Weight & Balance

[N3188A]

I had my plane weighed recently and it was no surprise to see my weight go up by about 90 lbs. What I didn't expect was the calculated CG change that occurred relative to my existing paperwork - it shifted forward by about 4 inches and now I'm way forward of the envelope. The plane flies great and does not feel nose heavy at all - by myself with 29" Gar Aero's, minimal baggage and 1/4 fuel I can do a tail-wheel landing with the mains farther off the ground than I care to have them and still have plenty of elevator.

Plane specs: '53 170B, 180 hp o-360-a1a, Hartzel CS prop, smaller battery on the firewall, C-180 gear legs, Gar Aero tail wheel, 8:00x6 tires, Cleveland double puck brakes, Selkirk extended baggage, Horton STOL kit, basic radio and a few instruments, full paint, plumbed for hydraulic wheel skis, Atlee Dodge folding rear seats. Measured weight is 1405 lbs, including unusable fuel and full oil.

The standard method of calculating the center of gravity is to use the distance from the datum to the center line of the axles as the basis for the moment - this was measured at 18.5 inches as opposed to the 22" used in the TCDS. Using the 18.5" puts the calculated CG at empty weight at about 32" where as using 22" puts it at about 36 inches.

So my questions are:
1) Do the C-180 gear legs have a greater forward cant than the stock 170 gear legs leading to a more forward measurement than listed in the TCDS? On paper, this becomes important as I put on wheel skis or transition back to the 29" Gar Aeros in the summer.

2) Has anyone else experienced this type of forward shift in GC after having their plane weighed. I've heard that the 180hp conversion leads to a forward CG problem but I've also heard it's rare that tail weight is needed. I hate to add 20 lbs in the tail for the sake of being "legal" given how the plane flies.

I'm tempted have the airplane weighed again but measure the weight at the gear attach points instead of the wheels - any thoughts on this?

[Bruce Fenstermacher]

If I understand yo correctly, you put the forward scales under the wheels which theoretically would be directly under the axles and these axles are now 18.5 from the datum rather than the stock 22. You then used 18.5 in your calculations. If this is correct, then you did the weight and balance correct and substituting anything else would be garbage. Mathematically if you could somehow properly weight the aircraft with the scales at a point at the 22" datum, the CG would be the same.

Fact is your airplane is nose heavy. And I'm not surprised it flies fine while outside the CG envelope. Flying inside the CG envelope assures that your airplane can perform all maneuvers required of certification. Apparently you haven't tried to fly all of those maneuvers required of certification while outside the CG envelope and so your plane flies just as expected, fine. Fact is you are a test pilot every time you do this.

I'd suggest 20 lb of survival equipment in the baggage compartment, if that is what it takes, is good investment of useful load. This way you can remove your test pilot hat and relax.

[n2582d]

... Do the C-180 gear legs have a greater forward cant than the stock 170 gear legs leading to a more forward measurement than listed in the TCDS? ...

There are two types of C-180 gear according to Tom Anderson, wheels aft (p/n 0741001-1 & -2) and wheels forward (p/n 0741001-3 & -4).

... The standard method of calculating the center of gravity is to use the distance from the datum to the center line of the axles as the basis for the moment - this was measured at 18.5 inches as opposed to the 22" used in the TCDS. Using the 18.5" puts the calculated CG at empty weight at about 32" where as using 22" puts it at about 36 inches... .

Confirm that you are weighing the aircraft with the upper door sill level? You've added a bunch of weight in the nose and taken out a lot at 70". If I recall correctly the 170B with the Avcon conversion I flew at MAF had a tire filled with cement tied down in the baggage compartment to help move the forward CG aft. Time for a lightweight starter and alternator, carbon fiber cowling, and titanium gear! Then move the ELT as far aft as possible and move the battery to the tailcone (you'll want the baggage door mod for that). An 18 gal. Javelin aux. tank in the baggage compartment will also help move the CG aft. It's on the TCDS; no STC is required to install it. Doing all these mods will lighten your wallet to such an extent that your CG will, if anything, be too far aft when you're flying!

[bigrenna]

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[canav8]

For understanding. The distance used for your W&B calculations should be the actual distance where your scales are. If the axles are 4 inchs farther forward then you must take a new measurement from the datum to get accurate numbers. make sure the upper horizontal portion of the door is level. You did not mention a Baby Bush Tailwheel. The ABC Bushwheel lengthens the distance from datum also so when your scale is under the tailwheel you will need to modify your distance from datum to tailwheel axle as well.
Have you considered talking to mechanics that have done these mods before? If you have to convince your mechanic that you know what you are talking about then I might consider getting a second opinion. Remember that it's their signature in your logbook.

Remember if you have a forward CG, It will stall at a much higher airspeed.

[bigrenna]

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[canav8]

I was modifying my comment while you typed yours. I agree with you, I just didnt know what his resources were.

[N3188A]

Thanks for the great comments. Just for clarification, all balance calculations have been done based on actual distance measurements. The Gar Aero fork, being longer than a stock 3200 fork, pushes the rear axle slightly farther back than the stock 3200.

I'm in discussions with my mechanic now on what to do next. Based on my calculations, 50 lbs. of survival gear in the extended baggage compartment (125") is not enough because the new calculated CG is so far forward - I'll need to add significant weight in the tail. If this is the case, so be it but it just seems odd that the GC would be that far off from the prior W&B calculations. Of course, it's difficult to reconcile all of the former W&B calculations given how they were done. And, as is often the case, errors can compound dramatically as subsequent modifications are done.

Again, thanks for the comments!

[bagarre]

This seems pretty exceptional, even with an engine swap and heavier prop.
That's just a whole lot of weight to be off by.

Are you positive the plane was level at the upper door sill when weighed?
It sounds like something else it off...or the cowl is lead lined and your exhaust is cast iron.

[bigrenna]

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[Bruce Fenstermacher]

Anything moved forward, like the gear, will make the airplane more nose heavy. Unless of course the weight of the part was reduced significantly.

[DaveF]

Your situation doesn't seem right. I have a '54 with an Avcon 180hp/CS conversion, newer front seats, 7.00x6 tires, and a firewall-mounted Concorde RG battery. Otherwise it's stock. Empty weight is 1410, per a 1993 weighing. With two aboard and full fuel I'm right at the forward edge of the CG envelope.

[c170b53]

Sounds like the datum distance was measured with the aircraft"s tail 6 ft or so in the air but is there that much of a difference between axle center line positions between lady legs and 180 gear?
I agree with Dave. My airplane with the 0-320 C/S does have a forward C of G but well within limts, the engine might be a bit lighter than a 0-360 but not by much. Replacing the stock bench seat with the aft jump seats would make a good change due to the arm.
Maybe a Mod to far?

[ghostflyer]

I have the 180hp. Mod done with 180 legs and double puk brakes . My c of g gave me such concerns that I put a fixed pitch prop on(Sensenich) . My battery is on the fire wall. Light weight starter and alternator have been fitted . I am now within limits regarding forward cof g. Yes I have found my stall is about 4knots faster. But I can live with that . Would I do these mods if I knew about the changes it does to the flying characteristics , YES . It has made it a even better aircraft overall.
Many people suggested putting the battery down the back ,it's not practical ,plus the battery gets bounced around too much . You must use the upper door sills for levelling as using the lower will give you the wrong result. The number of people using the wrong area for levelling is staggering .

[KS170A]

I am really curious what the results of only adding 180 gear legs and/or larger wheels would have. In theory, moving the main axle c/l forward should increase the amount of weight measured on the tailwheel with the airplane in level flight attitude (upper door sill level), which would increase the moment thereof, thus negating the forward movement of the main wheel measuring point. Sounds to me like a re-calculation would be prudent.

Just so I understand...take a stock 170, add larger tires, add 180 gear legs, add more HP engine, add C/S prop, achieve an extreme forward CG that increases stall speed, and you still have an airplane with 170 weight limits. Just seems illogical to spend that much money to achieve those results, when the proceeds of said stock 170 sale + money spent on parts & labor for those mods could get a reasonable C180 that already has the additional HP (even more, actually), c/s prop, larger wheels, more speed, AND higher gross weights? Just sayin'....