Talk:Glider (sailplane)/Archive 1

I've also been working on reconsiling Gliding and Gliders with Motor glider. I have moved most of the motor glider specific content there. I also agree that all of these overlapping articles need to be organized and expanded. Dhaluza 20:37, 23 November 2006 (UTC)

I've just sorted out the three competing articles "Gliding", "Gliders" and "Glider" and tried to give them some structure. But there is still some work needed. I notice that Ray Van De Walker has just added some content while I was doing this. I don't think I've mucked up your contribution, Ray, but apologise if I have. GrahamN

I'd like to add car-tow to the list of launch methods. I'm totally new to WikiPedia so I don't know how to add this, but basically the idea to have a pick-up truck tow a steel cable about 400m long with the glider attached to its other end. This gives the glider about 1100 feet of altitude above-ground-level (AGL) at the end of the tow. There it usually goes almost directly to the "down wind" side for landing. The climb is shallower than with a winch but the technique is similar ("stick back" for higher speed, the opposite of regular flight). With good conditions it's even enough to catch a thermal and go flying. Amos Shapira

Welcome to Wikipedia, Amos. Personally, I think we should try to keep the glider and gliding articles as distinct as possible, with a minimum of repetition, and I feel that descriptions of lanunch methods belong in the gliding article rather than here. Car-towing (aka auto-towing) is mentioned in the gliding article, but it could certainly use some more detail. Please don't feel you have to ask permission from anybody to edit any article - if you want to add more information or change something just go for it. That's the Wikipedia spirit, as I understand it. If you feel like it, you could read the Wikipedia:Help pages first, but I'd recommend you just to get stuck in. One tip - use the "Show preview" button. I remember when I was new here it took me a while to notice that it even existed! All the best. GrahamN 00:50 16 Jun 2003 (UTC)

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(moved comment to talkpage of Military gliders) Muijz 14:32, 23 December 2005 (UTC)

I have added some better images of gliders and generally tidied up. There is a natural tendency for the glider page to include material about gliding but some duplication explains what gliders are used for. May 2005 Jmcc150

Anyone mind if we zap one of the two pictures of touring motor gliders? I vote for deleting the Ximango because it is not flying. JMcC 18:38, 2 November 2005 (UTC)

Sailplane vs. Glider

The word sailplane is used repeatedly in the article, although searching on 'sailplane' on wikipedia brings you here. Shouldn't it be enough to mention at the start of the article that gliders and sailplanes are the same thing and then use the word 'glider' after that. It just seems a little confusing. ahpook 17:14, 8 May 2006 (UTC)

Quite right. I have changed the article so that 'glider' is used throughout. JMcC 23:49, 8 May 2006 (UTC)

Gravity

The section on moving forward is not the accepted view, Derek Piggott the famous British glider pilot clearly explains this in his book Understanding Gliding.The comment that gravity only acts downwards and therefore can not provide forward motion is easly disproved by anyone on a skate board who starts at the top of a hill. The lift from a wing never pulls you forward and in fact the lift is always directed backwards at some small angle. The confusion seems to stem from the fact that the lift vector is always drawn at right angles to the path of the glider and another vector is then drawn for the induced drag,in fact the total reaction is inclined towards the rear of the wing. The only way the wing can help you speed up is by a momentary redution in drag which occurs when the angle of attack changes but it is gravity that is providing the driving force. Proberts 23:10, 12 October 2006 (UTC)

Sorry, but you have not explained how a vertical force can provide a horizontal acceleration. Without wings the glider falls vertically. It is therefore the wings that provide the horizontal force. The lift is always at right angles to the relative air flow. The airflow usually comes from below the line of zero lift of the wing and so there is a forward component. Just as there is a lateral component from the wings to turn the aircraft when it banks. Analogies with skate boards and bicycles do not apply because there is no rigid medium to transmit force. Much as I like Derek (he saved my life once), he is just plain wrong, though and he can still beat me flying. Which particular element of drag do you think is reduced? Lift-induced drag or profile drag? If so, how? JMcC 23:45, 12 October 2006 (UTC)

In fairness to Derek Piggott he is not wrong. I checked his book and does not support Proberts arguments. The diagram on page 36 of my copy of 'Understanding Gliding' is virtually identical to my diagram below. JMcC 10:05, 17 October 2006 (UTC)

The forward trust on a glider is Mg sin Ø where M is the mass of the glider g is the force due to gravity and Ø Is the glide angle. The lift vector is Mg cos Ø. You can resolve this ( pun intended).Proberts 23:29, 14 October 2006 (UTC)

Before you start resolving forces, try starting with the basic force diagram of a glider flying at a steady speed. There is drag acting at right angles to the lift, there is gravity acting downwards and there is a lift force. If the glider is flying at a steady speed, these forces cancel. If lift acts backwards as you say, your version of the diagram would have the drag acting backwards & slightly downwards forward to maintain equilibrium. What force increases to cause acceleration when the nose is lowered? You say that it accelerates because lift induced drag is reduced. However this lift-induced drag falls towards zero as the glider exceeds min sink. Are you saying that if I am flying at 100 knots, there is no way I can speed up because there is no lift-induced drag left? I assure you that you can still speed up until the wings drop off. JMcC 09:44, 15 October 2006 (UTC)

A glider can not sustain Horizontal Flight. The angle between the flight path and the vertical is less than 90 degs on the other hand the angle between the lift force and the flight path is exactly 90 degs. The motive force is gravity. The article is spoilt by this which is pity.This should be fixed ASAP.

At last, you are supporting my argument. You correctly state that the flight path is not horizontal but slightly downhill. You also state that the lift force is at right angles to the flight path ie the lift force acts slightly forwards. QED. I knew rational argument would win in the end and I will remove the dispute tag from the article with your agreement. JMcC 12:59, 16 October 2006 (UTC)

I removed the disputed tag, if needed I can cite the SSA gliding text book (which I keep in my office for some strange reason), or I can cite the current FAA text book which I have at home. On another note I moved the picture to the right as it disturbs the section heading below it with it being on the left. PPGMD 15:26, 16 October 2006 (UTC)

I remembered that the FAA text book is available online so I added a Reference to it, since it depreciates the SSA text book for the most part. I also changed the section name as it didn't sound right being plural. PPGMD 15:37, 16 October 2006 (UTC)

No I do not support your view in any way whatever. Lift is defined as at 90 degs to the path of the glider not 88 degs or anything else. Drag is defined as directly opposite to the path of the glider. These are defined in just the same way as a kilogram is defined because this is what we find is useful. The real effect is the total reaction which leans to the rear of the flight path and is exactly opposite to the weight of the glider. Your ideas would allow the creation a perpertual motion machine. It should be obvious that if you try to move something through the air you will encounter air resistance but you seem to think that somehow that very air resistance can be made to move you forwards.Where is your source of power? . Why is it the higher I am the further I can fly.

Here is a quote from the SSA flight manual "Lift produced by the wings acts perpendicular to the modified relative wind." Page 1-5. Since the relative wind for a glider on the average soaring profile comes from below, the lift being perpendicular would have a forward component. Gravity only helps produce this lift by being a constant force pulling the glider down thus creating the relative wind, but gravity has no forward force on the glider. PPGMD 00:05, 17 October 2006 (UTC)

I am reading you post and about the only thing that matches up is induced drag. Induced drag isn't as big of a factor during soaring as it is for powered flight; instead it actually helps the glider (from what I understand from hanger flying) There is no energy generated by glider they are exchanging potential energy (altitude) for kinetic energy (airspeed). The only way for a glider to stay up indefinitely is to harness what is essentially the energy of the sun, through thermals, and ridge lift (at least for most of my soaring) to build up altitude (potential energy again) to release as airspeed as I try to get to the next source of lift before I have to consider to land. PPGMD 00:11, 17 October 2006 (UTC)

First the glider is using energy it is mixing up the air and creating turbulence. The only area we agree on is that you can not make something move forwards if the force is at right angles to its path. Take A glider on a glide path of 2 degs the angle between the vertical and its path is 88 degs the angle between the the lift vector and its path is 90 degs (no forward component here ). The angle between the wings total reation and the gliders path is 92 degs ( a net force to the rear of the glider. What is so hard about that ? the only time drag does not contrubute to overall lift is when you are flying level or climbing. If you know the weight of a glider and its current glide path you can calculate the force which is driving it forward. In the above the thrust force is MG sin 2 degs(Newton worked this out 400 years ago and the glider had not even been invented ). say the glider is 300 Kilogams then the force is .035 X 300 X 9.8 newtons that is 102 newtons or if you prefer 10.4 Kilgrams force . The motive force must equal the drag so we can now say that LD in this case is 300/10.4 or about 29 to 1. The article is way out, also a clear understanding of this area will help you to fly safely. It is no good just lowering the nose a touch when you need speed as in a winch cable break get it over firmly. Sin 30 is .5 that means the forward force at this angle is half the weight of glider that is 150 kgs force or 1470 newtons for the example above.The FAA must have been out to lunch when they came up with that nonsense. I will replace the disputed tag please do not remove it this time. Proberts 07:03, 17 October 2006 (UTC)

Forces on a glider

Once again User:Proberts has failed to respond to points made, but here is one last attempt. Please state what is wrong with this diagram or provide an alternative vector diagram that shows all the forces on a glider at a steady speed, before resolving them into components. Next, please then state which of the forces in the diagram increases for the glider to accelerate forwards when the nose is lowered. There is no free lunch, whether or not the FAA is already eating it! :-) No-one is saying that perpetual motion is possible. The potential energy that was given by the launch is all too soon used up unless lift is found. Incidentally I agree that lowering the nose after a cable break should be done rapidly but, please, not too rapidly. I also agree that drag contributes to the net upward force in the same way that a parachute slows a descent and it also reduces the forward acceleration, hence the name. JMcC 07:20, 17 October 2006 (UTC)

The diagram is also consistent with the following statements by Proberts:

• Lift is defined as at 90 degs to the path of the glider
• Drag is defined as directly opposite to the path of the glider
• The angle between the flight path and the vertical is less than 90 degs

An alternative diagram from Proberts that is also consistent with his own statements would be appreciated. JMcC 08:13, 17 October 2006 (UTC)

It least we agree about the flying aspect. I would love to show you the correct diagram but I am not computer savy so for the time being I will just have to explain. Draw a verticle line at the centre draw a line to represent your glide path at an angle of say 10 degs label the top of the line total reaction and label the bottom weight thats it done. Your diagram is missing the must important line the total reaction. It should be drawn in exactly opposite to the vector for weight. You are mixing up the lift vector with the total reaction vector.Any way it should be clear that if the wing is producing drag it can not be producing thrust at the same time it just does not make sense. You still have not explained where you get the power from to overcome the drag. In simple terms height is your gas tank and and the stick is the acceleration pedal and brake combined. The term horizontal only means at 90 degs to the vertical. The path of the glider is not horizontal under normal conditions. The statements in your article only prove you can not fly level or uphill but are not applicable when you are flying at angle less than horizontal.Gravity is the motive force, it has to be, no other forces are available.

For powered aircraft induced drag is a negative aspect of lift being generate from the wings because the relative wind comes from above the aircraft, so the lift generated from the winds pulls you backward. In a glider the relative wind comes from below, to the induced drag componet of the wing generated lift pulls you forward. Gravity OTOH is always pulling the glider straight down toward the ground, it provides no horitzontal movement what so ever. PPGMD 00:56, 18 October 2006 (UTC)

In Derek Piggotts book Understanding Gliding 1979 Edition on Page 22 under Propulsion he states Quote “In gliding flight, the aircraft must descend and use the force of gravity to maintain a steady speed.” I see no reason to argue with one of the best gliding instructors the world has know and nor should you .Proberts 02:02, 18 October 2006 (UTC)

Yes that is correct, except gravity doesn't provide a forward component for airspeed, it simply provides the downward component. Because a glider has no engine it has to fall by gravity at all times. PPGMD 02:08, 18 October 2006 (UTC)

Despite requests, Proberts did not even defend his own three statements that I had summarised for him and now has added to his own confusion by invoking additional forces. A simple question for Proberts: as well as counteracting gravity, does lift as shown in Derek Piggott's diagram have a forward component? Yes or no would suffice. JMcC 07:08, 18 October 2006 (UTC)

Yes When I figure out how to upload my drawing of the lift forces you can explain what if anything is wrong with it. I have not invented any new forces we agree I hope there is only one force on the wing which can be resolved into drag and lift that combined force is called the total reation of the wing Proberts 09:39, 18 October 2006 (UTC)

Your answer of 'yes' means that you have accepted there is a force that can move the glider forward. If your diagram combines the lift and drag vectors in the correct proportion, you will get a completely vertical resultant that exactly counteracts gravity. This predicts (correctly) that the glider flies at a steady speed in both the vertical and horizontal plane until it reaches the ground (Newton's First Law). However this is not helpful in deciding how it accelerated to its chosen horizontal speed to begin with. (You and I both are in agreement on how it got its vertical speed.) JMcC 10:10, 18 October 2006 (UTC)

I agree with all that I have added a link to my image Proberts 11:41, 18 October 2006 (UTC) http://en.wikipedia.org/wiki/Image:Wing_force_diagram-png.png

Bear in mind we are discussing horizontal acceleration, but you have attempted to draw a diagram of a glider at equilibrium. By combining the forces it will be difficult for you to use your diagram explain which of the three forces changes if you move the stick forward. When the wings are tilted forward, they produce less vertical force and so the rate of vertical speed increases. However the forward component of lift (see my diagram) also increases and this produces a horizontal acceleration. Note the angle between the lift force and horizontal is less than 90 degrees and so horizontal acceleration is entirely possible. With increasing speed there is also increasing drag until a speed is reached where there is a new equilibrium (unless the ground arrives first). The weight force remains constant in strength and direction throughout this event. JMcC 13:01, 18 October 2006 (UTC)

I accept that. In the past I took the view that if you had a device of some kind eg a pulley you could change the direction of a force and that the wing was in effect doing just that. For example tie a piece of string around a book and on the other end of the string a weight. Put the book on a table and the weight over the edge. The book moves across the table. Gravity is the source of the motive force. So your satement that a horizontal force could not produce a motion at right angles to it only seemed to make sense in very simple situations.So here is my diagram of the wing forces (resolved).

http://en.wikipedia.org/wiki/Image:Gravity_wing.png

Glad to have resolved that one at last. Not sure about the latest diagram, but I am happy to leave it here. JMcC 07:10, 19 October 2006 (UTC)

My diagram above is wrong ideally it should have appeared after my Post starting with YesProberts 09:43, 20 October 2006 (UTC)

http://upload.wikimedia.org/wikipedia/en/c/cd/Gliding_force_Vectors.png

On the other hand this diagram is correct and I am confident proves my case. Note Newtons 3rd law states all forces must come in pairs and be equal and opposite in value.

You must show the directions of all forces clearly in a vector diagram.

I would recomend the section on moving forward be rewritten.Proberts 23:31, 22 October 2006 (UTC)

Oh dear, mythical forces again. Every textbook shows there are only three forces. I hope you are not a teacher. I thought rational argument had prevailed, but now I can see that this is not possible. No further discussion will be entered into. JMcC 14:02, 23 October 2006 (UTC)

No Mythical Forces in My Diagram. Please Review "Appendix C Vectors and force diagrams" Understanding Gliding Derek Piggott. Proberts 04:24, 24 October 2006 (UTC)

Ok if you have a problem with what I am saying at least try the following simple experiment. Next time you are driving down the road at say 50 mph stick our hand out the window and hold your flat hand at a small angle of attack to the wind. You will feel your hand being lifted but you will not feel it being pulled forward! even when you drive down hill. Try it. The next time you are in a glider have look at the angle between wing tip and the horizon. This is my last post in this discussion. Proberts 10:02, 24 October 2006 (UTC)

That because your hand has a positive angle of attack compared to the relative wind, put your hand out with the negative angle of attack that replicates more closely to how the glider is being hit by the relative wind. PPGMD 14:08, 24 October 2006 (UTC)

As an engineer in air handling I know the forward speed comes from gravity no ifs no buts no question. CA

Gravity can only provide vertical acceleration. The relative airflow passing the wings caused by the vertical speed also generates lift. Without wings a glider would fall vertically - something that Proberts never acknowledged. Wings are what distinguishes a glider from a stone. The standard three force diagram above shows that the only force that can accelerate a glider horizontally comes the forward component of the lift generated by the wings. (By horizontal acceleration I do not mean acceleration down the glide-slope.) JMcC 16:48, 30 October 2006 (UTC)

I have spent the last few weeks studying this problem and can find no evidence to support User JMcC view. According to a physics professor I discussed this with if your view was correct you would attain an infinite speed in a vertical dive. For a powered aircraft in level flight the thrust force from the engine is exactly balanced by the drag force from the whole aircraft. Using JMcC diagram above of the forces on glider, what force balances the drag? Note that the lift component can not do this as it is a 90 degs to the flight path. If there is no force opposing the drag we are going backwards. That only leaves us with gravity. Therefore it is gravity that is pulling us along our glide path. If we increase our angle of descent the gravity component becomes steadily greater allowing us to fly faster (drag increases by the square of the speed).The idea that there has to be a horizontal force is not relevant as the path of the glider is not horizontal. The other idea that there is any forward force provided by the wings is also fundamental flawed. It like saying that pulling equally on both ends of a train will make it move. Further information and diagrams can be found by doing a search on the net for “Forces in descent aircraft”. 58.84.86.21 01:08, 22 November 2006 (UTC)

Firstly I have had to edit your contribution to remove the spaces at the start of each line. I suggest that you use 'preview' before saving your work. I also suspect you are just a sock puppet for User:Proberts, who I had planned to ignore as unable to answer a question. However to answer your points:

• You merely have to answer the following questions:

1 What makes a glider different from a stone in flight?(I suggest your answer will mention wings.)

2 What effect do the wings produce that a stone cannot creating a different flight path from the stone's? (I suggest your answer is lift)

3 As you say, lift acts at right angles to the airflow. In what direction is the airflow? (I suggest your answer is that the airflow is along the glide path and in the opposite direction. Now draw a line of lift at right angles to this airflow and tell me which way it points. (I suggest your answer will say that lift acts slightly forwards as well as upwards.)

4 If you lower the nose of a glider, the vertical component of lift is reduced. The forces are no longer in equilibrium and so an acceleration down a steeper glide-path will occur. Do you agree?

5 When the nose is lowered, are wings still generating force in the same direction? (I suggest that your answer will be no, lift is now produced at right angles to the steeper glide-path. At a higher speed more lift is also produced in total.)

6 Now that the glider is travelling at a steady higher speed with its nose lowered, has the horizontal component of lift increased? (You should agree.)

• If you agree with the suggested answers to all six points, then you have to decide how the higher horizontal component of the glider's velocity occurred. If not, you have to state which of these points you disagree.
• Pulling equally hard at both ends of the train will, as you say, produce no movement. This is known as an equilibrium, just as when a glider is gliding at a steady speed. Above I added a diagram of the forces that shows they are in equilibrium at a steady speed. (It is exactly the same as Piggott's book.) Drag acts upwards and backwards, ie in the opposite direction of the glide-path and so limits both vertical and horizontal speed. The diagram clearly shows that gravity and the lift balance the drag in steady flight. There are only three forces that you could invoke.
• Quite correctly you say that drag increases with the square of the speed. Consequently a terminal velocity would in theory be reached in a vertical dive, though the wings would drop off first if you tried this, but not when I am instructing, please.

I will happily answer further questions only on the condition that you fully answer my six questions. JMcC 09:31, 22 November 2006 (UTC)

I would like to see a glider move forward using its wings in the complete absence of gravity... that would be really amazing. 195.194.199.50 12:34, 23 November 2006 (UTC)

Your suggestion is analogous to a car moving forward in the absence of friction. We know that the motive force is provided by the chemical energy converted in the engine, but without the friction between the wheels and the road there is no mechanism to transfer the rotational into linear motion. In the same way that the energy I am using to write this response was obtained from a bowl of pasta I ate this morning, but the energy really came from the sun before being converted to carbohydrate by the cereal. Did the pasta power my brain or did the sun? One would not have happened without the other.--BWDuncan 14:59, 23 November 2006 (UTC)

The last two contributions have confused two entirely different things in physics: energy and force. No-one should dispute that the kinetic energy of a glider comes from converting the potential energy from raising its mass against gravity. The force that makes it go forward comes from the wings. In the complete absence of gravity, it would have no potential energy and so would not move in any direction. JMcC 15:55, 23 November 2006 (UTC)

First I must apologize it was not my intention to deceive you that I was someone else It was just sheer computer incompetence. I see that my last post was signed with a number and not my name as previously. I have no idea why, nor can I get preview to work. So here are my answers to your questions. (Proberts)

Not I use A as the glide angle, D as the drag force, and L as the lift component.

1 What makes a glider different from a stone in flight? (I suggest your answer will mention wings.) Ans: Its shape. I am tempted to say that in the case of some of the gliders I have flown not much! The dispute we are having is not about whether a wing produces lift. It is about does a glider wing produce a forward force I say no.

2 What effect do the wings produce that a stone cannot creating a different flight path from the stone's? (I suggest your answer is lift) Ans: None a stone can produce lift if it is the correct shape or is spinning see Magnus effect. EG Golf balls, tennis balls, Frisbees. I would add that of course wings can produce lift.

3 As you say, lift acts at right angles to the airflow. In what direction is the airflow? (I suggest your answer is that the airflow is along the glide path and in the opposite direction. Ans: Yes I agree. Now draw a line of lift at right angles to this airflow and tell me which way it points. (I suggest your answer will say that lift acts slightly forwards as well as upwards.) Ans: I say forwards is in the direction of travel and JMcC defines it as parallel to the ground which is bound to lead to confusion. If we use my definition the lift is at 90 degs to the direction of travel and can not provide any forward motion. In any event the horizontal drag component is at all times equal to the horizontal component of the lift.

4 If you lower the nose of a glider, the vertical component of lift is reduced. The forces are no longer in equilibrium and so acceleration down a steeper glide-path will occur. Do you agree? Ans: Aha gravity! Yes

5 When the nose is lowered, are wings still generating force in the same direction? (I suggest that your answer will be no, lift is now produced at right angles to the steeper glide-path. At a higher speed more lift is also produced in total.) Ans Yes and no. Yes the lift is at right angles to the steeper glide path No the wings are not producing any more lift. The angle of attack is less at the higher speed and lift is dependant on the angle of attack as well as speed. The lift plus drag must equal the weight at all times in unaccelerated flight. The lift has in fact decreased and the drag has increased. The net effect is the horizontal component of lift remains equal to the horizontal component of drag. On the other hand gravity’s forward component (my definition of forwards) can increase steadily as the glide angle becomes steeper. See below.

6 Now that the glider is travelling at a steady higher speed with its nose lowered, has the horizontal component of lift increased? (You should agree.) Ans: No the drag has increased and the lift has decreased. If it had not your l/d would remain the same so there would be no penalty for flying faster.

This is where I Disagree. The steeper the slope the faster you will go. I pointed out previously that forward force is equal to mg sin A where A is the glide angle and mg is the acceleration due to gravity or weight if you prefer. For example A=0 then sin A = 0 and no force available to equal drag in level flight. If A=2 deg then Sin A =0.035 indicates a small forward force. If A=10 deg then Sin A=0.17 indicates a force nearly 5 times bigger than above. If A=30 deg then Sin A=0.5 at an angle of 30 degs half the weight is supported by Drag and half by lift. The force is now 14 times bigger than when A=2 degs.

If A=90 deg then Sin A=1 in other words in a vertical dive the driving force is exactly equal to the weight of the glider which is in turn must be exactly equal the drag. The lift is Zero. So despite the lift force being 100% horizontal (forwards?) it is of no benefit as it is equal to Zero! To sum up drag component + lift component must equal the weight in a steady glide (unaccelerated flight). As the glide angle becomes steeper drag increases and lift is reduced. At the limits of glide angle=0 weight = lift, at glide angle =90 lift =zero and weight=drag in between these two extremes the proportion of weight supported by lift or drag is steadily changes. To put this in perspective in a modern machine a typical flight (apart from the final approach) the glide angle during the whole flight might only change by 2 degs on the other hand the attitude may change by over 15 degs the difference is in the angle of attack. I live in hope that the following points can now be accepted. The lift is a right angles to the direction of travel and so can not provide a force in that direction. The force of gravity is at less than right angles to the direction of travel and therefore can provide a force along the glide path. There are numerous problems with the idea that the wings pull the glider through the air one of them concerns your explanation of how the air brakes work. Using the wings as the diving force the lift becomes less the so the speed must drop but then we have to speed up again. Gravity is not allowed. You should end up concluding that it would be near impossible to maintain a steady speed on approach or that you can not control the glide angle. It is not a simple problem you will need to think about it. The point of the train example was this if you pull harder on your end I will pull harder at the other end. This becomes a tug of war with no winner.

Quite correctly you say that drag increases with the square of the speed. It is important to note the following Only if Cd (coefficient of drag) is held constant The same is true for lift but again only if Cl is constant (CL is coefficient of lift) Cl and Cd change with angle of attack. Cl varies from 0 to about 1.6 at high angles of attack for gliders through a range of about 15 degs.

In Derek Piggot book “Understanding Gliding” Note the following Page 25 fig 6 he shows the weight resolved into its component parts On pages146 he explains about the total reaction of the wing. The force which JMcC seems to think is mythical. Diagram 91 shows what a large proportion drag can make to supporting the weight. Unfortunately Derek refers to centre of pressure, a concept still found in just about all pilots handbook even today. This concept was discarded by aerodynamics people back in the 1930s. My question to JMcC is simple In what way is the physics of this problem different to a snow skier going down a slope? Note we can increase the speed by going down a steeper slope. Please explain where the increase in speed comes from, and also where the forward motion comes from. How are the forces balanced when we reach a steady speed?

Interestingly some gliders of the 1950”s could be dived vertically with full dive brake without exceeding VNE. In a modern glass ship VNE with full dive brake occurs at about 45 degs or earlier. I think JAR specs state 45 degs but I could be wrong. I would want to read the hand book before any experiments in that area. Proberts 58.84.108.74 22:56, 25 November 2006 (UTC)

I promised myself I would not respond but this is definitely the last time. I have never said that gliders travel parallel to the ground, but I have said that gliders have both a vertical and horizontal component to their velocity. As I understand it, you still disagree with the following statement: "Lift has a horizontal component". To me this explains how gliders can overcome drag and fly cross-country rather than landing direct below the point of release. Nevertheless I think we have always agreed that the energy is provided by converting the potential energy from raising the mass against gravity into kinetic energy. The air speeding past the wings creates a lift force acting at right angles to the glide-slope. Consequently the force is generated has both a horizontal component and a vertical component. If this horizontal force stopped, the glider's horizontal speed would be reduced by drag until it was descending vertically. The problem of the skier down the slope differs because there is a solid surface providing a normal force at right angles to the surface. This normal force prevents the skier disappearing into the mountain. See basic physics page [1]. Because it acts at right angles to the surface, the normal force provides an upward component and a horizontal component. On a steep slope there is less upward component and so the acceleration from gravity is greater. At equilibrium there are three forces: normal force, drag and gravity looking much like the three forces for a glider. Consider also a marble dropping onto an inclined plane. A force appears that suddenly changes its direction from vertical. This is provided by the plane. Similarly the initial horizontal acceleration to the skier is provided by the piste. Any further messages will not be replied to. A lack of response does not mean I agree with you. JMcC 16:26, 27 November 2006 (UTC)