Monokote or Ultracote? By James Goss      Over the years I have built and covered about 75 models that I can remember, and still enjoy building as much today as I did 30 years ago. My building skills are better than my flying ability, I can never seem to get that perfect landing. At my age I don't think I will be flying in the tournament of champions any time soon so I am content being a Sunday and sometimes Saturday flier.  Get it in the air, cutup a few aerobatic maneuvers, get it back on the ground in one or two pieces and I am happy for another week. For me there is something special about building the airplane and then taking it up for its first flight. You never know how it will fly until you see it in the air.  I think it is safe to say that all airplanes will fly, but some will just fly better than others. I have found that if you use a lot of care and patience while building a kit it will normally fly just fine.  In order to have patience you must really love what you are doing. To perch for hours and hours in the same spot putting sticks and sheets of balsa together does require being devoted to the hobby, but I get peace of mind when it all starts to shape up and I start thinking about flying it. This makes it all worthwhile for me.      Once the model is framed and sanded I really get excited because my favorite part of the project is coming up.  I have always liked covering a model with plastic film rather than painting it, even though I have painted many models in the past.  Nothing is faster than covering with monokote or ultracote.  I like all the coverings on the market today but mainly use these two. Covering material that ends in the word tex, such as colortex or worldtex, are also very easy to work with around compound curves and you will absolutely have no bubbles because the material is porous and will trap no air. One of the main things I like about ultracote is that it does not leave color residue on the sealing iron, as does monokote, which requires periodic cleaning.  Always clean your iron when it is cold and not hot if you use acetone, it will clean up much easier. You can also remove ultracote when it is stuck to itself by applying a little heat and it will usually come up nice and clean so you can use it again, while with monokote it is there to stay. Ultracote is a little thicker than monokote and may expose a little more seams than does monokote. If you plan ahead the seams will not show. Always have the seam facing to the rear of the fuselage and to the root rib of the wing. I also like how the backing is so easy to remove from ultracote. The backing is not that hard to remove from monokote if you know how, but it is still not as easy as ultracote. Another item that some modelers do not like about monokote is the clear plastic overhang on its side edge. This overhang must be removed before using the monokote so we loose a little material here. I have actually seen a few jobs in the past that left this strip on including the writing. This was probably a first time covering job for that modeler.     Just how thick is monokote and ultracote?  I thought I would run a few experiments with them to find out how thick they are and also which one will shrink the most.  And maybe answer the question “ which one is the strongest”.  Here are the typical questions modelers have about monokote and ultracote.   How thick is monokote, before and after shrinking? How thick is ultracote, before and after shrinking?                                                                       Are some colors thicker than others are?  Can some colors shrink more than others? Which has the greatest amount of shrinkage, ultracote or monokote? How much does the film weight per unit area?  One square foot. Is one stronger than the other is?  Which of the two has the greatest puncture resistance?      How thick are they before and after heating?  I used a standard 1-inch micrometer for this measurement that is calibrated in thousands of an inch. Here are the results: Ultracote is .0022 inches thick before shrinking and about the same after heating which measured   .0020 inches, two ten thousands of an inch less. Monokote measured .0015 of an inch before heating and .0014 after heating. They both were heated to their maximum shrink ability. So ultracote is .0007 on an inch thicker than monokote.  Seven ten thousands on an inch is not very much. With all the talk about the seams showing up with ultracote, I thought it would check to be a lot more than this value of .0007. Fabric covering such as colortex measures .0035 of an inch. This is over twice as thick as monokote but as you will see it may not be twice as strong in all cases.      Are some colors thicker than others are?  Over the years I have noticed that some colors just appear to go on better than others. I think this is true for ultracote and monokote.  By going on better I mean some appears to have less gas formation and some appears to shrink and hold better than others. The amount of moisture in the wood and how clean the surface is along with how porous the wood is will all play a role in how well the film bonds.  All colors that I checked measured about the same thickness. I did not check the metallic family of colors.  I have found that these colors really go on nice and seem to not wrinkle as much with time, but will not work around curves as well.      Which covering has the greatest amount of shrinkage?  For this test I used a strip of covering 8 inches long and 1-inch wide bonded to a sheet of balsa and heated for maximum shrinkage. Here are the results for ultracote: Length after heat being applied was 7 7/8 inch and width was 15/16 inch. For monokote the length was 7 7/8 inch and the width was 1-inch. Both had about the same shrinkage. Being bonded to the balsa did restrict the shrinkage.  This showed that monokote and ultracote really bonds to balsa because it would not let it shrink very much.   Next on an open frame of balsa, to simulate a rib bay, I used a 6 inch square piece of covering and placed a ¼ inch wrinkle in the center. This is equal to ½ inch of loose material and in a 6-inch square space that is a lot of loose material. Here are the results when using a heat gun: Monokote reduced down to 1/8 of an inch so it reduced the wrinkle by 50 %.  Ultracote removed 100% of the wrinkle and became drum tight.        How much does the film weight per unit area? Using a balance scale calibrated in grams here are the results: With the backing in place ultracote weights 13 grams for one square foot. Without backing it weighted 7.2 grams. Converting grams to ounces by multiplying each gram by .035 we will get a better picture of the results. Ultracote without backing weights .252 ounces per square foot.      For monokote the results are as follows: Each square foot of monokote with backing weights 11 grams or .385 ounces.  Without backing it was 6.7 grams or .234 ounces. So ultracote weights .5 grams or .0175 ounces per square foot more than does monokote.  A full roll of ultracote has 13 square feet on it and will weight 3.27 ounces. When you use one roll of ultracote and have at least 15 % of unused material, you will add 2.8 ounces of weight for each roll.  The average .40 size plane can be covered most of the time with two or three rolls of covering.  This will add 9.8 ounces to the plane. If you were using monokote it would add 3.04 ounces per roll or 9.145 for three rolls.  This is a difference of .65 ounces in the plane's weight. So as a general rule we can say it adds 3 ounces per roll for monokote, or ultracote, when estimating the added weight.  You may not have realized this but both ultracote and monokote has exactly the same amount of material on a roll, 13 sq. ft. But here again you can't use the full width of monokote because of the clear plastic edge that must be cut off.      Colortex measured in at 11 grams or .385 ounces with backing and 8.5 grams or .297 ounces without backing per square foot of material. So comparing it to the plastic coverings it weights 3.86 ounces per 13 square feet. This is only .82 ounces more than monokote per roll, but remember most of the time you will need to seal the fabric by using clear or colored paint. This will add about one ounce per roll.  Most of the time you will not paint the plastic coverings even though it will bond to paint if you wish to paint it.      I saved the best for last, which one is the strongest? For this test I cut the material into small strips and connected it to a hook with attached weight.  I wrapped the film around the hook so it actually gives us two sides of material supporting the weight. A true tensile strength test would only use a single piece of film 1-inch in width, but for this comparison test the results are still accurate. The idea was to pick up the weight and hold it for 10 seconds. Here are the results:                                                     Strip width in inches            Ultracote                   Monokote      3/16                            5 kg  ( 10.9 lbs.  )                 4 kg ( 8.75 lbs. ) 1/4                     7 kg  ( 15.3 lbs. )          5.5 kg ( 12 lbs. )           5/16                     8.5 kg  ( 18.59 lbs.  )          7 kg  (  15.3 lbs. )      1/2                     13.5 kg  (  29.5 lbs.  )                 11 kg  (  24 lbs.  )      Using a ¼ inch strip of colortex showed that it was much stronger in tensile strength. It supported 13 kg  (  28.4 lbs. ) with little stretching. It does not have the same stretch ability or elasticity as does the plastic coverings, when it breaks it is more of an instantaneous break. So this material is more than two times stronger than monokote or ultracote in reference to tensile strength, but you will be shocked to find out about its puncture resistance.        It is easy to see from the table above that ultracote is about 20% stronger than monokote.  A 1-inch strip of ultracote will support about 59 pounds and monokote about 48 pounds. Remember that I am using a 24-inch strip of material folded in the middle and placed over a hook. This will double the tensile rating so keep this in mind as you look at the chart above. On the monokote package it states that monokote has a tensile strength of 25,000 psi. Have you ever wondered about this statement?  Where did they get the value of 25,000 psi? Tensile strength is the amount of stretching force the material can withstand before separating.  To the average working class Joe, such as myself, it sounds like they are saying a 1-inch square piece of monokote will require 25,000 pounds of force to pull it apart. How can this be?       Actually the full definition for tensile strength is as follows: The maximum load a material can support without fracture when stretched, divided by the original cross sectional area of the material. Now the cross sectional area for the material tested would be a 1-inch strip of monokote .0015 inches in diameter.  So a 1-inch strip of monokote can lift 48 pounds, divided by its cross sectional area of .0015, which is the diameter times the length of a 1-inch square piece. All this adds up to equal 33,000 psi tensile strength. Divide this number by two, because of the double strips of film I am using, and you will get 16,500 psi. tensile strength. This is a little less than the rated value of 25,000 psi, probably due to the thickness and weight measurement not being under laboratory conditions. Based on these figures ultracote will have a tensile strength of 26,818 psi divided by two which gives us 13,409 psi tensile strength. At any rate ultracote will lift and support more weight than monokote, based on my experiments even if it has a lower tensile strength rating.  I guess it's more impressive to state it as 25,000 psi tensile strength rather than to say a one inch strip of monokote will support 24 pounds of load. By being a little thicker ultracote has a reduced tensile strength rating as compared to monokote, but I am now sure of one thing, ultracote will hold my planes together better than monokote.      Our models made from balsa are very fragile before covering. Plastic covering adds a great deal of strength to the model and transforms it into a finished product all in one step. Yet sometimes even the smallest mishap will total your plane. Remember that film covering only increases the overall tensile strength or separating resistance of your model and does not help much against the crushing forces encountered during a crash.       The strength test I was most interested in was the puncture test. How many times have you had to land that new plane in a distant field due to a dead engine? What looked like a perfect landing from your viewpoint turned out to be a disastrous occasion.  Damage to the wing and tail section has occurred because straw and sticks have punched multiple holes in them. Makes a grown man want to sit down on the ground and cry like a newborn baby. Having this happen to me many times in the past I have always wondered which type of covering would puncture the easiest. I performed the following puncture test by using a ¼ inch dowel reduced to a 1/8 inch round tip with a sharp point and a spring scale to apply force to the top of the dowel for the actual puncture pressure needed. The spring scale was calibrated in Newton's; one pound is equal to 4.445 Newton's. The material was placed over a six-inch square frame to simulate a rib bay in the wing.  Results: Ultracote required between 15 - 18 Newton's  ( 3.37 - 4.04  lbs. ) of force to puncture it.  Monokote requires 14 - 17 Newton's  (  3.14 - 3.83 lbs. ) of force to puncture it.  I performed numerous puncture tests for each material.  As you can see they both have about the same resistance to puncture with ultracote being just a little better. Now this is what I really was not expecting to see! When I executed the puncture test on a piece of colortex it had no better puncture resistance than did ultracote or monokote. It had about the same range of puncture, 15 - 20 Newton's  ( 3.37 - 4.49 lbs. ).  So even though colortex has a tensile strength of 16,250 psi, based on its lifting power of 113 lbs. for a 1-inch double strip divided by its cross sectional area of .0035, its puncture resistance is actually low in value. Its ability to hold your airplane together is great compared to the plastic coverings, but it will puncture just as easy. It must be due to the weave in the fabric separating. This test piece of colortex was not painted or sealed, so it may have a little higher puncture rating if painted. The sealant may help hold the weave together and not let it separate as easy.  Notice that the tensile rating for colortex was much lower than the tensile rating of the film. This is because colortex is much thicker than the film and has a larger cross sectional area.      As I said at the beginning of this article, I like to work with monokote and ultracote. I now use ultracote more than monokote because of the advantages listed above. I know some modelers that will use nothing but monokote and some that use nothing but ultracote and some that will fight you if you mention the wrong type of covering material. Now the finished product on both will look very nice, in fact many times I have to ask the builder which covering they have used. They are some colors unique to monokote and some for ultracote making them easy to identify. By and large, I am just happy we have two good coverings to select from and hope they last for another 30 years at a reasonable price. Good luck on your model finishing techniques.

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