abufletcher

The method described on previous pages is a stripped down version for modelers. It only needs two weights and three distances.

Rafael23cc

My Feedback: (6)

So both spreadsheets tell you that the CG is actually in front of the recommended CG. What did hanging the model tell you?

Rafael

abufletcher

At the recommended CG the nose was significantly down. I'd guess around a 10 degree angle. It seemed a bit much so I was considering putting 25g in the tail. That brings it up to about a 5 degree angle. Without the weight but balanced on the cones at a point about 20mm further forward, the model was almost level.

I'll recheck the weights once I get the better HF scale.

abufletcher

I've never had a model that was this "engine-heavy" and I think that's throwing off my thinking. I'm used to short-nosed WWI models which are almost always tail-heavy and need weight up front...and there's not much front there. Usually, on our models moving the CG back makes the model tail-heavy. That's because our model engines don't have anywhere near the scale weight of full-scale engine. Airplanes have the shape they do because engines were heavy. Our models are almost always "engine light" and therefore tend to be tail-heavy.

On this model, however, with its large 7-cylinder engine which with its mount weighs in at a hefty 2.6kg (not to mention the exhaust ring and on-board glow) moving the CG back makes it even more nose-heavy.

Note that the CG at 124mm back from the LE assumes the model is using a 50cc gas engine. And there's the problem with confusing the "aerodynamic CG" with the "balance point." The two are NOT the same. It may be perfectly true that this model BALANCES at 124mm with a 50cc gas engine. That does not mean it would balance at 124mm with the Seidel 770. However, in principle, the CG (as an aerodynamic feature of the design) should remain the same regardless of what engine is used.

Hmm...I may just have had a bit of an epiphany here. The recommended 124mm is where the model WILL BALANCE using the recommended 50cc engine. It is not (or not necessarily) the CG. Does this make any sense?

abufletcher

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

Propworn

My Feedback: (3)

Well let us know how it works out. When you fly it get it 3 mistakes high and slow it down see if it snaps/tip stalls. Try some abrupt elevator input while slow see what happens. Its after all supposed to fly like a Cub. From past experience balancing to far forward ahead of the center of pressure makes an aircraft prone to tip stalling a low speed and higher g inputs. I'm not convinced this spread sheet knows more than the manufacturer. Good luck.

Dennis

abufletcher

I know what you mean. It is worth noting that the wing cord is 440mm so that 124mm position is at 28% MAC. Many modelers prefer a conservative 20-25% MAC for a maiden flight. Moving the CG forward half an inch puts it around 24% and one inch forward makes it 20%.

Rob2160

Yes having the CG too far forward will increase stall speed. The reason for this is that a forward CG requires greater downforce by the horizontal stabiliser (& elevator) to compensate.

This extra downforce is like adding extra weight to the aircraft which the main wing must support in flight. It will need a higher angle of attack at any given speed and therefore will reach the stalling angle sooner.

Depending on how nose heavy and design, you may also run out of elevator effectiveness, which means the nose drops when the elevator runs out of travel - this looks like a stall in flight and is commonly mistaken for a stall but is not technically an aerodynamic stall.

MajorTomski

abufletcher, forgive me but you had me rolling on the floor in posts 95-97. It's NEVER as easy as we think it will be. Where will you be coming to in the US? It would be nice to meet you.

Propworn

My Feedback: (3)

HMM looked all over the net no pictures of a radial in that airframe so this is a bit of a kit bash I gather. Nothing wrong with that it’s what makes the hobby fun. The radial is a bit heavier than the recommended 80cc gas engine. The nose on the model seems to be a bit lengthened over the 3 views most likely to compensate for light weight RC engines. In your picture of it against the wall I notice the prop is quite a long way extended from the cowl. Moving the engine back so the prop just clears the cowl might help to alleviate the extra nose weight.

These airframes are very lightly loaded. The engine you have chosen is different but the extra weight should not be a problem. Heck 1/3 scale cubs fly off water with floats with an old G-62. Set it up as per the manufacturers balance point and you have a slightly heavier airframe. Set it up with the balance point forward and you have a nose heavy airframe. It’s apt to fly either way however it would probably fly much nicer set up with the manufacturer’s specifications even at an increased weight.

Dennis

Boomerang1

Well, you know it appears to be nose-heavy, no point wondering about it's effects, just fix it!

Just shift your flight (or whatever other) batteries rearwards.

As an extreme example I helped a friend out with his (now mine) Edge 240 which was very
nose heavy with a big petrol motor. The solution was to mount the batteries on a long plywood
strip which slid down the fuselage to secure the batteries just in front of the tailplane.

Different situation now, they are now up front over the fuel tank after fitting a lighter four stroke
engine.

John.

abufletcher

My family lives in SoCal, so that's home. Yeah, it would be nice to put a face to some of our online friends.

abufletcher

I've had the engine for several years anticipating its use in 1/4 scale WWI project. When I saw this ARF for sale, it looked like the perfect platform to get some flying time on this engine...and learn a thing or two about working with larger models.


The manual states: "50CC PAULISTINHA is specifically designed to be powered by 50c.c. gasoline engine..." I always knew that with the 770 in a WWI model, I'd be flying at less than 1/2 throttle most of the time so again this ARF seemed like an ideal platform to test that out.

Part of that might be the distortion from the wide-angle lens. But it may extend a bit further than typical gas engine. At any rate, I wasn't willing to do any major re-engineering (such as replacing the firewall and all the problems associated with that) on this ARF.

abufletcher

The simplest fix would be just to put a 50g weight in the tail section.

otrcman

So your model currently weighs about 370 oz. (sorry, I don't speak gram). And the tail weight necessary to bring the model to the CG as shown on the plans is slightly under 2 oz. That's an addition of about 1/2 of one percent. No big deal. Just put the 2 oz somewhere on the tail and go for it. That change isn't cast in concrete; you can always take the 2 oz. back off after you've flown the model a few times.

Unless your horizontal stab is quite small, or your tail length is exceptionally short, the model will have an acceptable CG RANGE. What that means is that the CG can be a little forward or a little back and still have acceptable flying characteristics. Very few airplanes have just one precise point where they must absolutely balance. That's why in full scale airplanes we have what is called a Forward Limit and an Aft Limit. Usually the forward limit will be a point where elevator effectiveness becomes marginal, as in not being able to do a 3-point landing. The Aft Limit can be defined by a number of factors, often stability related. Maybe the airplane won't come out of a spin easily if the aft limit is exceeded. Or maybe the airplane becomes unduly sensitive to elevator control and can't be flown with precision by a novice pilot.

Of course we don't even know what criteria the manufacturer of your model used to set the CG location on the plans. Maybe they just built the prototype, flew it and found that it handled satisfactorily, and said, "That's where the CG should be." Or maybe they really did do some testing with the CG more forward and more aft. But if they tested more than one CG, I would expect them to show an acceptable range on the plans rather than just a single point. Probably the CG shown on your plans was determined by "that looks about right".

Dick

Bozarth

My Feedback: (15)

+1..Well said Dick.

Kurt

abufletcher

I've got a pretty good handle on the balance now. I'll add the weight to the tail and then once I've got the HF digital balance run the numbers again and do a final visual inspection.

Propworn

My Feedback: (3)

These sound pretty popular and with all of them flying I'll bet 99% just stuck their hands under the wings next to the fuselage picked it up at the suggested balance point said good enough and have been flying every since. Sounds to me like to much over thinking increases the chance of making a mistake second guessing the manufacturers instructions.

Dennis

abufletcher

That's not the first (and probably not the last) time I've been accused of over-thinking a problem. BTW, in have this company's smaller P-56 (80" wingspan) and the suggested CG was good on that. Partly, I'm just taking things slow because I've got time to kill. I'm headed to the US on March 18th and probably won't be able to fly again until mid-April due to work. And before I fly I've got to get the engine running reliably.

And when it comes down to it, I'll trust my visual inspection over a numerical calculation. It would just be nice if both coincide.

Rafael23cc

My Feedback: (6)

This few lines effectively show that you ARE over thinking the problem. The CG shown on the plans should be the Aerodynamic CG and the Airplane should balance there, no matter how it is equipped.

Rafael

abufletcher

I was just confused because I've never ever had a model where I've had to add weight to the tail. Also the smaller 80" Paulistinha does indeed balance, without any added weight, exactly at the recommended CG with the recommended engine.

Anyway, I hot-glued some fishing weights into the tail section. It took a bit more than I had thought, about 160g (5.6oz). With this weight the model balances on the cones at the recommended CG with the stab level, or just very slightly inclined forward. If I later choose to remove a bit of this weight I can soften up the hot glue with a soldering iron.

Next weekend, I'll do an engine ground test. Then I'm off to the US for two weeks so the maiden will have to wait until April.

Rafael23cc

My Feedback: (6)

I know you were using basically Kilogram scales, but from this, I doubt that you will ever remove some weight. In fact, you may have to add weight to get closer to the recommended 240g of additional weight at the tail, not just 160g. The reasoning for the stab to be level is a good point tho.

Rafael

abufletcher

That 160g (plus the hot glue) was based entirely on a visual inspection. That is, I propped the model up on the cones at the 124mm mark and added weight to the tail until it was level. Once I bring back the more precise HF digital scale I will weigh everything again, including getting a more precise total weight. What I hope to find is that the calculated numbers correspond. If they don't, then I will trust the visual inspection and just go fly the thing.

otrcman

Here is an interesting little "science project" for you:

Is your smaller Paulistinha dimensionally the same as the large one ? That is, is the smaller model an accurate scale of the larger model ? If so, check the recommended CG of the smaller version to see if it's at the same location as the larger model. If they are the same (percentage wise), how does the small model fly ?

Next, add some nose weight to the small model to duplicate the nose heavy condition of the larger model. How does the small model fly now ?

Finally, if you wish to pursue it, try ballasting the small model a few percent rearward of the plans CG. How does it fly now ?

Having performed those tests, you have done some subscale research for use in predicting the behavior of the bigger model. And at the same time, you have given yourself some practice in preparation for flying the bigger model.

Dick

abufletcher

Funny you should suggest this. Last Sunday at the field I did indeed look at the CG on the smaller model. As far as I can judge eyeballing the two models they are the same. The smaller one is balanced at the recommended CG which looks very close to the 28% MAC numbers using on the larger P-56. It flies very well at this CG.

I haven't tried this. How might I expect a nose-heavy model to fly?

Haven't tried this either. I've done a lot of playing around with variables like this on RealFlight. Yeah, I know, I know. But the one thing is shows you is that you can go pretty extreme in the nose-heavy direction and still fly the thing. But go just a bit too far rearwards and it suddenly becomes a nightmare.