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LANDINGS YOU CAN WALK AWAY FROM....

By Chris Williams

At the White Sheet aerotow event here in the UK recently, I was approached by one of the chaps who wondered if he could suggest a topic to be covered in my regular scale column. I listened politely, for any writer knows the value of outside stimulation, and I am no exception. Why is it, he asked, that model glider pilots don't generally seem to plan their approaches in the manner of the full-size? This question was no doubt prompted by some rather dodgy arrivals during the course of the day and a couple of outbreaks of IMFD (Involuntary Modified- by- Fence Disease) This feller, and I apologise for not recording his name as I was flying at the time, went on to detail what happens when the full-size decide to descend to Terra Firma.

At a height that is predetermined by protocol and local conditions, the circuit commences by applying forward trim and upping the speed by a small but significant amount of knots. A square-ish pattern is flown which culminates in a controlled landing directly into wind with forward velocity reduced to a safe margin just above the stall. At the last moment the descent is checked by pulling back on the stick and 'rounding out', with the mainwheel gently and sweetly kissing the ground, the transition from flight to mere perambulation being indiscernible to even the keenest observer. (As a knowledgeable chap at the Dorset Gliding club was telling me recently, with the K13 it is entirely possible to pull up to such an angle of attack at the point of landing, that the tailskid will touch before the mainwheel. Apparently this is not cricket and causes much muttering and head-shaking back in the clubroom.) Why don't we modellers fly to the same standard, that is the question.

For accurate landings with a machine like this the approach speed needs to be well above the stall speed as the drag from the struts and wires causes the momentum to decay rapidly...(Author's 3rd scale T31 at Lordshill)
For accurate landings with a machine like this the approach speed needs to be well above the stall speed as the drag from the struts and wires causes the momentum to decay rapidly...(Author's 3rd scale T31 at Lordshill)
We'll start with the more obvious observations: models, even scale models, operate to an entirely different regime. If the full-size pilot has been taught, for instance that once down to 500 feet he must commence a circuit and land, and if the placard on the glider he is flying informs him that his approach speed must be in the region of 60 knots, why then this is what he does. One thing, or rather two things, help him to achieve this and they are, in no particular order, the altimeter and airspeed indicator. Now I'd be the first in the queue for the telemetry that would give me these instruments on my tranny, but until then I, like the rest of you, will have to rely purely on visual judgement.

This means that we have to judge (guess) the minimum height from which we can set up a reasonable circuit, we have to judge (guess) just how close the machine is to obstacles such as bushes, trees and fences, and we have to judge, (really guess or consult Mystic Smeg) where a model is going to come to rest after it has touched down.

On the day of this event it so happened that I had the last flight of the day with my Minimoa. It was not particularly difficult to set up a landing as described and roll her gently into the centre of the patch, and I reckon this was something that most of the flyers present could easily have accomplished too had they the advantage of an empty sky and such a modicum of peace and quiet. They say that the deck of an aircraft carrier during operations is one of the busiest places on earth, and I don't plan to argue with that. Anyone who has experienced a busy period at an aerotow event will know how to vote when it comes to deciding the second busiest place on earth. If you haven't had this experience, let me describe it for you:

Having fought your way through all of the frequency and queuing battles, you have moved to the front of the queue and the flightline controller bellows

'NEXT'. 'Flair, or loop' he enquires, referring to your preferred method of towing attachment.
'Er, Flair' you might say.

'Not on this tug', he replies, try the next one over'.
With a sudden roar, the next one over takes off with a glider on tow. You hear a shout:
'Tug landing, you're in the way, glider pilot, please move back'.
You move back as a tug burbles and swishes it's way on to the patch. An elderly gent earnestly asks you if you have seen Mabel.
'Flair?'...the flight line controller beckons you forward. You grab the proffered ball and attempt to stuff in the hole in the front of your glider.
'Glider landing!' comes a shout, followed by another, different voice: 'Glider landing...'
Bump, swish, rumble, a sleek glass ship comes to rest in front of the tug. The pilot starts to run forward to retrieve his precious creation, but is halted by a stentorian bellow:

'GLIDER LANDING', please keep to the bottom half of the patch'.

Faced with a rapidly diminishing area in which to land, the glider pilot tries to change direction, but catches a wing tip in the long grass and ground loops. With a sudden roar another tugs heads skywards with its charge hanging on behind, and the two glider pilots rush out to retrieve. After much fumbling, you still haven't managed to connect your glider to the line, and a tide of red slowly rises up your face, as the people in the queue behind shuffle and stamp impatiently. Finally, the job is done, and you stand next the tug pilot whilst someone else authoritatively holds your wingtip clear of the ground. That someone else turns out to be the elderly gent...he looks under you starboard wingtip.

'Mabel', he quavers, 'are you under there?'

'OK?' The godly figure of the tug pilot enquires, kindly, well mostly they're kind. As you nod nervously he calls 'all out' and tug and glider waltz merrily off down the patch. If there's a slight tailwind and your glider is heavy, if the temperature is high and the tug's engine sucking comparatively less oxygen, then those first few seconds can be pretty crowded. If your anhedralled gull-wing's tips barely clear the ground to start with, then the first task is to apply the necessary physic guesswork on the aileron stick to keep them clear, the second is get the thing off the ground before the tug overruns into the long grass. Once airborne, things become easier and it's usually a fairly simple ride up to altitude.

Skipping lightly over the period of soaring, you've lost the lift and now the model is getting larger and larger. Another glider has undergone the same experience, and is stooging about at the same altitude.

'Tug landing', shouts a voice, is the patch clear?'
'Are you coming down now?' asks the other glider pilot,' cos I think I'm gonna have to...'

It becomes obvious that Mr Gravity is asking both gliders for repayment of all outstanding sums.

'Me too', you reply.
'Tug landing', says yet another voice, 'is that tug clear?'
'Dunno' replies the flight line controller (bet you'd forgotten about him) 'there's a damn great glider pilot in the way.

A hand guides you gently out of the way, just as your final circling coincides with the other guy's and what looks like a potentially nasty mid-air passes of without incident. You decide that enough is enough, and push the stick forward and the spoilers out in attempt to get down first and out of the damned way. The speed builds up and you flatten out just over the grass and hold it there as the patch approaches. A bit more spoiler, a bit less, a flurry of small elevator adjustments and the skid finds the mown grass right at the leading edge of the square...lovely. But wait, the machine won't stop, it's slipping and sliding along according to the laws of Conservation of Momentum (not to be confused with the laws concerning the failure to abate a smoky chimney).

Still some way to go to reach the patch: spoilers still retracted...(Author's 1/4 scale bergfalke at Lordshill)
Still some way to go to reach the patch: spoilers still retracted...(Author's 1/4 scale bergfalke at Lordshill)
There's nothing you can do, it's all in the hands of Fate, and it's with no small relief that our machine rolls right in the pits, coming to rest amid half a dozen other gliders and someone's lunch box. Holding on the tranny firmly to hide the fact that your hands are shaking badly, you walk with all the insouciance of someone who meant that to happen all along, whilst all around people shake their heads with disgust or appreciation.

Compare the forgoing to that last flight of the day... there's something here holding up its hand to be noticed. Right, dead right in fact... when you fly at a busy aerotow event, there are sometimes quite a lot of things clamouring for your attention, and quite often it's this factor that leads to a degrading of a flier's otherwise flawless competence in the landing phase. So, part of the answer to the original question is that at an aerotow event, such as the one described, it's the unaccustomed pressure of events that causes many people, myself included, to do things that they normally wouldn't when bringing their machines back to earth.

But even under more normal circumstances, the question is still valid. Why do pilots sometimes get into such a mess in the landing phase?

Let me tell what I am convinced is one reason: putting off 'til tomorrow that which should be done today. To many people the landing is a difficult hurdle, something to be put off as long as they can, and then got over with as quickly as possible. No two landings are alike, and indeed if there wasn't something special about this procedure, people wouldn't burst into spontaneous applause when someone pulls off a good one.

So, a pilot gets a model up on the slope, for instance, soars to a great height, stooges around for half an hour, then lands and reaches for his flask of coffee. He may repeat this procedure five or six times during the course of the day and then wend his way home filled with the glow of satisfaction that is the reward of a day well spent. So what, you may ask, what's wrong with that? Regard now that same flier when the lift has suddenly died and the landing, which was previously a leisurely affair, now needs to be performed toot sweet. As often as not, the result will be a manoeuvre that is significantly degraded from its predecessors, sometimes leading to a confetti -strewn hillside and some surreptitious dustpan-and-brush work. The conclusion is obvious; any pilot who flies the same pattern day in, day out, is going to overwork his sweat glands when presented with unusual circumstances such as may be found at a busy aerotow event. (Or the sudden appearance of a low-flying aircraft, or a dense patch of mist flowing unexpectedly up the hill, or the horn of a maddened bull appearing between his legs...)

It's perfectly possible to complete this turn from this height and land towards the camera providing you have SPEED and GOOD AIRBRAKES! (Author's 1/4 scale Minimoa at White Sheet)
It's perfectly possible to complete this turn from this height and land towards the camera providing you have SPEED and GOOD AIRBRAKES! (Author's 1/4 scale Minimoa at White Sheet)
To help makes things clearer let's define three types of landing, then you can decide into which category yours will fit.

1.      Controlled crash: This is usually accompanied by some nifty legwork as people struggle to get out of the way. The approach is often totally unplanned, with the pilot's options diminishing throughout the procedure until the inevitable ensues.

2.      Adequate Landing: Most fall into this category; it's safe, it's geographically correct (in the same field as the take-off) and has all the style of a water buffalo passing wind.

3.      Greaser: No description required here, as we all know what a greaser looks like, right?

Well, you know what's going to happen now, don't you? Some git is going to tell you how to fly... don't you just hate it when that happens?

Getting a model from a particular point in the sky down to a particular point on the ground is basically an exercise in energy management. At 100 feet your model has a given amount of potential energy compared to its resting state on the ground. Looking at the extremes of your many choices of action, you can dive vertically downwards converting all that energy into velocity, or you can burble around on the stall trying to retain height by energy conservation instead. Neither is a wise choice of action, instead, this is where the circuit comes into play.

The idea of flying a circuit is a simple one, as the pioneer aviators at the turn of the century found out; this simple pattern gives you the time and the opportunity to adjust your glide path (which is determined by balancing speed, height, drag and direction) so that you arrive at threshold with just the right amount of remaining energy to pull off your high quality landing. As I mentioned before, without flight instrumentation we are going to have to rely on judgement to balance the factors involved, and good judgement can only come with practice.

What happens with practice is that some hidden part of your subconscious becomes a computer, it automatically correlates what the eyes see with the knowledge database that you have built up from previous experience. This will vary from model to model and will be refined with experience, the critical factor being the behaviour of that particular wing section upon which you model is relying. The other critical factor is the efficiency or otherwise of your glidepath control apparatus, the airbrakes/spoilers living in the wings.

So to recap, you control altitude and airspeed with the elevator, drag (and also airspeed) with the spoilers.

Traditionally, a circuit is square in shape: you fly overhead into wind passing to a point somewhere in front of you where you will commence your first crosswind leg, either to the right or left as the situation dictates. At the appropriate point you turn another ninety degrees in the downwind phase, losing altitude as necessary. The point at which you enter your final crosswind leg, known as base leg, is crucial, as it will determine the amount of energy you have left when you enter the last into-wind leg, known somewhat sinisterly as Finals.

Speed falling off, nose starting to rise and still some way to go...(MU13E at White Sheet Aerotow '99)
Speed falling off, nose starting to rise and still some way to go...(MU13E at White Sheet Aerotow '99)
If you turn too soon you will come over the landing area too high and with the distinct possibility that the brakes may not be man enough to kill off the excess energy. If on the other hand you turn too late, you will find yourself short of the required energy to reach the landing area and that's usually when the lurking fence leaps up and takes a bite out of your sailplane. The square approach is the simplest and best way to give you the time you need to sort things out and bring your flight to a suitable and satisfactory conclusion, but of course outside of competition, it doesn't have to be square at all, a large circle will do the job just as well.

Why then, does the pilot of the full-size add down trim and speed up in the landing circuit, surely this not the thing to do?

Actually, it's a pretty nifty idea, because it gives you options both sides of your rate of descent in this

configuration. If the computer in your brain beeps and says, 'too low too low... you can't get back from there' you can opt to leave the spoilers alone and rely on your extra airspeed to bring you home. If the converse applies (Beep! Beep! Too high) you can pull out the spoilers until things start looking better.

But, I hear those of you still awake say, surely if you are short of the height needed to reach the landing area then it's better to slow up and trim for minimum sink?

It depends on the strength of the wind of course, but most experienced pilots will, in this situation, stuff the nose down and head for the home plate. There are three possible explanations for this; firstly at higher airspeeds there is more positive control, which is something you might need on a blustery day. Secondly, as you lose altitude you get to that part of the wind gradient near the ground where the air is moving more slowly due to the drag of the grass, bushes, coffee flasks and antelope etc. This means that you can cover comparatively more ground compared to the higher altitude, even allowing for the extra drag at high speed.

Ah, home at last...Full spoiler (and up-aileron) kills the last of the excess speed prior to touchdown... (Author's Schweizer 2-32 at Lordshill)
Ah, home at last...Full spoiler (and up-aileron) kills the last of the excess speed prior to touchdown... (Author's Schweizer 2-32 at Lordshill)
Finally, as you get within a few feet of the ground, the aircraft enters what is known as ground effect where the due to the wings' proximity to the ground, it rides on a sort of cushion of air, something which also increases the wing's efficiency. If I was coy before in describing exactly what constitutes a 'greaser' then now is the best time to elucidate. A perfect landing, and I ought to point out that this is purely my definition, takes place when the transition from flight to flat is entirely seamless, one minute the sailplane is floating effortlessly within inches of the ground, the next the wheel is rumbling and you can actually see the lift dying from under the wings as she slows and comes to rest.

Hopefully, you will be able to hold the wings level until the last moment, and prevent the usual tip-snag and semi-ground-loop that bedevils a landing on a less than perfect surface. No doubt the full size practice is somewhat different, landing at unnecessarily high speeds may well cause dissension in the workshop, but I believe that for models, the best landings come from the benefit of a slightly higher airspeed. Then you can experience those final, satisfying seconds before touchdown, skimming the surface of the ground and utilising the spoilers to designate the exact point of contact.

This K18 ain't messing about on its approach at Romanway, near Oxford
This K18 ain't messing about on its approach at Romanway, near Oxford
There are no shortcuts to pulling off a good landing, not just now and again that is, but with consistency, and it boils down to this: practice, practice and more practice. First you have to get to know the efficiency of your airfoil, and this means deliberately setting the glider up to land short, and then pulling back to see how far you can stretch it. With many models what will happen as the airspeed diminishes is that a wing will suddenly drop, so get to know if yours falls into this category. Often pilots with such a model come to fear the stall and do everything to avoid it, which in many ways seems entirely sensible. More sensible still is to stall the machine repeatedly at a safe height under different conditions i.e. from straight and level, from a turn etc. This will enable the pilot to know precisely the time when it's going to happen, and thus avoid its consequences nearer the ground. You often see a model set up nicely on the approach suddenly drop a wing and sag to the ground in the final moment of landing, with practice this is quite avoidable.

As we reach the end of this little homily, I would beseech you not to be put off by my description of a typical aerotow event, you have to make allowances for artistic interpretation.

Although our aim is to emulate the full-size as much as possible, there are inescapable differences between the two, and it would not be sensible to try to take the comparison too far. When things go wrong for model pilots, nine times out of ten it's because unforeseen circumstances have suddenly cropped up, and the pilot has not got the experience to cope. This is why commercial pilots spend so much time with their bums strapped to a simulator, and why our simulation must come from practice.

I must thank that gentleman at the aerotow event for his suggestion, and I sincerely hope that by now he's managed to find Mabel...