Is it overweight?
 

Hi Guys,
I have just finished buliding or maybe better put the finishing of a Thunder Tiger ARF Cub but would still say a good 8 to 10 hours have been put into it, very enjoyable hours I will say,

I have done an electric conversion on it and have a question on weight,

Its total weight has come up above the weights stated in the manual,
The manual speaks of weights between 3 and 3.5 kgs and my finished plane with 4 cell lipo installed weighs 4.1 kg,
I have set up the COG perfectly but the fact it weighs heavier than the manual has spiked my question,
I have set it up with the most "nose heavy" COG being a learner so added some lead to the firewall after mounting the battery as far forward as practical,
Its suggested COG was 4 to 4.5 inches from leading edge so I set it at 4,
Somebody told me that sometimes the weights are a bit missleading in the manuals,

Will the extra 600 grams be a problem?
I cranked it up testing roped around the fuse and tailplane to the TV cabinet and gut feel tells me she wants to fly,
I just hope she isnt too heavy???

Cheers,Paul

RE: Is it overweight?
 

You may have to adjust your engine choice to match the weight and flying style that you ended up with, including the battery, and if necessary, the ESC and motor. There is a general formula for watts per pound for various styles of flying.

A rule of thumb is that a sport-type, advanced aerobatic trainer needs about 50 watts per pound and that 10-20 watts per pound is about right for intermediate arf electric rc airplanes. With rc electric aircraft better performance comes with more battery capacity, less weight, and more efficient motors.

It may fly the way it is now, but if it is heavier than planned, it may be sluggish, or consume your battery power very quickly. Do the math and see how it works out for your setup. If it is ok, then go fly it!!

CGr.

RE: Is it overweight?
 

Gee I though he was talking about me, when he wanted to know if it was overweight. Hummm!!! 15 more minutes on the air-dyne this morning. Went to my first indoor fly and the walls seems very close. That should sharpen one's flying skills. Freezing rain here so can't fly outdoors.

RE: Is it overweight?
 

G'Morning, Gene.

Yeah, it's been windy here for the past few days, so there has not been a lot of outdoor RC'ing here either. I did load up the simulator and played with one of their electrics "indoors". Pretty neat. Has to be a slow plane, though. Those walls do come up quickly!!

CGr.

RE: Is it overweight?
 

My rule of thumb is that if you have to add nose weight, you don't have enough engine. If the COG is in the range without the nose weight, I'd get rid of it.

Brad

RE: Is it overweight?
 

Brad: There are exceptions to every rule. For instance, have you ever owned a Goldberg Tiger 120? If not, they I assure you, 18 ounces of lead in the nose is needed to make that thing meet the CG requirements, safe enough to fly. Yeah, you read it right, 18 ounces. It has an OS 1.20 AX engine. Can you recommend an engine that is 18 ounces heavier than that one (be reasonable now.. :D ) ?

CGr

RE: Is it overweight?
 

Sometimes a sanding block and a stack of sandpaper is required.

RE: Is it overweight?
 

That's a lot of balsato sand-18oz That should make the stab and fuselage transparent.

RE: Is it overweight?
 

Perhaps you missed teeter-totter 101 in the second grade.

RE: Is it overweight?
 

No, I took it in the 8th grade, I was a little slow. If memory serves me correctly- 18oz is a pound and 2 oz. I don't care what kind of teeter-totter you rode on, but that ain'g gonna work. Might be able to lengthen the nose abour 4 inches, or shorten the tail up about the same distance.

RE: Is it overweight?
 

I guess most kids are fat these days, so the lesson is lost. So much so that the board bows so much that their feet stay on the ground.

Perhaps a better answer is to move the wing. I'm not really much of a fan of the Tiger series anyway. Not a great design structurally speaking. And the term "lite ply" is a classic oxymoron.

RE: Is it overweight?
 

The ARF was built according to the instructions. I removed the supporting hardware (flying wire as called in the manual) which supports the vertical and horizontal stabs, to help reduce weight in the back. I used a lightweight Sullivan tail wheel and associated hardware. It still needed 18 ounces of nose weight.

As Goirish said, ain't no amount of sanding that would help that thing. Heck, the entire tail doesn't weigh 18 ounces! It's just that the overall construction is solid, uses heavy wood with a lot of ply in the fuselage.

It flys like I would expect a Goldberg Tiger to fly ( I had a Tiger 60 and flew that for several years until I sold it, and I believe it still flys).. it did not require even a proportional amount of noseweight to make it balance. Another club member built one and, even with an OS 1.60 up front, it STILL required about 12 ounces of noseweight.

I never took Teeter Totter, but did take Basket Weaving 101 in college.

CGr.

RE: Is it overweight?
 

Gee CG. Why don't you move the wing back farther. It would only require that you fill in the original wing saddle and then re-cut them farther back. Of course you will have to experiment with the CG to find out how far you have to move the wing back. OR, you could leave it the way you have it and fly, fly, fly.

RE: Is it overweight?
 

My comment was made for the OP. I believe hes talking about the 40 size. He said he'd added weight to the nose to move the CG to the forward recommended point and he's over a pound heavy. That's a lot of weight (over 8 llbs total weight) and is to heavy for a 40 size plane. That weight is more the 60-90 range, and really needs to be brought down. He didn't say how much weight he'd added to the nose or which engine/battery combination he's using, and I'm sure its not the whole problem, but if he wants to add weight, I'd do it with battery, motor, or both, not lead.

A little more research shows that for the 40 size tiger cub, thats going to be 23 oz/sq ft wing loading, which is in the aerobatic approaching warbird range. Its going to land fast, and require more horsepower to get it off the runway. Not impossible to fly, but more difficult for a beginner.

As for the Goldberg Tiger and the OS-120 AX, if I knew I was going to have to add 18 oz to the nose, I'd seriously think about a 4 stroke 120, or maybe even go with a 1.40 (inverted if necessary for clearance. The specs call for upto a 120 four stroke, and do discuss putting a larger engine and using the tail bracing wires. No its not the full 18 oz difference, but its a lot of it.

And there are always exceptions to every thumbrule. That's why they're only thumbrules. :D

Brad

RE: Is it overweight?
 

You're right, Brad, however, I had no idea that the thing needed the 18 ounces of nose weight until after it was assembled, with the OS 1.20 AX that I bought specifically for that plane because that was the recommended engine for that plane.

And, you're right again... there are exceptions. It flys just fine, but is a tad bit heavier than I thought it would be.. 18 ounces in fact! ;)

There was another problem with this ARF.. the covering kept coming off. Lousy job by Goldberg, or whomever did the ARF part of that model. I was flying it one day, it was up about, oh 100 feet or so, going from left to right, when I heard this loud POP sound and saw a streamer behind the plane. I had no idea at first what it was, but knew I had to get it down... so I cut power, wanting to slow it down, and did a dead-stick landing. It flew fine, by the way. When I got it on the ground, I saw that ALL the covering on the top of the right wing was off and streaming behind the plane.

Just another problem with the Tiger 120, I guess. Lousy workmanship on the part of Goldberg.

CGr.

ps. sorry for hijacking this thread.. I will refrain from further comments in this one.

RE: Is it overweight?
 

Thanks guys for the info so far,
Some interesting reading,

I was initially aiming to get the COG to the front of the range but am feeling now that by leaving the weights off and setting th SG to the rear part of range it still may be better to fly for a beginner than the heavier alternative,
Being that any gain of stability by having frontward CG will be offset by the extra weight added to accomplish it?

I am going to have the plain flown by an experienced pilot first so might ask if he can fly it with and without the weights to make a judgement for me what is better,
I guess that if the plane is within its COG range it should still fly ok,
By moving the COG back by the half inch it requires no extra weight,

Cheers,Paul

RE: Is it overweight?
 

There is a saying among RC enthusiasts. A nose heavy plane will fly and land, and fly and land. A tail heavy plane will fly just once. (Or something like that). Moral to that story is that you do NOT want a tail heavy plane. For a beginner, it's better to have something that is slightly nose heavy than even slightly tail heavy. Even most experienced pilots will not like tail heavy.

Do the math on the power to weight ratio and see if what you have is within those limits. If it is, then shift the position of the battery until it works in your favor (move it foward to shift the CG forward.. rearward to move it rearward..) then, as you suggested you were going to do, have someone with experience fly the plane. See how it performs. If it's ok, then fly it. If it is not, then make adjustments as necessary then have him/her fly it again.

Best of luck.

CGr.

RE: Is it overweight?
 

He did say in the original post that the battery was as far forward as practical. I think the plan of removing the weight and having an experienced pilot perform the maiden flight is the most practical solution. Most manufacturers will have some margin for error factored into the rearward COG.

Having said that, I went and looked at the manual for this. I'm somewhat concerned about the spec for the CG. This plane appears to have an 11" chord, which puts the 4-4.5" cg range at 36-40 percent, which seems rather far back. Normal CG ranges run in the 25-33 percent range. I'm not saying it's wrong, I just think I'd try to do some more research on where others have set the CG.

Brad