Please fix that deflected propwash effect :-(

[Pilum]

Jcomm: I was actually writing this reply to your post in the Me109K4 thread but I see you updated that so I post my reply here instead. OTOH I guess this is the more appropriate place for it anyway :smilewink:

 

Weather the "reverse" weathercocking effect you refer to is a true representation of how a high hp single propeller fighter aircraft should behave or if the modelling in DCS is lacking I will not make a judgement on. Especially since I have not tried this out myself in DCS.

 

That being said, I do believe based on your description it may actually be correctly modeled: That a power application can act destabilizing in yaw is I believe well documented: You can find a number of NACA reports by Herbert Ribner on the subject, e.g. NACA Wartime Report L-219 Propellers in yaw etc.

 

So in a scenario where you have a strong sidewind from your 3 O’clock, this will cause a sideforec acting ahead of the c.g. to your 9 O’Clock if power is applied. If this is enough to overcome the weatherwane effect I will leave left unsaid but I don’t think it’s as implausible as it may sound.

 

While the NACA reports provide the theory, I think there is also an intuitive way to look at this: If you think of momentum theory and think of the aircraft being enveloped in a slipstream tube, the air is being sucked into this momentum tube from the right being this deflected to the rear. Looking at this picture from above it’s similar to the deflection of air by a wing and the resultant force should analogously be forward to the left, i.e. roughly to your 10-11 O’clock.

 

Since to my understanding the DCS model is quite accurate in modelling slipstream, it may be so that what we see in the sim is actually a correct representation since there is definitely a connection to theory on the destabilizing effect of high solidity propellers such as we see on the Dora and Pony.

 

OTOH a model is always a model and it may be there are simplifications that have resulted in this effect being more pronounced in DCS than IRL. However, I don’t see that we have any basis to say this so and it may very well be so that what we have in DCS is the real deal.

 

So to conclude, I think we should take care in passing judgement on the crosswind behaviour of aircraft currently modeled in DCS and consequently expect the Me109K4 to be any different.

[NineLine]

I have chatted with Yo-Yo on this, he doesnt feel there is an issue and that it is modelled correctly in the sim.

[Pilum]

I have chatted with Yo-Yo on this, he doesnt feel there is an issue and that it is modelled correctly in the sim.

 

Well yes and that was my point: I think what we see can actually be explained by theory and while it may be counterintuitive it may very well be the correct representation :)

[Anatoli-Kagari9]

Well yes and that was my point: I think what we see can actually be explained by theory and while it may be counterintuitive it may very well be the correct representation :)

 

Well,

 

I would really like this to be true, and look forward to a better understanding of the aerodynamics supporting it... I would gladly change the OP tittle to "Please DON'T fix that deflected propwash effect" :-)

 

Thx a LOT Pilum for your answer!

[Pilum]

Well,

 

I would really like this to be true, and look forward to a better understanding of the aerodynamics supporting it... I would gladly change the OP tittle to "Please DON'T fix that deflected propwash effect" :-)

 

Thx a LOT Pilum for your answer!

 

Well, note that I'm covering my bets cause I'm not saying it's right or wrong just that it's difficult to be sure either way and I believe an aquittal is the way to go if there is insufficient evidence ;)

Edited November 19, 2014 by Pilum

[Art-J]

I have chatted with Yo-Yo on this, he doesnt feel there is an issue and that it is modelled correctly in the sim.

 

Well, It would be nice If he could hop in and share a few thoughts about the subject then. Right now paper and net sources describing crosswind ops with "intuitive" controls input are dime a dozen, while one has to struggle to find a source describing something similar to "reversed" controls logic at low speed, required in DCS taildraggers.

 

I liked the way Yo-Yo explained a groundlooping phenomenon in take-off / landing the Dora-9 threads, also using some good youtube clips when necessary. But here, there are some legitimate doubts about details of slipstream modelling and from our point of view accepting it only because one "doesn't feel there is an issue" just doesn't cut it.

[NineLine]

I am sure if he has time he might, but honestly he is a busy boy... he already shares a ton on here, both English and Russian side... for now you will have to do with my response till or if he finds time to repeat himself and say its not an issue :)

[[DBS]TH0R]

Well, It would be nice If he could hop in and share a few thoughts about the subject then. Right now paper and net sources describing crosswind ops with "intuitive" controls input are dime a dozen, while one has to struggle to find a source describing something similar to "reversed" controls logic at low speed, required in DCS taildraggers.

 

I liked the way Yo-Yo explained a groundlooping phenomenon in take-off / landing the Dora-9 threads, also using some good youtube clips when necessary. But here, there are some legitimate doubts about details of slipstream modelling and from our point of view accepting it only because one "doesn't feel there is an issue" just doesn't cut it.

 

Seconded.

[Anatoli-Kagari9]

A forum member from another site, who happens to be an aeronautical engineer, and very experienced pilot too, is also at it, and reading the NACA reports mentioned by Pilum...

 

I hope I can get some additional info too from him, who is now busy with the maths on those NACA reports :-)

[Art-J]

I'm sure Yo-Yo is very busy now with upcoming 109K. I would also presume the handling aspect we're discussing here will be present when this plane is released, after all it's not airframe specific, but related to general prop slipstream modeling in our sim.

 

This means, however, that most probably this won't be the last topic related to the subject, until we get thorough response form the man himself, somewhere in the future.

[Yo-Yo]

I'm sure Yo-Yo is very busy now with upcoming 109K. I would also presume the handling aspect we're discussing here will be present when this plane is released, after all it's not airframe specific, but related to general prop slipstream modeling in our sim.

 

This means, however, that most probably this won't be the last topic related to the subject, until we get thorough response form the man himself, somewhere in the future.

 

What response do you wait for? I said all I could say after I checked the things and not once.

If somebody wants to discuss about this matter and where does the slipstream column hits the airframe dealing with x-wind of 20 knots - please welcome. The sources for necessary math are available, the math is not so complicated - come on... I am open for a discuss.

 

If the plane behaves properly in flight, in sideslip - why don't you think it is wrong at TO?

[BitMaster]

This is actually a quite dynamic process and it is very acceleration dependant when which force is holding the crown and has the longest vector, aka applied force.

 

When looking at Vector equation and acceleration with a wind perpendicular to the vector of acceleration you have quite a dynamic force of how much wind will effect your course as with higher G forward the resulting vector of a perpendicular force goes towards ZERO when reaching 1 G forward acceleration.

 

As a Pony can accelerate rather FAST, it does throw in some funny bonbons for you as the force you will need to compensate the wind are NOT static, or defined by the wind speed alone, but also and FOREMOST of how fast you ACCELERATE. While you are accelerating certain laws apply, one is power vectors or how it is named in english, at 90° working angle and 1 G or more things become zero, whatever it is coming from 90° looses it's effect.

 

The same law appllies to any car on a quarter mile drag race.

Leaving sticky tires ( toothing instead of friction ) aside, no car on wheels can accelerate faster than 1 G or 9,81m/s² as with this acceleration your masses downforce ( 90° angle to your direction of movement ) becomes a plain zero and your tyres spin without any grip. To achieve the results actual racers have you need additional downforce and/or special tyres that "glue" to the tarmac rather than rolling on top of them.

 

Understanding this behaviour of natural forces is cumbersome enough as it happens all in 1 pot in 5 seconds, going all the way from full wind force at zero speed to almost no or no wind force at all if you accelerate at 1G at any time on your way down the runway to an ever changing fraction of that force while you accelerate with anything from 0.1-1 G.

 

On top of that comes what you is part of the OT, the prop thrust and engine torque.

 

As a man of understanding I have to say, this is COMPLEX and cannot be overseen before what will happen when. This woud be true with less dynamic ingredients but on this special case...wow very very complex.

 

To add another one, inertia:

 

That whole plane has a certain mass and that needs a certain force and time of appliance to have effect.

That again adds a factor of uncertainty, delay and downgrade in of acting froces....LoL

 

If Yo-Yo says it's ok, it's ok for me cause whatever happens in those 10-15 seconds is anythimg but simple and explainable, it isnt. You cannot devide the forces and reverse engineer where that YAW came from, neither to what exact momentum. Maybe fairly close with a good nose of salting it so it matches what we expect but never in finite shares and values.

 

...and dont forget that other thing, tyre traction is also going towards zero while accelerated ( dependent on G of Earth = 1g ).

This is really really complex.

 

There will be a ceratin uncertainty in the game, that is really certain :)

 

Bit

Edited November 22, 2014 by BitMaster

[BitMaster]

To put it correct in vector physics, maybe this wasnt really clear.

 

When to forces ( vectors ) hit each other at a 90° angle ( car = forward, G = downward ) and the forward movement !!"DURING"!! acceleration is same or greater as the force perpendicular the resulting force of the perpendicular force = zero.

 

So when the wind would accelerate you with 0.15G you would need 0.15G speed 90° offset to the wind to zero it out WHILE you accelerate at least with 0.15G.

 

The same effect is used when accelerating vehicles out of orbit into interplanetary trajectories, just way more simple than this pony equation

with a Merlin up front and a 2 wheel tail dragger built-in-instability kinda thing.

 

= flying to the mars can be simpler in math than putting to paper what the Pony does while accelerating down the runway, haunted by the torque, pulled py the prop, rattled by the propwash and disturbed by the wind.

 

Haha

 

Bit

[BitMaster]

I tend to check my nose only in DCS to not bogglr my mind. forget the wind, it wont help you knowing where it come from if you cant read your plane.

You may think you can predict what will happen, but as you may have read above there are many factors deciding what the nose will do, forget to pre-calculate what will happen, you cant and no logic can as it is complex and complex things cannot be pre-thought.

 

Watch the nose and counteract with rudder, assist tracking with elevator up for low to mid speeds and hang your wing INTO the wind, on ground by just lowering that aileron ( like roll onto that side the wing comes from ) and when airborne actually lowering that wing half 1 foot down into the wind, that helps to fight it's effect and especially gusts if they occur. this is a very wise idea to do on approach with X-wind and gusts. you do NOT want to see that wing half flip over you which is way harder to control than having the wind blow it downward.

Steer INTO the wind as soon as you are airborne to stabilze and gain speed. The plane by itslef wants to go that way as soon as you hit anything below 90° AoA to the wind and the slower you are while having that attitude, windvane effect.

 

Always work with the wind and never against it when Landing or Taking Off.

[Anatoli-Kagari9]

If the plane behaves properly in flight, in sideslip - why don't you think it is wrong at TO?

 

Well, I guess that inflight, it is moving with the airmass, without any contact with a source of drag like the ground.

 

The strange thing Yo-Yo, is that when you come down for landing, as you approach the rw in a crab, with your nose pointing into the wind, the aircraft starts to automatically uncrab and align with the rw centerline, or even past it downwind, as it get's closer to the ground, without you having to input ANY controls! Strange, don't you think?

 

Is it because when it enters ground effect this additional slisptream effect gains force?

 

If you find the time, please do this test. Approach Batumi with a direct x-wind component of, say, 7m/s ( it doesn't have to be 10 m/s ). Your aircraft will approach in a crab, with it's track aligned with the rw, but, as you start losing altitude and come closer to the ground, you'll notice that your nose starts to automatically uncrab ( ? )

Edited November 22, 2014 by jcomm

[BitMaster]

Isnt this a nature of sims themselves, discussed over and over ?!

 

When looking at Yo-Yo and the dev team, knowing you have to make a compromise at some point from understanding the nature of complexity,

I can only say: The more extreme the situation is, the easier for the devs

tell the code what to do and have a pleasing result as, like Hurrican...heck, your Merlin wont matter, the whole thing will be blown over by a 440km/h gust, period, no head aches for the devs. The more the forces are equal to each other and the more there are in general, the more headaches.

In other words, it was obvios the hurrican tossed that pony over and broke it to pieces but what caused that mere yaw at Meter 139,75 down the runway, throttle at 42,38 MP, 2742 RPM, wind 2,5ms at 19°, 3879kg TO weight and good tyres ??? yes, it was only a little yaw and a cough, but what caused it ???? Many ingredients, many answers, many gradients of truth and uncertainty.

 

Some midnight thoughts while my son occupies my DCS machine with a thing called Dota-2 that also runs on Macs. How I miss those days when Mac only had 3 games... :)

 

Bit

[Murmur]

Is it because when it enters ground effect this additional slisptream effect gains force?

 

Have you tried to establish if it happens out of ground effect? For example trying a "virtual" landing at altitude, gradually decreasing airspeed, etc. In order to discern if it's triggered by ground effect or by a certain airspeed range.

[Anatoli-Kagari9]

Have you tried to establish if it happens out of ground effect? For example trying a "virtual" landing at altitude, gradually decreasing airspeed, etc. In order to discern if it's triggered by ground effect or by a certain airspeed range.

 

Excellent idea Murmur, but I never tested it that way...

 

I will try next weekend - about to leave for a whole week in EDDF at the trainning center ... well... the PC sims will not come wih me :-( ( well, PSX will :-))) ), but some pencil and paper will help for the lonesome nights...

[GGTharos]

I don't know if this applies to all in-game weather, but wind is shaped to match a realistic airmass profile. At lure agl the affirms isn't moving the same way as it is just 100' higher.

[Weta43]

^ Testing something else last week I got results appeared to confirm that at least with Dynamic Weather, wind speed is profiled and the effect on the player aircraft reduces as you get closer to the ground.

Edited November 22, 2014 by Weta43

[Anatoli-Kagari9]

Hmmm,

 

ok, it can be the wind speed being reduced by friction, but then please bare in mind that it should back, not veer....

 

Then I will have to test it with a jet, but the only I can use is the Su25 T

 

Lot's of tests to perform when I return :-)

 

P.S.: Ah! There's a surprise sale going on, so, just bought the Sabre, and will be able to test with this one next week :-)

Edited November 22, 2014 by jcomm

[Yo-Yo]

Well, I guess that inflight, it is moving with the airmass, without any contact with a source of drag like the ground.

 

The strange thing Yo-Yo, is that when you come down for landing, as you approach the rw in a crab, with your nose pointing into the wind, the aircraft starts to automatically uncrab and align with the rw centerline, or even past it downwind, as it get's closer to the ground, without you having to input ANY controls! Strange, don't you think?

 

 

I already wrote about it, possibly even twice: THE WIND GETS SLOWER NEAR THE SURFACE. And please do not tell that it is not so in real life... we use statistically averaged data from professional experiments and I personally checked it by myself. :)

[ec-swr]

I already wrote about it, possibly even twice: THE WIND GETS SLOWER NEAR THE SURFACE. And please do not tell that it is not so in real life... we use statistically averaged data from professional experiments and I personally checked it by myself. :)

 

 

of course anyone who says otherwise is that it has not flown in real life

 

 

http://en.wikipedia.org/wiki/Wind_gradient

[Bucic]

Isn't there a way to set up a FM testing environment with completely uniform wind profile?

[Flagrum]

The question at hand (#41) was why the aircraft aligns with the runway automatically.

 

But if the wind gets slower near ground, shouldn't the aircraft crab more into the wind - assuming unchanged rudder deflection?