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Wind won't affect pitot tube indication


hein22
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Did a test with a strong head wind and constant power setting, then turn to have a tail wind and the groundspeed changed (ok) but the CAS in HUD and HSI did not. I know the ADC plays a role in here but IRL both indication changes to show the pilot the airspeed above the wings so he can evaluate lift and others factors important. Track attached.

airspeed and wind.trk

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The best example I use with my students is to equate wind to a current in a river. Think about it like you’re swimming in a river at 5 knots (This is your CAS), and the river’s current is flowing at 2 knots. Attempt to swim upstream, and relative to the shore, you move more slowly (only 3 knots, which is our Groundspeed) but relative to the water, you’re still swimming through it at 5 knots (Our CAS still shows 5 knots). Turn around, and swim with the current, and you’re now moving at 7 knots relative to the shore (our Groundspeed is now higher). But relative to that stream of moving water, you’re still swimming at 5 knots through it (5 knots CAS, plus the 2 knots of current = total GS of 7 knots). Make sense?

 

When you’re in the air, the aircraft flies relative to that moving mass. Assuming constant power and negligible drag influences from the turn, changing from headwind to tailwind will not affect any airspeed (IAS, CAS, EAS, or TAS). The only thing that will change is your Groundspeed. And I’m not talking about edge cases like wind shear etc., because those aren’t applicable here.

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Wind doesn't affect airspeed!

 

If you fly at an indicated airspeed of 100 knots into 50 knot headwind, your indicated airspeed is still 100 knots ...... and flying downwind at 100 knots in a 50 knot tailwind is still 100 knots.

 

Suppose I tell you to walk 10 steps from the back to the front on a bus driving at 50kph. Then walk 10 steps towards the rear. You've still only taken 10 steps (and taken the same time to do it) even though the bus is travelling at 50kph.

 

So to summarise, 350 knots IAS is 350 knots IAS whatever the strength or direction of any wind.

 

:)

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These 3,000 hours are virtual... aren't they?

 

I unblocked you in a pursue to seeing you be useful in these forums some day, but I see you are still the same limited and not very bright guy you've always been. I see why people don't usually like you.

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The best example I use with my students is to equate wind to a current in a river. Think about it like you’re swimming in a river at 5 knots (This is your CAS), and the river’s current is flowing at 2 knots. Attempt to swim upstream, and relative to the shore, you move more slowly (only 3 knots, which is our Groundspeed) but relative to the water, you’re still swimming through it at 5 knots (Our CAS still shows 5 knots). Turn around, and swim with the current, and you’re now moving at 7 knots relative to the shore (our Groundspeed is now higher). But relative to that stream of moving water, you’re still swimming at 5 knots through it (5 knots CAS, plus the 2 knots of current = total GS of 7 knots). Make sense?

 

When you’re in the air, the aircraft flies relative to that moving mass. Assuming constant power and negligible drag influences from the turn, changing from headwind to tailwind will not affect any airspeed (IAS, CAS, EAS, or TAS). The only thing that will change is your Groundspeed. And I’m not talking about edge cases like wind shear etc., because those aren’t applicable here.

 

THanks Mika, but that's not how it works IRL, just in papers. 1+1 isn't 2 in the atmosphere. 1 knot headwind will not slow you down 1 exact knot. So IAS do change. I understand you teach it this way, but in practice is different. I guess ED simulated it like plain maths and I was looking to see a too high level of depth that pilots see IRL.


Edited by hein22

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An airplane in flight doesn't care what the ground below it is doing. That's why the fuel flow and KTAS changes with heading were legitimate bugs in the simulation.

 

Yes, in the air it's still true that 1+1=2. Also 1-1=0. And just about any other combination you want to try. I watched your track, there appears to be no issue.

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Wind doesn't affect airspeed!

 

If you fly at an indicated airspeed of 100 knots into 50 knot headwind, your indicated airspeed is still 100 knots ...... and flying downwind at 100 knots in a 50 knot tailwind is still 100 knots.

 

Suppose I tell you to walk 10 steps from the back to the front on a bus driving at 50kph. Then walk 10 steps towards the rear. You've still only taken 10 steps (and taken the same time to do it) even though the bus is travelling at 50kph.

 

So to summarise, 350 knots IAS is 350 knots IAS whatever the strength or direction of any wind.

 

:)

 

Thanks Meyomyx. Yeah, I know how it is, but IRL 30 knots head or tail wind won't slow or speed the object that exact amount of knots but it will modify the pressure reading with that precision; There is where you see small differences in indicated airspeed. But like I said, it seems that level of depth may not be simulated at all. What you guys kindly explained is correct for study cases, just not that exact in the real world.


Edited by hein22

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Thanks Meyomyx. Yeah, I know how it is, but IRL 30 knots head or tail wind won't slow or speed the object that exact amount of knots but it will modify the pressure reading with that precision; There is where you see small differences in indicated airspeed. But like I said, it seems that level of depth may not be simulated at all. What you guys kindly explained is correct for study cases, just not that exact in the real world.

 

Sorry mate, but in the real world, perfect scenario ( constant wind, alt, temp) you describe, wind will have the exact effect of “0” on IAS. A 30kt headwind will slow your GS by exactly 30kts, but your IAS will be exactly the same.

 

Don’t confuse IAS/CAS with TAS, which is dependent on ALT & temp. It’s is also how you measure the 30kt reduction. TAS +/- wind = GS.


Edited by norman99
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Sorry mate, but in the real world, perfect scenario ( constant wind, alt, temp) you describe, wind will have the exact effect of “0” on IAS. A 30kt headwind will slow your GS by exactly 30kts, but your IAS will be exactly the same.

 

Don’t confuse IAS/CAS with TAS, which is dependent on ALT & temp. It’s is also how you measure the 30kt reduction. TAS +/- wind = GS.

 

No problem norman, no harm done. I think there's been some confusion about the topic. If you're interested we can continue this over PM and I'll do my best to explain the point, but mainly all of you guys are describing pure plain maths (basic one) which is lovely and accurate, but the day you encounter a headwind that strikes your aircraft at exact 0° from your nose with 0 deviations per second give me a call and together we'll call the guiness so they can put that on paper. Unfortunately I was talking about something too practical for everyone to understand, at least those who are not involved in real life operations. The bug (or not) in place was supposed to be about a possible simulation of the small variations that the atmoshere has every second. Remember the wind isn't a magical blow, but a mass moving and you move with it. I appreciate your insight.


Edited by hein22

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Okay, I believe I understand what your question is.

 

You're asking about implementation of minor atmospheric variations even under steady state wind conditions in the sim. Let's just be clear that this has absolutely no bearing on Air Data Computers, Pressure Transducers, indication of IAS, or calculation of CAS, EAS, TAS or GS, whether through ADC input/output or Steam Gauge ASI/E6B (whether it be in sim, or in real life).

 

Additionally, this is already somewhat possible in sim. Add a small amount of turbulence in the mission editor (value of 10 or 15, which is multiplied by 0.1 feet) and you'll get small variations in alpha, and as a consequence, airspeed speed within the sim. Yes, it would be nice to also have a separate gust factor slider in addition to the steady state wind, but this is not a bug related to the F-18C, or any aircraft in DCS, but rather in relation to the atmospheric modeling and implementation of weather. I suggest you add a post to the DCS World 2.5 Wishlist for these additional parameters.

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Okay, I believe I understand what your question is.

 

You're asking about implementation of minor atmospheric variations even under steady state wind conditions in the sim. Let's just be clear that this has absolutely no bearing on Air Data Computers, Pressure Transducers, indication of IAS, or calculation of CAS, EAS, TAS or GS, whether through ADC input/output or Steam Gauge ASI/E6B (whether it be in sim, or in real life).

 

Additionally, this is already somewhat possible in sim. Add a small amount of turbulence in the mission editor (value of 10 or 15, which is multiplied by 0.1 feet) and you'll get small variations in alpha, and as a consequence, airspeed speed within the sim. Yes, it would be nice to also have a separate gust factor slider in addition to the steady state wind, but this is not a bug related to the F-18C, or any aircraft in DCS, but rather in relation to the atmospheric modeling and implementation of weather. I suggest you add a post to the DCS World 2.5 Wishlist for these additional parameters.

 

Hey Mika, thanks for your insight. I see what you mean about not being an f18 thing and you're right, I was looking into a different aspect of the whole dcs engine. Thanks and stay safe.

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No problem! I was just confused as to what you were asking for. I entirely agree that it'd be nice to have a more comprehensive set of tweakable parameters for Wx in the sim. Maybe this will come after they implement the revised cloud modeling.

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:D and this brings me to a loosely related item in ME weather setup menu. When you set the surface wind speed to a certain value, the wind speed at 1600 feet is automatically doubled and cannot be changed. Winds aloft at any other level can.

I tried to see if it makes any difference in flight and flew at 250 knots, perpendicular to wind direction passing 1600 feet with VV uncaged and there was no visible ghost VV wobble or shift.

So... I think this is some sort of 'left over' item in ME or... does anyone know what's up with that?

There is no change in ground speed when flying into or down wind through 1600 feet either.

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No problem norman, no harm done. I think there's been some confusion about the topic. If you're interested we can continue this over PM and I'll do my best to explain the point, but mainly all of you guys are describing pure plain maths (basic one) which is lovely and accurate, but the day you encounter a headwind that strikes your aircraft at exact 0° from your nose with 0 deviations per second give me a call and together we'll call the guiness so they can put that on paper. Unfortunately I was talking about something too practical for everyone to understand, at least those who are not involved in real life operations. The bug (or not) in place was supposed to be about a possible simulation of the small variations that the atmoshere has every second. Remember the wind isn't a magical blow, but a mass moving and you move with it. I appreciate your insight.

 

I think your right, and I may have been confused about your initial post. If your're talking about the transient effects of either maneuvering an aircraft within a constant airflow, or a changing airflow (wind) on a constant aircraft state, I believe you are correct, these do not seem to be modeled.

 

As you are probably aware, when an aircraft experiences a sudden increase in relative airflow, be it from a change in wind, or a climb/turn into a headwind, you can expect a temporary increase in IAS, until the aircraft has stabilised within the new airflow. The opposite occurs with a decrease/tailwind.

 

A significant factor that contributes to the magnitude of the IAS change experienced, is the inertia of the aircraft. The more inertia an object has the more resistant to change it is, and therefore the longer it takes to re-establish a balanced state within the new airflow. In a large aircraft, such as a 777 at MTOW (~773,000lbs) it is common to observe deviations of up to +/- 20 Kts IAS when experiencing significant wind changes. A 40,000lbs Hornet obviously has significantly less inertia, and therefore the effect of sudden airflow changes will be significantly less too.

 

These factors I have mentioned however, have more to do with the inertia/momentum modeling of an object within a changing airflow, rather than the airflow around a pitot tube and the resulting accuracy of the airspeed measurement. This may be where some of the initial confusion on my part came from. Then again I may still be confused, and talking about another issue altogether. Haha.

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As you are probably aware, when an aircraft experiences a sudden increase in relative airflow, be it from a change in wind, or a climb/turn into a headwind, you can expect a temporary increase in IAS, until the aircraft has stabilised within the new airflow. The opposite occurs with a decrease/tailwind.

 

This is exactly what I was talking about. Maybe I am terrible at making a point clear. I need to work on my english.

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