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

[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.

[Archer39J]

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

[Battson]

Thank you

[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.

[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]

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)

[Archer39J]

Yeah I was just pulling up your POH, thanks!

[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.

[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?

[Archer39J]

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

Plumb bob and tape, apparently.

[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.

[DanWard]

I am scratch building and have been for several years, so airplane not flying yet. However, I had carbon fiber doors, both baggage doors and cabin doors made. I also had carbon fiber window frames made. The rear window frame also made from carbon fiber. Weighing these items and weighing the same items made of steel (a friends project) I roughly have a weight savings of 25 lbs.
Dan

[svyolo]

I hadn't thought of that, but the first tool that I bought is a digital scale. Everything I bolt onto the QB airframe will get weighed, and recorded. Even fasteners. After it is signed off, and flying, I will look back at everything, from the standpoint of greatest weight first, and ease and safety of replacement, and slowly pull more weight out of the airframe. But I am going to get it flying first. The only "non-standard" things I am doing on the build are Ti firewall and tunnel, and I will replace the Al floorboards with composite. The rest of the interior will also be composite. But that is it. Everything else will get weighed, and installed. The only purchase I haven't decided on is the prop.

How did you do the doors and window frames? CF square or round tubing, or foam and CF skinned? I like it.

[DanWard]

Foam and CF skinned. CF square for windows. There is a local CF business on the field where I am based. I have always been interested in building light...this conversation has me thinking about building light and paying more attention to CG.

[Collin Campbell]

This has been an interesting thread! Somewhat baffling to me...I went back and looked up the weight and balance calculations for Bob's 0-540 prototype. (Oct. 1999 newsletter) Some things to consider...0-540 engine! 3 blade constant speed composite prop (homemade by Bob...assuming it to be very light) NO starter, NO alternator, NO elec... (all up front items that if added would add to a more forward c.g). no rear seats installed. 10.2" empty weight C.G... 1270 # Makes me wonder what the C.G. would have been IF Bob had used a heavy prop, starter, alternator etc..??

Collin






Bob's 0-540 Prototype.jpg

[Bdflies]

Battson, this has proven an interesting, active and thought provoking thread! One detail I haven't seen is initial build empty weights and post modification weights. I think you shared the post mod numbers, but not the first weigh numbers. I'm just curious.
Thanks!

Bill

[svyolo]

Extremely informative, for myself and many others I think. Thanks to all the contributors, especially Battson for starting it.

[Battson]

Battson, this has proven an interesting, active and thought provoking thread! One detail I haven't seen is initial build empty weights and post modification weights. I think you shared the post mod numbers, but not the first weigh numbers. I'm just curious.
Thanks!

Bill

I have learnt through the discussion too, so I am glad for all your input. All the reasons for the change are still somewhat of a mystery to me.

The initial weights were as follows,
Mains 325kg at about 1.5cm forward of datum (measurement difference there)
Tailwheel 38kg at 5.025m aft of datum (the difference length is because the tailwheel was facing backwards on this weigh-in)

The weight on the tailwheel is the big difference, in fact it's the crux of the whole thing.

[Bdflies]

If I'm adding correctly, your current empty weight is 716.5 kg. Your initial build weight was 688 kg. So, after the lighter prop and lighter ignition, the plane weighs 28.5 kg more. For us metrically challenged, it weighs 62.8 lbs more. I understand that the tailwheel accounts for most of the difference, but there's 2 kg more on the mains, after the weight reductions.
I'm not trying to be the smart alec here, but it really seems that the CG shift has it's roots somewhere aft of the datum, rather than forward.

Bill

[svyolo]

If I'm adding correctly, your current empty weight is 716.5 kg. Your initial build weight was 688 kg. So, after the lighter prop and lighter ignition, the plane weighs 28.5 kg more. For us metrically challenged, it weighs 62.8 lbs more. I understand that the tailwheel accounts for most of the difference, but there's 2 kg more on the mains, after the weight reductions.
I'm not trying to be the smart alec here, but it really seems that the CG shift has it's roots somewhere aft of the datum, rather than forward.

Bill

It might be more complicated than that. Battson mentioned his initial weigh in had the tailwheel turned backwards. I am not sure.

But if the tailwheel is in the same place on both weigh ins, he took 12 kg out of the prop and ignition, and the mains got HEAVIER on the second weigh in, then definitely weight was added to the tail. With no other change than the lighter nose, the mains would have gotten 4-5 kg lighter, each, and the tailwheel would have gotten 3-5 kg heavier.

[Battson]

That weight difference is the standard equipment I mentioned. Maps, flight guides, tools, spare parts, manuals, tie-downs, covers, headsets, etc. It's distributed through the forward cabin and when removed it causes practically no change to the CG location, only the weight. That was my first thought when the weigh came out funny, but no, life is not that simple!

I remain baffled how the tail ended up ~25kg heavier. Much of it's the lighter prop, but where the rest comes from I have no idea.