Glide path control on final.
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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
For example - fly at min drag speed - 30 minutes airbourne
Fly 15 knots above min drag speed or 15 knots below then the time in the air will not only be less but will be the same no matter if you fly 15 knots faster or 15 knots slower (assuming that drag is equal at both speeds).
SO why does changing the amount of drag change the time spent in the air?
Regards,
DFC
Last edited by DFC; 16th Jun 2009 at 19:49.
Unless the laws of physics have changed since I was at school there is no way that a force can cause an object to move in a direction it doesn't have a component in. So far as I am aware there is no downward component to drag so by the Newtonian physics I was taught there is no way that drag is responsible for the downward movement of the glider.
Think of my no-gravity and no-drag scenarious. Do you find a flaw in them? You also haven't disputed my statement that there is no downward component to drag, unless you are trying to do so by the 'guess what's in my mind' method sadly all too beloved of some instructors.
If you can produce a nice diagram showing how drag has a downward component that would be great.
PS try reading the following, especially page 4:
http://williams.best.vwh.net/smxgigpdf/SMX99tot.pdf
Think of my no-gravity and no-drag scenarious. Do you find a flaw in them? You also haven't disputed my statement that there is no downward component to drag, unless you are trying to do so by the 'guess what's in my mind' method sadly all too beloved of some instructors.
If you can produce a nice diagram showing how drag has a downward component that would be great.
PS try reading the following, especially page 4:
http://williams.best.vwh.net/smxgigpdf/SMX99tot.pdf
Last edited by cats_five; 16th Jun 2009 at 20:09.
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I think that you will find that when in descending flight drag has an upward component!!
Again I ask you to explain why reducing drag is so important to glider pilots and why if two gliders of the exact same weight and the exact same wing are released at the same height, the one that has the least drag will remain airbourne the longest?
Regards,
DFC
Again I ask you to explain why reducing drag is so important to glider pilots and why if two gliders of the exact same weight and the exact same wing are released at the same height, the one that has the least drag will remain airbourne the longest?
Regards,
DFC
Last edited by DFC; 16th Jun 2009 at 20:38.
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Sorry, my post crossed with your amendment.
I will take some time to read the pdf.
However, on quick inspection looking at page 2 there are some problems with his drawings;
Straight and level - OK
Descent - Thrust is less than drag and so this object will be decelerating. No force shown to oppose weight.
Climb - Shows an excess of Thrust which will cause the aircraft to accelerate. Again no force opposes weight in the diagram.
Angle of attack is only one aspect of Coeficient of lift and drag - other aspects can also have an effect.
The wing does not stall at CL Max. It stalls when the angle of attack for CL max is exceeded.
He is correct to say that the best glide is at a tangent to the drag curve.
Unfortunately his diagram is not of the drag curve!
The last page makes sense - in a glider - keep drag down to fly for longer.
As for page 4 - a bit incomplete but it does show that Drag has a verttical component.
Regards,
DFC
I will take some time to read the pdf.
However, on quick inspection looking at page 2 there are some problems with his drawings;
Straight and level - OK
Descent - Thrust is less than drag and so this object will be decelerating. No force shown to oppose weight.
Climb - Shows an excess of Thrust which will cause the aircraft to accelerate. Again no force opposes weight in the diagram.
Angle of attack is only one aspect of Coeficient of lift and drag - other aspects can also have an effect.
The wing does not stall at CL Max. It stalls when the angle of attack for CL max is exceeded.
He is correct to say that the best glide is at a tangent to the drag curve.
Unfortunately his diagram is not of the drag curve!
The last page makes sense - in a glider - keep drag down to fly for longer.
As for page 4 - a bit incomplete but it does show that Drag has a verttical component.
Regards,
DFC
Since drag has an upward not downward component it cannot make a glider go down. Flying at more or less than min sink decreases the upward resultant of lift & drag so that glider is affected more by gravity. The bigger the upward component the longer the glider stays aloft, the smaller the upward component the quicker gravity pulls it back to the ground.
As to which glider remains airborne longest (not the same as which goes furthest from a given height), lack of drag is clearly not the only influence. The glider which produces the biggest upward resultant of drag *and* lift is the one that stays up longest. So, drag is only the determining factor when they have the same mass *and* they are both producing the same lift. Possibly we should require they have the same wing loading as well....
(and gliders that stay up well are frequenty not gliders that can go places, especially not upwind places)
If you rephrase to say that drag has a big influence in how long a glider can resist gravity then yes, obviously so - as does lift. But since gravity is the only downward force, gravity is the force making a glider lose height. Unless Newtonian mechanics have stopped working.
As to which glider remains airborne longest (not the same as which goes furthest from a given height), lack of drag is clearly not the only influence. The glider which produces the biggest upward resultant of drag *and* lift is the one that stays up longest. So, drag is only the determining factor when they have the same mass *and* they are both producing the same lift. Possibly we should require they have the same wing loading as well....
(and gliders that stay up well are frequenty not gliders that can go places, especially not upwind places)
If you rephrase to say that drag has a big influence in how long a glider can resist gravity then yes, obviously so - as does lift. But since gravity is the only downward force, gravity is the force making a glider lose height. Unless Newtonian mechanics have stopped working.
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Drag has an upward component on every descending object.
Jump out of your glider at 10,000ft and you will accelerate at 9.8 m/s/s until drag acting opposite to gravity increases ehoung to stop the acceleration (about 200mph I think).
Now pop your chute.
The extra drag from the chute (acting upwards) reduces your descent speed and keeps you in the air longer than would have been the case without one.
But drag also has a component along the flight path. In the falling case the flight path is vertical and all the drag acts along the flight path. However, in the glide some of the drag acts upwards while the bigger part acts along the flight path.
You are correct to say that gravity is the force that acting on a glider in a downward sense. However, the glider's ability to resist that force is directly dependent on drag.
What I find amusing about this discussion is how the whole idiology can be changed. Point and power people would probably press the point that extending a speedbrake (not spoiler) would increase drag and thus slow the aircraft. You I think see the result of using a speed brake as causing height to be lost quicker.
Are you not thus saying that the increase in drag by extending the speed brake has caused the aircraft to descend more quickly and thus varying the drag using speedbrake has a direct effect on the amount of time that the aircraft can remain airbourne?
Regards,
DFC
Jump out of your glider at 10,000ft and you will accelerate at 9.8 m/s/s until drag acting opposite to gravity increases ehoung to stop the acceleration (about 200mph I think).
Now pop your chute.
The extra drag from the chute (acting upwards) reduces your descent speed and keeps you in the air longer than would have been the case without one.
But drag also has a component along the flight path. In the falling case the flight path is vertical and all the drag acts along the flight path. However, in the glide some of the drag acts upwards while the bigger part acts along the flight path.
You are correct to say that gravity is the force that acting on a glider in a downward sense. However, the glider's ability to resist that force is directly dependent on drag.
What I find amusing about this discussion is how the whole idiology can be changed. Point and power people would probably press the point that extending a speedbrake (not spoiler) would increase drag and thus slow the aircraft. You I think see the result of using a speed brake as causing height to be lost quicker.
Are you not thus saying that the increase in drag by extending the speed brake has caused the aircraft to descend more quickly and thus varying the drag using speedbrake has a direct effect on the amount of time that the aircraft can remain airbourne?
Regards,
DFC
Arghhh!!!! 
You asked:
That is awful English. I don't know why you used the word 'requires' - I suspect you haven't really asked the question you are trying to answer, which is what force or forces control the speed at which a glider descends.
If the word 'makes' was substitued for 'requires' (which is how I read it) the only possible answer to 'what makes the glider descend' is gravity.
Sheer utter b
s. It is mitigated by the total of all the upward component of the aerodynamic forces operating on the glider, conventionally described as lift and drag. Or do you think that that the upward component of the lift in the diagram in the URL I provided is a different kind of lift that cannot offset gracity?
You asked:
Which force requires the glider to descend and prevents sustained horizontal flight?
Hint - Gravity is not the answer!!
Hint - Gravity is not the answer!!
If the word 'makes' was substitued for 'requires' (which is how I read it) the only possible answer to 'what makes the glider descend' is gravity.
However, the glider's ability to resist that force is directly dependent on drag.
s. It is mitigated by the total of all the upward component of the aerodynamic forces operating on the glider, conventionally described as lift and drag. Or do you think that that the upward component of the lift in the diagram in the URL I provided is a different kind of lift that cannot offset gracity?
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Can I bring a bit of simpleton pilot into the discussion?
Four 'forces' act on a powered aircraft. Weight/Mass (not much that we can do about that), lift, thrust and drag. In any stage of flight where the aircraft is not accelerating (ie changing velocity - I'll sidestep the fact that gravity is actually an acceleration in space) these forces are balanced. So, if an aircraft is descending on final, we have lift & drag balanced against mass & thrust (this may not actually be the case as in some scenarios elements of the lift vector may act as thrust). In this scenario, and indeed almost all scenarios, lift does not equal mass and thrust does not equal drag - the forces involved are not at right angles to each other but the overall result is a steady state. (I've got some pretty atpl notes that show lift is not at right angles to relative air flow)
If Biggles changes one of these forces (either in size or direction) the whole system will adjust to re-achieve a balanced state. Whether the pilot induced change is pitch or thrust, the inter-relationship is such that the other will also change unless Biggles doesn't want it to.
What does that mean? If Biggles uses PnP or 'power for flightpath' he will have to use the other control to maintain one element of balance. If he pitches down to increase speed, he will have to add power to maintain flightpath. Conversely, if he uses power to increase speed, he will have to adjust pitch (AoA) to maintain flightpath. The order in which he does this is largely irrelevant and we should actually be teaching that the controls be used in unison.
Of course, I am just a simpleton so I may be completely wrong.
Four 'forces' act on a powered aircraft. Weight/Mass (not much that we can do about that), lift, thrust and drag. In any stage of flight where the aircraft is not accelerating (ie changing velocity - I'll sidestep the fact that gravity is actually an acceleration in space) these forces are balanced. So, if an aircraft is descending on final, we have lift & drag balanced against mass & thrust (this may not actually be the case as in some scenarios elements of the lift vector may act as thrust). In this scenario, and indeed almost all scenarios, lift does not equal mass and thrust does not equal drag - the forces involved are not at right angles to each other but the overall result is a steady state. (I've got some pretty atpl notes that show lift is not at right angles to relative air flow)
If Biggles changes one of these forces (either in size or direction) the whole system will adjust to re-achieve a balanced state. Whether the pilot induced change is pitch or thrust, the inter-relationship is such that the other will also change unless Biggles doesn't want it to.
What does that mean? If Biggles uses PnP or 'power for flightpath' he will have to use the other control to maintain one element of balance. If he pitches down to increase speed, he will have to add power to maintain flightpath. Conversely, if he uses power to increase speed, he will have to adjust pitch (AoA) to maintain flightpath. The order in which he does this is largely irrelevant and we should actually be teaching that the controls be used in unison.
Of course, I am just a simpleton so I may be completely wrong.
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Boofhead,
coming back to your original posts, the main points of which I've tried to list below,
As I've already posted, in the gliding community we teach elevator for speed, brakes for ROD. This is standard practice in our discipline and I appreciate that it is not always taught this way in power flying but I believe some of the issues you raise would be reduced by using the "traditional" method.
The first thing we teach is speed control on the approach. An appropriate approach speed is nominated and that is the speed required on the approach - not plus 10 minus 5 knots - the nominated speed. Clearly there will be some variance but the option is never offered - if you offer it, the student will take it and add some as well. At this stage it doesn't matter where they land (the instructor will be using the brakes initially anyway) - without the ability to fly a steady approach speed any reference point in the canopy will be up and down with the frequency of Paris Hiltons pants. Next we teach how to adjust the position of the reference point using the brakes for ROD.
Most people will recognise that in times of stress or danger a student pilot will want to pull back to avoid hitting the ground - getting them to push on the stick to recover from a deep stall or spin highlights this. Translate this to the scenario when pilots are moving back on the stick and losing speed as they approach the runway. By being in the habit of controlling speed with elevtor and correct monitoring of the airspeed the tendancy to ease back will be resisted. A common comparable phenomina in gliding is a field landing into a upward sloping field. The picture looks totally wrong and every instinct is to flatten the approach to make it look right, followed by the inevitable stall/spin/accident report/bodybag etc.
The other advantage that I consider for any pilot is that one day that fan on the front may stop turning. If you learn to rely on it to maintain approach speed, when the sh1t hits and the fan does stop, a pilot is faced with using an unfamiliar method at a time when workload is high and he needs less problems not more.
I totally agree that spot landings should be a matter of course and that is what we teach. Every pilot I fly with including the junior ones consistently land within a fuselage length of the takeoff point and often more accurately. After a particularly long day flying personnel around I flew with a power instructor who had never flown in a glider. When we landed, he commented about his amazment that all the pilots had finished every flight that day where it had started from at the launch point. He finished by adding that a lot of power pilots couldn't manage that consistancy. Clearly you are not the only one who gets frustrated at sloppy flying, and as we all know the flight only ends when the wing goes down.
I am sure that someone (I wonder who?) will tell me that I don't know what I'm talking about but I don't need to justify my theory or flying skills on this forum, I do that with my CFI.
What I have explained is:
how we do it,
why we do it and
why it translates into the power world, even if only on the GA kit.
Air Marshal or Airman, jet jockey or desk jockey they get taught the same and fly the same or they get the same debrief telling them why it's wrong
regards
coming back to your original posts, the main points of which I've tried to list below,
They do not know how to get onto a correct glidepath and canot maintain one even if they recognise it. The particular bane is the student (and some of these guys/girls are commercial pilots) who lets the nose come up approaching the runway and loses speed, then has to dive for the runway or drops on in a partial stall.
I am only talking about the last 500 feet. And only for students or people having trouble with landing approaches.
Speed can vary a little. Later on, speed becomes more important but initially I tell my students that so long as they are plus 10 and minus 5 let it go.
The first thing we teach is speed control on the approach. An appropriate approach speed is nominated and that is the speed required on the approach - not plus 10 minus 5 knots - the nominated speed. Clearly there will be some variance but the option is never offered - if you offer it, the student will take it and add some as well. At this stage it doesn't matter where they land (the instructor will be using the brakes initially anyway) - without the ability to fly a steady approach speed any reference point in the canopy will be up and down with the frequency of Paris Hiltons pants. Next we teach how to adjust the position of the reference point using the brakes for ROD.
Most people will recognise that in times of stress or danger a student pilot will want to pull back to avoid hitting the ground - getting them to push on the stick to recover from a deep stall or spin highlights this. Translate this to the scenario when pilots are moving back on the stick and losing speed as they approach the runway. By being in the habit of controlling speed with elevtor and correct monitoring of the airspeed the tendancy to ease back will be resisted. A common comparable phenomina in gliding is a field landing into a upward sloping field. The picture looks totally wrong and every instinct is to flatten the approach to make it look right, followed by the inevitable stall/spin/accident report/bodybag etc.
The other advantage that I consider for any pilot is that one day that fan on the front may stop turning. If you learn to rely on it to maintain approach speed, when the sh1t hits and the fan does stop, a pilot is faced with using an unfamiliar method at a time when workload is high and he needs less problems not more.
I totally agree that spot landings should be a matter of course and that is what we teach. Every pilot I fly with including the junior ones consistently land within a fuselage length of the takeoff point and often more accurately. After a particularly long day flying personnel around I flew with a power instructor who had never flown in a glider. When we landed, he commented about his amazment that all the pilots had finished every flight that day where it had started from at the launch point. He finished by adding that a lot of power pilots couldn't manage that consistancy. Clearly you are not the only one who gets frustrated at sloppy flying, and as we all know the flight only ends when the wing goes down.
I am sure that someone (I wonder who?) will tell me that I don't know what I'm talking about but I don't need to justify my theory or flying skills on this forum, I do that with my CFI.
What I have explained is:
how we do it,
why we do it and
why it translates into the power world, even if only on the GA kit.
Air Marshal or Airman, jet jockey or desk jockey they get taught the same and fly the same or they get the same debrief telling them why it's wrong
regards
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What does that mean? If Biggles uses PnP or 'power for flightpath' he will have to use the other control to maintain one element of balance. If he pitches down to increase speed, he will have to add power to maintain flightpath. Conversely, if he uses power to increase speed, he will have to adjust pitch (AoA) to maintain flightpath. The order in which he does this is largely irrelevant and we should actually be teaching that the controls be used in unison.
==============
Cats Five,
Can you please just answer the question;
Why is it that if the amount of lift remains the same and the weight remains the same but drag is increased then the aircraft will stay in the air for less time?
Think glider or B747 it does not matter more drag means less time in the air!!
Regards,
DFC
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Someone posted that lift & drag are interrelated - altering one alters the other - so talking about drag only is a nonsense.
Lift and induced drag are related but airspeed is also a factor.
However, form drag or parasite drag or interference drag as examples have no relation to lift.
As an example that is common to your gliding and also powered flying;
If you fly with the wheel extended, drag will be increased but there will be no effect on lift. The extra drag will reduce the time you can remain airborne.
If you fly a B747 with the wheels extended, drag will be increased and the amount of time that the aircraft can remain airborne is also reduced.
So why is it that powered or not, it is drag that an influence on the time that the aircraft can remain in the air?
Regards,
DFC
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<shrug>
For what its worth, I've always found it easiest to teach gliding performance by remembering that lift and drag are a merely a 'convenient' way of resolving the total aerodynamic reaction.
If we take thrust to be zero (since we're talking gliders) that then leaves us rather nicely with two forces; Weight and Total Aerodynamic Reaction. If they are equal and opposite then the flight path is constant; we are in a steady glide.
This could be a house brick falling at terminal velocity, or a theoretically perfect glider flying level.
Gliding range is all about how that total aerodynamic reaction is provided. Since drag is defined as parallel to the flight path, and lift perpendicular, our house brick clearly must have an L/D ratio of zero.. entirely reasonable. Likewise, our perfect glider must have an infinite L/D ratio, again entirely reasonable (if not quite achieveable in reality).
Gliding endurance is all about the ratio between weight and the vector addition of CL and CD. If an object can generate a big total aerodynamic force from a small airspeed then its weight will be balanced out with a lower terminal velocity, so its 'gliding' endurance is larger. e.g. a lead ball versus a feather.
So, I'm with DFC in that no drag = no descent.
But I think you are both talking slightly at cross purpose regarding gliding because you're swapping interchangeably between considerations of range and endurance.
Adding drag decreases range but increases endurance (assuming of course that the optimum speed is flown in each case), this is best understood by reducto ad absurdum; deploy an infinitely large parachute... what happens? Glide path tends to vertical, endurance tends to infinite.
(All of which diverges someone from original topic... but so what... I'm bored)
regards
pb
For what its worth, I've always found it easiest to teach gliding performance by remembering that lift and drag are a merely a 'convenient' way of resolving the total aerodynamic reaction.
If we take thrust to be zero (since we're talking gliders) that then leaves us rather nicely with two forces; Weight and Total Aerodynamic Reaction. If they are equal and opposite then the flight path is constant; we are in a steady glide.
This could be a house brick falling at terminal velocity, or a theoretically perfect glider flying level.
Gliding range is all about how that total aerodynamic reaction is provided. Since drag is defined as parallel to the flight path, and lift perpendicular, our house brick clearly must have an L/D ratio of zero.. entirely reasonable. Likewise, our perfect glider must have an infinite L/D ratio, again entirely reasonable (if not quite achieveable in reality).
Gliding endurance is all about the ratio between weight and the vector addition of CL and CD. If an object can generate a big total aerodynamic force from a small airspeed then its weight will be balanced out with a lower terminal velocity, so its 'gliding' endurance is larger. e.g. a lead ball versus a feather.
So, I'm with DFC in that no drag = no descent.
But I think you are both talking slightly at cross purpose regarding gliding because you're swapping interchangeably between considerations of range and endurance.
Adding drag decreases range but increases endurance (assuming of course that the optimum speed is flown in each case), this is best understood by reducto ad absurdum; deploy an infinitely large parachute... what happens? Glide path tends to vertical, endurance tends to infinite.
(All of which diverges someone from original topic... but so what... I'm bored)
regards
pb
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our house brick clearly must have an L/D ratio of zero.. entirely reasonable. Likewise, our perfect glider must have an infinite L/D ratio
I was always talking glide endurance since glide range is a trade-off between forward speed and endurance just like powered flying for range.
Gliding endurance is all about the ratio between weight and the vector addition of CL and CD
Regards,
DFC
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However, isn't endurance at a maximum when total drag is at a minimum?
Your glider has a certain ammount of Gravitational Potential Energy.
To maximise the endurance you need to be losing that GPE at the lowest possible rate. Energy / Time = Power.
Since power = force x speed you need to fly the speed that gives the smallest product of Drag x TAS.
(You'll find this below minimum drag speed, its the point where a line from the origin to the Drag versus IAS curve is perpendicular to the tangent. Being at a lower density altitude helps since this will give the smalled possibe TAS associated with that condition.)
So in fact we are not aiming for Min drag as such, which may influence the rest of your observation.
But anyway... where does CL come in? Well, lift forms part of the total aerodynamic reaction, but from an endurance point of view we are not concerned with efficient lift production. The more drag you add, the less relevant CL is. Add infinite drag, and no lift is required yet the object will stay up there all day.
pb
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No, for this you need a minimum power speed.
This Thrust force uses up stored energy.
To stay in the air as long as possible we need to use the energy as slowly as possible. This is done by using the minimum thrust.
In order to use the minimum thrust at a constant speed the drag must also be at a minimum at that speed.
Thus isn't your minimum power speed also the minimum drag speed.
So looking at the total drag curve (not including CL) there will be an airspeed where total drag is at a minimum. Faster or slower than this speed causes more drag.
Is it not true then to say that when flying at this minimum drag speed minimum thrust will be required since to fly faster or slower requires more thrust to offset the increased drag?
This is a basic foundation in powered flying regarding endurance, minimum sink and the old 2 speeds fro every power setting except when at min-drag.
Regards,
DFC
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DFC,
Unfortunately you appear to have some gaps / confusions in your understanding of basic mechanics.
For starters, there is no thrust in a glide. Although I suppsoe you might be talking more generally now, so I won't debate this point any further.
Forces don't really use energy. They might be a mechanism that's involved in transforming energy from one form to another, but you can have a force existing quite happily without any energy being used, transferred or transformed.
It would be better to say something like "The effect of drag is to cause disturbances in the air as the object passes through it, so we can see that the air has gained some kinetic energy. Consequently the object must have lost some energy, so it must either descend or decrease in velocity. If it descends at a rate that compensates for the rate that energy is lost due to drag, then the objects speed will be constant."
Agreed, but.....
No. This is the key to where you are going wrong.
The work done by a force is the product of the force multiplied by the distance the object moves in the direction of the force.
This is often difficult for people to visualise; it is a very common misconception that the work done by a force must somehow be related to the duration of application. This is not the case:
Force and Time are relevant when you want to know the relationship between an unbalanced force and a change of velocity. i.e. Forces and Times are related to Momentum changes.
If you want to know about energy transfer though, you need to know the force and the distance.
Since Work (Energy) done = Force x Distance, we can divide both sides of the equation by time.
This gives us Energy / Time = Force x Distance / Time
But Energy / Time = Power
and Distance / Time = Speed
So Power = Force x Speed
So we can say that if we want our store of Gravitational Potential Energy to last as long as possible, we must be converting GPE to KE (disturbances in the air due to drag) at the lowest possible rate.
So we do NOT need minimum force (drag), but rather we need whatever speed gives us the smallest value for Drage multipled by Speed. This will be below Vmd.
OK, well, I was talking about gliding, but the principle is exactly the same for the aerodynamic considerations of flying for endurance (rather than gliding for endurance).
The key point about the discussion is that we are talking about power REQUIRED to compensate for teh effects of drag without a decrease in the aircraft velocity being needed.
As soon as you add engines into the situation its crucial to differentiate between what the airframe needs and what the engine can provide. The latter is also influenced by airspeed, strongly so if its a propeller engine.
Under power, the optimum range and endurance speeds are by necessity a compromise between what the airframe needs and what the engines can provide.
No, as discussed above. Although in a Jet, it'll be approximately so. In a propeller aircraft all bets are off; Propeller efficiency (especially for a fixed pitch prop) is a huge deal and so power plant optimisation often overwhelms aerodynamic considerations. Hence why in a jet you typically fly a target speed, but in a prop set a target power (or a parameter that is esentially a Power setting, e.g. RPM or MAP or Torque/RPM) and then set what speed you get.
Agreed.
Yes.
But be careful... minimum thrust does not imply minimum power.
We know that best gliding range is achieved at the best lift drag ratio. Condider also that for a shallow glide angle, weight is very close to equally lift, and weight is fixed. Put this altogether, and we can see that Vmd is actually the speed for best glide angle, not best endurance.
For best endurance, (from a purely airframe point of view because we are mostly talking about gliders here) you need to fly slower than Vmd. You'll work your way back up the drag curve, and initially becuase you are close to the bottom of the U, you get a proportionally big reduction in speed with only a small increase in drag.
i.e. in the power = force x speed equation, force (drag) goes up less than speed goes down, so the power required value decreases.
Well, this demo illustrates a powerful point, namely the reverse side of the drag curve, so i would not dispute it as a teaching exercise. But at the risk of being pedantic, it's two speeds for a given throttle setting. Typically RPM and Prop efficiency will both change as the speed changes so its not really one power setting.
Hope that's of some help.
pb
Unfortunately you appear to have some gaps / confusions in your understanding of basic mechanics.
The way that I picture the situation is that an aircraft has to produce a force (let's call it thrust) to oppose the drag force.
This Thrust force uses up stored energy.
It would be better to say something like "The effect of drag is to cause disturbances in the air as the object passes through it, so we can see that the air has gained some kinetic energy. Consequently the object must have lost some energy, so it must either descend or decrease in velocity. If it descends at a rate that compensates for the rate that energy is lost due to drag, then the objects speed will be constant."
To stay in the air as long as possible we need to use the energy as slowly as possible.
This is done by using the minimum thrust.
The work done by a force is the product of the force multiplied by the distance the object moves in the direction of the force.
This is often difficult for people to visualise; it is a very common misconception that the work done by a force must somehow be related to the duration of application. This is not the case:
Force and Time are relevant when you want to know the relationship between an unbalanced force and a change of velocity. i.e. Forces and Times are related to Momentum changes.
If you want to know about energy transfer though, you need to know the force and the distance.
Since Work (Energy) done = Force x Distance, we can divide both sides of the equation by time.
This gives us Energy / Time = Force x Distance / Time
But Energy / Time = Power
and Distance / Time = Speed
So Power = Force x Speed
So we can say that if we want our store of Gravitational Potential Energy to last as long as possible, we must be converting GPE to KE (disturbances in the air due to drag) at the lowest possible rate.
So we do NOT need minimum force (drag), but rather we need whatever speed gives us the smallest value for Drage multipled by Speed. This will be below Vmd.
In order to use the minimum thrust at a constant speed the drag must also be at a minimum at that speed.
Thus isn't your minimum power speed also the minimum drag speed.
Thus isn't your minimum power speed also the minimum drag speed.
The key point about the discussion is that we are talking about power REQUIRED to compensate for teh effects of drag without a decrease in the aircraft velocity being needed.
As soon as you add engines into the situation its crucial to differentiate between what the airframe needs and what the engine can provide. The latter is also influenced by airspeed, strongly so if its a propeller engine.
Under power, the optimum range and endurance speeds are by necessity a compromise between what the airframe needs and what the engines can provide.
Thus isn't your minimum power speed also the minimum drag speed.
So looking at the total drag curve (not including CL) there will be an airspeed where total drag is at a minimum. Faster or slower than this speed causes more drag.
Is it not true then to say that when flying at this minimum drag speed minimum thrust will be required since to fly faster or slower requires more thrust to offset the increased drag?
But be careful... minimum thrust does not imply minimum power.
This is a basic foundation in powered flying regarding endurance, minimum sink
For best endurance, (from a purely airframe point of view because we are mostly talking about gliders here) you need to fly slower than Vmd. You'll work your way back up the drag curve, and initially becuase you are close to the bottom of the U, you get a proportionally big reduction in speed with only a small increase in drag.
i.e. in the power = force x speed equation, force (drag) goes up less than speed goes down, so the power required value decreases.
and the old 2 speeds fro every power setting except when at min-drag.
Hope that's of some help.
pb
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Capt P.B.,
Sorry for the poor discription with regard to energy "conversion"!!!
If you don't have an engine to provide that nice arrow forward labled "thrust" then you are in a glider and you need to use gravity insted.
I have always tried to use "thrust" since that avoids efficiency issues.
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C. G. B.
Thank you for the diagram.
As shown in your diagram, total drag varies with airspeed. In order to maintain a constant speed, the thrust force along the flight path must be equal to the drag force which acts opposite along the flight path.
In your diagram, the drag at the point you marked "Minimum Power Speed" is shown to be greather than the drag at the point marked "Minimum Drag Speed".
If the drag is greather then in order for the speed to be constant, the opposing force must be equal. Thrust opposes drag.
Perhaps you prefer to lok at the diagram as follows;
An aircraft is flying in steady flight at the minimum drag speed.
If a gust disturbs the aircraft and causes the speed to increase by say 10 Knots, then since the drag has increased while thrust has remained the same, the speed will tend to return to the minimum drag speed (stable).
If however, a gust causes the speed to decrease by say 5 knots then drag has increased and again thrust has remained the same, speed will reduce further (unstable).
The above speed stability cases (front and back of drag curve) show that the thrust required for a constant speed is higher at speeds both above and below the minimum drag speed.
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The bottom of the drag curve is minimum drag and hence the best endurance speed for a jet aeroplane (B747 in my examples above) and speed for the flattest glide (glider).
If you want to talk about minimum power for a piston engine then you need to plot a power required curve which will be different from the drag curve and that is a different debate I think!
Regards,
DFC
Sorry for the poor discription with regard to energy "conversion"!!!
If you don't have an engine to provide that nice arrow forward labled "thrust" then you are in a glider and you need to use gravity insted.
I have always tried to use "thrust" since that avoids efficiency issues.
--------
C. G. B.
Thank you for the diagram.
As shown in your diagram, total drag varies with airspeed. In order to maintain a constant speed, the thrust force along the flight path must be equal to the drag force which acts opposite along the flight path.
In your diagram, the drag at the point you marked "Minimum Power Speed" is shown to be greather than the drag at the point marked "Minimum Drag Speed".
If the drag is greather then in order for the speed to be constant, the opposing force must be equal. Thrust opposes drag.
Perhaps you prefer to lok at the diagram as follows;
An aircraft is flying in steady flight at the minimum drag speed.
If a gust disturbs the aircraft and causes the speed to increase by say 10 Knots, then since the drag has increased while thrust has remained the same, the speed will tend to return to the minimum drag speed (stable).
If however, a gust causes the speed to decrease by say 5 knots then drag has increased and again thrust has remained the same, speed will reduce further (unstable).
The above speed stability cases (front and back of drag curve) show that the thrust required for a constant speed is higher at speeds both above and below the minimum drag speed.
-----------
The bottom of the drag curve is minimum drag and hence the best endurance speed for a jet aeroplane (B747 in my examples above) and speed for the flattest glide (glider).
If you want to talk about minimum power for a piston engine then you need to plot a power required curve which will be different from the drag curve and that is a different debate I think!
Regards,
DFC
I am a mere private pilot and shall not attempt at being superb or wise, but I learned to adjust pitch to control airspeed and throttle to control sink. This works perfectly for me, even at bumpy, very short grass airfields.
I have a tendency at pulling power early, arriving VERY low at the threshold, making for safe landings. (Scared a bus driver once). Even at my home airport's hard surface (practically infinite) runways, I arrive just above the runway light poles. An instructor has reprimanded me for this. Yep, one day I may be caught by windshear and land before the threshold, but I follow the POH's speeds. So it be.
I have used the same techique in MS flightsim for heavy aircraft like the 747, and have discovered that landing a 747 is just like cooking ! Whatever you do won't have any effect till in 45 seconds or so. So, it's about compensating for that, which is exactly what such pilots do (me knowing from sitting in back and listening). Full throttle for 10 secs, and then wait and see the result. Once you're good at it, it's automagic.
To widen this, an approach is like cooking. You need to plan in advance to have everything fall into place at a specific time. A pilot starts backwards, thinking about what she'll need in the final phase. A cook has everything lined up, because she knows that it'll be busy around the "climax".
I'd be interested in any intelligent replies on this, as this is the way i fly and cook. Realise where you'll be in 10 mins and make sure for that. Anything that can be done now, do it.
Now, where is my beer....
I have a tendency at pulling power early, arriving VERY low at the threshold, making for safe landings. (Scared a bus driver once). Even at my home airport's hard surface (practically infinite) runways, I arrive just above the runway light poles. An instructor has reprimanded me for this. Yep, one day I may be caught by windshear and land before the threshold, but I follow the POH's speeds. So it be.
I have used the same techique in MS flightsim for heavy aircraft like the 747, and have discovered that landing a 747 is just like cooking ! Whatever you do won't have any effect till in 45 seconds or so. So, it's about compensating for that, which is exactly what such pilots do (me knowing from sitting in back and listening). Full throttle for 10 secs, and then wait and see the result. Once you're good at it, it's automagic.
To widen this, an approach is like cooking. You need to plan in advance to have everything fall into place at a specific time. A pilot starts backwards, thinking about what she'll need in the final phase. A cook has everything lined up, because she knows that it'll be busy around the "climax".
I'd be interested in any intelligent replies on this, as this is the way i fly and cook. Realise where you'll be in 10 mins and make sure for that. Anything that can be done now, do it.
Now, where is my beer....