SkyDemon Forums

W&B - does Skydemon calculate properly?

By Daniel_KDF - 10/2/2017 8:11:21 PM


I‌ am trying to get the Cessna 152 W&B from the flight manual into Skydemon.
F‌irst I don't get it why Skydemon only offers a weight vs lever arm model, while a lot of W&B flight manuals have a weight vs ‌moment model. But ok, lets just divide the weight out of all the flight manual drawings and hope for the best....

U‌nfortunately ‌once I was finished, the Skydemon calculations did not match my manual calculations according to the Cessna flight manual. For debugging purposes, I set all all the weights and fuel to zero. So I was expecting to find the empty weight in the W&B chart. 
B‌ut as it can be seen, the W&B charts shows 539,8 kg as takeoff weight, while the empty weight is actually set to 545,5 kg.

A‌m I doing something wrong / am I not getting something here? Or is Skydemon not able to calculate W&B properly?


By jfw - 10/5/2017 7:52:52 AM

GEAR - 10/4/2017 4:14:01 PM
SemperFi - 10/4/2017 8:21:38 AM
Shouldn't the levers for a 150/172 be unit cm instead of meters, to get the W&B moments calculated correct for a ramp check / compliant to the POH? AFAIK, ‌Cessna does do W&B on 1xx series on <kp> for weight and <cmkp> for moments. Not sure right away, but I to believe to remember that the moment change due to fuel was calculated wrong when we tried meters.

Mathematically, it does not matter what units are used for lever arms, providing that all lever arms are expressed in the same units.  However, if you want to make direct comparisons with the POH data and charts it is easier if masses/weights and lever arms are expressed in the same units as those used in the POH, otherwise you need to be absolutely sure that you are applying the correct conversion factors.

My POH uses Lbs for weight, US Gallons for volume and inches for lengths.  However in the UK we buy fuel in litres, express mass in Kg and use either Inches or cm for length.  My SkyDemon W&B model uses litres for volume, kg for mass and inches for length.  I can readily compare my cg location with the POH to ensure compliance.

Hence my suggestion (that was never picked up) to have the possibility to specify per aircraft in which units you enter the info. which makes it easy to compare with the POH. cfr