View Full Version : Glide path control on final.
boofhead
1st Jun 2009, 22:14
I get into discussions with instructors and others and usually find myself to be outside normal thinking, but this one I can't let go of. I see students screw up landings because they cannot fly a final approach. 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.
The traditional advice is to use attitude for speed and thrust or power for glidepath. But that does not work on an ILS, except in a very rough and ready manner, since the response is too slow, so I have always flown the glidepath with attitude (elevators) and maintain the speed with small changes in thrust.
Most variations from the normal glidepath are pilot induced, so that if the airplane goes a little low, the speed will be a little high as a result. Getting back on the glidepath using elevators will simultaneously correct glidepath and speed.
Of the two parameters, speed and glidepath, glidepath is the more important. The airplane can safely land if a little fast or slow, but it cannot safely land if the landing is short or long.
Many pilots don't seem to give a damn about where on the runway they land, so long as there is tarmac under them, but I like to teach my students that there will not always be excess runway available and they should make every landing as a precision landing.
And a glider does not have power to adjust the glidepath (sure it has speed brake but I am sure most glider pilots do not pedal their way down final with that control).
I therefore teach that when on final, the pilot should establish the glidepath and speed, and maintain the glide path using elevator, adjusting power as needed to control airspeed. For every airplane, not just the B737.
Some of the junior instructors I have spoken to about this claim I am talking heresy, but cannot give me a sporting argument.
I am not talking about normal descents or climbs, just on final. If turning base you see you are high, of course drag back the throttle or select flap early, and if low, push the throttle forward. I am only speaking about a glide path, whether electronic or visual.
You'll get a sporting argument on here, boofhead! Do a search for 'point and power' and you'll find a lot of stuff already written. I'm with you though for the simple reason that P&P works in any aircraft, big or small, whereas attitude for airspeed won't if you have to maintain an accurate glidepath in anything with reasonable inertia.
Cows getting bigger
2nd Jun 2009, 07:44
Point and power is a valid technique that many students warm to. Indeed, I think that most of us develop the technique with experience although, as stated on a previous thread, it's all about the elastic band connecting the left hand to the right. Shooting off at a bit of a tangent, one of my favourite niggles is the ability for a student to completely Horlicks a great approach by killing the power as they cross the threshold, thus demanding a rearwards lunge on the elevator as the nose pitches down rather rapidly towards the ground.
It does not matter what flight path we are talking about be it descent, cruise or climb, it all comes down to the four forces acting on the aircraft in flight.
People fail to understand the difference bewteen flight path - the path through the air that the aircraft is following and attitude - where the nose is pointing.
It is usually only when CPLs transition onto something that flys along a -3 degree path with a +4 degree attitude that they learn to totally separate the two.
Put any aircraft on a 3 degree approach path at approach speed and power setting.
If the aircraft is now moved above the desired flight path and the pilot does nothing the aircraft will parallel the desired path.
To regain the desired path the pilot has to steepen the aircraft flight path. The pilot wants to do this while maintaining the approach speed.
It matters little how you visualise the control movements, it is the same - the flight path is steepened and to avoid the resultant increase in speed, the thrust is reduced.
So do you reduce the thrust - aircraft slows due to drag greather than thrust and then lower the flight path and use gravity to regain the airspeed;
or
Do you lower the flight path - aircraft speed increases due to assistance from gravity and in response you reuce the thrust to regain the desired speed.
Overall, the problem is that many people do not fully understand either way of doing it.
There is no way of getting away from the major principle when flying on the ideal flight path towards your aiming point, if you become high or low then you are going to have to fly a new flight path that intercepts the desired flight path until it is regained. That means that the aiming point will have to change to some point before the ideal point when high and some point beyond if low.
Think of the extreme situation - if you get so low that you have to fly level what are your doing - you are adjusting your flight path to be horizontal (aiming point is the horizon) until you regain the desired approach path.
Having established that we are using different flight paths to correct and maintain the desired approach path then we have to agree that we will use all the controls - flying controls and thrust controls to acheive those flight paths.
The only difference between big and small aeroplanes is that in the big case, due to momentum one can make some small temporary changes in flight path without having any immediate change in airspeed. In something like a small microlight, any change in flight path will result in an almost instant change in speed.
Does this make a difference to how the pilot flies a particular flight path - only in the amount of throttle movements required if one wishes to maintain a very accuarte airspeed.
So forget - point and power or power for height and attitude for airspeed - it is the flight path of the aircraft that is getting you to where you want to be.
For those that argue attitude for speed is safer - can they explain why recovery from the stall enforces reduction of angle of attack rather than increasing airspeed?
Regards,
DFC
TheChitterneFlyer
2nd Jun 2009, 10:13
For those that argue attitude for speed is safer - can they explain why recovery from the stall enforces reduction of angle of attack rather than increasing airspeed?
I feel sure that you fully understand all of the principles, but the final statement regarding reduction of angle of attack rather than increasing airspeed (with power) is the basic fundamental for teaching new pilots the art of stall recovery; in fact, they are taught to do both... reduce angle of attack (to unstall the wing) and simultaneously applying power. An advanced flying student, or a more experienced pilot, would of course understand the merits of what we're trying to achieve. However, as anyone will tell you, if you're revalidating your PPL and you choose to carry out stall recovery by the application of power alone... your examiner will fail you!
Our RAF bretherans will tell you that 'point and power' is the technique taught to military pilots; for after all they are (in the main) going to be flying those sharp pointy and slippery things.
I guess that the argument is 'twofold'... we're not routinely expected to stall on the glidepath, so why not use 'point and power' for glidepath control. However, during the final approach phase; revert to 'attitude for speed' and 'power for attitude'. The 'twofold' argument is really only applicable to advanced students/experienced pilots and not for the part-time weekend flyers.
I wholeheartedly agree, that larger aeroplanes (with the attendant inertia) and 3 degree path with a +4 degree attitude require more refined handling skills. Having said that, an aeroplane such as the TriStar was designed for 'point and power' from the onset... too high on the approach path; push on the yoke, where the Direct Lift Control would maintain the attitude but increase the ROD by modulating the speedbrakes.
In conclusion, it all rather depends upon which flying community we're trying to teach! If you designed DLC into a Warrior... well, we'd end up with more accidents!
Horses for courses.
TCF
I feel sure that you fully understand all of the principles, but the final statement regarding reduction of angle of attack rather than increasing airspeed (with power) is the basic fundamental for teaching new pilots the art of stall recovery; in fact, they are taught to do both... reduce angle of attack (to unstall the wing) and simultaneously applying power.
Unfortunately however, very few seem to understand the two totally separate and unrelated things acheived by such actions even when done in parallel.
Application of power has absolutely nothing to do with stall recovery. Stall recovery involves unstalling the wings by reducing the angle of attack. No more and no less.
A PA28, A Glider and a Tornado all recover from a stall in exactly the same way - the angle of attack is reduced below the stalling angle of attack.
Power is simply used to reduce the amount that the subsequent flight path has to point towards earth in order to regain suficient airspeed to a) avoid re-entry to the stall and b) climb away with less drag than one has 1kt above the stall.
With no power, gravity is all one has to accelerate the aircraft.
With full power less use is made of gravity because the thrust accelerates the aircraft and only a very temporary shallow descending flight path is required to regain suficient speed.
People may think that they are recovering from a stall with power alone but they are not - they are simply maintaining a constant (balistic) flight path and ensuring that thrust exceeds drag. As the aircraft accelerates, the angle of attack required to maintain that flight path reduces.
Examiners are quite right to fail anyone that uses any method to recover from a stall other than reducing the angle of attack.
As I said, any pilot flying an approach will be using all the controls incvluding power to maintain the desired flightpath and airspeed. One will say that they are using point and power. Another will say that they are using something else. However, when you sit back and watch they fly a correct approach with whatever adjustments are necessary they both do exactly the same thing with the controls - if they did not then either the flight path would end up being wrong or the airspeed would wander all over the place.
So what we have is simply two camps that do exactly the same thing but think of their actions in different ways.
Regards,
DFC
Cows getting bigger
2nd Jun 2009, 13:51
My elastic band analogy?
TurboJ
2nd Jun 2009, 14:29
can they explain why recovery from the stall enforces reduction of angle of attack rather than increasing airspeed?
Quite simply you need to reduce the angle of attack in order to allow the air to flow over the wings. Full power is applied to merely reduce the amount of height lost.
Its also important to get the student to reduce the angle of attack of the wings before applying the power as done the other way round can cause the problem to get worse. In certain circumstances I get the student to recover by merely reducing the angle of attack of the wings and then later on introduce the power.
Cows getting bigger
2nd Jun 2009, 14:33
When teaching stalls I do exactly that. Stall recovery without power - note altitude loss. Do the same stall but this time recover with power (as well as reducing AoA) and note a smaller altitude loss.
Getting back to PnP or the more traditional method, the reality is as DFC says. To maintain a particular glidepath (an unfortunate word) you may need to alter the picture. If your aircraft is nicely trimmed at approach speed (which it should be) then any attempt to change the picture will require the use of both pitch and power. Which one you apply 'first' is probably something of an academic argument. More interestingly, if your not flying at the desired approach speed then you do have to ask how best to alter the speed. Assuming you are on glidepath in this scenario, the most natural thing to do is alter the power (to change speed) then countering any power induced change of picture with pitch. However, the reality should be that the two actions are coordinated. After all, we do teach students to alter speed whilst maintaining altitude in Ex6, don't we? To me there is no difference between Ex6 and Ex13 as far as controlling the aircraft is concerned.
boofhead
2nd Jun 2009, 15:48
I am only talking about the last 500 feet. And only for students or people having trouble with landing approaches.
I consider the most important parameter to be glide path control. 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. I also tell them that there is a minimum speed that they are not to go below at any time until over the fence.
The first part is getting them to understand what the glide path is and how to determine it. Then how to fly it.
If the airplane is above the glide path, put it back on. The easiest and fastest and most accurate way to do that is by using the elevator. If the diversion was small, the speed should not be much affected and will settle down once the glide path is re-established. No need to touch the throttle unless the average speed is not within the parameter established.
Keep the sight picture using elevator and the speed stays the same.
Later, I expect coordinated use of both controls but intially students find it difficult to do this and also intially they do not have the discipline to even be aware that they are off the glide path and do not appreciate how important it is to land at a nominated point on the runway. Good habits should be learned early. Landing accurately is an essential skill.
It is like parking your Toyota in the garage; you simply MUST do it right or you will hit the side or back walls. Landings should have the same discipline and this should be inculcated right from the beginning. I remember doing this in the Air Force and we were not allowed any variance from the standards. I believe it made me a better pilot.
All that you guys have given me is great stuff and reassures me that I am not so far off the well-trodden path.
TheChitterneFlyer
2nd Jun 2009, 20:06
So... I'm correctly assuming that we're talking about 'student pilots' and not the experienced flyer i.e, that the runway is always in view and that it's not an instrument approach?
'Student Pilots' must adequately demonstrate their ability to safely recover from a stall condition; be it at a safe altitude or during the last 500 feet. In my view; and I don't necessarily subscribe to the syllabus; we must provide a common method for doing so. When the student has shown an appreciation of what is trying to be achieved; and only then; we can then allow the introduction of alternative methods of flightpath control.
I'm sure that you will agree, that the student's ability to fully understand the mechanics of flight requires a modicum of repetitave input. If you start swapping and changing the rules, at a much too early stage, you're going to end up with a confused pupil!
Give him/her the basics, but when the time is right... show them the alternatives! Food for thought...
TCF
betpump5
3rd Jun 2009, 12:34
attitude for speed and thrust or power for glidepath
Ah yes, the third most discussed statement on aviation after:
1. Integrated or Mod for training?
2. What makes a plane fly? Newton or Bernoulli
It seems to me that your post is to have a go at the most basic lesson which is Attitude for Speed, Thrust for blah blah blah.
And I agree with you. In my 9 year career of questioning this, no one has ever been able to back up this principle with any answer worth listening to. And you are right, on an ILS if you use attitude for speed, then forget about making a good landing.
The taught concept goes against every single human instinct. If you are going too slow, then you increase power. Cessna 152 or a 744. No argument.
Its about time someone got rid of this ridiculous concept - unless they can come up with an excellent unquestionable reason as to why you would NOT increase power if you are going too slow.
When the student has shown an appreciation of what is trying to be achieved; and only then; we can then allow the introduction of alternative methods of flightpath control.
There is no "alternative method of flightpath control".
One controls the flight path by moving the elevator, ailerons, rudder and (for powered aircraft) using the throttle to vary the force along the thrust vector.
For the constant correct approach you need;
1. The flight path to be towards the aiming point or put another way, you need to be on a collision course with the aiming point and to hit it (like anything else) it must remain static in the window.
2. The angle between the flight path and the horizontal must be correct
3. The track over the ground must equal the runway centerline; and
4. The airspeed must be correct.
No matter how you think of it, how experienced or not, you can not complete a stable approach without the above.
So when you want to teach a person how to do number 1 above is it not very simple to say - use the controls to keep the aim point steady in the window?
Then one can remind them of the secondary effects that they covered some time earlier and remind them that then the flight path angle steepens gravity accelerates the aircraft along the flight path and vice versa so if they need to correct by using a different flight path then there will be a need for a thrust change so that speed remains constant.
I can explain - pitch up and increase thrust to conteract the secondary effect of reducing airspeed in 10 seconds and it has already been covered on exercise 4 as the primary and secondary effects of moving the elevator.
In exercise 4 the student will also have been shown the primary and secondary effects on increasing and decreasing the thrust settings.
I can also explain why one can think of it as increase thrust to change the flight path and adjust attitude to counteract the change in airspeed but it takes longer, doe snot relate to the previous lessons and is harder to relate to what one is trying to acheive.
In the end however, if a "point and power" pilot and an "power for height" pilot both fly the same aircraft along the same approach they will both manipulate the controls in the exact same way. The only difference is in the mind!
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The main reason why this issue rarely comes up until pilots start flying ILS approaches or something for which a constant approach angle is essential is that for PPL and CPLs in training, instructors simply require that the flight continues towards the aiming point at a constant speed.
Too often if the aircraft becomes a bit high, the answer is to fly a steeper flight path towards the aiming point with reduced thrust to offset the resultant increase in airspeed. If the aircraft is a bit low, the flight path is aning flown towards the aiming point but with higher thrust.................and people are thus dragged into the mindset of power for height.
In effect, PPL and CPL training do not concentrate on re-intercepting the correct approach angle and everything is based on travelling towards the aiming point.
When it comes to flying the ILS, then if the aircraft is not on the correct path, a new path must be flown that does not take the aircraft towards the aiming point (they may already be heading there) but to regain the correct glidepath.
Far too many PPLs and CPLs do not understand that travelling towards the aiming point is not all there is to flying the correct approach.
While every instructor will draw lovely intercept lines for getting back onto the centerline, very few do the same in a vertical sense whan talking about the appropriate approach angle.
Regards,
DFC
Meikleour
3rd Jun 2009, 17:30
This is a perennial `chestnut`!
I have always assumed that the Attitude for Speed technique was introduced early on in pilot training because usually the training aircraft is single engined. Thus the forced landing scenario has to be taught early on. However what you advocate is exactly correct for almost all operations, the problem arises in how one weans the tyro pilot away from the early training. Incidently, John Farley`s book has some excellent views on this subject.
boofhead
3rd Jun 2009, 23:13
I work with experienced pilots as well as tyros and both types have trouble at times. Generally pilots who are familiar with short fields find it easy to make a precision landing, but those who only fly C182 or the like have never learned how to do it. Students cannot handle two things at once and I am of the opinion that glide path control to a landing on the nominated spot is the most important part of learning to land. Using the elevator to establish and remain on the glide path seems to work.
I am grateful for the advice and information given to me here. I am relieved that I am not completely off the wall.
Power for rate of descent and elevator for speed are used to ACHIEVE the correct glidepath. Once on the correct glidepath, power is used for speed and elevator is used to keep the touchdown point in a fixed position in the windscreen.
chrisN
4th Jun 2009, 14:52
I offer nothing about the arguments re powered aircraft, but the original poster was completely wrong to say “And a glider does not have power to adjust the glidepath (sure it has speed brake but I am sure most glider pilots do not pedal their way down final with that control).”
I don’t know why the word “pedal” was used, but for the record, glider airbrakes or spoilers are hand controls.
All gliding operations in the UK teach that you do indeed use the brakes continuously if necessary, to maintain glide path towards a reference point, in particular below 500 feet. It is often the case that they have to be reduced when descending through a wind gradient.
The elevator is used to maintain airspeed. With some gliders, a small change in elevator is required to maintain airspeed when the brakes are opened significantly more, but the essence is always elevator for airspeed, brakes for rate of descent.
Trying to point to where you want to go in a glider would lead to potential disaster in attempting a field landing with limited room for flare and ground run.
If you want to teach something else for power, or (worse, IMHO, teach different things at different stages of learning), feel free, and be aware that converting glider pilots to power you will need to explain why it is different, but please don’t propagate misleading advice to anyone learning gliding. There is standardised training of gliding instructors to a standardised syllabus in the UK, and we do what I have said, and not what Boof says..
Chris N.
boofhead
4th Jun 2009, 19:10
I stand corrected on the use of speedbrakes. Indeed that is the way I fly gliders on approach, but I thought I might be translating powered techniques onto the glider. A better example is a powered airplane on final, high and a little fast. Throttle is already at idle and flaps are full. Too late to sideslip.
Choices are to land long or land fast.
I maintain that one should land fast, maintaining the aiming point.
Of course it might be necessary to hold off in the flare until the speed and attitude are acceptable for touchdown, but changing the touch down point should not be acceptable.
There is only one control available and only one that needs to be used.
This is not meant to cover a gross error, when either a go around or a changed aiming point would be better.
The elevator is used to maintain airspeed. With some gliders, a small change in elevator is required to maintain airspeed when the brakes are opened significantly more, but the essence is always elevator for airspeed, brakes for rate of descent.
I think that you will find that the elevator varies the flight path i.e. if your current flight path is giving you 80Kt and you want 100Kt then you will use the elevator to steepen the flight path so that the airspeed reaches 100Kt.
If the elvator is used to make the flight path steeper then thanks to gravity a secondary effect is that the airspeed will increase.
The airspeed acheived in a glider is a result of how steeply the aircraft is descending along the flight path less the drag which is a function of both airspeed and configuration.
Trying to point to where you want to go in a glider would lead to potential disaster
Of course it would because in the absence of an engine, the only force available to provide "thrust" is gravity. However, if there was no variation in the wind, the constant point would remain that - constant and if the aircraft is clean and at the best glide speed then by definition there is no way to move the constant point forward. One can however move that point closer by steepening the flight path.
So again there is two ways of looking at the same actions;
Do you use elevator to steepen the flight path and add drag to prevent a speed increase or do you add drag and use the elevator to steepen the flight path in order to maintain speed?
When it comes to engine failures, I can confirm that people who use point and power as a mind set during powered flight unually perform far better.
Why? - simply because if you ask a person to use the controls to keep a point constant in the window then as an unavoidable consequence if they can do that task then they have to be able to recognise when the constant point is moving or no longer coincides with the place they wish to travel towards.
They usually understand that airspeed is as result of the thrust / drag balance coupled with the flight path and obviously in the absence of an engine all they have is gravity to provide "thrust".
Regards,
DFC
TheChitterneFlyer
4th Jun 2009, 23:23
Forgive my earlier posts on the subject, but it was my understanding that stalling was the consideration, and not flightpath control, i.e. increasing power rather than reducing the angle of attack?
I believe that we're all in agreement here; there's a difference between 'point and power' as opposed to 'power for ROD'. It is of course important to maintain the correct 'approach path'; albeit in a simple single engine or a heavy multi. The difference being, of course, is that if you're very high (in a simple single), and all you have is a visual reference, you cannot maintain the speed and increase the ROD (to the same aiming point)! Actually, in some of my experiences in a heavy B747, you can't do that either... unless you wish to prematurely hit the ground with idle power! The vital part is assessing whether or not it's 'do-able' within the distance remaining!
Actually, this is a great topic for discussion, but not in a closed forum; because all of us need to be adequately understood (and we're mostly missunderstood within this forum), which cannot be easily undertaken by typing words into a little box... I'll probably be berated for my short missives! However, keep your thoughts coming... they're ALL interesting, and, all have their merits.
TCF
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
Whispering Wings
10th Jun 2009, 19:58
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
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.
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.
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
chrisN
10th Jun 2009, 22:11
"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.
"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.
Perhaps you would like to explain?
Regards,
DFC
chrisN
10th Jun 2009, 22:29
WW already did. You just seemed to think he was wrong and you had a better explanation. He was right.
Chris N.
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
chrisN
11th Jun 2009, 09:27
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.
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
chrisN
12th Jun 2009, 01:30
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.
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
chrisN
12th Jun 2009, 22:12
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.
Stick to power. You don’t know enough about gliding to pontificate on it.
I know that and airframe will glide the same no matter if you have an engine at the front or a lump of concrete and as I pointed out you in your own explanation used the "point" from "point and power" to land your glider!
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
cats_five
15th Jun 2009, 09:43
<snip>
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.
There is no problem whatsoever maintaining a constant airspeed in a glider - one of the early parts of glider ab initio training is learning to control airspeed and do it by attitude.
The engine lets you do it without losing height.
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.
Chris,
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.
----------
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.
----------
Cats five,
Which force requires the glider to descend and prevents sustained horizontal flight?
Hint - Gravity is not the answer!!
Regards,
DFC
cats_five
15th Jun 2009, 16:58
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 (http://www.grc.nasa.gov/WWW/K-12/airplane/glider.html)
(unless of course your answer is weight rather than gravity - but remember, no gravity, no weight)
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.
--------
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
poina
15th Jun 2009, 21:35
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.
Cows getting bigger
16th Jun 2009, 07:21
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. :)
cats_five
16th Jun 2009, 12:49
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.
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
Why then does the amount of drag directly affect the amount of time that a glider can stay in the air?
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
cats_five
16th Jun 2009, 19:58
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
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
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
cats_five
16th Jun 2009, 20:44
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.
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
cats_five
17th Jun 2009, 11:01
Arghhh!!!! :ugh: :ugh:
You asked:
Which force requires the glider to descend and prevents sustained horizontal flight?
Hint - Gravity is not the answer!!
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.
However, the glider's ability to resist that force is directly dependent on drag.
Sheer utter b:mad: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?
Cows getting bigger
17th Jun 2009, 19:02
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.
Whispering Wings
17th Jun 2009, 19:22
Boofhead,
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.
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
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.
Totally agree. Either option is just a different mindset.
==============
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
cats_five
18th Jun 2009, 06:54
DFC, why not answer your own question?
Someone posted that lift & drag are interrelated - altering one alters the other - so talking about drag only is a nonsense.
Someone posted that lift & drag are interrelated - altering one alters the other - so talking about drag only is a nonsense.
Who posted that?
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
Capt Pit Bull
18th Jun 2009, 13:49
<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
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 like that explanation.
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
That comment makes the driagrams provided earlier a little clearer. However, isn't endurance at a maximum when total drag is at a minimum? If this is the case then where does CL come into the equation? Or are you using CL to determine the angle of attack for min drag speed which of course varies with weight?
Regards,
DFC
Capt Pit Bull
19th Jun 2009, 11:32
However, isn't endurance at a maximum when total drag is at a minimum?
No, for this you need a minimum power speed.
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
No, for this you need a minimum power speed.
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.
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
Capt Pit Bull
19th Jun 2009, 17:04
DFC,
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.
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.
This Thrust force uses up stored energy.
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."
To stay in the air as long as possible we need to use the energy as slowly as possible.
Agreed, but.....
This is done by using the minimum thrust.
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.
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.
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.
Thus isn't your minimum power speed also the minimum drag speed.
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.
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.
Agreed.
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?
Yes.
But be careful... minimum thrust does not imply minimum power.
This is a basic foundation in powered flying regarding endurance, minimum sink
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.
and the old 2 speeds fro every power setting except when at min-drag.
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
Cows getting bigger
19th Jun 2009, 19:37
I think PB has said what I'm thinking. Vmp is not the same as Vmd.
http://me-wserver.mecheng.strath.ac.uk/group2003/groupf/images/lab/drag3.gif
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.
--------
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
Gargleblaster
20th Jun 2009, 00:34
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....
Capt Pit Bull
20th Jun 2009, 02:27
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.
People get fixated on the idea that there has to be some thrust somewhere.
Stop looking for the thrust. There isn't any. You don't need to find it, its not gone AWOL ;)
I can understand what you're thinking. You understand that if you resolve forces along the flight path then Drag = W Sin (Glide angle). You're trying to relate the situation to something the student already knows - always a good plan. But when you refer to it as 'Thrust' you're being incorrect and you aren't actually doing the student any favors.
The particular danger is that you'll get someone that's shaky on newton 1 and is still latched onto the (incorrect) idea that if an object is moving there must be a force acting in the direction of movement. This is the sort of student thats happy with the idea that you found the 'thrust' because that fits right in with their misconception.
If you want to prove the derivation of optimum glide angle, there is basically two ways.
a) Resolve the weight, Write down some equations about lift and drag. Each will involve some trig. Then set yourself the task of combining those equations and finding the condition that makes the glide angle smallest. This requires either a knowledge of trigonometry and algebra or some rote learning.
or
b) Establish that in a steady glide (T=0) Weight is equal and opposite to total aerodynamic reaction. Then recall (the student should already know) how lift and drag are defined in relation to direction of moving. It is then immediately obvious (with a couple of sketchs, one for good L/D and one for bad) that the L/D ratio determines the direction of movement.
Although (a) is a very commonly used method, (b) Simply requires the application of things already known with no need for algebra or trigonometry.
I've been using (b) for years and its quick and works a charm because its visual rather than equations. But you have to be happy to let go of thrust ;)
Bottom line is that you should teach it a way that you're happy with, and if the students get to the end result, its all good.
Think of option B as a Dr Pepper moment... try it... you might like it ;)
Sorry for banging on about this, but I enjoy it and don't get much student contact these days :(
pb
Cows getting bigger
20th Jun 2009, 06:51
DFC, thanks for the lesson in stability. However, I think you are confusing thrust with power.
Capt. P. B.,
Yes I see your point about Thrust being AWOL. I should have spoken about the effect of gravity along the flight path.
The problem is not contact with students - the student can't really debate the issues. It is contact with other instructors. Even the "refresher" seminars lack any serious discussion on matters such as these.
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C. G. B.,
DFC, thanks for the lesson in stability. However, I think you are confusing thrust with power.
Not at all.
The total drag curve show the total drag at various speeds.
To maintain a constant speed in a powered aircraft, one has in simple terms to make thrust equal to the drag.
In a jet aircraft it is (relatively) simple - make thrust equal to drag and you are a happy bunny.
However, in a propeller aircraft as you have quite correctly pointed out, the power required curve is not the same shape and in fact can be very different from the total drag curve due to propeller efficiency etc.
That is why I have tried all along to use Thrust and have tried to only use B747 (jet) or glider as examples.
So let's forget about "power" since that is a different debate.
Regards,
DFC