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16 Answers

W&B upgraded 172 n 180hp and aux tank

Asked by: 6200 views , ,
Aerodynamics, General Aviation

I have recently started flying a 172n that has a 180hp and a 18g aux tank.  The plane has the 30 flap limit and gross increase to 2550.  Here is the problem, the EW is recorded at 1550.13 with a new Cg of  48.25 on the latest W&B.   Unless I am doing something wrong, I can't find anyway to load the plane with the aux tank filled and it is almost impossible to load the plane with any weight behind the front seats.  I am considering having the plane re weighed but if I do, can the CG be moved by adding counter weights?  

https://www.dropbox.com/s/r6l0r4ibis0s9u8/photo%20%284%29.JPG

 

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16 Answers



  1. Nibake on Aug 10, 2014

    Can you upload the w&b info?

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  2. tim shivers on Aug 10, 2014

    trying to figure out how to copy or link a scan? can you point me to directions

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  3. Jim F. on Aug 11, 2014

    You can scan in and upload to the “public” folder in a free Dropbox account, then post the public ling to the document.

    As for your ballast question, sure, why not? I used to fly a PA-44 Seminole that needed about 90lbs of sandbags in order to fly it solo… As long as you don’t exceed any other limitations such as floor load limits or max weight, put in all the ballast you want. Just be sure to properly secure it to ensure it doesn’t shift and cause a catastrophic in-flight CG change.

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  4. Nibake on Aug 11, 2014

    The poster says that he can’t put weight in the aft seats, which means that the CG is already near the aft limits. I don’t think this is correct, that sounds extremely unlikely for a 172 or any trainer. Unless he meant “without” instead of “with.” It is much more likely that a couple of heavy guys in front could put the CG too far forward.

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  5. tim shivers on Aug 11, 2014

    I admit I may be totally confused. I have really struggled with these numbers but look at the W&B report I have and see if i am mising something. I have flown the plane with the owner and the AUX tank full and two 180-200lb front seat passengers. But when I started to fly as PIC and did the numbers i was a little concerned. The AUX tank is 18G O&N. See attached.

    https://www.dropbox.com/s/r6l0r4ibis0s9u8/photo%20%284%29.JPG

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  6. John D Collins on Aug 11, 2014

    Tim,

    You are not providing all the necessary data to calculate a W&B. The stock 172N has a Maximum GW of 2300 pounds and a CG envelope of a forward CG of 38.5 inches 2300 pounds and reduces to 35 inches uniformly at 1950 pounds. Below 1950 pounds the forward CG is fixed at 35 inches. The rear CG limit is fixed at 47.3 inches for all weights, 2300 pounds or lower.

    So what is missing are the CG limits forward and aft above 2300 pounds up to 2550 pounds. Also missing is the arm used for the 18 gallon Aux tanks. Without this information, one can’t determine the CG locations or if they exceed the limits.

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  7. tim shivers on Aug 11, 2014

    The tank was placed in the plane before the attached W&B was done. The tank was installed at least 6 months before the engine so the attached is the latest.

    It was my understanding the STC for installing the engine would not change the performance charateristics, so would the CG limits change? I’ll have to look through all the paperwork again and see if I see any changes in limits.

    I am still learning a lot. Thanks for all the info so far.

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  8. John D Collins on Aug 12, 2014

    Tim, what is the moment arm for the new tank, it is not in any manual I have.

    The CG limits are absolutely needed as they are not specified above 2300 pounds. You need three items.

    1) Aux tank moment arm location
    2) Forward CG limits above 2300 pounds
    3) Aft CG limits above 2300 pounds

    Without this information it is impossible to do a W&B.

    It is likely that the aft CG limit will remain at 47.3 inches, but the forward CG limit is slanted and changes with weight along a linear line above 1950 pounds.to 2300 pounds, so it is not likely that it would continue from that point the same as it is at 2300 pounds.

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  9. tim shivers on Aug 12, 2014

    Thanks I’ll have to look at all the W&B docs when I get back in town, but I think I did a poor job of asking my question.

    If the attached W&B is correct with the tank installed, and the ew 1550 cg is 48.25, even if you don’t fill the aux fuel tank or any of the luggage area, I can’t seem to get the cg to work for most normal loads for example 2 x 180 in front and 140 passenger and 240lbs fuel in wings keeps the plane under 2300 but I calculate the cg out of range. If pilot and copilot are are 180 and 135 like my wife and I and you have 100lbs in back seat, I am calculating cg of 47.7. With and empty cg of 48.25, the plane appears to have a cg almost 10 inches aft of every 172 I have researched and that’s with a 16lb heavier engine than stock.

    I’ll go back through docs when I get back next week.

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  10. Steve Butler on Aug 12, 2014

    From looking at the link you provided, I suspect that the original moment from which the new calculations were made was incorrect.

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  11. John D Collins on Aug 13, 2014

    I have to agree with Steve. The addition of the engine moves the CG forward, so if anything you would expect to have a forward CG problem. The sample airplane in my manual has the empty Wt 1454 and CG at 39.61 which is almost 10 inches forward of your empty CG location. Although these can be assumed to be marketing numbers and not real aircraft numbers, your aircraft started out (pre engine upgrade) only 79 pounds heavier but already 1.77 inches aft of the rear CG limit. The only station that is forward of the CG that has a possibility of moving the aircraft within the CG limits are the pilot/copilot stations located at station 37. All the other stations (rear passengers, fuel, and baggage) are aft of the CG limit and by definition move the CG further aft. Check the math for any upgrades since leaving the factory.

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  12. tim shivers on Aug 13, 2014

    Thanks all. You confirmed my thoughts. I will check the paperwork and or have the plane weighed.

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  13. Sam Dawson on Aug 14, 2014

    The problem may be the combination of STCs- the 180 HP conversion and the O&N aux tank. I’ve seen one installed in a 152 but not a 172 so I am not familiar with this instillation and the company website seems to be dead so they may be out of business. Putting this setup in the cargo area would shift the CG aft. I don’t see a W&B for the installation of the aux tank so it is difficult to tell.

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  14. Bill Zaleski on Aug 18, 2014

    I have the same 172N with 180 H.P. and O&N 18 gal aux fuel tank. My Empty Weight data is 1511# @ 38.23 CG

    I am not believing that the subject aircraft has an empty weight CG 10″ aft of mine. The aux fuel is at the same arm as normal baggage. Combined baggage and fuel may not exceed the normal baggage limit of 120#

    In 25 years as an A&P I have not come across an aircraft that is out of CG when empty, but I suppose that it is possible. If that were so, the aircraft loading would mandate a minimum pilot weight, as some helicopters do.

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  15. tim shivers on Aug 22, 2014

    https://www.dropbox.com/s/bmihjbdcn2yx08i/2014-08-22%2022.45.51.jpg?dl=0

    Please take a look at this weight. The CG looked normal up until this weighting. Just glancing, take a look at the math where they apparently weighed the plane full and then subtracted the fuel. They list the fuel arm as 11.5. Shouldn’t that be more like 48? If so that changes the math pretty dramatically. I have yet to find the stc for the tank but because it is in both baggage areas I would guess it’s more in the 108 range since that appears to be the center.

    Those two changes puts the CG back in the 40 range before the engine change.

    Comments

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  16. John D Collins on Aug 23, 2014

    The shop or mechanic that did the weighing calculation had a major error in his calculations and some minor ones. First, as you note the 11.5 is the moment/1000 and not the arm. The arm should be as you state close to 48 inches. A proper weighing requires that all the fuel and oil be drained from the aircraft and not subtracted after the weighing as this introduces error. The aircraft also needs to be leveled in accordance with the weighing instructions. When I had my airplane weighted, I cleaned it, drained all of the fuel, removed everything from the airplane that was not specifically included in the equipment list (this can easily be 20 to 40 pounds in a typical aircraft and includes items in the pockets and glove box such as spare batteries, chocks, head sets, fuel sampler, tools, cloths, …). You can either have an A&P recalculate the existing weighing data and create a new W&B or better yet, get the aircraft weighed by a shop that knows what they are doing.

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