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C.G. Location on a scale model

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C.G. Location on a scale model

Old 09-01-2007, 01:12 AM
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EF
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Default C.G. Location on a scale model

Looking at my Ryan STA kit as an option for my first CL scale model, I noticed an instruction to move the C.G. location forward when flying it on CL vs. the location to be used for flying it RC.

Is this unique to the Ryan or is this done to all scale models converted from RC to CL, and if so - why?
Old 09-01-2007, 07:13 AM
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Default RE: C.G. Location on a scale model

I'm not an expert, but I think it's done to all C/L models. It has to do with the difference in the way the plane flys with the lines connected ... the plane has to pull on the lines and track around the circle, but without looking like it's flying sideways.

Phil
Old 09-01-2007, 07:37 AM
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Default RE: C.G. Location on a scale model

Only interested in scale CL?

I've only built a few, but the last came from my drawing board. And I've flown more than one scale model for scale builders who needed help on maidens and for trimming. And flown a ton of stunters, and a bunch of racers (trimming there pays big dividends), and trashcans full of combat wings (pitch trim is very important). But scale trim can be very critical. It can let you carry lots more weight if you do it the right way. Not the common way, but the right one.

With scale CL, you usually do not need much out of the pitch axis of the model. They seldom do anything that even approaches tight corners. Most never wind up inverted. And the ones that do, don't do square corners to get there.

But there are two ways to get the model to be very "slow and steady on the handle". (seems there are often things that have two correct ways to do them)
You can move the CG farther and farther forward. The handle response slows as the CG moves forward, which everyone knows about and often teach others. What's actually happening is that the tail has only so much "power" to cause changes on the pitch axis. And as you take some of that power to carry the load caused by moving the CG forward, the elevator movement gets more and more sluggish. Everybody is familiar with this, and equates it to stability. It is "handle" stability, but it's not really pitch stability. but that's another deal...... So anyway........ When you solve the twitch handly syndrome with forward CG, you wind up needing more and more elevator deflection. So the elevator deflection is easier and easier to control with the handle. And the elevator trim is easier to nail too. So all this winds up giving a lot wider target to hit. Easier to hit, less problems if you don't the first time.

There is a downside, but one that's almost never noticed by most. The more the CG moves forward the heavier the airplane flies. This happens to also improve the handling for the average flyer, btw. There is a real increase in wingloading when the tail has to carry the load imposed by the forward moved CG. And the wing winds up having to carry the artificial load of the CG's forward movement AND the real download the tails having to produce. AND the airplane sees lots more induced drag, which tends to smother the acceleration you experience. So the already heavy scale planes wind up flying even heavier thanks to that CG move. whatever..........

On the other hand, if you balance the airplane with the optimum CG, you wind up with a tail that is better able to stabilize pitch and to pitch the airplane. It takes less elevator deflection to pitch the airplane. And most flyers don't rerig the connector locations or change the handle spacing to tune for that. The less experienced ones decide the airplane is less stable and start to dumb it down with the CG. If you simply move the elevator horn connection out, or the bellcrank drive hole in, you wind up with a model that carries it's own weight and no artificial excess, and flies steady and true, but responds to the handle cleaner. It's less apt to slow over the top of loops, or for that matter, whenever it's pitched any amount. But there is a downside........

You've got less elevator throw to work with. So your initial trim is more critical. If you do detune the throw, the trim is harder to hit on handle setup when you swap lines for example. It simply takes more experience to deal with this situation, so what's the natural result? Everyone is advised to go the first route, and move the CG forward.

I cannot imagine how many, many modelers have been led astray by this deal. Darned if everyone nowadays is absolutely positive that dumbing down the elevator with forward CG is the road to stability. The only road. And it isn't. It is actually a road that costs you.

BTW, I used to fly semi-scale warbirds in stunt competition. Even drew up and flew a few Dewoitine520s. As I evolved that little beauty, they became more and more scale. I wasn't Al Rabe, but darned if I wasn't getting there.

From what I've seen and done, I'd suggest that you do a couple of things about this CG thing.
Start with a CG with about a 15% static margin. If the airplane feels twitchy or too quick for you, look very hard at how much elevator movement you're getting. Reduce it. Don't touch the CG. At 15%, it's more than stable enough to be very safe, and you've just got stupid rigging. fix the problem, don't dump dirt on it. If the airplane scared you, cut the throw in half. If it was just uncomfortably quick, cut it by about a third. Look at what you're doing and what it does. We learn that way.
Old 09-01-2007, 07:45 AM
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Default RE: C.G. Location on a scale model

BTW, learn to use the application at the included link. That sound byte wisdom about CL CGs is certainly safe but just not too bright.

http://www.geistware.com/rcmodeling/cg_super_calc.htm
Old 09-01-2007, 07:49 AM
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Default RE: C.G. Location on a scale model

BTW, my advice is that you do what lots of experienced modelers do. Both methods at the same time.

With experience, you'll learn which is needed when, and how much.

Our hobby isn't sound byte simple.
Old 09-01-2007, 08:11 AM
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Default RE: C.G. Location on a scale model

da Rock's comments seem to make sense. I have, or have had, 4 r/C to C/L conversions--Top Flite Gold P-40, SkyShark Val, Hobby Hanger Typhoon, and Taylor Typhoon. CG as called for in the R/C plans and insensitive elevator control linkages. I use some stability calculations from an old Model Aviation R/C pattern column for my scratch built models and found that they yield a very stable model. I tried for this on my dH Hornet and Hs 129 but limited CG movement foreward by the amount of weight I was willing to put in the nose. I suspect you will have a safe starting point using either the R/C location or the forward C/L suggestion. Then trim to suit your style per da Rock's suggestions.
Old 09-01-2007, 08:44 AM
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Default RE: C.G. Location on a scale model

Thanks for the very detailed and informative response!

I am not too happy about the extra forward CG location since bearing in mind I'm considering a taildragger, it could lead to a stronger tendency to nose over.

Seems to me the RC balance point, or somewhere in between the RC and CL recommended points, would be the reasonable place to start.
A more forward CG would also mean a higher stall speed so - why spoil the model?

Anyway, what is also obvious is that adjustable leadouts will be necessary even though not called for on the plan, in order to get the best out of every CG position.

BTW does all this mean that short nosed protoypes e.g Sopwith Pup/Camel, or Gloster Gamecock, being difficult to balance for RC, would be even worse candidates for converting to CL?
Old 09-01-2007, 03:26 PM
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Default RE: C.G. Location on a scale model

Plenty of short nosed models been flown over the years. However in my opinion, gained from judging lots of C/L contests, models with narrow landing gear tend to go up on the inboard wingtip when taxying.
Old 09-03-2007, 05:30 PM
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Default RE: C.G. Location on a scale model

EF,

Most CL Scale models I've seen were fairly heavy for their wing area, by comparison with most other CL type models. They fly "more realistically" as the nose must be brought up as you throttle down. You can land tail-draggers 3-point, not just run them onto the mains and let speed bleed off until the tailwheel drops...

CG in the range 15% to 20% of mean aerodynamic chord is not so far forward as to be a nose-over risk, particularly with a tail-dragger trying to land 3-point... The STA has a nicely proportioned longer nose, but yes, the WWI biplane fighters had very short front ends, and that can be a problem to balance. The originals of those were very light; the heaviest objects in them, in order of weight, were the engine, the pilot, and the guns/ammo or the fuel tank. And those were clustered right aroound the CG...

The STA main gear is a bit narrow spaced. Keep that in mind while taxying, and also it might help to lower your handle so you're not pulling the inboard wing too high up. It helps to build some drag into the outboard wheel, too.

Luck!

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