Can someone who has done extensive weight and balance work on a 170B assist me with this info request, thanks sincerely in advance.

I need to do some calculations for a tailwheel spring material selection (172 to 170 conversion project). As part of my STC conversion upgrade, I will be going from steel to composite springs, which (if STC-PMA approved) will deliver several very useful benefits in weight savings, maintenance, corrosion, resistance to yield/breakage and longer service life. Even better than the 180 tailwheel stinger.

We'll eventually be doing FAA drop tests and all sorts of official mayhem. But to keep the cost of this down (for me and for the customers), I need to get pretty close on paper before I start writing checks for the "official" testing.

What I need is to get the tail weight of a 170B at GROSS WEIGHT in the 3 point attitude. Not empty weight, gross. This of course will allow me to burn up some calculator batteries and figure out reasonably close approximates of the correct spring rates, and thus determine the spring dimension and material requirements in composite. Then the actual FAA testing can be done once or twice, instead of five or six times and breaking parts in front of the feds.

Thank you sincerely in advance for providing me with an accurate tail weight to streamline this process for me.

I would post this on the 170 association site if you haven't already. Someone over there may be able to help.

I know you already thought of this, but in case you hadn't.... perhaps a CG to go with that weight, because as you know gross weight at a fwd CG is going to yield a different T/W weight than gross at an aft CG....

Take care, Rob

This formula might help you, works for all TW planes, just plug in your numbers. You'll need to know the plumb distance from the firewall to your main axles, and to YOUR TW axle (which may be different than a 170 leaf spring type tail). Then figure gross plus 5-10% and slightly behind the CG limit for safety sake.

http://sparky.supercub.org/photopost/da ... a_info.pdf

Also, get a solar calculator.

Well, thanks much for all the replies.

I have posted it on the 170 group, waiting for any help from either direction.

Cannot do the plumb bob measurement method because there are no 170's at my airport

I have located a very good existing candidate for a composite tail spring, which I will be able to modify for this use. Before I go off and assume that it will work, and set up an expensive FAA drop test, I want to make sure I'm in the general ballpark close enough to be able to carry the loads.

I already know the spring/deflection rate of the part I want to use.
I already know how much height I need on the drop test will require to certify it.
I know what the gross weight of the airplane is, and I know what the FAA says is the proportion between gross weight and tail weight, as a general starting point.
But that is not really good enough for my inquisitive mind, because various airplanes' landing gear geometry will make the 3 point tail weight a lot different even at the same gross weight

Having the actual tail weight of an actual airplane (instead of extrapolation, interpolation, or reverse engineering) will allow me to run fairly simple calculations and very quickly see if the proposed part I want to use is even in the right ballpark... hopefully verifying the "use 10% of the gross weight" rule of thumb from the FAR's.

The difference in yield strength between composite and steel makes reverse engineering from a metal spring somewhat irrelevant. The spring rates can be figured out easily enough, but the steel's yield (basically a permanent kink in the steel at a certain load) is why there is a stack of 4 or 6 leaves on a typical steel tail spring. With composite we will be able to have a lesser number of leaves, perhaps even one, compared to a stack of several clinking clanking rusting steel boat anchors. But this difference in stack versus mono-leaf also makes it very complicated to reverse engineer one into another.

So... that is what made me realize that a simple numerical data point on an actual 170B at gross, sitting in a 3 point attitude, will help me get to the party sooner and easier than any other way.

I've never owned one, but I helped a friend replace his tail wheel on his 'B', and he had to do another W&B. With the wheel on a sawhorse/plywood affair, door frame level, no gas or oil, it came in at 120#, which he thought was a bit light. I recall having to do it a few times since the force was really sensitive to how level things were.

Good luck on the composite tail spring. Sounds fun!

Thanks... I have found several references to tail weights of 114-120 pounds in level attitude with nobody on board. But as you point out, tail up or down a few degrees changes the "weight" a lot, and the tailwheel is never going to hit the ground when the plane is in the level attitude (HUGE rocks and BENT props, vol. 10???)

That is why I am looking for weights in 3 point attitude, so I can compare with the level measurements and see how much tail "weight" is increased.

You should see the fatigue and environmental test requirements of the composite spring I'm proposing to use... 200,000 cycles to max deflection with no appreciable loss in spring rate, thousands of hours of salt spray and humidity with no change in properties, high and low temperatures, etc. etc. No way an old stack of steel truck springs could make that IMHO.

Don't know of too many people who've weighed the airplane tail in a three-point attitude, as all CG calc's need to be with the airplane levelled. I did it once just to determine the total aircraft weight, but never wrote down any figures. I do recall that it is quite a bit more than in the level attitude.
Checking my records, FWIW my old ragwing came in at 1318# empty, with 115# of that on the tail (levelled), for a CG of 38.65.

There are also a lot of variables in c 170 configuration. Also, the cg can move around a lot even at gross weight, depending where the weight is located. I assume you would want worst case scenario...cg at aft limit?.

MTV

mtv wrote:There are also a lot of variables in c 170 configuration. Also, the cg can move around a lot even at gross weight, depending where the weight is located. I assume you would want worst case scenario...cg at aft limit?.

MTV

Yes, worst case is likely what the FAA would want to see. So I'm starting to think that a 250+ pound tail load for impact consideration is not out of line.

EZ-

It sounds like for your rough envelope, you don't exactly need precision aircraft-weighing scales, do you? A bathroom scale would work if I filled my plane up first, right? I might be able to do that for you.

I know that in the winter when I'm fully loaded with fuel and survival gear and a passenger, and I have to get out and horse it around by the BAS handles, it feels like between two and three sacks of flour. That figure should probably do for the FAA, yeah?

-DP

it feels like between two and three sacks of flour

What size sacks?

Cary

Cary wrote:

it feels like between two and three sacks of flour

What size sacks?

Cary

PMA'd sacks, naturally.

Unless I missed one, the weights you are getting are empty. Don't you need gross weight and as somebody mentioned aft CG? No doubt you can extrapolate the difference and add whatever buffer the FAA wants for your drop test.