Trimming a model airplane CG and thrust angle

Here’s a brief article I wrote in response to someone misunderstanding the effect of CG on high-speed model aircraft handling. I know regular readers of barnson.org may not be into this, but it’s a place I can point other enthusiasts to in order to correct a common misconception regarding the Center of Gravity.

The only issue i have at wot is that it wants to climb even though I’m a tad nose heavy.

No, it wants to climb BECAUSE you are nose-heavy. Think about the aerodynamics. Let’s assume you trim your bird for straight-and-level flight at about mid-throttle. I usually trim somewhere between 50% and 75% throttle. Whatever you do with trim, you want trim to be well above ‘waddling near a stall’, and well below the speed it would achieve in a power-off dive, for reasons outlined below.

Now let’s climb. Really good and high, and fairly far out. Then turn OFF your motor, and put your Stryker into a 45-degree dive either with or against the wind. Take your hands off the controls and just watch what it does for a few seconds.

If your bird is nose-heavy, in this power-off dive, it will tend to pull out of the dive fairly sharply… on mine, stock, it will pull out of a 45-degree 200 foot dive long before hitting the pavement. It seems backwards, I know! The reason for this is because at higher speeds, you elevons exert more aerodynamic force on the tail of the airplane than they do at traditional cruising speed. So since that power-off dive has it going faster than where you trimmed it for cruising speed, those elevons are pushing down harder, causing the nose to raise.

If your bird, instead, steepens the dive, you are tail-heavy. This is for the same reason as why it climbs when nose-heavy: you’re flying faster than your trim-speed, so your elevons exert more up-force, causing your nose to pitch downward. The elevons were literally “propping up” the tail of the aircraft at cruising speed.

Now, you ask, “what about my thrust angle?” Well, we can figure out if that’s your problem too, by finding a way to increase thrust from cruising-trim speed without actually increasing speed. It’s much easier to sort out motor alignment problems once your CG is correct, so fix that first. Use weights if you have to, but at the very least, remove that big screw which is under the sticker in the nose of your Stryker-C.

Do this test several times before coming to a conclusion, and then after making any CG changes, do it again several times. You’ll see the difference, and know whether more modification is needed. I actually have a small 1/4 oz lead weight near the tail of my Stryker-C to get the CG where I like it.

OK, so you’ve sorted out your CG issue, right? Put it into a power-off forty-five -degree dive, and it should VERY SLOWLY start to pull out on its own. That’s pretty much proper trim on the Stryker. It’s almost impossible for a Stryker to not pull out of a dive when the CG is correct, primarily because the airfoil is semi-symmetrical. Your goal with balancing your CG is to eliminate the elevons as a source of that differential lift.

All right, now how do we eliminate excess speed in figuring out motor thrust angle problems? Well, once again, let’s start out straight and level, properly trimmed for flight at around 50% throttle, and going farly slowly.

Now, punch the throttle on your Stryker C (about 75% to 85% will do fine on this bird, it has a thrust-to-weight of very nearly 1:1, particularly with an APC 6×4 prop) and aim it skyward at about 45 degrees. Once again, take your hands off the stick and watch what it does.

Now, ideally, the speed you’re achieving in this climb should be the same as your trim speed. Do the test several times at varying throttle settings to confirm your results before you make any changes.

It should *very slowly* lower its nose towards horizontal, at a rate similar to that in your dive test. If it drops the nose very quickly toward horizontal, or if it steepens the climb, your thrust angle is off. To remedy nose-dropping, lower the front of your motor and raise the rear (backwards from what you’d do on a tractor setup). To fix nose-steepening, raise the front and lower the rear.

In any of these cases, if your plane starts to roll one direction or the other, you have an issue with thrust angle left/right, wing warping, or transverse CG. You want to double-check that your machine is balanced laterally (wing-to-wing) on the bench, and this is very easy just by holding the nose and the tip of the prop nut (upside down, of course, since this plane is effectively a low-wing warbird). A nail stuck into the high wing balances you laterally. If you’re still rolling after lateral balancing the bird, then you have a wing or control-surface warp you need to figure out and fix.

Hope this helps. The use of some simple climb and dive tests from a cruising-speed trim really helps to figure out CG and thrust angle issues.

In short, the Stryker-C flies just fine, stock. For pilots interested in more axial rolls and high-speed upright and inverted passes without having to apply much down-stick, a small change to the CG helps sort those issues out pretty well. The downside of scooching the CG back is, as always, stability. Other pilots complain that my birds are “twitchy”, because I like a fairly far aft CG (still within “normal”, but just a bit further back than most pilots are used to) for manueverability and hgh-speed handling.