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.
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.
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?'
It becomes obvious that Mr Gravity is asking both gliders for repayment of all outstanding sums.
'Me too', you reply.
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).
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...)
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.
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 thisconfiguration. 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.
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.
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...