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

[kestrel]

I have done a bit of digging. Not yet enough to present a full analysis and proof, but I think I've found the bit the makes Bob right. Though a moment is mass*arm, rotational inertia is mass*arm^2. 1/2 the mass at twice the arm ends up having twice the rotational inertia. I'd like to do a bit more analysis of this and also look at the percent difference in rotational inertia vs. the base air frame.

[Battson]

This is almost a separate thread... The rotational inertia is calculated based on the arm from the center of rotation, which will be some imaginary point - which is not equal to the CG (is some rare cases it may overlap the CG). The forces acting on the aircraft are important. As per the link shared above, the arms, forces, and axis of rotation are constantly changing during a spin. I think it's very complicated to calculate. I am not an expert on spins or aircraft stability.

[kestrel]

The wing is not horizontal and is producing lift, so the path will be a spiral. However, I believe that forces acting on the free rigid body of the aircraft will produce rational torques/accelerations about the CG. The forces/torques of the ballast will act on the CG because the aircraft is "free".

[Battson]

Yes - he concurs that unless you're in a perfectly flat spin, the aircraft does not spin around the CG.

[kestrel]

Search for "ball" (the formatting of that page is rather unfortunate):

https://www.avweb.com/features/bill-kershner/

[svyolo]

Agree 100%. I can fix nose heavy. I can only try to fix tail heavy, limiting what i carry. I will use repositionable ballast/survival/camping gear.

Where i will be flying, I should be carrying that gear on almost every flight. Similar to your south island, but a lot bigger.

[Battson]

You're assuming the spinning aircraft rotates around the centre of gravity or maybe the datum... It doesn't?

[kestrel]

Battson, you are also making the same incorrect analysis. It is NOT the same weight. It is the same moment with a much longer arm. Bob may have a point that I've missed, but it hasn't surfaced yet.

[Battson]

Bob is right. If you have the same weight distributed differently, that effects the rotational inertia. The more inertia, the more force required to overcome it and stop the spin.

[jaredyates]

Hmm, you guys make a good point.

[kestrel]

Jared, your analogy doesn't fit. It would be 20 lbs each side of center on the stick. It would be exactly the same difficulty to "twirl" it, except that you'd have to be lifting 40 lbs instead of 10.

[Battson]

The main point of my post is to draw awareness to the fact that on the B model, there's no longer a need to build with all the weight as far forward as possible if using an IO540 etc, and that if you do, you might end up having to add aft ballast (well in excess of a 50lb survival/tool kit) just to get close to being within a normal range).

Well, there are factors to consider before a builder reaches that conclusion - I don't see the B model being materially different - the builders choice depends on:
- the builder's mission
- whether the builder wants to store gear further aft, baggage tube
- the individual build
- flying style, preference for power on approach, steep power off descent etc.

If the pilot can use heavy essential gear as repositionable ballast, then you've achieved the holy grail of CG flexibility.
I think having a CG fixed further aft is a limitation you don't want, as you say - I am stuck with my CG a lot further aft than yours. I can never carry as much useful load stowed in the same way. Who's better off?

[Nev]

The main point of my post is to draw awareness to the fact that on the B model, there's no longer a need to build with all the weight as far forward as possible if using an IO540 etc, and that if you do, you might end up having to add aft ballast (well in excess of a 50lb survival/tool kit) just to get close to being within a normal range).

There seems to be a change somewhere that I can't put my finger on. The stations used for CG calculation seem to vary between aircraft in a way that isn't necessarily explained by having the tail higher or lower during the weighing process. Maybe someone can shed light on this. Perhaps having CG locations (main gear, front seats, cargo area etc) supplied on the plans might help so we're all using the same data.

[svyolo]

I dissagree with Bob with great reservation. He knows every part on his airplane. I

[svyolo]

I agree Jared, but in this case it is taking 50 pounds from close to the middle, and replacing it with 12 pounds 3 or 4x away from the rotation axis. I believe the polar moment of inertia is the same if you reduce the weight as you move farther from the CG.

I would still rather have the extra gear.