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when i actuate flap down from full retract to first step in level flight, why does the nose pitch down abruptly?....the downward trajectory worsens if you go

to step 2, 3, etc.

all other planes lift their nose which seems reasonable as you are adding lift.

devs pls explain.

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Generally the overall flight model in BOX is excellent.  But it is not perfect.  Reading an article (sorry it has been a while so I can not remember where to provide a link) regarding the BOX P51 flight model written by a pilot who flew the real aircraft his comments were very positive but he did specifically mention that on the real aircraft, when set up for landing with full flaps, it was not necessary to dial in full elevator up trim as in BOX.  Even with full elevator up trim in BOX it is necessary to use back pressure to the stick to maintain glide slope.  He said in the real aircraft this is unnecessary and it can be trimmed to a fairly neutral elevator setting during the landing approach.  I think the need to add back pressure with full flaps and full elevator up trim is a bit over done.  It just doesn't feel right. 

 

Then again I have never flown a P51 so I may be wrong.   It would be interesting to hear the developers perspective on this as I am sure they have done extensive research on the P51 when developing the flight model.

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I just looked at my DCS P51D flight manual, and it also states in general terms that the plane becomes nose heavy

with flaps engaged or landing gear down, also increase in drag lowers the nose and reduction in drag raises the nose.

It just seems odd, that this plane is the only exception to the rule.

The whole reasoning behind combat flaps, takeoff flaps is to increase lift,

why does it have a reverse effect on this one plane?

Just curious....can someone give a complex answer pls?

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So out of curiosity I tried landing the P51 in DCS and it behaves the same as BOX with respect to lowering the nose when I extend the flaps and the gear.  However where it differs is that I can fully correct for this with elevator up trim so I can land with little or no back pressure on the stick.  This feels much better and more realistic but that does not mean it is correct.  BTW, overall I much prefer the P51 flight modle in BOX over DCS except for this one minor issue. 

 

Would be interesting to know how the real plane behaves.  Can the elevator be trimmed to neutral stick pressure during landing or is back pressure needed on the stick in addition to full up elevator trim?    

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agreed, but in level flight at 300mph, if you drop the flaps 1 step, which should be combat or takeoff

position, why on this plane does the nose dive, whilst on all other planes it lifts?

i have tried at similar speeds with different fuel loads from 11% to 100%, same result,

in DCS, i do not get this behavior.

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10 hours ago, genelovesjezebel said:

agreed, but in level flight at 300mph, if you drop the flaps 1 step, which should be combat or takeoff

position, why on this plane does the nose dive, whilst on all other planes it lifts?

i have tried at similar speeds with different fuel loads from 11% to 100%, same result,

in DCS, i do not get this behavior.

Because every aircraft is different? Many planes have a nose down moment, as the air resistance increases on the lower part of the plane and rotates the plane over the nose/tail axis.
Not everything is lift in that equation. Yes flaps increase lift but they also add drag.

The commenters below are ofc right. I had it wrong out of the top of my head. Flaps create lift and that lift vector in relation to the CG of the aircraft results in either a pitch up or pitch down movement.

Edited by ACG_DerSheriff
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45 minutes ago, genelovesjezebel said:

agreed, but in level flight at 300mph, if you drop the flaps 1 step, which should be combat or takeoff

position, why on this plane does the nose dive, whilst on all other planes it lifts?

i have tried at similar speeds with different fuel loads from 11% to 100%, same result,

in DCS, i do not get this behavior.

 

I tested P51 flaps in both BOX and DCS at 5500 feet 300 MPH indicated and level flight.  Then I added one notch flaps to each and both exhibited a strong nose down tendency for me.  Actually DCS may have an even stronger nose down tendency in this test.  The only difference is at low landing speeds where BOX doesn't have enough elevator trim authority to correct for the nose down tendency and DCS does.

 

Not sure why the P51 exhibits this behavior  but possibly the unique laminar flow wing design has something to do with it?  

Edited by S10JlAbraxis
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With regard to needing full nose up trim and lack of elevator authority in the landing configuration, the P-51 has the exact same problem at high speed - not enough elevator authority, and needs significant trim input to maintain control.  This is almost certainly wrong as I documented here: 

 

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7 hours ago, genelovesjezebel said:

when i actuate flap down from full retract to first step in level flight, why does the nose pitch down abruptly?....the downward trajectory worsens if you go

to step 2, 3, etc.

all other planes lift their nose which seems reasonable as you are adding lift.

devs pls explain.

 

You're adding lift and drag, but you also change longitudinal (pitch) moment of the airfoil, which many seem to forget about. In planes the interaction of all three factors is not always the same (depends on airfoil & flaps type plus general airframe layout) and can result in some pitch down or some pitch up. 

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6 hours ago, ACG_DerSheriff said:

Many planes have a nose down moment, as the air resistance increases on the lower part of the plane and rotates the plane over the nose/tail axis.

 

I've read this several times on this forum now. The height at which drag is created has minimum effect on the nose up/down trend. You'd need to massively decelerate or to massively create thrust to feel that effect. In practice it is nearly irrelevant when compared to the change of the centre of lift.

 

As the flap is extended, the wing not only does create more lift, it also changes the (longitudinal) position of where it is created. As a trend, it is created further back, so obviously the nose wants to go down for balance reasons. The extension of the flaps, and also the change of attitude of the entire aircraft coming with it, also changes the airflow over the tail, which as a trend will create a more downforce (or less lift), which leads to a nose up reaction of the aircraft. These two major changes in lift distribution work against each other and therefore it is a question of design what happens to the aircraft when flaps are lowered.

 

Art-J posted while I was typing, saying the same in a much shorter text. Thought I'd post mine nonetheless.

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absolutely agree vbf-12_kw, you are close to me as i am in Vancouver.

i have been playing all flight sims since Jane's ww2 fighters circa 1998.

flown the mustang in every iteration of any flight sim since.

none, produced the behaviour as modelled here.

can we somehow ask an actual p51 pilot what is the

real behaviour of the flaps.

i do not believe that these so called laminar wings,

which is just adspeak, make a diff on lift.

all wings produce lift, if you extend them you create more lift

to the point where drag overtakes lift.

no way, that a 10% drop of the p51's flap creates more drag than lift.

not buying it.

something is seriously wrong here.

so on take-off, fully loaded with bombs to the hilt.

a mustang pilot would use no flaps on take-off?

come on, that's not reasonable.

riddle me this batman...what are laminar wings?

just thinner maybe....come on ppl, let's get to the bottom of this.

 

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See How It Flies

 

Link to section 5.5 of “See How It Flies”, an excellent text that is free online (and a great read), where vertical damping, roll damping, and stalls — and in particular how those things are affected by flaps is discussed. Of interest to the posters in this topic may be section 5.5.5, which talks about the effect of flap extension on trim speeds. The gist of it is there are a several factors that come into play when discussing whether or not an aircraft will pitch up or down with flap extension, and it really depends on the design of the aircraft.

 

Great book. I used to point students to it all the time when I used to instruct as it often helped them understand certain concepts by presenting them in ways that were orthogonal to traditional texts.

 

 

 

 

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56 minutes ago, Kablamoman said:

Great book. I used to point students to it all the time when I used to instruct as it often helped them understand certain concepts by presenting them in ways that were orthogonal to traditional texts.

 

Your link is blocked by Browser Guard for hosting malware.

 

I see you "used to instruct". Perhaps that was due to an insistence on using the word "orthogonal"? 

Edited by unreasonable
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1 hour ago, genelovesjezebel said:

flown the mustang in every iteration of any flight sim since.

none, produced the behaviour as modelled here.

That is because they were are badly modelled in your games (which comes as no surprise). Flaps indicing nose down attitude is a very basic feature of most aircraft. It even works both ways. In gliders you even exploit that fact to ajust your flight attitude to always match perfect level flight over most of the speed range. It increases gliding performance considerably when going fast and lifting flaps above neutral to make the nose come up again from the downward position induced by high speed flying to minimize frontal area.

 

The Mustang gets progressively nose down attitude when deploying the flaps. It was something that was also used as a convenience when doing strafing runs. You can fly rather fast while having the flaps one notch down. Having guns pointing down will make strafing much safer and easier.

 

Things are as they should.

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22 minutes ago, unreasonable said:

 

Your link is blocked by Browser Guard for hosting malware.

 

I see you "used to instruct". Perhaps that was due to an insistence on using the word "orthogonal"? 


Heh.

 

Nice to meet you, too.

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44 minutes ago, unreasonable said:

Perhaps that was due to an insistence on using the word "orthogonal"? 

It is a bit orthogonal indeed. I quote from there:

 

" [...] On most airplanes, extending the flaps tends to lower the trim speed, just as if you had dialed in some nose-up trim. [...]"

 

I haven't come across a single one that goes nose up when setting flaps down. I mean, there can be, haven't seen a single one though. Anyone has a PN of an aircraft describing such behaviour?

 

Until then, I file it under  O for "orthogonal".

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7 minutes ago, ZachariasX said:

It is a bit orthogonal indeed. I quote from there:

 

" [...] On most airplanes, extending the flaps tends to lower the trim speed, just as if you had dialed in some nose-up trim. [...]"

 

I haven't come across a single one that goes nose up when setting flaps down. I mean, there can be, haven't seen a single one though. Anyone has a PN of an aircraft describing such behaviour?

 

Until then, I file it under  O for "orthogonal".


In that same section you quoted it listed two real world examples. Unless you’re talking about the sim?

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3 minutes ago, Kablamoman said:


In that same section you quoted it listed two real world examples. Unless you’re talking about the sim?

The Mooney is quoted as "nose down" (as I should think is normal) but the nose up trim is referred to as "in most aircraft". I haven't sat in any of these then.

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9 minutes ago, ZachariasX said:

The Mooney is quoted as "nose down" (as I should think is normal) but the nose up trim is referred to as "in most aircraft". I haven't sat in any of these then.


you haven’t sat in a Cessna?

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IIrc, the Bf110 had a nose up reaction to lowering flaps. The manuals I just checked unfortunately don't tell, they simply say "re-trim". Not sure where I got it from, but maybe it's worth checking.

 

Edit: B-25 manual says nose up when flaps down. Got one. :)

 

image.png.65f6580a2746d4af9f753529c9b014ea.png

Edited by JtD
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17 minutes ago, Kablamoman said:


you haven’t sat in a Cessna?

Haven't landed any myself. But you did? And did the author of the linked text mention a Cessna in that text? Or where did that come from?

 

But ok yes, one then.

 

48 minutes ago, JtD said:

IIrc, the Bf110 had a nose up reaction to lowering flaps. The manuals I just checked unfortunately don't tell, they simply say "re-trim". Not sure where I got it from, but maybe it's worth checking.

 

Oh yes! I checked Eric Browns account of piloting the Bf-110 and he says that as well.

 

"[...] Once off the ground the flaps were retractedat 150 m, this amount of altitude being necessaryfor safety because of the considerable nose-down change of trim that accompanied upward flap movement. [...]"

 

Two.

 

All aircraft then? Certrainly not the Spitfire. (And others.) But said notion certainly impacted its initial FM. Thankfully it was corrected. None of the gliders with flaps. It is their main purpose adjuststing flight attitude in said manner.

 

It is ok for there being (even many) exceptions to the rule. But if a generalization is far from helpful, on the contrary even, then I'll leave it under "O".

 

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23 minutes ago, ZachariasX said:

Haven't landed any myself. But you did? And did the author of the linked text mention a Cessna in that text? Or where did that come from?

 


Yes.
 

The text specifically used examples of a Cessna 152, a Piper Cherokee, and a Mooney to illustrate the point that the trim response to flap deployment is wholly dependent on the airframe design.

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16 minutes ago, Kablamoman said:

Yes.
 

The text specifically used examples of a Cessna 152, a Piper Cherokee, and a Mooney to illustrate the point that the trim response to flap deployment is wholly dependent on the airframe design.

That is a very good point indeed.

 

 

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14 hours ago, Art-J said:

@genelovesjezebel - these are the comparison videos some guys above meant I guess. I don't remember what Agathos said about trim changes caused by flaps, but you'll probably find what you need there:

 

https://www.youtube.com/watch?v=NDCkexCZrpA

 

https://www.youtube.com/watch?v=TR6PuZ2IKjk

 

 

Interesting after listening following is the high level conclusion for BOX:

 

1) Real plane rolls on the ground easily at 900 RPM, BOX requires 1500 RPM to get moving and seems to lack thrust and or experiences too much ground resistance. 

2) BOX take off is fairly close to the real plane but the rate of speed gain once airborne is too low vs real plane - seems to lack thrust.

3) Also on take off it takes more rudder input to control prop torque effects than in the real plane.

3) In level flight BOX seems to point too high on the horizon vs the real plane.

4) The real plane and BOX correctly nose down when flaps are deployed.

5) In the real plane this nose down is easily corrected with trim.  In BOX it is not easily corrected and even with full trim, back pressure on the stick is required to maintain correct glide slope.

6) On touch down the real plan is quite stable rolling down the runway and in BOX tends to wander more requiring correction with the rudder which due to it's sensitivity leads to over controlling.

 

From my perspective the issues with the BOX P51 that I notice the most are the excessive thrust required to get moving on the ground, the excessive rudder sensitivity making it difficult to control on the ground and by far most importantly the inability to trim during landing to allow neutral input to the stick when controlling the elevator.

 

Overall Agathos indicates that the P51 in DCS can be landed using the same procedures (flying by the numbers) as for the real plane but this is not possible in BOX due to its tendency to nose down into the ground even with full up elevator trim.  I agree with this conclusion.

 

However despite the issues with the P51 flight model in BOX outlined above I have to say I much prefer dogfighting in the BOX P51.  The DCS P51 seems to loose energy very quickly, maintains turns poorly and enters stalls very quickly to the point it just frustrates me.  I think this aspect of the DCS P51 is out of wack.  If the P51 really flew this poorly at it's limits I think the pilots would have refused to go into battle with it.  

 

 

 

 

 

Edited by S10JlAbraxis
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21 hours ago, S10JlAbraxis said:

Overall Agathos indicates that the P51 in DCS can be landed using the same procedures (flying by the numbers) as for the real plane but this is not possible in BOX due to its tendency to nose down into the ground even with full up elevator trim.  I agree with this conclusion.

 

I‘d be carefull applying the videos 1:1 on this sim for the simple reason that this sim does not model pitot errors. In consequence, we have tend to have the lift modelled for our planes on the high side and we land the plane on the slow side of things. 100 mph in this sim are not 100 mph on a P-51 dashcam. This can produce differences in visible trim as well.

 

It is great and very instructional that we have Agathos videos and comments, but before finding other peoples errors we really should be aware of our own errors. This regardless of „how other sims do it“ as any programmer may have his own ways approximating what he deems right (enough), often making other compromises.

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2 hours ago, ZachariasX said:

 

I‘d be carefull applying the videos 1:1 on this sim for the simple reason that this sim does not model pitot errors. In consequence, we have tend to have the lift modelled for our planes on the high side and we land the plane on the slow side of things. 100 mph in this sim are not 100 mph on a P-51 dashcam. This can produce differences in visible trim as well.

 

It is great and very instructional that we have Agathos videos and comments, but before finding other peoples errors we really should be aware of our own errors. This regardless of „how other sims do it“ as any programmer may have his own ways approximating what he deems right (enough), often making other compromises.

 

Who knows how accurate Agathos comments about the P51 flight characteristics are.  Since he actually flew the real aircraft that lends some credence to his observations but things can get lost during translation from actual experience to words on a page. 

 

There is only one point I will personally take from this post and my personal experience which is that I feel the landing characteristics of the P51 in BOX are a bit off. 

 

For example:  I set up a flight with a P51 with 50% fuel at 2000 feet.  I reduced speed to 150 MPH and lowered flaps to max and lowered gear.  The P51 immediately pitched down which seems to be correct.  Then I tested what would be necessary to counteract this downward pitch.   As it turns out to remain in level flight (with no back pressure on the stick) I needed to increase MP pressure to 58 at 3000 RPM and apply full elevator up trim.  At this point I remained in level flight without touching the stick (still at 150 MPH).  If I reduced MP to 3000 I entered an immediate dive and by the time I hit the ground I was descending at 4000 feet per minute.  In order to keep descent in the 500 foot per minute range (normal for landing approach) I had to keep manifold pressure at somewhere around 50 (still at 150 MPH which is too fast for final approach).  Of course with substantial back pressure on the elevator I could counteract this downward pitch tendency and reduce MP pressure to normal levels at a reasonable approach speed but that is not necessary to land most aircraft if properly trimmed.  But the BOX P51 can not be properly trimmed for landing and it is impossible to set up a final approach at the recommended 110 MPH without pulling the stick way back and holding it there.

 

Most every flight instructor I am aware of indicates that with most aircraft you should set up for landing with the correct RPM/MP/flap settings then trim the elevator to close to neutral and control the descent rate at 500 FPM mostly with the throttle.  Substantial back pressure on the stick should not be necessary prior to the flare. Running the engine at full RPM and close to maximum MP to maintain a 500 foot per minute descent rate is usually not part of this process at least not that I have ever heard of.  Is this elevator back pressure necessary when landing the real P51 aircraft?  I can't say for sure but highly doubt it.

 

So I think there is an issue with this aspect of the P51 flight model.  While I would enjoy the sim more if this issue were addressed it does not ruin my overall enjoyment and so many other aspects of the flight model are great so I can mostly ignore this.  But it would be nice if it could be addressed at some point in the future keeping in mind that there are much higher priorities to addressed first.

 

Edited by S10JlAbraxis
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6 hours ago, S10JlAbraxis said:

Who knows how accurate Agathos comments about the P51 flight characteristics are.

I certainly agree with his diagnosis, my point was that whe he is comparing the trim of the aircraft at an indicated 100 mph, the real aircraft is in probably fact going 120 mph or so TAS while the sim aircraft is doing closer to 100. At such slow speeds, that may well an effect on trim and I am not sure that he was aware of this. Just pointing out some variables here that might be considered as well, that‘s all.

 

As for how things should be, I don‘t know yet. But I have booked a Mustang as well for this year. As soon as travel is reasonably possible again I (hopefully) should know better.

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On 1/27/2021 at 12:12 AM, ACG_DerSheriff said:

The commenters below are ofc right. I had it wrong out of the top of my head. Flaps create lift and that lift vector in relation to the CG of the aircraft results in either a pitch up or pitch down movement.

 

A more complex approach:

 

In the literature you will find flaps generate lift AND a moment around the aerodynamic centre.

 

A more intuitive way to think about it that the lift vector (w/o moment) is exerted on the centre of pressures which is a moving point. So the lift vector increases but also moves forward and backward. This changes the pitching of the plane not only by the increase of force around the CG but also how much leverage it has (like a wrench).

 

When the flaps are deployed the downwash changes and affects the tail, which will now also produce a different magnitude of lift. Reaching the equilibrium here is not instant and it dips into the theme of stability. Regardless, this new change will also change the pitching moment of the plane.

 

Downwash is completely different from plane to plane and I doubt they got this right in IL-2 because you need to use empirical formulas combined with the propeller effects and probably shoud use CFD. It's surprising how different the downwash can be based on small changes to the geometry. Anyways, I would imagine if there's something wrong with the pitching attitude it may boil down to this most likely due to the difficulty it entails in depicting properly.

 

I would imagine in the game it's limited to those two factors, but there's more. It's highly complicated.

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Initially the Spitfires had it wrong, pitching up when flaps deployed, but it was fixed.

 

The Hurricane is still wrong - pitches up when flaps deployed, while it should pitch down, just like the spitfires, for the very same reasons ( aerodynamically ).

 

Yet another of many details that I would love to see addressed sometime ahead - the Spitfire usually cruises with nose heay trim settings, thus implying that we should be able to see the elevator hinges slightly to the up side of the horizontal stab, but they're always perfectly aligned...

 

Things...

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12 hours ago, ZachariasX said:

I certainly agree with his diagnosis, my point was that whe he is comparing the trim of the aircraft at an indicated 100 mph, the real aircraft is in probably fact going 120 mph or so TAS while the sim aircraft is doing closer to 100. At such slow speeds, that may well an effect on trim and I am not sure that he was aware of this. Just pointing out some variables here that might be considered as well, that‘s all.

 

As for how things should be, I don‘t know yet. But I have booked a Mustang as well for this year. As soon as travel is reasonably possible again I (hopefully) should know better.

 

Ok I see your point.  Modeling flight is certainly a very complex topic and also very interesting.  If you have a chance to see for yourself in the real aircraft would be very interested in your thoughts.

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