As Prof. David Burton explains, twin engine aircraft can bring a whole new dimension to flying, especially when an engine stops!
There’s something magical about the sight and sound of a multi-engine model. But twins are troublesome, right? They need an experienced hand at the controls and, of course, there’s the fear of what might happen if one engine cuts!
Well, yes, twins do pose their own unique challenges and losing an engine is certainly a scenario that should be treated with respect. However, a lot of the fear is, perhaps, based on not knowing what to expect. So what are the aerodynamic issues with a multi-engine model? Just what can you expect to happen if one engine cuts? And, most of all, is there anything you can do to save the day? These are the questions we’re going to look at this month. Coffee at the ready? Biscuit tin on stand-by? Then I’ll begin...
FLYING A MULTI
Fundamentally, there isn’t much difference in flying a two, or even a four-engine model compared to its single engine cousin. The basic principles of flying are just the same, although there are a few particular areas to consider.
First: this type of model tends to be bigger and heavier and so might not be quite as agile as you think. As a result, you may need to plan ahead a bit more when flying. Also, the wing loading may be higher than you’re used to, and if this is the case you can expect higher airspeeds, particularly on landing.
Second: some familiar effects are amplified on multi-engine models. For example, a single-engine tail-dragger has a tendency to swing to the left on take-off, and with a multi this tendency is stronger. The reason for this is that you have more propellers contributing to the effect, a fact that isn’t helped by a rudder that’s likely to be less effective. This takes us nicely to what is, perhaps, the biggest, difference, i.e. the controls may be less effective in general. In the case of a single-engine model, the propeller and tail are generally in line down the length of the aircraft so the tail benefits directly from the prop-wash. This may not be the case with a multi-engine model.
Designers have done their best to address these problems. For example, prop driven multis often have twin fin assemblies, one at each end of the stabiliser – the Lancaster, P-38 and Mitchell being cases in point. This, of course, puts the rudders back into the prop-wash.
Boeing adopted an alternative solution for the B-17, just look at the size of the tail assembly, it’s huge! Being so large enables it to work effectively with what is largely airspeed-induced flow alone, without quite so much prop-wash benefit. For the Mosquito, de Havilland found yet another solution. Placing the engines far forward, and as close together as possible ensured that the fin and tailplane remain in the prop-wash to a significant degree.
These design observations aren’t just curiosities, we can draw valuable lessons from them. If you have your heart set on a multi-engine model you can use this knowledge to aid your selection. Designs with above average tail area, twin fins, or with the engines very close together are good potential candidates for a model.
Recent progress in electric flight has, for many, made electric power their first choice for multi-engine models. Whilst by no means immune from failure, electric power is undoubtedly more reliable. It also allows the possibility of having the motors rotating in opposite directions, provided you can find a left-handed prop or reverse a pusher. We shall see that many of the problems associated with multi-engine aircraft stem from having the engines rotating in the same direction, so this modification is definitely worth considering.
WHAT IF IT GOES WRONG?
It’s all very well working out what configurations make for good multi-engine models when all the engines are running, but what if one of the engines cuts? What can you expect to happen and what are your chances of bringing the model back down in one piece?
We need to use our aerodynamics knowledge and a bit of common sense to answer these questions and figure out just what’s likely to happen and why. Then we need to work out how we might give ourselves the best chance of successfully coping with the situation. Let’s begin by looking at what’s going to happen when one engine cuts. For the purpose of this explanation we’ll assume that the model is a twin, however all the ideas we’ll cover could equally apply to a four-engine machine.
ENGINE FAILURE ANATOMY
Engine failure produces a very asymmetric thrust arrangement wherein the model will yaw towards the dead engine. It will yaw, but it’s still travelling in the same direction, i.e. it’s not turning (yet!) so it must be side-slipping, as in Figure 1.
This side-slipping is the major problem. A side-slipping aircraft experiences far more drag and this, coupled with the fact that it has just lost half of its thrust, means it’s going to slow down. But it doesn’t stop there. Cast your mind back to RCM&E’s 2011 Special where we considered aerodynamic stability. We said that when an aircraft with dihedral side-slips, it rolls in the direction of the yaw. We’re already yawing towards the dead engine and now we’ll start rolling towards it as well. What prop wash we had over the tailplane will decrease and the tailplane will not produce as much aerodynamic force. As we’ve seen in previous articles the tailplane is usually producing a downward force holding the nose up. Because our tailplane is no longer working so well this trim is disturbed and we will start to dive.
It’s not looking so good is it? A yaw, decreasing airspeed, a rolling tendency and now we’re on a nose-down heading.
THE CRITICAL ENGINE
Does it matter which engine we lose? Yes, it does. The fact that our engines normally rotate clockwise, when viewed from behind, means that it’s much worse if we lose the left engine than the right; this makes the left-hand engine our critical engine. To understand why, we need to examine four factors in light of either left-hand or right-hand engine failure.
1. The P-factor effect. If the propeller’s disc of rotation is tilted backwards, i.e. the model’s attitude is nose-up, then a down-going blade will be at a higher angle of attack than an up-going blade. Therefore, the propeller produces more thrust in the right-hand half of its arc than in the left-hand half. This is shown in Fig.2 with the thrust difference from either side of the props shown as blue arrows.
If you fly a tail-dragger you’ll have met this effect in the early stages of the take-off run. The tail is down, so the propeller disc is tilted back and we experience a noticeable tendency for the model to swing left. Once we get the tail up, and the prop disc is vertical, this tendency lessens to just the torque effect.
As a consequence, loss of the left engine results in a larger asymmetric thrust effect because it acts over a longer lever arm.
2. The prop-wash effect. If the prop produces more thrust, then it produces more wash as well. So, associated with the differential thrust across the propeller disc, you’ll see a differential prop-wash – shown as the red arrows in Fig.2. Greater prop-wash over some sections of the wing means more lift from those areas.
In conclusion, failure of the left-hand engine results in the model experiencing a greater roll tendency than would be the case with a right-hand engine failure.
3. The slipstream effect. So, what do we see here?
4. The torque effect. Last month we saw that our engines produce a natural torque reaction that tends to roll the model to the left.
We can conclude, then, that for every one of these four effects the situation is worse when we lose the left-hand engine.
WHAT TO DO?
Well, sadly there is no golden rule. The best course of action is going to depend on airspeed, altitude, attitude, rudder authority, design factors and which engine has quit.
If you have plenty of height, let the nose come down a little and keep your speed up. This will reduce any undesirable p-factor effects and a higher airspeed will also make your controls more effective.
Stop the side-slipping, if you can. This will reduce the drag and help you to maintain airspeed. Note that simply using lots of rudder will not remove the side-slip. It will straighten the model’s nose up, but it will still be flying sideways. You’ll need to feed in some aileron to counteract the yaw-induced roll and so eliminate the side-slip. The rule ‘lift the dead engine’ applies here in that you should aim to be flying with the dead engine wing very slightly higher. This makes the dihedral effect work for you rather than against you.
Consider your throttle options for the remaining engine. Full throttle to make up for lost thrust is not necessarily the best choice as this will increase the thrust asymmetry, aggravating the yaw and hence the side-slip. You may be better off more nose-down and at a lower throttle. Don’t rule out cutting the good engine and going dead-stick. It may be your best option if you can’t control the side slipping.
Try to win control of the model before you even think about attempting to land it. I know that sounds obvious, but it’s amazing what we’ll try when we’re panicking! When you do attempt a landing be ready for the p-factor effect as you flair. You might want to consider coming in fairly fast and flat with no flair – possibly undercarriage up if it’s retractable. You might also want to consider throttling right back and even cutting the remaining engine just before landing if you haven’t already done so.
GOT YOUR HANDS FULL!
All this advice is fine while we’re sipping coffee but it’s a very different matter when it happens. Unlike pilots of full-size aircraft we can’t really practice engine loss. However, you’ll have noticed that a lot of this is about handling side-slip and we can practice that. Try flying flat turns using the rudder to turn and countering the roll tendency with opposite aileron. You’ll find that you will need to add some throttle and / or up elevator as the drag caused by the resulting side-slip will slow you down and you’ll lose lift. Don’t push your luck too far with this and remember that up-elevator with crossed controls is a very good spin entry technique! When you can do flat turns, move on to flat figure-eights, then try flying a side-slip approach for landing – that’ll concentrate the mind.
These drills will mean that if you do face the dreaded engine out scenario then at least the flying techniques won’t be alien and you’ll stand a chance of pulling off a landing that will be the envy of your mates. And if you do get it down successfully then mine’s a large scotch in the bar afterwards!
Next month we’re going to be looking at the aerodynamics of biplanes. Until then, happy landings!
|Missing Article - aerodynamics of biplanes|
By Chris Walby
by Chris Walby
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