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Thread: Help moving engine for CG....

  1. #11

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    Quote Originally Posted by Frank Giger View Post
    Kind of a stupid question - you did raise the tail to flying configuration when weighing, right?
    Next question: Was whatever used to raise the tail somehow included in the tail wt? There is too much wt on the tailwheel.

  2. #12

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    cwilliamrose......

    Plane is a restoration-----has about 300hrs TT. All we're doing is replacing the engine that was separated from the airframe by the previous owner (actually the guy before the immediately previous---but that's not important..:-) ) and cleaning her up, repaint, minor repairs of hangar rash, through inspection of structure.... The plane is built according to the plans. Maingear and TW are in correct positions per the plans.
    Last edited by Marlhalbrook; 08-09-2018 at 08:33 AM.

  3. #13

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    Quote Originally Posted by martymayes View Post
    Next question: Was whatever used to raise the tail somehow included in the tail wt? There is too much wt on the tailwheel.
    Marty----

    No. Built a tower of concrete blocks--then used thin pieces of ply to shim the scale to level the plane with the tailwheel directly on the scale---no tare involved. Took 5 weights and averaged to 48 lbs....(all were 47.9-48.1)

  4. #14
    cwilliamrose's Avatar
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    Quote Originally Posted by Marlhalbrook View Post
    cwilliamrose.......... The plane is built according to the plans. Maingear and TW are in correct positions per the plans.
    Just a thought. I have seen a lot of people go to all the trouble of leveling the airplane to do a W&B and not take a few minutes to measure the weight points. I hope you find the answer.

  5. #15

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    Steve...

    That's a simplification of the formula w/W=d/D where "w" is weight to shift, "W" is total weight of aircraft, "d" is change in CG, and "D" is distance the weight is moved.

    Look at it another way, D= (W x d)/w if do the calculation, just to move the CG 6 inches forward from 39 to 33, D= (509 x 6)/180 = 16.9 inches (!!)

    I think the error in this is that the engine is such a high % of the total wt that moving it with this simple equation doesn't take into account the reduction in the weight of the tail by moving the weight forward. If i move the engine forward, the weight on the maingear will increase and the weight on the tail decrease---as the moment for the tailwheel decreases, the need to balancing weight on the nose decreases also......I don't see how this ratio equation accounts for that-----but it's been a LOOOOOOOOOOOOOONG time since I studied math!


    I did find this spreadsheet on the KC Dawn Patrol webpage (www.dawnpatrol.org/Wghtbal.htm). It is VERY handy. Helps crunch the numbers of a lot of variables and keep it all straight.....playing with it last night, I found some really interesting info.

    Looks like pilot weight makes minimal change to CG---fuel, on the other hand, is more dramatic.... Also, adding oil and prop, plus placing a battery on the firewall (which now has room for one since the engine is moved forward 3 1/2 inches from the "normal" position.....plus possibly a starter...the CG moves mathematically to a range that is totally acceptable---but in the back half of the allowable CG range....all this---AT WORST CASE OF HEAVY PILOT AND ENGINE NOT MOVED FROM PRESENT LOCATION, FULL FUEL TO EMPTY........the biggest limitation becomes the gross weight of the plane (the original paperwork states gross at 850 lbs with a reduced "G" loading to essentially the equivalent of Utility Catagory (+4/-2 G).) and climb performance.

    I've found multiple photos of other VP-1s with engines that appear to have engine mounts up to about 10 inches ----so, obviously, this is not the 1st plane to have to address aft CG. I've read in many places about VP-s tending to be tailheavy----makes me wonder how many are flying that have never had a good examination of the actual CG of that particular plane.......hate to say, but many of these birds were built in a time when folks didn't have all the resources we have today---so many may have just "build it like the plans"(for the most part), hung an engine on it, and ASSUMED that it was good to go. Copy the CG info from the plans for the FAA paperwork requirements and off you go into the wild blue yonder........you KNOW that happened, I just wonder how common that may have been. The benign flight characteristics and very powerful controls of the design prolly hide a multitude of sins on some of these early homebuilts......

  6. #16

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    Quote Originally Posted by cwilliamrose View Post
    Just a thought. I have seen a lot of people go to all the trouble of leveling the airplane to do a W&B and not take a few minutes to measure the weight points. I hope you find the answer.

    Know what you mean.....I'm old school....don't trust the plans dimensions. Dropped plumb-bobs with plane level---have centerline, datum, CL of mains and tailwheel drawn on my shop floor. (actually still have the same for our Starduster Too, Kolb MK III Classic, Rans S-9, and Long EZ on the shop floor---and EAA Biplane in our garage.......maybe I need to repaint the floor!!

  7. #17

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    I still have one little piece of tape on the floor of my hangar for when I marked for my W&B...and hollered at someone who was going to pull it up the last week.
    The opinions and statements of this poster are largely based on facts and portray a possible version of the actual events.

  8. #18

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    The CG numbers for the engine sound a bit off. From your description, the CG for the engine and prop combination is 9.5 inches from the firewall? That would indicate that the the CG point of the motor would be just about at the trailing edge of the back cylinder. CG should be more like the aft side, or center of the forward cylinder.

  9. #19

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    Quote Originally Posted by CraigCantwell View Post
    The CG numbers for the engine sound a bit off. From your description, the CG for the engine and prop combination is 9.5 inches from the firewall? That would indicate that the the CG point of the motor would be just about at the trailing edge of the back cylinder. CG should be more like the aft side, or center of the forward cylinder.

    CG of the engine sans prop --- you are very close to spot on. It balances almost exactly between cylinders 1 and 2----actually somewhat closer to the leading edge of 1. (this includes the dizzy, oil pump, y-pipe, Revmaster carb, but not the prop. We actually balanced it on a dowel rod on the welding table to prove it to ourselves....).

    As the engine is mounted on our temporary mount---the balance point of the engine is exactly 10.5 inches ahead of the front face of the firewall (our datum) so (-)10.5 inches for the moment.

  10. #20

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    What happens with the numbers when you add the prop in?

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