Math Formula

[Casey]

Hey Guys

I am replacing my worn out Norvel 061 with a brand new Norvel 074. As the Norvel 074 is slightly heavier than the Norvel 061, is there a Math formula I can use to determine how far back I should position the engine to get the model to balance at the same CG point.

[kenneth cook]

       Weigh them and subtract the difference which becomes your tail weight.

[George Truett]

Weight x Arm = Moment.  Pick a spot as your reference datum line, CG should work fine.  Multiply weight of .061 times the distance from datum line (Arm) to a point on engine (drive washer should work).  That number will be your Moment.  Divide that Moment by the weight of your .074, that will give you the distance it needs to be from your datum line to achieve the same balance.  PM me if you need more help.  GT

[John Rist]

Going from an 061 to an 074 is a big jump in power! Wow! 

[FLOYD CARTER]

Calculating moments is valid.  Easier just to install the .061, mark the CG.  Then install the .74 and add tail weight until CG falls at previous mark.  See, no math!  So easy , a caveman can do it.

[frank mccune]

      Hello All:

     Awhile ago, there was a posting as to how to do this.  It was nothing more than ratio-proportion using a similar method as to nose  moment and tail moment as George posted. It was so simple, that I forgot how to apply it. Lol Perhaps someone could post this solution again.

[Fredvon4]

Floyd is best opinion....dif between NV .074 and NV .061 is less than 1/2~3/4 oz*........20 inches aft the added ballast will be insignificant

my only CL .074 has a new home.....but when I fondled each of my NV .049/.061/.074S   I did not have any wow...this one is very heavy....sense or thoughts

apples  vs oranges because the .074 is R/C With carb

.061 CL....57 GRAMS  OR 2.01 Oz

.074 RC....83 grams  2.92 Oz

[Howard Rush]

George is correct, but:

a) you gotta pick the airplane CG (with .061) as reference datum.  Otherwise you also have to consider the weight and CG location of the airplane without engine, which complicates the formula.

b) the drive washer won't work as a point on the engine.  You gotta pick the engine CG.  Include mounting bolts in engine weight.

[Tim Wescott]

Calculating moments is valid.  Easier just to install the .061, mark the CG.  Then install the .74 and add tail weight until CG falls at previous mark.  See, no math!  So easy , a caveman can do it.

Or take of the 061, and rubber-band the 074 to the nose.  Move it back until everything balances, and that's where it goes.  Note that you'll be off ever so slightly if you have to saw a bit off of the nose to make it fit.

As an alternative:

• Weigh the plane plus the 061.  Don't forget to include the mounting screws and the prop.
• Weigh the 061 with mounting screws and prop, and muffler if you used it.
• Determine the fore-and-aft CG of the 061 in the condition that you weighed it.  It should be somewhere around the front of the cylinder
• Determine the fore-and-aft CG of the plane with the 061 mounted.
• Determine the distance between the all-up plane CG and the motor CG
• Weigh the 074 in the same condition as the 061 (prop, screws, muffler)
• Calculate the new distance between the 074 CG and plane CG.  It's equal to (old distance) * (061 weight) / (074 weight)
• Confidently rework your airplane with this new distance

OK, that was the theory part.  Now, here's the real part -- and remember, Floyd Carter and I are both engineers:

• Measure your wonderful new plane's CG
• Note that it is off
• Curse
• Rip the 074 off the nose, install it with rubber bands, and slide it fore and aft until the proper CG is found
• Mount it in that location
• Spend the rest of the life of the plane looking at the scars of the second rework and cursing yourself

Note that this method includes Floyd's method, and a whole bunch of work that is, ultimately, pointless.  Weight and balance are never, ever right the first time -- this is why one of the valid civilian uses of uranium (well, depleted uranium) is as weights on aircraft, because it's dense and thus easy to find a spot to tuck it into an airframe.  I've worked on teams that included mechanical engineers using new, fancy CAD programs with super-exact mechanical models of mechanisms and the materials that they were made of -- and when we shipped product, it always had weights attached to it.  The engineering and pre-production prototypes had tire weights epoxied on; the inital production runs had brass or tungsten weights (our machine shop balked at depleted uranium) bolted on and a few tire weights scattered here and there, as the product matured the weights would get refined so that less hand-balancing was necessary.

But they always shipped with weights.  And those weights were never found computationally.

[Mark Mc]

I'm with George, as qualified by Howard.  Instead if the drive plate, I would use the front mounting holes on the lugs as my forward datum, and the c.g. per plans to do the calculation.  Only takes a minute with the calculator you have on your computer or phone.

Mark

[Serge_Krauss]

Insert mounting bolts in holes and bolt on the prop. Then just see where along the mounting lugs each engine balances. Then set the moments about the c.g. equal and solve, as stated above. BUT,...this assumes that you have located the c.g. right. It might be easier, as suggested to just add tail weight until the plane balances in the same spot, if the weight penalty isn't too great. The point is that you don't have to guess at the location of each engine's c.g. 

[Howard Rush]

Yep. I forgot the prop.

[Howard Rush]

Here's how to find the CG of something: an engine and pipe in this case.  The strings holding the engine and the string holding the plumb bob emanate from the same knot.