Request Data: 170B Weight and Balance question

[EZFlap]

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 Fravel STC conversion upgrade, I will be going from steel to composite tail 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. While admittedly not "original", this will eventually be another option for 170 owners who may value these benefits for their situation.

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.

[akclimber]

Why don't you just get a bunch of real aircraft weight and balance numbers (EW and arm) and then run a computer simulation with different loadings at gross within the flight envelope and some outside of it. You will have to use simple vector math to convert the level attitude to 3 point so you'll need to know the 3 point angle dependent on the height of the airplane - the gear (180 gear conversion) and tire type, as well as tailwheel (baby bushwheel or 3200).
After doing this simulation, you'll end up with a probability distribution of different tailwheel weights. Seems like that would be a much simpler way of doing it than loading and unloading a bunch of airplanes on the scale.

[Bruce Fenstermacher]

I'd look into what akclimber is talking about as we are not likely to have a lot of examples of tailwheel weight at gross in the tree point but we all should have a W&B which of course would be empty weight at level (upper door frame) attitude.

[EZFlap]

Why don't you just get a bunch of real aircraft weight and balance numbers (EW and arm) and then run a computer simulation with different loadings at gross within the flight envelope and some outside of it. You will have to use simple vector math to convert the level attitude to 3 point so you'll need to know the 3 point angle dependent on the height of the airplane - the gear (180 gear conversion) and tire type, as well as tailwheel (baby bushwheel or 3200).
After doing this simulation, you'll end up with a probability distribution of different tailwheel weights. Seems like that would be a much simpler way of doing it than loading and unloading a bunch of airplanes on the scale.

All that work and mathematics, arriving at a theoretical tail weight rather than an actual one... is exactly what I was trying to avoid.

Perhaps I had been asking for something none of you guys have... so let's amend my request and I will ask for the numbers from a NORMAL weight and balance.

Can anyone give me the weight at the tail of a 170B, during a normal weight and balance, both in level attitude and 3 point attitude?

UPDATE: Never mind, I found a thread on this forum that shows a "normal" weight and balance of seeing 115 pounds or so on the tail. But that is empty, and leveled. I believe that in the 3 point attitude the tail weight might be more, because the aircraft CG swings slightly rearward and the main wheels swing slightly forward... as the tail comes down. And when you add the passengers, baggage, fuel etc. there is a gain in tail weight proportional to the main wheel weight.

So at least I have a starting point, although I was hoping not to have to extrapolate into guesswork.

[Bruce Fenstermacher]

Perhaps I had been asking for something none of you guys have...

It would be very unusual for someone to have the information as you asked because a 170 is not weighed at gross and three point attitude.

Arriving at the theoretical weight is probably easier than you think and you will probably want it anyway to confirm the few actual weights you should also get before committing to anything.

115lb is about right give or take for the tail but you also need the weight on the mains and type of main gear so you know the axel arm for the mains. Then you should have enough info.

In your figuring you would also want to load for the worse case which would be the rear most CG.

[falco]

UPDATE: Never mind, I found a thread on this forum that shows a "normal" weight and balance of seeing 115 pounds or so on the tail. But that is empty, and leveled. I believe that in the 3 point attitude the tail weight might be more, because the aircraft CG swings slightly rearward and the main wheels swing slightly forward... as the tail comes down. And when you add the passengers, baggage, fuel etc. there is a gain in tail weight proportional to the main wheel weight.

So at least I have a starting point, although I was hoping not to have to extrapolate into guesswork.

In order to know how far aft the CG moves in the 3 point attitude you have to know the vertical position of the CG as well. Or you could weigh both 3 point and level and figure out the vertical position. Higher the CG is above the mains, the further aft it moves as the tail comes down. fuel load will have biggest effect on this.

I'll have a go at calculating some scenarios tonight or over the weekend.

[GAHorn]

According to records made by the previous owner, my tailwheel wt is 149 lbs, when at gross wt in the 3-point attitude. (I have a Scott 3200 with std 2.50X4 tire.)

[akclimber]

"All that work and mathematics, arriving at a theoretical tail weight rather than an actual one... is exactly what I was trying to avoid."

Hmm, so do you think doing a W&B as required by FAA is just theory unfounded in reality?

It's a very simple mathematical problem and you will come up with much more realistic data from the frequency distribution of tailwheel weights than just loading an airplane once and weighing it... If you are able to navigate FAAs engineering requirements for the STC, you surely can do the math here quicker than it would take to load to full gross and weigh an airplane.

[EZFlap]

George thank you for checking the previous owner's records, I do appreciate it.

AKClimber, I have no desire to circumvent the FAA weight and balance process at all. Weight and balance is both theory and reality. As one of the early competitors experimenting with a tail ballast water tank (contest sailplanes circa 1985), I had a very good grasp of the concepts and risks, even without a hint of an engineering degree.

And trust me, where structural certification is at stake, the FAA considers actual hands-on physical test results much much more relevant than paper calculations. In fact, event though I am going to submit a full engineering analysis, prepared by people whose resume's I'm not strong enough to lift, the FAA will still have me drop test it.

[EZFlap]

I'll have a go at calculating some scenarios tonight or over the weekend.

Thank you sincerely, I truly appreciate it !