Weight & Balance
Posted: Thu Nov 13, 2014 8:43 pm
by 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.
Re: Weight & Balance
Posted: Thu Nov 13, 2014 9:21 pm
by 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.
Re: Weight & Balance
Posted: Thu Nov 13, 2014 9:27 pm
by Aldo
Thank you for that explanation. It was very easy to understand! Really appreciated the time you took to write that.
Aldo
Re: Weight & Balance
Posted: Thu Nov 13, 2014 9:34 pm
by 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
Re: Weight & Balance
Posted: Fri Nov 14, 2014 12:16 am
by daleandee
Aldo wrote: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
Re: Weight & Balance
Posted: Fri Nov 14, 2014 2:48 pm
by 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!
Re: Weight & Balance
Posted: Fri Nov 14, 2014 4:00 pm
by 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.)
Re: Weight & Balance
Posted: Fri Nov 14, 2014 8:44 pm
by 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.
Re: Weight & Balance
Posted: Fri Nov 14, 2014 11:29 pm
by 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).
Re: Weight & Balance
Posted: Sat Nov 15, 2014 6:20 am
by 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
Re: Weight & Balance
Posted: Tue Sep 29, 2015 12:12 am
by rizzz
I’ll be doing my W&B this weekend and I’ve run into a question about the mean aerodynamic cord on a Sonex, which I understand starts on the leading edge of the wing and ends on the trailing edge.
However, given my unconventional engine the tip of my spinner which will be my datum point might be slightly forward compared to the 53" in the manual, hence I need to measure this again to correctly position my MAC and then calculate the corresponding 20-32% MAC positions.
However, which leading edge do I measure to? On the Sonex one wing is positioned slightly forward of the other isn’t it? (Can’t remember which is the forward one).
Should I measure both and get the average position? (Which would make sense given it’s called MEAN aerodynamic cord).
Re: Weight & Balance
Posted: Tue Sep 29, 2015 8:37 am
by DCASonex
Average of two leading edges sounds reasonable. I have a 5/8’ prop spacer and simply established the datum point for my plane as 5/8" behind the tip of the spinner. That keeps all other reference dimensions same as Sonex handbook. Should be able to do same (with different dimension) for yours.
David A.
Re: Weight & Balance
Posted: Tue Sep 29, 2015 8:49 am
by Rynoth
Good question! I just watched Joe Norris’ W&B video and he doesn’t mention using a specific wing or combining the wings, though he does use the right wing when demonstrating how to measure (which was probably just convenient to the camera angle.) I don’t think the leading edges are offset by much more than about 1/8 - 1/4" (the left wing is forward of the right wing.)
About 6 minutes in:
Re: Weight & Balance
Posted: Tue Sep 29, 2015 8:59 am
by fjdoug
I left the datum where it is and re-drew the spinner, my spinner is 1/2" behind the datum.
Re: Weight & Balance
Posted: Tue Sep 29, 2015 6:52 pm
by rizzz
Response from Sonex, “It does not matter”:
It won’t matter which leading edge you measure from as the difference is .125” at the most and that is level of accuracy beyond what matters. However, if you really want to get down to the nitty gritty just for the sake of the math, you could measure both and average. Just know that the it won’t matter as nothing on the Sonex is that critical.
Good to know, that explains why Joe does not mention this either in his EAA W&B videos.
Re: Weight & Balance
Posted: Sat Oct 03, 2015 5:02 pm
by rizzz
I did my Weight and Balance yesterday, here’s how she turned out:
Left Main: 143 kg (315.2 lbs)
Right Main: 142 kg (313.0 lbs)
Tail Wheel: 17 kg (37.5 lbs)
Total: 302 kg (665.7 lbs)
Empty CG: 22.3% MAC
Most adverse forward CG: 22.7% MAC
Most adverse aft CG: 31.8% MAC
Re: Weight & Balance
Posted: Thu May 23, 2024 5:48 pm
by STONE
gammaxy wrote: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).
Thank you very much Chris.
Are the data you posted referred to the Sonex-B?
Mario
Re: Weight & Balance
Posted: Thu May 23, 2024 6:58 pm
by Bryan Cotton
STONE wrote:Thank you very much Chris.
Are the data you posted referred to the Sonex-B?Mario
Welcome Mario! If you check out Chris’s blog or posts here, you will see he has an A model. The B will have the same datums. Check out this graphic from Sonex:
If you do a non-standard engine you may need to figure out an updated datum.
Re: Weight & Balance
Posted: Thu May 23, 2024 8:15 pm
by Skippydiesel
I stand to be corrected - Datum point is just the point at/from which all measurements are taken/rferede back to.
You do not have to use the Sonex recommended tip of spinner (especially if not using the Sonex spinner).
In fact most homebuilts use the leading edge of the wing, in recognitions that there can be diffrent engined/propeller/spinner combinations.
Re: Weight & Balance
Posted: Thu May 23, 2024 11:15 pm
by Bryan Cotton
Skippydiesel wrote:I stand to be corrected - Datum point is just the point at/from which all measurements are taken/rferede back to.
You do not have to use the Sonex recommended tip of spinner (especially if not using the Sonex spinner).
In fact most homebuilts use the leading edge of the wing, in recognitions that there can be diffrent engined/propeller/spinner combinations.
You can absolutely set any datum you want. The key thing is to make sure you have the correct percentage of MAC (mean aerodynamic chord) so you are in balance. And under gross of course.
If I had a Sonex with an engine sticking way out, I’d still use the stock location as the datum. Why confuse things? Also it was an older post but we absolutely used plumb bobs off the spinner, wing leading edges, and so on. I used to be responsible for W&B at our Sikorsky facility in NY.
Re: Weight & Balance
Posted: Sun Jun 02, 2024 9:48 am
by STONE
Bryan Cotton wrote:
STONE wrote:Thank you very much Chris.
Are the data you posted referred to the Sonex-B?Mario
Welcome Mario! If you check out Chris’s blog or posts here, you will see he has an A model. The B will have the same datums. Check out this graphic from Sonex:
If you do a non-standard engine you may need to figure out an updated datum.
Thank you very much Bryan
Mario
Re: Weight & Balance
Posted: Mon Jun 03, 2024 6:26 pm
by flyguy0609
I agree with Bryan.. The datum location is arbitrary. I’m defining the datum as being 54" forward of the wing leading edge. Then one can lift the moment arms directly from the Sonex book. I have a W&B excel file I play with.