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

[rodsmith]

Oratex weighs about 5 oz per square yard. The 4 place has about 25 installed yards of cloth, for a total of 125 oz, or just over 8 lbs. "Lightly" finished traditional fabric is double that, or 16 lbs. "heavy" is triple or quadruple. Taking the middle of triple the weight, and traditional is 16 lbs heavier than Oratex, and the centroid of that weight is probably 7-9 feet behind the CG. You can also probably take a pound out of the lead in the elevator due to lighter cloth. A rough guess is that this all has a similar cg effect to a 20 lb difference in prop weight. It only moves the CG an inch or so. But recovering an already covered aircraft in Oratex is a 6500 dollar endeavour. Not worth it to me. But since mine isn't covered, I will spend the extra when the time comes.

If you are doing a fabric interior you can use the lightweight Oratex and that will be an additional weight savings. I will be doing my fabric this summer after Oshkosh so will be taking a good look at Oratex covered airplanes.

[Archer39J]

I happen to be reading a related thread: https://bearhawkforums.com/forum/bea...=7014#post7014

Plumb bob and tape, apparently.

[Chewie]

My measurements are as follows:

Main wheels -55mm (minus)
Tailwheel 5,025mm
Fuel tanks 610mm
Front seats 690mm
Rear seats 1,450mm
Baggage area 1,960mm
Baggage tube 3,300mm (tube full length back to LE of horizontal stabiliser)

How does one get these arms, by physical measurement or by tossing your dog in the back and backing out the calcs from the scales?

[svyolo]

So far my purchase decisions based on CG and cost drove me to a Bob rebuilt O-540. I think a 390 or XP-400 is the perfect BH engine, but I thought the extra weight would help with cg. The 540 is also cheaper. The only other place where I might purposely add weight is the prop, and I haven't made my choice between a Trailblazer or 3 blade MT for that reason.

I will put my battery on the firewall, but I don't think I will use a lead battery. The firewall just isn't far enough forward to justify adding weight in the form of a heavier battery, or at least that is my opinion.

BTW the installed weight of the 540 is much more than the dry weight difference. 12 quarts of oil vs 8 (32 lbs), 2 more exhaust pipes, more baffling, 4 more spark plug wires, etc. Plus your prop is probably bigger and heavier. I think the installed 540 weight is more than 150 lbs different than the 360, given similar choices in accessories and installation.

Oratex weighs about 5 oz per square yard. The 4 place has about 25 installed yards of cloth, for a total of 125 oz, or just over 8 lbs. "Lightly" finished traditional fabric is double that, or 16 lbs. "heavy" is triple or quadruple. Taking the middle of triple the weight, and traditional is 16 lbs heavier than Oratex, and the centroid of that weight is probably 7-9 feet behind the CG. You can also probably take a pound out of the lead in the elevator due to lighter cloth. A rough guess is that this all has a similar cg effect to a 20 lb difference in prop weight. It only moves the CG an inch or so. But recovering an already covered aircraft in Oratex is a 6500 dollar endeavour. Not worth it to me. But since mine isn't covered, I will spend the extra when the time comes.

I am starting to think I might want the engine mounted an extra inch farther forward.

[Archer39J]

Yeah I was just pulling up your POH, thanks!

[Battson]

Thank you

[Archer39J]

Oh yeah it is, I recall seeing it. Have to look when I get home.

[Battson]

Speaking of, is there a list of the moment arms somewhere? I recall Mark maybe mentioning forward seat positions, but it would be nice to have defined stations for fuel, front and rear passengers, and baggage. My cursory search didn't turn up anything.

My measurements are as follows:

Main wheels -55mm (minus)
Tailwheel 5,025mm
Fuel tanks 610mm
Front seats 690mm
Rear seats 1,450mm
Baggage area 1,960mm
Baggage tube 3,300mm (tube full length back to LE of horizontal stabiliser)

[Chris In Milwaukee]

Speaking of, is there a list of the moment arms somewhere? I recall Mark maybe mentioning forward seat positions, but it would be nice to have defined stations for fuel, front and rear passengers, and baggage. My cursory search didn't turn up anything.

Bergy through together a calculator for the 4-place recently:

http://poplar.us/BWB.html

It also shows a graphic from the Patrol. I haven't looked lately for the 4-place diagram. Is it in the Bearhawk Book that came with the plans? I presume it is, but will have to dig it out.

[Archer39J]

Speaking of, is there a list of the moment arms somewhere? I recall Mark maybe mentioning forward seat positions, but it would be nice to have defined stations for fuel, front and rear passengers, and baggage. My cursory search didn't turn up anything.

[Archer39J]

And it's a point well taken, thank you.

[Battson]

Agreed, it's something that warrants closer attention, especially prior to major changes. But it sounds, to me at least, like this is quite a-typical. We're apparently unable to justify what's being observed by calculation knowing the weight of what changed and the respective moment arms. A CG too forward is a problem too, though easier to deal with, but it's probably not prudent to swing too far the other way based on this one account. IMHO.

Not to say I don't think a slightly forward CG at empty is a bad idea, math is math and if you've run those numbers and that's what it looks like great (I will for my plane too).

The point of this thread is not about the reasons for my CG change, although it has raised an interesting question for me seeing as I didn't add weight on the tail (as far as I know) and the numbers don't add up. As per my original post, the numbers never felt right. But that is not the point!!!

Rod has totally understood the point. Adding a few kilograms on the tail combined with using a light prop can cripple your plane's ability to haul a heavy load. Running some scenarios on paper demonstrates the point clearly. That's what I really want to draw attention to.

[rodsmith]

Agreed, it's something that warrants closer attention, especially prior to major changes. But it sounds, to me at least, like this is quite a-typical. We're apparently unable to justify what's being observed by calculation knowing the weight of what changed and the respective moment arms. A CG too forward is a problem too, though easier to deal with, but it's probably not prudent to swing too far the other way based on this one account. IMHO.

Not to say I don't think a slightly forward CG at empty is a bad idea, math is math and if you've run those numbers and that's what it looks like great (I will for my plane too).

There are a lot more things you can do to fix a forward CG. A bag of tools/survival gear attached at the rear of the baggage compartment for example. I agree that the numbers don't add up with Battson's situation.

[Chewie]

Archer, using your numbers I get the same result.

[Archer39J]

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.

Agreed, it's something that warrants closer attention, especially prior to major changes. But it sounds, to me at least, like this is quite a-typical. We're apparently unable to justify what's being observed by calculation knowing the weight of what changed and the respective moment arms. A CG too forward is a problem too, though easier to deal with, but it's probably not prudent to swing too far the other way based on this one account. IMHO.

Not to say I don't think a slightly forward CG at empty is a bad idea, math is math and if you've run those numbers and that's what it looks like great (I will for my plane too).