Weight and Balance

[thammer]

I recall some discussion of factory weight and balance not too long ago, I thin it was on the 140 site. The gist of it was that only every 5th or 10th aircraft required actual weight and balance to be done. I'll see if I can find that info and where it came from.

[johneeb]

Bruce,

I think Eric is right about the Factory calculating the weight and balance, I have heard this before and if you look closely at the factory W&B form not only are there no numbers in the weight columns the form says calculated where the numbers would be if the airplane had been weighed and the math had been done.

Eric,

I used the numbers you posted (1229lbs and 37.6ins) and assumed standard main gear position (22" aft of datum) and Scott 3200 tailwheel postion (249" aft of datum) I came up with 572lbs on each main and 85lbs on the tailwheel. Your new straight 170 EOW is quite a bit lower the my new B model, 1283 - 1229 = 54lbs so thats your speed secret!

[zero.one.victor]

John, who even knows if that factory figure of 1229 is accurate? A 1229 pound 170 would be a pretty good performer though! That 1229 must be for a stone stock, bare, factory product: no super-homer, no paint (except maybe trim), thinly-doped wings,no beacon,etc.
The lightest 170 I've heard about (the lightest believable one anyway!) was Kelly Mahon's B model: 1250# with stripped ext, gutted interior, 3 seats,minimal panel. I'm betting most are in the 1350-1400 pound range, with plush interiors, vacuum systems,radios, paint,180/Ponk gear mods, etc.
c170B53, you asked about pilot & pax "moments"- I'm guessing you actually meant "arms". Weight times arm equals moment. FWIW, my ragwing W&B shows front seaters at 36" aft of datum (firewall),backseaters at 70",baggage at 95",fuel at 45",and oil at <20">. The loading graph shows acceptable CG (normal category) at 1700# to be anywhere from 36.3" to 45.2", and at 2200# anywhere from 40.9" to 45.2".
When I measured station arms for calculating the W&B change from replacing the bench rear seat with the single jump seat, I came up with about 76" for the bench and about 72" for the jumpseat. This was about where I figured the actual CG of the seats themselves are.

Eric

[c170b53]

Ooops made a few mistakes on my weigh and balance report for my machine thanks to all for the info. I made a mistake when I used the term moment as Eric rightly deduced I wanted to know arm data for passengers and fuel. As for the arm for my gear s/n 25656 I measured it at 19" from gear centre to the datum. The engine is an 0-320 and I'm thinking that my aircraft would weigh the same as 180 hp powered 170's.
Also I did not add the 30lbs and the appropriate moment to the weighed empty weight per the Type data sheets therefore my empty weight should be 1445 Lbs. Yikes

[zero.one.victor]

If your MLG is at 19" I'm betting you have 180 legs. Maybe the Ponk gearbox mod too? It all adds up, along with paint,gyro's, lighting, electronics, bigger engine,carpet, upholstery,longer battery cable (if the battery's been moved to the back),etc etc. And pretty soon that nice light bare-bones airplane which left the factory weighing around 1283 or even 1229 weighs 1300-1400 pounds, or more.
This is why some of the backcountry guys pull their electrical systems and all other non-essential goodies: light weight = better performance. My next airplane is likely gonna be a tube & fabric, light-weight,negative-electric two-seater of some sort.

Eric

[Robert Eilers]

Today I met with the A&P who wighed my 170B and prduced the 1376 EW and 37.26 EWCG which caused me loading problems. We went over the numbers and his method of obtaining them in detail. He was able to document how he arrived at the numbers to my satisfaction. However, I am left with an EW and EWCG that require ballast aft even with just two 180 pounders and full fuel. His number for the main gear was 21" vs. Cessna's 21.375. His number for the tailwheel was 248" Vs. Cessna's 249". I found it amazing how much difference such small measurements can make. I am not certain why my aircraft is nose heavy, perhaps the instrument panel. The A&P showed me examples of several other 170B's he had weighed and they all seemed to be at or very near the forward CG limit. Thanks for all the advice and recommendations regarding this matter. Looks like I am just going to have to live with the numbers.

[blueldr]

Robert Rilers
Be sure that you understand that the loading index shown in the POH is a function of the moment and is NOT the CG. Your comment about needing aft ballast sounds as though there may be some confusion.

[GAHorn]

And Robert,...what was his explanation/approval basis for using ARM's different than the data Cessna publishes? (Has someone relocated the tailwheel?) Did he use Cessna ARM's for the seats, etc.... or his own?

[c170b53]

I'm replying with some numbers for comparison. My Wheels are at 19" as I have the lady legs. My tailwheel is at the 249 cessna dimension and I did a physical measurement as well. The tailwheel has new springs and a scott 3200. I did use H.D's STC to move the battery aft to compensate for the Compact Hartzell on the nose. In terms of weight I'm guessing that there's at least 15lbs of battery cable. The battery is in the back as I couldn't except flying around with a bag of lead to trim the airplane, but I guess really all I've done is trade copper for the lead.

[N170CT]

Well, FWIW...c170b53 has described the most accurate method to weigh any aircraft. Believe it or not, Boeing (Long Beach) weighs each and every commercial aircraft after painting and before fuel is introduced into the tanks. Unusable fuel is calculated. We use a plumb bob, chalk, tape measure and a spirit level. This is because a) there are always variations in manufacture/tolerance, and b) the axle centerline or midpoint between axle centerlines on multiple bogeys is the precise reaction point on roll-on scales. Assuming the a/c is level in pitch, this method will nail the longitudinal cg providing one uses the manufacturer's datum point (plumb bobbed to the floor) as the reference for all measurements. Lateral cg is generally ignored after checking and weighing the first two or three examples of a model. In actual practice, large differences in MLG weights usually makes one suspicious of the scales. The airlines don't weigh their aircraft often. They are generally satisfied using our "as delivered" weight and cg values with calculated corrections for addition or removal of equipment. Curiously, the same facility usually weighs every fifth USAF C-17. The Military has different requirements.
If I were weighing my 170B, I would concentrate on defueling and leveling the a/c followed by accurate measurements ON THE HANGAR FLOOR between the datum point, the main gear axles centerline, the tail wheel axle centerline and with eight quarts of oil in the sump. Empty fuel tanks are better because the manufacturer's unusable fuel values are much more accurate (+/- 0.5 gallon) compared to one's idea of "Full Tanks" which can vary by as much as 2 or 3 gallons. Please note, I am addressing accuracy not the amount of unusable fuel here. Don't forget to remove everything that ain't bolted down/permanently installed from the interior of the aircraft/baggage compartment and place the seats in a normal flight position. My two centavos worth....chuck

[zero.one.victor]

I agree with Chuck, when I did the W&B on mine I got out the string, plumb-bob, and tape measure. That way I knew the arm's were correct. I have tthe later 180 legs on mine so the MLG arm is 18.5, not 22. And the t/w arm was 249.5, not 249.
BTW, the biggest pain was trying to weight the tail. It's really a hassle to try to level the airplane using blocks, etc under a scale. I ended up rolling the mains onto the scales, then picking the tail up using an engine hoist. I hung the t/w from the hoist using a deer scale, that made it real easy to get the pitch attitude set just right for level. Then just read the scales. If you insisted on a regular scale, you could level the airplane using the engine hoist, that'd give you the total amount of blocks/scale you needed for that level attitude. Then re-rig using the blocks/scales. Most people seem to use a shop table, rollaway, or something like that under the scale.

Eric

[johneeb]

Eric,

You know what I like is a stack of what ever than a motor cycle jack and than the scale. Once you get this all under the tail wheel you can make your small height adjustments by turn the jack screw on the jack.

We just happen to have a couple of these motor cycle jacks aroud the hangar not for motor cycles but because they work well jacking up anphibious floats to do a gear swing.