Incorrect Pitch Moment for flaps being deployed

The F-86F is in great shape. I truly enjoy flying it. But one flight model feature bothers me compared to the flight manual (and as far as I know, reality). With flaps deployed, there should be a fairly strong nose down pitch moment. It is for this reason that the manual procedure for takeoff has you put in a lot of nose up trim (until the trim light comes on). As currently programmed, the aircraft will leap off the runway and continue to pitch up without even putting any nose up trim.

A simple test is to get the aircraft trimmed for straight and level flight at 150-170 knots with flaps up. Deploy the flaps. Which way does the nose go? Level or worse yet, up, which is the wrong way! The pitch down moment should be strong enough that it requires quite a bit of nose up trim to neutralize it. Only after retracting flaps, the aircraft should want to pitch up and require getting rid of the trim setting used for takeoff.

If you have aerodynamic data that contradicts what I have said, please cite a source. For most aircraft this is true, and per the USAF flight manual, it is definitely true of the F-86F.

To be fair: the manual doesn't specify one way or the other for the pitch moment in response to flap position changes. But most aircraft with a low wing (where the tail is not affected by the airflow from the flaps) will suffer from a nose down moment, and the express requirement for excessive nose up trim at takeoff per the manual is the dead giveaway that the flaps cause a nose-down moment.

I also find this strange, although I don't have any RL experience with swept wing aircraft.

In the DCS F-86, there is first a nose down moment when the flaps are a few degrees down, and then a very noticeable nose up moment all the way to full flaps.

Whether this is correct or not, I don't know. It would be nice to hear from someone with real knowledge..

EDIT:

just to add this:

1. The increase in lift created by the increased wing area and camber will lead to a pitch-up moment if the centre of pressure remains in front of the centre of gravity.

2. If the associated rearward movement of the centre of pressure is behind the centre of gravity, then this will produce a nose-down pitch.

3. The flaps will cause an increase in the downwash, and this will reduce the angle of attack of the tailplane, giving a nose-up moment (doesn't seem to be the case with the F-86 since the tail is higher that the wings)

4. The increase in drag caused by the flaps will cause a nose-up or nose-down moment depending on whether the flaps are above or below the lateral axis.

This and more recent discussion of tailplane trim sensitivity in the sim got me interested enough to look for sources describing the response in pitch when the RW Sabre's flaps, gear and speedbrakes are extended or retracted.

I expected to find some PIREPs on the 'Net which would clarify things, but haven't located anything suitable. The best and clearest source I've come up with to this point is RAF Pilot's Notes for the Canadair Sabre Mk 4 (F.4 in RAF terms), dated June 1953. This doc was found on a website dedicated to RAF/GAF Station Jever, here: http://www.rafjever.org/sabrepictures.htm

The Sabre Mk 4, roughly equivalent to the F-86E-10, had a less powerful version of the J47 engine than the F-86F-35 in DCS and initially at least, a slatted wing. See pp. 21-22 of the DCS F-86F Flight Manual for a description of the differences between the slatted wing and the “6-3” wing on our F-86F-35. For the most part, the Pilot's Notes on the Jever site deal with the slatted wing Sabre; Appendix I provides supplemental info for the Mk 4 updated with the slightly larger “6-3” wing.

Here's what the “slatted wing” portion of the Sabre Mk 4 Pilot's Notes says regarding tailplane trim and the response in pitch to flap, gear and speedbrake movement:

a. trim sensitivity – p. 46 para 58 (iv) notes all three trims are “powerful and quick in operation” and suggests tailplane trim in particular be applied in blips to avoid over-controlling;

b. pitching moment due to flaps, landing gear and speedbrakes – p.47 para 58 (vi) describes the reaction when undercarriage extends as “weak nose up initially and finally weak nose down.” Response in pitch as gear retracts “slightly stronger nose up.” “Slight nose down” reaction as flaps extend; “slight nose up” as flaps retract. Strong nose up reaction as speedbrakes extend at high speed, decreasing to mild at low speed. Strong nose down response when speedbrakes retract at high speed, decreasing to mild at low speed;

c. takeoff tailplane trim setting – p. 42 para 56 (viii) notes that the tendency to pitch nose up when flaps are raised will be reduced if the forward limit of the trim indicator light is used for the takeoff setting.

Appendix I dealing for the Mk 4 with a “6-3” wing provides no new info regarding the response in pitch to flap, gear or speedbrake movement. An argument likely could be made that this indicates the reactions are at least approximately the same as seen with the earlier wing. With regard to the takeoff tailplane trim setting, Appendix I para II (ii) (a) goes even further than the recommendation in the “slatted wing” portion of the Notes, suggesting nose up trim be applied until the indicator light just comes on, at which point you stop, reverse and trim the stick one and a half to two inches forward.

If we're inclined to accept these excerpts from the Mk 4 Pilot's Notes as being generally applicable to the F-86F-35, tailplane trim sensitivity and power, and the response in pitch when speedbrakes are opened or closed seem reasonably well modeled in the sim. The trim changes in the sim when flaps and gear are lowered are problematic, however. Extending full flaps at 180 KIAS currently produces a mild nose up response for the first second or so, followed by a couple of seconds of stronger nose down, finally changing direction again and settling into a sustained moderate nose up reaction as full flap extension is approached. Clearly not the “slight nose down” described in the Notes. Dropping the gear at 180 KIAS generates a moderate to strong nose down response, which steps up to full strength abruptly less than a second after selecting gear down, at a point in the cycle where the nose gear is only partly extended and the mains have just begun to drop. Again, not a good fit with the description in the Pilot's Notes (“weak nose up initially and finally weak nose down”), or with expectations.

My feeling at the moment is that the pitch trim changes due to flaps and gear are not realistic, and twice per flight detract from an otherwise very satisfying and immersive flight model. That said, I'm new here and may have missed an earlier post which cites more definitive sources and proves me wrong. If so, perhaps someone could point me in the right direction. Alternatively, if Belsimtek based this portion of their work on a better source than the one I've offered above, it would be good to hear from them, to put these concerns to rest.

Why don't the Dev's react on this bug !!

It is important that the flight model is as realistic as possible...specially in DCS, they must know !!

The pitch moment with flap movement feels wrong, that should be reason enough to investigate this by the Dev's.

They themselves should be annoyed by the fact that this does not feel good or is not explainable when flying this beautifull module.

Hope the dev's share some info on this.

Why don't the Dev's react on this bug !!

 

It is important that the flight model is as realistic as possible...specially in DCS, they must know !!

 

The pitch moment with flap movement feels wrong, that should be reason enough to investigate this by the Dev's.

They themselves should be annoyed by the fact that this does not feel good or is not explainable when flying this beautifull module.

 

Hope the dev's share some info on this.

If you want it fixed you need to find concrete proof to back up your claim.

It's been said time and time again "feels wrong" is not good enough.

If you can find some pilot accounts, along with data on pitch moments (ie, degrees per second under the specific flight regime), then, as shown on other threads, when presented with correct data BST will fix stuff.

If you want it fixed you need to find concrete proof to back up your claim.

 

It's been said time and time again "feels wrong" is not good enough.

 

If you can find some pilot accounts, along with data on pitch moments (ie, degrees per second under the specific flight regime), then, as shown on other threads, when presented with correct data BST will fix stuff.

Understood, Buzzles. That said, this isn't a quibble over a couple of knots of airspeed or a few feet per minute in climb rate – it's a bit more fundamental. The qualitative data cited in my earlier post suggests the incremental pitching moment due to flaps, in the sim, seems to be acting in the wrong direction. In this circumstance, I don't think it's unreasonable to expect at least a brief response from BST. As matters now sit, we're not even sure our concerns have registered.

I' am very aware that a flight model should not be changed just because it doesn't feel good to someone.

But if there is a doubt about it, I think that the guys who determined that flight behavior, should be interested in that doubt.

They could look into it, and at least explain why they programmed that behavior.

They are the specialists, and aim on having delivering realistic flight behavior I presume.

A reaction from them is not to much to ask for in my opinion.

In the worst case they could say that they don't know either !!

By the way, for me it is more than just a feeling, the reaction on flapsetting is unexplainable, and not being backed up by anything I can find on internet.

So ....Still hoping on a reaction from those who know .:music_whistling:

Greetz

I' am very aware that a flight model should not be changed just because it doesn't feel good to someone.

 

But if there is a doubt about it, I think that the guys who determined that flight behavior, should be interested in that doubt.

They could look into it, and at least explain why they programmed that behavior.

They are the specialists, and aim on having delivering realistic flight behavior I presume.

 

A reaction from them is not to much to ask for in my opinion.

In the worst case they could say that they don't know either !!

 

By the way, for me it is more than just a feeling, the reaction on flapsetting is unexplainable, and not being backed up by anything I can find on internet.

 

So ....Still hoping on a reaction from those who know .:music_whistling:

 

Greetz

It's always the same thing here, YOU have to provide proof. But As you are saying, if there is a doubt of this kind, wouldn't it be easier for BST to chime in and say "Hey, we modelled it like this because of x..y..z..". Their silence is only increasing the doubt that they actually don't have any proof for how it's modelled now.

Incorrect Pitch Response to Flaps

Note: this issue was previously reported by several users at https://forums.eagle.ru/showthread.php?t=163309. I am repeating the bug report and providing additional supporting data in the hopes of seeing the problem resolved. It detracts from an otherwise outstanding flight model.

The issue is simple – in the sim, when the flaps are lowered in preparation for landing, the nose of the model tries to rise. A moderate amount of forward stick pressure is needed to compensate. Conversely, when flaps are retracted after takeoff in the sim, the nose wants to drop and a moderate pull on the stick is needed to maintain the desired attitude. In the real world F-86, the trim changes due to flaps are less pronounced and in fact act in the opposite directions to what is currently modelled in the sim.

Clear evidence of this is found in A.P. 4503D-P.N. (Pilot's Notes for the Sabre Mk 4) found here http://www.rafjever.org/sabrepictures.htm Note in particular the description of pitch trim changes found on page 47. For those not familiar with RAF Pilot's Notes, please bear in mind that the term “pitch trim changes” refers to the reaction of the aircraft when the systems in question (flaps, gear, speed brakes) are activated – not the changes in elevator trim setting needed to compensate. The real world Sabre's response to flap extension is described as “Slight nose-down” - conversely, for flap retraction the response is “Slight nose-up.”

To confirm the validity of these notes, and their applicability to later Sabre models, I contacted an ex-Air Force friend and former display pilot who has flown “Hawk One” here in Canada (see http://www.hawkone.ca/). He reviewed this portion of the Pilot's Notes, reassured me that the comments are “spot on” and described the nose movement with flap selection as “slight but noticeable.”

Having searched long and hard, I don't believe we'll find any better data than this, nor do we need it in this specific case. Certainly, the differences between “weak”, “slight” and “strong” pitch trim changes can be debated endlessly. They are subjective to begin with and the outcome of attempts at simulating any of them likely will vary somewhat based on the controller in use and related settings. That said, IMHO the pitch response to flaps we see now in the sim is clearly more than “slight” and more importantly is in the wrong direction – nose up when it should be nose down, and vice versa. This at least should be fixed.

Well put, bbrz. You and I are on the same page here. The incremental pitching moment due to flap extension can act in either direction; on some aircraft it is nose up, on others nose down. On the real world F-86 it seems clear it is nose down.

For Yo-Yo, I think part of our disagreement here may be due to language. In particular, the term “trim change” perhaps is confusing. To bbrz and me (and to the author of the RAF Sabre 4 Pilot's Notes) “trim change” means a change in the balance of the aircraft around its pitch axis. When we describe this change as “nose down” we could also say “nose heavy.” We are not referring to the act of changing the elevator trim to reduce the stick forces. That comes later.

In the real world F-86, as the flaps extend the nose wants to drop – additional aft stick is needed to maintain the desired pitch attitude, and eventually nose up elevator trim is applied to compensate and re-balance the aircraft. We don't see this in the sim. In fact we see the opposite. This is what needs to be fixed.

Do you have stick-fixed and stick free trim charts for F-86? To avoid language problems - I mean elevator angle vs CL or IAS for the first case and stick force vs the same values - for the second.

No I don't, Yo-Yo. All I have is RAF Sabre Pilot's Notes which advise that the aircraft has a mild tendency to pitch nose down when the flaps are extended, and confirmation from a real world pilot who has flown the aircraft recently that this description is accurate. In my opinion, bearing in mind that we're dealing with a transient effect that is only encountered a few times a flight, this evidence should be sufficient. If anyone has similar or better information which suggests the F-86's nose pitches sharply up as flaps extend, this would be a good time to share it.

For bbrz, thanks for the links, your Googling skills clearly surpass mine. I'll take a good look through this info when time permits. I expect support for our concern, in the form Yo-Yo seeks, is found in there somewhere.

...

edit: found this after a 1min google search

http://naca.central.cranfield.ac.uk/reports/1958/naca-report-1369.pdf

...

Thanks, made interesting reading

Thanks, made interesting reading

Cheers Ramsay, you've clearly got too much free time on your hands, to have gone through all that info so quickly.

This table speaks directly to our concern that the response in pitch to flap extension is incorrectly modeled in the sim. and is consistent with the supporting info I've already provided. The second line says it all - when flaps were lowered to 55 degrees down from a previously trimmed condition (140 knots, 80% power, gear down, BLC off), a pull of 7 pounds was needed to maintain constant altitude.

Let's not confuse the issue with discussion of stick movement versus force, or pitch angle versus velocity vector. Let's just talk stick force, and more importantly pull versus push.

Returning to the table it means, that for the stick-free level flight trimmed aircraft after flaps are down you need to apply 7 pounds of pulling force.

We all agree on this point. NACA flight test showed that seven pounds of pulling force were needed to maintain the same altitude, when flaps were dropped.

Let's try a simple experiment. Go into the sim, select the F-86 Instant Action "takeoff" mission. Get airborne but immediately throttle back to approx. 80% and remain at low level, say 200 feet AGL. Raise the flaps fully but leave the gear down. Establish straight and level, fully trimmed flight at approx. 140 knots. When you are satisfied that both airspeed and altitude are constant and you are properly trimmed, lower the flaps and attempt to hold a constant altitude using stick movement alone. Do you find you need a sustained pull on the stick to remain at your entry altitude? Or are you pushing?

Let us know how you make out.

I guess you are so deep in the DCS world, that you forgot the profound difference between the reaL STICK and the joystick. Let me refresh it... the plain joystick without FFB can not behave like the real stick. PULL the joystick means MOVE it. PULL the real stick means that something changed beside your back at the stabiliser, and some force (I mean changed downwash) bends the elevator DOWN and you have to pull JUST TO HOLD the stick in its place (remember "Through The Looking Glass"...)

 

It is due to the main consideration that the virtual stick corresponds the plain joystick POSITION.

 

Yes, the FFB joystic can do the trick but even powerful MS FFB 2 does it in a manner that is very far from the prefect RL manner, because of reduced forces.

 

And, by the way, I mentioned it some times ago, when the poll was started about stick retrimming due to flaps lowering. This feature never was applied in sims before, I think.

Thank you for your recent attention to this thread, Yo-Yo. After a seven month wait for any acknowledgement from ED or BST, it's good to have you here to address this concern. I must say you have a rare talent for diverting the discussion in unexpected directions, and for making the simple appear very complicated. It seems at least one of our recent participants may have fallen by the wayside as a result of this, along with others before him that simply lost faith during the prolonged wait. That is unfortunate; they are paying customers who appreciate the high quality of the F-86, and raised a concern not in an attempt to score points on the devs, but in the interests of improving the fidelity of the model. They do not deserve to be ignored, or worse, belittled.

If I understand your most recent post correctly, you are suggesting there is no point in attempting the experiment I proposed, because the joystick I am using (a very good stick, by the way, equal to or better than that used by most of your customers) by its very nature depends upon displacement rather than force, and is therefore a rather poor simulation of the real world item.

In the interest of keeping our discussion rolling, let's say I accept your reasoning. We'll set the PC joystick, with all its shortcomings and complications, aside. For my sake let's keep things very simple, and just talk about airplanes and flight models.

I think we both agree that in the real world, NACA found that if their F-86 was fully trimmed, in straight and level flight at 140 knots, and flaps were lowered 55 degrees, a seven pounds pull on the stick was needed to maintain altitude. What does this tell us about how the real world aircraft had reacted to flap extension? It can't have merrily continued along undisturbed, or there would have been no need to pull on the stick. It can't have tried to climb, because having to pull on the stick to arrest a climb would be totally unacceptable from the perspective of aircraft handling qualities (as you well know, from your large experience). We can only conclude that real world F-86's immediate response to flap extension was to descend, and that a seven pound pull on the stick was needed to overcome this tendency and maintain altitude.

I invite you again to try the experiment I proposed earlier, in the sim. With one change. Once you're in straight and level, fully trimmed, low level flight at 140 knots, release the stick (it only complicates things), lower the flaps, clasp your hands together behind your head, lean back in your chair and watch. If your trusty Sabre noses over, descends and perhaps even flies into the ground, our flight model has adequately simulated what the NACA report implies and what the RAF Pilot's Notes describe. If it doesn't, IMHO we have found a small problem that should be fixed.

Again, let us know what you find. On my installation, after a brief, transient up and down nose movement while the flaps are in motion, the nose rises of its own accord and the model climbs steadily away.

The table, you probably refer to, says nothing about STICK MOVEMENT - only about forces. So, your statement, that APPLYING THE STICK is necessary, is not correct at all. Maybe yes and maybe no... it says that you need to apply additional FORCE. This is exactly the same I wrote before about changing of the hinge moment due to extended flaps.

 

Returning to the table it means, that for the stick-free level flight trimmed aircraft after flaps are down you need to apply 7 pounds of pulling force. Nobody says, that the stick is at the same position, if this force is applied. Rather it means, that the stick now has the zero force point more forward due to increased downwash - and you have to pull even to hold it in the new, more forward position, that gives you the same CL, as it was before flaps lowering.

It seems to me what you are describing here would happen to an aircraft with unboosted elevator control, lowering the flaps would change downwash and that would change airflow direction over elevator, producing an elevator hinge moment that would translate into a force that would be transmitted via elevator control linkage back to the pilot.

As you know, this does not apply to the F-86 which has an irreversible hydraulically actuated elevator. It doesn't matter if due to flaps lowering the elevator hinge moment changes; at 140kts the actuator will easily keep the elevator in the same place and no force will be transmitted back to the pilot.

The F-86 having an irreversible hydraulically actuated pitch control with no pitch ratio changer or anything (just a classic force simulator spring and a trim actuator), this means applying 7 pounds of pulling force on the stick will ALWAYS produce an elevator leading edge down deflection, no matter what the elevator hinge moment is, what the airspeed is, etc. Only exception might be at very high speeds where elevator actuator might have insufficient force to deflect the elevator the commanded amount, but that's definitely not the case here, we're talking about 140kts experiment here. The actuator at that kind of speed would have no problem coping with any kind of elevator hinge moment.

The table, you probably refer to, says nothing about STICK MOVEMENT - only about forces.

The pitch control being what I described above, with an F-86 aircraft trimmed for horizontal flight (as the table in discussion shows) applying 7 pounds of pulling force on the stick will ALWAYS cause a stick back movement, and that will ALWAYS produce an elevator leading edge down deflection.

A quick update for everyone contributing to or following this item.

Four days ago, with assistance from Google Translate, I posted this message on the Russian language F-86 forum:

“For Belsimtek, please read the final post on the English forum here: https://forums.eagle.ru/showthread.php?t=163309

I believe the flight model requires improvement. The response in the pitch axis to flap movement is incorrect. It acts in the wrong direction.

Thank you.”

I received a very gracious and positive reply from Foxhound_vva (Third Party Developer) today, which you can review here: https://forums.eagle.ru/showthread.php?t=176196

Given this development, I'd suggest our spirited discussion on this topic has gone as far as it needs to for the moment, and we can all move on to things we'd sooner be doing.

blue six