flyboy1963

flyboy1963

Well Known Member
Can't seem to find any reference to these on the forum.....

what is everyone using for the moment to the CG of the fuel tank?
I see most using 70" as the datum, and 76 or 77 as the moment arm of the fuel tank. This seems MUCH too close to the leading edge; my best guess for the CG of the tank as more like 6 or 7" ahead of the spar, which would make the arm something like 79.5".

I don't have my plans handy.....can anyone tell me how they arrived at their calculation? Even if the tank were a rectangle, it's profile is about what, 9" x 16" or so, then the moment arm would be half that, or 8" from datum, not 6!

Likewise, the pilot/pass moment seems to ignore the significant weight of your LEGS!....the 'published' moment seems casually placed at the seat back!

confused and doubting my numbers!!!!

thanks
 
I use 76.75" that is taken from the builder's manual. All my numbers are from the manual.
 
need some NASA engineers here! :)

tc1234c said:
I use 76.75" that is taken from the builder's manual. All my numbers are from the manual.
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I see Ted that almost everyone uses this; even the CAFE testers! My question is where the heck it came from???? It appears totally illogical to me.
 
is this math or ????

rockwoodrv9 said:
I haven't done W&B, but the comment about your legs made me think. Since they are just "resting" and not supporting your weight, the weight they transfer would be at your seat. The more you recline, the less weight on your heals. I suppose you could use 4-5 pounds for the resting weight of your legs, but probably not enough to matter much?
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so 'Rocky', I took a couple years of statics in engineer skool 30 years ago, and all I can recall ......is that there is no weight 'transfer'.....the mass is accelerated downward by gravity in a theoretical vertical vector. Yes, your entire body has a C.G., and it will move aft as you recline, and the RV seats travel will move your belly button a few inches; my question ...... where IS the real c.g. when seated...I believe it is a bit forward of your belt buckle.
 
flyboy1963 said:
I see Ted that almost everyone uses this; even the CAFE testers! My question is where the heck it came from???? It appears totally illogical to me.
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Perry, since Van's wrote the manual where the 76.75" value is found, they're probably in the best position to answer your question. In fact, they may be the only people who could answer where that value came from.

Having said that, I too use 76.75" and it works well, at least so far. My unvalidated guess, and you'd need to check with Van's, is the number is an average value that considers what the range of fuel tank conditions could be. The tanks are not symmetrical and I think the forward part of the tank empties pretty early in a flight. So I'm guessing, that full tanks may have an arm forward of 76.75 value and at some point, as the tanks empty the arm moves aft.

To me, what's important is the weight and balance calculations work. The plane is controllable and flies well within the CG range Van's describes.
 
Moment arm reality

While getting a new Airworthiness certificate for a tube and fabric antique, I checked all the moment arms by calculation based on actual weights ( certified scales). EW, full fuel, pilot in cockpit etc just to know truth. I have since purchased some 750lb digital scales for my RV3. For a few hundred dollars you can answer all these questions. Certification ( or not) does not change the moment arms. Any error is only relevant to total weight.
 
Fuel tank arm

http://www.vansaircraft.com/pdf/RV9Awb.pdf

Here is a link to the Vans web site W&B page. Note that the leading edge of the wing is 70" aft of the datum which will put the fuel arm (tank centroid) pretty darn close to 76.75" by my eyeball. Note that this is the calculated centroid of the tank not the physical center of the tank.
 
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In the real world it is pretty hard to load my rv9a out of CG range. I am running an 0-320 with wooden prop. My wife and I both weigh under 300 lbs. by 40 lbs. I have run a bunch of different fuel and baggage scenerios and it always comes in within limits. Every time I fly my 9 I like it just a little bit more.:D
 
I too have not found a realistic weight/loading scenario that puts the 9A out of CG range. ... Another nod to superb design by Vans. :D
 
I did W&B calculations to check CG and nose gear load under different scenarios. I was satisfied that under all my operating conditions I am fine. In real life I have loaded the plane to the gross and flew at high elevation (Leadville, CO at the extreme). I found the plane performed to my satisfaction.

My W&B spreadsheet
 
tanks everyone! :)

Ted, so much great info on your site!

I'll use the W&B spreadsheet to satisfy myself, at least until I can get on the scale.
As everyone notes, even if it isn't perfect, you are unlikely to get into trouble!
heck, the calculation show the gross at something like 2100 lbs!:eek:

as you fill the tank, the weight moves forward, then slightly back as the tank is topped. The average of all this, with the dihedral taken into account, is probably within a few tenths of an inch!

but to ask another question, what's all this 'baggage #!, and Baggage #2" moments? I know I count the tools stuffed right behind the seat differently than the stuff way in the back, just to attempt to be more accurate.
.....but what are you guys talking about????
 
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