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How to Balance Your Model Airplane. (Read 16091 times)
Reply #8 - Jan 29th, 2012 at 8:56pm

C.L. Chennault   Offline
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Springfield MO.

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 Here is an excellent article that goes deeper than I ever could on this subject.
http://www.bakehead.com/rc_cg.htm

  Special thanks to those who did not sue me for stealing their videos. If you see a video that you like, please, feel free to thank them for making it.    
« Last Edit: Feb 1st, 2012 at 8:29am by C.L. Chennault »  

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Reply #7 - Nov 15th, 2011 at 11:00am

C.L. Chennault   Offline
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  OK, We have discussed how to balance the front and the rear of your plane.
  There is one more thing to check for. Balance from side to side. I balance my main wings before I cover them {weight match the two halves} and again after I cover them. {extra layer of dope on the light one}  If your wing is already built, you can still check it for balance. One way to do this is to attach a couple strings along the center line on the top of the main wing.  Hang it from the strings and add weight to the side that is light.  Remember to add the weight at the farthest point away from the center. That way you add less to get the same effect. Some folks like to add weight to one side or the other to cause the plane to bank. In a case like this, ballast helps turn the plane.

  That's about it.  Balance is everything.
Now go have fun.
« Last Edit: Feb 1st, 2012 at 8:14am by C.L. Chennault »  

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Reply #6 - Nov 13th, 2011 at 11:09pm

thymekiller   Offline
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Missouri

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   OK, break time again.  My favorite time of all.....

   What I am going to do is throw a couple vids at ya just for fun.  We are NOT going to discuss airflow over various air foils.  Thing is, you don't have to know much about that stuff just to enjoy building and flying model airplanes.  Some folks argue for hrs about Reynolds numbers and stuff that don't make much sense to me.  I cant build a computer, but I know enough to run one.  That's all I need to have fun with it.  If you want to know more about airfoils, feel free to chase links on U-tube or Google.  All I want to do is help get that plane in the air.
 
   Please keep in mind that balance is not the only thing you should know about flying a model airplane, but I think its the first.  Without balance, nothing else will matter.  If you are building from a kit, most of the rest has been engineered for you.  Build it straight, balance it, and it will most likely fly.

http://www.youtube.com/watch?v=jWMC27GVtzE&feature=related

http://www.youtube.com/watch?v=Jfkyp89LLTE&feature=related

http://www.youtube.com/watch?v=-WVrPgG0y9U&feature=related

http://www.youtube.com/watch?v=xh4t9yiD-Rg&feature=related

« Last Edit: Feb 1st, 2012 at 8:16am by C.L. Chennault »  

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Reply #5 - Nov 13th, 2011 at 10:43pm

thymekiller   Offline
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Missouri

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  Looks great doesn't it?  HaHa, tricked ya'.  Its nose heavy. Yes, it is balanced level, but the balance point, CG, is in the wrong place.

...

  Here's a video that will outline what I'm talking about when I say "Center of Lift". [CL] The music is kinda cool and its brief.  What you are looking for is when the little lines go over the top, the faster they go, the bigger the gap in-between the wing and the lines.  That is lift. Cl is the center of that bubble.

http://www.youtube.com/watch?v=uUMlnIwo2Qo&feature=related

  Here's how it looks from the side. See how it is balanced on the Leading Edge? {LE}  That means the CG is in front of where the wing lifts evenly. [CL] It's like if you picked up that stick and you can't pick it up in the middle because one end is more heavy than the other. That means the nose is heavy.

...

  The picture above shows the plane with the weight distributed evenly.
  The picture below points more toward where the Center of Lift is located.  See the difference in where the pencils hold up the plane? On this plane, the CG is way to far forward of the CL.    

...

  Not balanced well, is it? And that's pretty much how it would fly. Now, using a couple razor blades, I add ballast to the tail....

...

  And presto!!!  The CG is MOVED closer to the correct relationship to CL. This plane might fly level. Again, we will talk about lift and such later.

...

  Notice in the last picture, the pencils are holding the wing at the fattest part of the wing.  As a rule of thumb, that's the best place to start if its not marked on your plans.  Normally, that happens around 25 or 30 % of chord. Each plane has its own sweet spot.  You must test glide to find yours.

  You can experiment with CG location by adding just a touch of ballast to the nose or tail. That will cause the plane to balance on the pencils in a slightly different place.  Minor changes here make a BIG difference in flight, so don't do anything drastic.  Baby steps. What you are looking for is the best, smoothest, straightest, longest glide you can get. You can't really change the CL unless you alter the shape of the wing. Better off not to. You CAN change the CG of the airplane to match, or come close to, where it is supposed to be by simply adding or subtracting ballast.  

  Here's a video that I hope will entertain and help explain.    

http://www.youtube.com/watch?v=P0iEIus4ZmY

   That video always makes me grin.   
« Last Edit: Feb 1st, 2012 at 8:12am by C.L. Chennault »  

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Reply #4 - Nov 13th, 2011 at 10:00pm

thymekiller   Offline
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Missouri

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  Balancing on CG is critical.  Now I am going to try to explain where CG should be.  We are not going to get all caught up in aerodynamics. That is for another thread.  For now, all I want to do is give you a general idea what to shoot for.  

  Most Stick and Tissue plane have flat tail feathers.  We will start there.
  On most plans you will see a circle that is divided into 4 sections. Opposite sections will be black and white. This circle lines up pretty close to the high point of a main wing IF the tail surfaces are flat. Sometimes there is a black arrow that points to a certain place on the wing.  Sometimes you don't get any indication at all.  
  In that case, a good place to start is around 25% of "chord". That means the width of the wing. The length of a wing is the "span".  The high point of the airfoil on the top normally happens at about 25 or 30 percent of the width, from the front to the back.  {Leading edge to trailing edge}

  This is where most of the action is.  This is where you want your plane to balance in the air.  If you balance to far forward, we call this nose heavy and your plane will dive.  If you balance too far behind the high point, we call that tail heavy.  In the air, a tail heavy plane will throw the nose up, slow down too much, and stall {or "dive"}  If the tail imbalance is minor, it will fly up and down and up and down. Mostly down. If it is major, it will fly straight up, and come back to earth like a rock.
  Sometimes it is difficult to see the difference between a nose heavy plane with too much trim, and a tail heavy plane with good trim. If you only move the CG, and that alone fixes it, you can bet she was nose heavy.  Remember, most planes are just a tad nose heavy. Try different combinations and the right one is the one that glides the farthest and smoothest.       

 Think of balance as equal lift, fore and aft.  If you pick up a stick that is 10 foot long, you would pick it up in the middle so that each end weighs the same.  That way you can carry it without dragging one end in the dirt. That's balance. That's the CG of the stick.

  Most airplanes don't have the wing in the middle so what do we do to balance the CG?  We add weight to the short end so that it equals the weight of the long end.  On real airplanes the reason the nose is shorter is because of the heavy engine.  Our models rubber powered free flight don't have that problem because the engine { rubber band } runs all thru the airplane and weighs less.  The weight is distributed more evenly.  

  Try this.  Lets say that stick we spoke of weighs 10 pounds. If you pick it up in the middle, each end weighs 5 pounds. You have balance. If you pick it up 25% of the way from one end, you get 2.5 pounds on on end and 7.5 pounds on the other.  Makes it hard to carry.  If you add 5 pounds to the short end, now both ends of the stick weigh 7.5 pounds. Presto, equal balance. Cool, huh?  
  Lets try again, but backwards.  Lets say the stick you need to pick up is different on one end than the other.  Lets say that one end weighs 7.5 pounds and the other weighs 2.5.  Obviously, you cant pick it up in the middle and have balance.  So you LIFT it closer to the heavy end. That distributes the weight more evenly.  In fact, you would pick it up around 25 or 30 % back from the heavy end.  

  On a real airplane, the motor might weigh as much as the whole rest of the plane. That's why the main wing is closer to the motor. If you are building a scale model, and you want it to look right, you need to add dead weight to the nose in order to balance it.  Just like that  stick we spoke of.  Dead weight on a model is called ballast.

Still with me?  Here's the point of all that,:  

The CL is the point of equal lift caused by the wing.
CL is built into the plane.  Tough to move.
The CG is the center of the planes weight.
You change that by adding or removing Ballast.  VERY easy to move.

  The more control you have over where these two things are, the better your plane will fly.  


« Last Edit: Feb 1st, 2012 at 7:44am by C.L. Chennault »  

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Reply #3 - Nov 12th, 2011 at 8:17am

thymekiller   Offline
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Missouri

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  Welcome back.  Not sure if your still here because this thread is helping or because you are laughing at my woeful lack of skill. Whatever, your back.  Cool !!!

  Here's an EXCELLENT video from the Harfang boys to show a cool and simple tool for balancing your plane.  Basically, its a wood block with holes for pencils. Simple and very effective.
 Of special note, these folks have the largest collection of "How To" videos in the world, as far as model airplanes. I highly recommend that you subscribe to their series or at least mark them in your favorites. There is a vast wealth of good information here, and alot of it is just plain fun to watch.          

http://www.youtube.com/watch?v=hVl1-3XV3c4
« Last Edit: Feb 1st, 2012 at 7:10am by C.L. Chennault »  

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Reply #2 - Nov 11th, 2011 at 1:21pm

thymekiller   Offline
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Missouri

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   Here are a few glider videos to inspire you.  Remember, more drag is natural if your plane has a fuselage.
Some of these are AWESOME !!!

http://www.youtube.com/watch?v=zYzXhgI1RaI&feature=related

http://www.youtube.com/watch?v=akdlnJsDDDU&feature=related

   This video shows how sensitive balance is. Just opening the retracts changes CG.  I know free flight planes don't normally have retracts, but you can see that every little bit counts.  The pilot of this plane would be able to compensate for the change in flight.  We don't have that option.  

http://www.youtube.com/watch?v=i9bS7R8RD58&feature=related

  While on U-tube, feel free to hunt around and look at other videos. This is not a required course in school. This is just having fun. We will be here when you get back.

   Next, I will show you how to choose WHERE to put your CG.  In the meantime, take a break, refresh your beverage, and enjoy the ride.

« Last Edit: Feb 1st, 2012 at 7:01am by C.L. Chennault »  

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Reply #1 - Nov 11th, 2011 at 12:40pm

thymekiller   Offline
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Missouri

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  Its not as difficult as you may think to balance your plane.  Thats good because R/C or free flight, you MUST balance.  Its one of those things sound weird until you do it once.
 With free flight, you must balance for level flight.  The method I use is to glide the airplane, balanced where I think it might be best, and see how it flies.   If it "noses in" your plane is nose heavy.  If it "noses up" your plane is tail heavy.  On some planes, you move the wing to adjust the location of CG.  On most planes you add weight.  [ Ballast]    If you have a model of a jumbo jet, where the wing is pretty much in the middle of the plane, you probably won't need much ballast to balance it.  A ww1 bi-plane fighter, with its short nose and comparatively long body will require a lot more weight in the nose to balance in the correct spot.
 Here is a fairly/ sorta  balanced glider.  Remember, this is not a scale model so there is not as much drag involved.  There is always some drag involved, but not much here.  A model bi-plane with a million feet of wing and wire and landing gear ect, will have more drag. It will not glide as far, but still be a stable glide to earth.  

http://www.youtube.com/watch?v=NeW9zJrg9U8  

 This plane is a bit more nose heavy than it should be.  The trim is set to compensate.  Trim is speed sensitive. You can see the effect of speed on the trim by watching the glide. Watch it again if you need to. When I first launch it it tends to go up because it still has the force from the throw.  That extra speed causes extra trim to raise the  the plane.  Nose up, it bleeds off speed.  As it slows down, the trim becomes less effective,  she drops the heavy nose,  and picks up speed as it heads towards earth. When it gets back up  to a speed it likes, it levels out and glides into the grass.  
 If there had been more altitude, the plane would repeat the process over and over till it touches earth. Nose up, slow down, drop and glide, nose up, slow down, ect.  Not the best glide, but it is close.  If I readjust the CG, and re adjust  the "trim", it will glide smoother and farther.  That means bringing the CG and CL closer together.  The result will be a long, stable, steady rate of decent to the ground.  
 
  On a side note, before I got the bugs out of it, I accidentally stepped on it. Some times that's how it goes, when reaching for the stars. All you can do is grin, and go build another one.  

  If I add a prop and rubber motor, the same thing happens but the speed is now affected more by the thrust of the prop than the strength of my toss.  That is another subject that we will get to later.  The goal for now is to obtain the best glide you can.  Work with it and try balancing it in slightly different places.  With practice, you will learn to see what it wants and putting the CG where it should be,  will be a snap.

  If its out of balance, it wont glide well.
If it wont glide, it wont fly.

  Of course, there is more to flying than just that, but this will get you started.  If you skip this step, the rest of it wont matter.  You cant build a square house on a crooked foundation.  

 
« Last Edit: Feb 1st, 2012 at 6:56am by C.L. Chennault »  

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Nov 11th, 2011 at 12:28pm

thymekiller   Offline
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Missouri

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How I balance a model airplane.

By Eric McAuley



  Balancing  a model airplane is the most important and easiest part of building and flying a model airplane.  I have found this to be THE most overlooked  part of flying your plane. My intent is to explain it in as simplest terms I can.   I will include videos from several folks and pics also.   This is not meant to be a "Total Package", but rather a starter course.
  The thing to remember is that your plane MUST balance in the correct place, or it wont fly no matter what else you did.  Sometimes, the plane will not fly so well even if it balances where the plans show. Sometimes you have to find your own CG.  {CG = Center of Gravity.} Hopefully, these videos and pictures will help you do that.
  Please remember, the following are only MY thoughts on the subject. As with any topic, its best to gain knowledge from several sources so that you can get a balanced {LOL get it? Balanced? LOL} view.  I will try to provide at least a few links for further study.  My plan is to outline the topic and get you in the air as quick as possible. No muss, no fuss, no details to hold you back.  You don't need a Masters Degree to fly model airplanes.  If you build true to a good kit, most of the smarty stuff has been worked out for you. I just want to get you flying. Where you go after that is up to you.  

  Here's a video that talks about balancing R/C craft.  If you have an R/C and you want to fly stunts, you can use IMBALANCE to aid you.  Thing about free flight is that you want NEARLY NEUTRAL balance.  You want it to float, flat and true.  With R/C you can adjust trim in flight to suit the needs at that time.  With free flight, that is not an option.  

http://www.youtube.com/watch?v=m-MUYmrh63M&feature=related

Short but sweet and to the point.  ALL aircraft MUST balance.  From a real 767 to the tiniest rubber powered job.  

http://www.youtube.com/watch?v=xFnZORtpggE&NR=1

  This next video is good because it explains why a poorly balanced free flight plane wont fly as well.  We all use the tail to guide our planes. This is called "trim". If you use the tail to compensate for being out of balance, excessive trim, it creates drag. Drag slows down the plane.  As the plane slows down, the effect of the trim becomes less and the "out of balance" comes more into play.   Trim can help, but wont fix being out of balance.   Kinda like a band-aid on a shotgun wound.  Grin

http://www.youtube.com/watch?v=cyC0i_zOqkg&feature=related  

  I would like to add that what she is calling "center of pressure" is also called "center of lift"  {CL}   I prefer to keep my CG as close as possible to, but slightly in front of, CL. That way, I can use less trim to maintain level flight and my planes don't seem to be as speed sensitive.  Faster craft might put the CG a hair farther forward of CL than I do.
« Last Edit: Feb 1st, 2012 at 6:30am by C.L. Chennault »  

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