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

[Battson]

I do not know for sure Jonathan. But Simon has both and he could probably help by weighing them. In your case you would need to include the adapter with the weight of the round spring to know the weight savings. I had a set of the flat springs but just took them to my ranch where a customer is supposed to pick them up later this week. So I do not have a set to weigh. If for some reason Simon can not help - let me know. Mark

SimonNicholson, are you able to help?

[Mark Goldberg]

If you do not have contact info for Simon - let me know. MG

[svyolo]

Battson;
I don't have a CG program, or even my plans in front of me so that I could do it manually, but 10kg out of the prop arc is about 1/3 of the cg effect of taking 60 or 70kg out of the back of the engine (540 vs 360). If you took your cg movement, and multiplied it X 3, a O-360 powered BH, using the same engine datum, would be way beyond the aft limit sitting on the ramp, empty. I still think there is a math error if your CG moved as much as you wrote earlier.

[Chewie]

Bob said the tail is 3X as far back from the datum as the prop is forward.

By "datum", don't you mean "CG"?

[Battson]

No, datum is correct.

[whee]

Battson, I have the tapered rod spring and adapter in hand. I can weigh them tomorrow. I will also try to remove my flat spring and weigh it so we can compare.

[marcusofcotton]

When you weigh them, please report weghts for adapter and spring separately. Thanks!

[Chewie]

This thread has brought some interesting points to a head. It seems that it's more common to hit CG limits than gross weight. This practically puts the nail in the coffin on light engines such as UL Power, and makes a great case toward Oratex, since the majority (all?) of the fabric on a bearhawk moves the CG back.

[jaredyates]

We are still waiting for the supporting data, and I would be reluctant to form hard and fast conclusions until it is posted. I'm surprised that our requests for the data have not been answered, especially in the presence of such strong statements about the capabilities of the design and its CG capabilities.

[n144sh]

Battson, would you mind posting more details for CG? What were the weights on each wheel and at the tail in level attitude? Also, maybe I missed it - what prop did you change to? Is it the Catto or MT?

Thanks,
Steve

[whee]

Adapter: 0.90lbs
Tapered rod spring: 3.3lbs

Flat spring: 4.5lbs

[whee]

This thread has brought some interesting points to a head. It seems that it's more common to hit CG limits than gross weight. This practically puts the nail in the coffin on light engines such as UL Power, and makes a great case toward Oratex, since the majority (all?) of the fabric on a bearhawk moves the CG back.

This has has been my experience but that was with a O360 powered BH. I tried to shoot the gap between the nose heavy O540 and the CG limited O360 planes. Don’t get me wrong, the O540 BH flies great, I just preferred the feel of the lighter engine but wanted more utility. This is similar to the early Cessna 180 vs later C180 argument.

Just my opinion after limited experience messing around with Oratex samples; if really needed weight loss and the covering system was my best option I’d be apply new fabric and spraying it with exterior latex house paint. No Oratex for me.

[Battson]

Battson;
I don't have a CG program, or even my plans in front of me so that I could do it manually, but 10kg out of the prop arc is about 1/3 of the cg effect of taking 60 or 70kg out of the back of the engine (540 vs 360). If you took your cg movement, and multiplied it X 3, a O-360 powered BH, using the same engine datum, would be way beyond the aft limit sitting on the ramp, empty. I still think there is a math error if your CG moved as much as you wrote earlier.

No, we have checked the math several times. Checked the measurements. We have flown the plane to confirm. The CG has moved almost halfway through the envelope. Like I mentioned earlier there was a small mistake but it only accounted for a few percent.

Remember the prop is about 60 inches forward of the datum - maybe 80 inches forward of the CG location. That is a very long arm, even a little weight makes a big difference.

Wheels 55mm forward of datum, weights 325kg and 327kg. Tailwheel aft of datum 4.77m, weight 64.5kg. Do the math if you like.

Note the weight includes standard equipment, unusable fuel, flight manuals, tie-downs, etc.

[Battson]

This thread has brought some interesting points to a head. It seems that it's more common to hit CG limits than gross weight. This practically puts the nail in the coffin on light engines such as UL Power, and makes a great case toward Oratex, since the majority (all?) of the fabric on a bearhawk moves the CG back.

You have fully understood the point of this thread.
I've been saying for years that the CG is the limiting factor for the Bearhawk. I am keen to raise awareness of that fact.
The 2700 max gross is a non-event, you could never use it on a land-plane. I have flown at or above 2500lbs on perhaps 5 occasions, whereas I've flown with the CG near or at the aft limit nearly 50 times.

But I don't think it precludes lighter engines, you could put a turbine in a Bearhawk if you wanted. You would just need an unusually long engine mount, like all retrofit turbine engines. You would have to live with the associated disadvantages too.

[Archer39J]

Well thanks for sharing that here, eye-opening for me at lest...

[Battson]

Battson, would you mind posting more details for CG? What were the weights on each wheel and at the tail in level attitude? Also, maybe I missed it - what prop did you change to? Is it the Catto or MT?

Thanks,
Steve

Hi Steve. I have posted the weights and arms above. My original CG position was 250mm aft of datum. The new CG position is about 380mm aft (rounded). You will note it's different to my first post, I corrected for some small errors.

The prop is the Hartzell Trailblazer, and it's a great prop so far. I don't think the prop is at fault, I simply need to re-balance my plane by changing a few minor things.

That is really the key message. For a Bearhawk the balance is as important, or even more important, than the weight.

[Bcone1381]

I think I should get some of scales for my work shop. I think I should declare a goal for the empty CG when its finished. I think I should weigh everything, and keep an eye on where its at in relation to my goal. Then I can make educated equipment decisions based on the calculated effect of the empty CG position. I do not know how far aft the empty weight CG can be and still load it to MTOGW. I do not know who far forward the CG can be and fly solo with 6 gallons of fuel on board. Those two numbers will be used for the target of my Empty CG envelope. I will be calculating those numbers today.

[Archer39J]

I've been reading that the pilot and fuel is all aft CG, so I figure shoot for an empty+pilot CG at the forward limit?

[Battson]

Mine was originally forward of the limit without pilot and fuel. Even with only a few gallons and 160lbs of me, I could easily control the aircraft on finals at or below the stall warning. You just need a spurt of power to flare, or VGs / gap seals on the tail instead.

[Archer39J]

And here I was looking at putting a servo and some linkages in the rudder...

[Battson]

And here I was looking at putting a servo and some linkages in the rudder...

There's nothing wrong with that, or with using a light prop.
It just means you need to put other heavy things forward to compensate.
But what I failed to do was calculate the consequences ahead of time. I can't just move my battery or something simple. Even moving the battery, removing the baggage tube, and chopping off the whole tailwheel(!!) wouldn't be enough weight savings to rebalance the plane in my case! I will have to find ways to make the nose heavier and the tail lighter wherever I can.