W&B envelope

[jfraizer]

Hi all,

I'm seriously considering purchasing a used Legacy SONEX. To that end, I'm trying to work out a W&B profile for ForeFlight and I'm sorta at a loss.

W&B info for the airplane

Empty Weight & CG
Left Wheel WT: 216lb ARM: 78.5" MOMENT: 16956
Right Wheel WT: 255lb ARM: 78.5" MOMENT: 20017
Nose Wheel WT: 176lb ARM: 26.5" MOMENT: 4664
Empty Weight: 647lb
Empty CG: 64.3"

Most Aft Weight & CG
Aircraft Empty = 647lb Arm: 41637
Front Seat = WT: 170lb ARM: 91" MOMENT: 15470
Bagage = WT: 0lbs ARM: 105" MOMENT: 0
Fuel = WT: 60lbs ARM: 47" MOMENT: 2820
Weight = 879lb MOMENT: 59927
CG = 68.3"

Most Forward Weight & CG
Aircraft Empty = 647lb Arm: 41637
Front Seat = WT: 340lb ARM: 91" MOMENT: 30940
Bagage = WT: 0lbs ARM: 105" MOMENT: 0
Fuel = WT: 80lbs ARM: 47" MOMENT: 3760
Weight = 1067lb MOMENT: 76377
CG = 71.5"

Does anyone have ForeFlight W&B numbers I can plug in for Forward / Aft CG limits and the station arms for the cockpit seat, baggage station and fuel tank? I have tried doing this myself but, I'm ending up with a max TOW of 960lbs (16gals + 310lbs PAX) to stay in balance. Anything over 310lbs of PAX puts it out of balance with AFT CG. I've obviously got something wrong because I can't imaging SONEX approving 1150lbs Max Gross (JAB3300 engine) if it threw CG so far AFT.

Here is what I have in ForeFlight. I'm obviously doing this wrong.

BEW CG: 64.3"
BEW: 650lbs

Stations: Pilot = 91" Baggage = 105" Fuel = 47"

Forward CG Limits:
64.3" 675lbs
64.3" 1100lbs
64.3" 1150lbs

AFT CG Limits:
68.5" 1150lbs
70.25" 1075lbs
71.5" 675lbs
71.5" 1025lbs

Thanks for your help!

John

[Bryan Cotton]

John,
There are a few spreadsheets out there.
viewtopic.php?f=3&t=7071&p=53400&hilit=Spreadsheet#p53400

http://www.sonex604.com/misc/N604CM_WB.xls

Mine is here:
viewtopic.php?f=58&t=6965&p=52750&hilit=Weight+and+balance#p52750

Edit: your arms for pilot, baggage, and fuel are different from what Sonex uses.

[Bryan Cotton]

One thought, the datum is specified as the tip of the spinner. That can change based upon the build. My wing leading edge is 52" behind the spinner tip.

[jfraizer]

John,
...
Edit: your arms for pilot, baggage, and fuel are different from what Sonex uses.

Hi Bryan. Thanks for the reply. I plugged in 77" for pilot 102" for baggage and 45" for fuel. Now, I can "just" in balance at max gross. One more question: This is a tri-gear Legacy Sonex. Is there a difference in these arms for the tri-gear? I wouldn't think there would be since the datum is the tip of spinner but, there might be a difference between the Legacy and the "B" model Sonex.

Thanks,

John

[Bryan Cotton]

John,
My Waiex is a legacy. Your three wheels will have different arms than my taildragger, but the datum should be about the same. Part of the W&B process is to figure out the relationship between the arms and the Mean Aerodynamic Chord (MAC) of the wing. The CG with respect to the MAC is the most important thing but by convention we calculate CG with respect to the datum.

My understanding is the B models are just wider in the nose, but should have the same arms for pilot, fuel, and baggage. More or less.

[jfraizer]

One thought, the datum is specified as the tip of the spinner. That can change based upon the build. My wing leading edge is 52" behind the spinner tip.

One thought, the datum is specified as the tip of the spinner. That can change based upon the build. My wing leading edge is 52" behind the spinner tip.

I used the numbers from the N604CM W&B to get the ARMs since they have a Jab3300 as well. Unless there is a difference in the ARMs for Legacy vs "B" model, those should be correct. The W&B info I have for the airplane is now in serious question based on the ARM they used for the pilot especially.

[Bryan Cotton]

I am pretty sure Jeff's Sonex is a legacy, not a B model. That's for N604CM.

[Bryan Cotton]

B model vs legacy:


Should be no difference in the arms. That pic is from the Sonex website.

[jfraizer]

OK. Now, to recalculate the max forward and max aft CG using the real ARMs. lol. There is no way the current W&B doc is accurate. The current owner flew the airplane with two "normal" sized people in it (close to max gross if not over). According to the current W&B documentation for the airplane, it would be impossible. Not enough down elevator in the world to overcome the supposed AFT CG. I don't know how it passed muster to even do test-flights.

[Bryan Cotton]

It does sound like they screwed up. Check out our builder W&B database:
viewtopic.php?f=21&t=2933

Also I believe nosegear aircraft are typically more forward CG than tailwheel.