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

[Battson]

Well,

In the "never-ending pursuit" of lightness we recently swapped out to a two blade carbon fibre constant speed prop, also light weight electronic ignition. All the weight came off the nose.
I need to double check all the measurements from the weigh-in, but they were done professionally. At this stage it's been a disaster from a CG perspective.

The empty weight CG location has moved from 15.8% TOMAC to 26.1%. The limits are 16% to 34%.
This means I've wasted half my CG envelope before I even put myself or any fuel in the plane.

Before, I could load full fuel and four 90kg (200lbs) people into the plane and then still have room for 20kg (44lbs) of luggage.
Now, I can't put ANY 90kg (200lbs) people in the back, not even one. Forget about any luggage....

What a disaster. I've no idea what I will do about this.

To the people using fixed pitch props..... sell them immediately! Quite apart from the performance sacrifices, you might as well have a two place aircraft.... Seriously. The empty CG needs to be forward of the limit to get any real utility from the aircraft once you start loading it up.

[Mark Moyle]

Where did you put the prop flange ahead of the datum line? 56-1/2” or 58-1/2”.

[Chewie]

Stupid question, where are front seat occupants in relation to cg?

[Battson]

Where did you put the prop flange ahead of the datum line? 56-1/2” or 58-1/2”.

Our prop has a 3" extension, so it's blade centreline is over 60" ahead of the datum.

[Battson]

Stupid question, where are front seat occupants in relation to cg?

So CG ranges from 26 to 57 cm aft of datum, and the front seat is at 69cm aft of datum. Meaning the front seat is always aft, adding weight in the front moves the CG aft.

Everywhere you can add weight is aft of datum.

I will be moving my battery forwards, as far as possible... if possible I'll hang it inside the nose bowl...!!! Right now it's 7kg about 12" aft of CG.
I will also be scrapping my baggage tube (5.5kg a shocking 3.3m aft of datum), and looking to shave weight off the tailwheel if possible.
Let me know if you have any other ideas.

[whee]

Something seems off to me. 6MK had enough CG range to fly with 200lb people in the rear seat. Surly a o360 with a CS prop weight less than your setup. Besides your baggage tube, what do you have in the back that weighs so much?

[Bdflies]

Battson commented that the plane was weighed by proffesionals, so I'm VERY reluctant to comment, 'cause I'm not a professional anything. The CG shift seems more than I could imagine, from the changes described. The only thing I could imagine is that the plane was weighed with the tail on the ground, rather than level. Such an oversight would explain the CG shift.

Bill

[Battson]

Battson commented that the plane was weighed by proffesionals, so I'm VERY reluctant to comment, 'cause I'm not a professional anything. The CG shift seems more than I could imagine, from the changes described. The only thing I could imagine is that the plane was weighed with the tail on the ground, rather than level. Such an oversight would explain the CG shift.

Bill

Nope it was weighed correctly, in terms of technique. My only question is the resolution of the scales at low values, but it seems correct.

The only change was hanging a prop which weighs over 20 lbs less, one inch aft from the old prop (shorter extension), and then losing 1kg from the mags.

[Battson]

Something seems off to me. 6MK had enough CG range to fly with 200lb people in the rear seat. Surly a o360 with a CS prop weight less than your setup. Besides your baggage tube, what do you have in the back that weighs so much?

Did it have a carbon prop....? Because mine was great with a metal prop. I have nothing else aft except the tube.

I know what you mean though, when Iook at the math it seems like it doesn't add up... Which is why I put the underlined caveat on my original post. However the signs and symptoms when I fly the plane corroborate the CG moving a long way aft. We are doing a practical test this evening.

I should probably have run the numbers beforehand, and moved the engine forward with bushes at the firewall. If I want to do that now, I need to disassemble the whole firewall forward and put a new prop extension in. Huge job and $,$$$.

[rv8bldr]

Man, does that ever SUUUUCK. However, you just made be feel better about the used Hartzell I bought and just sent in for overhaul. I had been planning on a Catto fixed pitch like on my RV-8 but changed my mind after reading some of your other posts in the past regarding CS versus FP.

[whee]

Bummer deal that the flight characteristics corroborate the major move aft. It will be interesting to see how it is resolved.

I guess I'll get to work on the nose locker I've been thinking about to help bring my CG forward.

[Chewie]

Jonathan your plane flew well before. Did you actually crunch the numbers from the swap analytically to cross check?

[jaredyates]

I'm thinking the same thing!

[Archer39J]

Counterweights? Would adding weight at an appropriate arm at least get you back to where you started?

[jaredyates]

If you'd like for us to check your math, post your weight at each wheel at the first weighing, and at the recent one.

This is just arithmetic, not like the black magic of oil temps and cooling drag. You've still got way more weight forward of the datum than any of we 360 operators, signs are definitely pointing to a measurement or calculation discrepancy. We should be able to account for the weight changes in calculations to verify the measured results.

[Archer39J]

Yeah we're still talking about a 540 here right?

[svyolo]

Battson,
If your math is correct, an o-360 Bearhawk would be unflyable. I don't have the measurements, but 10 kg out of the prop does not equal 2 90 kg people in the rear seats from a cg perspective. The Surfly ignition is only slightly lighter than mags. I think I read 5 or 6 lbs each.

Just a swag: the prop looks about double the distance from he cg than the rear seats. 10kg out of the prop would mean 20kg less weight available to sit in the rear seats.

[Bdflies]

I'm with you! Haven't done the hard numbers, but at first blush - something doesn't pass the litmus test of "what am I missing here?"

Bill