build mockups - today they no longer do that. even on this site
some in the scratch built treads do so.
Me old school and always build my mock up - which is also used
as a cg glider yes I know how to find the correct CG but the glider
can tell a lot of different things . I start with simple glide test's
then on some I use rubber band launcher's .
My latest project flys great-very stable slows with no hint of
tip stall. computer design is great but I think it takes way too
long and since this works and its fun -

Quick Reply
Thread Tools