Taildraggers © Chief Instructor

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Taildraggers © Chief Instructor

Beitrag von I/JG27_Zimmi » Mo 26 Dez 2016, 08:30

Part 1

If I may make a little contribution here… Hopefully some of you who are finding the Spit, Dora, 109 or P-51 a bit of a handful during take-off and landing, may find it useful!

What do I know about it? Well, I have spent a significant proportion of my professional flying career teaching both experienced and novice pilots how to fly and handle tail-dragging aircraft. This amounts to several thousand hours of tailwheel training alone, though who’s counting! These aircraft include among them modern high performance aerobatic aircraft and a variety of more vintage types from DH Tiger Moths, to Harvards. I can’t recall off the top of my head exactly how many students I’ve worked with over the years, but it’s well over 200! Best of all, they have all gone on to fly extensive tailwheel ops in a variety of types and to the best of my knowledge, only 2 of them have crashed anything since!

As a significant number of pilots here are expressing difficulties with tailwheel handling, particularly with the early access (and quite wonderful) Spitfire now being available, I hope that my experience of teaching real pilots to overcome these challenges may be beneficial to some here. After all, the majority of difficulties being described are exactly the same as you’d encounter in the real world, and therefore, most of the solutions and techniques used to overcome them are also highly applicable. A real testament to the fidelity of the DCS offering.

Having read all of the comments on the new Spitfire forums here, as well as most of the other forums, I wanted to reassure those of you who are finding the ground handling difficult, that what you are experiencing is quite normal and to be expected. I’ve read several posts which question the overall difficulty levels involved and how they are modelled. Some suggest that the DCS Spit (and other TW aircraft) are perhaps too difficult. On balance, I think that they are pretty representative of what real world TW ops are like, and here’s why…

Firstly, the behaviour of a TW aircraft on the ground is fundamentally different to that of a “nosedragger” aircraft. This is because unlike a tricycle configuration, a TW aircraft is positively directionally unstable. This describes a condition where the aircraft wants to turn left or right on the ground, whilst the effect is almost exponential in its manifestation. In contrast, tricycle configurations are almost all positively directionally stable; meaning that any tendency to turn left or right on the ground will, for most part damp itself out until its directional tendency is to remain straight.

The reason for this is simple: A TW aircraft’s C of G is behind the main wheels, where a tricycle aircraft’s C of G is in front of the main wheels. Once the aircraft has kinetic energy and is in motion, the C of G (the point where the aircraft’s mass acts) will always want to “lead” the aircraft. Or, better put, it will want to overtake the point of the aircraft where most of the directional influences are concentrated, which is usually the point of contact with the ground. I.e. the main wheels. There are numerous factors acting on the airframe which will cause it to either swing left or right when it is in motion. Some of these forces will act to make the aircraft swing left and some to make it swing right. Some of them are strong and some are weak. Some of them increase with ground speed and/or airspeed, and some decrease. In addition, some of these forces act together and multiply their combined effects, and some cancel each other out. The overall effect is that in some form or another, the aircraft WILL want to swing one way or another. Indeed, during one part of the take-off or landing roll, the tendency to swing left or right may reverse depending on what direction the lack of equilibrium is strongest at any particular point.

One of the greatest challenges facing pilots who are new to this is knowing instinctively what control inputs are required to maintain a straight roll. An important point to remember (and the one which causes the most difficulty) is that to be successful, it is essential that corrective inputs are made decisively BEFORE the motion they are correcting has developed. This is because of what I described earlier – namely that swinging moments develop at an almost exponential rate. In a nutshell, the control input required to cancel the first split second of a swinging moment is a fraction of that required to cancel the same moment once it is fractions of a second more developed. Simply put, if you cancel a developing swing to the right at 0.3 of a second after it starts, it may only require the smallest and shortest of touches on the required control [*usually we’re referring to the rudder here, but ailerons do play a part.] However, if that same swing is allowed to develop for a full second, the chances are that it will require a far greater rudder deflection to counter it. Probably 2 or 3 times the magnitude.

Now, here’s the real crunch point… You must learn instinctively both the required magnitude of the input that is required (i.e. how much deflection) AND importantly, what duration that input should last for. In nearly all cases, the single most difficult thing to master is modulating the duration, as this tends to have a direct effect on the magnitude, more so than the other way around. The phrase “dancing on the rudder pedals” is often used to describe the kind of control inputs required. Indeed, I recently read a post where Wags said just that, and it’s a doctrine well worth embracing. This describes a situation where you are making instantaneous and very short duration inputs, both left and right in a manner that almost predicts the swinging motions before they actually happen. For the reasons explained above, if you can “kill” a swing the instant it starts, or even better, before it starts, then by definition it will only require the smallest of inputs over a very short duration. By that I mean fractions of a second. If you are having to apply large deflections to counter a swing, then you are behind the situation.

The reason why this then leads to the inevitable loss of directional control is because every input you make WILL have an effect directly proportional to its magnitude and duration. The later you leave your response to a particular swing, the longer the effect of that control input will take to manifest itself, but it WILL happen. This makes the next movement even MORE difficult to predict and after several cycles, the chances are that the swinging moment will have exceeded the control authority of the rudder, meaning you’re going for an early shower.

Everything that I’ve described here is somewhat different to the control sequences required to keep a tricycle configuration straight. Firstly, lateral directional stability is convergent, so the timing is less critical because the deviation from your intended path will not (in most cases) increase in rate. So, as long as you eventually steer the aircraft back to the correct path, it will just follow in a predictable and stable manner. This is fairly easy to master because it relies more on a cognitive response from the pilot, and one which is fairly forgiving, allowing time for the pilot to assess the deviation and make a conscious decision as to what control inputs are required.

Response inputs in a directionally unstable aircraft are less instinctive and require a far greater element of “motor skill learning”, particularly as to be successful, you have to be able to predict what the aircraft will do next and react preemptively, rather than retrospectively. Motor skills take more time to learn. They need to be hard coded into your hands and feet and this takes time. Though some element of natural ability will help, it will at best only reduce the time taken for your motor responses to embed themselves. It will NOT shortcut the process. Therefore, no matter how much you understand it (though it helps), and no matter how much you beat yourself up to do better, everyone will have to put in enough cycles to nail it, because it is not a skill we come blessed with by default.

Is it more difficult in DCS than in the real world?

For those who doubt the validity of how DCS TW aircraft behave, and the difficulties they simulate, I can say with some authority that they are pretty much spot on. Therefore, the problems being described on these forums are also very much representative of the real world. The thing that sets DCS apart from other PC based simulations, and a number of commercial simulators, both leisure and certified which I have experienced, is that so many of these factors deemed undesirable in the real world, are faithfully modelled, along with the challenges they bring.

I have read a number of contributions which ask the following question:
“Is it more difficult in a fixed base simulation such as DCS because you can’t “feel” the physical movement of the aircraft?” Well, you may find it surprising but my answer to this is a fairly emphatic “NOT REALLY!”, and here’s why…

As we’re talking about the specific difficulties associated directional control on the ground with tailwheeled aircraft here, a very significant factor comes into play. According to what I’ve described previously, the only ideal response to a directional deviation is to counter it either prior to, or, immediately in response to its initiation. At this stage, there is so little lateral acceleration as to be usefully perceptible through the “seat of the pants”. In real life, if you’re responding to that said lateral movement at the point where you can really feel it, you’re way behind. This is particularly the case because in the real world, where those sensations are also being combined with other movement sensations from the airframe, ground and air, which can have the effect of masking or even confusing what you are feeling, especially in lighter weight aircraft.

Of course, if you ARE responding that late to a swing, however successfully or otherwise, the fact is that the deviation in question is now easily perceptible visually, regardless of how much you can actually feel it. Therefore, in a fixed base simulator, your eyes have already told you something needs your attention, provided, that is, you’re looking in the right place!
This is especially true if you’re lucky enough to be flying in VR. The added depth perception and more lifelike peripheral visual cues are more than adequate to detect movements as they develop. But remember, the most important thing is to be ahead of the aircraft before it deviates that far. Of course, there are many other differences in the experiences that both real life and simulation can offer but my belief is that most of those differences are not so fundamentally influential in the context of this subject, though some may be more so in other aspects of flying. One other obvious difference is control force and feedback, though in the phase of flight we’re talking about here, the feedback element is less useful and the force element can be “tuned out” to a great extent by using control curves.

I can’t emphasise this enough. You need to “dance” on the pedals… Like you’re tiptoeing over hot coals in bare feet. Trying to make the tiniest, quickest movements you can before getting your foot off again. Most pilots new to tailwheel aircraft, particularly those with extensive tricycle experience (which probably accounts for most people here) are programmed for making long smooth pedal inputs during the ground roll. This I liken more to wading through deep mud in wellies, rather than dancing on hot coals! It simply won’t work, though your instincts may be very persuasive in telling you otherwise. After all, it’s what you’re used to! I can’t explain just how you reach the point where you instinctively know what’s coming in time to make these tiny, fast and decisive inputs. I can say however, that if you practice the correct technique, with the right mind-set and an understanding of what is happening, you’ll get it eventually, but for most humans it will take a lot longer than learning to control a tricycle. It’s simply a very different type of learning, both physically and mentally.

In summary, here are a few pointers:

1. There are an almost infinite number of things which will cause a swing. Engine torque, asymmetric prop effect, prop-wash, asymmetric ground resistance, wind, slope, gyroscopic effects, control asymmetry, and of course pilot input! These are just a few of the significant ones but remember, they can combine in any number of ways to produce numerous effects. The only thing you know for sure is that it WILL swing at some point.

2. The earlier you respond, the more instantaneously your input will take effect, requiring much smaller inputs and therefore, much fewer counter inputs. Over-controlling is one of your biggest enemies and becomes more likely as your inputs increase in magnitude and duration. Remember, if you’ve put in a large boot full one way, the chances are that it’ll take some time to be fully felt, by which time you’re focussed on the next movement. Then, it’ll be too late and it’ll get you! If you’re on top of it, there should be no need to make anything other than very small, very short inputs.

3. Some swinging tendencies ARE predictable. Use this to your advantage! If you can remember how increasing power makes the aircraft respond, you should be countering that motion in anticipation and stopping it before it happens. Similarly, experiment with setting the rudder trim to lessen the effect of whatever is causing the most significant swing at the beginning of your roll.

4. Primary causal factors vary according to ground and airspeed. For example, the gyroscopic effect of raising the tail during take-off, or lowering it during the landing roll, will be more pronounced at lower airspeeds because you have less aerodynamic control authority. Similarly, increasing the power causes torque effects which in turn cause a swinging moment. However, with increased power comes increased slipstream effect from the prop and therefore, greater rudder authority to deal with it. DCS models this effect very well. My advice here is to be confident with the application of take-off power. The key is to get take-off power set promptly but as importantly, smoothly. Again, use your experience to preempt the effects before they cause problems. Do everything SMOOTHLY and CALMLY.

5. The left swinging effect of engine torque and the ground interaction as a result can be countered with the application of right aileron during the take-off roll. However, you must be proficient at modulating this input as aileron authority increases with airspeed. Failure to do so will only result in a swing and/or roll in the opposite direction, which you may not be expecting. Overall, that may add to your problems rather than helping, so experiment with applying right aileron and holding it prior to starting the ground roll, OR starting neutral and feeding in some aileron immediately prior to the torque causing a swing. There’s no right or wrong way here, it’s down to what works for you.

6. RELAX! It’s a physical fact that when your muscles are tense, you can’t make quick accurate and short inputs. This is a major consideration because the nature of the scenario makes you tense and it’s difficult to counter that! It’s so important though. Before you start your take-off roll, chant a little mantra to yourself… “wake up feet! Wake up feet! Wake up feet!” Do this while doing a little light dance on the rudder pedals. It may sound daft but subconsciously, it will focus a little bit of your brain on the job in hand. It will also help to relax your muscles which have probably already tensed up without you realising it. Do this on final approach to land as well. It helps!

7. SIT BACK IN YOUR SEAT! When faced with a situation that you perceive as being difficult, your tendency is almost always to lean forward and focus on what’s immediately in front of you, effectively narrowing your field of view. Believe me, I’ve seen pilots almost planting their faces in the panel as workload increases. It’s just what we do but the effect of decreasing your field of view and by definition, your peripheral vision, makes detecting lateral movement very much more difficult. It also has the secondary effect of re-tensioning your muscles, something we want to avoid!

Does VR help?

VR technology is still in its early stages of development. However, if you’re are lucky enough to have an effective VR rig that works well, use this to your advantage. The sensory effects gained through vision are much underestimated and VR’s increased depth perception and peripheral cues are singly more persuasive when it comes to movement perception than many physical movements. That might sound far fetched but it is true. Evidence to support this is in the large number of people who appear to experience motion sickness when using VR. Counter intuitively, motion sickness is poorly named and is not a direct result of motion, rather it’s a physiological response to conflicting sensory stimulation. The fact that these effects are so readily triggered by the use of visual equipment alone is a powerful demonstration of just how much we rely on our visual senses to make sense of our surroundings, both on a conscious and subconscious level.

I hope that some of this helps to ease your concerns and to develop the right technique for enjoying the DCS taildraggers. Just remember, if it’s not working for you, the very worst thing is frustration. It will numb your senses and be highly detrimental to your motor skills. The best thing you can do is take a break and do something else for a while, then go back to it. You may have to do this quite a few times over several hours, days or even weeks. If you do though, I can almost guarantee that at some point, you’ll come back to it and it will just click into place. When it does, you’ll wonder why it was so difficult whilst enjoying a whole new aspect of your simulator.

Don’t underestimate the cognitive complexity what you’re teaching yourself to do, and don’t fall into the trap of assuming that handling a taildragger on the ground is just another component of what you’re already used to, because it isn’t. It’s an entirely new skill, almost independent of all of the other skills you’ve amassed. It’s a new skill that’s as difficult to master (if not more so) than all of the other things you’ve had to learn and it’s going to take some time. Of course, there is a simple way to shortcut all of what I’ve said… Start your missions in the air! You’ll be missing out though!

If you’re interested in some further reading and to add more detail to what I’ve written here, I can’t recommend this book highly enough. I used it as my tailwheel teaching bible for many years and it’s as relevant and helpful today as it was then: https://www.sportys.com/pilotshop/th...ger-pilot.html

Happy ground looping!
OLt. Zimmermann

Beiträge: 1311
Registriert: Sa 05 Sep 2015, 09:30

Re: Taildraggers © Chief Instructor

Beitrag von I/JG27_Zimmi » Mo 26 Dez 2016, 08:31

Taming Taildraggers – Part 2

This is the second installation of my essay which seeks to shed some light on the idiosyncrasies of tail-wheeled aircraft; why they behave as they do on the ground, and how best to handle them.

I previously wrote about the fundamentals – why tail-draggers differ from tricycle (nose-wheeled) configurations, ways in which these differences manifest themselves in the practical sense, and best practices for those learning how to manage them. In this instalment I’d like to explore the merits of raising the tail during take-off and why it’s a good thing to do in most tail-wheel types. In a further part I’ll move on to handling the aircraft during the landing phase and explore some of the most common techniques including the physics and aerodynamics behind them. So, here we go…

Many a time I’ve heard the expression “just keep it going like that, and it’ll lift off by itself…” Sounds easy doesn’t it? If only it were that simple! [Note: The best take-off technique for any aircraft is the one which produces the most consistently safe outcome].
Initially, let’s talk about more heavily wing loaded aircraft like the Spitfire, 109, or the majority of metal skinned fighters of that era, simply because it bears the most relevance to the kind of aircraft DCS is simulating. Then, let’s ask ourselves the question: “Should we raise the tail during the take-off roll prior to lifting off, and if so, why?”
The short answer to the first part is most certainly “YES.” With that established (you’ll have to take my word for it at this stage) I’ll endeavour to explain why. Firstly, let’s establish something which will place us all on the same page...

Imagine a tail-dragger, viewed from exactly side on, sitting on the ground on all 3 wheels. Now, draw a line (called a chord line) from the trailing edge of the wing through the centre part of the leading edge, and extend that line to infinity. Also, extend that line from the trailing edge of the wing to where it intersects the ground. Clearly, the chord line and the ground line will not be parallel and the angular difference between them represents the aircraft’s angle of attack as it sits on the ground (Otherwise referred to as “Alpha.” [Note: Alpha describes only the relative angular difference between the wing’s chord line and the airflow over the wing as it is presented to the wing, nothing else].
You’d probably expect this angle to be around 5-8 degrees. By the way, the relationship between the aircraft compared to the ground is referred to as “attitude”, and in the condition described here, it’s further qualified as the “3-point attitude”. Now, for comparison, repeat the same exercise for something a little more vanilla – a Cessna 172 with a nose wheel for example. Again, sitting roughly level on all 3 wheels. If they were to start moving forward, which has the higher Alpha? Not surprisingly, the taildragger has. If you can’t picture it, do a quick sketch.

In ideal conditions, an aircraft will get airborne once the amount of lift being produced by the wings exceeds or overcomes the force of gravity acting on the aircraft’s mass. The amount of lift being produced by the wings (assuming a sufficient constant airflow) is directly proportional to the wing’s Alpha. This is true ranging from a degree or so of positive Alpha, right up to the Alpha at which the wing will stall (Also known as the “critical Angle”). For an average wing this critical value is around 14 degrees. The alpha at which a wing starts to produce lift varies from wing to wing, and particularly depends on whether the wing has a symmetrical or cambered section. However, within our context of interest, the difference doesn’t really matter.

So, imagine the 2 example aircraft referred to above, both starting their take-off runs on all 3 wheels and both maintaining a constant alpha during this initial phase. Which one will start producing lift first? Obviously, the tail-dragger will because it’s wings already have a significantly positive Alpha value, owing to the aircraft’s 3-point attitude. More critically, which one will get airborne first? Let’s assume that they both weigh about the same and that their performance and other parameters are roughly equivalent. Again, the tail-dragger will. Now the really important bit… In reality, the runway surface will not be perfectly level and smooth. There will come a point where each aircraft will climb bumps and descend down the other side. The effect of climbing a bump will compress the tyres and undercarriage structure to some degree, in addition to slightly raising the nose, which in turn will slightly and momentarily increase the aircraft’s Alpha. The further effect of the undercarriage then releasing this energy will create an opposite moment which will push the aircraft upwards slightly. Now, in a tail-dragger you already have a positive Alpha so you are already producing lift. At a given point when your airspeed has built sufficiently, this Alpha combined with the reactionary force from riding the bumps will momentarily produce sufficient force to overcome the force of gravity acting on your aircraft’s mass. Hence, you get airborne. In the nose wheeled aircraft, you’ll just ride down the other side of the bump, but essentially remain planted on the ground.

Why is this significant? Well, in a nose wheeled aircraft, you elect to stay on the ground until you have sufficient airspeed to produce an excess of lift once you start increasing your Alpha by pulling back on the control column, lowering the tail and increasing the angle at which you are presenting the wings to the relative airflow. This a safe condition because with higher speed and therefore a larger volume of airflow, the wings can produce an excess of lift meaning you can keep away from that “Critical Angle” I mentioned earlier. Remember, the critical angle is where the wing will stall and stop producing lift. In the case of the tail-dragger, allowing it to get airborne in the 3-point attitude will invariably mean flight occurs either slightly before (due to bumps) or exactly at the point where the lift being produced equals the mass of the aircraft, leaving no margin. No margin means that during the first few seconds of flight, there is a likelihood that you’ll need to further increase the wing’s Alpha just to maintain level flight, let alone climb. With take-off power applied, the elevator will be very effective in the prop’s slipstream, meaning that large Alpha increases can occur very quickly, with very little control force. In addition to this, low airspeed combined with large volumes of high energy prop-wash air over the horizontal stabiliser (tail wing) will create a downward pushing moment on the stabiliser, further encouraging a nose pitch up and increased Alpha.

If you haven’t managed to keep up with all of that, I’ll summarise it very simply by saying that if left in the 3-point attitude throughout the duration of the take-off run, a taildragging aircraft with a reasonably high wing loading is likely to get airborne at a speed less than is required for safe flight and sufficient climb performance. This condition means there exists a high risk of dynamically stalling the aircraft which at low airspeed will invariably lead to a wing drop and incipient spin entry. Put bluntly, at best you’re going to be an embarrassed pilot, and at worst, a fairly thinly spread out one.

Now, there has been much talk and many recommendations made regarding the best and easiest way to get the Spitfire, 109, P-51 and 190 airborne. Most of them involve continuing your take-off run with the tail on the ground until… here it comes… “it’ll lift off by itself…” Without the real-life risks associated with what I’ve explained above, this is perfectly fair enough and after all, if it gets you airborne most of the time without incident, then life is good. However, you may feel slightly differently in a real aircraft with similar behavioural characteristics, knowing that you’re hanging on the very edge of disaster each time you take off! You’ll also have noticed that though airborne, the aircraft flies horribly; yawing all over the sky, huge trim changes taking effect, oh yes, and a lack of forward field of view. This is because you’ve managed to get airborne but without sufficient airspeed for the control surfaces to be fully effective, or to damp out aircraft movement in any of the 3 axis, particularly the “normal” or “yaw” axis owing to the vertical stabiliser’s lower surface area. You’re flying too slowly! [Note: An identical condition is likely to occur following a large bounce on landing]. Sound familiar? If you really want to explore the full envelope of your “pucker apparatus”, try this for real in an Extra 300, or better still, a Harvard, particularly if it’s sprung on you by an over enthusiastic student pilot!

Why does raising the tail help?
Cast your mind back to the nose wheeled aircraft in our scenario. Now, compare that to an image of the tail wheeled aircraft on its take-off roll, but with the tail raised level, or just shy of level. If the tail-dragger pilot can get it up and keep it up, (the tail that is) they’ll suddenly find themselves benefiting from a number of highly desirable rewards.

Firstly, they’ll be able to see where they’re bloody going! Don’t you miss that since you’ve started messing with tail-draggers? This in turn means that you have a far larger number of visual references at your disposal, particularly valuable when it comes to countering the ever present tendency to swing left or right.

Secondly, their tail will be up and firmly embedded in the high energy airflow from the prop and the building airspeed from the aircraft’s acceleration along the ground. This vastly increases the rudder and vertical stabiliser (fin) authority, which in turn increases directional stability and the pilot’s ability to control it. Highly desirable, yes?

Thirdly and probably most importantly, the pilot can keep the aircraft firmly on the ground until it has reached sufficient airspeed to maintain the required level of performance for safe flying and climbing. This is because a tail-dragger with the tail raised will behave in a similar fashion to the tricycle wheeled aircraft regarding lift, meaning that the PILOT controls the point at which the aircraft gets airborne by gently lowering the tail slightly, increasing the Alpha, producing lift and hey-presto… In reality you can, if you feel so inclined, achieve an even higher lift-off speed by raising the tail even higher and producing a very slight negative Alpha. The effect of which keeps the aircraft more firmly planted on the ground for longer. Not the kind of thing you need to do for every day flying, but useful if you need a higher energy take-off at an air show!

How high should you raise the tail?

With all new students, when introducing them to the aircraft for the first time, I’ll sit them in the cockpit on level ground and ask them to “look at the picture” ahead, and memorise what they see. I’d explain that this is the 3-point attitude, and that they’re going to need to know what this looks like when it comes to landing.
The second thing we do is prop the tail up on a trolley (or suitably volunteered member of the ground crew) and again ask them to memorise the picture ahead. I’d explain that this is the “tail up” attitude, and this is what you want to see during the second part of your take-off run with the tail up.

Having seen this first hand and despite having listened to me waffling on about it mercilessly, nearly all students are reluctant to really get the tail up during take-off. For starters it just feels weird if you’re not used to it. The real fear though in most cases, is that of striking the prop on the ground, and rightly so. Once expressed, this concern is quickly countered by repeating the hangar exercise with the tail propped up level, but this time I invite the student to exit the aircraft and to see for themselves just how much clearance there is between the prop tips and the ground. Despite the sensation of riding on the back of a very tall and thin donkey they perceive from the cockpit, most are quite surprised that at this attitude, how far the prop remains from the floor. Of course, a number of WWII warbirds have far less clearance than say a modern Extra 300L but on the whole, the aircraft we are flying in DCS have plenty enough to get the tail up.

So, the answer to the question of “how high?” is determined primarily by how far you need to raise the tail to maintain a sufficiently low Alpha during your take-off roll prior to lift-off. And secondly, by the diameter of your propeller.

Now that we’ve established why it’s a good thing to raise the tail, we should conclude by examining the difficulties associated with “getting it up on demand” – Something close to every male pilot’s heart no doubt. Incidentally, some of the very best pilots I’ve had the pleasure of flying with are female. Less puerile in most cases and usually able to behave more responsibly when their egos come knocking, but that’s a whole different discussion…

When you raise the tail, you are effectively rotating the prop disc (the plane in which the propeller rotates) so that the lower half of the disc moves backward, and the upper half moves forward. The laws of “gyroscopic precession” rule that a rotating mass (i.e. the prop disc), when displaced in such a fashion will displace or “precess” that moment through 90 degrees in the direction of rotation. If you can’t dig that, put more simply it means that in the case of a propeller turning clockwise when viewed from the cockpit, raising the tail and therefore forcing the prop disc into a new plane, will have the effect of the same prop disc trying very hard to twist through 90 degrees. The result is a very strong force which will yaw the aircraft to the left. It doesn’t matter if you don’t understand this, you just need to know it happens. However, if you’ve got a bicycle wheel hanging around, try spinning it as fast as you can whilst holding each end of the axle with your hands. Then, try yawing the wheel through about 30 degrees firmly and promptly. You’ll soon get the message!

Clearly, this yawing force is significant and needs to be caught BEFORE it happens. With a large prop on a powerful engine spinning at high RPM, this may take a considerable boot full of rudder initially, but be warned, once you’ve countered the swing be prepared to back off with the rudder as quickly as you applied it. Why..?

2 reasons – Firstly, once the tail is up and is no longer climbing or descending, the gyroscopic force causing the swing will disappear as quickly as it appeared. Secondly, as described earlier, your vertical tail surfaces are now in the high energy airflow, making the effect of that rudder input suddenly very much more powerful.
The good news here is that unlike many of the other factors which cause a swing, the gyro effect caused by raising the tail is entirely predictable, so you can plan for it and start from an advantageous position. Once the tail is up and you’re stabilised, you’re in a good place, with a good field of view, better control authority, less tendency to swing and a better thrust to drag ratio because you’re free of the drag induced by an Alpha you don’t yet need. When you want to get airborne, simply lower the tail a fraction to increase your Alpha. Importantly, the precise timing of this is entirely at your command. Incidentally, lowering the tail slightly is exactly what you’re used to doing in your nose wheeled aircraft when you rotate, so you’re already on familiar ground.

Finally, I’ve mentioned aircraft with a high wing loading and a low wing loading. Simply put, this is just an expression of how highly loaded the wing structure is in normal flight conditions. For a given weight, a faster flying aircraft can utilise a smaller wing area than a slower flying one. Therefore, the faster aircraft has a higher wing loading. The calculation is determined by simply dividing the loaded mass or weight of the aircraft by its total wing area. Now that’s something you can impress people with at a party, NOT. The reason it is significant in this context is because a lightly wing loaded aircraft will tend to stall at a lower airspeed than a higher wing loaded example. Indeed, stall speed increases as wing loading increases. Therefore, it is perfectly acceptable and sometimes advantageous to fly a Decathlon or Tiger Moth off the ground in the 3-point attitude because they are comparatively lightly wing loaded and likely to have sufficient energy at the point of lift-off. Not so a more highly loaded example such as a Spitfire. Hoover up some YouTube clips and you’ll rarely see aircraft such as these taking-off with the tail fully planted on the ground. It’s usually best to raise it to some degree, even if not to a completely level attitude.

Have fun!

OLt. Zimmermann

Beiträge: 1311
Registriert: Sa 05 Sep 2015, 09:30

Re: Taildraggers © Chief Instructor

Beitrag von I/JG27_Zimmi » Mo 26 Dez 2016, 08:32

So here's part 3 of my conscious stream...

Taming Tail-draggers Part 3 – Landing

Landing a tail-dragger is no more difficult than landing a nose-wheeled aircraft. Maintaining control afterwards however is another story all together. Before we go any further, I’d like you to remember this: ATITUDE, ATITUDE, ATITUDE. Then I want you to remember: FEET, FEET, FEET. Then finally: TRIM, TRIM, TRIM!

When you’ve got that, carry on reading…

Essentially, there are 2 kinds of landing technique which are best suited to this configuration:

1. The three pointer, the one which most of you are already pretty familiar with and...
2. The wheeler. Wheeler landings are performed on the main wheels only, until such point in your landing roll where you lower the tail onto the ground before decelerating to a stop. Of the two, it is generally considered that the wheeler landing is the more difficult to master, at least to a point where it becomes a useful tool for the pilot. More about that in the next part.

Let’s start with the 3 pointer. Every good landing starts with a good approach. Nay, every good landing requires a good approach. Look at it like this – for the duration of your landing approach, always assume that you’re going to “go around” at the end of it, then only actually land if everything is perfectly balanced and in harmony. Sounds obvious but one of the leading contributory factors in landing accidents is a failure to go around when appropriate. I’m not going to go into detail about how you approach, but I am going to assume that you can fly an approach in the correct configuration, on a suitable glide path and at the correct speed.

This is where trimming the aircraft correctly comes into its own. Nearly all pilots are guilty to some extent when it comes to failing to trim their aircraft accurately. You may not think it’s a big deal but it really is. If you’re a bit out of trim, you probably think to yourself “it’s only a bit of stick force, I’m just going to compensate and hold the stick where it is…” Nothing could be more wrong. Imagine, during very delicate phases of flight, landing being one of the most poignant, you are required to make some very slight and gentle inputs. If you are to avoid over controlling, then those inputs should amount to little more than pressures. The majority of those pressures are likely to be less than the equivalent pressure required to compensate for a slightly out of trim airframe. Therefore, if you’re already applying a larger pressure to the stick than the small inputs you’re trying to make, you’ve basically got no chance of succeeding to the level of accuracy required.

Here's another very important point: Your aircraft should be so well trimmed during your final approach, that you ought to be able to hold the stick still, moving only occasionally to correct for small bumps or other minor displacements. This also means having the power set correctly, and not jockeying the throttle up and down continuously. Every time you change the power setting, you need to re-trim. Simple. If you’re having to do much more than this, then you’re not stable and not configured correctly. There is simply no substitute for accurate flying.

Over the years I have sat next to countless pilots on approach, stirring the controls around like they’re trying to unblock a toilet full of elephant dung. All of this in calm stable weather conditions. When asked to remove themselves completely from the controls, the aircraft settles down and flies smoothly down its approach course. What they didn’t realise is that they were putting nearly all of their effort into fighting their own over controlling inputs, whilst assuming they were actually doing something useful. I suspect that some of them even felt that doing this in some way makes them a better pilot because they’re having to work so hard to control the beast. It’s not just pilots in smaller aircraft either, have a hunt around on YouTube and I bet you’ll find plenty of airline pilots doing it too! Even in really turbulent approach conditions, there’s simply no need for it. You can’t possibly react to that many course and attitude corrections in such rapid succession, so you may as well ride it out and correct half as many times. My old flying instructor (who was a less than charming Kiwi) used to shout at me “If you grip like a w****r, you’ll fly like a w****r.” As much as I despised him, he was right.

Undoubtedly, in an aircraft with a Barry Manilow sized nose (such as the Spitfire) your field of view forward is limited, which limits the useful visual cues available for correcting drift and ensuring runway alignment. So, you have a number of choices: A steeper, close-in curved approach, or a crabbed, side slipping approach. Of course, you can fly straight in without crab, if you feel so confident and it works for you, carry on. In aircraft like the Spit though, curved is undoubtedly the way to go.
Remember, you have plenty of runway, so if anything, stay on the side of “a bit too steep” during your final turn to aid field of view. The Spitfire is very draggy in the full landing configuration, which makes life a little easier. If you are too shallow, you’re entering a whole world of pain which no amount of accurate flying will overcome, so just don’t do it – you’re not landing a 747.

Fast forward to the threshold. Most people’s tendency is to cross the runway threshold going too fast. +/- 5 kts can make all the difference, so don’t be complacent and work on your accuracy. The only way you’re going to get good at this is by being pretty brutal with yourself about constant trimming and a stable descent. There are no shortcuts – accept nothing but complete accuracy and once it becomes second nature, life will be sweeter! This guide is fairly generic, so I’ll avoid exact numbers because all aircraft differ, but about 100mph over the threshold is a good starting point. Be clear though that I mean OVER the threshold, we’re not landing yet.
So what attitude must we land the aircraft in? Well, the 3-point attitude of course. If you want to know what this looks like, look out of the window when you’re in the cockpit, stationary on the ground. That’s it. Nothing more, nothing less. Unlike a tricycle wheeled aircraft where the actual landing attitude at touchdown may throughout a significant pitch range, in the case of a tail-dragger executing a 3-pointer, there is precisely 1 attitude.

As you cross the threshold you must have cancelled, or be about to cancel ANY drift (meaning lateral movement relative to the runway as relates to the centre line). You must only be travelling ALONG not across the runway. Landing with drift is like releasing a jack-in-the-box. There are various ways of ensuring you have no drift but essentially they all involve looking out of the aircraft and assessing it visually. The slip ball can be helpful, but only earlier in the approach, as you certainly do not want to be looking inwards in the final stages.
As you are crossing the threshold, you should be decreasing your power ideally to idle and maintaining a very gentle descent. As you decelerate, the diminishing airflow over the wings will result in decreasing lift. Therefore, in order to maintain a constant descent rate, you will need to coordinate this deceleration with a constant increase in Alpha by progressively raising the nose.

Now then, if your threshold speed was correct, and the height at which you started reducing the power was about right, and your chosen rate of descent is spot on, and your rate of pitch change is perfect… all things being equal, you should have made the transition into the 3-point attitude at the precise moment where your descent intersects with the runway and the throttle hits the idle stop!

That’s the theory and it’s a lot of “ifs” isn’t it? Yes, it is, but that is how it’s done. Now, I can’t tell you how to do this because it takes practice, experience and a certain amount of trial and error. However, what I can tell you about is some of the factors which come into play that may put a spanner in the works. Incidentally, I going to assume that we’re dealing with zero cross wind. Namely, because in a simulator we can elect to keep the wind out of it while we’re learning. In any case, you have no business trying to learn how to land or take-off in a tail-dragger with any significant cross wind component. It’s fairly futile.

I’m assuming you have a little power applied during your crossing of the threshold, but it’s not mandatory depending on how you have elected to make your approach. However, a little power is a good thing because it keeps your control surfaces alive.
So, some things to be aware of:

1. Getting on the ground is the first bit. Keeping it on the ground and more importantly, on the runway, is the second. Once your wheels have touched down and you’re apparently rolling along nice and straight… THAT’S when it’ll bite you hardest. Do not relax until you’ve stopped.

2. WAKE UP YOUR FEET! Remember this from part 1? You need to relax and get your feet ready for those quick dancing moves. Tense feet means slow rudder inputs and lack of dexterity when you need it most.

3. Make all of your power changes very smoothly. Every increase or decrease in power setting will cause swing/yaw/drift to develop. It will also cause slight pitch changes which equate to trim changes. Use the throttle but do so smoothly and progressively, trimming all of the time.

4. At threshold speed, any movement of the ailerons will cause significant adverse yaw (yawing right for a left aileron input and visa-versa). This will in turn cause drift and lateral misalignment. Make sure your wings are level as you cross the threshold and try to avoid the use of aileron other than tiny inputs to prevent any rolling tendencies.

5. Be aware that the REAL 3-point attitude is often slightly nose higher than you may think as you touch down. This is because you’ve spent the last few hundred feet flying relatively nose down, so the transition to a 3-point nose high attitude can be alarming.

6. Following on from point 5 – If you don’t get the 3-point attitude exactly right, it’s better to land slightly tail first than main wheels first. Why? Well, because as your tail-wheel touches the ground first, the downward momentum of the aircraft is acting around the centre of gravity. This remember is situated behind the main wheels and in front of the tail wheel, so it will form a moment arm which will bring the main wheels down for you. Provided you don’t allow the tail to bounce up in the air again, or land so hard on it that you break it, you should be fine.

7. Bouncing – Bounces are caused by the main wheels touching first without any further corrective control inputs to counter the effect. The effect is the reverse of landing tail first because this time, the downward momentum of the aircraft’s mass acting on the centre of gravity will serve to bring the tail down, pivoting around the main gear. The result is an instantaneous increase in Alpha, causing an immediate increase in lift, followed by you getting airborne again. Many people assume this bouncing is due to the main wheels “springing you back into the air”, but it’s almost irrelevant. You now have only 2 choices…

8. If it’s a significant bounce, smoothly apply power, rudder and aileron to counteract the swing and/or roll. Adopt your normal climbing attitude and climb away for a go around. Trying to rescue a large bounce is pointless, and you’ll feel dirty even if you manage to pull off a survivable landing out of it.

9. For a small to medium bounce, don’t touch the throttle. Simply ensure that you’re still flying straight by using corrective rudder, and then, most importantly, RESET THE 3-POINT ATTITUDE AND HOLD IT THERE! I shouted that bit because it’s crucial. Most pilot’s instinctive response to a landing bounce is to start stirring the stick around wildly because it makes them feel as if they’re taking control back. You’re not, and the reason I know this is because if your speed was even nearly correct, then you’re not properly flying, you’re following a ballistic trajectory back towards the ground with some (but not much) aerodynamic help. Large macho control inputs in this situation are likely to end in over control and tears. Definitely tears. If you simply set and maintain the 3-point attitude and don’t do anything else, the aircraft WILL land. It may not be a greaser, but the chances are that you’ll be down on all 3 wheels.

Right then. Let’s pause for breath. We’ve got the thing on the ground, and all 3 wheels are turning. Now you have to stop it. All of the non-power on related factors which can cause a swing (and inevitably a ground loop) are now lining up to have a pop at you. Asymmetric braking, runway surface, pilot input, puddles, uneven grass lengths, some residual gyro effect from the idling prop, plus a whole heap of others which by now, you should be convinced exist. Some of these will increase in influence as you decelerate, and some will decrease. What you must remember though is this… As you decelerate, the aircraft’s interaction with the air around it changes, meaning that all of those lovely things which help you to maintain directional stability when a large volume of air is passing over the aerodynamic surfaces, including the fuselage itself, are fast becoming ineffective and frankly, useless.

This is why the biggest “gotcha” is saved for the time when you least expect it; namely when you’ve slowed down, and you’re patting yourself on the back. THIS is the time where you have the least control over your direction, yet you still have sufficient momentum to bend a lot of metal. How many times have you made a reasonable landing, slowed down nicely, only to tip a wing and go bumbling off the runway edge at a speed no more than you could run at? The problem with this kind of departure in the real world is purely that of repair bills and embarrassment, and believe me, no repair bill in the world can equal the pain of embarrassment like that!

Think about the aircraft a little like a car pulling a trailer. Imagine the engine is the car and that the mass of the aircraft, acting at the C of G is the trailer. When you have power applied and other factors are roughly in equilibrium, the car (engine) is towing the trailer (aircraft). Happy days. When you close the throttle, the propeller is no longer pulling the aircraft (trailer). In fact, it’s doing the opposite and creating lots of drag. That’s like applying a good boot full of brake in the car. What happens when you brake in a car that’s towing a heavy trailer, and the trailer has got a bit sideways? The answer is that it wants to ground loop your car and trailer rig. It’s a bit like that during your landing roll out, though your idling prop isn’t creating as much drag as it would in flight under the same circumstances, the ground interaction with your undercarriage IS creating a lot of braking force before you even apply the brakes. Therefore, if your C of G manages to get even 1 degree out of alignment with your direction of travel, it will excerpt a force that creates an arm which wants to swing the aircraft around. As with all swings, the further it goes, the faster it will go because the length of that moment arm increases as the aircraft swings further.

So, what can be done? As always, here are a few pointers to chew on. Possibly not a comprehensive list as every aircraft, situation and day is different, but practice these things and you’ll be on your way to mitigating the lion’s share of the risk.

1. Keep the control column FIRMLY at the back stop. This assumes your tail is already on the ground by the way! The effect of full aft stick increases the rolling resistance of the tail-wheel. As the tail-wheel is behind the C of G, it serves to greatly increase directional stability. It’ll also help prevent the tail from bouncing should you hit bumps, but that’s something more likely to happen in real life rather than our simulator.

2. Keep off the brakes for as long as you can. As a rule, tail-draggers are pretty aerodynamically draggy when they’re on the ground in the 3-point attitude. Remind yourself of why you carry out aero-braking in the fast jets after touchdown, and that pretty much sums up the situation here. Unless you’re landing on a very short strip, or indeed at the wrong end of a long one, then you probably won’t need to brake at all in reality. Braking is fundamentally risky because it is one of the leading causes of asymmetric force likely to provoke a ground loop. Add to this your tense, nervous legs, and likelihood is that you’ll be harsher than you intended to be. This is an area where real life trumps the simulator for 2 reasons – mainly because it’s much harder to judge your longitudinal rate of deceleration without a positive physical cue and secondly, only the very most expensive simulator rudder pedals you can buy can provide a lifelike feedback to braking pressure.
Here’s a tip: Adjust your pedal saturation to provide only roughly 70% of full braking when your toe brakes are at maximum travel. That way you can step on them a lot harder without inadvertently applying too much braking force. It also provides greater resolution enabling you to modulate the input more easily. The effect also makes it FEEL like you’re pressing harder, which is a bit more realistic. You’ll have to play with the values to suit your setup, and it might not be for you anyway, but it’s worth a try even if you don’t have any issues. I don’t recommend it for the jets, but for a tail-dragger in which you’re unlikely to ever be braking as hard as possible, I find it works well.

3. Don’t let a wing drop! As you slow even more, the ailerons become completely ineffective, particularly in the absence of any inboard flow courtesy of the propeller. Without the balancing and damping effect of more airflow across the wings and ailerons, the centre of pressure will move inboard, making the aircraft much less stable in roll. Then, you have a situation where you effectively have two long levers sticking out from each side of a trolley riding on two very narrow and closely spaced wheels. It’s not difficult to imagine why the wingtips are dying to snog the ground. If your wingtip does touch the ground, momentarily apply full rudder on that side and full brake. That’ll help lift it up. Most importantly, make a face like you meant to do it and hope no one saw it!

The rest, my friends, is up to you. Practice circuits and full stop landings as much as you can. On most of DCS’ runways you can easily come to a stop then start another take-off run. Do it over and over until it’s easy, but when it feels easier, don’t relax! Do touch and gos too if you like, but they’re nothing like as useful because they bypass one of the most difficult phases.

By the way, if anyone can suggest a good way of capturing a decent video whilst flying with my Rift, do let me know and I'll try and do some demos when time allows. I'd do it with replays after the fact, but they seem to be a bit broken with the Spit at the moment.

Happy landings!

OLt. Zimmermann


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