Wind breaks FM

While certain FM components are behaving correctly in windy conditions, others are not.

 

Since I could not get a track to replay properly, attached instead is a short mission from 1.5 effectively demonstrating the issue within seconds. All conditions are default with wind set to 50m/s from the south.

 

The MiG starts facing south directly into the wind, a boat to the left near the shore. Place the nose over the boat and the MiG enters a heavy slip.

MiG-21_wind.miz

Identified as not really being a bug. 50 m/s winds is essentially a hurricane jetstream that will have adverse effects on an airframe, especially one shaped as the MiG-21.

:lol: 50 meters per second = 111 miles per hour! What are you trying to do in those winds ;)

that will have adverse effects on an airframe, especially one shaped as the MiG-21.

That is incorrect. a 50 m/s wind will not cause adverse effects on the structure or control of an aircraft. the aerodynamics of a plane are only concerned with the speed of the plane relative to the air. The plane doesn't care what that wind is doing relative to the ground, so a 50 m/s wind speed will only cause a 50 m/s deviation in the ground speed of an airplane. Judging from external landmarks it might appear like slip to the pilot, but it should not cause any slip relative to the wind, so no abnormal slip should appear on any cockpit instruments.

 

I just checked the manual, and the ADI is in fact showing slip in this case. This is a bug.

 

The EFM API passes in both the aircraft ground velocity and wind velocity from DCS. This bug should be resolved by using the aircraft velocity relative to the atmosphere as aircraft velocity in the aerodynamics. If you represent ground velocity by 'v' and wind velocity by 'w', then this is simply the vector equation v - w

Edited October 7, 2015 by VincentLaw

Identified as not really being a bug. 50 m/s winds is essentially a hurricane jetstream that will have adverse effects on an airframe, especially one shaped as the MiG-21.

 

:lol: 50 meters per second = 111 miles per hour! What are you trying to do in those winds ;)

The speed of the wind only serves to magnify the problem so it is more immediately visible :) It is still present even at 1m/s and is most certainly a bug!

 

As mentioned by Vincent, steady 50m/s wind should have no impact on the handling of the aircraft - if we were to fly under the hood and remove all visibility of the outside world we should not be able to notice any difference in any instruments or controls whatsoever aside from those making use of ground speed/track (RSBN etc). There is no airframe design in the world that will change aerodynamics in such a way as that simply makes no sense; it would mean the aircraft is no longer using just airspeed to fly, which brings me to my brief findings...

 

At present, the aircraft will fight against the pilot in impossible ways in any sort of windy conditions. From cursory testing, it appears that some FM components give an effect similar to using ground speed for FM calculations as the MiG will always attempt to roll towards the track over the ground, "weathervaning" away from the wind (much like the pitot tube vanes). Application of rudder and aileron is required simply to fly straight and level. Steady control inputs are able to stabilize the aircraft, demonstrating that the wind itself is steady. The unusual roll behavior stops somewhere between fully slipped and zero slip when certain airspeed and ground speed conditions reach equilibrium, further demonstrating the issue.

 

Think of this issue in another way: every time an aircraft approaches certain jet streams, it is approaching winds in the neighborhood of 50m/s. In no case is an adjustment in coordinated control inputs required. No aircraft requires rudder inputs based on wind! If Novak has spent much time above 8-9km then he has more than likely spent some time in winds over 50m/s, and I would ask if he has any experiences of the aircraft gradually transitioning into heavily uncoordinated flight. Without rudder trim, cruise in such conditions would be tiring!

 

The MiG-21 is the only aircraft exhibiting this behavior. This means that in any mission that implements realistic winds aloft, the MiG is the only DCS aircraft requiring heavy rudder/stick in simple symmetric cruise. Hopefully this is recognized as a bug and fixed :)

 

Thanks for your time!

Identified as not really being a bug. 50 m/s winds is essentially a hurricane jetstream that will have adverse effects on an airframe, especially one shaped as the MiG-21.

 

A strange thing to say given that transport category aircraft routinely fly in the north atlantic jet streams which are considered slow when 'only' 100kts, and that the North Atlantic tracks are updated daily to either take advantage of (eastbound) or avoid (westbound) them...

 

You can see from today's map that the eastbound jetstream varies from 100kts (two triangles) to 140kts (two triangles, 4 lines).

 

 

An aircraft in flight cannot possibly want to turn into the wind - or away from the wind, or at all - as there is no difference to the aircraft between travelling at 500kmh IAS in a 250kmh wind, and travelling at 750kmh IAS in a 10kmh wind.

 

The high wind speeds are exposing a bug that is masked under slower conditions.

 

Please do check further.

Edited October 8, 2015 by Lascaille
Broken map (fixed map)

Yeah, the way I see it a slip should not be happening - maybe only when changing direction, but not flying straight (provided wind speed is more-or-less contstant).

 

Commercial airliners are flying into 100km/h + winds on a daily basic in the jetstreams. The only thing that's changing is that if you have a strong side wind component - you need to compensate by pointing your nose at an angle to desired track to maintain it.

Yeah, the way I see it a slip should not be happening - maybe only when changing direction, but not flying straight (provided wind speed is more-or-less contstant).

 

May we avoid the word 'slip' - that is a word which has a meaning referring to the flight conditions deliberately created by crossed controls, where the angle of attack deviates from the perpendicular to the wing plane.

 

While there may be slip occurring the common word for the behaviour we are seeing is 'weathercocking'. This refers to the tendency of aircraft parked on the ground to point into the wind as a result the reaction of wind forces on the vertical stabilizer vs friction forces against the tires. However weathercocking is not used to describe the behaviour of aircraft in the air as it does not occur in the air... I would still want to avoid the word 'slip' however as it describes something that is possible, this behaviour is impossible.

 

However back to the matter at hand, having loaded the mission and experimented there is no argument from me: this is a bug.

Edited October 8, 2015 by Lascaille
Slip...

Something is wrong here.

 

The aircraft is influenced by crosswind like it's on the ground, it weahtervanes as someone said.

I obviously should refrain from passing on second hand information which I am poorly informed about.

It is possible there was a misunderstanding on the track did not play correctly.

 

For now, it's been noted and issue reopened.

Though I believe the smoke trail is more DCS in general.

I don't know for my winding arrows, but the cock looks pretty cool. Difference in surfaces size and shape around CoG, will create forces that will try to rotate the aircraft along any of axis provided mentioned asymmetry exist, moving the focus (aerod. centre) to new position.

I ran out of paper for the images 5 and 6, side wind effect on wings.

 

ADDED /edited/: This is the (simplified) situation in the air (aircraft airborne). Situation on the ground is more complex since the aircraft have 3 contact points which also affect the forces distribution around the CoG, which - in this case - resides in front and above the main wheels line.

 

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.

Edited October 19, 2015 by Dolphin887

Does this behaviour afflict version 1.2.16 also?

I don't know for my winding arrows, but the cock looks pretty cool. Difference in surfaces size and shape around CoG, will create forces that will try to rotate the aircraft along any of axis provided mentioned asymmetry exist, moving the focus (aerod. centre) to new position.

I ran out of paper for the images 5 and 6, side wind effect on wings.

 

On the ground, sure, but flying the plane should only react only to the dynamic change of flow or turbulence, which is not really modelled in DCS AFAIK. Or did I miss something?

As SNAFU is saying it is the aircraft reaction to crosswind in the air which is the problem.

 

If you fly in a smooth 40 knot crosswind you should not notice this in any way except in reference to the ground (RSBN, Doppler, visually, RADAR etc). Right now the MiG behaves like others described above, ie. it yaws into the wind.

Yawing into the wind sounds like broadly proper behaviour for an aerodynamically stable object doesn't it?

Yawing into the wind sounds like broadly proper behaviour for an aerodynamically stable object doesn't it?

 

While airborne? No.

While airborne? No.

 

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

 

Having said that... there's one thing I do find odd. I would've expected the Mig to roll to the left (with the wind coming from the right). It has a huge vertical stabilizer and the R-wing (upwind) is being blown at right angles (provided you turned 45 degree left away from the wind) _and_ the L-wind is pretty much shielded from any airflow because of the fuselage.

Edited October 19, 2015 by chaos

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

 

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

 

Let's look at a hot air baloon on a windy day. Tethered on the ground, the baloon is subject to the force of the wind. Once it gets airborne, the wind is accelerating the ballon. Once the ballon has reached the speed of the wind, no wind force will act on it anymore. It will move along with zero velocity relative to the air, while the air itselfe moves along the ground (and with it the baloon). In that condition, the wind has no aerodynamic effect on the baloon, just as uniform wind aerodynamically doesn't affect an aircraft*.

 

Edit: *Well, except for the transition phase after takeoff, as seenin the balloon example. Just wanted to add that for clarity.

Edited October 19, 2015 by MBot

But it should not. A plane in the air is like a fish in the water. Neither ones dynamics should be effected by the current for non-transient situations.

 

PS: MBot was faster... Post is referring to chaos`post.

Except the Mig is hardly a uniform shape like the balloon. Wont a crosswind for example 'pushing' harder against the vertical stabilizer than the actual fuselage of the plane, and thus cause it to weather-vane?

There is no crosswind for the plane. It moves relative to the air mass and the air-mass moves relative to the ground. The last one movement is what we call wind. Once in the air, the plane moves only relative to the air and therefore relative to the ground.

Edited October 19, 2015 by SNAFU

There is no crosswind for the plane. It moves relative to the air mass and the air-mass moves relative to the ground. The last one movement is what we call wind. Once in the air, the plane moves only relative to the air and therefore relative to the ground.

 

Ok got it now, makes sense. It seems Novak's later drawings under 3 and 4 are wrong then. (For the plane in the air).

Except the Mig is hardly a uniform shape like the balloon. Wont a crosswind for example 'pushing' harder against the vertical stabilizer than the actual fuselage of the plane, and thus cause it to weather-vane?

 

Nope. If there was no wind at the nose then it would turn. Otherwise the object in a fluid will simply move relative with the fluid.

Except the Mig is hardly a uniform shape like the balloon. Wont a crosswind for example 'pushing' harder against the vertical stabilizer than the actual fuselage of the plane, and thus cause it to weather-vane?

 

No, because the whole aircraft is moving with the wind (just as the balloon is), therfore between aicraft and air, no crosswind exists.

 

Or another famous example. You are on a boat in the middle of a large, uniformly flowing river (let's say the Amazon). Your engine is out. Your boat can point forward or sidewarts, your boat remains in the center of the river and will not move towards eithether shore. You can also drop into the water and take a swim around without fearing that your boat is washed away by the current. To both your boath and yourselfe, the surounding water looks as it is not moving at all. No hydrodynamic force is acting on you, even though you are moving down the Amazon.

 

Edit: Multisniped :)