Center of Gravity - Need Help
 

I am preparing a 35 year old Goldberg Sr. Falcon for it's first flight in 30 years. The old World Engine receiver and servos have been replace with a Spektrum receiver and Hitec servos. The old 10 oz tank replaced with an 8 ozpart and the ni-cad 4-cell battery pack replaced with a NiMH 4-cell part. Engine has been refurbished but otherwise the same. Nothing in the front of the plane was replaced with a "heavier" part. When I checked the CG, using 4 3/4" from wing leading edge as shown on the 35 year old plans, the plane is very nose heavy. I moved the battery pack from under the fuel tank to just forward of the servos but little change (as expected). If I place3 oz of weight just forward of the verticle stabilizer,about 19" rear of the CG, the plane balances. This does not make sense to me!! I did not build the plane; bought it used but Ihave the old plans. The plane flew OK before (30 years ago) but I did not check the CG before removing the old hardware.

I have found a .pdf copy of the Assembly Manual for the Goldberg Sr. Falcon ARF. I do not know if this plane is an "identical" copy of the kit Sr. Falcon. The ARF CG is 3 1/2" from leading edge with a 3 1/4" to 3 3/4" range.

Do I (can I) assume that the kit Sr. Falcon CG is wrong???

Any thoughts or ideas?? Help would be appreciated as I am otherwise ready to head to the flying field.

Many Thanks!

Bob Guerity

RE: Center of Gravity - Need Help
 

Keep in mind that normally, the plane is balanced with an empty tank. If you replaced the tank with something that weighs 8oz, it like you are balancing it with a full tank

RE: Center of Gravity - Need Help
 

You can use this calculator to determine where your CG should be so there won't be any question.

http://adamone.rchomepage.com/cg_calc.htm

It's possible that the original owner just flew it nose heavy. It's also possible if there is some sweepback in the wings that you are measuring it wrong. If you really do need to move the CG back, I would make an extension for your battery pack and move it back to where it needs to be. It's a simple task to fly the plane with removable lead on the tail to verify the CG, then take it off and make a hatch for the battery.

RE: Center of Gravity - Need Help
 

If I remeber correctly if you ballpark the cg at the main spar it should be flyable. I once helped a friend who bought a used plane from another flyer once. the plane was sold with everything still in it but rx and we replaced the batteries. We were told the plane flew great but we ended up having to pull more then 5 oz of lead out of the nose to even get the plane to the point where we could taxi the tail dragger reliably on a grass surface. Point being is that unless you have really seen it fly or really know and trust the original owner don't bet on anything being right.

RE: Center of Gravity - Need Help
 

MinnFlyer thanks for the comments. The 10 oz tank I removed was 10 oz capacity and the 8 oz tank I installed was a smaller 8 oz capacity. I guess what I was saying was that the replacement tank was lighter than the tank originally installed and should not have made the plane nose heavy. All attempts to check the CG were done with a dry tank. I think this is the correct proceedure.

Thanks again,

Bob Guerity

RE: Center of Gravity - Need Help
 

jester s1 many thanks for the link to the CG calculator. The results look more like what I expected and I will follow this for first flights.

Thanks again,

Bob Guerity

RE: Center of Gravity - Need Help
 

I was given a mostly built Sr. Falcon back in the eighties and balanced it at the spar as I did not have the planes or instructions. It flew great, it was definitely not overly nose heavy. It would land slower than my trainer and flew inverted rock solid with very little down elevator.

RE: Center of Gravity - Need Help
 

I would trust that the kit CG is correct. Many thousands of Sr. Falcons and Falcon 56 were kitted, and when built true to the plans, all flew well. The Falcon series was introduced in 1964, long before the advent of mufflers and heavier engines. Even the size of engines were smaller then, with the original kits specifying a .35 to .45 engine, so achieving the balance would have been easier, especially with the heavier servos of the period.

Many people thought that the Falcon design was strongly influenced by Doug Spreng's Flattop Stormer that won the Nationals in 1961. The Flattop was a simplified version of his earlier Stormer that he won the Nats with in 1960. Doug's biggest contribution to the hobby was his invention of the digital proportional servo with the 1 to 2 mS control pulse scheme that has lasted to today, 50 years later.

RE: Center of Gravity - Need Help
 

Regardless of whatever you read, hear, or do, there is one rule that will get you flying very well and VERY SAFELY.

Your Falcon is a straight wing airplane. No need to worry. SIMPLY, measure the chord length that is the distance from leading edge to the rear most of the wing's trailing edge including ailerons and flaps. Do whatever is needed with batteries, weight or such so the balance point is 25% OF THAT total CHORD DISTANCE, back from the leading edge. If the chord is 10" come back 2.5 inches, 12" chord, back 3", etc. etc. No need for any aeronautical degree. :eek:

That is all there is to it when using a straight wing within subsonic convergent airflow.

Now later as things get going well, you can do small things to adjust the Center of Gravity if you feel the need. Move it forward, airplane gets more stable. Move CG back and stability becomes less.

One last point: Never trust any kit stated CG point. Some are so ridiculous that it is appalling.
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RE: Center of Gravity - Need Help
 

My limited experience building and flying my own designs, kits, and RTFs has been that balancing on the spar (for a straight spar) or wing tube if there is one is normally very reliable, and will get you very close to the final CG. The spar is typically placed at the thickest part of the air foil, which in most air foils (flat bottom or symmetrical excluding more exotic wings) is close to the center of lift. I have yet to find a plane that balanced on the spar and flew tail heavy.

Brad