OK, let's compare 170's. Use the example of my old ragwing, with an empty CG @ 38.65". Now look at a 170 with the empty CG @ 40", and another one with an empty CG @ 36". To me, the CG location doesn't mean much unless you consider what you can do with it.
It's obvious that the 36" CG airplane will take more of an aft load of camping gear & stay in limits, but otherwise what do the CG locations tell you?

The rule of thumb is that an aft CG will fly faster & stall slower, but these rules of thumbs often don't work out in real life when comparing different airplanes to each other.

Zane, I think you're going to have to have a REALLY fine touch to be able to "feel" a significant difference in how a particular airplane flies by just moving the battery, particularly if you also change to a much lighter battery. The feel and performance differences are going to be kinda subtle, I suspect.

Also, comparing one old airplane to another is a fool's game. There are so many other things that can and do affect performance that it's pretty unrealistic to try, unless there are some pretty big differences in the two planes, in which case, you're not comparing apples to apples.

Again, the question I have is: Have you actually calculated the CG location with a couple of different loadings?

I agree with Hotrod too: Many weight and balance data on these old airplanes are inaccurate, often significantly.......another wild card when trying to compare one plane to another.

MTV

To make accurate conclusions in science or engineering, you have to level the playing field, and knowing what the base configuration and balance is is the beginning of that. If you're not using your empty W&B to calculate your per flight CG with your known load, what good is it? Why even weigh an airplane and calculate the empty moment?

Why would Alpina even bother moving his battery or making any other common weight relocation mods if empty CG doesn't matter? It's where it all starts; it's THE airplane. Then go load it how you will but you have to have a starting point, and if two 170s are different as you say they are, then you've supported my point that the base config is important. Whether you believe the evolutionary calculation of W&B sheet for the aircraft or not is beside the point. Just weigh it and get a real number.

There's no empty CG range specified on the TC, but that empty CG certainly plays into where the loaded CG ends up, and there are specified ranges for that in the TC, for normal and utility categories at various weights. I don't honestly believe I'm telling you anything you don't know, but for the spectators...

One thing I agree on is to leave it as is, unless you want to save the weight of the heavy gauge battery wire at that length, which might be something worth while.

I think we're arguijg over semantics. I sat "empty CG doesn't matter, but loaded CG does", and you say "you have to start with the empty CG to get to the loaded CG"...I think we're saying the same thing.

As far as installing a lightweight battery- not gonna mame much difference in CG in the existing location. Might make a significant change if also moved to the firewall. IMHO that's the way to go if the loaded CG will stay within limits. Here's why:

1) moving CG forward allows for staying within limits with heavier loads aft
2) shorter battery cable run means less weight and less voltage loss (better starting)
3) removing battery from aft area allows for a flat-floored extended baggage if desired.
4) easier access to battery for jumping, servicing, etc

The only downside I can see is the paperwork, which may or may not be a big PITA- depending on your IA & your FAA inspector.

Zzz wrote:Eric, while I agree with most of what you've written, I have to stick with my opinion on this. When we're talking about fixed equipment, we have to consider them independent variables. They aren't changeable on a per flight basic like fuel or cargo loading. Two 170's with all other things being equal, yet having different battery locations, should show different empty CGs. We can assume those aircraft will behave slightly differently with the same loading configuration. It's the starting point that can't be altered.

That's why I still think it's a good metric for evaluation.

I think there are two different disciplines discussing the same subject "Pilot" and "Mechanic".

Mechanic:

Definitely! Empty matters. There has to be a reference datum at some point to evaluate the build, EMPTY WEIGHT AND BALANCE! "What's in your log booK"?

Pilot:

Don't give a %#<>£ as long as I stay legal during the flight!

From a pilot's standpoint larger aircraft have zero fuel W/B for symmetric wing loading but, that is a different subject.

hotrod180 wrote:I think we're arguijg over semantics. I sat "empty CG doesn't matter, but loaded CG does", and you say "you have to start with the empty CG to get to the loaded CG"...I think we're saying the same thing.

Probably, but my original point was to ask Alpina23 about his aircraft so that we, the peanut gallery, could get an idea of how his basic CG compares to other 170s given his battery location. What if he had given us a value that was super aft compared to all other 170s? That would be evidence that maybe the deck is stacked against him in aft loading, trim authority, elevator authority.

OK. I guess I need to get scheduled with someone to weigh this thing properly. Any suggestions in my area?

Alpina23 wrote:OK. I guess I need to get scheduled with someone to weigh this thing properly. Any suggestions in my area?

My question was whether you've computed some sample loadings, with the current weight and balance paperwork. Assuming you have a current W/B, that's a good first step. If you don't have a current W/B, your plane isn't legal to fly. Before you rush out to have it weighed, do some head scratching and see if what you have makes sense. Maybe post what you have empty weight numbers here and ask others to compare what they have.

I will almost guarantee that if you re-weigh it, it will suddenly get heavier.....

But, sometimes, it's best to just weigh it, and get on with life. Before I weigh it, however, I'd do what I suggested earlier: Compute a couple sample loadings to see where your LOADED C/G is at various configurations. Then, estimate how much your battery and a lightweight one weigh, and what station they'd be at, and see if it makes sense to move the battery. If so, do that FIRST, then weigh the plane. No sense having a mechanic compute E/W C/G twice. If you're contemplating any other mods, like interior mods, or removing old radios/wiring, etc, it'd be a good idea to get that all done prior to weighing as well. Less math means less chance for errors, and less $$$ to the mechanic.

MTV

mtv wrote:Zane, I think you're going to have to have a REALLY fine touch to be able to "feel" a significant difference in how a particular airplane flies by just moving the battery, particularly if you also change to a much lighter battery. The feel and performance differences are going to be kinda subtle, I suspect.

I tend to agree, which lends to my very first reply in this thread-- Something to the effect of technique/practice with wheel landings in an airplane that's new to you.

Alpina23 wrote:OK. I guess I need to get scheduled with someone to weigh this thing properly. Any suggestions in my area?

I'm not saying it's gonna be spot on (although it might be), or even close, but as MTV pointed out you need to have a current W&B sheet to be airworthy. In fact, weight and balance is the "W" in the ARROW documents which need to be carried in the airplane.

I don't know who might have scales etc down your way. I know Paul Nyenhuis (Air Inspection & Repair) at Arlington has a set, but IMHO he charges pretty dearly for a W&B. I'd be glad to help you weigh your airplane in my hangar some time-- one of my hangar neighbors has a set of good certified digital scales, but it might cost you a rental fee to use them.

Ok..... finally. Sorry its taken so long. I know you guys asked for a W&B right from the beginning. It was in the plane which is not at my house. This plane is kept at the Elma Airport until my hangar is completed (which is finally underway!!! Woohoo).
I found two W&B sheets in the plane this is the newest one. I went through the complete logs and cant find anything that declares that it was actually weighed so I have no idea if this is anywhere close to accurate or if it was updated when the battery was moved etc. I can't image any modifications would have been done off the books (wink wink).



This weight and arm figure is forward of the CG limitations in the owners manual.


Maybe I need a refresher lesson in how to do this because somehow this just doesn't seem quite right to me.....

[quote="Alpina23"]Ok..... finally. Sorry its taken so long. I know you guys asked for a W&B right from the beginning. It was in the plane which is not at my house. This plane is kept at the Elma Airport until my hangar is completed (which is finally underway!!! Woohoo).
I found two W&B sheets in the plane this is the newest one. I went through the complete logs and cant find anything that declares that it was actually weighed so I have no idea if this is anywhere close to accurate or if it was updated when the battery was moved etc. I can't image any modifications would have been done off the books (wink wink).



This weight and arm figure is forward of the CG limitations in the owners manual.




If I'm reading that right, with full fuel, and a 200 pound pilot and nothing else in the airplane (which is likely the worst case scenario), you're well forward of the CG envelope.

Pilot at 200 lbs = 7 index units
Full fuel at 220 lbs = 11 index units
Aircraft empty weight = 57.5 units (index units = moments divided by 1000)

So, All Up Weight = 1428 + 200 + 220 = 1848

Total index units = 57.5 + 7 + 11 = 75.5 index units.

Place the weight and the index units on the graph, and you wind up well forward of the forward limit.

Unless I'm doing something wrong, which is a possibility. In fact, it looks like, again if my calculations are right, that with two in the front seats and full fuel, you'll still be close to the forward limit.

Correct me if I'm wrong, gang. I personally hate those Cessna graphs, but.....

MTV

My plane gained 80 lbs. after getting it CORRECTLY weighed. It had not been actually weighed in 25+ years. Just calculated weights and I knew those were suspect. Knowing what the true weight and moment are, is where I would be starting from to figure out all the other info you desire. STEP ONE: get it correctly weighed. I know of too many planes that are way off on what they really weigh and the owners still swear by their calculated W&B. "My 170 with an O-360 on 31" bushwheels only weighs 1360 lbs" I call BULLSHIT!

mtv wrote:Pilot at 200 lbs = 7 index units
Full fuel at 220 lbs = 11 index units
Aircraft empty weight = 57.5 units (index units = moments divided by 1000)

So, All Up Weight = 1428 + 200 + 220 = 1848

Total index units = 57.5 + 7 + 11 = 75.5 index units.

Place the weight and the index units on the graph, and you wind up well forward of the forward limit.

MTV, I read that is being smack dab in the middle of the CG range. 1848 lbs at 75.5 index units for landplane.

Oops, yep. Reading glasses let me down again with those itty bitty weight numbers.

I'd be moving that battery forward post haste, otherwise you won't be able to carry much aft.

MTV

When my P172D was actually weighed, it weighed quite a bit more than the calculations had it up until then--it probably hadn't been weighed for 35 or 40 years, maybe since it left the factory in October 1962. I don't remember the difference, but it had a forward CG and enough more actual weight that I was surprised--my IA wasn't, though, as he said it was pretty common. It lightened up a little and the CG moved a little forward when I had the panel redone--newer avionics are lighter and the 6# glideslope receiver was pulled out of the back end--but not enough to make any discernible difference in how it flies. There's no way I'd move my battery out of the back end without ending up with too much forward CG, if I didn't have stuff in the baggage compartment, and I'd much rather it is within the envelope regardless of where I put things.

Ideally, the only way to know exactly what will happen when the batter is moved is to weigh it as is, then actually do the change and reweigh it. Just rounding numbers will let errors creep in. But that's a little scary without a pretty good guesstimate about the results.

Cary

Commercially we do an actual weighing every 36 months. Private? My Citabria had never been weighed ever until I did it myself last year. Original paperwork from factory delivery says "estimated". With 40 plus years of A&P pencil-whipped estimates. Probably most of your airplanes are the same.

Borrowed some high end aviation scales from the local EAA. Did the plumb bob and x inches from the leading edge of the lower strut fitting - bah, just put a level on the upper door frame, same thing.

I changed battery, location, removed the float kit, removed wiring, unused instruments, changed instrument panel, swapped to ABW, lightweight fire extinguisher, better seatbelts. The actual weighing W&B was 20 heavier than 40 years of estimates and calculations. I'm nudging the front of the graph by myself and low fuel, so I did a bunch of wheelie hard brake landings like that just to see what the plane felt like. Didn't Zane just recommend that? Always gear or gas or passenger in the back when I fly for real anyway.

If I ever go lightweight starter it just improves things.

I would leave it aft unless you plan to carry something in the baggage compartment all the time. I have a 54 170B with a O360 and a 80 inch Hartzell with the damper. I know it is the worst of the prop choices as far a weight forward but it came on the plane when I bought it. My airplane is a bit on the heavy side at 1499 with 30 pounds of unusable fuel. The empty cg is 37.58 inches. This is an actual weight on certified scales when a previous owner had a ski pump and ski axles and fittings installed. It also has the factory camera hole aft of the pilots seat, a Javelin fuel tank in the baggage compartment that weighs 27 pounds with unusable fuel and a field approved extended baggage which weighs 4 pounds, steps and handles, double puck brakes and 8.00 tires. Of importance to you the battery is still on the firewall. To be legal I have to carry 50 pounds in the baggage compartment. I plan to pull the old fiberglass insulation and vinyl headliner to help get the weight down but will leave the interior.

My suggestion would be to pull the back seat. If you like the way it flies move the battery forward. If you can't tell the difference leave it alone.

When I wheel land, which is what I do most of the time, I hold the airplane off to make a tail low landing. As I feel the mains touch I visualize touching down on the aft side of the tire, at that point I move the yoke forward which i visualize as rotating the airplane forward around the axle and pin the mains on the ground which reduces the angle of attack and reduces lift on the wing. I have never been able to make smooth wheel landings by leveling off and then moving the yoke forward to pin the gear on the ground without the mains touching first. Excess speed is not your friend. Touching down without zeroing out the rate of decent will result in a ballooning of the airplane which some mistakenly think is a rebound of the spring gear. What is actually happening is as the mains touch the nose of the airplane can no longer continue downward, inertia will cause the tail to continue downward, increasing the angle of attack and a balloon results. Practice, practice,practice.

Tim

Tim,

As you noted, with that prop, you're in the worst possible forward C/G situation. I had the same prop as you, and switched to an MT composite prop. That took 29 pounds right off the very front of the plane.....magic, and no estimates needed. Pt hat 10 pound harmonic damper you have on that prop is a brick, right up front.

MTV

Ya, I know Mike, just not worth the money to me for a airplane I only plan to fly for a couple of years. Probably would never be worth the money to me, I spent my whole career flying tramped out old freighters, piston Convairs, Metroliners, and DC8's, you just learn to compensate for the way the airplane flew. No two flew the same and the cg moved all over depending on the way the airplane was loaded. One leg was at the forward limit and the next might be at the aft. The load plan was for the convenience of the loaders not for how well the plane flew for the pilots. Try hand flying a DC8 with an aft cg for a few hours at FL350 and you will appreciate being loaded forward.

Even right up against the forward limit in 170A and B models I have never trimmed full nose up to be able to make a full stall 3 point landing with full flaps and 800 tires. The A model elevator control pressure will higher than the B but both are not a problem as you slow below trimmed approach speed. Personally, I don't want to be trimmed below 65 mph on approach even though I will be slower than that on short short final. Just me, each of us gets to fly the way they like.

Tim