Quick CG lesson, please

[DesertBearhawk]

Ok, I know the basics and I've balanced a ton of model airplanes so I know the goal, but you have a ton of latitude in moving stuff on a model, not so much on a full scale airframe. There has been much discussion in relation to the 540 vs the 360/390. Some of it had to do with the CG and useful load, the jist of it being that because the 540 weighs more you can add more weight aft of the balance point. Makes sense to me.... But it got me thinking, as I build the airframe I should be conscious of the balance point and how the location of equipment will effect the final balance point. Can I get some insights from those that have completed their BH and the methods used? Hope my question makes sense....

[Battson]

Hi Dave,

If you follow the standard airframe design per the plans, you are very unlikely to have any CG problems. The CG envelope is generously sized.

In my humble opinion, a CG issue is more likely to arise due to specific "finishing out" choices. For instance, I have been told that one or two machines with large and heavy three blade alloy props have been nose-heavy enough to cause a handling problem. I believe this issue was somewhat mitigated by moving the battery behind the aft bulkhead, if I recall correctly.

I know of other planes which tipped the scales well outside of CG limits - these were not on Bearhawks - it added months or years to the build in each case. They both had to make a new motor mount and modify cowling to fix the problem properly. One older guy actually missed his chance to fly the plane with the extra year's work (lost his medical). Real sad story...

[Enewton57]

Build it to the plans unless you are planning to add something unusual. If needed there are few tricks to move the CG but it probably wont be necessary. If you go with a lightweight engine, move your CG forward with a constant speed metal prop, old fashion prestolite starter and regular alternator, plus put your battery on the engine side of the firewall. If you have a heavy engine you can move your CG aft using a composite or wood propeller, light-weight skytec starter, lightweight alternator and put your battery, Elt, etc, further aft, like under your seats. If your CG is is still too far forward when you fly alone, you can put some gallon jugs of water in the baggage compartment. Then, if you carry people in the backseat or baggage, you can just pour out the water. As a last resort you can add a pound or two of lead back in the tail, but its doubtfull that will be needed. In my experience Forward CG is not usually the problem. Its more common that people have too aft CG because of thier heavy engine with heavy propeller. That then, limits what they can carry in the baggage compartment.

[rv8bldr]

Good stuff deleted....

Its more common that people have too aft CG because of thier heavy engine with heavy propeller. That then, limits what they can carry in the baggage compartment.

Uh, Eric, don't you mean "light engine and light propeller"? Otherwise they wouldn't have an aft CG problem.

[Battson]

Easy mistake to make in a hurry. I made it yesterday

[Enewton57]

Whoops - yes I meant forward CG - what limits what they can carry is gross weight not CG Eric Newton - Long Beach, MS http://mybearhawk.com

[Helidesigner]

I have an IO-540 and three bladed composite prop. My battery is on the firewall and my interior is probably average weight. Just me at 205 lbs and half fuel the plane still lands fine. I did install simple tape gap seals and that increased my elevator authority some. Unless you have a 540 engine AND a three bladed metal prop, I would put the battery on the firewall and keep other weight as far forward as possible. The water jug trick sounds like a good one.

Mark
BHQB73