Wind breaks FM

[chaos]

I think Justin Case is correct, there is no weathervaning while you are airborne as there is no aerodynamic force.

 

No, because the whole aircraft is moving with the wind

 

I see what you're saying now... yes, I don't have an explanation for that. Odd is all I can come up with :-)

Edited October 19, 2015 by chaos

[VincentLaw]

Yes it does, actually. The aircraft will yaw into the wind because it acts as a weathervane. No input to rudder or aileron required.

Imagine, if you will, that the wind was moving 1000000000000 km/h. It doesn't really matter what the speed is, just an arbitrarily large number. For simplicity, ignore effects of the speed of light, structural integrity, and gravity, and assume the air is infinite and with uniform velocity.

 

Now, if you start with an airplane at zero km/h in this massive wind, it will rapildy weathervane into the wind because it's relative velocity to the wind is 1000000000000 km/h... however, without any other applied forces, drag from the wind on the airframe will soon cause the airplane to reach equilibrium with the wind until there is 0 drag. At this point the airplane will be traveling in the same direction and speed as the air particles.

 

Now, from the perspective of the pilot, the ground will appear to whiz by at incredible speed, but there will no longer be any weathervaning effect no matter what orientation the plane takes because as far as the plane is concerned it is traveling ZERO km/h relative to the wind.

 

At this point, if the pilot turns on the engine and tries to fly around, all of the instruments (sans GPS, inertial navigation, and similar) will read like the pilot is in normal flight. There will be no abnormal side slip. The pilot can fly around and the plane will not weathervane towards the wind because the plane is already flying relative to the wind. The plane will point in any direction the pilot commands as easily as it would without wind.

 

In aerodynamics only the speed of the plane relative to the speed of the air matters. It does not matter how fast the wind is moving compared to the ground. A pilot in a 100 km/h wind effectively cannot tell if the wind is traveling 100 km/h or if the Earth itself is moving 100 km/h and the wind is still. It wouldn't make a difference.

 

If you still think it should make a difference, consider that Earth is currently orbiting the sun at close to 108000 km/h. That means that relative to the surface of the sun (ignoring rotation of the sun), the average wind speed on Earth is close to 108000 km/h. Now, why don't we weathervane that direction?

Edited October 19, 2015 by VincentLaw

[Dolphin887]

I forgot to mention that drawings 1 and 2 refer to "static" condition, while 3 and 4 refer to "dynamic". There is no point to edit the post once again.

[Lascaille]

I forgot to mention that drawings 1 and 2 refer to "static" condition, while 3 and 4 refer to "dynamic". There is no point to edit the post once again.

 

There is a need to clarify turbulence vs wind. Turbulence is 'wind within wind' i.e. the air as a whole has a speed of 50kmh at 0 degrees, a small 1m^2 sector of that air might be moving downwards at 20kmh but another sector next to that might be moving upwards at 20kmh. Turbulence ofc will rock the aircraft and can cause it to roll - e.g. if one wing is in an updraft and the other in a downdraft.

 

We are not discussing turbulence, but wind. Wind with zero turbulence (both inside and outside of DCS) involves the entire air mass moving stably as one. In that wind there is NO effect upon the airborne aircraft (yawing, rolling, pitching, no effect at all) because it is impossible, there is no reaction point or pivot point for the aircraft. The wind does not 'blow' on the aircraft, the wind just moves the air and the aircraft just moves inside the air. If there is delta (moving from a region of 50kmh wind to 150kmh wind) there will be inertia forces but that is not the case for this scenario, this involves a static constant global wind.

[OnlyforDCS]

There is a need to clarify turbulence vs wind. Turbulence is 'wind within wind' i.e. the air as a whole has a speed of 50kmh at 0 degrees, a small 1m^2 sector of that air might be moving downwards at 20kmh but another sector next to that might be moving upwards at 20kmh. Turbulence ofc will rock the aircraft and can cause it to roll - e.g. if one wing is in an updraft and the other in a downdraft.

 

We are not discussing turbulence, but wind. Wind with zero turbulence (both inside and outside of DCS) involves the entire air mass moving stably as one. In that wind there is NO effect upon the airborne aircraft (yawing, rolling, pitching, no effect at all) because it is impossible, there is no reaction point or pivot point for the aircraft. The wind does not 'blow' on the aircraft, the wind just moves the air and the aircraft just moves inside the air. If there is delta (moving from a region of 50kmh wind to 150kmh wind) there will be inertia forces but that is not the case for this scenario, this involves a static constant global wind.

 

I think Novak has already acknowledged this, hence the "static" (i.e. uniform or constant wind) and "dynamic" (i.e. increasing or decreasing wind) conditions.

[Izgud]

Thanks for your response, Novak! Unfortunately, I think there is still some sort of misunderstanding here. I like your idea of illustrations, so I made something that should help explain the problem.

 

Regards.

[Lascaille]

As for the significance of the wind effect on the aircraft, note that every aircraft have strict takeoff/landing wind speed limit, for the desired velocity vector have to be several times larger than the wind vector in order to maintain satisfactory stability/agility aircraft reserve.

 

Crosswind speed limits are because the aircraft has to be brought level and oriented to the runway heading centerline (decrab) before it contacts the runway, or the gear will shear off. With crosswinds greater than a certain amount the aircraft cannot safely decrab without running out of runway width or a wing strike.

 

Headwind: I read what you say but B777 has NO headwind limit for manually flown takeoff and landing! (FYI crosswind limit 37kts, autoland headwind limit 25kts)

 

Tailwind limits are due to various technical limitations, runway length and gear speed limitations. B777 10kts.

[Dolphin887]

Thanks for your response, Novak! Unfortunately, I think there is still some sort of misunderstanding here. I like your idea of illustrations, so I made something that should help explain the problem.

 

Regards.

 

This helps a lot if this is what happens. I will check the vectors operations once I get home. Thank you.

 

 

Headwind: I read what you say but B777 has NO headwind limit for manually flown takeoff and landing! (FYI crosswind limit 37kts, autoland headwind limit 25kts)

 

I found this strange but it might apply to big aircraft with reverse thrust... All aircraft that I have flown had 180 degrees wind speed limit for TO/LNDG procedures. In case of 21, 90 degrees side wind limit is 15m/s for the approach and landing without the drag chute use, and 10m/s with the drag chute upon landing. Tail wind is limited to 4m/s.

[Lascaille]

I found this strange but it might apply to big aircraft with reverse thrust... All aircraft that I have flown had 180 degrees wind speed limit for TO/LNDG procedures. In case of 21, 90 degrees side wind limit is 15m/s for the approach and landing without the drag chute use, and 10m/s with the drag chute upon landing. Tail wind is limited to 4m/s.

 

30m/s is the crosswind limit? What is the headwind limit?

 

I would guess small and light aircraft have limits due to ground handling. If you were to land C172 for example in 50kts headwind (I think you would land at full throttle to achieve positive ground speed) I don't see how you would get it off the runway. Either the wind would blow it straight whenever you tried to turn (that weathercocking effect) or it would turn upside down once the wind to the downwind wing was blocked.

[Manuel_108]

Thanks for your response, Novak! Unfortunately, I think there is still some sort of misunderstanding here. I like your idea of illustrations, so I made something that should help explain the problem.

 

Regards.

 

Now this is a proper illustration! Hats off, I'm interested to see where this observation will lead.

[Bestandskraft]

I would like to point out that this issue has already been in part discussed in this thread, and was specifically mentioned in this post. I suspect the "uncommanded roll" issue and the wind issue that this thread here deals with are in some way linked.

[Hummingbird]

This is how crosswind affects an aircraft:

 

 

Since a crosswind strikes all the vertical parts of an aircraft, most of which are located aft of the center, this causes a turning moment aft of the center which will want to swing the nose towards the opposite direction of the wind, i.e . from the ground it will look like the aircraft is flying slightly sideways (sideslip) in comparison to the direction the aircraft is moving relative to the ground.

Edited October 20, 2015 by Hummingbird
Removed a superflous "of"

[Hummingbird]

A similar illustration:

[VincentLaw]

Since a crosswind strikes all of the vertical parts of the aircraft, most of which are located aft of the center, this will cause the aircraft to swing its nose towards the direction of any crosswind.

A crosswind gust will cause the airplane to yaw, but a steady crosswind will not. This is a well established fact that is not up for debate. Try reading my previous posts. Your second figure shows the aircraft heading necessary to maintain the same ground speed. The airplane does not automatically yaw this direction.

 

your graphics are not illustrating a crosswind "affecting" an aircraft. They are illustrating the pilot compensating for crosswind to achieve the desired ground speed.

[OnlyforDCS]

A crosswind gust will cause the airplane to yaw, but a steady crosswind will not. This is a well established fact that is not up for debate. Try reading my previous posts. Your second figure shows the aircraft heading necessary to maintain the same ground speed. The airplane does not automatically yaw this direction.

 

your graphics are not illustrating a crosswind "affecting" an aircraft. They are illustrating the pilot compensating for crosswind to achieve the desired ground speed.

 

Yes, this is correct, Hummingbird is mislabeling those graphs, only the very first top down view graph is correct. Unfortunately not everyone is familiar with fluid dynamics, and I too reached an incorrect conclusion initially since it's been ages since I've studied that stuff called 'physics' at university. I went ahead and read up a bit in the meantime but it's actually really simple stuff conceptually.

 

Basically a steady, non-changing wind condition will approximately equal a steady ideal (or Newtonian)fluid flow. A body in contact with the ground will try to weathervane because of Newton's 3rd law of motion, however once in the air it can be viewed as being in the same frame of reference as the air-mass around it.

Edited October 20, 2015 by OnlyforDCS

[Hummingbird]

I thought we were talking about maintaining the same direction of movement relative to the ground?

 

If you don't have a specific direction you want to maintain, well then this will happen:

 

Strong blowing winds however will cause some natural crabbing by acting on the vertical stab, as I tried to explain.

Edited October 20, 2015 by Hummingbird

[OnlyforDCS]

Yes that is correct, however that is not what is happening with the Mig. Look at this post:

 

http://forums.eagle.ru/showpost.php?p=2524430&postcount=31

 

The second picture specifically. The Mig is apparently behaving very oddly in crosswinds at the moment.

Edited October 20, 2015 by OnlyforDCS

[Hummingbird]

Yes that is correct, however that is not what is happening with the Mig. Look at this post:

 

http://forums.eagle.ru/showpost.php?p=2524430&postcount=31

 

The second picture specifically. The Mig is apparently behaving very oddly in crosswinds at the moment.

 

So crosswinds make the ingame MiG automatically turn into the wind without any pilot input, am I understanding that correctly? Or does it simply require more rudder to stay on path?

[OnlyforDCS]

So crosswinds make the ingame MiG automatically turn into the wind without any pilot input, am I understanding that correctly?

 

Yes, or rather away from the crosswind.

[Hummingbird]

Yes, or rather away from the crosswind.

 

If the aircraft yaws into or away from the crosswind without any pilot input then something is indeed very wrong.

[Lascaille]

The actual situation seems to be both - if you have the crosswind directly on your right wing (88-92 degrees) wing the aircraft will yaw right _into_ it, but will stabilize at some angle to it. If you have the crosswind barely off the nose (i.e. 2 degrees) the aircraft will yaw right _away_ from it and also end up stable.

[SinusoidDelta]

This problem exists with the F-15 as well. The FM is unpredictable if you deviate from a STD day.

[cofcorpse]

This problem exists with the F-15 as well. The FM is unpredictable if you deviate from a STD day.

Can you describe it in some F-15 topic?

[NoJoe]

Something worth noting, just to avoid confusion while people test this: The smoke effects in DCS World 1.5 appear to stream from the aircraft along its ground track (incorrectly), and it looks like this effects multiple aircraft, not just the MiG-21. You can see this in Justin Case's screenshot in post 9 of this thread.

http://forums.eagle.ru/showpost.php?p=2510253&postcount=9

 

Contrails seem to behave correctly, though. So if you fire your guns at high altitude you can get the gunsmoke and the contrail moving away from the airplane at different angles. Anyway, something to keep in mind to avoid confusion while testing windy conditions.

 

(the sideslip the MiG-21 experiences while flying in steady wind is definitely not correct though, and the other planes don't seem to do this. So there is something strange going on with the MiG flight model)

[chaos]

Sorry to drag this out again but something is bothering me about all this...

 

MBot wrote in a previous post "No, because the whole aircraft is moving with the wind (just as the balloon is), therfore between aicraft and air, no crosswind exists.".

 

It doesn't do that in the sim because the airpseed indicator would have to read '0' if it moved along with the wind. Also, it would crash because no lift is created. Something is amiss with the FM but it's not what you guys are suggesting...