Capt Jim

My Feedback: (168)

Every kit or ARF that I have worked with specifies a CG point, and wants the plane balanced at that point...empty of fuel. However, the fuel tank is just about always well forward of the specified CG.
It seems to me that under actual flying conditions...that is loaded with fuel, the plane would then become quite nose-heavy. My question is very simply...why do they want to allow every plane to become quite nose heavy?....and what would be the result of balancing it with a load of fuel on board? Is it the danger of the plane becoming tail-heavy as fuel is depleted?
This puzzles me, and I thank you all for some insight.

GaGeeBees

My Feedback: (23)

Avoiding a tail heavy condition when fuel is low or empty would be my guess. On most of my models the fuel tank is located pretty close to the CG and don't influence things very much one way or the other.

Richard L.

My Feedback: (24)

I'm sure they have factored in the fuel weight when they calculated the CG range. It's always better to err on the nose heavy side anyway. For instance, I can fly any nose heavy plane. It's just mean the landing speed is a little faster that's all.

Capt Jim

My Feedback: (168)

Yes, a higher landing speed is one of the issues. Just about every plane that I have seen has the tank right up at the firewall, and the CG is naturally way back at about 25% or so on the wing chord. In terms of weight and balance, thats a BIG moment.
As I watch the mayhem that so often occurs at the popular flying fields, I see that the flying field is usually a very limited length...be it grass or pavement. To add to the difficulty, many fields have a tree line, or other obstruction right on the approach, and relatively close to the runway. These factors dictate a close call over the trees, and a last minute dive for the runway, (picking up additional unwanted speed), and then due to the speed, having to deal with a longer runout...sometimes longer than the runway permits.
Flying fields are being lost to development, etc, and new ones are difficult to obtain.
As a result, we are forced to tolerate these much less than ideal conditions.
Recognizing that most flyers do not have a full scale runway to deal with....and without going into little electric foamys...wouldn't it be better to work on the design of the airframe to accomodate the limited flying space available.
Placing the fuel on the designed CG would help maintain a consistant balance throughout the flight. This may in some cases require a fuel pump to assure sufficient fuel flow, but all this seems to be a better plan at the design stage, making the plane a more comfortable flyer.
Personally, I am not having problems with the changing trim, but I do feel for so many less skilled pilots who are frustrated with their failed attempts to land gently.
...Just my thoughts on the subject...whats your opinion...is there any merit to this?

Strat2003

My Feedback: (1)

There's an old addage that says, "A nose heavy airplane flies poorly, a tail heavy airplane flies once." The reason we balance airplanes without fuel is to avoid a tail heavy condition on landing, when the tank is near empty.

CG location can be a rather personal taste once a pilot has some experience, but for trainers and first flights it's usually best to avoid even a hint of tail-heaviness, thus the practice of balancing with an empty tank.

ZT-inactive

Most fuel tanks are located near or forward of the CG point. The manufacturer wants to make sure that when you come in for landing or near the end of your flight, your CG hasnt' shifted past the aft limit. Stall/spin on landing that will be unrecoverable from a low altitude will ruin your day. Granted, with a more forward loading, you will be less responsive, but will benefit from added stability.

Montague

As for the concern about not being able to slow the plane down for landing, a big part of it is a lack of pilot skills. My field actually has a rather easy approach, no obstructions. There is a bit of a dip that can cause some turblance if the wind is blowing the wrong way, but that's about the worst of it. And yet I see guys driving their planes in to the runway way faster than they need to all the time. And it's not just the CG either, I've landed their plane nice and slow and nose-high, then watched them land the same plane much faster. We spend a lot of time on this during our flight instruction, but it doesn't stick for everyone.

It it safer to measure the CG with the tank empty. But most sport planes can have their CGs moved way back with out problem anyway, and this goes double for trainers.

Bax

My Feedback: (11)

So the CG changes as the tank drains. For the vast bulk of models, it's not really an issue.

For full-size aircraft, the most-forward CG limit is determined by the ability of the elevators to raise the nose to a proper landing attitude. The rearward limit is determined by the amount of stability margin the airplane is intended to have.

Many modelers tend to land too fast. That's because they really don't know the speeds at which their model works. They tend to not spend time with the model in the air doing slow flight and stalls. They don't learn what the best attitudes for the model to have during landing approach. Once these are known, a model with a forwar CG will land just fine...you need to have enough elevator to allow the nose to be raised.

Now if you're discussing aerobatic aircraft, then it's a different story.