Difference between revisions of "Polar, Performance, and Water Ballast"

Revision as of 06:49, 24 December 2019

Have you ever wondered what differentiates the 'high-performance' gliders from the normal two-seaters, apart from the price tag and the rather fragile appearance? What are the ridiculously long wings of a Duo-Discus good for? Or more importantly, after you have paid the launch fee, what can you do to stay in the air for longer? The answers to these questions require an understanding of the performance metrics of the glider.

You might have heard of more experienced pilots talking about 'polars', or you might have seen the convex curve which is confusing to get started with. You might also have seen people adding or dumping water into and out of their gliders. Building on the knowledge of glider performance, we can have a closer look at how these tools help cross-country pilots to fly faster and further.

There is a wealth of text, published or online, discussing the topics mentioned above. However, some of these are rather scattered pieces of discussions on the forums, or they can be written in another system of conventions than what is adopted in Cambridge. Some are sloppy about their assumptions and approximations, and some dive straight into the calculus making it impossible to follow. This work aims to present the derivations of the governing equations and the polar functions in a clear and detailed manner, and summarises the implications for those who would rather not follow the mathematics.

The road map of this article is as follows:

1. Consider the forces acting on the glider in unaccelerated flight: lift, drag, and glide ratio.
2. Aerodynamic coefficients: definitions and meanings.
3. Relationship between lift and drag.
4. General method of solution, and assumptions necessary to simplify it.
5. Analytical form of the glide polar.
6. Implications of the polar: minimum sink speed, and best glide.
7. Adjustments to the polar: headwind and sinking air.
8. Adjustments to the polar: change of glider weight, first purpose of water ballast.
9. More effects of water ballast and recommended readings.

This article is a major project which will take me at least a month to complete. I cannot save a draft on WiKi, so if you accidentally come here and see this page in its very much incomplete form, please bear with me and come back after some time.

Glider in Unaccelerated Flight in Still Air

We start by considering a glider in unaccelerated flight in still air. We shall assume the following:

1. The aeroplane in question is a glider, i.e. it creates no thrust.
2. The flight is unaccelerated, i.e. the glider is flying straight and level without changing its airspeed.
3. The air is still, i.e. there is no macroscopic movement of air in forms such as wind, thermals, ridge lift, etc.
4. The air is homogeneous in its thermodynamic properties, especially, it has a uniform density \( \rho \).

Governing equations from a force perspective

Hopefully you already understand how a glider can remain airborne, but just in case you are in confusion, consider an unpowered glider in unaccelerated flight in still air: three forces act on the glider, namely:

1. Gravity (weight), pointing vertically downwards.
2. Lift, pointing upwards and perpendicular to the flight path.
3. Drag, pointing backwards and along the flight path.

By Newton's first law, in order for the glider to stay unaccelerated, these three forces must balance. Imagine the glider is flying horizontally. If this is the case, then the lift force must point vertically upwards. We then have a drag force pointing horizontally backwards with no force balancing it, because the other two are both in the vertical direction.

Therefore, the only way for the forces to balance is that, the glider cannot be flying in the horizontal direction. The flight path must be at an angle to horizontal. We shall denote this angle as \( \theta \). By experience, a glider in unaccelerated flight in still air keeps descending, rather than climbing. Therefore, we know the flight path is inclined downwards. We shall define this direction as positive \( \theta \).

With this made clear, the gravity (\( W \)) can be decomposed into two components, one to balance the lift (\( L \)), and one to balance the drag(\( D \)). The following relationship holds:

\[ W \sin(\theta) = D \] \[ W \cos(\theta) = L \]

Dividing these two expressions, \(W\) can be eliminated, giving:

\[ \frac{L}{D} = \frac{1}{\tan(\theta)}\]

The quantity \( \frac{L}{D}\) is referred to as the Lift-to-Drag Ratio.

Governing equations from an energy perspective

An alternative way to think about this is from an energy perspective. Because the drag force wants to slow down the glider and take its kinetic energy away, the glider must keep descending, so that it releases its gravitational potential to make up for the loss, otherwise it cannot remain at the same speed. Consider riding a bicycle: if you stop pedalling on level ground, you will gradually slow down and eventually stop, this is because drag force steals your kinetic energy away and you have no means of replenishing it. However, if you cycle downhill, you will not stop even if you do not pedal.

Therefore, we conclude that a glider flies downhill. This is in agreement with the conclusion of the previous section. We can borrow the notation and call the slope angle of this imaginary hill \( \theta \). Geometrically, if we travel for a unit distance on the face of the hill, then in the horizontal direction the distance travelled will be \( \cos(\theta) \) and in the vertical direction the height drop will be \( \sin(\theta) \).

From an energy conservation point of view, the following expression holds (it means the energy that the drag force uses up equals to the energy the gravity must provide):

\[ D \times 1 = W \times \sin(\theta) \]

This is the same result as in the last section.

Glide ratio

The glide ratio is a measurement of the efficiency of the glider. It means 'how many feet can the glider travel forward for every foot of altitude drop?' If a glider has a glide ratio of 50:1 (fifty-to-one), it means the glider is capable of travelling 50 feet forward for every foot of altitude drop, the same thing applies in meters, etc. We want the glider to travel as far as possible while losing minimum altitude, therefore, a larger glide ratio is favourable over a smaller one.

This measurement we can vaguely call 'performance', although strictly speaking this is only one aspect of it. We can say glider A (50:1) has a higher performance than glider B (30:1), for example.

In the last section, we concluded that, for each unit distance travelled on the hill, the horizontal distance covered is \( \cos(\theta) \) and the vertical distance covered is \( \sin(\theta) \). Therefore, the glide ratio is:

\[ \textrm{Glide ratio} = \frac{\cos(\theta)}{\sin(\theta)} = \frac{1}{\tan(\theta)} = \frac{L}{D}\]

This is a very important result. It is also worth noting that, up until now, we have made no approximations.

Glide ratio: influential factors

If you are familiar with calculus you will already have noticed that everything derived above is valid in an instantaneous sense. If you are not, take a minute to appreciate that the scale of time does not play a role in the process explained above: a glider with a glide ratio of 50:1 can travel fifty feet while dropping by one foot, it can also travel for fifty miles while dropping by one mile (slightly more than 5000 ft). If we extend this to the other extreme of the length scale such that the time associated is very small, we can see that the glide ratio is defined for any instant of the flight process.

There is no restriction on the glide ratio changing from one instant to another either, otherwise we will have no need to consider the polar if the glider flies the same at whatever speeds. Before proceeding into the more detailed discussions, some of the most important factors are presented here.

The glide ratio is mainly affected by:

1. The aerodynamic design of the glider. The more streamlined, sleek, and aerodynamic-looking a glider is, with long slender wings and smooth gel coating, the more likely it is to have a larger glide ratio.
2. The configuration of the glider. The glide ratio is almost always the highest in the clean configuration, i.e. with nothing sticking out or deployed. Lowered undercarriage, extended brakes and spoilers, deployed and windmilling propellers, opened or lost canopies, attached ropes are things that will reduce the glide ratio. Generally speaking, having the flaps set to other angles than neutral is not good for the glide ratio, but this very much depends on other factors.
3. The way the glider is flown. For a fixed glider mass (which is generally the case with the exception of jettisoning water), the glide ratio is a function of indicated airspeed, giving rise to the polar which is the immediate next topic. Moreover, if the glider is not flown straight (with sideslip), or flown otherwise than normal (e.g. stalled, inverted), the glide ratio can decrease drastically.

Lift and Drag Coefficients

Definitions

In aerodynamics, the lift coefficient (\( C_L \)) and drag coefficient (\( C_D \)) are defined as follows:

\[ C_L = \frac{L}{\frac{1}{2} \rho V^2 S} \] \[ C_D = \frac{D}{\frac{1}{2} \rho V^2 S} \]

Where:

• \(\rho\) is the true density of air.
• \(V\) is the true airspeed of the aeroplane.
• \(S\) is the area of the wing (projected onto the ground), a fixed value for a given glider.
• \( \frac{1}{2} \rho V^2 \) is collectively known as the dynamic pressure, or dynamic head.

A comment on the dimension

You should notice that, for both coefficients, the unit of both the numerator and the denominator is the unit of force (Newton in SI units). The denominator comprises \( \frac{1}{2} \rho V^2 \) which has the unit of pressure, and \( S \) which has the unit of area, so the product yields a force.

Consequently, both \( C_L \) and \( C_D \) are non-dimensional. These quantities have no units. Non-dimensional quantities are the language of aerodynamics: it allows us to study the underlying physics without being distracted by how things are measured. A K-21 is heavier than a Junior, therefore, in unaccelerated glide with the same angle-of-attack, the wings of the K-21 produces more lift than the Junior wings. What causes this? Is it because the design of the K-21 is aerodynamically superior? The answer is not necessarily, as the K-21 can be flying faster, for instance, or has larger wings. The comparison only becomes meaningful when the lift is non-dimensionalised into the lift coefficient.

A comment on dynamic pressure

The true air density and the true airspeed always appear together as a compound quantity \( \frac{1}{2} \rho V^2 \) which is referred to as the dynamic pressure or dynamic head.

The density of air is not a constant: it depends on pressure (which most notably depends on altitude) and temperature. This causes a major inconvenience as we have to assert a value to it in order to arrive at any numerical results directly useful for flying: e.g. you do not check any non-dimensional quantities in the cockpit, you read the instruments instead which tells you the airspeed in knots or the altitude in feet.

To overcome this problem, we notice that density only appears within \( \frac{1}{2} \rho V^2 \). Therefore, we can define an equivalent density \( \rho_e \) and an equivalent airspeed \( V_e \) such that:

\[ \frac{1}{2} \rho V^2 = \frac{1}{2} \rho_e V_e^2 \]

We can assert a value to \( \rho_e \) and arrive at a value of \( V_e \) such that, when used together, they produce the same amount of dynamic head, therefore, the aerodynamic effect is exactly the same.

The most reasonable value to assign to \( \rho_e \) would be the density of air at some standard conditions. This can then be implemented into some instrument that tells you \( V_e \) (all this instrument has to do is to measure the dynamic head). So long as all the manuals and polar charts express airspeed in \( V_e \) assuming the same value of \( \rho_e \), the change in true air density will not cause these performance guidelines to vary.

In practise, the instrument that tells you \( V_e \) is the air speed indicator (ASI), and \( V_e \) is known as indicated airspeed. Based on the discussions above, you should realise that:

1. Indicated airspeed is directly related to the dynamic head.
2. The dynamic head is the only way airspeed affects glider aerodynamics.
3. Therefore, the glider's aerodynamics is affected only by indicated airspeed, not true airspeed (apart from the never-exceed speed).
4. We should tabulate performance figures and draw polar graphs using indicated airspeed.
5. We do not need to adjust the performance tables or polar graphs to compensate for non-standard atmospheric conditions.