Weight and Balance

Dave Clark · Post by **Dave Clark** » Tue Mar 04, 2003 1:05 pm

I am interested in getting away from the owners handbook method of weight and balance with the goofy "index" value and going to a more conventional system. Any comments pro or con? Does anyone have the datum data for the front seats, rear seats, baggage and fuel? I could measure it but was wondering if there was anything published

Also if anyone has a form or fillable form I would be interested in that. Thanks

Post by **Bruce Fenstermacher** » Tue Mar 04, 2003 3:53 pm

All the datums your are looking for are found on the TC. They are the plus or minus numbers found in the parentheses. You can also use most of the datums from the CM100 manual for the 172. All of the datums I have checked from this manual are the same but some could be different so use caution here.

The front seats are said to be at +36. Of course the TC doesn't say where the seat is adjusted in the seat track at +36 but I beleive it is in the middle.

As I recall the 170 instrument panel is at +16 (0 is the front of the firewall) so if you really wanted to you could figure out where in the seat the center of mass would be and measure from the instrument panel to that point and add 16 inches. You could also do a worse case scenerio with the seat all the way forward and all the way back.

I have created an Excel spread sheet for my aircraft which allows me to plug in various weights. The CG is shown both numerically and graphically. I'd be happy to share this with anyone who emails me privately

zero.one.victor · Post by **zero.one.victor** » Tue Mar 04, 2003 11:06 pm

Dave,the original factory W&B for my ragwing (dated 9/13/48!) uses the firewall for a datum.

pilot/co-pilot @ +36" arm
rear seat pax @ +70" arm
baggage @ +95" arm
fuel @ +45" arm.
Legal CG per POH is between +36.3" and +45.2".

Hope this helps. It might be a little different for a B model.
Is it flying yet?

Eric

Dave Clark · Post by **Dave Clark** » Wed Mar 05, 2003 1:07 am

Thanks Bruce and Eric that will fix me up. Buce I'd love the spreadsheet and will email you for it. Eric- I just got approval from Avemco for $60k limit and expect to fly it late this week. I spent ALL day Sunday on paperwork and am not done with that yet. It's sooooo close to ready... and I'm really horney to get in the air again!

Dave Clark · Post by **Dave Clark** » Thu Mar 06, 2003 1:08 am

Hey gang this spreadsheet is the neatest thing since sliced bread!!!!! I reccommend you try it even if you don't know excel it's real easy. Oh it's so neat!!!

wa4jr · Post by **wa4jr** » Sat Mar 08, 2003 3:25 pm

I may get into trouble here, but I am just curious as to what other 170 owners are doing with regard to W&B. Is the spreadsheet thing something that you do before every flight? I do have current W&B figures for my 170B, but as long as my total weights of passengers, baggage, and fuel do not put me more than 50 or so pounds over max gross weight, I usually go...depending on density altitude and runway length. I don't worry about CG as I find it difficult to envision loading the aircraft out of CG unless of course you do something really dumb like put a 300lb engine block in the baggage area...in which case the weight limit is also exceeded. Since I am not loading a stretch DC-8, I don't see the need for figuring a full W&B for every flight...especially the CG portion. OK....let me have it!

Post by **Bruce Fenstermacher** » Sat Mar 08, 2003 9:47 pm

John
I don't use the W&B spread sheet before every flight cause I have already used it to look at several loading scenerios. This takes no time at all with the spread sheet. I have created and done this with all 6 of the aircraft that I am current in.

I would only then use the spread sheet if i was going to do something out of the ordinary like tieing a row boat under the airplane or filling the cargo area with bowling balls.

Now lets see. On my 170 if I have 8 quarts of oil, a pilot and front seat passenger both weighing 220 lbs and full of fuel I'd be forward of the CG limit. We all know it will fly. I'll just have to be extra careful the engine doesn't quit causing a forced landing and an investigation. That could be a problem.

dkalwishky · Post by **dkalwishky** » Sun Mar 09, 2003 3:43 pm

John;
I use a spreadsheet to calulate my weight and balance before I fly. I keep several sheets in the plane that show different loading configurations. I'm a bigger guy so I always have to have some balast in the baggage compartment to keep me from being to nose heavy.

Mon Mar 10, 2003 11:55 pm

I now have a 48 170 version of this spread sheet. I hope all those I sent out already where for the A and B model. If not email me again and I'll send the correct one out to you.

I found an inconsistency in the TC with regards to the unusable fuel in the 48 170. The max fuel is 37.5 with usable fuel being 33.5 which leaves 4 gallons unusable. But note 1 of the TC says that the 48 170 would have 19lbs of unusable fuel at +53. Problem is of course that at 6lbs a gallon that is only 3.167 gallons of fuel. Am I missing something here. Wonder how these planes have stayed in the air safely this long.

zero.one.victor · Post by **zero.one.victor** » Tue Mar 11, 2003 6:12 pm

Bruce,my 48 has the second LH tank for a total of 50 gallons. It's placarded for 46 usable.
I did some measuring on my 170 yesterday,I now have my doubts about the arm shown by Cessna for the different stations--pax,baggage,etc. I measured aft from the firewall and marked the outside of the fuselage for the various stations--36" for pilot/copilot,70" for pax,95" for baggage. In every case,this measurement appeared to be a little too far forward.For example,the pilot arm works IF the seats are all the way forward AND the occupants sit on the edge of their seat. Same edge of the seat deal for back seaters,and baggage arm was at the extreme forward edge of the baggage compartment. Kinda makes the whole W&B thing suspect.
The reason I was checking this out,my new jumpseat positions the passenger approximately 5" farther forward than the stock bench seat. I will need to revise my empty W&B using different arms for the (removed) bench seat and the (added) jump seat,and also use a lesser arm for figuring passenger into loaded W&B calc's.

Eric

Tue Mar 11, 2003 10:22 pm

I haven't checked all the datums on my plane but the ones I did seem pretty accurate. As for the datum positions being all the way forward this might have been on purpose. After you do a few W&B with the spread sheet you will see it is very difficult to get the plane out of CG range to the rear but is easy to go to far forward. If people adjust their seats or sit back further there is roam for error.

Also you may have some error measuring around the outside of the fuselage (maybe not 5 inches) . The datum needs to be measured straight down the center.

I just rechecked my service manual and realized the instrument panel datum is 16.66 not 16 inches as I instructed some people that I sent the spread sheet. So those that I instructed wrong please take note.

Bruce

zero.one.victor · Post by **zero.one.victor** » Wed Mar 12, 2003 6:51 pm

Dave,after a little investigation,I found that Cessna sez the arm for fuel in the A & B models is 48",not 45" like the ragwing. I also found that while the ragwing fuel arm is 45",the arm for unusable fuel is 53". Interesting....
I'm planning on weighing my airplane and doing a whole new weight and balance sheet. The current W&B sheet is not at all accurate,but I don't have the sheet from when the thing was actually weighed back in the mid-1980's. I suspect they did not properly deduct the weight of gas that was in it,or something like that,since it sez my 170 weighs 1460#. I weighed it (but not levelled) a few years ago and it was more like 1330#. Doing a new W&B has been on my list of things to do ever since,so I finally decided to quit putting it off and just do it!
I plan to drain all the gas but weigh it including engine oil,and add 4 gallons of unusable fuel to the results (24# at 53").
Any thoughts or suggestions on W&B procedures?

Eric

Post by **Bruce Fenstermacher** » Wed Mar 12, 2003 8:11 pm

How are you going to remove all the fuel from you tanks?
Having just weighed my plane several times (don't ask) this is what I did. Place your plane on the scales then level it. Drain all the fuel that will drain from the gasolator. This should leave pretty close to the unusable fuel. Weigh the plane. Or you could level the plane and fill the tanks to the very top. Assume this is 42 gallons or 37.5 for a '48 (50 with the 4th tank.) Weigh the plane and subtract just the usable fuel.

zero.one.victor · Post by **zero.one.victor** » Thu Mar 13, 2003 6:18 am

I planned to hoist the tail up to a approximately level position and drain the fuel at the gascolator. Then lower the tail,pull the tank sump drains, and catch the remainder of the gas from each tank in a bucket. Then up onto the scales,level,and weigh. I had a brainstorm to use a scale under each main,but weigh the tailwheel by picking it up with a hanging beam scale (like you'd use to weigh a deer) suspended from an engine hoist. I borrowed one that goes up to 200 pounds that oughta work slick. I figured it would be very easy to precisely level the airplane that way,instead of trying to crib up a scale "just so"under the tailwheel.
I did think about not draining the tank sumps,and figuring the results as including unusable fuel,like you describe. I may go that way yet.

Eric

n3833v · Post by **n3833v** » Thu Mar 13, 2003 1:13 pm

When you drain the tanks through the gasolator, it acts as a siphon and gives different amounts. It would probably be best with full tanks and deduct weight. I went through this when I was repairing the fuel valve and had to drain about 3 times. Twice up level at flight position and another on all three wheels. Where is level flight position? I could guess.