Spitfire Pitch up / Pitch down

[HagarTheHorrible]

Just curious:

 

When extending the flaps / air brakes on the Spitfire the nose of the aircraft pitches up.  I would have thought that the air brake / barn door element of the flaps would have created enough drag to the underside of the wing to either cancel out the pitch up or even, maybe, pitch the nose down ?

[=TBAS=Sshadow14]

wait what..

Its a Low wing plane..
when you extend the flaps it should pitch down (around the CG) not up..

Its not a high wing like cessna (they pitch up when lowering flaps) as the centre of mass is under the wing

But low wing planes pitch down as Mass is above the wings..

Will have to retest this plane now (i flew it but not used flaps)

[unreasonable]

Gavrick (dev) addresses this question in this thread.

 

https://forum.il2sturmovik.com/topic/28175-news-our-beloved-spitfire/page-14

[HagarTheHorrible]

Thanks.

 

I'm not entirely sure what he said ( or what he meant ) but I'll take his word for it..

[Finkeren]

The thing is: Gavrick's reply might show, that this behavior is correct (I don't believe the devs would release a model that behaved like this unintentionally) but it doesn't explain why the Spit behaves in this rather counter-intuitive way.

Edited July 8, 2017 by Finkeren

[unreasonable]

True - I read it as saying that with flaps and gear together, the overall pitching is minimal: but that does not answer the question Hagar posed.

 

What I really meant by my post, however, was just that if you answered/questioned Gavrick in that thread he might be more likely to see the question and reply in more detail. So, obviously, that is what I should have said. 

 

Early morning when I posted, still dozy.....

[69th_chuter]

Flaps down cause a vastly increased downwash on the stabilizer more than offsetting the initial pitch down force of he wing.

[Guest deleted@50488]

The overall pitching being minimal, at the speeds mentioned in the article whose link I had posted at that other thread, and Gavrick cited, are nonetheless indicatory of a pitching down moment, not a pitching up...

 

In fact, on a table at the bottom of Page 11, for stick force variation with gear and flap deployment, at 120 KIAS, we can observe, between lines 5 and 7 that the forces are:

 

5) 120 mph ( kias )  Flaps Up, Gear Down,       Stick force 1 pound ( pull ) 

 

7) 120 mph ( kias )  Flaps Down, Gear Down   Stick force 3 pound ( pull )

 

So, you have to pull, not push, more on the stick in the flaps down situation, which is imo indicative of a pitching down moment, not pitching up  ( ? )

 

Addendum: http://www.warbirdalley.com/articles/spitfire-flight-report.htm

 

"The Spit has only two flap settings, Up or Down to 64 degrees. With the split flaps going down, the Spit pitches noticeably nose-down, improving your view, and making speed control on finals relatively easy. This was fun, as later on with experience, I could set a trickle of throttle, flying a “tight-ish” base leg, then when I got to the point of TLAR (an RAF highly technical term meaning “That Looks About Right”), I would select flap, she would pitch nose down into a lovely curved approach to the runway, normally needing no further throttle adjustment. I was able to do this after only about 7-8 hours on the aircraft -- the point being that’s what the Spitfire is like!"

Edited July 8, 2017 by jcomm

[6./ZG26_5tuka]

The behaviour is correct. The manual does mention that although the aircraft requires relatively little trim changes during flight it is required to adjustit for landing when deployong flaps.

[Holtzauge]

In addition to the added drag below the CG lowering the flaps also adds lift behind the CG so you get an additional nose down pitching moment from that. In addition, the increased wing lift causes a change in downwash at the stabilizer/elevator which both changes the lift on the tail and the elevator hinge moment. IRL a pilot would feel this as the stick "wanting" to move forward and the nose pitching down. However, how to model this in a sim is not altogether straightforward since we don’t have the force feedback on most sticks. I can understand the argument for having it modeled the way it is now but TBH I would rather see a nose down pitching even though the ingame stick is not moved when flaps are deployed.

 

Some previous arguments for this can be found here and here. The former link also contains a link to a post where the IL-2 developers outlined their view and the latter a link to where this was discussed over at DCS forum. IIRC then the conclusion of the DCS discussion was that the DCS Spitfire Mk9 was modelled with the pitch down when flaps were deployed.

 

So no clear right or wrong here as far as I can see but a matter of taste on how you would like to see this modelled. My opinion on the matter is quite clear from the posts I linked so I won't be tedious and repeat myself here.

[HagarTheHorrible]

Jcomm missed a bit, in that article he linked to, towards the end, the pilot says "you would need to be unconscious not to notice the pitch change with flaps extended." or words to that effect.

[Guest deleted@50488]

In addition to the added drag below the CG lowering the flaps also adds lift behind the CG so you get an additional nose down pitching moment from that. In addition, the increased wing lift causes a change in downwash at the stabilizer/elevator which both changes the lift on the tail and the elevator hinge moment. IRL a pilot would feel this as the stick "wanting" to move forward and the nose pitching down. However, how to model this in a sim is not altogether straightforward since we don’t have the force feedback on most sticks. I can understand the argument for having it modeled the way it is now but TBH I would rather see a nose down pitching even though the ingame stick is not moved when flaps are deployed.

 

Some previous arguments for this can be found here and here. The former link also contains a link to a post where the IL-2 developers outlined their view and the latter a link to where this was discussed over at DCS forum. IIRC then the conclusion of the DCS discussion was that the DCS Spitfire Mk9 was modelled with the pitch down when flaps were deployed.

 

So no clear right or wrong here as far as I can see but a matter of taste on how you would like to see this modelled. My opinion on the matter is quite clear from the posts I linked so I won't be tedious and repeat myself here.

 

Brilliant - as usual in your posts Holtzauge !!!  And yes Hagar, I did read that too...

 

I think the link from the Devs has just answered my questions - it's a modelling option, given the limitations imposed by not using Force Feedback hardware... 

 

Probably, the ideal solution would be for a flight simulator to dynamically create stick curves. On such a situation, for a non FF stick, the stick would of course stay put at it's neutral ( central ) detent, the aircraft would pitch down, but dynamically the pitch curve would be adapted ( truncated at the full forward end ) to account for the reduced travel then allowed, because even if the joystick is deflected fully forward, the actual travel would then be shorter IRL...

 

A permanent adaptation of the pitch curves / limits computed on-the-fly to take into account stick-free changes in pitch and roll, would be required, and I believe would allow for a much more realistic fine tuning of the "feel of control" in a PC-based flightsim, even when a non FF joystick is used.

 

Now, I wonder if those of you with FF sticks do notice the stick moving fwd when you deploy the flaps ?

[Holtzauge]

Yup, I think the option to model the pitch down also for non-FF sticks would be preferable since it would be consistent with the effects of changing the elevator hinge moments: Does not matter if this is due to a change in hinge moment due to a trim tab being deflected or a change in downwash due to the flaps being deloyed: IRL the effects on the pilot would have been the same: the stick "wants" to change it's position and you would need to apply a force to counter it just like your post here indicates jcomm.

[6./ZG26_5tuka]

You could acchieve that with artificial adjustments of the elevator trim tab inflight for different flap states.

 

If I'm not mistaken CloD does model that to some extent.

[HagarTheHorrible]

Apart from being pedantic. Because the flaps are only used for landing and fluffing a landing in a sim is hardly a matter for concern, it's not like body parts or expensive aircraft parts are at stake, then as an issue it really doesn't rate, the last question I have though is:

 

Why would a pitching down of the aircraft, when flaps are deployed be any more of a concern or less problematic for non FFB users than a pitching up ? Wouldn't a pitching down be of greater benefit, because the sight line would be improved, rather than damaged, for no required control input or stick pressure ?

[216th_Jordan]

Apart from being pedantic. Because the flaps are only used for landing and fluffing a landing in a sim is hardly a matter for concern, it's not like body parts or expensive aircraft parts are at stake, then as an issue it really doesn't rate, the last question I have though is:

 

Why would a pitching down of the aircraft, when flaps are deployed be any more of a concern or less problematic for non FFB users than a pitching up ? Wouldn't a pitching down be of greater benefit, because the sight line would be improved, rather than damaged, for no required control input or stick pressure ?

 

Yes but the pitching up movement comes from keeping the stick at the centered position whereas a FF stick will move forward and thus create a pitch down movement. So the only way to model this for non FF sticks would be to take the position of the joystick as force applied by pilot, that however would be incredible nonlinear as at low speeds you just need very little force to move the stick to any position. (So not an option)

 

The best thing of course would be more FF sticks out there

Edited July 8, 2017 by 216th_Jordan

[ZachariasX]

Yup, I think the option to model the pitch down also for non-FF sticks would be preferable since it would be consistent with the effects of changing the elevator hinge moments: Does not matter if this is due to a change in hinge moment due to a trim tab being deflected or a change in downwash due to the flaps being deloyed: IRL the effects on the pilot would have been the same: the stick "wants" to change it's position and you would need to apply a force to counter it just like your post here indicates jcomm.

This!

 

I find also "simulating" the requirement for induced pull back on the stick preferable on non FFB sticks. It (IMHO) feels much more like the real aircraft. The real aircraft (especially aerodynamically) neutral aircraft often for most parts of the flight don't give me much of a sense moving the stick at all. There is no center anyway. Just the postion where the aircraft flies straight. Correctly trimmed, you shouldn't have any pressure on the stick (or yoke) then. All I do sense is pressures on the stick induced by (change of) attitude and veclocity. It is more like the stick being firmly bein stuck to the aircraft, but what I have to do is "like holding the whole aircraft on the stick" in the desired attitude. It takes more coarse maneuvers (or aueobatics) to really get the sense of moving the stick.

 

I feel gliders give more the feeling of stick movement as you have to work the ailerons a lot for maneuvering in thermals etc, and the trim changes on the elevator can be huge with changing speeds.

 

Making the plane nosing down when deploying flaps basically makes the sim-pilot fighting pressures he would experience to hold the stick in place. This I find feels more natural, more like sitting in the real aircraft. Like that, I get much more "what I feel" in the real one. It is commonly done like this in many sims, not only DCS.

 

However, as mentioned above, as we have it now is correct.

 

 

The best thing of course would be more FF sticks out there

 

A FFB stick firmly installed between the legs like in an aircraft and it should be able to induce at least 1 kN/m torque. Then the issue of us happily maneuvering at 700 km/h or pulling out of a dive with such ease that we can tear wings off our aircraft would be alleviated as well. And folks would LOVE the 190 even more.

[unreasonable]

 

Now, I wonder if those of you with FF sticks do notice the stick moving fwd when you deploy the flaps ?

 

I have a FF stick : so naturally I had to check.... but remember it also has some self-centering spring action. In a normal landing, with wheels down, lowering the flap does seem to cause a very slight nose up movement - but it is the equivalent of about 1mm of stick travel, so hard to be too accurate about it. Especially as I am a little drunk after watching a rugby match You will be pleased to hear that I did not crash. 

[Holtzauge]

<snip> However, as mentioned above, as we have it now is correct.

 

Um, well I think that's actually a matter of opinion Zacharias:

 

I still think the best way would be to get a pitch down when they are deployed: I think the argument that we don’t “feel” forces in non-FF sticks and consequently would not perceive the trim change is flawed since we don’t perceive the aileron forces at higher speeds either and that is still modeled in non-FF sticks by decoupling the stick controller and the virtual in-game cockpit stick: You can bottom out your desktop controller and still hardly move the virtual stick at high speeds. By the same token the trim change when deploying flaps should be there IMHO. I think we need to get away from the 1to1 mapping between the controller stick for trim just as we accept this for the ailerons at high speeds. But I digress: I don’t think there is any right or wrong here: It’s a matter of taste how you want to see it modeled in-game because whichever way you do it it won’t be totally realistic.

 

Anyway: All this is like Hagar said a bit pedantic and I just want to add that I love the IL-2 Spitfire!

[Holtzauge]

I have a FF stick : so naturally I had to check.... but remember it also has some self-centering spring action. In a normal landing, with wheels down, lowering the flap does seem to cause a very slight nose up movement - but it is the equivalent of about 1mm of stick travel, so hard to be too accurate about it. Especially as I am a little drunk after watching a rugby match You will be pleased to hear that I did not crash. 

 

Lucky you: The sun is not yet over the yardarm here so I have to wait a few hours more. Rugby: Now there is a man's sport: Not like those sissies on the other side on the pond who have to wear a lot of protection to play "football" ........

[216th_Jordan]

The decoupling of stick movements with high forces is logical, but how would you implement your solution in particular? Its a difficult task. You either have a realization that follows the input as far as possible (what we have now) or you model forces on the stick, now you can guess that its quite hard when you need 50N for a full deflecton at low speed versus 500N at high speed. You would need scaling of input deflection, but this is dependend on a dynamic process that only produces the values you are searching for after the inputs have changed. What you will get is oscillations and a lot of input lag as the new force setting need time to settle in with the scaling. Additionally the forces are nonlinear as they increase exponentially with deflection. I just don't see this working without a proper hardware FF. I'd be interested to know your solution as I have been thinking about that problem for quite some time now. (last year)

[cellinsky]

Used to XPlane and DCS low-wing  behavior when setting flaps on a Spit, I nearly stalled the aircraft on my first approach. Because I expected the nose pitch down as in RL or in this other sims. The arguments above are all valid, I aggree. But we all have no real sticks and forces even with FB and it just feels wrong to me. Shame, because otherwise this plan is a true masterpice. Well, but that is just me and I can live with it easely.

[Holtzauge]

The decoupling of stick movements with high forces is logical, but how would you implement your solution in particular? Its a difficult task. You either have a realization that follows the input as far as possible (what we have now) or you model forces on the stick, now you can guess that its quite hard when you need 50N for a full deflecton at low speed versus 500N at high speed. You would need scaling of input deflection, but this is dependend on a dynamic process that only produces the values you are searching for after the inputs have changed. What you will get is oscillations and a lot of input lag as the new force setting need time to settle in with the scaling. Additionally the forces are nonlinear as they increase exponentially with deflection. I just don't see this working without a proper hardware FF. I'd be interested to know your solution as I have been thinking about that problem for quite some time now. (last year)

 

I don't think it has to be that difficult: It's basically the same as changing the trim tab: In that case the controller stick stays in position even though you move the trim tab which causes the virtual stick to move to a new position. The same thing could be done for the change in elevator angle due to flap deflection. In both cases the balanced angle of the elevator is changed due to a change in elevator hinge moment even though the controller stick is not moved. So IMHO it would be more consistent to model a change in trim in both cases rather than in just the one we have today.

 

Used to XPlane and DCS low-wing  behavior when setting flaps on a Spit, I nearly stalled the aircraft on my first approach. Because I expected the nose pitch down as in RL or in this other sims. The arguments above are all valid, I aggree. But we all have no real sticks and forces even with FB and it just feels wrong to me. Shame, because otherwise this plan is a true masterpice. Well, but that is just me and I can live with it easely.

 

Yes but before this was implemented in the DCS Spitfire Mk9 Yo-Yo asked the community there for input on how it should be modeled and he said exactly the same thing as the IL-2 devs have said here:  That if the elevator angle is kept constant then you get the pitch up moment but if you take the change of elevator hinge moment into account then it should pitch down. I linked to that discussion above and the conclusion there was to take the changed elevator hinge moment into account: In DCS IIRC correct then it pitches down and you have to trim up to counter that and I think that is the way to go. This would also make it consistent for the Me-109 since if you take this effect into account, lowering the flaps gives a pitch down moment which you need to counter with turning the trim wheel in the same direction as the flap wheel which you need to do IRL but if this is not modeled in-game then you get a pitch up, not pitch down effect meaning you need to turn the trim wheel in the wrong direction as compared to IRL.

 

But like you say, it's not a big deal and one can live with it but that still does not change the fact that I would have preferred to see the elevator hinge moment taken into account for flaps just as it is done for trim tabs.

[ZachariasX]

Um, well I think that's actually a matter of opinion Zacharias:

 

I still think the best way would be to get a pitch down when they are deployed: I think the argument that we don’t “feel” forces in non-FF sticks and consequently would not perceive the trim change is flawed since we don’t perceive the aileron forces at higher speeds either and that is still modeled in non-FF sticks by decoupling the stick controller and the virtual in-game cockpit stick: You can bottom out your desktop controller and still hardly move the virtual stick at high speeds. By the same token the trim change when deploying flaps should be there IMHO. I think we need to get away from the 1to1 mapping between the controller stick for trim just as we accept this for the ailerons at high speeds. But I digress: I don’t think there is any right or wrong here: It’s a matter of taste how you want to see it modeled in-game because whichever way you do it it won’t be totally realistic.

 

Anyway: All this is like Hagar said a bit pedantic and I just want to add that I love the IL-2 Spitfire!

Agreed. I would also have made it the other way, by letting the aircraft pitch down on non FFB sticks, beause I too think it feels more correct like the real plane. But just looking at the stick input alone, it's another story. Looking just at that, the devs had good reason to program behaviour as we have it.

 

Generally, I am very much impressed with what the devs came up with "in their first try".

 

 

But like you say, it's not a big deal and one can live with it but that still does not change the fact that I would have preferred to see the elevator hinge moment taken into account for flaps just as it is done for trim tabs.

+1

[Holtzauge]

Generally, I am very much impressed with what the devs came up with "in their first try".

 

Amen to that! I'm also impressed and I really like the way there is so little adverse yaw when you roll: Looks a lot like the in cockpit vids you can find on YouTube for the Me-109 and Yak!

[unreasonable]

Amen to that! I'm also impressed and I really like the way there is so little adverse yaw when you roll: Looks a lot like the in cockpit vids you can find on YouTube for the Me-109 and Yak!

 

It is incorporating the new calculations - confirmed by one of the testers, but also pretty obvious when you fly.  Will be interesting to see how the stable of old models behaves when they get the tweaked FMs. I have no idea about realism, but it certainly makes for more fun flying!

[Holtzauge]

It is incorporating the new calculations - confirmed by one of the testers, but also pretty obvious when you fly.  Will be interesting to see how the stable of old models behaves when they get the tweaked FMs. I have no idea about realism, but it certainly makes for more fun flying!

 

Well I'm keeping my fingers crossed for no more rubber band Me-109's wobblying  around in 2.012! As for realism, this vid has been shown a lot before but it still illustrates the point very well IMHO......

[Finkeren]

I didn't really do proper tests, but I've flown the Spit all day and landed it multiple times, and when I drop flaps at the speeds they are supposed to be lowered, and I noticed that I do get a distinct nose-down movement that has to be countered by a bit of trim or pull on the stick.

 

Could it be that people are dropping flaps at too high speeds? 

[HagarTheHorrible]

When I checked what happened or the effect the flaps had ( I was actually looking to see how much they affected speed), it was just what I noticed, the speed would probably have been within, if at the top end, of regulations or 130-140 knots but certainly no more.

[Guest deleted@50488]

I'm lowering the flaps at or bellow 120 mph IAS, and I definitely get the pitch up effect.

Edited July 8, 2017 by jcomm

[dburne]

I'm lowering the flaps at or bellow 120 mph KIAS, and I definitely get the pitch up effect.

 

I do as well...

[unreasonable]

Speeds in this Spitfire are all mph, not knots, and PEC is +4 at this speed, so if the manual says maximum 140 then for us it should be 144, since our game instruments do not have a PEC.  Which is all too small to make any difference, so I can take off my Pedant's Outfit. 

 

I would note that whenever I have got to the dropping flaps stage I am always at maximum tail heavy trim already - I find that the plane is generally nose heavy and cannot be trimmed to cruise hands off - I always need a little back pressure. I have no idea if this is just my stick etc or normal, and actually do not mind it at all - the feeling of slight back pressure on a FFB stick is good, makes you feel in control, but it is very different from some other a/c which I could trim to land almost hands off.  

[Guest deleted@50488]

Well,

 

first a remark regarding my completely inconsistent writing of the speed units in posts above, where the early morning writing made me write something as ridiculous as 120 mph kias ... DUH!  Shame on me :-/

 

Now, back to the notes I posted regarding the Spitfire in IL2, please do not take them as expressing that I do not like it !  Quite on the contrary, and just as I feel about IL2 Battle of as a whole ( flight simulation wise ) I really appreciate what the talented 1C / 777 Team has made available to the user community.

 

It's a really good model, and I am enjoying it a lot.

[-TBC-AeroAce]

This behavior is to do with the neutral or even negative longitudinal stability of the Spitfire.

 

Read https://en.m.wikipedia.org/wiki/Longitudinal_static_stability

 

Basically an aircraft can behave in three different ways to a disturbance of lift\pitching moment.

 

If it is positively stabbut (like most aircraft) a pitch up disturbance will lead to the aircraft creating a pitch down response. This is what most people are used to!

 

If it is neutrally stable when disturbed the aircraft will stay in that disturbed state and it will not exacerbate or decrease the disturbance.

 

If it is unstable a pitch up disturbance will will lead to the aircraft responding by further departing in the direction of the disturbance instead of in opposition to the disturbance.

 

The relationship between the wing, tail, cg and cp forces and geometry are factors of what stability you will get. Traditionally planes are designed to be positively stable as this is safe for the pilots but stable aircraft are slow to react to control inputs. Once computers flight control systems came into play fighter aircraft were designed to have unstable airframes with the computer keeping it going where the pilot want as this increases maneuverability.

 

The spit for most of its flight envelope is positively \neutrally stable but at slow speed with gear and flaps down it becomes negatively stable. Hence the pitch up force cause by lowering the flaps causes the plane to keep pitching up. This pitch up force on positively stable planes(what the majority of people are used to) will cause the familiar pitch done response.

 

This behavior for the spit if correct and quite advanced.

[Wolfstriked]

The nose pitching up is realistic?Every real flight document I read says a plane will pitch down when flaps are deployed.I always thought that flaps had the simple job to allow the pilot to see the runway with same amount of lift.In the IL2 Spitfire I am having a hell of a time landing because I come in nose high which limits forward visibility drastically.

[Wolfstriked]

This behavior is to do with the neutral or even negative longitudinal stability of the Spitfire.

 

Read https://en.m.wikipedia.org/wiki/Longitudinal_static_stability

 

Basically an aircraft can behave in three different ways to a disturbance of lift\pitching moment.

 

If it is positively stabbut (like most aircraft) a pitch up disturbance will lead to the aircraft creating a pitch down response. This is what most people are used to!

 

If it is neutrally stable when disturbed the aircraft will stay in that disturbed state and it will not exacerbate or decrease the disturbance.

 

If it is unstable a pitch up disturbance will will lead to the aircraft responding by further departing in the direction of the disturbance instead of in opposition to the disturbance.

 

The relationship between the wing, tail, cg and cp forces and geometry are factors of what stability you will get. Traditionally planes are designed to be positively stable as this is safe for the pilots but stable aircraft are slow to react to control inputs. Once computers flight control systems came into play fighter aircraft were designed to have unstable airframes with the computer keeping it going where the pilot want as this increases maneuverability.

 

The spit for most of its flight envelope is positively \neutrally stable but at slow speed with gear and flaps down it becomes negatively stable. Hence the pitch up force cause by lowering the flaps causes the plane to keep pitching up. This pitch up force on positively stable planes(what the majority of people are used to) will cause the familiar pitch done response.

 

This behavior for the spit if correct and quite advanced.

Interesting....so will the other planes get a pitch down attitude with flaps if they are balanced differently?

 

The one issue I am having in landing the spit is that if I have to come in with a pitch up attitude I would land with some altitude and then nose down to point plane at runway and cut throttle.Problem is the plane does not slow down at all then.

[Guest deleted@50488]

Well,

 

there are all sorts of high wing and low wing aircraft that react differently to flap depoyment.

 

There are actually low wing aircraft that pitch up when flaps are deployed, and there are high wing aircraft that pitch down with flap deployment. Various factors including the tail geometry, ammount of downwash, CoG ... are responsible for this.

 

For instance the Cessna C310, Beechcraft B200 amd B1900 are exemples of low wing aircraft that exhibit a positive pitching moment due to flap deflection, and while the B200 and the B1900 have "T" tails, the C310 has a "low horizontal stab" geometry.
 

Some texts I've read about the Spitfire mention a pitch down tendency, but the devs explained that that would have to be accompanied by a stick fwd tendency due to the effects of the defflected propwash, that can't be simulated on the vanilla joysticks or even mouse most simmers use, so...

 

I accept that the IL2 Spit pitches a little bit up - it was a choice among two possible and after all equally valid options, I believe.

Edited July 8, 2017 by jcomm

[-TBC-AeroAce]

Interesting....so will the other planes get a pitch down attitude with flaps if they are balanced differentlyn.

Yes and it is not just in reaction to flaps but to any thing that changes the lift of the wing for example a gust of wind, pilot input, flaps gear ,.........

 

It is the combination of the aerodynamic forces/moment contributions from the wing and tail, their geometry and the mass properties of the aircraft.

 

A common cause of instability is that the cg starts to move towards or in front of the lift vector from the main wing or what I think is happening with the spit is the lift vector from the wing moves towards the cg after deploying the flaps . Commercial airlines have an adjustable tail plane that helps maintain stability as the fuel is depleted (hence the cg is changing)

 

Most conventional aircraft are designed to be positively stable but some times it is not possible to have this true for the whole flight envelope. Meaning that some configurations may be unstable. This is a compromise the designer may have to make. As in the case of the spit it is stable or neutral for most of its envelope with stability compromised for the landing phase. Why this particular compromise was made I do not know. It may have been an acceptable over sight, it may have been to get better stability some where else in the envelope, it could be loads of things.

 

But this is very realistic behavior that needs to be carefully designed and flight tested. In fact I think it is one of the first things they assess in a flight test.

Edited July 8, 2017 by AeroAce

[Wolfstriked]

Makes sense guys.

 

Just did three perfect landings after adopting a fly to the runway angle and all without a single ground loop.If all the coming planes will have odd landing quirks all the better!

Edited July 8, 2017 by Wolfstriked

[Guest deleted@50488]

Actually, under some speed and trim situations I am getting the pitch down moment Finkeren mentioned he was getting too !

 

This happens, for instance, when I make my approach with the aircraft trimmed slightly tail-heavy, lower the gear, bleed off speed bellow 120 mph IAS, and then lower the flaps. It pitches down in such circumstances.