CoosBayLumber

My Feedback: (1)

Old C.G. location


I have predominantly been working with older models, the type that used tube receivers, B batteries, escapements, and other heavy items lying forward of the wing. As built this may have worked, but now radio gear weighs in grams instead of pounds.

I find it difficult to balance the model as per the original plan. The estimated C.G. has shifted to the rear. In order to bring it forward again, I shove everything possible right up behind the firewall. On certain models this is infeasible, and looks as if I need to add a goodly amount of ballast in the form of some heavy lead perhaps. For tri-cycle gear models this C.G. problem has effects in that the nose gear does not touch very heavily too.

The thought came, what about shifting the wing towards the rear by up to one inch?

Or worse yet shifting the engine forward more by about one inch by increasing the overall nose length?

Wm.

rmh

do what is the easiest - -all of these early types were simply free flight designs with some trim control -- the Radart - and Rudder bug - whatever your design.
Power (engine)should be kept low as these models again were setup to work in a fairly limited speed range .
So far--no one has introduced a Chinese Arf of the Rudderbug so minor deviations will likely go unnoticed -by the current crop of flyers -----

BMatthews

Coos, I'm assuming that you're working with modern radio gear. If so then there's your problem. The new stuff just isn't heavy enough to get the CG in the proper location. But don't be afraid to add some nose weight. It'll still be a way lighter than the old models with old gear.

Also if you're flying rudder only you need the forward CG and resulting stabilizer trim to give the model the strong nose up effect when spiral dived for speed. It's that buit in stron pitching damped with lots of downthrust that makes the rudder only models fly as intended.

But if you're adding elevators then there's no need to keep the CG that far to the front. You can easily let it come out a good 5% further back and possibly even more depending on the design.

I agree that the current crop of modellers won't know the difference but if you're wanting the models to fly as per the originals then altering them this way will make them fly a little differently. I'd say keep them as per the prototypes and just be prepared to add some nose weight. It'll still come out lighter than the originals since the nose weight can be further forward and will have more effectiveness thanks to the leverage arm so you will be using less weight than the difference from the old gear to the new gear.

But if you're using old gear and similar weight of batteries then you're just using too heavy a wood grade....

CoosBayLumber

My Feedback: (1)

Bruce....

Am working on an old design that specifed ten channel radio and Bramco servos. Yes, it has ailerons, plus retracting gear. The whole A/C was designed around such equipment. My version has not flown yet, but in bare shape sitting on table I have to push the nose down for the tire to make contact. This is why I am considering to shift the wing a good bit, as it will take the C.G. along with it. Am not too happy about adding in couple pounds of ballast up near the firewall to get a good balance.

Had been considering extending the nose too, but that may make it too obvious.

How much off in overall length could I be off for C.G. position and still have a good steady flier?


Wm.

rmh

On a plane that small - 1/2" is noticeable
Your "oldtimer" is nothing like I had envisioned .

To me an oldtimer is a 1950's tube radio setup -
Yours is a "multi" stunt job .especially one with retracts - which were at that time , pretty lame mechanical thingies.
assuming it has a straight rectangular wing - set the CG for 25%.

RaceCity

Agreed with DH.

25% or so on the CG. This point (CG) is not negotiable regardless if the radio gear weighs 5lbs or 5oz.

Modifying the airplane to shift the wing rearward would probably work, but isn't that a lot of effort now that the airplane is built, and everything is shaped?

Models of the type you describe just weren't terribly large, so I'm puzzled by the need for "pounds" of weight in the nose to balance, unless something went horribly wrong in the construction somewhere. OSB ply instead of balsa in the tail perhaps?

Liquid nails, and copper flashing for the tail fillets will do it everytime.....<GG>

Stick to the plans, and there's a good chance the model will live to fly twice.

Good Luck!

CoosBayLumber

My Feedback: (1)

'lemme give you some facts and figures here.
The overall length of the fuselage is 45 1/4" long.
The C.G. (Per the plan) sits at 30% and 70% of that.

Wing chord is 11.9 inches, and the C.G. sits at 4.25 back from leading edge or at about 35% of the chord. This is not at Max. thickness, for that is at about 37% back.

Problem being is that due to lighter weight equipment now, the C.G. measures out to about 9/16" aft of the plan noted C.G. To which I think if I move the wing towards the aft about 1/2 inch, plus add a bit of ballast forward of the C.G. I then would be able to bring the nose tire in contact with the asphalt.

As the whole aircraft is lighter now, the general performance ought to improve aerobatically.

Wm.

rmh

Let us know how it works out -

RaceCity

Exactamundo. As a general rule, lighter always flies better.

9/16" shift in the CG shouldn't be terribly difficult to accomplish. If the model is "bare"...perhaps there's something you could do to remove a little weight from the tail.

Barring that, you may be able to simply shift the radio gear slightly forward. Further, as an alternative to installing dead-weight to achieve balance, perhaps a larger NiCd battery installed as far forward as is feasible would help you out.

It bears repeating however, that it is far preferable to "build in lightness"...ie: removing weight from the tail where possible.

In the end, so long as the model balances within the CG range specified on the plans, you should be fine.

Tall Paul

I've extended the motor mounts on some planes when changing from slimer to electric, and there's no room for a heavier battery where the fuel tank was.

feihu-RCU

Wm:

I assume there is no way of lightening the tail.
Move the wing back, moves the CG back, but to add ballast fwd will move the CG fwd again.
Rather than add fwd ballast, you might work with the landing gear.
The landing gear geometry was designed for the CG in the original position, so you really need to change the lg geometry to fit. Perhaps making the mains slightly longer and swept back about 1/2 in might not be too obvious?

feihu

CoosBayLumber

My Feedback: (1)

Preview This preview windows does not show attachments
RE: Old C.G. location Via using masking tape, I held the wing in place, at about 9/16" more towards the rear. Then hung the radio gear into new forward location, and added a couple fishing weights at bottom up near the firewall. Used a standard plastic fuel tank (w/o fuel) but plumbed into the engine, instead of the metal clunk one called for on the plan. The wing is angled a bit towards the rear, thus making the center of the wheels yet a bit further aft by yet another 1/2" and everything to the rear of the wing is using contest grade wood, instead of the slightly heavier standard grade.

It sort of balances, but they too the wing has not been fit or everything flown.

Wm.