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Yakcopter or Automatic flaps? + Video


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I ran some of it through google translate- it is more about the change of AoA and pitching movement of the La5 and Yak when applying flaps.

This is the discussion they mentioned in the latest dev diary.

 

Overall, for me this shows again how very very deep the knowledge of Devs in aerodynamics really is. The discussion in the russian thread was pretty rough at times with Han saying some serious stuff, and in the end the devs prove their depitction of the plane behaviour in the game to be spot on.

 

As much as I am critical about the flaps, I personally trust the Devs more now.

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I ran some of it through google translate- it is more about the change of AoA and pitching movement of the La5 and Yak when applying flaps.

This is the discussion they mentioned in the latest dev diary.

 

Overall, for me this shows again how very very deep the knowledge of Devs in aerodynamics really is. The discussion in the russian thread was pretty rough at times with Han saying some serious stuff, and in the end the devs prove their depitction of the plane behaviour in the game to be spot on.

 

As much as I am critical about the flaps, I personally trust the Devs more now.

With all due respect, that's nonsense. I also have high respect towards the devs, that is why it shocks me how they can leave such a basic anomaly untreated. Flaps increase the AoA and if you drop them near the critical AoA (low speed, climbs, turns) you exceed it, and the airfoil HAS TO stall, not produce even more lift. This is super basic.

 

 

I'm more and more certain that an overdone propwash effect is the cause of everything. It's always so strong that the wings in BoS seldom exceed the critical aoa. It comes from RoF, where it produced enough airflow for a wingless Camel to fly at full power. As soon as the engine was off it fell like it should. Same here. You should be way past critical aoa, but the prop supplies the wing with airflow infinitely. That's why flaps actually help in such situations.

Edited by Reflected
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With all due respect, that's nonsense. I also have high respect towards the devs, that is why it shocks me how they can leave such a basic anomaly untreated. Flaps increase the AoA and if you drop them near the critical AoA (low speed, climbs, turns) you exceed it, and the airfoil HAS TO stall, not produce even more lift. This is super basic.

 

 

I'm more and more certain that an overdone propwash effect is the cause of everything. It's always so strong that the wings in BoS seldom exceed the critical aoa. It comes from RoF, where it produced enough airflow for a wingless Camel to fly at full power. As soon as the engine was off it fell like it should. Same here. You should be way past critical aoa, but the prop supplies the wing with airflow infinitely. That's why flaps actually help in such situations.

 

I didn't say that I think flaps behavior is correct, only that I don't think anymore that the issue is so basic and easy to understand. If it was, we would have definite proof that sth is wrong for a very long time.. If it is "super basic", please put together a proper, scientifically sound FM report. As the Devs prove in the last diary, they are willing to implement changes, even going so far as testing something IRL after a discussion became ugly.

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I didn't mean the problem was basic. I wish it was and we could find the root cause. What i meant was that if an airfoil exceeds the critical aoa it should stall. When near critical aoa lowering the flaps is a sure way to exceed it. This is super basic, like first class of aviation theory.

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I didn't mean the problem was basic. I wish it was and we could find the root cause. What i meant was that if an airfoil exceeds the critical aoa it should stall. When near critical aoa lowering the flaps is a sure way to exceed it. This is super basic, like first class of aviation theory.

 

http://www.boldmethod.com/blog/2013/10/how-does-lowering-flaps-affect-angle-of-attack/

 

The key point is that although lowering flaps increase AoA they also reduce critical AoA at the same time

 

Cheers Dakpilot

Edited by Dakpilot
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Thanks mate. This illustrates well what I wanted to explain. Now imagine that AoA1 is very close to the critical AoA. Increasing it to AoA2 (by lowering the flaps) makes the airfoil exceed the critical AoA. Instant recipe for a stall. But not in BoS.

 

aoa_clean.png

 

aoa_dirty.png

Edited by Reflected
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But at the same time the critical AoA is reduced when lowering the flaps 

 

  1. The angle of attack is the angle between the cord line and relative airflow, extending the flaps will change the cord line in such a way that the normal angle of attack of the wing relative to the airflow is increased, so now the change in angle required to reach the critical AOA is less hence the critical AOA is deceased. 

post-6177-0-64730600-1450630092_thumb.jpg

 

Cheers Dakpilot

 

*Edit*

 

Not wanting or enjoying getting into complicated discussions of moving CoL/CoP and how different types of flaps will react etc....I will recount

 

I once had to do a slightly overloaded EVAC from a place we did not want to be for long, the dirt runway was wet and the takeoff roll long

 

the tree line at the end of the makeshift runway was getting closer and the stall warning was becoming irritating, we would not clear it, dropping a bit of flap allowed us to clear the trees without stalling

 

Cheers Dakpilot

Edited by Dakpilot
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Dakpilot, when you drop flaps a bit necessary AOA to produce lift decreases so an airplane flying at low speed can decrease its AOA and use the abundant energy to clear an obstacle. That doesn't contradict the fact, that a wing flying at a very high AOA will probably stall if you suddenly drop flaps.

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See the diagram I just added to post  above, as flaps increase the AoA you decrease the critical AoA, you cannot do one without the other

 

In my example,I know I am reaching critical angle due to stall warning (and paying attention to the airspeed) an aircraft stalls due to exceeding critical AoA

 

Cheers Dakpilot

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Together with the critical AoAs the wwing profile also changes with landing flaps deployed. The relation of lift vs drag per increasing AoA is completely different than on a wing in normal configuration.

Furthermore different flaps have different aerodynamicle effects. The Yak's split flaps don't behave the same as flower flaps on a Boeing for example.

 

The fact is landing flaps are not meant for any other flight state than that, landing. Even a laymen without any aerodynamic or engineering background can see that in a game where this is being abused to a skyhigh limit must not be perfectly accurate.

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Dakpilot, thanks for the diagrams and the clarification. Something is still not clear:

 

Doesn't your graph say that Flaps down critical AoA < Flaps up critical AoA ? That is what I've heard too.

 

If so, the wing should stall all the more! Near stall, we're almost at the critical AoA. We lower the flaps, which increases the actual AoA, surpassing the critical AoA of the flaps up wing. But according to your graph, the critical AoA of the flaps down wing is even lower, hence easier to exceed - you know what I mean?

 

Let's say our AoA is 15 degrees. Critical AoA is 17 degrees. We drop the flaps, that inclines the wing cord by 5 degrees, and voilá, our AoA is 20 degrees now. 3 degrees past the critical limit. There is no lift whatsoever beyond that. If the critical AoA of the flaps down wing is lower than normally, then let's say it's 16 degrees. But our 20 degrees ought to exceed that too, just like the original 17, doesn't it?

 

Therefore dropping flaps near critical AoA should be very, VERY counterproductive. Unless of course, and the same moment you pitch forward and decrease the AoA - then the new camber will give you more lift. But if the wing is held there, it ought to stall, according to your graph, and to plain logic too. 

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Flaps only decrease critical AoA if the reference remains the original chord line.

 

How so? Can you please elaborate?

 

According to this graph, the critical alpha is lower with flaps down:

 

post-6177-0-64730600-1450630092_thumb.jp

 

Our new chord line forms a larger angle with the airflow that remains the same. I don't see why the AoA shouldn't increase when the flaps are down, and I don't see what can prevent it from decreasing the limit.

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herefore dropping flaps near critical AoA should be very, VERY counterproductive.

 

For large deflections of flaps the general effect is very counterproductive.  Small deflections of flaps can be beneficial because of the way in which realize the coefficient of pressure changes.  Small deflections will result in 75% of the flap design lift changes for only ~25% of the drag changes of the flap design.  Large deflections see the other 25% lift and 75% drag.

 

 

 

2wdsduo.jpg

 

 

Flaps can be useful for maneuvering but must be used correctly.  They are not an "instant turn performance button".

 

Study of turn performance flaps and supercharging.pdf

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Well it does not drop 80 degrees flap in one instant, as the flap is dropped AoA is increased but incrementally at the same time CL is increased and critical AoA is reduced

 

The pitch angle of the aircraft is pretty irrelevant, aircraft are not designed to stall in landing or take-off procedure when flaps are deployed when nearing critical AoA

 

Cheers Dakpilot

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Ok, 50 then, this is totally beside the point as long as it's not 10-20 degrees of takeoff flap. Also, you keep saying critical aoa is reduced. This means it's more prone to stall. A lower critical limit means it's easier to exceed. I really can't explain in other words, and I really want to understand why you guys think it shouldn't stall, despite all the graphs that have been posted here.

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Okay think of it a different way, it reduces your stalling angle of attack

 

" If you are out practicing power off stalls in your favorite GA airplane (level flight) you'll notice that your pitch angle (in level flight equal to your geometric AoA) at stall is greater in the no-flap config than with flaps extended. You can also see this effect in that a fully flared off no-flap landing will result in a higher pitch angle at touchdown."

 

Cheers Dakpilot

 

*EDIT* 

 

As i said in an earlier post this is a confusing subject with many contradicting terms, which is why I related a story of approaching "critical AoA" (stalling) and used flaps, this did not cause the aircraft to stall, in fact it delayed that situation and enabled me to clear the trees

 

For further confusion/unconfusion please read all of this small thread  ;)

 

http://www.pprune.org/flying-instructors-examiners/153457-angle-attack.html

 

Cheers Dakpilot

Edited by Dakpilot
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Ahaa, so you're saying that the pitch angle  is lower when you stall with flaps down. That's because the angle of incidence of the flaps down wing is higher. If that's what you are sayin, I understand now, and I see why it is so.

But please note that the graph is about the AoA of the wing, it doesn't say anything about pitch. Wing chord vs airflow. Period.

 

I read the thread you linked, very interesting to see that so many "professional pilots" have serious lack of understanding of aerodynamics. There were some though, who explained very well what I've been trying to clarify.

 

Your story is indeed very interesting, but I can think of 2 explanations:

a) those were fowler type flaps

b) you extended them just a little, so that the new chord vs airflow didn't exceed the critical AoA yet, but it was enough to produce sufficient extra lift to clear those trees.

 

However, extending 50 degrees of flaps all of a sudden on a Yak, right at stalling speeds (prop hanging, or super tight turns near stall speed) MUST make the wing exceed that critical AoA. Which it never does.

 

Another real life example:

When I fly a glider, I bleed the speed, and right before it stalls, I throw the stick to the left. What happens? The plane spins down to the right. Why? Because the right aileron moves down, increasing the AoA of the right wing that was already at the edge of critical AoA. Now it exceeds it ,so it stalls. It's the same principle with flaps, only it's not just one wing, but both.

Edited by Reflected
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Yes critical angle of attack wing with flaps down is lower then with clean wing. It is clearly seen on polares AoA vs Cl.  It simple that with flaps lowered wings create much more drag and much faster turbulent airflow over wings airfoil.

 

In old Il2 sim there were obiously error in fligh model data for all planes where with flaps down planes got the same cAoA as clean wings thats why planes there with flaps down could do super manouvers without penalty ( only more drag cofficent).  In some mods for Il2 1946 these bug was corrected by some modders.  I think in CLOD it was also corrected.

 

iIf i  remember coreectly from real life flying plane with flaps down ( even Cessna) got more faster and nasty stall characterisic then with clean wing during during dynamicall stall.

 

 

BTW another interesting fact in BOS is that Yak-1 with flaps down could turn easy inside Yak-1 with clean wings and still retain energy good  where for example 109 or Fw 190 with similar situation can't.  We made simple test where one start with flaps down in front of clean plane and start to turn. Yak-1 with flaps down always outurn Yak-1 with clean config and could dominate fight with easy.   Other hand Fw 190 or 109 with flaps down cant do the the same.

Edited by 303_Kwiatek
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How so?

Because that's how it is. :)

Can you please elaborate?

I don't really feel like it right now, maybe another day. Maybe in the meantime you can look at detailed fluid dynamics to understand the theory or just dig into some airfoil data that is specific to how chord is defined for the x-axis on a chart, and you'll get practical evidence.

 

Hope you don't mind me leaving this at that for now.

 

Edited by JtD
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Last time I tested, the critical angle of attack is lower with extended flaps, in a power off condition. I don't think that changed.

Edited by Matt
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It would help to understand effective vs absolute angle of attack.

 

Absolute angle of attack = Effective angle of attack + Induced angle of attack

 

Induced angle of attack is a result of drag due to lift.  The heavier and higher wing loaded the aircraft, the larger the induced angle of attack.

 

In very low aspect ratio wings (3 or less) this is an issue.  

 

Effective angle of attack is derived for each section by processing the 2D polar data thru a set of formulation designed for wings with Aspect Ratio's of three or less.

 

In High Aspect Ratio wings the formula for our absolute angle of attack changes and becomes:

 

Absolute Angle of Attack = 2D polar angle of Attack + Induced Angle of Attack

 

In other words, Absolute angle of attack and effective angle of attack based on the 2D polar data are one and same for the purposes of discussing most World War II fighters.  

 

Induced Angle of Attack only becomes important when discussing the Body Angle in High Aspect ratio wings.

 

 

 

iIf i  remember coreectly from real life flying plane with flaps down ( even Cessna) got more faster and nasty stall characterisic then with clean wing during during dynamicall stall.

 

Flying around with 80 degrees of flaps maneuvering is laughable and ranks up there with taking an airplane into the sky with greatly reduced fuel onboard.  It is just not something that is an issue in aviation for good reasons.

 

Large deflections of flaps will not help an aircraft turn better except at speeds below normal stall speeds.  At those speeds it will only turn better because the clean wing cannot fly there.  It is not something that is combat useful or has any practical use.  End of Story.

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Thanks, it's a very interesting video, but it still doesn't prove my argument pro nor con.

 

I just don't understand what is not understood here.

 

1) AoA = chord vs airflow

2) chord = straight line between leading edge - trailing edge

3) lowering the flaps will change the chord line

4) the angle between the airflow and the flaps down chord line is higher than it was with the original chord line

5) The AoA of a wing has to be BELOW the critical angle, otherwise the wing stalls

6) The critical AoA of a flaps down wing is lower than that one of a flaps up wing

 

Which one(s) are incorrect? If all of the m are correct, which they are, I can't, for the life of me, see how these don't add up to my original statement.

 

Just for the heck of it, let me post the graph again. As you lower the flaps you instantly jump to the right on the x axis. Hence, if you are close enough to the critical limit, you are bound to exceed it. Been there, done that. In real life.

 

post-12951-0-37525100-1450694856.jpeg


 

 

Large deflections of flaps will not help an aircraft turn better except at speeds below normal stall speeds.  At those speeds it will only turn better because the clean wing cannot fly there.  It is not something that is combat useful or has any practical use.  End of Story.

 

Exactly!

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Thanks, it's a very interesting video, but it still doesn't prove my argument pro nor con.

 

I just don't understand what is not understood here.

 

1) AoA = chord vs airflow

2) chord = straight line between leading edge - trailing edge

3) lowering the flaps will change the chord line

4) the angle between the airflow and the flaps down chord line is higher than it was with the original chord line

5) The AoA of a wing has to be BELOW the critical angle, otherwise the wing stalls

6) The critical AoA of a flaps down wing is lower than that one of a flaps up wing

 

Which one(s) are incorrect?

That list makes it easy to pin it down: For the chart you are posting, 3) is not correct. Charts that show you how the critical AoA flaps down is lower than flaps up, always (I haven't seen a single one different) use the original flaps up chord also for the flaps down chart lines.

 

In other words - additional camber does not lead to a decrease of the stalling AoA.

 

Or to make it easier to visualize - if the chart you posted is supposed to be applicable and you have an untwisted wing with a flap system that does not go full span, the wing section that has flaps down has the same angle of attack as the wing section without the flap.

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I see your point, thanks! While I agree that it doesn't change the chord line of the entire wing, it's enough for the wing to stall partially, that should decrease the total lift heavily, and invariably increase the AoA (the plane starts to sink).

 

In my example with the glider, only the wingtip stalls where the ailerons are, and yet the glider spins down.

 

I also see now what you mean: so the red curve's x axis refers to the angle between the airflow and the chord of the flaps up wing? Makes sense now. (It is lower on the chart, because we practically increased the angle of incidence of the wings, right?)

 

HOWEVER: even if we only consider the flaps up graph, lowering the flaps close enough to the critical AoA should still make the wing stall - at least the part where the flaps extend.

Edited by Reflected
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Some input from two sources on the change of trim due to flaps on the Me-109:

 

British Royal Aircraft Establishment report RM2361 Me109E handling and performance evaluation, by M B Morgan, September 1940:

 

”The juxtaposition of the tailplane-adjusting wheel and the flap-control wheel was also considered an excellent feature, as the wheels may be operated together with one hand and the change of trim due to flaps thereby automatically corrected.”

 

From article ”Four of the finest” in Royal Air Force Yearbook 1975: English test pilot Captain Eric Brown on the captured Me109G6 he flew in 1944:

 

”The flaps were raised manually by means of the outer of two concentrically-mounted wheels to the pilot’s left, the inner wheel adjusting the tailplane incidence. Thus the wheels could be moved together to counteract the change in trim as the flaps came up.”

 

Now compare this with BoS: Which way (as seen by pilot) do you turn the wheels to trim and deploy flaps in the BoS Me-109?

 

The flap wheel goes anticlockwise when deploying the flaps which is correct. However, you need to turn the trim wheel clockwise to counter the nose-up trim we see in BoS.

 

So something basic is obviously off with the flap modeling in BoS since the trim change due to flaps leads to the opposite effect as IRL……

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Some input from two sources on the change of trim due to flaps on the Me-109:

 

British Royal Aircraft Establishment report RM2361 Me109E handling and performance evaluation, by M B Morgan, September 1940:

 

”The juxtaposition of the tailplane-adjusting wheel and the flap-control wheel was also considered an excellent feature, as the wheels may be operated together with one hand and the change of trim due to flaps thereby automatically corrected.”

 

From article ”Four of the finest” in Royal Air Force Yearbook 1975: English test pilot Captain Eric Brown on the captured Me109G6 he flew in 1944:

 

”The flaps were raised manually by means of the outer of two concentrically-mounted wheels to the pilot’s left, the inner wheel adjusting the tailplane incidence. Thus the wheels could be moved together to counteract the change in trim as the flaps came up.”

 

Now compare this with BoS: Which way (as seen by pilot) do you turn the wheels to trim and deploy flaps in the BoS Me-109?

 

The flap wheel goes anticlockwise when deploying the flaps which is correct. However, you need to turn the trim wheel clockwise to counter the nose-up trim we see in BoS.

 

So something basic is obviously off with the flap modeling in BoS since the trim change due to flaps leads to the opposite effect as IRL……

 

 

Remember.  Coefficient of moment is dependant upon coefficient of lift...

 

If players are  deploying flaps outside of the POH recommended speeds, then the Coefficient of Moment may not reflect the behavior noted in the Operating Instructions.  

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Remember.  Coefficient of moment is dependant upon coefficient of lift...

 

If players are  deploying flaps outside of the POH recommended speeds, then the Coefficient of Moment may not reflect the behavior noted in the Operating Instructions.  

 

Yes, but even if flaps are deployed at low speeds within limits this effect shows up so there is some fundamental issue with the BoS flap FM since trim effects are reversed as compared to IRL.....

Edited by Holtzauge
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I see your point, thanks! While I agree that it doesn't change the chord line of the entire wing, it's enough for the wing to stall partially, that should decrease the total lift heavily, and invariably increase the AoA (the plane starts to sink).

 

In my example with the glider, only the wingtip stalls where the ailerons are, and yet the glider spins down.

 

I also see now what you mean: so the red curve's x axis refers to the angle between the airflow and the chord of the flaps up wing? Makes sense now. (It is lower on the chart, because we practically increased the angle of incidence of the wings, right?)

 

HOWEVER: even if we only consider the flaps up graph, lowering the flaps close enough to the critical AoA should still make the wing stall - at least the part where the flaps extend.

 

If I understood your point earlier you were saying that the stall aoa will decrease if flap is deployed and I think this is correct:

 

When it comes to the stall aoa for a given profile this will be lower with flaps deployed than with flaps up: The reason for this is that both increasing aoa and deflecting flaps increase the circulation lift of the wing. Now when circulation lift is increased, this moves the stagnation point backwards on the lower edge of the wing which increases the leading edge suction on the nose of the profile. However, since too much leading edge suction leads to flow separation there is only so much aoa and flap deflection in combination a profile can take without stalling. Hence if the flap is deflected, the max aoa will be lowered as the diagram shows.

 

However, this is assuming the same aoa reference as with the wing with no flap deflection which is also the reference angle that should and usually is used just for the reasons to avoid the confusion that changing the reference point introduces as this thread shows. ;)

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Yes, but even if flaps are deployed at low speeds within limits this effect shows up so there is some fundamental issue with the BoS flap FM since trim effects are reversed as compared to IRL.....

To me this seems to be a small problem with FM of flaps. In real Bf 109 flaps deflection leads to shift of center of lift backwards which creates nose heavy momentum. Hence you need to trim nose up. It seems this shift is not modeled in IL2 BOS.

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We use flaps for landing and some time on short fields takeoff but rarely the best is taking off clean... .. never for turns IRL ( In game the flaps use is used way to many time and the Yak in RL do not need it ) Flaps create a pitch down that is controlled with throttling. 

 

Here some old video back in 2004 and the last time I touch a Yak, I filmed with a small bad video take recorder for my very good friend Jim. Basically at the time the only 9 Yak-9U's in the planet are from there. I played and left my mark on all of them since very early 90's. I never touch a Yak9 or 3 ever since, This video's was the last time and a good bye to our best Yak the latest and we just mounted a new rebuilt engine and the most powerful of all the Yak9 flying today a first start of a two years work. This aircraft maintain the same royal air guard colors today but with a different number on it's fuselage overseas.

 

You can see some flaps activated ;)

 

New owner starting our baby... https://www.youtube.com/watch?v=H40_AovSQOc

Edited by GOZR
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To me this seems to be a small problem with FM of flaps. In real Bf 109 flaps deflection leads to shift of center of lift backwards which creates nose heavy momentum. Hence you need to trim nose up. It seems this shift is not modeled in IL2 BOS.

 

That's true. The resultant lift from the flap is well behind the 1/4 point on the chord so it leads to a nose down momentum and this can be seen in wing profile figures as well. However, in addition to this there is also the effect of the downwash on the tail so this needs to be taken into account as well.

 

 

That has just been explained in the latest dev update. 

 

http://forum.il2sturmovik.com/topic/168-developer-diary/page-3?do=findComment&comment=315273

 

Yeah, I saw that but I would rather the Me-109 behaved like the quotes I made. Also, I don't think the plane in the devs post is  "similar to WWII era fighters modeled in the sim" really. Can you see it the other way around? We make a claim about how the sim should behave and make a reference to that plane? Honestly, I don't think we would have been taken seriously if we did....

 

Anyway, we know there is not always a one to one connection between the stick deflection on our desktop and what the controls do in the sim, e.g. high speed pullout and roll performance is doctored to simulate the real airplane behaviour. We should see the same type of modeling for flap deflection: If the plane IRL (without any input from the pilot) dips it's nose when we drop flaps it should do the same in the sim IMHO.

Edited by Holtzauge
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