Empty Weight and CG Data Collection

[whee]

On a side note, I wonder where the wheels end up on the CG range. Mine are pretty heavy being 31's.

Main wheels are 3" in front of the datum. I think I'll put some 40" BFGs on my plane to help with CG.

[yateselden]

Found my W/B sheet from inspection time. Empty was #1281, pilot 6 gal fuel #1552 at 12.34. Thats going to change with 31s. Wish I knew then what I know now it would be lighter.

[Bcone1381]

School us.

[yateselden]

If I had a do over. No Insulated cabin, aluminum fuel lines instead of SS, no auxiliary tanks lines or pumps, smaller radiator, no sky light. But then I would likely have a three rotor, so I guess it would be a wash. weight wise

[Nev]

Great feedback. Interested to know what engine you’re running? Are main tanks only sufficient for an IO-540 ? Thanks.

[Battson]

Main wheels are 3" in front of the datum. I think I'll put some 40" BFGs on my plane to help with CG.

As a point of comparison, that's not the way they measured on my plane, 0.5" in front.

But it's the distance from the CG which matters, the datum is arbitrary. I know that's what you meant Jon

[yateselden]

Batson why the difference, your BH scratch built?

[Battson]

Mine is a QB kit. We measured very precisely too, so I can't say where the difference is.

[yateselden]

That's interesting, I measured my kit at 3.5 but factory spec is 3 according to Bob

[yateselden]

Nev, I tried to post a response last night. It didn't post. I'll try again, my BH is at the lighter end of things because I use a Mazda RX7 13B Rotary engine. Expense and weight are the reasons for 13B decision. It already has dual plugs, you can't lean it out and burn a piston, it has none. It just stops running with inadequate air fuel ratios. Done correctly can make 180/200 with more rpm. It's very smooth and a lot less weight. A three rotor can make 250 hp and then there is turbocharging. Not a complexity I'm interested in. Not trying to talk anyone into this engine, just my reason's for the installation.

[Nev]

Very interesting - thanks!

[zkelley2]

Weighed mine today. 1413lbs with oil and unusable fuel. 8.00 Tires, O-540, 2 blade c/s composite prop, aux tanks. I know where there's a couple lbs I can shave in steel that shouldn't be there(lb or two), not much. Was shooting for high 1300. Perhaps if I rebuild with 1/2 in fuel lines and ditch the fuel pump and aux, that's probably 5 or so.
I don't really want any weight off the nose though, C.G came to 11.37 empty.

[AKKen07]

Nice work! I hope I’m within 50 lbs of that!

[svyolo]

Nice. Any other details? Electrical, type of covering and paint? Skylight? I left off the aux tanks, and shaved about 20 lbs off by changing material. I don't know what a "baseline" would be, so I don't know what it will weigh.

[zkelley2]

GRT HXr, EIS, xponder, navcom in the panel.
Oratex fabric, areothane on the metal, no skylight.

[whee]

Nicely done zkelley! I’m curious what caused the CG to be that far aft.

[whee]

Understood, thanks. I kinda figured 11" CG was about where I wanted it to be. I came in at 11.25 so I've been looking for ways to move it forward. A firewall mounted tool kit and a smaller tail wheel might get me there. Less paint on the fuselage next time would help. Eliminates the possibility of composite prop probably, a 3 blade might work.

[zkelley2]

Did the math on that steel plate. If I cut it out, it would move my C.G. forward about a half inch. That's worth it, so I'll be doing it. Going to the 3 blade version of my prop, and I would assume another brand of composite would be close adds 18lbs to the nose, at a -58.5in arm, that moves the C.G to 10.15, which would be awesome.
Guess I need to save up for that 3 blade reversing MT.

[Nev]

Any idea why the steel plate was welded in the tail ? Does it have a leaf spring or stinger arrangement ?

[Battson]

4 place with IO-540


2013 original build (details in post #7 above):
679kg and 0.271m (1,496 lbs, 10.67 inches)


2017 rebuild, added 31" bushwheels, lighter prop & ignition, and extended wingtips. Doubled down on paint in on the bottom skin of the wings. Removed baggage tube and installed lightweight fibreglass aft bulkhead.
699kg and 0.340m (1,537 lbs, 13.38 inches)


I still need to shift the battery forward onto the firewall (inside cabin)

[kestrel]

13.38 inches)

Really? It moved 2.7 inches?

[Battson]

Unless the scales were wrong. It feels that way, when I fly.
I couldn't believe it either, we never could calculate why it happened exactly. There are plenty of threads about it on this forum.

[Nev]

Bearhawk Bravo 4-place

1498 LBS
CG 8.3 Inches aft of leading edge.

3 bladed Hartzell Trailblazer
IO-540
Dual PMAGS
EarthX battery forward of firewall
Fabric interior in back
Kydex in front
Corflute rear bulkhead
Polyfiber covering

[Mark Goldberg]

That is a very respectable empty weight Nev. For a plane finished out as nicely as yours. Mark

[LukeFrog]

Great numbers. All that Kydex paid off eh Nev?

There's a t-shirt idea: "Kydex in the front, party in the back"

[Nev]

As a point of comparison, that's not the way they measured on my plane, 0.5" in front.

This is interesting. Mine is different again. The most obvious answer would be if we are not all measuring at the same angle, but I also wonder if there have been changes to the kit itself.

Does anyone know if the main landing gear is located in the same place on all kitsets ? Or has there been a change over the years to accommodate the heavier engines ?

Same with the wing leading edge. Has there been any change that would affect the leading edge position ? Did the B model wing change the leading edge position at all ? (Which would affect the datum point that we use for comparison). I recall there being some issue with leading edge to windshield fit on some early B models.

[Mark Goldberg]

The landing gear position did not change with the B model. As to whether the leading edge of the B model wing is different than the A model wing - I doubt it but do not really know. Mark

[Battson]

The most obvious answer would be if we are not all measuring at the same angle, but I also wonder if there have been changes to the kit itself.

Yes - that would be my guess. An error of just 2 degrees would more than explain the differences we are seeing. That is around 200mm at the tailwheel, which sounds like a lot, but it's hard to know exactly how level the steel member is that we're measuring the master deck angle from. A two degree difference in one short steel members, like the top window frame, is very believable.

[Nev]

I'm working on a spreadsheet to show the location of all the arms, including a number of additional arms such as under-seat avionics, battery aft of firewall etc so that builders can simply type in the weight of the component they plan to add, and see how it affects weight and CG. An example would be adding a 3-bladed prop, autopilot servos, baggage tube with an IO540. It won't be perfect but should be very close, and I've done one for both an IO360 and an IO540.

This week I will "lift the tail" with my hoist and measure all the arms from scratch. What would be really neat is if Mark or Bob could then double check the measured arms and confirm them as correct. Then, all the kitset arms for the 4-place would already be known, thereby simplifying the process.

9E8E3C05-8A0B-43AD-83A5-5BD8566B7B78.jpg

[jaredyates]

I'm happy to help spread the word via the newsletter and KBM once you have it together.

[davzLSA]

help need some LSA numbers, Im not flying yet so I dont have any numbers yet. Hey BTAZ, Stinger where are you??????