While anyone who has been in the RC plane game for long knows, there are some things that have a pretty standard "Rules of Thumb". Sure, there are exceptions to every rule, but there some things that serve as a good starting point when setting up a ARF or even that plane you've spent months or even years to build.

Time and time again the same old questions are asked. I'd almost bet that a lot of the guys with experience don't even take a second look at some of the new threads askin' the same ole thing.

While building a plane from scratch or a kit is seeming to become passé, those planes did come with a lot of valuable information. The Centre of Gravity being the one of the first hurdles. Wing Incidence (both main and tail). Thrust angles for the power (engine or electric).

I am working on a kit built plane (built in the early '80s by someone else) that I tried to sell. I am re-powering it with the engine that came from a crash that I didn't see coming! The general idea was to take a proven airframe that was slightly under-powered and give her a heart transplant with the salvaged engine.

My problems start with the lack of information on my particular plane. All the Google and Bing searches in the world result in frustration about the CG. Most of us have learned that you never want to Maiden (or Re-Maiden) a tail heavy plane. My research said "25-35%" of the wing chord. This seems to be pretty standard protocol, and guess what? That percentage puts the CG right on the spare!

This might be a pretty good rule of thumb #1 for someone asking "where is the CG supposed to be". From previous experience with this particular plane that's a great place to start.

So the new-to-it engine is all mounted up. I weighed the old and new engine to see how much difference there was. New power is 4oz heavier, not a big deal. Thats until I remembered the fuel tank! New engine is gas, old engine is glow. Lines and bung have to be changed, that's when I remember the original builder wrapped at least 8oz of lead around the tank (32oz that was stuffed right up to the firewall) and the 4oz of lead that I had added to the firewall to make it balance.

I mentioned the tank is 32oz. In my book, and from experience flyin' this thing, when you consume 28oz out of that tank (at the firewall) this thing flies differently. I want to make this thing fly better. I know a lot of the Precision aerobatic (old school flyin' glow engines) guys kept the fuel tank as close to the CG and centred on the Datum line as possible. Hmmmm, good idea, but my plane suffers from "excessive weight" in the tail. How do ya fix this dilemma???

One question recently asked was about thrust angles of the engine. I have built quite a few kit planes. I was shocked when I came across a set of planes that had 8 degrees of down. Built it that way and she flew like a dream. I guess its best to stick with the program (plans), maiden the plane and tweak to taste from there. I have met a few RC pilots that never considered thrust angles as a solution to bad flight habits. Most of them added ballast to make their plane fly to their liking. I try to do all I can to avoid any added dead weight, especially if I am building a plane out of sticks.

As usual I have more questions than answers, can some please help me tune up my bird so it flies better?

[fixer]

I'm just throwing some stuff out. I'm new to this. Is it possible to move the engine forward. The further it gets from the CofG the more effect it has. I agree with keeping the plane as light as possible, but not at the expense of progressive tail heaviness (28oz). With a more forward engine that is heavier than the original, and some ballast lead sheet tucked in behind the engine, I would hope to get that tank much closer to CofG. You did not mention which model you have. Can tail heaviness be compensated for by increasing tail plane incidence???
Oh, one last question, did you mean the cg was on the wing spar at 25-35%?

[michaely]

Brent,

As already suggested, identify the 1980s kit built plane.... there are lots of people on this and similar, U.S. based websites, that are still around from that time and I would not be surprised to hear you get "this is my experience with this actual model" answers.

good luck

Michael

[CHEVPRO]

Google Alan's hobbies. There is a database of rc and modeling links. Should be able to find what you need in there.

[Trumpetman]

You are right that with glow motors the tank needs to be as close as possible to the engine as possible. The good news with gas motors is you can put the tank pretty much anywhere you want as the carb has a pump that will pull the fuel. This means you can put the tank right on the CG and you will not have any change of CG as the fuel is used up.

[fixer]

Originally Posted by CHEVPRO

Google Alan's hobbies. There is a database of rc and modeling links. Should be able to find what you need in there.

Thanks for that link to Alan's, Chevpro, what a gold mine!

[kip51035]

Try the attached PDF file.