Somebody who know this?
 

Hello all

I have 2 questions I would like to figure out so I hope there are some smart brains out there.

What effect does headwind have on max angle of climb (Vx) speed ?
I found out there is an increase in the climb angle but what about the speed?


What gives the worst icing condition:
Largewater droplets with temp. below 0
or
small water droplets with temp. below 0?

Cheers

No change to Vx since that is referenced to the still air distance covered during the climb afaik...

I was flying a J3 cub playing in a 40+ knot wind and if you can slow it down to the wind speed you can climb with no forward movement so guess on that day slowing down the climb speed increased Vx to 90 degrees.

The air-speed for max angle of climb reduces with headwind and increases with tailwind. By how much depends on how thrust and drag change with air-speed.

I don't think that the question about small or large supercooled water droplets can be answered in general. It depends primarily on the water content and on the type of de/anti-ice protection of the airplane (e.g. rubber boots versus thermal). Larger droplets due to their inertia tend to be less deflected by the airflow than smaller droplets, which affects the catchment-efficiency and the extent of the area of impingement on the airplane.

Regards,
HN39

Sorry, it does not, with regard to airspeed.
Groundspeed, on the other hand, is a different kettle of fish.
Don't make the mistake of confusing the two.:ugh:

411A;

Sorry, it sure does (airspeed), and any glider pilot can explain it to you.

Regards,
HN39

hi Hazel

just imagine you are flying the glider with only 4 instruments

altimeter, ASI, VSI and compass

so you are "connected" to air and have no clue of wind or groundspeed

so how will ???

when you do not know the wind ?

Besides gliders NEVER climb.
They constantly descend even in a an updraft. Right ? :hmm:

Talk about confusing the poor guy. Vx is not an angle, it's a speed. :cool:

Wiki:
So Vx remains constant regardless of wind. It is purely aerodynamic in nature, being the speed where excess thrust/power is greatest, and is not affected by the wind. The actual angle achieved will vary: more headwind, the steeper the climb. This is because while the rate of climb eg 500ft/min is constant, the groundspeed has reduced (because of the wind. Over say 1nm, the aircraft will spend longer to get there, therefore allowing more climbing time. Higher after 1nm means a steeper angle/gradient achieved. The pilot would still only see the "normal" rate of climb on the VSI though.

Green Guard;

I wasn't referring to gliders, just to glider pilots, because they should be familiar with optimizing their airspeed for vertical and horizontal air movement.

I suggest you make a plot of rate-of-climb on the vertical or y-axis versus airspeed on the horizontal or x-axis for whatever airplane you are interested in. You'll get a curve shaped somewhat similar to one of the curves shown here. Then draw a straight line from the origin and tangent to your curve. The angle of that tangent to the x-axis represents the angle of climb, and the point it shares with your curve is your speed for max angle of climb in still air. 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.

Regards,
HN39

Gliders are generally not used in airline operations...as the paying passengers would undoubtedly be slightly less amused at the thought.:}

I have been thinking about this all day and still do not understand. If you change the airspeed axis to groundspeed (ie applying wind to the IAS), I agree you may be showing the achieved gradient but that must be still at the same groundspeed.

Are you suggesting that if you slow down twenty knots (IAS), the gradient will increase?

Capn Bloggs;

Does this graph answer your questions?

Regards,
HN39

I am struggling to get my head around this a little bit. I came up with the following thought experiment. Imagine an aircraft that has a Vx of 60kts. The aircraft is flying in a 50kt headwind. So the aircraft has a ground speed of 10kts, and one could imagine that it is climbing farily steeply (relative to the ground). If the pilot were to now slow down to an airspeed of 50 kts, thus reducing the groundspeed to 0, provided the aircraft can climb at 50 knots - it will now have a climb angle of 90 degrees.

Does this make sense?


Golf-Sierra

Hazelnuts,
Sorry, but no it doesn't. All that graph shows is that, as the thrust-to-weight ratio increases, the vertical speed increases (as we would expect), and that there is only one airspeed for each T to W that will provide maximum climb performance ie top of each curve. The curve clearly shows that if you change the airspeed then the ROC will reduce.

The 20m/s headwind speed bar merely shows the groundspeed, not the new airspeed as you claim, in that wind; all the groundspeeds are simply 20m/s less than the airspeed. What is does show is that in a headwind, you will get a higher gradient when at the best IAS for the ROC, but only because same ROC is being achieved at a slower groundspeed, not a lower IAS.

So I maintain that a headwind has no effect on the Vx speed. It affects the resulting gradient but not the speed itself.

Correct. See Bubbers post above. Tigers used to do it. A vertical circuit. Takeoff, Up, fly "backwards", down, touch and go, up...

Reducing your speed from Vx of 60KIAS to 50 would problably reduce your climb performance somewhat (as per Hazelnut's graph), but any climb would in effect be vertical.

Thanks for producing another of your superb graphics, HN39. Prior to that, trying to get my head around your written description was only partially successful, and cost me several minutes sleep at bedtime!

The mists are now slowly lifting, although you haven't drawn us a sample tangential line. 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? But what if headwind equals TAS (see Cpn Bloggs et al)?

Don't worry, you'll get us there in the end. ;)

IMO, the gradient will increase as the windspeed increases until the windspeed reaches 142-ish m/s. At that point, you'd be going vertical (in the climb attitude, of course, with 142m/s showing on your ASI). Your groundspeed would be zero.

Great answers guys:ok:, AIR speed will be unchanged but groundspeed will decrease makes perfectly sense. About the water droplets I guess there is just to litle information in the question to make a general conclussion.

Let's suppose that we are climbing at 100 kts airspeed, producing our best angle of climb in still air.

If we meet a 90 knot headwind and reduce our airspeed to 90 knots will be in a vertical climb. So going slower in a headwind increased our climb angle.

If the headwind then increases to 110 knots we will need to increase our airspeed to 110 knots to restore our vertical climb. So going faster in a headwind increased our climb angle.

Curiouser and curiouser!!

So depending on the relationship between our airspeed and our headwind, we may need to increase or decrease our airspeed to maximise our climb angle.

Capt Bloggs and 411a,

Sorry, you are both incorrect.

Bloggs- think about the senerio above- the aircraft that has a STILL AIR Vx of 60 kts will climb MORE STEEPLY wrt the ground at 50 kts in a 50 kt headwind.

Vx is max excess thrust- IN STILL AIR.

reference:-

Performance of light aircraft - Google Books

Nonsens
 

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

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.

EAS
 

But what EAS (IAS,CAS or TAS) are you going to use if the Drag-Tas curve is a function of the wind (Vertical as well as horizontal), altitude and acceleration?

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!!!!

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.

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! :D

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

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! :{

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 ?

Here's John Lowry's chart showing the effect:

http://i521.photobucket.com/albums/w...nbloggs/Vx.gif

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.

I know, thrust to weight ratio. Still impressive.

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.

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

Speeding up in a headwind !!! ( and reducing speed in a tailwind )
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.

At some point you will encounter the stall speed, as shown for example in this modified graph or, in a new graph
Regards,
HN39

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.

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.

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