Glide path control on final.
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For most airspeeds in a powered aircraft a range of flight paths are available. For example at 70 Kt and aircraft can fly a steep descending path with the throttle closed and gradually bring that flightpath through level flight to a climbing flight path all the while maintaining 70Kt by adjusting the thrust. The limits being set as the point where the throttle is closed in the glide (a steeper descending flight path will cause a speed increase above 70Kt) or where full power is selected in the climb (any steeper a climbing flight path resuts in a speed decrease below 70Kt).
The flight path of the aircraft is often not properly explained to the student. There a lots of talk about "relative airflow", "angle of attack", "power gives you height attitude gives you speed" but few instructors seem to explain that the general relative airflow arrives along the flight path and it is attitude and flight path that cause the aircraft to fly.
Why does pulling the control column back increase the angle of attack - the answer is that the attitude changes in response to the elevator movement but that momentum keeps the aircraft moving along the original flight path. Since the relative airflow comes along that flight path then it is easy to see that the new attitude and old flight path have to give us a different angle of attack.
Why does pushing the control column forwards reduce the angle of attack - the answer is that the attitude changes in response to the elevator movement but that momentum keeps the aircraft moving along the original flight path. Since the relative airflow comes along that flight path then it is easy to see that the new attitude and old flight path have to give us a different angle of attack.
So what does flight path have to do with stall recovery - well simple, when stalled the relative airflow is coming along the flight path. The reason why you are stalled is that the angle between the chord line and the flight path is greather than the stalling angle of attack. Assuming that the flightpath is steady, you have to change the attitude enough so that the angle between the chord line and the flight path (relative airflow direction) is less than the stalling angle of attack.
Thus when it comes to flying, flightpath and attitude are king.
Of course the queen has to be drag - a function of guess what - the flight path / chord lind relationship (angle of attack) and airspeed which in the absence of an engine relies on the interaction of the fligt path with gravity.
So. Did someone say that fligth path only mattered on approach?
Regards,
DFC
The flight path of the aircraft is often not properly explained to the student. There a lots of talk about "relative airflow", "angle of attack", "power gives you height attitude gives you speed" but few instructors seem to explain that the general relative airflow arrives along the flight path and it is attitude and flight path that cause the aircraft to fly.
Why does pulling the control column back increase the angle of attack - the answer is that the attitude changes in response to the elevator movement but that momentum keeps the aircraft moving along the original flight path. Since the relative airflow comes along that flight path then it is easy to see that the new attitude and old flight path have to give us a different angle of attack.
Why does pushing the control column forwards reduce the angle of attack - the answer is that the attitude changes in response to the elevator movement but that momentum keeps the aircraft moving along the original flight path. Since the relative airflow comes along that flight path then it is easy to see that the new attitude and old flight path have to give us a different angle of attack.
So what does flight path have to do with stall recovery - well simple, when stalled the relative airflow is coming along the flight path. The reason why you are stalled is that the angle between the chord line and the flight path is greather than the stalling angle of attack. Assuming that the flightpath is steady, you have to change the attitude enough so that the angle between the chord line and the flight path (relative airflow direction) is less than the stalling angle of attack.
Thus when it comes to flying, flightpath and attitude are king.
Of course the queen has to be drag - a function of guess what - the flight path / chord lind relationship (angle of attack) and airspeed which in the absence of an engine relies on the interaction of the fligt path with gravity.
So. Did someone say that fligth path only mattered on approach?
Regards,
DFC
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I come from the Gliding community with a limited experience of power (mainly SLMG). We always teach elevator for speed, brakes for rate of descent as has already been described. A "problem" we have with new students in the early stages of teaching approach and landing is that they want to aim with the elevator and control speed with brakes (similar to your point/power question). This raises a couple of issues in our aircraft types. Firstly, if the approach is started too high/close, the brakes cannot reduce the speed sufficiently and 5-10kts excess speed in something as slick as high performance glider results in a massive overshoot-bad news in a short field landing. The other "gotcha" of this method is a long/low approach where the risk is that the glide is stretched with no brake resulting in, at best an under shoot or decaying airspeed and a stall/spin .
A final consideration is that whilst some gliders are flapped a great many are not, therefore when the brakes are opened they spoil the lift and create drag but give no increase in lift, resulting in an increase in ROD (steeper glide path) and elevator used only to maintain approach speed. Whilst it can be argued then that the elevator contributes to the increased rate of descent, with small brake settings the affect on airspeed can be almost negligible. This is most readily seen in low level launch failures where cracking the brakes open will risk slamming the glider into the ground whilst still having flying speed.
As for landing fast, landing on is a no-no, with a fully held off landing and the stick on the back stop required, in which case the touchdown spot will inevitably be further up the runway.
I would say that most tug pilots I know use elevator for speed and power for ROD but then most I know glide in on idle, with a dead stick style approach,
Ian
A final consideration is that whilst some gliders are flapped a great many are not, therefore when the brakes are opened they spoil the lift and create drag but give no increase in lift, resulting in an increase in ROD (steeper glide path) and elevator used only to maintain approach speed. Whilst it can be argued then that the elevator contributes to the increased rate of descent, with small brake settings the affect on airspeed can be almost negligible. This is most readily seen in low level launch failures where cracking the brakes open will risk slamming the glider into the ground whilst still having flying speed.
As for landing fast, landing on is a no-no, with a fully held off landing and the stick on the back stop required, in which case the touchdown spot will inevitably be further up the runway.
I would say that most tug pilots I know use elevator for speed and power for ROD but then most I know glide in on idle, with a dead stick style approach,
Ian
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I really think that most people do not understand what is really being spoken of when people use the terms "point and power".
Point and power simply means using the various controls at one's disposal to ensure that the constant point and the desired aiming point coincide and remain so.
Having sat through various instructor explanations (good and bad), experienced and ab-initio recently, not one could correctly draw the following;
Draw a diagram of the forces acting on an aircraft in steady straight and level flight at x speed (back of the drag curve close to the stall) and a separate one for y speed (same power but on front of drag curve)
All of them said that both drawings are the same since lift and and weight are the same in each because level flight and since thrust is the same then drag is the same since constant airspeed.
They are wrong.
The common issue is that in both cases the aircraft is being pointed at the horizon and the power is suficient to maintain the airspeed.
However what was lost on them was that while the flight path is horizontal, the direction of thrust is totally different in each due to the different attitudes.
Day after day students are shown diagrams of the four forces. Very few of them show the flight path.
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if the approach is started too high/close, the brakes cannot reduce the speed sufficiently
That is not a function of "point and power" or "elevator for airspeed" mindsets.
It is simply a function of gravity and drag (or in this case not having enough drag to counteract the effect of gravity on the overly steep flight path that is required.
when the brakes are opened they spoil the lift and create drag
That is my understanding also.
Could the real reason be that the flight path to keep that speed is steep enough to "slam the aircraft into the ground".
Ask yourself - should you do that, in the brief period before impact where is the constant point?
Just because the airspeed is x and the attitude is y it does not mean that the aircraft is travelling in the direction you expect - take a flat spin as a perfect example regarding attitude, gravity, drag and constant point!!
Regards,
DFC
Point and power simply means using the various controls at one's disposal to ensure that the constant point and the desired aiming point coincide and remain so.
Having sat through various instructor explanations (good and bad), experienced and ab-initio recently, not one could correctly draw the following;
Draw a diagram of the forces acting on an aircraft in steady straight and level flight at x speed (back of the drag curve close to the stall) and a separate one for y speed (same power but on front of drag curve)
All of them said that both drawings are the same since lift and and weight are the same in each because level flight and since thrust is the same then drag is the same since constant airspeed.
They are wrong.
The common issue is that in both cases the aircraft is being pointed at the horizon and the power is suficient to maintain the airspeed.
However what was lost on them was that while the flight path is horizontal, the direction of thrust is totally different in each due to the different attitudes.
Day after day students are shown diagrams of the four forces. Very few of them show the flight path.
-------------
if the approach is started too high/close, the brakes cannot reduce the speed sufficiently
It is simply a function of gravity and drag (or in this case not having enough drag to counteract the effect of gravity on the overly steep flight path that is required.
when the brakes are opened they spoil the lift and create drag
This is most readily seen in low level launch failures where cracking the brakes open will risk slamming the glider into the ground whilst still having flying speed.
Ask yourself - should you do that, in the brief period before impact where is the constant point?
Just because the airspeed is x and the attitude is y it does not mean that the aircraft is travelling in the direction you expect - take a flat spin as a perfect example regarding attitude, gravity, drag and constant point!!
Regards,
DFC
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"Could the real reason be that the flight path to keep that speed is steep enough to "slam the aircraft into the ground"."
No.
Chris N.
No.
Chris N.
Regards,
DFC
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What slams the aircraft into the ground other than gravity?
In a glider it sure is not thrust!
Can you explain how any aircraft gets from a point above the surface to impact the surface (hard or soft makes no difference) without following a flight path between the start point and the impact point?
or
How if that flight path is constant that the constant point is not coincident with the impact point?
Like I said - think of a flat spin in simple terms - draw attitude, thrust, drag, lift and weight. Then draw the flight path.
Even WW made it clear that in a glider it is possible to start at Vne pointing the flight path at a chosen point and by using the drag devices to control drag reduce airspeed and using the controls to keep the flight path constant towards the point. All that is required is the correct distance to accomplish the task and good judgement.
As a glider pilot, I hope that you understand that in this example, the attitude is constantly changing as speed reduces but that the constant point is being kept in the same place by......
Does it make any difference is you think of it as "point and power" or "elevator for speed" because no matter how your mind works, your hands will do the exact same (if not you will not end up at the aiming point).
The difference with non-powered flight (regardless of aircraft type) is that if one has expended the energy too soon, there is no throttle to add thrust and the only way to keep the minimum speed is to steepen the flight path and use gravity.
In the local air Gliders can never have a steady horizontal flight path. In other words, you can not point a glider at the horizon or above. Why not? because if you point the glider on a horizontal flight path you do not have any thrust to oppose the drag and consequently airspeed will reduce. Og course, if you had an engine then you could add thrust to oppose that drag allowing you to point the flight path horizontally or even vertically if you have enough power (F16).
Regards,
DFC
In a glider it sure is not thrust!
Can you explain how any aircraft gets from a point above the surface to impact the surface (hard or soft makes no difference) without following a flight path between the start point and the impact point?
or
How if that flight path is constant that the constant point is not coincident with the impact point?
Like I said - think of a flat spin in simple terms - draw attitude, thrust, drag, lift and weight. Then draw the flight path.
Even WW made it clear that in a glider it is possible to start at Vne pointing the flight path at a chosen point and by using the drag devices to control drag reduce airspeed and using the controls to keep the flight path constant towards the point. All that is required is the correct distance to accomplish the task and good judgement.
As a glider pilot, I hope that you understand that in this example, the attitude is constantly changing as speed reduces but that the constant point is being kept in the same place by......
Does it make any difference is you think of it as "point and power" or "elevator for speed" because no matter how your mind works, your hands will do the exact same (if not you will not end up at the aiming point).
The difference with non-powered flight (regardless of aircraft type) is that if one has expended the energy too soon, there is no throttle to add thrust and the only way to keep the minimum speed is to steepen the flight path and use gravity.
In the local air Gliders can never have a steady horizontal flight path. In other words, you can not point a glider at the horizon or above. Why not? because if you point the glider on a horizontal flight path you do not have any thrust to oppose the drag and consequently airspeed will reduce. Og course, if you had an engine then you could add thrust to oppose that drag allowing you to point the flight path horizontally or even vertically if you have enough power (F16).
Regards,
DFC
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DFC, I would be happy to discuss it with you if you want understanding and not an argument, and I tried to send a private message with my phone number so you could call me - but your profile says you don't accept pm's.
I don't think it lends itself to ping-pong essays, and anyway I am fed up of writing long diatribes on PPRuNe.
Send me a pm with an email address or something, and I will give you my phone number if you want to talk. It needs to-and-fro, to get anywhere.
Chris N.
I don't think it lends itself to ping-pong essays, and anyway I am fed up of writing long diatribes on PPRuNe.
Send me a pm with an email address or something, and I will give you my phone number if you want to talk. It needs to-and-fro, to get anywhere.
Chris N.
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Chris,
Why exclude everyone else from your explanation?
Go ahead, I am sure that others may wish to read it and / or comment also.
I don't agree with such explanations being purely private conversations. Lots of people will benefit from it I am sure!
Regards,
DFC
Why exclude everyone else from your explanation?
Go ahead, I am sure that others may wish to read it and / or comment also.
I don't agree with such explanations being purely private conversations. Lots of people will benefit from it I am sure!
Regards,
DFC
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As I said, I am fed up of writing long diatribes on PPRuNe.
However, suffering from insomnia, I will do a little.
First, throw away any misconceptions you bring with you from power flying. I don’t care what you were taught there, or what you teach if you are a power instructor. This is a theory course on your conversion to gliding, assuming you have not flown a glider. The differences, if you find any, are important.
DFC, your point challenged what WW wrote about how to handle a low cable break or winch launch failure, so first come with me on a virtual winch launch. We are going to do a simulated power failure. (i.e. I am PIC/PF and I know its coming. By the way, I am always prepared for it even if I don’t know.) I will do this as a demonstration. This exercise is sufficiently dangerous that gliding instructors are told ONLY to do it as a demonstration, and not to let student pilots have a go. It is taught only on the last day of an instructor course – so that if the would-be instructor crashes the glider, it does not ruin the whole week!
The wind is nil, or very light and down the runway. As the launch starts, the wingtip runner lets go at about 10 knots of ground/air speed, and we keep the training glider balanced on its wheel by stick/elevator in about its mid position, adjusting very slightly if we need to, to keep the fuselage level. The tail skid is just off the ground.
WE DO NOT ROTATE IN ORDER TO LIFT OFF.
As the airspeed builds up, the airflow meets the wings, which are set with a positive angle of incidence. At say 37 knots (a little above normal stalling speed of say 32 knots), the glider lifts off the ground, still fuselage level. Can you tell me why, from your power flying experience?
[In a full stall, at altitude, the wings will reach about 15 degrees of alpha before stalling. That is achieved there, as a demonstration, by raising the nose above the horizon and letting speed bleed away at 1 knot per second, typically, until it drops to 32 knots. In that attitude, the tail skid is below the main wheel. On the ground, we cannot get the tail lower than the wheel. So from a lower alpha we have to have a higher airspeed to achieve sufficient lift to become airborne.]
Now we are airborne, just above the ground, fuselage still level, and accelerating. As Instructor/PIC/PF I jam the airbrake with my elbow, while holding the cable release with my left hand, while I keep the stick in the same mid-elevator position.
I jam the airbrakes because I must prevent you, the newbie in the front seat, opening them. If you did, you would do what WW cautioned against. You would slam the glider onto the ground and probably break it. (I don’t care if your power flying expertise leads you to think otherwise, you are not going to learn you are wrong in my glider by breaking it. We shall come back to that topic later, on the next flight.) If I were not an experienced instructor, and had not got through my course doing the demo on the last day and succeeded in not breaking the course glider, I might get it wrong myself.
With power from the winch, we accelerate further, stick still in the middle, and the glider without our further intervention starts to rotate gently into the climb. WE MUST NOT INCREASE THE RATE OF ROTATION BY STICK BACK/MORE UP ELEVATOR. This beautifully harmonised training glider can manage a winch launch very well on its own.
If the winch launch fails in, say, the first 50 feet of climb, it really is important to handle it very carefully. In this pre-arranged failure, the winch driver chops the power when we are about 10 feet above the ground. The fuselage is still only slightly nose high, and speed is around 40 knots. I release the (now slackened) cable, and adjust the attitude to slightly nose down before we lose flying speed. At the normal flying attitude, at 40 knots, we are on a 1 in 30 glide angle. As we approach the ground, I round out (power pilots may use the expression “flare”, but we call it rounding out). We fly parallel with the ground, main wheel about 6 inches off. As the speed bleeds off, we prevent the glider landing by easing back the stick to increase alpha and maintain lift equal to weight, The drag slows us, and eventually, the change in alpha means nose higher, tail lower, and - - the tailskid touches the ground. We cannot bring the nose up and increase alpha any more, so as the speed drops further, lift can no longer hold up our weight. The main wheel drops the last 6 inches, gently, not fast, and we have a perfect landing. Now we fully open the airbrakes to stop as soon as we can.
Second flight. We have an uneventful launch, get quite high, and I am going to demonstrate the use and effect of airbrakes. We fly straight into the (almost nil) wind. We fly at a typical approach speed, 50 knots. Check your ASI and vario as we call it (the latter is our VSI) and note the sink rate: 2.5 knots. Yes, it is in knots, not feet per minute in hundreds. Divide one by the other and we see our instantaneous glide angle – 1 in 20, about 3 degrees.
Look over the nose. In front of us is a field. It is sliding down the canopy, showing that we are overshooting it. Look in the distance. That town with the white warehouse is sliding up the canopy. We will never reach it. Now look at that water tower. As we get nearer, it is in the same position on the canopy. If we do nothing else, we will reach the ground just where it is. That is where our 1 in 20 glide slope will take us to.
When I open the airbrakes, what do you think will happen to out airspeed, our attitude, and our sink rate and glide slope, if I don’t also move the stick? Be ready for a surprise.
OK, half airbrake. (Actually, air“brake” is a bit of a misnomer at this speed, as you see.) Our attitude has not changed! Our speed is still 50 knots; the “brakes” do not slow us down perceptibly. What has changed is the sink rate – now about 5 knots. At 5/50, we now have a glide angle of about 1 in 10. Why is this so?
The “brakes” spoil some of the lift. To maintain a steady speed, and steady albeit greater sink rate, alpha has to increase on the unspoilt part of the wing.
Yes, we have more drag, but we are now going down a steeper glide path, ALTHOUGH THE ATTITUDE HAS NOT CHANGED. So the gravity component has a greater vector along our glide path, and that makes up for the increased drag so the forces are still balanced..
(There are some gliders where opening the brakes DOES cause a SMALL speed reduction at normal approach speed, and would need a very minor correction with elevator. There are others, particularly an old favourite trainer with spoilers, where they cause the nose to drop and speed to increase, and a bit of up elevator/stick back is needed. Do what you have to do to maintain approach speed, but in this trainer the stick movement required and attitude change is very slight.)
In that demo, half brake increased the sink rate, virtually instantaneously, to 5 knots. I will show you full brake. Now the sink rate is - - 12 knots.
You remember that I jammed the airbrake lever when we had a low cable break/power failure. If you had just opened the brakes, we would have experienced an immediate increase in sink rate. Hitting the ground with a vertical component of 10 or 12 knots would break the wings. Even 5 knots sink rate would bend the undercarriage and damage the fuselage.
If I ever have another virtual flight with you, I might be able to show why we always use elevator to control speed and the brakes to control sink rate to achieve a stable final approach, contrary to what a boofy power pilot said, but that will do for today. And I am fed up with instructing, so I probably won’t.
Chris N.
However, suffering from insomnia, I will do a little.
First, throw away any misconceptions you bring with you from power flying. I don’t care what you were taught there, or what you teach if you are a power instructor. This is a theory course on your conversion to gliding, assuming you have not flown a glider. The differences, if you find any, are important.
DFC, your point challenged what WW wrote about how to handle a low cable break or winch launch failure, so first come with me on a virtual winch launch. We are going to do a simulated power failure. (i.e. I am PIC/PF and I know its coming. By the way, I am always prepared for it even if I don’t know.) I will do this as a demonstration. This exercise is sufficiently dangerous that gliding instructors are told ONLY to do it as a demonstration, and not to let student pilots have a go. It is taught only on the last day of an instructor course – so that if the would-be instructor crashes the glider, it does not ruin the whole week!
The wind is nil, or very light and down the runway. As the launch starts, the wingtip runner lets go at about 10 knots of ground/air speed, and we keep the training glider balanced on its wheel by stick/elevator in about its mid position, adjusting very slightly if we need to, to keep the fuselage level. The tail skid is just off the ground.
WE DO NOT ROTATE IN ORDER TO LIFT OFF.
As the airspeed builds up, the airflow meets the wings, which are set with a positive angle of incidence. At say 37 knots (a little above normal stalling speed of say 32 knots), the glider lifts off the ground, still fuselage level. Can you tell me why, from your power flying experience?
[In a full stall, at altitude, the wings will reach about 15 degrees of alpha before stalling. That is achieved there, as a demonstration, by raising the nose above the horizon and letting speed bleed away at 1 knot per second, typically, until it drops to 32 knots. In that attitude, the tail skid is below the main wheel. On the ground, we cannot get the tail lower than the wheel. So from a lower alpha we have to have a higher airspeed to achieve sufficient lift to become airborne.]
Now we are airborne, just above the ground, fuselage still level, and accelerating. As Instructor/PIC/PF I jam the airbrake with my elbow, while holding the cable release with my left hand, while I keep the stick in the same mid-elevator position.
I jam the airbrakes because I must prevent you, the newbie in the front seat, opening them. If you did, you would do what WW cautioned against. You would slam the glider onto the ground and probably break it. (I don’t care if your power flying expertise leads you to think otherwise, you are not going to learn you are wrong in my glider by breaking it. We shall come back to that topic later, on the next flight.) If I were not an experienced instructor, and had not got through my course doing the demo on the last day and succeeded in not breaking the course glider, I might get it wrong myself.
With power from the winch, we accelerate further, stick still in the middle, and the glider without our further intervention starts to rotate gently into the climb. WE MUST NOT INCREASE THE RATE OF ROTATION BY STICK BACK/MORE UP ELEVATOR. This beautifully harmonised training glider can manage a winch launch very well on its own.
If the winch launch fails in, say, the first 50 feet of climb, it really is important to handle it very carefully. In this pre-arranged failure, the winch driver chops the power when we are about 10 feet above the ground. The fuselage is still only slightly nose high, and speed is around 40 knots. I release the (now slackened) cable, and adjust the attitude to slightly nose down before we lose flying speed. At the normal flying attitude, at 40 knots, we are on a 1 in 30 glide angle. As we approach the ground, I round out (power pilots may use the expression “flare”, but we call it rounding out). We fly parallel with the ground, main wheel about 6 inches off. As the speed bleeds off, we prevent the glider landing by easing back the stick to increase alpha and maintain lift equal to weight, The drag slows us, and eventually, the change in alpha means nose higher, tail lower, and - - the tailskid touches the ground. We cannot bring the nose up and increase alpha any more, so as the speed drops further, lift can no longer hold up our weight. The main wheel drops the last 6 inches, gently, not fast, and we have a perfect landing. Now we fully open the airbrakes to stop as soon as we can.
Second flight. We have an uneventful launch, get quite high, and I am going to demonstrate the use and effect of airbrakes. We fly straight into the (almost nil) wind. We fly at a typical approach speed, 50 knots. Check your ASI and vario as we call it (the latter is our VSI) and note the sink rate: 2.5 knots. Yes, it is in knots, not feet per minute in hundreds. Divide one by the other and we see our instantaneous glide angle – 1 in 20, about 3 degrees.
Look over the nose. In front of us is a field. It is sliding down the canopy, showing that we are overshooting it. Look in the distance. That town with the white warehouse is sliding up the canopy. We will never reach it. Now look at that water tower. As we get nearer, it is in the same position on the canopy. If we do nothing else, we will reach the ground just where it is. That is where our 1 in 20 glide slope will take us to.
When I open the airbrakes, what do you think will happen to out airspeed, our attitude, and our sink rate and glide slope, if I don’t also move the stick? Be ready for a surprise.
OK, half airbrake. (Actually, air“brake” is a bit of a misnomer at this speed, as you see.) Our attitude has not changed! Our speed is still 50 knots; the “brakes” do not slow us down perceptibly. What has changed is the sink rate – now about 5 knots. At 5/50, we now have a glide angle of about 1 in 10. Why is this so?
The “brakes” spoil some of the lift. To maintain a steady speed, and steady albeit greater sink rate, alpha has to increase on the unspoilt part of the wing.
Yes, we have more drag, but we are now going down a steeper glide path, ALTHOUGH THE ATTITUDE HAS NOT CHANGED. So the gravity component has a greater vector along our glide path, and that makes up for the increased drag so the forces are still balanced..
(There are some gliders where opening the brakes DOES cause a SMALL speed reduction at normal approach speed, and would need a very minor correction with elevator. There are others, particularly an old favourite trainer with spoilers, where they cause the nose to drop and speed to increase, and a bit of up elevator/stick back is needed. Do what you have to do to maintain approach speed, but in this trainer the stick movement required and attitude change is very slight.)
In that demo, half brake increased the sink rate, virtually instantaneously, to 5 knots. I will show you full brake. Now the sink rate is - - 12 knots.
You remember that I jammed the airbrake lever when we had a low cable break/power failure. If you had just opened the brakes, we would have experienced an immediate increase in sink rate. Hitting the ground with a vertical component of 10 or 12 knots would break the wings. Even 5 knots sink rate would bend the undercarriage and damage the fuselage.
If I ever have another virtual flight with you, I might be able to show why we always use elevator to control speed and the brakes to control sink rate to achieve a stable final approach, contrary to what a boofy power pilot said, but that will do for today. And I am fed up with instructing, so I probably won’t.
Chris N.
Last edited by chrisN; 12th Jun 2009 at 02:35.
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OK. I see that you are talking about spoilers and not about airbrakes.
However, you have to admit that when you roundout that you use the controls to point the aircraft at the horizon and keep the aircraft pointing at the horizon until the tailskid touches?
Yes, spoilers will reduce lift and the flight path will be changing until the gravity versus lift and vertical component of drag balance.
However, if your descent velocity with 50Kt and spoilers extended will hit the water tower and you turn on your engine and apply full throttle, what controls will you use to keep the aircraft travelling towards (pointing at) the water tower (while accelerating)?
but we are now going down a steeper glide path, ALTHOUGH THE ATTITUDE HAS NOT CHANGED
Please note that the "point" in "point and power" does not refer to attitude (where the nose is pointing). It refers to where the flight path is taking you - the constant point and using the controls to put the constant point where you want it to be.
To explain further, imagine an approach with full flap on a 3 deg path. This requires attitude of x at speed y to point the flight path at the aiming point.
If the approach is flown flapless then the attitude will be higher eventhough the aircraft is still flying the 3 degree path and is stillhaving it's flight path pointed at the aiming point.
Regards,
DFC
However, you have to admit that when you roundout that you use the controls to point the aircraft at the horizon and keep the aircraft pointing at the horizon until the tailskid touches?
Yes, spoilers will reduce lift and the flight path will be changing until the gravity versus lift and vertical component of drag balance.
However, if your descent velocity with 50Kt and spoilers extended will hit the water tower and you turn on your engine and apply full throttle, what controls will you use to keep the aircraft travelling towards (pointing at) the water tower (while accelerating)?
but we are now going down a steeper glide path, ALTHOUGH THE ATTITUDE HAS NOT CHANGED
To explain further, imagine an approach with full flap on a 3 deg path. This requires attitude of x at speed y to point the flight path at the aiming point.
If the approach is flown flapless then the attitude will be higher eventhough the aircraft is still flying the 3 degree path and is stillhaving it's flight path pointed at the aiming point.
Regards,
DFC
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DFC, I was talking about airbrakes and spoilers (UK terminology) or spoilers (covers both in US terminology). I am not going to debate further – you would not have a 15 minute 2-way telephone call and I am not going to spend another 2 hours writing. Stick to power. You don’t know enough about gliding to pontificate on it.
Chris N.
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Stick to power. You don’t know enough about gliding to pontificate on it.
The only thing an engine gives us is the thrust force acting along the thrust line which we can use to maintain a constant airspeed without having to rely on gravity.
Regards,
DFC
The engine lets you do it without losing height.
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As we approach the ground, I round out (power pilots may use the expression “flare”, but we call it rounding out). We fly parallel with the ground, main wheel about 6 inches off. As the speed bleeds off, we prevent the glider landing by easing back the stick to increase alpha and maintain lift equal to weight, The drag slows us, and eventually, the change in alpha means nose higher, tail lower, and - - the tailskid touches the ground.
Let me put the above part of your quote in the context of pointing the aircraft's flight path;
As we approach the ground you round out. You change where the flight path was pointing (towards the constant point) so that it now points at the horizon. This is done by adjusting the attitude (raising the nose) with the elevator.
Now that the flight path is pointing at the horizon the aircraft flies level (the flight path is horizontal).
As the aircraft slows, the attitude must be gradually increased to keep the flight path pointing at the horizon. Eventually the tail being in contact with the ground no further increase in attitude is possible and as the speed reduces further the flight path gradually changes from horizontal to a descending one.
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If you had an engine you could have increased the power from idle to add some thrust and travel along the runway with just the tail in contact. What height have you gained in return for that increase in thrust?
As I said "point and power" or "attitude for speed, power for altitude" are simply two different mental pictures of the same actions and reactions.
Going from idle to full throttle has the exact same effect on the aircraft and requires exactly the same control inputs no matter if you think point and power or power for height.
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Cats five,
Which force requires the glider to descend and prevents sustained horizontal flight?
Hint - Gravity is not the answer!!
Regards,
DFC
I was taught that there are three forces working on a glider - gravity, lift and drag. Since there has to be a force in a given direction to accelerate something in that direction, and since the only force with a downward component is gravity, I therefore struggle with what force is responsible for the downward component of a glider's path (that is a 1g, steady speed glide if it's not gravity.
The forward component of the lift makes it go forwards. The speed is whatever it settles to when the forward component of the lift is balanced by the backward component of the drag. The lower the nose the more forward component the lift has so the faster the speed the glider settles to.
BTW NASA agrees with me about the three forces though for some bizarre reason they rename gravity to weight:
Forces on a Glider
(unless of course your answer is weight rather than gravity - but remember, no gravity, no weight)
The forward component of the lift makes it go forwards. The speed is whatever it settles to when the forward component of the lift is balanced by the backward component of the drag. The lower the nose the more forward component the lift has so the faster the speed the glider settles to.
BTW NASA agrees with me about the three forces though for some bizarre reason they rename gravity to weight:
Forces on a Glider
(unless of course your answer is weight rather than gravity - but remember, no gravity, no weight)
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Cats five,
The answer I had in mind was of course drag.
Why is it drag and not gravity that forces a glider to descend and prevents it from sustained horizontal flight?
The First answer is that if there was no drag then there would be no requirement to have a thrust force to oppose it. Imagine getting a tow along at 100Kt in a glider with zero drag. When that pull force was removed i.e. thrust disapears, since you have zero thrust and zero drag you can fly level for ever!!!!! You would have to increase drag to get yourself down!! - think BRS !!!
Secondly but more relevant to reality - if you fly at the speed for minimum drag, then you can stay airbourne for say 30 minutes. Any speed above or below that speed means that you spend less time in the air because you are forced to descend in order to harness the force of gravity and use it to offset the drag.
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To explain the NASA terms - Gravity is the acceleration force caused by the mass of the earth attracting other bodies. This force varies over the surface of the earth but on average is 9.8 m/s/s near the surface.
The mass of an object is simply the amount of matter in an object which would not chnage even if you put it in outer space (almost zero gravity).
Weight is the force caused by the combination of the mass and gravity.
Hence why you weigh x on earth and somewhat less on the moon despite your mass being the same in both places.
Therefore, your aircraft is loaded to a certain mass (airframe, balast, you) and that mass combined with gravity gives a force called weight which requires a force to oppose it for steady flight.
If you had the same atmosphere on the moon, you would use less lift for level flight than you do on earth!!
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Regards,
DFC
The answer I had in mind was of course drag.
Why is it drag and not gravity that forces a glider to descend and prevents it from sustained horizontal flight?
The First answer is that if there was no drag then there would be no requirement to have a thrust force to oppose it. Imagine getting a tow along at 100Kt in a glider with zero drag. When that pull force was removed i.e. thrust disapears, since you have zero thrust and zero drag you can fly level for ever!!!!! You would have to increase drag to get yourself down!! - think BRS !!!
Secondly but more relevant to reality - if you fly at the speed for minimum drag, then you can stay airbourne for say 30 minutes. Any speed above or below that speed means that you spend less time in the air because you are forced to descend in order to harness the force of gravity and use it to offset the drag.
--------
To explain the NASA terms - Gravity is the acceleration force caused by the mass of the earth attracting other bodies. This force varies over the surface of the earth but on average is 9.8 m/s/s near the surface.
The mass of an object is simply the amount of matter in an object which would not chnage even if you put it in outer space (almost zero gravity).
Weight is the force caused by the combination of the mass and gravity.
Hence why you weigh x on earth and somewhat less on the moon despite your mass being the same in both places.
Therefore, your aircraft is loaded to a certain mass (airframe, balast, you) and that mass combined with gravity gives a force called weight which requires a force to oppose it for steady flight.
If you had the same atmosphere on the moon, you would use less lift for level flight than you do on earth!!
-------------
Regards,
DFC
Last edited by DFC; 15th Jun 2009 at 23:06.
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G/S control
No need to make it hard, realize flying is a continual series of SMALL corrections which are fixed by small changes in both pitch and power before the big corrections which require the above plus trim when a large power change is required. Easiest to have a reference to power requirements for the landing weight and pay particular attention to deck angle and try to minimize excursions from that pitch reference. Both go together whether 150 or 744.
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I remember doing my CPL skills test with an ex-RAF CFS chap. Anyway, casually discussing his career whilst on the navex he told me that he had been with CFS at Little Rissington. I offered that he must have loved his time there. His response:
"Not really. Too many A1 QFIs pontificating with each other about how aircraft actually work, producing complex theories that just confuse the students".
I passed my skills test and learnt something at the same time.
"Not really. Too many A1 QFIs pontificating with each other about how aircraft actually work, producing complex theories that just confuse the students".
I passed my skills test and learnt something at the same time.
Sorry, I don't buy that drag causes a glider to glide down unless it has a downward component, which I don't think it does. To get an object to move in a direction you have to impart a force in that direction. Basic Newtonian mechanics surely?
Consider a glider in a gravity-free situation. Import some energy and it will continue in whatever direction it was pushed in until the drag stops it. (free-fall is NOT a zero-gravity situation) It still has drag, but with no gravity will only change course towards the lift generated by the wings. If you stop thinking about a glider and start thinking about (for example) a small steel ball - something that generates no lift - it simply continues in a straight line until drag stops it.
If you have no drag but gravity and lift, impart another glider with some energy. It doesn't matter what it's initial path is or how much lift the wings are generating, sooner or later gravity will have it on the ground - unless you gave it so much energy the wings were able to generate enough lift to take it into orbit!
Of course no gravity is an absurdity since a glider is forever trading potential energy for kinetic energy - managing that trade-off is the skill of soaring. And with no gravity there is no potential energy to trade, plus also no atmosphere to fly in to generate lift - and drag...
Back to the real world - in my view my glider goes down because of gravity, the relationship between lift and drag at that speed determine what the sink rate is.
The relevence to landing a glider? If ab initios flying draggy trainers get in the habit of pointing at the ground with the nose (e.g. lowering the nose) then it will work to a degree. The drag will stop speed building up to a degree, it builds up relatively slowly, that sort of glider will have much more sink at faster speeds. Do the same thing in a Discus and instead you simply get faster whilst continuing to go down at much the same rate... In other words it bites you, especially if trying to squeeze the glider in a rather small field. BUT they need know nothing about the aerodynamics (IMHO) to be able to do accurate field landings, just of how to fly the glider.
Since I don't fly power I have no idea what (if any) the relevence for power flying this has, except in the broadest sense. Learn the right way of doing things as whilst the wrong way may work, sooner or later it will come back and bite you.
Consider a glider in a gravity-free situation. Import some energy and it will continue in whatever direction it was pushed in until the drag stops it. (free-fall is NOT a zero-gravity situation) It still has drag, but with no gravity will only change course towards the lift generated by the wings. If you stop thinking about a glider and start thinking about (for example) a small steel ball - something that generates no lift - it simply continues in a straight line until drag stops it.
If you have no drag but gravity and lift, impart another glider with some energy. It doesn't matter what it's initial path is or how much lift the wings are generating, sooner or later gravity will have it on the ground - unless you gave it so much energy the wings were able to generate enough lift to take it into orbit!
Of course no gravity is an absurdity since a glider is forever trading potential energy for kinetic energy - managing that trade-off is the skill of soaring. And with no gravity there is no potential energy to trade, plus also no atmosphere to fly in to generate lift - and drag...
Back to the real world - in my view my glider goes down because of gravity, the relationship between lift and drag at that speed determine what the sink rate is.
The relevence to landing a glider? If ab initios flying draggy trainers get in the habit of pointing at the ground with the nose (e.g. lowering the nose) then it will work to a degree. The drag will stop speed building up to a degree, it builds up relatively slowly, that sort of glider will have much more sink at faster speeds. Do the same thing in a Discus and instead you simply get faster whilst continuing to go down at much the same rate... In other words it bites you, especially if trying to squeeze the glider in a rather small field. BUT they need know nothing about the aerodynamics (IMHO) to be able to do accurate field landings, just of how to fly the glider.
Since I don't fly power I have no idea what (if any) the relevence for power flying this has, except in the broadest sense. Learn the right way of doing things as whilst the wrong way may work, sooner or later it will come back and bite you.
Last edited by cats_five; 16th Jun 2009 at 13:59. Reason: freefall is not zero G