Barrie James explains why taking flight smoothly is never as easy as it looks
1946. A small boy hastily dismounts from his bicycle in a narrow lane that skirts Pendeford airfield, home of the Bolton Paul aircraft factory, and lets the machine fall against the perimeter fence. His eyes are fixed on a nearby Tiger Moth, its engine idling as it stands at the threshold of a grass runway. Here, the boy knows, is where the aeroplanes stop to prepare for take-off, the wash of their propellers flattening the close-cropped nap of the runway, and sometimes raising miniature rainbows from its rain-wetted grass. Engine checks complete, the Tiger’s control surfaces are methodically exercised as the pilot is limbering up ready for the off, and then, with a roar and a sweep of its rudder, the aeroplane moves slowly forward. As the take-off run gathers pace, the fishtailing of the rudder slowly diminishes and the boy watches, open-mouthed and unblinking, as light appears between the wheels and the runway. As the machine lifts into a steady and deliberate climb that tracks neatly along the centreline of the runway, the boy sighs and absently reaches into his trouser pocket for a dusty sweet.
It will be another 20 years before this lad is able to afford flying lessons of his own at the airfield, but every time that he pauses beside the fence that borders the narrow lane, and then lines up on the grass runway, he’ll remember these moments.
Even now, I often recall those far off days when I’m preparing to take-off with a model aircraft because, as with full-size flight, taking off in a controlled manner that demonstrates good airmanship is never as easy or simple as it looks. All too often, of course, model flyers’ take-offs look as though they’re simply doing it the easy way: slam the throttle wide open, let the machine charge forward and weave down the runway as though it’s chasing chickens, then drag it off the ground and climb-out too steeply and too slowly in a display of thrust over thought.
There are times, it’s true, when an aggressive take-off with lots of power and maximum lift is necessary, such as when your model patch calls for a steep climb-out. In these cases, many models have a tremendous power-to-weight advantage over their full-size cousins, and it makes an interesting exercise to apply full power with the machine restrained, then release it; you’ll find that most will leap off the ground in a very few feet. To carry out this manoeuvre safely, though, calls for the help of an assistant, and plenty of practice, initially using lower throttle settings.
In most circumstances I believe that model aeroplanes should fly the take-off manoeuvre in a manner that closely resembles a full-size aircraft - a process that starts with the taxi and ends (unless your model is following a full-size flight path to 1500ft!) when the climb-out levels out at circuit height.
Watch a full-size light aircraft as it taxies, and you’ll see that its speed is both constant and conservative, allowing the pilot to judge the surface ahead and react to obstacles in good time. Unless you fly from a smoothly paved surface, this steady progress isn’t so easy to achieve with a model: grass offers varying resistance to small wheels, an overly sensitive throttle will make speed control tricky and the ambient wind won’t scale itself down to flatter your efforts.
Even so, you should practice taxiing your model, making the small throttle changes necessary to maintain your speed and to assist when turning, at the same time paying attention to the way that the prop wash will act on control surfaces. The elevators, for example, should be used to keep all three points firmly on the ground. With nose wheel equipped aircraft, this means taxiing with the stick forward, whilst tail-draggers require the stick pulled back so that bursts of extra power don’t lift the tail wheel. If you have a tail wind, on the other hand, you need to balance the down force that the prop wash on the elevators exerts on the tail wheel with the area that the deflected surfaces present to the following wind: a sudden gust getting under a tail-dragger’s raised elevators can up-end you embarrassingly!
The wind will play other tricks, too. A quartering cross-wind, for instance, will generate lift on the into-wind wing, which can cause the wing to lift, and will do nothing for directional control. Remember, then, to lower the into-wind aileron to spoil this unwanted lift. When moving cross-wind, for instance, it will lean on the fuselage aft of the main wheels and use this area to weather-cock the aircraft into wind. Without brakes, the only thing you have to counter this is the rudder, though to be effective you may need a fraction more throttle to increase the wash over its surface. When it comes to making turns, you can exploit this weather-cocking tendency by turning the nose towards the wind and letting it assist your rotation. It may mean turning 270° to the left to achieve a 90° turn to the right, but your turn will be neater, tighter and safer than fighting your way round with rudder and a wide-open throttle.
When taxiing after landing, where possible you should avoid taxiing towards yourself or others. If a throttle linkage is going to come adrift or the transmitter is going to suffer a glitch, it’s better if the model’s pointing away from people. When the taxi’s complete, hand your transmitter to a friend or put it down, wait for a safe moment and after announcing, “On the patch” and making sure that all other pilots have heard your call, retrieve your model before reporting, “strip clear,” to those still flying.
Okay, now let’s examine what I call a proper take-off.
WHAT GROUND EFFECT?
In general terms, a wing that’s generating lift will create a downwash from its trailing edge which, when aggregated with the relative airflow at the leading edge, results in a reduction of the wing’s effective angle of attack. That’s to say, it will produce less lift. To compensate, the pilot increases the actual angle of attack, which increases lift but also increases drag. When you’re flying within about half a wingspan of the ground, however, this downwash is suppressed, which significantly increases the wing’s effective angle of attack, which means it generates more lift, doing away with the need to pitch up, and doing away too with the extra drag that goes with that pitch-up.
The practical application of all this is that, by holding the aircraft in ground effect, you can sustain lift with less energy, which means you have more energy to expend in accelerating the model. Hence the expression, ‘accelerating in ground effect’.
At the other end of the flight, the reduced drag and increased lift that comes with ground effect will help to cushion the landing because the aeroplane generates more lift for a given speed, and doesn’t lose that speed so quickly.
After preparing your model for flight you can taxi out to the threshold or, if your club’s rules prohibit this, carry your model out to the runway. From the pilots’ box you’ll be able to take stock of the situation in the circuit, and whether fellow pilots are flying a left or right-handed pattern. When you consider that it’s safe to go, have a good, last look at the finals leg to be sure that it really is clear and that there isn’t an unannounced model coming over the hedge, before either asking, “Okay to go?” or announcing, “Taking off.”
It’s not necessary to be standing behind your model during the take-off; in fact, you should be able to control the model from any angle. Concentrate on opening the throttle smoothly and be ready to correct any initial swing. I’ve often been asked the reasons for this swing, and I’ve explained some of the causes in the panel Effects of Power; for now though, let’s keep our mind on using the rudder to hold the aeroplane in check and keep it tracking along the centreline of the runway. As with those Tiger Moths I watched as a boy, you may need quite a lot of rudder until the model gathers speed and the control surface gains authority.
At this stage, if anything starts to go amiss - you apply rudder in the wrong direction, say, or the engine misses a beat or two, then abandon the take-off. You will not lose face in front of your friends because this is good flying practice. Rather than struggling into the air wildly off heading, or with an engine that is about to fail, it is far better to simply cut the throttle, maybe using a little elevator if necessary to stop the machine nosing over as it decelerates. With this you can cut the engine and, after the usual safety checks, retrieve the model and try to determine the fault.
Let’s suppose, though, that you’re tracking straight and the engine is on song. In a full-size aircraft, the pilot would know when he’d reached flying speed as much from the seat of his pants as from the airspeed indicator, but the sum of these inputs will tell him just when to raise the nose / tail. The model pilot, on the other hand, has to judge this moment by experience and by ‘testing the air’ with the elevator control.
If the airspeed will indeed support flight, then a well-trimmed model will now start to rise from the ground without your assistance. In any event, as soon as the model lifts off, you should ease the stick forward so that it barely climbs. Holding it down in this way allows it to accelerate in ground effect (see What Ground Effect?) and build up to a proper, safe, climb-out speed. After a few moments you’ll be able to ease back on the stick and allow the model to climb at a scale-like angle. Throughout all of this, you should have maintained your position over the centreline of the runway.
THE CLIMB OUT
Now, if your power fails at this point, do not try to turn back. Simply close the throttle, call out, “dead stick,” and land straight ahead. Tempting though it may be to turn back, the fine judgement of height and speed in making a 180° turn to land downwind stacks the odds heavily against success.
In all likelihood, however, the engine will be fine and all you’ll have to worry about is laying off any crosswind (which will only grow stronger with increasing height) to maintain the centreline until you reach about 50 - 100ft, where you can commence a turn onto the crosswind leg of the circuit and start enjoying the rest of your flight. Ain’t this a great hobby?
THE EFFECTS OF POWER
In the single-engine prop-driven aircraft that many modellers fly, as many as four factors can add up to create a swing on take-off.
The first of these is the slipstream of the prop, which takes the form of a vortex that runs around the fuselage in a helical path and eventually encounters one side of the fin, setting up a yawing moment which causes a tendency to swing.
If you’re a tail-dragger pilot, you’ll also have to contend with the effects of blade asymmetry, which will be most marked when the prop axis is not in line with the flight direction of the aircraft, that is, at the start of the take-off run, when the tail’s down. In this condition, the length of the path of the blades through the air will vary as a result of two factors. One of these is the difference in the angle of attack of the blades: The down-going blade meets the relative airflow at a higher angle of attack than the up-going blade, and so generates more thrust. The other cause is the fact that, in their tilted state, the blades are effectively travelling forward through the air at different speeds. The easiest way to visualise how this comes about is to imagine an aircraft moving forward in a nose-high attitude. In the time taken for a blade starting at the top of the tilted disc (the rearmost position) to rotate to the bottom (the foremost position), it will have travelled further through the air (distance travelled by the aircraft + distance of forward rotation) than a blade starting at the bottom of the disc (the foremost position) and rotating to the top (the rearmost position), which amounts to distance travelled by the aircraft - distance of backward rotation. This gives rise to extra thrust on the long-path half of the disc, and so to more yaw.
In a right-handed engine, then - one in which the top of the prop moves to the right when viewed from the cockpit - the extra thrust is on the down-going right-hand side, yawing the model left. The unfortunate thing, of course, is that blade asymmetry starts doing its thing from the start of the take-off run, when you’re winding on power - increasing slipstream effect, in other words - and when your rudder is at its least effective.
You’d think that picking up the tail would at least put you on a level footing with nose-wheel pilots, but no - at least, not until you’ve dealt with the third consideration - torque gyroscopic effect. The force applied to move the axis of the prop’s rotation - that is, to tilt the aeroplane’s longitudinal axis - will be modified by precession so that it acts at a point displaced by 90° in the direction of rotation. Think of it as a finger pushing the top of the prop disc from behind; in a right-hand engine, the precessed force acts as a push on the right-hand side of the disc, compounding the leftwards yaw.
The fourth and final factor is torque effect. The equal and opposite reaction to the force which rotates the propeller tries to rotate the airframe in the opposite direction. On take-off, this causes one wheel to be pressed more firmly onto the ground than the other, giving rise to more friction on that side and therefore another yawing force. On a right-hand engine, you’ve guessed it, it’s the left wheel that drags, compounding the leftward yaw even more.
For all of these reasons, then, you can see why on the take-off run, the throttle should be advanced at a rate such that the reactions are never greater than the ability of the rudder’s increasing authority to control them.
WHAT ABOUT FLAPS
What part do flaps play in the take-off? Well, in principle, the effect of flaps is to increase the camber of the wing’s airfoil, which increases its coefficient of lift. In practical terms - and within the flaps’ normal operating ranges of 5 - 20°, say - this means that the wing will generate more lift for the same speed, or the same lift at a lower speed.
In the take-off, then, the model can become airborne at a lower speed. I know, I know, you’re probably saying that flaps must add drag that slows acceleration. On balance, however, the lift that they produce outweighs the profile drag they create and a take-off run will be shorter with a little flap than without.
In full-size aircraft, the profile drag of the flaps becomes an issue when considering the angle of climb, which depends on the excess of thrust over drag. If you’re climbing at full power, then, any extra drag is going to reduce that excess and make the climb gradient worse. Once again, the fact that model aircraft usually have an abundance of power means you can add a touch more throttle to maintain a healthy climb angle, especially if there are trees or obstacles to encourage you. For scale-like flap handling without worrying about minimum flap retraction speeds and what have you, when established on the climb-out you should lower the nose and accelerate, retracting the flaps and correcting for the change in trim that this will cause, to maintain a steady climb profile.
Want the latest issue of RCM&E? Use our magazine locator link to find your nearest stockist!