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There is something seriously wrong the FM when there is some wind.

Just try making a mission with 15 kts wind straight down the runway with ~50% fuel, no stores and try to land both directions.

Landing with headwind i can maintain no less than 300 kph without reaching 20 degrees AoA.

Turning around and landing with tailwind however i can maintain 220 kph no problem.

 

As far as i know i should be able to maintain the same speed both directions.

 

Something is wrong :P

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Are you sure you're setting the wind correctly. In DCS mission editor (and I think also in auto generated briefing data) wind direction is the direction the wind is blowing into and not as IRL, where the wind is blowing from.

 

I.e. to get a headwind when landing/taking of rwy 27 (a/c heading 270 degrees) you need wins set to 90 degrees.

 

But I agree something seems odd as you should be able to maintain indicated airspeed just the same regardless of wind direction.

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Are you sure you're setting the wind correctly. In DCS mission editor (and I think also in auto generated briefing data) wind direction is the direction the wind is blowing into and not as IRL, where the wind is blowing from.

 

I.e. to get a headwind when landing/taking of rwy 27 (a/c heading 270 degrees) you need wins set to 90 degrees.

 

But I agree something seems odd as you should be able to maintain indicated airspeed just the same regardless of wind direction.

 

Yea just try it for yourself dude, its weird :P

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  • 3 weeks later...

This is a very real bug. I don't know if it's because the ASI is a showing ground speed instead of airspeed, or if it's because the FM is using ground speed instead of airspeed to determine lift. But if you set a strong wind straight down the runway you have to maintain a considerably higher IAS with a headwind than you do with a tailwind when you're on final in order to maintain an appropriate glide path and angle of attack.

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Are you sure that the wind you put into the mission is blowing in the right direction?? The ME is backwards as far as placing wind into the mission is concerned. If you want to fly off of runway 27, you will need to place the wind at XX velocity and 090 degrees. The ME requires that you specify which direction the wind is blowing, not from which direction it is blowing FROM.

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There is certainly a problem with wind in the Viggen. Can't even take off with a 97kn headwind at 400km/h IAS. Nor does the ASI read anything when stationary with such a strong headwind.


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I was interested in this thread because the MiG-21 used to have a similar bug regarding cross wind flight where the MiG-21 was using ground speed in some of the aerodynamic calculations, resulting in continuous sideslip any time the airplane had a component of velocity that wasn't parallel to the wind velocity.

 

I can now confirm there is definitely a bug with the Viggen. I am aware of the DCS wind vector conventions, and I just tested taking off the Viggen clean with 20% fuel and a 97 knot headwind. The Viggen was barely able to to take off by the end of the runway, and needs about a 20 degree angle of attack to maintain level flight at 356 knots airspeed and full afterburner.

 

My appeal to Leatherneck is to ensure that ground speed is never used in aerodynamic calculations. Airspeed, or more specifically aircraft velocity relative to the wind velocity should be calculated as soon as the simulation inputs are received through the API. This wind relative velocity can be calculated by subtracting the velocity of the wind from the velocity of the aircraft relative to the ground. The result of this computation is what you use as the velocity of your aircraft in aerodynamic computations.

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Are you sure that the wind you put into the mission is blowing in the right direction?? The ME is backwards as far as placing wind into the mission is concerned. If you want to fly off of runway 27, you will need to place the wind at XX velocity and 090 degrees. The ME requires that you specify which direction the wind is blowing, not from which direction it is blowing FROM.

 

Wind direction is irrelevant in regard to the indicated airspeed at which an aircraft will stall in unaccelerated flight.

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Wind direction is irrelevant in regard to the indicated airspeed at which an aircraft will stall in unaccelerated flight.

 

I don't follow that.

 

Just for example, If your stall speed is 100 knots IAS and you are flying directly into a headwind of 20 knots, then surely your IAS stall speed is 80 knots IAS?

 

Similarly, if you have a tailwind of 20 knots, your stall speed will be 120 knots. At least that's my understanding.

 

However, in this topic, if the airspeed is not reported as such on our ASI, then there's a problem. :cry:

 

Off topic the L39 has a dual readout, IAS and ground speed....very handy! But not in our Viggen!

 

regards,

 

David

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I don't follow that.

 

Just for example, If your stall speed is 100 knots IAS and you are flying directly into a headwind of 20 knots, then surely your IAS stall speed is 80 knots IAS?

 

Similarly, if you have a tailwind of 20 knots, your stall speed will be 120 knots. At least that's my understanding.

 

 

Wind has nothing to do with stall speed. IAS is the speed you are moving through the air.

In your example, what would happen if you have a 100 knot headwind? would your stall speed be 0? No, it would still be 100 but your GROUND SPEED would be 0.

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I don't follow that.

 

Just for example, If your stall speed is 100 knots IAS and you are flying directly into a headwind of 20 knots, then surely your IAS stall speed is 80 knots IAS?

 

Similarly, if you have a tailwind of 20 knots, your stall speed will be 120 knots. At least that's my understanding.

 

However, in this topic, if the airspeed is not reported as such on our ASI, then there's a problem. :cry:

 

Off topic the L39 has a dual readout, IAS and ground speed....very handy! But not in our Viggen!

 

regards,

 

David

 

No, the stall speed concerns the speed of the aircraft relative to the surrounding air (the IAS). How fast the aircraft is moving relative to the ground is irrelevant. In your example, with a headwind of 20 kts and IAS 100 kts, you are moving 80 kts relative to the ground, but the stall speed is always given for IAS.

 

Re: the L-39, I'm pretty sure there's no readout for ground speed, although there is a separate readout for true airspeed.

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Off topic the L39 has a dual readout, IAS and ground speed....very handy! But not in our Viggen!

The dual readout in the L-39 is for IAS (Indicated Air Speed) and TAS (True Air Speed).

 

At standard sea level pressure and temperature, IAS is approx. equal to TAS (the speed the aircraft flies through the air mass).

 

At higher altitudes, lower pressures, etc. the IAS differs from TAS (less air molecules hitting the pitot tube, etc.). The L-39 takes these changes into account to display TAS.

 

TAS by it's self, does not compensate for a head/tail wind and read ground speed.

 

However TAS will equal ground speed if the air mass is stationary i.e. there is no head/tail wind.

 

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I don't follow that.

 

Just for example, If your stall speed is 100 knots IAS and you are flying directly into a headwind of 20 knots, then surely your IAS stall speed is 80 knots IAS?

 

Similarly, if you have a tailwind of 20 knots, your stall speed will be 120 knots. At least that's my understanding.

 

However, in this topic, if the airspeed is not reported as such on our ASI, then there's a problem. :cry:

 

Off topic the L39 has a dual readout, IAS and ground speed....very handy! But not in our Viggen!

 

regards,

 

David

 

In this situation the aircraft would be still flying at 120 knots with that headwind. Your ground speed would be what gets reduced to 80 knots. (Although this depends on your altitude but we'll assume that you're not doing this at an altitude too high over sea level)

With the tailwind it would be the opposite. Your ground speed will increase because the air is providing 20knots-worth of less resistance, making your ground speed faster, but 20 knots-worth of IAS will be lost because of this. Also, the second needle in the L-39 I believe is TAS or TRUE airspeed. Not ground speed.

 

*EDIT* alftand beat me to it :)

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Wind has nothing to do with stall speed. IAS is the speed you are moving through the air.

In your example, what would happen if you have a 100 knot headwind? would your stall speed be 0? No, it would still be 100 but your GROUND SPEED would be 0.

 

So what would your airspeed indicator show, given a 100 knot headwind, in order for you aircraft to stall?

 

Indicated Airspeed is the speed of the wind entering the Pitot tube and thus the airflow over your wings.

 

Your ground speed would indeed be 0, but your airspeed, referencing your airspeed indicator, would be 100 knots. Or if you went below stall speed, 99 knots.

 

I will have to get to differ with your statement that wind speed has nothing to do with stall speed.

 

Airbus build a safeguard into their landing speed, called Mini GS. The computers detect wind speed (head wind, crosswinds or tail wind and factor the wind speed into the final approach speed. The reason is to avoid approaching stall speed. Thus, Airbus believes wind speed components have an important affect stall speed.

 

Just my opinion sir, not everyone can agree in this world.:-)

 

 

On the subject of the Viggen. At moderate speeds 5 m/s headwind, I couldn't tell much difference on take off or landing. The flight path indicator was doing a fan dance as the aircraft settled on course. Could be the Doppler adjusting the course calculations. My wing man crashed on landing, but that's nothing to go by..:doh:

 

Regards

 

David

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Airbus build a safeguard into their landing speed, called Mini GS. The computers detect wind speed (head wind, crosswinds or tail wind and factor the wind speed into the final approach speed. The reason is to avoid approaching stall speed. Thus, Airbus believes wind speed components have an important affect stall speed.

 

I do not fly Airbus so i dont know specific systems. I do however fly Boeing 737s and if there is strong winds we usually add a couple of knots on top of our final approach speed to have a bit of a safety buffer in case the winds change rapidly. Our actual stall speed is the same tho, regardless of wind.

For example if u fly at IAS 100 kt and have a 10 knot headwind which rapidly Changes to 0 wind then your IAS would drop down to 90 and you might stall.

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This is not a matter of opinion, they are simply telling you how it is.

A head/tailwind has no effect on stall speed what so ever and Airbus is very much aware of this.

 

 

What GS Mini does is introduce a safety margin so that with gusting and changing winds you maintain a safe IAS when/if the wind changes. This is done by setting a minimum GROUND SPEED, which means you will be flying with a higher IAS during an approach with headwind.

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I was interested in this thread because the MiG-21 used to have a similar bug regarding cross wind flight where the MiG-21 was using ground speed in some of the aerodynamic calculations, resulting in continuous sideslip any time the airplane had a component of velocity that wasn't parallel to the wind velocity.

 

I can now confirm there is definitely a bug with the Viggen. I am aware of the DCS wind vector conventions, and I just tested taking off the Viggen clean with 20% fuel and a 97 knot headwind. The Viggen was barely able to to take off by the end of the runway, and needs about a 20 degree angle of attack to maintain level flight at 356 knots airspeed and full afterburner.

 

My appeal to Leatherneck is to ensure that ground speed is never used in aerodynamic calculations. Airspeed, or more specifically aircraft velocity relative to the wind velocity should be calculated as soon as the simulation inputs are received through the API. This wind relative velocity can be calculated by subtracting the velocity of the wind from the velocity of the aircraft relative to the ground. The result of this computation is what you use as the velocity of your aircraft in aerodynamic computations.

May be a stupid question, not sure if it's even relevant to a flight model or might be na unit conversion issue but isn't the 97 knots wind a too extreme one? 63 knots is a 12 in Beaufort scale which basically is a hurricane wind.

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An aircraft, or a wing, stalls at a certain angle of attack. AoA is the angle between the relative airflow and the chord of the wing. An aircraft can stall at any airspeed.

However, the AoA at 1G, where lift equals weight, you find the Vs1, or Stall speed at 1G.

In theory, this stall speed is only dependent of the relative airflow. It doesn't matter if the aircraft is travelling at this speed in still air, a huge fan is blowing the wind at the aircraft or the airmass is moving. It's the wind speed that meets the wing that is important.

So, if your stall speed is 100 kt, and you're facing a 20 kt headwind, the indicated airspeed (IAS, the speed relative the air) will be 100 kt, but the ground speed (speed in relation to the ground) will be 80 kt. Wind does not affect the IAS stall speed. But, it's good airmanship to allow for shifting winds and gusts, so you should add speed so that the aircraft won't stall because the heavy aircraft won't change it's momentum as fast as the light air will.

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Just my opinion sir, not everyone can agree in this world.:-)

 

You don't get to have opinions about facts. You're just wrong here, sorry.

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