Winds
Discussion of Vector Winds, Component Winds, and Speed to Fly, including how to compute True Airspeed

Revised 24 Feb 2000

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This info is considered Public Domain, so use it as you wish, but not for commercial purposes. It was created to increase the joy and enjoyment of flying gliders/ soaring. WARNING: All info is unsubstantiated, so use it at your own risk.

Wind Calculations. There are two kinds of winds calculated by the cambridge L-NAV or S-NAV computers, namely - Vector wind and Component wind. Vector wind tells you where the wind is coming from and at what strength - such as 240 degrees at 15 knots. Component wind tells you how much of a headwind or tailwind you are experiencing at any given moment, while flying in that given direction - such as HeadWind 12 knots.

It is important to be aware of the general wind pattern at each of the altitudes your are flying through for two primary reasons. One, you need to know the general wind that exists from where you are - to where you intend to glide to, because it affects your glide angle! Two, you need to know where the wind is coming from so you can find lift.

Surface wind is critically important to formation of thermals. If you are looking for thermals, you can go to the top of any sunny canyon which has the wind blowing into it from the low ground, and you will find lift. That's a fact, and you can take that to the bank. Of course, the trick is to select the sunny canyon that is facing into the wind, and you can only do that if you know where the wind is coming from. The wind on the surface is not always the same as the wind at altitude, either in direction or in strength.

Vector wind is a computation of what the wind is at a given altitude. Vector wind is nice to know, as you head out in different directions. It helps you to guess the surface winds, and thus the location of lift, and helps you in estimating height requirements for gliding, but to know how much to adjust your speed to fly, so that you have the best glide angle, you need to know the headwind/tailwind component. Vector wind is computed for you. If you have a gps with a computer that accurately tells you what the wind is at any given moment, that's great, but that info may or may not be useful as you change location and/ or altitude. Besides, you always know what the winds are doing at any given moment as soon as you complete two or three circles. You can see your drift! What is more important, something that you really need the wind for is - what is the speed that you should be flying as you proceed from one thermal to another, or as you do your final glide? The primary determinant of the speed to fly is the wind component! So, that is what I want to discuss. Granted, you can determine your wind component from the vector wind, but that vector wind is old, and it is for the altitude you have left. What is your wind component at this very moment as you pass through this altitude? That's what you need to know. Before explaining how to compute Component Winds, let's discuss why they are important.

Using winds for Speed to Fly.
It is my contention that component wind (the amount of headwind (HW) or tail wind (TW) you have on your present heading) is the most important wind to a glider pilot, because it determines what speed you should be flying!

The rule of thumb, which everyone seems to agree upon is that you should fly the best glide speed (L/D) for the flap setting you are using, adjust that for the netto (the air is going up or down), and then add one half the headwind component if you have a head wind. If you have a tail wind, then maybe you should just fly your best L/D, and not add or subtract anything. Any agreement on this? or disagreement?

Certainly we adjust our speed and always fly faster in sinking air and fly slower in rising air, but those are momentary excursions, and you are deciding that by feel and by observing if the vario says the air is going up or down, and you adjust accordingly, but that is an adjustment from the correct speed to fly. Since well over 50 percent of your time is spent in level flight, you need to know the wind component, because you need to fly faster through a head wind, and slower in a tailwind. Like I said. the basic rule is to divide the headwind component in half, and add it to your airspeed. That will give you the best speed to fly, ie the best glide ratio -all other things being equal. For tailwinds, you should fly slower. So how do we calculate the Component wind?

A note on the "uncorrected speed to fly". For any given flap setting and wing loading, there is a correct speed to fly in stll air. Just take it out of the flight manual. Every glider pilot who flies cross country knows these speeds for the ship he/she is flying. Now back to the wind discussion.

To calculate the component wind, just look at your ground speed, and take the difference between that ground speed and your true airspeed. The difference is your existing headwind (HW) or tailwind (TW) component. If you have a GPS, you know your ground speed. The trick is to come up with your True Airspeed (TAS).

TAS is computed inside the computer. Both the Cambridge L-NAV and S-NAV compute your true airspeed by reading your indicated airspeed and adjusting it for altitude and adjusting it for temperature. This TAS is compared to your ground speed and a headwind or tailwind component is displayed. However, you must calibrate your ship before that info is accurate. There is a calibration maneuver which must be done for you to get the correct TAS computation. I will discuss that in another section.

If we can compute our own TAS (True Airspeed), we can validate that the Cambridge is doing a good job, and then rely on it more confidently, or, if we don't have a Cambridge, we can just get into the habit of computing our own wind components.

Manual computation of TAS.
Indicated airspeed (IAS) needs to be changed to Calibrated airspeed (CAS), which is changed to Equivalent airspeed (EAS), which is them changed to True airspeed (TAS). I learned this stuff back inthe 60's, so it isn't totally clear anymore, so I'd appreciate corrections as I go along.

Changing IAS to CAS.
Indicated is changed to Calibrated when you apply the correction for installation error. All ships have an installation error of some amount. The Cambridge computer takes care of this during the calibration maneuver. Dick Johnson always illustrates the CAS corrections on each glider he evaluates, so maybe that will help you with your ship. I don't know if there is an index yet of all old SSA Soaring Articles, but maybe one exists?? Your flight manual may happen to mention the correction for installation error (which may only be a knot or two and may be added or subtracted from indicated). IAS, corrected for such an installation error is then called Calibrated Airspeed (CAS).

Changing CAS (Calibrated) to EAS (Equivalent) and then to TAS.
I know there is a factor for compressibility, which is the F factor on the back of the E6B flight computer wheel. In any case, the F factor doesn't come into play unless you are over 200 kts, and above 10,000, so I will ignore the F factor. Even at 20,000 ft and 200 kts, the factor is a decrease in the True Airspeed of one percent. That now leaves correction required only for temp and density altitude.

What is the outside air temperature?
Even if you are having the Cambridge compute your component wind, it must know the temp. How do it know? One of two methods are available. You can feed the temp to the computer manually, or you can have a probe installed to sample the temp. If you feed it, you only do it once, at a given altitude, and the computer adjusts that temp using standard lapse rate adiabat from that given altitude. (The latest version of Cambridge asks you to insert the temp for 3,000 ft, and then it uses adjustment from there) If the probe installation is accurate - that would be the better method of determing the outside temp. But how important is the temp?.

I ran out a bunch of temps and found that as a general rule, when changing IAS to TAS you need to add one knot to your Indicated airspeed (IAS) for each five or six degrees the temp is above standard day, at altitudes from 5,000 to 15,000 ft. eg. if your IAS is 60 and the temp is 10 degrees hotter then standard, then you should add 2 kts to IAS. ((I wish someone would validate this for me)).

Need to know what standard day temps are?
Standard day temperature is 15 degrees celsisus at sea level (that's 59 degrees). (You can download my adiabat chart if you want a scale that converts celsius to farenheit. However, you really don't need to convert one to the other. We are talking about temps aloft, not temps near the surface.) We only use celsius in temps aloft. From 15 degrees at SL, the standard lapse rate is 2 degrees per thousand feet, thus the temp at 5,000 feet should be 10 degrees colder, or +5 degrees.

Deviation from Standard Day.
I have been looking at computerized flight plans for at least the past 20 years, and in the summer at this latitude I have rarely seen the temp deviation from standard greater than 15 or 16 degrees, and routinely it is between 12 and 15 on summer days. Granted, the temp near the surface varies greatly from standard, but once you are in the upper air (more than 3,000 above the surface) it tends to become a certain deviation from standard day, and remains that amount of deviation as you go aloft up to 20,000 ft.. If you look at the soundings for Reno and look at the soundings for Oakland on any given day, you will see there is hardly more than one degree difference in the temp of the air at 12,000 ft between the two locations, unless there is a winter cold front about to pass through. The air mass is always warmer than standard during the summer time here at this latitude. I have been looking at that for the past several years. To know what the deviation is on the day you plan to fly, just take a glance at the sounding, or call FSS at 1800WXBRIEF, and get the temp at the altitude you will be soaring. It'll be nearly the same deviation at all altitudes once you get clear of the surface effects. Here is a table of the standard temps at a few altitudes.

Standard Day Temperatures
| Celsius
Sea Level | +15
3,000 | +9
5,000 | +5
10,000 | -5
15,000 | -15
20,000 | -25

So now you know how to compute temperature deviation from standard day and can now make that two or three knot correction (typically) for temps.

Correction for Density Altitude.
Use the following table, and add the appropriate amount, and you have the TAS. You can see the adjustment factor varies with IAS and with altitude, so interopolate.

Amount to add to your Adjusted Calibrated Airspeed to get TAS
| 60 kts | 90 kts
5,000 | +5 | +15
10,000 | +10 | +15
15,000 | +15 | +25
20,000 | +22 | +34

eg if you are at 15,000 ft, and you're IAS is 75 kts, and it is 5 degrees warmer than standard day, then add 1 kts for the temp correction, and 20 more for the density altitude (mid-way betweeen +15 and +25), and you know your TAS is 96 kts (75 + 1 + 20 = 96).

To simplify things, make up you own table, plan on a temp of 10 to 12 degrees hotter than standard, by adding 2 kts to all of those numbers, and it is merely a one step process.

The significane of accurate TAS computations.
Let's take an example of how you would adjust your best L/D speed, that is, your speed to fly.

Say your Indicated Air Speed (IAS) is 60 kts, the True Air Speed (TAS) is 100 kts, and your Ground Speed (GS) is 70 kts. This means you think you have a 30 kt headwind (HW) (100 - 70 = 30), thus you would normally increase your wings level flying speed by 15 kts (one half of the headwind) and fly an IAS of 75 kts.

However, say the calculation of your TAS by you or your computer is off by 20 kts - ie you have a 20 percent error in you calculated TAS. Does that mean you have a 20 percent error in the wind component? The answer is NO!! - It is much worse!!

In this second example, with a ground speed still reading 70 kts, you really have a TAS of 80 kts -not 100 kts and thus a headwind of 10 kts (80 - 70 = 10), not 30kts (100 - 70 = 30). In the first example, with an incorrect TAS calculation, you think the HW component is 30 kts, not 10. You think the wind is 300 percent stronger than it really is! The good news is that since you are dividing that headwind in half to adjust your flying speed, the efect of the error is diminished.

You should be flying 65 kts IAS (60 + 5) not the 75 kts (60 + 15) that you thought. The effect of flying 10 kts too fast over one hour may cause you to be 1,000 to 1,500 ft below the others, thus you will have to take one more thermal than them, and be landing five or ten minutes later, or if there is no more lift, you will land 6 miles short of the airport. That's not good.

I contend that the Component wind is the most valuable wind to have in the cockpit. If you know it accurately, you can fly the optimum speed for the given conditions. Without it, you are floundering. We always guesstimate the wind, that is basic, but if we can have an accurate component displayed, that would be ideal.

Computing Component Wind from Vector Wind. If the wind is right on the nose, or right on the tail, you have the component easily calculated, however it is rarely a direct head or tail wind. There was an easy way to remember how to convert the wind strength into a component, if it was 30 or 45 degrees off the nose or the tail (basic trignometry), does any one remember that rule? I read it in AOPA magazine some ten years ago. I think it was called the $5.95 rule, or some other amount, that was an easy way to rembember it - which I have forgotten. (There's a testimonial).

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