POLL: How do I know if I'm flying into a headwind/tailwind?

[FO Gyro]

I would like to see how most pilots on this forum work out if they are flying into a headwind or tailwind. Do you normally compare your indicated airspeed to your groundspeed on your GPS, or do you actually know what your True Airspeed is and then compare that to your groundspeed on the GPS?

[Gyronaut]

Interesting topic FO Gyro but perhaps this is wrong forum?
Microlights and Gyro's fly at such low speeds and low altitudes that IAS/TAS (nevermind Equivalent and Calibrated airspeeds) are so close its not worth bothering with the difference.
Agreed? Or am I missing something?

[Loco]

Headwind/tailwind makes a big difference to trikes

Sometimes headwind can give me TAS 35kts (or even less) and tailwind TAS 70kts

I use the IAS vs. GPS method...

[FO Gyro]

For me it's interesting, and I'd like more response first before commenting. Once it's run a lot longer I'll tell you why I ran it.

Anyone on this forum can vote, no matter what you fly. Unless one is flying a turbine, EAS (Equivalent airspeed doesn't really come into it as that is the airspeed corrected for compressibility and temperature rise as a result).

[D2O]

There should be other options in the poll, (e.g. if I decide to plan a nav using a map, protractor, scale ruler, and the latest TAF report) then I should be fully armed with calculated magnetic heading and true airspeed (wind corrected) information for my flight. If the TAF report was accurate, then my true airspeed will be known even before I take off. Once in the air, I can judge if I've got a headwind or tailwind by the time difference it takes me to get to each waypoint. If I arrive faster, I've got a slight tailwind, if slower, I've got a headwind.

In a WCM I think it's easy to see (when you've been flying in smooth conditions for a while) whether you're in a headwind or not. Just look down to see if you're going backwards
Of course if I were to select the options in the poll, I'd choose the option where I can see the difference between the IAS on my instrument, and the GPS true ground speed.

[FO Gyro]

Ok, I'm seeing a trend so far that confirms what I thought, and that is that microlighters generally don't really understand the difference between IAS (Indicated airspeed) and TAS (True Airspeed). TAS is not one's indicated airspeed corrected for the wind (that is groundspeed). It's the indicated airspeed corrected for temperature and pressure. Whether they really need to know this it debatable, but I've heard some very incorrect things on the radio because of it.

An example would be from when I flew my gyro. I would cruise at 90 mph indicated (IAS). When I flew with other gyro's in formation, they would compare this speed to their groundspeed (same units obviously so we'll keep it in mph) and then declare whether they had headwind or tailwind.

What most microlighters don't realise is that in an open cockpit, due to the localised low pressure within the cockpit, causes the airspeed indicator to overread.The ratio of dynamic pressure to static pressure that the airspeed indicator uses becomes distorted.

Getting back to my gyro, my 90 mph indicated, at sea level, on a standard day of 15C would give me a TAS of around 80 mph. I'm actually travelling 10 mph that I thought. Guys then make the false deduction that, in this example, that if their groundspeed is 90 mph, the headwind/tailwind component is zero, whereas it is actually 10 mph (which should be converted to kts to be correct when comparing to the forecast wind).

So the point of all this is the bad news is that all open cockpit aircraft suffer from over reading airspeed indicators, and the faster one flies, the greater the error. In a trike it might only be 5 mph, but in a gyro it is 10 mph. If you fly a faster machine this error will be more. This is corrected when static ports are fitted on the fuselage, so that the airspeed indicator reads the correct static pressure when it works out your airspeed.

[ZULU1]

FO Gyro, quite correct if the cockpit is a bath tub type, however if its a Aquilla type where the air speed indicator is in a pod. No reason for the airflow over the cockpit to create a negative pressure? I have experimented with similar but for wind and noise over the mic holder years back. The airspeed over a trike pax is a vortex, exaggerated when approximately 1/2 metre off the ground..good topic..

[Asterix]

Just check the windsock man!

[FO Gyro]

Zulu I think you are spot on. Any cockpit with a curved surface like a fish bowl will create a local low pressure, so I would imagine the airspeed indicators on aircraft that have a nose cowling will all over read by a large margin.

[Bundy]

Very interesting topic FO, I like such debates which inevitably result in learning.

I, like the majority I believe, chose option 1 above. I understand the difference between TAS and IAS but I share the view (perhaps mistakingly so) with Len above in that at our speeds (50kts or less) and altitude (even up on highveld I'm normally below 7500 ft) that the margin or error should be relatively low?

I tend to be far more focussed on looking for clues on what the wind is doing on the ground while I'm buzzing around. Smoke stacks, veld fires, wind ripples on water, etc etc etc. I then check the GPS to acertain what the wind is doing at my current altitude. Quite a few times I have had as much as 180 degrees difference in wind direction between that at altitude and that which is prevalent on the ground. The "Tailwinds" will remember our trip home from Blue Max last year where we had a 15-knot tailwind at 6500 heading roughly SW yet landed on rwy 21 when we got there!

Very interesting reading the comments on the shape of the cockpit and how it affects the IAS... I had never thought of that.

Thanks guys, good read.

[FO Gyro]

Figures from my gyro (before I sold it):
At Sea Level: 90mph IAS (which equals 144.8 km/h) gives a TAS (or Groundspeed with no wind) of 130km/h. Thus overreading by 14.8 km/h.
Flying at Joburg elevation: same 90 mph gives a TAS of 140km/h.

To summarise then, the guys at the coast are not travelling quite as fast as they think.

For those that are not sure at SL, IAS = TAS when the QNH is 1013 and the Temp is 15C (that's a standard day).

On the highveld, to achieve the same IAS, the aircraft has to fly faster to achieve the same dynamic pressure in the pitot tube, because the air molecules are further apart from each other.

An extreme example of this which microlighters don't get to see often is for high altitude flying jets. In the cruise we might be cruising at Mach 0.81 with an IAS of 255 Kts IAS , but our TAS (the true speed of the a/c through the air) is actually 463 kts (see top left hand side). Ground speed caused by the upper air wind is only 404 kts caused by a 60 kts headwind.

[Asterix]

Very insightful - thanks FO Gyro! I have often wondered about the difference between IAS and TAS - the windsock remark is obviously !

I think we do not understand the importance of the difference between the two in our "slow airspeed world". I use my airspeed indicator mainly for one reason - " How long is this going to take!" I don't even use it for "stall assessment" purposes - I judge that on the feel of the wing "in my hands"!

( And I REALLY hope that nice screenshot is NOT from your Gyro - that would REALLY start me thinking of getting one!)

(and BUNDY: I chose none of the above. I just check the windsock! ) I lie of course - I do the GPS thing, until I'm overhead. Then - I promise - I check the windsock! Once, about three years ago, while in the ML GF, a CB miles over to the East caused me some discomfort and I turned long finals for ML - anywhere - just to get on the ground. Stopped Geelkerneels with my front wheel on Johannes's porch. Since then, I checks the windsock.. )

[Gyronaut]

I've always thought you just add 2% to your IAS per thousand feet of altitude.

Thats what I was taught years ago before GPS's.
FO Gyro, won't you please check for us in practice whether this actually applies?

Interestingly my enclosed machine, with static ports, underreads by 10mph while the open one with no side static ports overreads by 10. Not at sea level @ 15 exactly but close enough and no wind compared to GPS.

[ZULU1]

The location and design of the static port will in itself make a contribution to the gauge reading. Bernoullis equation on fluid flow, reference orifice plate design. in plain english the coke bottle effect. Air or fluid movement over an orifice and direction of this can make a negative prssure. A simple example is a pneumatic suction device used in automation systems. And of course refrigeration systems. On atrike, the asi mounted in the pod is plain perfect.

[Dobbs]

FO Gyro - thanks for the question, certainly thought provoking, even if it means that my super fast BushBaby is now not so super fast

Paul, apologies for being pedantic, but we must not confuse an orifice placete with a venturi. An orifice plate is used in situations where you want to reduce flow in other words a restrictor, where the flow on the low pressure side is turbulent, whereas on a venturi, as your correct analogy of a coke bottle is more laminar, and the intention is to reduce pressure. Maybe that is what you meant?