CG and engine thrust
Dean Pappas | [email protected]
Hi, gang. The last time we got together, we began a discussion of five commonly seen—and easily fixed—maladjustments. Of course, it was necessary to wax philosophical for a while about what a well-tuned airplane can do for you.
As with a finely crafted sword (or any well-built tool), a well-set-up airplane falls to hand readily and is comfortable to use. It makes he who wields it more effective—or at least it helps him or her to fly better.
The big five
- Control surface hinge-line gaps
- Heavy wingtip or lateral misbalance
- Balance point, usually referred to as CG
- Engine thrust—both downthrust and right thrust
- Aileron differential and adverse yaw
We discussed the first two items last time, so here we’ll examine CG and a bit about downthrust.
Remember that example of the airplane that lands nicely if the airspeed is kept up a little, but if slowed down too much the elevator would lose the control authority needed to flare out? Last time we sealed the hinge-line gaps and made the elevator work as it should, but that was only one element that could have caused the problem. The airplane also could have been extremely nose-heavy.
The CG (center of gravity) or balance point of an aircraft affects every flight regime as well as operation on the ground. The effects of CG position show up everywhere; this opens the possibility of checking several different flight characteristics to determine the optimal CG position.
There isn’t a single optimal CG for any particular design. It depends on the type of flying you are doing, your piloting skills, and to a certain extent how the airplane is equipped. Some designs have wide CG envelopes—the range of useful CG positions—while others have a very narrow sweet spot. Within that envelope, fine-tuning the CG may even be a matter of taste. A wide sweet spot might be necessary; take fuel consumption, for example. If the fuel tank is right behind the firewall, as it should be for a non-pumped engine, then as the fuel burns off the CG will move aft.
If the tank is large (and we are often tempted to install the biggest tank practical!), it will be necessary to compromise and take off nose-heavy so the airplane is not dangerously tail-heavy when the tank is nearly empty. Being tail-heavy can lead to a loss of control and very short airframe life.
Measuring the CG
Before flight testing, measure the CG so you can make changes systematically. Nothing overly rigorous is needed, but don’t lose your place as you make adjustments.
- If your airplane has a high or shoulder-mounted wing, support it at the fuselage center and the tip. Measure 1/4 of the way back from the LE (leading edge) on the Mean Aerodynamic Chord (MAC) and you are done.
- The MAC is the chord line where half of the wing area is inboard and half is outboard; that places the MAC slightly inboard of halfway out.
- On a constant-chord wing the 25% point is exactly a quarter of the way back from the LE to the TE (trailing edge). Most trainers have constant-chord wings.
- For tapered and/or swept wings, find the chord halfway between the centerline and the tip; measure 1/4 of the way back from that chord’s LE and you have a good approximation.
A typical safe starting point for almost any airplane is CG at 25% of the MAC. The CG on a typical trainer is normally between 1/4 and 1/3 of the MAC. The balance point on 3-D aerobatic airplanes is often around 40% of the MAC.
CG positions near the front of the envelope make it easier to fly smoothly; a CG near the back gives more maneuverability. If you want to maneuver smoothly, compromise is necessary.
Start by placing the balance point where the plans or instructions specify. If the plans show a range, shoot for somewhere in the forward half of that range. A slightly nose-heavy CG is best for test flying and familiarization. Subsequent flight testing will help you decide whether to move the CG aft and by how much.
If you have a low-wing model, you may find it easier to flip the airplane over and balance it upside down. Place both fingers the same distance back on each wing panel and move them until the airplane hangs level. Several balancing stands have been sold through the years to avoid using your thumbs!
CG testing methods
- The climb-and-glide test — most useful with trainers and aircraft intended for loitering flight. This test gives information about engine downthrust as well as CG.
- The inverted-flight test — useful for sport and precision aerobatics. Coupled with the chop-the-throttle test for downthrust, it gives a more complete picture of how CG and pitch trim interact.
- The landing/flare/slow-flight check — note whether the elevator has enough authority to flare. If elevator power runs out before you can flare, that indicates nose-heaviness.
Tail-heaviness makes the elevator plenty powerful and a bit touchy until the last moments. An airplane must be quite tail-heavy before this characteristic shows clearly. If the airplane doesn’t reliably settle to neutral elevator after you release it, check both the control setup and possible tail-heaviness.
Climb-and-glide test
Start by trimming the airplane for level, cruise-speed flight. Once cruising, smoothly advance the throttle to full. Without making elevator corrections (but keeping the wings level), watch the resulting climb.
- If the climb is too steep and airspeed decays to the point where control authority suffers, you have an overly steep climb. Two remedies are possible: move the CG aft and add some down-elevator trim, or add more downthrust.
- If the climb is too shallow and fast, do the opposite.
To decide whether to change engine downthrust or the balance/aerodynamic trim, perform the low-throttle glide test.
For the low-throttle test, establish cruise flight at roughly 100 feet. Trim for cruise-power level flight, then smoothly reduce the power to slightly above idle—the throttle setting used during final approach before the runway threshold. Watch the glide slope after the airplane slows. Keep the wings level and don’t make elevator corrections.
Does the airplane settle into a nice glide, or does it assume a very steep glide? Maybe the glide slope is very shallow. Now combine the observations from both tests to decide on adjustments. CG and elevator trim both strongly affect glide behavior; since you set elevator trim for cruise, assume glide behavior is strongly influenced by CG.
- If the airplane climbs too steeply at full power and glides steeply, it may be nose-heavy. Nose-heaviness makes the airplane overly stable in pitch; it responds to more airspeed by trying to climb too much at full power. If insufficient downthrust is the cause, the same steep climb will result, but the glide should not be too steep.
- If at full throttle the airplane’s climb is slightly shallow and it glides nicely or a bit shallow, you want the model to change pitch trim more with airspeed. Accomplish this by moving the CG forward and re-trimming with a small amount of elevator for level flight.
After each adjustment, repeat the entire test sequence: trim for cruise-power level flight, perform the full-throttle and low-throttle tests.
If the problem is downthrust rather than CG, the climb will be more affected and the glides less so. For example: if downthrust is insufficient, the elevator will have to be trimmed slightly down for level flight compared to where it would be with correct downthrust. At full power, an airplane needing more downthrust will climb too steeply; if it has too much downthrust, the climb will be shallow and you’ll add up-elevator trim for cruise, making the glide shallow. It can take a few adjustments to tease these effects apart, but it’s fun to experiment.
Chop-the-Throttle test
This test is most useful for high-performance sport, scale, and some aerobatic airplanes—especially those with semisymmetrical or symmetrical airfoils.
Assuming you generally fly at full throttle, trim the elevator for a hands-off level pass at approximately 50 feet AGL. As you fly straight and level with the airplane trimmed out, suddenly pull the power to idle.
For a second or two the airplane will still be at cruise speed and downthrust is effectively removed; the aerodynamic trim at full speed predominates and the remaining difference is the effect of downthrust.
- Ideally, the airplane will fly straight and level without a twitch for a second or two, then slow and settle into a glide.
- If, after pulling the throttle back, the nose twitches up and then the airplane slows into a glide, you have too much downthrust. The downthrust forced you to add up trim to counteract it; when thrust is removed, that trim makes the nose rise.
- If the nose abruptly drops and the airplane instantly assumes a fast, nose-down glide, you need more downthrust. Your elevator trim had been fighting an engine-induced climb during the flight.
- If downthrust is correct, the airplane continues straight for a second or two and gradually fades into the glide angle.
You will also see wrong downthrust when power is applied: if downthrust is insufficient, the airplane will go abruptly nose-up when you pour the coals on for a missed approach. That can be bad news with a heavy scale model.
Inverted-flight test
For high-performance airplanes with semisymmetrical or symmetrical airfoils, CG is the dominant factor determining how much elevator input is needed to sustain inverted flight.
- If it takes too much down elevator to fly inverted, the airplane is likely nose-heavy.
- If it takes no down elevator or the airplane even climbs, it is definitely tail-heavy. It is not normal to fly inverted without needing any down elevator input—even Precision Aerobatics and Pattern fliers trim as tail-heavy as they dare.
Landing / flare / slow-flight check
You should already be familiar with this check. If the elevator power runs out before the airplane has slowed enough to flare nicely, the airplane is nose-heavy. It is normal to lose some control authority before the stall, but as long as you have enough to flare for a good landing, the CG is within the sweet spot.
Tail-heaviness makes the elevator very effective and a bit touchy until the last moment. If the airplane doesn’t reliably settle to neutral after returning the elevator to center, inspect both control setup and possible tail-heaviness.
The next time we get together we will discuss adverse yaw and aileron differential. These are key to good directional control, especially at low speeds.
I’m out of room for now. Next time we’ll spend more time sharpening our flying weapons. See you then—have fun, and take care of yourselves.
Transcribed from original scans by AI. Minor OCR errors may remain.
