Hi All,

I purchased an F27C Stryker back in 2008, and then R/L got in the way of R/C, so it went on the shelf. I'm wanting to resurrect the old girl, and have hit what may be a technology progression issue.

The Stryker CG was balanced based on the shipped battery, which was a 3S 2200mAh, 12C LiPo, but only weighed 117g out of the total plane flying weight of 620g.

Todya's 2200mAh batteries weigh in starting at about 180g, due to the higher C ratings pushing up the density. Given this means roughly 10% extra overall flight weight being added several inches in front of the stock CG, what effect will this have on the flight, and should I do anything to try and compensation for it?

Thanks.
Russell.<br type="_moz" />

well Bro, the plane still must balance at the CG.. only choice is to position the batt to obtain CG or go to a lighter batt..you could allways beef up the motor/fan unit /esc to handle the extra weight, or bungee launch to get airborne..no matter what direction you take, the plane has to be CG'd where its gonna fly right...sometimes I add weight to the tail if its nose heavy..there's no getting around correct CG..I know , it sucks but no other way. but get CG correct Bro....


guamflyer

P-40 Bro #5 - #6

VKRUSS

C.G. is only a RECOMMENDATION not an absolute.

It is definitely more important in a flying wing, but you DO have some leeway.

You should put one of your old batteries in, and mark the balance location.

Then put a new pack in and mark that location.

I'll bet the difference will be less than 1/4" forward which will make ABSOLUTELY NO DIFFERENCE to the handling of the Stryker.

I've flown it with even heavier packs and I could not see any change in characteristics. The C.G. moved forward about 1/2" with the even heavier packs.

If the difference in CG turns out to be only 1/4 inch then I'd tend to agree that the change won't do much. But it will be noticeable. But if the CG shift is more like 1/2 inch you're most certainly going to notice that it's acting differently in the air. On flying wings, even "long" swept wings like the Stryker, a small CG shift produces a lot of change in how the model handles.

Ideally you would alter the insides by cutting away some foam to allow you to locate the pack farther back so the CG remains constant. If that simply isn't practical because the pack will intrude on some other components then you may need to resort to some tail weight. Or go with a slightly lighter pack to get back down closer to the original spec pack weight.

As a P-3 Flight Engineer one of my primary duties was to calculate the weight and balance before each flight so I do have an extensive knowledge on the effects on CG. Opjose is right, CG is not an absolute, but there are limits to CG. If you exceed these limits it will have a profound effect on the aircraft limiting the control of the elevators. If the CG is too far forward or aft it can even make the plane unflyable (or briefly flyable, just prior to the crash). The best way to find out the CG rage would be to contact the manufacture to find out the forward and aft limits for the CG. To understand the concept of this range think about this: on most glow or gas aircraft a person balances the plane without fuel. When the plane is flown it starts out with a full tank of fuel which moves the CG forward. As the plane flies it burns off the fuel and the CG moves aft. Obviously on electrics there is no change in CG during the flight, but you get the point. Just things to think about when balancing an airplane.

Torque = weight x distance. I'm not sure how it works in metric, but inches and ounces works, as do feet and pounds. For example, say your design battery pack weighs 3 oz and its center of gravity is 6 inches in front of your airplane CG. 3 oz x 6 in gives 18 inch ounces. Say you replace it with a 5 oz battery. 18inch ounces / 5 ounces = 3 3/5 inches. So the CG of your 5 oz battery pack should be moved 2 2/5 in back of your design battery back position to preserve your original airplane CG.

Thanks guys,<div>
</div><div>I just did a rough desk-check and eyeballed the difference at  the new pack weight of about 1/2" - 3/4" forward. There's no room to move the battery back, so I guess my only option is to try and add some weight to the rear.</div>

You guys are very correct that there is no absolute CG. There's a range that can be used. But within that range the characteristics of how the model behaves in the air will change quite a bit. On a conventional 2 meter glider a shift of 1/4 inch has noticeable effects in how the model trims out and how it behaves in pitch response and pitch stability. I've proven that to my self on more than one model. On a smaller flying wing like the Stryker in this thread a 1/4 inch displacement is proportionally both bigger and more important given that the design is a smaller size and a flying wing.

So in order to correctly maintain the same flying habits the balance point will need to be pretty darn close to where it was.

A sort of related issue - - -
Many of the current ARFs are designed for both electric and engine power.
The specs specify a single CG, possibly with a +/- limit for both versions.
The engine powered version CG, by convention, is measured with an empty tank.
The electric version CG is usually checked with the battery in place.
The engine powered version CG measurement makes sense, in that it is the most tail heavy
condition, and is important for, if nothing else, landing.
Using the same CG location for an electric version often results in adding weight to offset the battery.

Fuel tanks usually end up forward of the desired CG, as does the main battery on electrics.