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

[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

[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.

[marcusofcotton]

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

[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.

[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"?

[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.

[Mark Goldberg]

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

[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]

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

[Battson]

The four place tail spring is 6150 steel hardened to around 60 Rc. It is made from 1 inch bar stock which tapers down to 3/4" where it fits into the tailwheel. You can run the numbers. It is my understanding from Bob that going this route does not lessen the toughness of the tailsprings. MG

What is the weight saving with the round spring?

[Mark Goldberg]

The four place tail spring is 6150 steel hardened to around 60 Rc. It is made from 1 inch bar stock which tapers down to 3/4" where it fits into the tailwheel. You can run the numbers. It is my understanding from Bob that going this route does not lessen the toughness of the tailsprings. MG

[Battson]

Bob recommended that you look at lightening the tail of the plane. The tail is so far back that for the 20 lb less prop - you could reduce the weight in the tail by 6 lbs and it would be the same as before the prop change. How could you do that? Bob said the tail is 3X as far back from the datum as the prop is forward.
1) go with the round tail spring. Simon has one and the adapter to use it. But be sure to contact me about this if you decide to go this route.
2) Remove the heavy fabric on the tail surfaces and replace with Oratex. There would be a 2-3 lb minimum reduction in weight on the tail from just this. The elevator especially since using the lighter fabric would allow you to remove lead counterweight that you currently have to balance the heavier fabric.
3) Floorboards in the baggage area - replace .032 that came with your kit with .020 kevlar
4) What do you have for the back bulkhead of the baggage compartment? If it is aluminum, then you could replace with canvas as a lot of guys have done.
5) I know you have electric trim which you are probably very happy with - especially if it been well behaved for you. But the servo in the tail does add weight.

That is all the specifics of what Bob said. But his emphasis was reducing weight in the tail. Mark

Thanks for sharing Bob's thoughts Mark. He is always full of good ideas!

I am tempted by the tail spring change.... however on days like yesterday when we load our plane to the maximum and take-off from a very rough airstrip, I think about all the punishment which the tail spring tolerates. Having a heavy-duty spring is my main concern.

[Battson]

As far as lengthening the nose - two inches would fix the CG issue but Bob thought it a poor idea because yaw stability would for sure worsen. You would have your old CG range but at the cost of a plane that wallows in the sky much more. It has been tried by others and it was not good.

bergy FYI - you might be interested.

[Mark Goldberg]

I have been following this thread but reluctant to comment as my CG calculations etc are very lazy and lacking. I did have one idea that I ran by Bob before posting this.

Of course Bob didn't think too much of my idea so I will not even mention it. But Bob, being the design engineer that he is - came up with some very interesting ideas/thoughts.

As far as lengthening the nose - two inches would fix the CG issue but Bob thought it a poor idea because yaw stability would for sure worsen. You would have your old CG range but at the cost of a plane that wallows in the sky much more. It has been tried by others and it was not good.

Bob recommended that you look at lightening the tail of the plane. The tail is so far back that for the 20 lb less prop - you could reduce the weight in the tail by 6 lbs and it would be the same as before the prop change. How could you do that? Bob said the tail is 3X as far back from the datum as the prop is forward.
1) go with the round tail spring. Simon has one and the adapter to use it. But be sure to contact me about this if you decide to go this route.
2) Remove the heavy fabric on the tail surfaces and replace with Oratex. There would be a 2-3 lb minimum reduction in weight on the tail from just this. The elevator especially since using the lighter fabric would allow you to remove lead counterweight that you currently have to balance the heavier fabric.
3) Floorboards in the baggage area - replace .032 that came with your kit with .020 kevlar
4) What do you have for the back bulkhead of the baggage compartment? If it is aluminum, then you could replace with canvas as a lot of guys have done.
5) I know you have electric trim which you are probably very happy with - especially if it been well behaved for you. But the servo in the tail does add weight.

That is all the specifics of what Bob said. But his emphasis was reducing weight in the tail. Mark