In this article, we’ll look at how the forces of nature can make a difference to an aircrafts aerobatic performance.
Flying aeros on a calm day is one thing, but flying them in the same manner on a windy day is another and will invariably lead to your stunts and tricks being bent out of shape. Loops cease to be circular, rolls can drift, and you may get caught out by a sudden gust and find yourself in a tricky situation. However, if you have an awareness of what’s happening and how to compensate then you’re in a good position to make your aerobatic figures look right, whether in calm conditions or a 30mph gust.
INTO WIND & DOWNWIND
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One of the first snippets of knowledge we gain when learning to fly is that the aircraft should be facing into wind during the take-off phase. You’ve probably noticed that a model takes off in a shorter distance into a breeze than it does in calm conditions, which is due to the effect of the wind blowing over the model’s wing, increasing the amount of lift available; the aeroplane’s air speed is effectively greater than if there was no wind at all. Air speed, related to ground speed, is a term that you may not have come across – let’s have a look at what both of these mean and their effects in aeromodelling terms.
Ground speed is the speed at which an aircraft travels over the ground. When you watch a model aircraft fly past, you’re watching it travelling at its ground speed. Air speed, on the other hand, is the speed at which air travels past the aircraft, and the way an aeroplane behaves is dictated by air speed. Say a model was flying along on a windless day at an airspeed of, say, 40mph. The wings would produce lift corresponding to 40mph, and the ground speed and air speed would be exactly the same. But on a windy day, this isn’t necessarily the case.
When flying into wind, the air speed of a model will be larger than its ground speed. To explain this, let’s imagine a little scenario. You’re flying your aircraft from right to left, directly into wind (so this wind is coming from the left). The model is flying along at 40mph ground speed, and the wind is blowing at a steady 10mph. The ground speed may be 40mph, but the air speed is 50mph, because the wind is blowing additional air (at 10mph) over the model.
This is why aircraft take-off in a shorter distance on windy days. Your model may take-off at, say, 20mph, but with a 10mph headwind it only needs 10mph ground speed to get enough air speed to take-off.
When flying downwind, air speed will be smaller than ground speed. For example, if you’re flying downwind at a ground speed of 40mph with a tailwind of 10mph, then air speed is only 30mph; the direct opposite of the ‘into wind’ scenario, and the controls may feel more sluggish and less responsive than if you were flying into wind (See Fig.1).
Key things to remember:
* Air speed and ground speed aren’t always the same.
*Air speed is what really matters to the model and determines how it feels and how it behaves.
*Ground speed is the speed over the ground, i.e. the visual speed.
There are a few things to bear in mind here. If you leave the throttle on your model at, say, half power, then (more or less) whatever direction you fly, whether into or downwind, drag will dictate that the air speed remains the same. Only ground speed will change. Assuming you’re flying straight and level, the throttle is the only thing that will change your air speed. The model will appear to be flying faster downwind than upwind, but if the throttle is in the same place then air speed won’t have changed, only ground speed. Altering power, or the model’s attitude (e.g. diving), is what changes air speed.
Some beginners make the mistake of reducing the throttle too far when flying downwind, because the model is rushing away from them. Sure, the ground speed looks quick, but if it’s a particularly windy day then the airspeed won’t be all that great, and this can result in the model stalling. The beginner may well exclaim: “But it was flying fast!” Alas, the fact is that the model stalled at the same air speed it always does, but the ground speed was faster due to being downwind.
Whilst learning to fly full-size I was told that regardless of what my ground speed appeared to be I should always fly the aircraft according to its air speed, because that’s what dictates the aeroplane’s performance. Sure enough, even on very windy days I’d always approach for landing at 65 knots, and whether into wind, crosswind or ever-so slightly downwind, the aircraft behaved in the same manner.
The way to measure air speed is with a special little device called a Pitot tube. A standard bit of kit on full-size aircraft, there are some available for model aircraft, too. Personally, I’ve never used or felt the need for one, but if you’re interested to see the effect of air speed on your model, then feel free!
CROSSWIND
A crosswind is where the wind is acting across a model’s flight path (Fig.2). In a pure crosswind the air speed of the aircraft is unaffected, because no wind is acting along the model’s flight path, it’s all hitting the sides. Imagine if you were sitting in the aircraft, flying at 40mph, with a 10mph wind coming at 90° from your right. The air speed would measure 40mph but the wind would be pushing the aircraft to the left at a steady 10mph. The track of the aircraft over the ground would change, and so, therefore, would the ground speed, but airspeed in its direction of travel would remain constant.
If you remember Pythagoras’ theorem from school, you can figure out the ground speed with a little simple trigonometry. Full-size pilots use trigonometry in navigation to correct for any wind drift, to ensure that they’re always travelling towards their destination. For model pilots a simple appreciation of what a crosswind can do is enough, so there’s no need to worry about maths!
Some winds aren’t pure crosswind or into / downwind but a mixture of both, acting at an angle both along the model and perpendicular to it. Suffice to say, you need to take both these components into account when flying aerobatics.
COMPENSATION
In model aerobatics, pilots are expected to compensate for the wind such that the aircraft maintains constant speed, i.e. it doesn’t accelerate or decelerate through a manoeuvre. So, if flying a loop into wind, the model should have a nice, constant speed throughout, as viewed by anyone observing it. If you enter a loop into wind flying at 40mph then you should aim to fly at 40mph throughout the loop, despite the change in air speed.
Furthermore, a good pilot will compensate for any drift due to a crosswind. So, again, using the loop as an example, the model shouldn’t be blown toward or away from the pilot, but should stay at a constant depth. All inputs by the pilot should be to achieve a perfectly round loop that’s not skewed or distorted in any way. A loop flown on a windy day should appear as though it were flown on a calm day, although, of course, sudden gusts, bumpy air and general turbulence will throw the model around a bit. There’s only so much that can be done to anticipate these, but we can make some effort to take into account the constant effects of wind (Fig.3).
To achieve full compensation the pilot will have to manipulate all four controls, i.e. throttle, elevator, rudder and aileron. I mentioned last month that the responsiveness, sensitivity and power of the control surfaces are affected by air speed, so if you try and maintain a constant speed in a wind where the air speed is changing, then the way the aeroplane feels will also change. As much as possible you should try to take this into account. The change could be subtle, almost imperceptible, but it could be quite extreme. Every model and every wind is different.
Remember I also mentioned that a good pilot will constantly adjust to changing conditions? That’s especially true here; when flying a loop the elevator may not be as powerful in the downwind section as it was into wind, so you should be ready to apply more elevator to get the response required and achieve that nice, round figure. Analyse, react, adjust and repeat – that’s the key here.
Now we’ve learnt something about the wind, let’s tie this knowledge in with an aerobatic manoeuvre. Yes, it’s the loop again!
LOOP INTO WIND
Let’s look at the corrections needed to fly a nice circular loop that begins into wind. By now you should be familiar with the necessary use of all four controls to carry out the loop, with due attention to throttle and elevator to keep a constant radius and control the size of the loop; now you just have to work those controls a little more to address the presence of wind.
We begin the loop flying from right to left, with a steady headwind. Keep in mind that the wind will constantly be trying to blow the aircraft back, to the right, which you need to compensate for (primarily) by using elevator and throttle.
What we’re looking for is to see the path of the aircraft prescribe a loop; this is an important point. With wind present, the direction in which the nose of an aircraft is pointing may not actually be the direction in which it’s flying! Try to focus instead on the path the aircraft actually draws in the sky. To help me gauge this, I like to focus on the model’s C of G. As the aircraft passes in front of you at the 6 o’clock position (Fig.4), begin applying up elevator and increase the throttle, just as you would when sport flying. Keep in mind that in this first quarter of the loop a headwind will be pushing the model back, flattening out this part of the manoeuvre. You’ll therefore need to apply more power than usual to help penetrate into wind, and you may also want to back off the elevator slightly, such that the aircraft’s nose isn’t following the loop’s path. By giving a little less elevator than on a calm day, the aircraft will have a slightly lower nose attitude. This helps combat the wind forcing the model back.
You might well be thinking that having a lower nose attitude doesn’t make much sense, but the need for this will become apparent when you hit the 9 o’clock position. If the aircraft was to be pointing directly vertical, then it would get blown across (downwind to the right), and the loop will lose shape. So, at this point the nose should be canted slightly into wind to keep a true vertical path.
From this point continue pulling the model around; when it enters the downwind portion of the loop, the ground speed will start to pick up. Back off the power to try and maintain the same speed relative to you. Be careful, though, because you’re reducing air speed here and will need to be wary of a stall, especially if the wind is particularly strong. Sometimes it’s just not possible to slow the aircraft enough to maintain a constant speed without getting into a dangerous situation, and a compromise has to be made. Also, be aware that at around the 12 o’clock position, the controls become a little more sluggish due to that air speed drop. Be proactive with the elevator to get the response from the model you require, in order to maintain that circular loop. Regardless of how the sticks feel, put an input in, observe the effect, react and adjust. It’s a recursive process.
Now, coming around toward three o’clock, the technique for combating the wind is very much like the first quarter. Here we pointed the nose a little into wind and here we want to achieve the same thing, so point the nose of the model a little into wind to prevent it from getting blown downwind. By the time you reach the 3 o’clock position this correction will be at its maximum.
When dealing with a crosswind (or a crosswind component) you’ll need to use rudder to cock the aeroplane into wind. For example, we’re again flying from right to left and about to start a loop. The wind is coming from the left, but there’s also a slight wind blowing on your face; you’ll need to apply some right rudder to prevent the model from drifting toward you throughout the loop. Exactly how much rudder is a matter of trial and error and, of course, depends on the wind strength. Use left aileron to correct for the bank induced by the right rudder, and keep those wings level!
