Weight & Balance

[Aldo]

Can some one explain to me where the CG is when the specification for it's location says "20% - 32% Wing Chord? All of the aircraft I am familiar with always express the CG in terms of inches from the Datum. The Sonex spec expresses CG as mentioned here.

[GordonTurner]

First a few terms...

The percent is a percent of "MEAN AERODYNAMIC CHORD" (MAC). In a straight wing airplane like the sonex this is easy, the aerodynamic chord is the distance from the leading edge to the trailing edge. Since not all wings are straight (delta, swept, double swept, ellyptical, etc), the MEAN is the average of the chord over the whole length.

For a basic aeroplane the CG needs to be somewhere around 25% of MAC for the aeroplane to balance. This means 25% of the distance from the leading edge to the trailing edge. There is always some acceptable range depending on the characteristics of the airfoil and a few other factors, in this case anywhere from 20% to 32% is acceptable.

The aerodynamic engineers are mostly concerned with w/b in this way, as a function of the wing. But once that range is determined, it can sometimes be more understandably expressed as a range of distance from some DATUM point like the nose cone or the pilots relief tube, whatever is a convenient point to measure all ARMS to calculate the MOMENT of each MASS, which is it's effect on the CG. Regardless of how it is expressed, any of the methods of w/b calculation are calculating the same thing.

[Aldo]

Thank you for that explanation. It was very easy to understand! Really appreciated the time you took to write that.

Aldo

[tonyr]

Hi Aldo,

If you need further clarification have a look at the EAA weight and balance video presentations by Joe Norris.

this is part 2 but they are all about the same date December 13 http://eaavideo.org/video.aspx?v=2897335227001

They are excellent and use a Sonex specifically for the example aircraft, computing the arms, moments, weights mac percentage ranges etc.

Regards
Tony

[daleandee]

Thank you for that explanation. It was very easy to understand! Really appreciated the time you took to write that.

I believe that every builder and pilot must have a good working knowledge of weight and balance and CG for any aircraft they are going to fly. During my airworthiness inspection my DAR grilled me on the numbers I had, how I got them, and the effect the different numbers would have on CG. We went methodically through all of my numbers so he could see that I knew what I needed to know to be safe.

Having said all of that ... there is a weight and balance calculator on the ASA site that's on the "builders page" under "downloads." This is not an easy way out as you are still required to confirm that the information given to you is correct for your aircraft. But once you have done that this spreadsheet can make it easy to see the effects of moving weight around.

http://www.americansonexassociation.org/builderspage/index.html

Dunno if this helps,

Dale
N319WF

[Pickleman]

Mr, Turner-- I re-read your post several times just to marvel at the clarity of thought! What a clear, concise, and accurate description of the process!

[Sonerai13]

In my years as a flight instructor giving many flight reviews, my years at EAA giving forums at various events, and now my time here at Sonex giving transition training, it has continued to amaze me how many pilots and builders don't really have a good understanding of weight & balance. Especially why it is important and how it actually works as related to the controls and control surfaces of the aircraft. This is why I continue to give the presentations, why I did the EAA Hints for Homebuilders videos, and why I continue to encourage pilots and builders to really get their arms around what weight and balance is all about. It's really not a hard concept at all, but my experience has shown me that many flight instructors have not been taking the time to help their students fully understand it. (I fear that these instructors don't have a clear understanding themselves.)

[Aldo]

Perhaps the point of my question is being missed. I am very comfortable with my understanding of weight and balance. Coming from the world of certified aircraft, however, I have always worked with datums, moment arms, Etc. I can calculate a CG and know how it's movement will affect the aircrafts performance. What I find confusing is the way Sonex expresses CG as a % of the wing cord. I had not seen it expressed that way before.

To be clear, if someone could tell me the datum, arms and fore and aft limits - I would be able to calculate the CG and know if I am within limits.

[gammaxy]

The Sonex wing chord (Mean Aerodynamic Chord or MAC--distance from leading to trailing edge) is 54". The 20%-32% utility CG limits correspond to 10.8" and 17.3" aft of the wing's leading edge (54" * 20% and 54" * 32%). You could use the leading edge as your 0 datum, but most people (everyone?) follow the example in the flight manual: http://www.sonexaircraft.com/support/manuals.html where the 0 datum is the tip of the spinner and is defined in the manual as being 53" ahead of the wing's leading edge. Putting the datum at or beyond the spinner makes it so you never have to mix negative and positive datum locations.

In this case, the forward and aft limits are 63.8" and 70.3" (10.8" + 53", 17.3" + 53") aft of the tip of the spinner.

To complicate things slightly, I doubt most people plumb bob from the tip of the spinner. They probably either use the leading edge of the wings or more likely the forward corners of the aluminum fuselage sides and add an offset to get the distances relative to the spinner (the fuselage corners are also defined in the manual, but I don't have my manual with me now, but it was the most convenient measurement for me to use).

[Aldo]

Thank you Gammaxy! Exactly what I was looking for. Appreciate it very much. Along with the concise explanation of how to use the % wing cord presented earlier, I have a clear picture of what is being calculated.

Aldo