Somebody who know this?
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Actually, it's about Excess Power
Folks,
Vx and Vy are determined by excess power vs EAS.
I have a neat diagram, but can't insert it (sigh!) - but look at Figure 91 in Preston:
http://selair.selkirk.ca/Training/Ae...l%20pilots.pdf
I'm with Bloggs on this. If you fly off speed either way, your air mass-referenced gradient reduces. If you are now earth-referenced and increase your EAS, you will increase your groundspeed and amplify your gradient reduction. If you do it downwind, it's an even more dramatic reduction in gradient. Between you and me and the gate post Hazlenuts, that is not how I plan to clear obstacles!
Stay Alive
Vx and Vy are determined by excess power vs EAS.
I have a neat diagram, but can't insert it (sigh!) - but look at Figure 91 in Preston:
http://selair.selkirk.ca/Training/Ae...l%20pilots.pdf
I'm with Bloggs on this. If you fly off speed either way, your air mass-referenced gradient reduces. If you are now earth-referenced and increase your EAS, you will increase your groundspeed and amplify your gradient reduction. If you do it downwind, it's an even more dramatic reduction in gradient. Between you and me and the gate post Hazlenuts, that is not how I plan to clear obstacles!
Stay Alive
4Dogs,
As those obstacles are attached to the Earth, your Earth-referenced gradient is the one you are interested in- and it is maximised at a different IAS if there is wind.
As those obstacles are attached to the Earth, your Earth-referenced gradient is the one you are interested in- and it is maximised at a different IAS if there is wind.
PUA,
Repeat that in English and I'll attempt to address it.
A question was asked- it has been answered. Perhaps it is only a technicality- but kindly read the title of this sub-forum!!!!
Repeat that in English and I'll attempt to address it.
A question was asked- it has been answered. Perhaps it is only a technicality- but kindly read the title of this sub-forum!!!!
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Chris Scott;
The graph was intended as a schematic illustration of the text in my post #9, i.e. assuming you make a plot of real data for a particular airplane. The curves represent an arbitrary 'parabolic drag polar' (*) and don't show how a real aircraft departs from that near the stall and at high speed. Another schematization is that each curve is for a fixed thrust-to-weight ratio, whereas the thrust of a real aircraft at either rated thrust/power or any fixed throttle setting (except, of course, for the 'glider') varies with airspeed.
If you will kindly keep that in mind, we can nevertheless draw a few tangents to the curves in my graph to illustrate the principle.
The 'point of contact' of tangents drawn from the origin (x=0, y=0) i.e. for still air, is 100 m/s for all T/W, the 'minimum drag speed' (max L/D) or best-glide-angle speed (Wiki's Vx ?).
If you draw the tangent from (x=30, y=0) to the curve for T/W=0,2 you'll find the speed for max flight path angle relative to the ground for 30 m/s headwind at about 75 m/s air speed (45 m/s ground speed).
If you draw a tangent from the same point to the curve for T/W=0, you'll find the speed for minimum power-off glide angle in 30 m/s headwind at about 112 m/s airspeed.
Finally, as Bubbers and Capn Bloggs have observed, if headwind equals TAS then ground speed is zero and FPA is vertical.
Regards,
HN39
(*)Parabolic drag polar: cD = a + b*cL^2. The wing loading was chosen so that the speed for max L/D is 100 m/s.
P.S. As a point of interest, albeit off-thread, I rather liked the way this graph illustrates how and why the speed for max rate-of-climb increases with thrust-to-weight ratio, following discussion of that topic some time ago on another tech-log thread.
The graph was intended as a schematic illustration of the text in my post #9, i.e. assuming you make a plot of real data for a particular airplane. The curves represent an arbitrary 'parabolic drag polar' (*) and don't show how a real aircraft departs from that near the stall and at high speed. Another schematization is that each curve is for a fixed thrust-to-weight ratio, whereas the thrust of a real aircraft at either rated thrust/power or any fixed throttle setting (except, of course, for the 'glider') varies with airspeed.
If you will kindly keep that in mind, we can nevertheless draw a few tangents to the curves in my graph to illustrate the principle.
... you haven't drawn us a sample tangential line ...
If you draw the tangent from (x=30, y=0) to the curve for T/W=0,2 you'll find the speed for max flight path angle relative to the ground for 30 m/s headwind at about 75 m/s air speed (45 m/s ground speed).
If you draw a tangent from the same point to the curve for T/W=0, you'll find the speed for minimum power-off glide angle in 30 m/s headwind at about 112 m/s airspeed.
Finally, as Bubbers and Capn Bloggs have observed, if headwind equals TAS then ground speed is zero and FPA is vertical.
Regards,
HN39
(*)Parabolic drag polar: cD = a + b*cL^2. The wing loading was chosen so that the speed for max L/D is 100 m/s.
P.S. As a point of interest, albeit off-thread, I rather liked the way this graph illustrates how and why the speed for max rate-of-climb increases with thrust-to-weight ratio, following discussion of that topic some time ago on another tech-log thread.
Last edited by HazelNuts39; 8th Mar 2011 at 20:25. Reason: PostScript
typo
I stand corrected. I still don't understand why, but the graph in figure 7.20 of Wizo's link shows the effect. The effect of the headwind more than compensates for the decreased climb performance when at less than "the best" nil-wind Vx. Probably similar to the max range speed increasing in a headwind.
I learn something every day!
I learn something every day!
Last edited by Capn Bloggs; 8th Mar 2011 at 23:55.
Thanks HN39,
Nearly all of that makes sense. Still unhappy on one point that I raised earlier:
Do I infer correctly that at a thrust/weight ratio of, for example, 0.2, a H/W of more than 30m/s (60kts) does not further improve my chances of missing the top of the mountain?
My reference to the looming mountain range was rhetorical, of course. But you are talking about tangents, and I cannot draw a tangent to any of those T/W curves from any point on the x-axis to the right of about 30m/s. Do you see what I mean? So how do we find the best climb speeds for the various T/W ratios when H/W > 30m/s?
Chris
Nearly all of that makes sense. Still unhappy on one point that I raised earlier:
Do I infer correctly that at a thrust/weight ratio of, for example, 0.2, a H/W of more than 30m/s (60kts) does not further improve my chances of missing the top of the mountain?
My reference to the looming mountain range was rhetorical, of course. But you are talking about tangents, and I cannot draw a tangent to any of those T/W curves from any point on the x-axis to the right of about 30m/s. Do you see what I mean? So how do we find the best climb speeds for the various T/W ratios when H/W > 30m/s?
Chris
For the original question, then, what is the practical application of this? Do light-aircraft pilot's handbooks have any guidance/charts (ala Figure 7.20 of Wizo's link), or is this all just theory? You'd be a brave pilot to reduce the Vx from say 60 to 50 thinking you were going to outclimb a hill because you had a bit of headwind. For example, John Lowry (in Wizo's link) is suggesting that, in a 30kt headwind, the Vx reduces from 64 to 48. Surely he can't be serious? I don't know a lot about flying light aircraft, but I would have thought that if you reduced Vx by that much you'd be dead in a flash if on one engine.
Certainly, I wouldn't have a clue in my Boeing whether my takeoff performance data reduces the V2 slightly in a headwind. I just fly at the speeds the FMS determines.
PS: John has "Bootstrap" on the brain!
Certainly, I wouldn't have a clue in my Boeing whether my takeoff performance data reduces the V2 slightly in a headwind. I just fly at the speeds the FMS determines.
PS: John has "Bootstrap" on the brain!
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Apples and pineApples
Hi Hazel
OK let's imagine this ( Still Air graph ) is actual and correct.
Tangent on a curve ( 0.15) ~Airspeed 100. No wind. We get V/S ~ 8 m/s
If we have 20 m/s head wind the Gradient is 8/80= 10 %
What you suggest is to add wind to this “still air” graph. Hence move zero to right by 20.
Then tangent on a curve ( 0.15) ~Airspeed 90. Now we get V/S ~ 7 m/s
So now we fly 90 m/s with 20 m/s head wind the Gradient is 7/70= 10 %
Other curves similar thing. And where we put tangent for a 60 kts wind ?
Also if you have a tailwind, would you increase the speed for better gradient ? !!!
All other arguments about 90 deg vertical climb actually do not care about Vertical Speed.
Even with V/S is as low as 1 ft/min ( 5 mm/sec) who cares about obstacles ?
Now draw another tangent to your curve from a point at 10 kt on the x-axis. The point where the second tangent touches your curve is the airspeed for max angle of climb in a 10 kt headwind.
Tangent on a curve ( 0.15) ~Airspeed 100. No wind. We get V/S ~ 8 m/s
If we have 20 m/s head wind the Gradient is 8/80= 10 %
What you suggest is to add wind to this “still air” graph. Hence move zero to right by 20.
Then tangent on a curve ( 0.15) ~Airspeed 90. Now we get V/S ~ 7 m/s
So now we fly 90 m/s with 20 m/s head wind the Gradient is 7/70= 10 %
Other curves similar thing. And where we put tangent for a 60 kts wind ?
Also if you have a tailwind, would you increase the speed for better gradient ? !!!
All other arguments about 90 deg vertical climb actually do not care about Vertical Speed.
Even with V/S is as low as 1 ft/min ( 5 mm/sec) who cares about obstacles ?
Last edited by Green Guard; 9th Mar 2011 at 05:36.
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I think Vx is a fixed speed for still air but if your obstacle was power lines then you don't have to factor in descending air off the obstacle so headwind would benefit your climb angle if you were slower. As a crop duster sometimes I decided to go over or under the power lines depending on my climb rate. It didn't really matter much but with normal flight operations going under the lines was not an option. Flying for the airlines we don't think of this because we have different rules. It is fun to climb straight up in a J3 with 65 HP and make multiple landings without having to fly the pattern though. Not much point in it except it was a lot of fun.
Yes, Vx is a speed, not an angle, but for a J3 cub to have a 90 degree climb angle on those days is quite impressive. We watch that F35 at Reno every year at the air races do that flip loop and climb vertically with zero airspeed. Now that is angle of climb profile. Lots of factors fit in to best angle of climb.
Yes, Vx is a speed, not an angle, but for a J3 cub to have a 90 degree climb angle on those days is quite impressive. We watch that F35 at Reno every year at the air races do that flip loop and climb vertically with zero airspeed. Now that is angle of climb profile. Lots of factors fit in to best angle of climb.
Bloggs,
In practicality, I doubt you'd ever be able to have the calculative ability at hand to adjust your IAS to achieve a better angle. Fly Vx as published would almost always be the safest solution (though, of course, how many light aircraft pilots actuaaly know their current Vx for their actual weight?)
As has been pointed out, gliders, with their much larger ranges of L/D and quest for ultimate efficiency DO make alowences to achieve best glide range adjusted for wind, which is roughly analogous (and, as I write, I realise that speeding up in a headwind to achieve best rang in a powered aircraft is basically the same princliple) and a gps linked glide computer will supply the best speed to fly.
In practicality, I doubt you'd ever be able to have the calculative ability at hand to adjust your IAS to achieve a better angle. Fly Vx as published would almost always be the safest solution (though, of course, how many light aircraft pilots actuaaly know their current Vx for their actual weight?)
As has been pointed out, gliders, with their much larger ranges of L/D and quest for ultimate efficiency DO make alowences to achieve best glide range adjusted for wind, which is roughly analogous (and, as I write, I realise that speeding up in a headwind to achieve best rang in a powered aircraft is basically the same princliple) and a gps linked glide computer will supply the best speed to fly.
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Green Guard (#29);
Apples and oranges: As you rightly point out, together with PUA and Wizofoz, the difference between two apples can be quite small and may well be considered insignificant for practical purposes.
Regards,
HN39
Apples and oranges: As you rightly point out, together with PUA and Wizofoz, the difference between two apples can be quite small and may well be considered insignificant for practical purposes.
Regards,
HN39
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I realise that speeding up in a headwind to achieve best rang in a powered aircraft is basically the same princliple)
everybody else including John Lowry was insisting to REDUCE speed in a headwind !!
Well, this one is a different "cattle of fish",
makes sence
and would need entirely different graphs from what we have seen so far.
Green Guard,
BEST RANGE speed DOES increase in a headwind.
The reason you will sometimes REDUCE speed in a head-wind is because you were planning to fly at a speed HIGHER than nil-wind best-range in any case.
For example, best range in something like a Barron might be around 130kts IAS at say 40% power- but nobody operates at that speed, as the increased fuel-burn is worth the faster speed, so everyone, in normal ops, goes for 65-75% and 175kts.
Now throw in a headwind that means range become critical- sure, NOW reducing to 160kts IAS at 55% will increase range over a normal cruise, but it is still faster than still air, max range cruise.
In jets, we DO always operate quite close to min-consumption speeds, as jet fuel is a very major percentage of overall cost and- hey presto- the FMC schedules higher speeds in head-winds and lower in tailwinds.
BEST RANGE speed DOES increase in a headwind.
The reason you will sometimes REDUCE speed in a head-wind is because you were planning to fly at a speed HIGHER than nil-wind best-range in any case.
For example, best range in something like a Barron might be around 130kts IAS at say 40% power- but nobody operates at that speed, as the increased fuel-burn is worth the faster speed, so everyone, in normal ops, goes for 65-75% and 175kts.
Now throw in a headwind that means range become critical- sure, NOW reducing to 160kts IAS at 55% will increase range over a normal cruise, but it is still faster than still air, max range cruise.
In jets, we DO always operate quite close to min-consumption speeds, as jet fuel is a very major percentage of overall cost and- hey presto- the FMC schedules higher speeds in head-winds and lower in tailwinds.
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Originally Posted by Chris Scott
So how do we find the best climb speeds for the various T/W ratios when H/W > 30m/s?
Regards,
HN39
Last edited by HazelNuts39; 11th Mar 2011 at 15:14. Reason: graphic added
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Especially for Wizofoz
If you want to make an estimate of wind effect on the choice of optimum Vx the first thing to do is to see what happens if you use the Vx from the manual.
The change in climb angle will then be proportional with the wind-component as a percentage of the TAS corresponding with Vx (IAS, EAS or CAS converted to TAS).
Then comes the question what will happen if I use another Vx. Changing Vx will change your drag and hence your excess thrust. It will also change your body attitude and hence the vertical component of your thrust and will also change your trust, this may be significant in the lower sped range.
If you decide to use another Vx speed then the one from the manual, you will have to accelerate or decelerate, this also will have an effect on your angle. So what might look, as an advantage in the long run may be a disadvantage on the short term.
In the end what counts is the true relation between TAS, Drag, Lift, both components of the Thrust. Just to calculate a correct TAS while taking into account all the possible errors is already a big problem and you have to do it since you need it to integrate the wind factor. The graph of Cpn Bloggs clearly shows the effect of increased TAS with altitude for a given CAS. It also reveals that the effect on optimum Vx is only significant at very low speeds.
Now Mr Wizofoz: AOA does affect IAS be it only to integrate the effects of position error and Mach and yes on a heavy jet when clean the Vx goes up to 280 kts. Some Airdata computers have software to deal with this and some do not.
So as a general rule one could say that increasing or decreasing your speed away from Vx by a value that results in minor changes in body attitude, will not significantly affect you angle once the new speed has been attained.
There are simply too many variables in the game to draw any conclusion. Those nerds that want to find out simply will have to try it out in practice, and their conclusion will only be valid for those condition.
On a high bypass engine you loose about 30 % of your thrust simply by accelerating down the runway. And to add to this I would like to mention that in our airline we even took into account the change of TAS with constant IAS while calculating the climb angle to cope with an N-1 take off.
So Wizofoz, before you go on rambling on another forum about what I wrote on this one you may want to think twice.
The change in climb angle will then be proportional with the wind-component as a percentage of the TAS corresponding with Vx (IAS, EAS or CAS converted to TAS).
Then comes the question what will happen if I use another Vx. Changing Vx will change your drag and hence your excess thrust. It will also change your body attitude and hence the vertical component of your thrust and will also change your trust, this may be significant in the lower sped range.
If you decide to use another Vx speed then the one from the manual, you will have to accelerate or decelerate, this also will have an effect on your angle. So what might look, as an advantage in the long run may be a disadvantage on the short term.
In the end what counts is the true relation between TAS, Drag, Lift, both components of the Thrust. Just to calculate a correct TAS while taking into account all the possible errors is already a big problem and you have to do it since you need it to integrate the wind factor. The graph of Cpn Bloggs clearly shows the effect of increased TAS with altitude for a given CAS. It also reveals that the effect on optimum Vx is only significant at very low speeds.
Now Mr Wizofoz: AOA does affect IAS be it only to integrate the effects of position error and Mach and yes on a heavy jet when clean the Vx goes up to 280 kts. Some Airdata computers have software to deal with this and some do not.
So as a general rule one could say that increasing or decreasing your speed away from Vx by a value that results in minor changes in body attitude, will not significantly affect you angle once the new speed has been attained.
There are simply too many variables in the game to draw any conclusion. Those nerds that want to find out simply will have to try it out in practice, and their conclusion will only be valid for those condition.
On a high bypass engine you loose about 30 % of your thrust simply by accelerating down the runway. And to add to this I would like to mention that in our airline we even took into account the change of TAS with constant IAS while calculating the climb angle to cope with an N-1 take off.
So Wizofoz, before you go on rambling on another forum about what I wrote on this one you may want to think twice.
Last edited by Pitch Up Authority; 9th Mar 2011 at 14:35.
HN39,
Extending the curves to the left has solved my problem, thanks. Should have realised that they would rapidly head "south"!
(For those who haven't been following my dialogue with HN39, that enables a tangent to be drawn for any headwind up to the stall TAS.)
Chris
PS
This fascinating discusion has crept well into the theoretical. Am concerned that some readers may not be aware that even flying at the "minimum-drag speed" (roughly speaking, at the angle of attack for the highest lift-to-drag ratio) is not normally recommended on public transport. (I think Concorde, on take-off and approach, was an exception.) And minimum-drag speed is itself way above stall speed. Current jet airliners use a "minimum clean speed" which is just above minimum-drag speed. Flying below minimum-drag speed is a tricky business.
Extending the curves to the left has solved my problem, thanks. Should have realised that they would rapidly head "south"!
(For those who haven't been following my dialogue with HN39, that enables a tangent to be drawn for any headwind up to the stall TAS.)
Chris
PS
This fascinating discusion has crept well into the theoretical. Am concerned that some readers may not be aware that even flying at the "minimum-drag speed" (roughly speaking, at the angle of attack for the highest lift-to-drag ratio) is not normally recommended on public transport. (I think Concorde, on take-off and approach, was an exception.) And minimum-drag speed is itself way above stall speed. Current jet airliners use a "minimum clean speed" which is just above minimum-drag speed. Flying below minimum-drag speed is a tricky business.
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Speed stability criteria affect the use of Min Drag speed. This is why some jets have a speed-trim function build into the AFDS that is active when flying manual and some even when the autopilot is engaged. So when reducng you Vx you might be faced with chasing your speed with all the effects on the climb gradient.
Good Luck
Good Luck
Last edited by Pitch Up Authority; 9th Mar 2011 at 14:19.