facebook js

Follow Planet-Soaring on Facebook

Sunday, February 12, 2017

[HOW TO] Calculate the CoG from the weight


Given the number of questions I still receive :) , I see that the concept of measuring the CoG location by measuring the weight at the leading edge and X mm behind is not fully understood, so let's explain again with a real case, my Stribog CoG:

2 kitchen scales measure the weight of the plane, separated in my case by 293 mm. The first scale measure the weight at the vertical of the leading edge, the second one 293mm behind, so behind the trailing edge.
The sum of the 2 weight measured give the weight of the model. Now we apply the formula of a barycenter of a barrel that says: a x p = b x p'.


In other words, the if I measure p and p' , and also that I know the distance a+b (293mm in my case), so that b = 293 - a, I can easily calculate the CoG location.
So now, let's do it for real. My Stribog is placed on the device: I measure 1475gr at the leading edge, and 834gr at 293mm of the leadging edge. After entering the formula in Excel, it gives me a CoG located at 105.83 mm from the Leading Edge.



Now if you want to set the CoG of a Plane, the formula give you the ratio between the 2 weights. The only inconvenience is that as you add or remove some weight in/from the nose, you need to recalculate every time you do an action.


Also, I noticed something critical, at least with the model of kitchen scales I used. The link between the 2 scales must be the most free possible (vertically and in rotation) to not disturb the measure of one scale. Also, I had to remove the scales from the wood plate I prepared as I suspect the scale to need only 4 contact points to work properly. The weight reading was totaly false until I realized that.


17 comments:

  1. Hi Pierre, It's a good explanation, but I think it's important to mention that the fuselage must be horizontal during measurement. Otherwise you can measure with a small mistake. In real aircraft weighing the aircraft pitch plays an important role. It is always taken into account during center of gravity calculation. (Boeing is using a correction table for this and Airbus has a formula with the alfa pitch value.)

    ReplyDelete
    Replies
    1. Thank Roland! This is a very valid point that I forgot to mention.

      Delete
  2. This comment has been removed by the author.

    ReplyDelete
  3. Hi Pierre,
    I made a similar CoG tool, with the automatic calculus of the additionnel weight to obtein the desirated position.
    You can find it here : https://drive.google.com/file/d/0B3DuQRBtCWWJSG9zMlU0WjV3N2M/view
    Franck

    ReplyDelete
  4. Hi Pierre,

    it looks good. Can you provide me a source data for the 3D print, it's possible?
    Thank you.

    Petr H.

    ReplyDelete
  5. Bonsoir Mr RONDEL,

    J'aimerai reproduire votre balance de centrage. Acceptez vous de partager les fichiers STL des pièces imprimées en 3D?

    Merci,

    ReplyDelete
  6. This comment has been removed by the author.

    ReplyDelete
  7. Now I have the idea that how to calculate cog weight from weight counting machine. Pallet Scales are also widely used to check weight accurately.

    ReplyDelete
  8. This particular looks for example certainly superb. Most of these slight tips are made using availablility of cosmetic foundation interest. I like to him or her appreciably. Tulobuterol HCL

    ReplyDelete
  9. These machines can measure the weight of the objects weighing between 150 grams and 600 pounds. Further, precision scales are used in laboratories and jewelry shops. best kitchen weighing scale

    ReplyDelete
  10. While all manufacturers will proclaim that their products offer high levels of accuracy, only the top brands will usually be able to deliver the best accuracy. load cell

    ReplyDelete
  11. However, the inquiry is the reason are probiotics useful for weight reduction? 60 kg to lbs

    ReplyDelete
  12. A good blog always comes-up with new and exciting information and while reading I have feel that this blog is really have all those quality that qualify a blog to be a one. как да отслабна в краката за 1 седмица

    ReplyDelete
  13. This, quite simply, is incorrect as we could never recommend using a pallet scale to weigh letters, or vice versa. scales for weighing

    ReplyDelete

F3G and F3B in Kulmbach / Germany, a tradition continues!

From May 10th to May 12th, 2024, the Kulmbach model aircraft association organized a combined competition on its own model aircraft site nea...