Making your Bearhawk too light - the best way to ruin a good airplane!

[rodsmith]

This discussion got me calculating weight and balance under different loading conditions, making some assumptions about what my plane will weigh. Battson is right, you definitely don't want an empty cg much past 11", preferably less, if you want to fill the back seats or fill the back with a load of camping gear. I know my plane is going to be on the heavy side, I had beefed up the tail some anticipating lots of off field use, that now looks like a real bad idea. I'm now thinking of taking a hard look at using Oratex fabric. Considering that all the fabric weight is behind the cg that could make a big difference. Another thought is to build composite rear and baggage doors, I'm pretty sure I am going to remake my floorboards in composite. On the plus side my 3 blade MT prop is on a 3" extension so that helps. Prop, spinner and extension weigh 58#. I was considering an EarthX battery. Not now, will have the 15# battery on the firewall.

[Archer39J]

Yes that is true.....

If you think about it, the CG of the 540 and 360 are both in the middle of the engine. The middle of the 360 is further forward than the 540. So the CG of the 360 must be further forward than the 540, by quite some distance for that matter, its engine mount is about twice the length. So the 360's moment arm is longer, which offsets the lower weight of the 360. At least that's my logic. Someone may care to run the numbers and post them.

Just running some numbers based on what I was able to find online. Assuming the CG of each engine is in the center (probably not, due to the heavy bits on the front). Using the prop flange as the fwd point at 58.5", an IO-540D4A5 @ 412# and 39.34" in length will have a moment of -15998 in-lbs (412*(-58.5+(39.34/2))). A IO-360-L @ 330# and 31.33" in length will have a moment of -14136 in-lbs (330*(-58.5+(31.33/2))). Saying the 540 is in an aircraft with an empty weight of 1550# at the fwd CG limit of 10.5", a moment about datum 16268 in-lbs, this translates to the 360 shifting the empty CG aft 1.85" at a gross weight of 1468# and moment about the datum of 18130 in-lbs.


Please correct me if I've made any errors.

[Archer39J]

Just looking at the prop I get 1.44" aft CG movement between new and old. Something else is putting weight somewhere far aft...

[svyolo]

Battson;
I absolutely believe you that your tail got 27 kg heavier. A rough SWAG says only 3.5-5 of that came from the change in prop and ignition. CG creep over the years? Dirt, etc? Osmosis? Does fabric slowly absorb moisture over the years and get heavier? Lots of patches from damage? Electric trim added after the initial 35 kg weigh in? All of the above?

[Chewie]

Yes it is useful and the math works out but you have to be consistent on your choice of a datum. What I'm saying is if you add 5 lb on one side of the datum at a given distance and then you add 5 lb on the other side at the same distance that does not mean it is balanced. The CG is the real teeter-totter that we need to be thinking about.

[Bdflies]

Yeah, I meant 13 cm... A number of years back, I remember a big push to bring the USA to the metric system. It didn't work, did it?

[Battson]

Your empty CG moved aft 130 cm (5.12")? How much lighter is the Trailblazer prop? I guessed 20 more pounds, at 60" fwd of the datum. Rough figuring, that moved the CG 1" fwd. To move 5" aft, from your original position, by my very rough figuring, would mean the new prop is 100# lighter.
I hear you, that it feels aft CG. I also hear your disappointment. Hard to see how the prop and ignitions made that much change. You mentioned that new numbers included "tie downs, etc.". I'm thinking about the etc..

Bill

Not sure where the 130cm came from, did you mean 13?
Yeah I tried removing the tie-downs etc, has almost no effect / makes no difference.

I am with you guys on the math, the prop does make a large difference but I cannot account for all the movement with the prop alone. That is a real measurement too. So while the prop accounts for most of the CG move, something else must have contributed too.

All I know for sure is my tailwheel was originally 38kg, now it's 65 (round numbers). That drives the CG move.
I know I haven't added weight aft of the CG, so I can only assume it's a) my efforts to lighten things, or b) some error in the official measurements. But the CG has moved a long way in real life. I can see it when I fly, it's obvious. So I am left confused.

[Bdflies]

I seem to have been typing when Chewie posted and you replied. Seemingly redundant post...

[Bdflies]

Your empty CG moved aft 130 cm (5.12")? How much lighter is the Trailblazer prop? I guessed 20 more pounds, at 60" fwd of the datum. Rough figuring, that moved the CG 1" fwd. To move 5" aft, from your original position, by my very rough figuring, would mean the new prop is 100# lighter.
I hear you, that it feels aft CG. I also hear your disappointment. Hard to see how the prop and ignitions made that much change. You mentioned that new numbers included "tie downs, etc.". I'm thinking about the etc..

Bill

[Battson]

Thanks for running the numbers, but I cannot replicate that result mathematically.

That being said, I can't determine where all the balance change has come from either. The prop and mags alone don't account for it by my math, but then again my math is one big unknown.

All I know is I have a certified legal balance position before and after, calculated by real weighs using professionals and calibrated equipment, and it's come out with a big difference, between 4 or 5 inches.

Further - I have done a quick sensitivity analysis. The weight on the tailwheel seems to be the most sensitive variable. Large errors in all other measurements have little effect. Not that it help me much at this stage.

[marcusofcotton]

Using your numbers above, 40# for new prop at 60" vs 70# at 61" for old prop, 2# at 24" for mags, I come up with 1.5" cg change (12% of cg range). 14.93" new vs 13.47" old.

This was done with plenty of distractions so mistakes could have easily been made in my above work.

Hope you come with a good solution soon!

[Battson]

Remember, the datum can be a completely arbitrary point, so it doesn't make sense to talk weight changes by balancing about the datum. The datum can be at the prop, or the tailwheel, or a mile ahead of the airplane.

I think this might be what's caused some disbelief / confusion in this thread.

The prop is a long way forward from the datum, but even further away from the physical CG. Almost 2 meters at aft CG locations. That is a huge moment arm, and the prop weight change is significant. These two big factors are then multiplied together.....!

A change in prop weight has a huge impact on the balance as a result.

I was unsure about the results to begin with, but now it's all confirmed.
I really think builders need to be aware of this when making design changes to save weight. Two-blade composite props and fixed pitch props come to mind.
The Bearhawk has huge utility, but it's all dependant on your ability to load the plane up.

[Battson]

The prop flange for either engine is at 58 1/2". The engine mount is longer because the 360 is shorter.

Yes that is true.....

If you think about it, the CG of the 540 and 360 are both in the middle of the engine. The middle of the 360 is further forward than the 540. So the CG of the 360 must be further forward than the 540, by quite some distance for that matter, its engine mount is about twice the length. So the 360's moment arm is longer, which offsets the lower weight of the 360. At least that's my logic. Someone may care to run the numbers and post them.

[Archer39J]

Great resource, I was just looking at that earlier. We're all using the same datum though, so I think it's useful for our purposes.

[Chewie]

Remember, the datum can be a completely arbitrary point, so it doesn't make sense to talk weight changes by balancing about the datum. The datum can be at the prop, or the tailwheel, or a mile ahead of the airplane.

For the truly inquisitive, the FAA managed to fill 114 pages on the subject:

https://www.faa.gov/regulations_policies/handbooks_manuals/aviation/media/FAA-H-8083-1.pdf