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

[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]

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!

[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.

[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

[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]

In talking with Bob just now - he had no idea how your tail weight changed so much. When the plane is leveled properly for weighing - the Bushwheels would not make a difference. But lifting the tail off the ground with your arm strength certainly got way harder with the big tires. Mark

[Battson]

I think I might weight it again, just to confirm there was no mistake.

[Chewie]

This discussion should motivate anybody to do a weigh in immediately prior to any alteration.

[marcusofcotton]

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

Svyolo said it well. Agreed that it is not a simple calculation to get it exactly right. The rough 5kg was quick and very rough. Only slightly more refined to answer your question of how it was arrived, picture a teeter-totter in equilibrium. 6' to the left of the pivot is point a with 30kg on it. 15' to the right of the pivot is point b with 12kg to balance the 30kg at a. (ratio of 6/15 = 12/30). Remove 10 kg from point a and you now have a downward force of 4kg at b. (6/15 = 8/20) The pivot of the teeter-totter would be the rough equivalent of your cg. Yes it is an over simplification, but it is roughly correct.

I've very much appreciated this thread as well, and honor your reason for starting it. I in no way mean any attack or negativity. The problem is not lightness though, it is cg (often overlooked and very important to consider). Lightness does have disadvantages when it comes to smoothness in thermals and turbulence. Bob once said (roughly quoted) "The 540s will get you out well, but you still need to get it stopped when getting in". Lightness can result in slower landings with less momentum.

It will be interesting to see how mine comes out (don't hold your breath, definitely a very slow build...). Bob designed it for O-320 as well, and that is what I'm planning (160hp) with Catto recommended 78x54 prop (12 lbs???). I'm very much paying attention to aft weight. And yes, I don't believe I'll be able to haul 1300# of people and supplies due to limits of the resulting aft cg.

No promises that I'll never go for more power or different prop.

Thanks for the discussion and sharing, you've inspired and helped many of us over the last few years! Looking forward to hearing what sneaky combinations have added up to your increased tail weight.

[Bdflies]

I'm not very good at copying and pasting various snippets, but I noticed something that I'd like to clarify.
If I read correctly, your initial weigh was done with the mains centered 1.5cm fwd of the datum (wing LE). But, the recent weigh shows the mains 55mm (5.5cm) fwd of the datum. If these measurements are correct, which I suspect is probably true, I think I can explain some of your additional tailwheel weight.
If the aircraft was not level, but tilted back, indicated by the mains being farther fwd of the datum, then more of the weight will be on the tail. Every ounce of the additional maps, tools, parts and residual fuel will bear just a bit more on the tailwheel, as well.
I'll leave it to you engineers to figure what the additional 4cm might do to the CG. I suspect that difference will be surprising. Jono (I hope you don't mind me using that), if you re-weigh, try to replicate the 1.5cm mains fwd of datum and take out the tools and parts. If your initial weigh was with dry tanks, replicate that as well and point the tailwheel fwd, if that's how it was initially. By the way, you didn't install a bigger tailwheel at some point, did you? We're all curious and staring over your shoulder, from several thousand miles away!

Bill

[svyolo]

This thread made me think a lot. Here is the summary.

1. The BH is not "CG limited". It just can carry so much between its empty weight and gross, that it is easy to get CG limited, before you run out of load. I think all 4 place airplanes have this problem. If you need to carry weight, keep the tail light.

2. There is not a lot you can do to keep the tail light. The basic structure is already very efficient. Don't install a heavier tailwheel assembly than you need. Unless you are trying to win an award at Oshkosh, or get bragging rights at the next campout, don't put a showplane finish on the airplane. Be careful what you install aft of the CG.

3. Put the battery on the firewall. Period.

4. Oratex. The following numbers come from a quote from Bob on Oratex's website. What I wrote is not a quote, but the numbers came from Bob.
It takes 30 sq yards of Oratex to cover a BH, at a material cost of double what traditional material and paint costs. Assume 25 "installed" yards. That is probably a few yards high, but it makes the mental math easier. 25 yards of Oratex weighs about 8 lbs. Traditional fabric/finish, done light according to Bob, is double that. So 16 lbs. So there goes 8 lbs of useful load, centered 10 feet aft of the CG. That might cost you 12-20 lbs of "CG" affected load in the baggage area. That is about 400 dollars/lb of useful load, or 200-350 dollars/pound of CG affected pounds, compared to "light" traditional fabric. Pretty expensive. Too expensive for me, based only the weight saved. But it also saves me 150-200 hours of labor, and makes repairs a lot quicker. For me, that pushes it over the top, and I am using Oratex.

A heavy showplace finish could have a fairly painful effect on the useful load of the aircraft because of the aft CG moment of the fabric and paint. But once again, this is about the same problem for all airplanes.

If I was a scratch builder, and had to justify every dollar spent on my "silly" project to my wife, I would not even consider Oratex. If I was paying a A&P to finish the airplane, Oratex is a bargain. The labor cost saved would be a slam dunk.

5. A light engine? Just put the engine a little farther forward. You WILL give up some CG, but you will also have a delightfully light, fun airplane to fly. Constant speed prop? Very nice to have, but if you are a scratch builder it might add 25% to the cost of the airplane. If you want to carry more weight, a heavier engine will help you do it, if all other things remain constant.

6. Did I mention, put the battery on the firewall?

That is what I have learned. There are not that many decisions to be made aft of the CG. A heavy back seat, and heavy floorboards and aft bulkhead do add weight, but they are not that far aft of the CG. They will cost you weight, pound for pound, but the CG effect is about the same. Basically pound for pound. From the front seats to the firewall, added weight is mostly just added weight. Their moment is not that great. Until you get to the engine (lots of weight), and especially the prop(not as much weight, but more moment), an extra pound in weight doesn't have a huge effect.

The worst thing you can do for the CG and useful load, is a 100 lbs of paint and fabric.

[whee]

Svyolo, You last statement is bang on. It’s easy to end up with a heavy paint job. I fear that even though we made efforts for a light paint job we ended up with a heavy one because of the issues we had.

We wanted to use Oratex but decided it wasn’t right for us. Cost wasn’t the determining factor. To anyone planning to use it I’d suggest getting samples in the colors you want, put them on frames and let them see some sun. Afterwards make sure you are still happy with the color and a punch test.

[schu]

I wasn’t happy with oratex finish either.

[Archer39J]

Definitely get a sample and see for yourself. But for me not having to dope and paint makes Oratex worth the cost. I wonder though, if you’re finishing the interior with another layer would you really see the tubes? Personally it wouldn’t bother me anyway, I’ll take that for the weight savings alone.

[Chewie]

Speaking of fabric weight... Probably more trouble than it's worth, but have you guys seen that picture of a kitfox with a partially exposed aft fuselage?

[svyolo]

There are a few people planes running around without covering on the aft fuselage. I have seen some videos of the STOL contest guys with the same setup. I am not STOL contest guy.

Archer39j. I never enjoyed using dope and fabric on model aircraft as a kid, I can't imagine I would enjoy it now. I was taken aback at the cost of Oratex, but I am still going to use it as well.