N9149A wrote:
Do you have other than stock main gear that would shift the main wheels from other than the stock position? This would do it as well.
This is something that's never ocurred to me before. Is is standard practice to use actual measurements for scale locations? I and the A&P's that have ever done a W&B on my airplane have always used the book numbers for the location of the main wheel and tail wheel weights. It would probably be a good idea to actually measure the distance that the axle centers are behind a plane through the firewall. Who is to say that just because the logs don't show any changes to the gear, that they haven't been changed, or at least tweaked? It might even be a good idea to measure all three locations separately. One main might be a half inch off of the other, but wouldn't it make a substantial difference in the calculated CG to have 600 pounds or so off by that much?
Miles
Miles
“I envy no man that knows more than myself, but pity them that know less.”
— Thomas Browne
Robert Eilers wrote:My partner arranged to have our C170B weighed by an A&P who has a reputation for accurate EW & CG calculations. The documented aircraft empty weight and CG before being weighed was 1308 & 38.05. After being weighed the EW and CG became 1376 & 37.26. The empty weight included unuseable fuel and 8 qts of oil. I believe the 8 qts of oil should have been deducted from the EW and the minus moment of the oil added to the total EW moment - otherwise a minus moment weight is included in the total moment as a positive. After correcting, what I believe is an error, the EW and CG are 1361 & 37.89. Even with this correction, if I load the front seats of the aircraft with two 180 lb adults and full fuel - no baggage - no pax, the CG is forward of the forward limits. I did not have this problem with the orginal EW and CG. Anyone else experiencing this W&B problem
Hi, Robert!
This sort of mistake is not all that uncommon. The difference is: The def'n of EMPTY versus BASIC OPERATING weights.
Empty wt (EW) is the wt of the aircraft, empty, except for undrainable/unuseable fuel and oil.
Basic operating weight (BOW) usually INCLUDES full oil, unuseable fuel, plus the essential crew (pilots) and ordinary equipment (such as req'd manuals, crew baggage, etc.)
Most light aircraft do not normally compute Wt/Bal using BOW. Typically light aircraft begin the computations with EW only. Perhaps your man is used to BOW rather than EW.
In either case, the ordinary computations may still be made. Just add in/subtract out the oil, depending upon the starting wt utilized.
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
Not too long ago someone on this site posted a real good w&b calculator. It is easy to use. Maybe your a&p put a number in the wrong place or something.
When I did my W&B two months ago, I rolled it up two 2 x 10 ramps to get the mains on the scales. Blocked the wheels, then lifted the tail up in about 4 seperate lifts adding wood blocks with each lift until it was level. Amazing how high the tail is to get the top door jamb level, assistance is required at this point (4 bodies works great). Then I strung a string across from the centre of one axle to the other. Using a plumb bob I marked the location of the firewall, centre of the string between the axles and the centre of the tail wheel onto the floor. I then used a tape measure for the measurements. The last 6 inches to level the upper door jamb did not seem to alter to any great extent the measurements nor the tailwheel weight. Obtained the scale readings, lowered the plane, weighed the wood blocks and chocks and deducted them from the weights and called it good.
Vic
N2609V
48 Ragwing
A Lanber 2097 12 gauge O/U Sporting
A happy go lucky Ruger Red label 20 ga
12N Aeroflex
Andover NJ http://www.sandhillaviation.com
The lateral level for the aircraft is determined by placing a level across the seat tracks. (Not across the area between axles, which would not necessarily level the aircraft, depending upon several factors, including the fact that the gear may not be exactly symmetrical with the airframe.)
'53 B-model N146YS SN:25713
50th Anniversary of Flight Model. Winner-Best Original 170B, 100th Anniversary of Flight Convention. An originality nut (mostly) for the right reasons.
Below is the original Factory Weight and balance I scanned from a Micro Fiche copy (not the greatest quality). The factory computed the W&B for my airplane so I had to work backward to find what weights they used and I came up with 593lbs for each main and 97lbs for the tail wheel, these weights include 5 Gallons unusable fuel and no oil. The comparison of my 97lb and your 120lb tail wheel weights is curious, I wonder if the difference is between a straight 170 and a B model?
John E. Barrett
aka. Johneb
Sent from my "Cray Super Computer"
The reason for the higher tailwheel weight was because the airplane was weighed with full fuel, why we did it this way I really dont remember
Vic
N2609V
48 Ragwing
A Lanber 2097 12 gauge O/U Sporting
A happy go lucky Ruger Red label 20 ga
12N Aeroflex
Andover NJ http://www.sandhillaviation.com
My tail a B model weighed in at 127 lbs, with a total empty of 1415. I mentioned an earlier post that I used a string between the gear axles, but I assumed that my madness in my method was obvious. I needed to locate the weigh points with respect to the datum point to calculate the moments. Because I only fly with one other person and my bikes in the back I must admit that I never really ever calculated my loading of the aircraft. Since the engine conversion my empty weight CG is now forward by 2" inches and the airplane feels nose heavy, I'm looking at the charts. The owners manual reproduction from Univair weight and balance is way off for a "B". Looking at the type sheets, W&B shows a range of approx 36.5 to 46.5., moments for fuel + 46" and unusable qty. @30 lbs. What's the moment for pilot and passengers?
c170b53 wrote:My tail a B model weighed in at 127 lbs, with a total empty of 1415. I mentioned an earlier post that I used a string between the gear axles, but I assumed that my madness in my method was obvious. I needed to locate the weigh points with respect to the datum point to calculate the moments. Because I only fly with one other person and my bikes in the back I must admit that I never really ever calculated my loading of the aircraft. Since the engine conversion my empty weight CG is now forward by 2" inches and the airplane feels nose heavy, I'm looking at the charts. The owners manual reproduction from Univair weight and balance is way off for a "B". Looking at the type sheets, W&B shows a range of approx 36.5 to 46.5., moments for fuel + 46" and unusable qty. @30 lbs. What's the moment for pilot and passengers?
c170b53,
If you go to page 35 of your Owners Manual and look at the table on the top of the page you can figure out the moment for each of the seat rows in your airplane. Example the index for the Pilot and Passengers is 12.2 multiply that number by 1000 and devide by the weight shown for the Pilot and Passenger, (12.2 x 1000) / 340 = 35.88. If you do this with the fuel numbers you will realize that there is an error in the either this page 35 table or in the type sheets as one says fuel is at 46" and the other says fuel is at 48".
Which engine conversion do you have?
John E. Barrett
aka. Johneb
Sent from my "Cray Super Computer"
Vic
It is easier to fill the tanks and be reasonable sure of what you have than it is to drain the fule to the unusable fuel point.
I choose to drain my fuel buy first using the tank drains to empty the tank. Then with the aircraft in the flight attitude I drain all that would drain from the gasolator.
CAUTION - My forum posts may be worth what you paid for them!
Bruce Fenstermacher, Past President, TIC170A
Email: brucefenster at gmail.com
John, when you reverse-calculated the factory weights for the mains & t/w, was 593/97 the only possible combination that worked out? I also have an original factory W&B sheet, and mine too is missing the actual weight-at-wheel figures. I'm guessing that the factory weighed the first couple off the line for each year or run, then just used those figures for the rest.
Miles, I would say that it's a good practice to use actual (measured) arm figures instead of book fiures. Mine for example measured out different for the tail: a half-inch farther aft (249.5"), maybe cuz I have a Scott 3200 and the factory issue was a Scott 3-24B. My mains are also different, due to the later model 180 gear legs installed: 3-1/2" farther forward (18.5"). This also accounts for the heavier than normal tail weight on mine.
BTW, mine empty except for oil was 1318# (CG @ 38.9"), with the mains at 606/597 and the tail at 115. This was before I removed the gyro's and venturi's, and installed the single jump seat in back. It weighs 1,310 now with the CG @ 38.4".
Bruce, that may be true about the CAA requirement to weigh each airplane. But look at the above photo of the factory W&B sheet for N4557C. Note the blank spaces on the top of the form. The factory for my airplane has similar blank spaces. To me, that sez they didn't weigh my airplane and calculate it's actual weight and balance. Just my cynical point of view,I guess- maybe you're more of an optimist than me.
BTW, the factory figures for my ragwing are 1229# including unusable fuel, with an empty CG of 37.6".
Also, it's interesting to actually measure the station for the back-seaters and for baggage. What I came up witth wasn't quite what the factory W&B sheet sez to use. Something to consider if you're gonna be pushing the envelope. As I recall, where I figured the pax & cargp weight would be is somewhat aft of where the factory sez.
