Fornula for lengthening the fuselage when switching to a heavier engine?

[frank mccune]

     Hello All:

     I would like to know if the formula for shortening the nose could also be used for making the fuselage longer when changing to a heavier engine.

    The formula was posted a few days ago but I can not remember where it is to be found.  It went something like this.  Multiply the distance from the cl of the engine to the balance point in inches, by the weight of the engine in ozs.   Take this product and divide it by the weight of the heavier engine and this will provide the number of inches that the engine must be moved to balance the plane.

    If I wanted to increase the length of the fuselage to accommodate a heavier engine, could I not use a similar method to determine the length of the length of the fuselage with a heavier engine?  For example. measure the length of the fuselage from the balance point to rear of the fuselage.  Divide this distance by the weight of the engine and get a value.  Take this value and multiply the additional weight of the heavier engine to provide the additional length needed to balance the airplane. 

     What I was attempting to do is to fabricate a profile fuselage body that would balance a heavier engine without changing the nose moment(length) or adding tail weight.  This may give me a shortcut to having the fuselage correct in length from the beginning to avoid making changes in "trim" later by adding tail weight etc.

      Is this correct?

     Suggestions / comments.

                                                                                                                    Tia,

                                                                                                                    Frank McCune

[George Truett]

From Larry Fruits on Stuka Stunt Forum:

Weight of original engine times length of glow plug to CG = inch ounces.

Inch ounces divided by weight of new engine = new length of plug to CG.

Larry

[Fredvon4]

Frank

While I think your formula is rational... and basic...from my reading, there is a LOT more to it----moments, tail volume, front end mass, wing loading, flap efficiency,  etc

If the exercise is to just take any old model... make a longer or heavier fuse to balance a newer heavier engine.. on the previous CG ...your math will work

Now ----if the new plane will fly and trim as well ----is questionable

I took a well flying Rugged Stunt Trainer with a OS LA 25 and decided to try my Evo .36... placed it further back worried about added nose weight

Balanced on the previous CG...flew like crap....yes, I know I did not change the fuse length...so sort of apple oranges comparison

[frank mccune]

         Hi Fred:

          Thanks for the reply.

          You brought up many other considerations that may be considered.  Nothing is as simple as it seems. Lol  Perhaps some of those injunears could make a few suggestions.

          I too have added a heavier engine to a good flying plane and thought that by just adding tail weight to get it to balance, it would fly very well.  WRONG I will bet that if one just moves the engine back to get it to balance, it still will not fly as well as the original engine.  I have done this and have blamed it on the increase in wing loading. 

         I have swapped a .15 engine into a previously .35 powered combat plane and the lighter plane responded much better.  Less wing loading?  Of course the speed was less with the .15.

        I hate to add tail weight!

                                                                                                           Have a nice day,

                                                                                                            Frank McCune

[Tim Wescott]

It's complicated, both because the weight of your fuselage extension matters, and because moving the tail back will change the aerodynamics of the plane.

IF you're flying something like a Twister or a Banshee or other pre-1980 (or whenever it was) airplane that has a short-ish tail moment arm, then you'll improve the aerodynamics by moving the tail back.  IF you have something that already flies like gangbusters, then unless you're really super-advanced, you can move the engine backward without changing the aerodynamics to any noticeable degree.

HOWEVER:

Just to figure out how much to move the tail back:  Weigh the portion of the plane from the CG back, and find it's CG, independent of the rest of the plane.  Then use that weight and CG as the weight and center line of your "engine" and apply the formula.  Putting all the pieces back together is left as an exercise to the reader.

I'm an engineer, so I weigh each bit and make a spreadsheet of weights and moments around the CG, with careful accounting for the weight of the finish, etc., etc.  I get this one right just about as often as if I'd just eyeballed things and guessed.  So I generally plan on getting the CG of the second prototype about right -- the first one will have some brass or lead on it somewhere.

[Tim Wescott]

What works best for me with a profile is to build it with a longish nose, up to bare wood.  Then I guess at the total weight of covering and tape on that much weight an inch or two ahead of the stabilizer.  Then put on a fuel tank, mounting hardware, wheels, etc., and slide the engine (with muffler) back and forth until it all balances.  Whack the nose shorter as required, and proceed.  I still get the balance wrong in the end, but the amount of lead seems to be less that way than with all the calculations in the world.

[Trostle]

     Hello All:

     I would like to know if the formula for shortening the nose could also be used for making the fuselage longer when changing to a heavier engine.

(Clip)

         Is this correct?

     Suggestions / comments.

                                                                                                                    Tia,

                                                                                                                    Frank McCune

Ah, I think you have something backwards here.  If you want to change the length of the nose for a heavier engine, you would probably want to shorten the nose, not lengthen it.

Keith

[frank mccune]

      Hello Keith:

      It is my lack of writing skills that perhaps hidered me explaining the question. What I hoped people would grasp was that I was using the formula for making the nose shorter to balance for an engine that was heavier than the one that it was replacing.  I assumed that the reader would make the connection about making the tail longer by rearranging the original formula.  That was my mistake.  I remember learning things that were named reciprocals used when using math.  I attempted to do this with the length of the fuselage question.  It made sense to me at the time but...

     After spending more than three decades in the classroom. I forgot the most basic premise of education and that is to make each lesson as simple and foolproof as possible for all people.  Do not assume anything! After teaching Grade 12 General Math, to students who were more challenged than others, I ignored what I had learned. That being the basic rule of education and let my guard down with that post an attempt to communicate with all of the people all of the time on this site. 

       The old saying,"Don't assume anything. It makes an ass out of you and me."

                                                                                                                        Mea culpa,

                                                                                                                        Frank McCune 

[Trostle]

      Hello Keith:

(Clip)

      It made sense to me at the time but...

(Clip)     

                                                                                                                        Mea culpa,

                                                                                                                        Frank McCune

Frank,

I am sure everyone knew what you meant when you posted.  I apologize for my dumb and unnecessary comment.

Keith

[PJ Rowland]

Having built a few planes designed for .35s and putting up to a. 76 in it I can say this.....

A plane wants thrust and a good balance point;  thats it. 

Getting dirty with torque and trim is more important than trying to factor 1/4" difference in nose length for a given engone weight..

Its just not THAT important.