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

[Battson]

It has happened that air-plane mechanics leave a wrench inside of a closed up bay. In your case - it seems they left the entire tool chest in the tail of your BH. Mark

I wondered if maybe the fuselage tubes have filled up with water or something....

It's a total mystery.
Maybe bushwheels have some magical effect whereby they make the tail heavy.

[Mark Goldberg]

It has happened that air-plane mechanics leave a wrench inside of a closed up bay. In your case - it seems they left the entire tool chest in the tail of your BH. Mark

[svyolo]

I know your an engineer, but me too, by education. To find something small use a magnifying glass and a calculator. To find something big, take 5 steps back, and look with your eyes. You miss big things, looking closely. Until you posted your moment arms, I never even bothered with numbers. I just looked, and did visual math.

It is easy to get lost in numbers crunching a CG off of a randomly selected datum. it is like filling out a US tax form using the instructions. You just blindly follow what they tell you, and when you are done you have no idea how you got there. I am sure KIWI tax forms are simple. You should see how stupid ours are.

Once again thanks for the thread, and your time. I am here to learn, and I do almost every day.

[Battson]

Actually, only very roughly ~5kg would be attributable to the lighter prop. Mud daubers busy in the tail using depleted uranium?

I would love to know how you guys figured that out. It's not a simple calculation!

[marcusofcotton]

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.

Actually, only very roughly ~5kg would be attributable to the lighter prop. Mud daubers busy in the tail using depleted uranium?

[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.

[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.

[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

[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.

[svyolo]

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

[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

[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

[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.

[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]

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