Piper baggage limitation

[Jim K]

Working w&b balance calculations for our Texas trip; it will be the first time carrying the whole family at one time. We are within the envelope even with full fuel, but I'd prefer the cg to be further aft, which caused me to ponder the 100lb rear baggage limitation and the reason for it.

At first I thought maybe it was an issue of that part of the fuselage not being built as heavy, but it occurred to me that the stabilator must apply much more force than that when you're loaded to forward cg. Also, there's no real limitation on weight in the aft seats, although the line in the chart stops at 450lbs. I have two sub-100 lb girls back there, so it seems like a few inches from the datum can't make much difference, structure wise.

So is it a matter of the strength of the floor? Is it the strength of the plastic bulkhead? It would certainly spell disaster if something broke through there into the tail.

When we took the Big Tylenol to Florida last year, we had <100lbs baggage, so it's an academic exercise, but I am curious as to the reason.

 

[flyingron]

Point loads on the floor I would suspect.

 

[kyleb]

Any chance it is caused by the propensity of aft loaded aircraft to bash their tail on the ground when a fat passenger puts his weight on the step?

 

[GRG55]

Point loads on the floor I would suspect.

^^^This.

It's a floor loading limitation.
In the battle between weight and strength every solution for airplane designers is a compromise.
Most low wing Pipers have quite large (and useable!) rear baggage compartments. But to keep the weight of the airplane down the floors are made from a thin sheet of aluminum supported by lightweight extrusions.

 

[WDD]

So would a small sheet of 3/4 plywood on the floor spread the load enough?

 

[GRG55]

So would a small sheet of 3/4 plywood on the floor spread the load enough?

It would help if you are carrying a gold brick.

Ordinarily though, no. The extrusions to which the floor surface is attached carry the shear and bending loads.
You could put in a 3/4 plywood floor. The baggage compartment would then be able to carry 100 lb less the weight of the plywood.

 

[Jim K]

I wondered about the point load, and had the same idea of spreading the load. I had not considered the structure underneath being the limiting factor.

I also had not considered the g-loading of the aircraft. 100 lbs becomes 380 at the limit of normal category certification, and i could could see that kind of weight doing damage.

 

[hindsight2020]

No such structural limitation on the PA-28 though. 200# in my certified 'death-winged' trap. Ditto for Comanches. The difference in area of the -28 floor in that compartment is not double the PA-32 (though obviously larger), so it cannot account for the additional 100#.

With the PA-32 the limitation is most likely born out of the meatbags in the 3rd row, considering all they did was stretch the PA-28 (looking at the stock arms of the PA-32). -32 aft bag comp has a smaller volume: 17ft^3 vs 24ft^3 in the -28; 26ft^3 if you include hatshelf. So they made up the deficit with the forward compartment where you have an additional 8ft^3 and another 100# of capacity.

At any rate, based on that CG spread, the answer is self-evident: third row is really the baggage compartment when used as a 4 seater. Or alternatively, chuck seat 2A or 2B, stack the heavier items in its place and use it as an ottoman for pax in seat 3A or 3B as applicable.

 

[GRG55]

No such structural limitation on the PA-28 though. 200# in my certified 'death-winged' trap. Ditto for Comanches. The difference in area of the -28 floor in that compartment is not double the PA-32 (though obviously larger), so it cannot account for the additional 100#...

The early fixed gear Cherokees, including my 1961 Cherokee 160, had 100 lb baggage limits. I think they increased it to 200 lbs about the same time they raised the gross from 2150 to 2200 lbs sometime in the mid-1960s.