Essay, PART 2: Getting the tail up... - ED Forums
 


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Old 12-19-2016, 06:30 PM   #1
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Smile Essay, PART 2: Getting the tail up...

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!

CFI.
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Last edited by Chief Instructor; 12-20-2016 at 08:52 AM.
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Old 12-19-2016, 07:07 PM   #2
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Thanks! I was having trouble maintaining a straight course after takeoff so I'll try the tail-lift to get more authority.
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Old 12-19-2016, 07:07 PM   #3
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What a great essay! The first part is already being translated into Russian.

The only thing I would like to add about this matter is about P-factor, that is more noticable before the tail is raised, and as you have counteracted the gyro effect and the plane is steady at 2-point attitude at low AoA, this P-factor diminished, so the required right rudder amount is not so high as it was before the tail raising.
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Old 12-19-2016, 07:17 PM   #4
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Very well explained Chief, but do you mind if I disregard the last line - it would guarantee my tail stayed down I suspect! I do however consider that to be a good thing.

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Old 12-19-2016, 08:04 PM   #5
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Great stuff again. Gonna read once more about the prop disc
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Old 12-19-2016, 08:23 PM   #6
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Great post, thank you
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Old 12-19-2016, 09:26 PM   #7
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Great explanations. Haven't heard it so comprehensively explained since I did basic basic flying training ground school in 1962.

I hope you're going to cover 'asymmetric blade effect', and the different wheel loading due to the torque.

I seem to remember that there was a list of 10 causes of swing on take-off with a tail-dragger, but I'm damned if I can remember them all now. And I certainly couldn't explain them so well.
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Old 12-19-2016, 09:34 PM   #8
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How do you get the back wheel off then? Bunt to stick forward?
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Old 12-19-2016, 09:39 PM   #9
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Quote:
Originally Posted by Wayc00lio View Post
How do you get the back wheel off then? Bunt to stick forward?
Bunt...no, ease the stick forward as required yes.
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Old 12-19-2016, 09:39 PM   #10
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Thanks Bongo :-)
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