Sink and Glide angle

[Olendirk]

Hi!

I have a question: What is the difference between the glide angle and the sink angle? Is it right that the glide angle is only affected by configuration settings and not by altitude and weight? And what happens with the best glide angle and the sink angle if i fly faster?

thanks!

od

[silentwitness]

The best glide angle only occurs at a specific speed. If you fly faster than this, you won't glide as far as your attitude will be too low. If you fly slower than this speed you won't glide as far as your drag will be too high.

Weight doesn't affect the glide distance as the 2 factors that do affect the distance are lift and drag. Where the resultant from L and D is the best glide angle. If you have a lot of lift and a little drag then the angle will be shallow. If you have a lot of Drag and not much lift then the angle will be steep.

More weight just alters the length of the L/D vectors which means the angle will stay the same, however the speed will be increased.

You can find all this information in PPL books. It pays to read!! :ok:

[Tinstaafl]

w.r.t. Silentwitness' first sentence: More correctly, best glide occurs at a specific Angle of Attack - which will correspond to a specific speed for a given set of conditions eg configuration & weight,

[Keith.Williams.]

The glide angle is between the horizontal and the flight path. For maximum glide range you require minimum glide angle. If you could achieve a gllide angle of zero you would never reach the ground, so your glide range would be be infinite. But in the real world your glide angle will usually be more than zero. so your glide range will be limited.

If you do the maths you will find that glide range (in still air) from any given height is equal to the lift to drag ratio multiplied by the height at which the glide starts. So if you fly at the speed at which lift to drag ratio is greatest, you will get best glide range. This will also mean that you have the minimum achievable glide angle.

Best lift to drag ratio occurs at VMD, so you get best glide range by flying at VMD. If you change the weight or configuration, you will of course change your VMD. But provided you fly at the VMD for that weight and configfuration, you will get your best glide range and minimum glide angle.

The weight of an aircraft does not affect its lift to drag ratio at VMD, so glide range is not affected by changes in weight. Increasing weight for example, simply means that provided you fly at VMD, you will fly down the same slope faster. You hit the same spot on the ground but get there sooner.

If your sink rate is zero you will never reach the ground, so your glide endurance will be infinite. This is possible in updrafts, but you will never achieve zero sink indeffinitely. so your glide endurance will be limited. But you will always get best glide endurance if you minimize your sink rate.

To get maximum glide endurance you must fly at VMP. To understand this we must consider the energy situation. Flying through the air requires energy, which is usually provided by the engines. So when the engines fail we can get no more energy. Whatever energy we have stored in the aircarft is then dissipated in the remaining gliding flight.

At the time of engine failure the total energy of the aircraft is the sum of its kinetic energy (1/2mv squared) plus its potential energy (wieght x height). After it has landed and come to rest its energy is zero. So to maximise glide endurance, we must minimise the rate at which we dissipate the energy that is stored in the aircraft.

The power required is a measure of the rate of energy dissipation, so to mimise the rate of energy dissipation we must minimise the power required. This is achieved by flying at VMP. But changes in weight or configuration change the power required, so varying aircraft weight or configuration will change the glide endurance.

[subsidence]

hy

I have a question relating to that topic!


The Problem:

The Rate of climb is the vertical component of the aircraft`s velocity and depends on the aircraft`s velocity and the climb angle,

I don`t understand why the rate of climb is independent of the wind speed.
A headwind for instance will give a steeper climb angle(relative to the ground), and a tailwind a flatter climb angle and thus it should influence the rate of climb. But obviously it doesn`t.

Can anyone clear this up, it would be much appreciated,

bye

[jockeymon]

Think of rate of climb along the lines of time to climb not distance to climb, a wind will effect distance (or as you pointed out angle) of climb but as rate of climb is "relative " to air or airspeed it makes no difference (a 500fpm climb is a 500fpm climb in still air or a 20knot headwind/ tailwind) your ground speed will be different but why worry as you`ll be flying at an indicated airspeed it all ties into the formula;
NAM/TAS=NGM/GS
nam-nautical air miles, TAS-true airspeed,ngm-nautical ground miles,gs-ground speed
But thats only if you want to work out climb distances

[Keith.Williams.]

Subsidence

Your statement that "The Rate of climb is the vertical component of the aircraft`s velocity and depends on the aircraft`s velocity and the climb angle" is true but if you are using speed relative to the air you must also use angle relative to the air.

A headwind increases climb or descent angle because the air is moving in the opposite direction to the aircraft. But if it is a steady wind it will not affect the airspeed, nor the angle of climb or descent relative to the air. So it will not affect the ROC or ROD.

To understand the situation in a glide for example, we should image that we are descending through a rectangular block of air. We start the glide at the top left corner and end it at the bottom right corner. The glide angle is the angle between our flight path and the top of the block. Our ROD is the height of the block divided by the time it take to reach the bottom. Let us suppose for example that it takes us 2 minutes.

Now consider the case where we descend with the same TAS and ROD as before, but this time with a headwind that is equal to half of our groundspeed. The block of air is now moving across the ground in the opposite direction to that of our aircraft. Because our TAS and ROD are unchanged, our dradient and endurance are unchanged.

But by the time we reach the bottom right hand corner of the block, the block itself has moved half a block to the left. So we have moved only half of the previous distance over the ground. From the point of view of an observer on the ground, looking from the side of our flight path, our glide angle appears to be greater because our ground distance is less. But the TAS and ROD were unchanged so the time taken will be unchanged. It will still have taken the same 2 minutes to reach the ground, but will have landed in a different place.

The problem with your argument is that glide angles are measured relative to space (the earth horizontla) whereas TAS is measured relative to the air. To do this calculation properly we must use our TAS, still air glide angle and windspeed to calculate our ground speed. Then using this and our ROD we could calculate our new glide angle.

[Bre901]

Weight doesn't affect the glide distance as the 2 factors that do affect the distance are lift and drag. Where the resultant from L and D is the best glide angle.
I'd like to make a slightly pedantic comment :

This is true only if the lift and drag coefficients are constant, which might not be exactly true. For gliders, it has been shown that the maximum L/D increases by a few percent with increasing wing loading.

Fact is that given the small chord length and low velocities, the Reynolds number (in the 700 000 range) is at the lower boundary of "developed" turbulence.
This is even worse for ultralights and models as shown here (http://marlongofast.tripod.com/zipped_aeronotes/clarky.htm)

On the other hand, the constant Cl and Cd assumption is indeed valid for airliners, where Reynolds numbers are at least an order of magnitude higher.