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Thread: Help moving engine for CG....

  1. #1

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    Help moving engine for CG....

    OK-----I need some help...

    I'm restoring a VP-1. Have it basically finished (just need to install "new" engine and prop, make cowling and take to hangar....).

    Decided to temporarily mount the engine and do a preliminary W&B to see where the empty CG falls as of now........got some interesting results.

    Using the front face of the firewall as the Datum...main gear is at 23.8125", tailwheel at 187.875". Leveled the plane and got 48.0 lbs on TW, 235 lb Rt main and 226 lb Left Main. (discrepancy in rt and lt---not sure our leveling dude got the side-to-side level exactly right----for and aft much easier to assure correct!)

    I got an empty CG of 39.3"------?? The acceptable CG range in flight is 31 to 39", best CG empty is 30-32". The only variable weights are the Fuel (8 gal at 14") and the pilot at 44. These #s just don't work out..........

    So----how do I calculate the distance to move the engine forward to place the empty CG at the forward limit of acceptable range--ie, 31"?? (Engine is 180-186 lbs depending on adding the starter or not----at (-)10.5" without starter and (-)9.75" with--starter moves the CG of the engine itself 1.25 inches aft)

    I've just not been able to make the math work......

    Here's what we have:

    Rt Main --- 235 lbs x 23.8125" = 5596 moment
    Lt Main --- 226 lbs x 23.8125" = 5382 moment
    TW --- 48 lb x 187.875" = 9018 moment

    Total wt = 509 lbs
    Total Moment = 19996
    Empty CG 39.3"

    Best Empty CG range 30-32"
    Allowable in flight CG range 31-39"

    Engine wt (as weighed in above calc) 180 lbs at (-)10.5"

    * Will have a small battery to play with to fine tune CG (for ignition and starter if used)--weight/location are undetermined at this time.

    Is there an equation that I just haven't found to help with moving the engine and doing the calculated change?

    Help!!

  2. #2

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    Quote Originally Posted by Marlhalbrook View Post
    ...main gear is at 23.8125"...
    * It's accepted and expected to measure longitudinal positions no more accurately than 0.1". Measuring any finer is for the most part a waste of time and digits. Given the relatively limber nature of lightweight aero structures, you can probably grab your landing gear and flex it by hand a tenth of an inch, and that kind of blows the 0.0125" out of the water.

    * One common approach to this situation is to make a free-body diagram, evaluate the masses, arms, and moments, and solve for the engine arm that give it the moment required to place the CG were required.

    * Another common approach, and the one easiest to implement with a minimum of algebraic manipulation, is to just make a Google Sheets spreadsheet for the CG calculations and just play around with the engine arm until the summation of moments puts the CG where you want it.

    --Bob K.
    Bob Kuykendall
    HP-24 kit sailplane project

    HP-24 Project Facebook Page
    http://www.hpaircraft.com/hp-24
    EAA Technical Counselor

  3. #3

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    In order to get the empty c.g. to 32":


    Your number suggest the engine needs to move forward an additional ~20".........so I'd say something is amiss.

    48# is a lot of weight on the tailwheel, that's almost 10% of the total airplane wt. If the wt on the tail was halved if would almost nail it.

  4. #4
    cwilliamrose's Avatar
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    I go cross-eyed when there are negative numbers in CG calcs. You might consider using 100" forward of the firewall as the datum so all your numbers are positive and there's less chance of making a error. Back when we did this stuff on paper it made huge sense but even using spreadsheets I still keep everything positive.

  5. #5

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    I'm really starting to doubt our weights..........I weighed the plane myself--saw the numbers on the scale--but I'm REALLLY starting to think that there is a bad apple in the numbers somewhere.

    I come up with 22 or 22.3 inches from every calculation that I do.......that cannot possibly be right!

    This plane has almost 300 hrs on it with a 1600 cc VW that was mounted 1 inch ahead of the firewall (so CG of that engine was about 6 or 7 inches ahead of the firewall). The paperwork for the plane claims that it was within CG limits with a 240 lb pilot and full fuel. We've added nothing to the airframe---except a coat of paint and varnish (with attention to being as light as possible!) and have actually moved this engine--that is a little heavier--- forward to put it's CG 10.5 inches ahead of the firewall.

    Something is just not right.....this has been very frustrating!

  6. #6

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    Kind of a stupid question - you did raise the tail to flying configuration when weighing, right?
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  7. #7
    robert l's Avatar
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    I've always loved the Volks Planes, I & II so I'm following this. Thanks
    Bob

  8. #8

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    Frank----Yep-----

  9. #9
    cwilliamrose's Avatar
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    And a follow up question; Did you locate the main gear and the tailwheel yourself or did you get the numbers from the drawings?

  10. #10

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    Aug 2018
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    Marl,

    A formula for CG change was posted on the Yahoo Volksplane forum.

    Here it is:

    Engine weight X How far you want to move it divided by airplane gross
    weight = CG change

    ie. 175 pound engine X 2 inches = 350 moments. 350 moments / 750 pound
    gross weight airplane = a CG change of 0.47 inches.


    Hope this helps.

    Steve H

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