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01-12-2004, 10:58 AM
Bug Report - flaps create pitch up moment.

1. Version 1.21 - Tested on all flyable aircraft with flaps in version 1.21.

2. Computer type - irrelevant. Flight model bug unrleated to computer.

3. Bug/Defect Description

Deploying wing flaps causes an incorrect nose-up pitching moment. At first observation this

appears to be a sign error in programming.

In reality flaps should cause a nose-down pitching moment. The sim appears to have things

backwards, or worse.

I compared aircraft with different types of flaps to see if there was a difference in the

magnitude of the pitch-up moment. Various flap types do not seem to be modelled, as all

aircraft behave similarly in regard to incorrect nose-up moment when flaps are deployed.

Magnitude of the nose-up moment does not appear to be related to flap type.

If this were a simple sign error, aircraft with plain and slotted flaps (P-51, Ju-87, etc)

might be expected to have a greater pitching moment than aircraft with split flaps (Yak, Fw

109, etc). However this is not the case. All aicraft have a similar pitch-up tendency when

flaps are deployed. Therefore if this is a sign error, the flaps are modelled with less

detail than expected.

Please look into this problem. Thanks.

4. Describe steps to reproduce the bug (detailed).

Step 1: fly aircraft which has flaps.
Step 2: At safe airspeed, deploy flaps.
Step 3: Observe incorrect pitch-up moment requiring nose-down trim.
Step 4: Retract flaps.
Step 5: Observe removal of incorrect backwards pitching moment as flaps are retracted.

01-12-2004, 10:58 AM
Bug Report - flaps create pitch up moment.

1. Version 1.21 - Tested on all flyable aircraft with flaps in version 1.21.

2. Computer type - irrelevant. Flight model bug unrleated to computer.

3. Bug/Defect Description

Deploying wing flaps causes an incorrect nose-up pitching moment. At first observation this

appears to be a sign error in programming.

In reality flaps should cause a nose-down pitching moment. The sim appears to have things

backwards, or worse.

I compared aircraft with different types of flaps to see if there was a difference in the

magnitude of the pitch-up moment. Various flap types do not seem to be modelled, as all

aircraft behave similarly in regard to incorrect nose-up moment when flaps are deployed.

Magnitude of the nose-up moment does not appear to be related to flap type.

If this were a simple sign error, aircraft with plain and slotted flaps (P-51, Ju-87, etc)

might be expected to have a greater pitching moment than aircraft with split flaps (Yak, Fw

109, etc). However this is not the case. All aicraft have a similar pitch-up tendency when

flaps are deployed. Therefore if this is a sign error, the flaps are modelled with less

detail than expected.

Please look into this problem. Thanks.

4. Describe steps to reproduce the bug (detailed).

Step 1: fly aircraft which has flaps.
Step 2: At safe airspeed, deploy flaps.
Step 3: Observe incorrect pitch-up moment requiring nose-down trim.
Step 4: Retract flaps.
Step 5: Observe removal of incorrect backwards pitching moment as flaps are retracted.

Fennec_P
01-12-2004, 01:50 PM
You know, delpoying flaps is supposed to cause a nose-up change of trim. Simply, the lift of the wings increases, so the nose will rise.

The elevator must be trimmed to counteract this, or reduce speed.

This has nothing to do with the change in chord line, which causes a nose down attitude in level flight. I think is what you're referring to, and it is modelled in FB.

01-12-2004, 02:15 PM
Lift does not increase. A 5000 kg aircraft in level flight needs 5000kg of lift to remain in level flight, regardless of the flaps.

Deploying flaps causes a nose-down pitching moment about the center of gravity.

Your claim that the flaps "increase lift" is baseless. Flaps permit more lift for a given AoA. To remain in level flight with flaps, our 5000kg aircraft reduces the AoA.

Check the pilot's manual for any warbird. You will find recommendations to roll in nose-up trim when deploying flaps.

Jaws2002
01-12-2004, 04:53 PM
Go back to flight school buddy. http://ubbxforums.ubi.com/infopop/emoticons/icon_biggrin.gif Joking ,Sorry. Check this site:
How flaps work (http://travel.howstuffworks.com/airplane4.htm)
Flight Controls (http://www.centennialofflight.gov/essay/Dictionary/Flight_Control_Surfaces/DI104.htm)

Aeronautico
01-12-2004, 04:56 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by cosmokart:
Lift does not increase.<HR></BLOCKQUOTE>

Sorry but I don't follow you here.
What flaps are meant for then?!

Of course they increase the curve on the upper surface of the wing, inducing the air flow to run a longer distance, therefore producing a much higher lift. At the price of an even grater increase of drag.

What Fennec said about the change in chord line might explain my doubts on this matter.

Aero

Kasdeya
01-12-2004, 04:58 PM
Quote from cosmokart--
"Your claim that the flaps "increase lift" is baseless. Flaps permit more lift for a given AoA."

Please explain this statement. Yes the flaps might not increase lift 'directly', they do help to add more lift to the airfoil. I am interested. So if they permit more lift wouldn't then pitch nose up rather than nose down at certain speeds.

http://www.smokinhole.com/demon.jpg (http://www.IL2airracing.com/)

Jaws2002
01-12-2004, 05:08 PM
Actually I think you are right in a way. The flap creates more lift but also more drag, so not necessary a pitch up movement. http://ubbxforums.ubi.com/infopop/emoticons/icon_confused.gif

AcesHigh_AVG
01-12-2004, 06:41 PM
On the planes I have flown deploying flaps increases lift and causes a pitch up motion. As you lower the airspeed the nose will come down and the flaps will act as both lift increasing devices and speedbrakes.

If you were flying with full flaps and at 60 knots and you raised the flaps you would need significant backpressure on the controls to keep the same decent rate and speed and you would be close to a stall. With flaps full lowered you can come down at a steeper angle of attack and maintain a slower airspeed than if you were in a no flaps configuration.

The flaps are basically altering the AOA, but they are definitely providing more lift for a given airspeed.

I have not noticed them as being wrong in FB. Besides if deploying flaps causes a downward pitching movement then why would anyone drop them when engaged in tight turning combat? According to your theory they wouldn't as deploying flaps would only decrease their ability to turn. However, real life accounts and FB prove otherwise as flaps can be used, often with great effect in a tight turning dogfight, especially when airspeeds are bleeding off.

WB_Outlaw
01-12-2004, 08:45 PM
Some VERY general thoughts...

Lift is a vector and thus has a direction. It is not always pointing up. For example, if you dive straight down, your wings are generating lift (perpindicular to the wings for the sake of simplicity), but the lift is pointed in the horizontal direction. Therefore, if you dive vertically over point A on the ground, you will not impact that point if you simply point straight down. The lift of the wings will translate your aircraft to some other point. If you begin your dive over point A and wish to impact at point A, you will have to push over past the vertical and fly inverted to it.

Lift does not necessarily affect pitch. If the center of lift acts at the center of gravity, then increased lift will not affect pitch. If the center of lift is aft of the center of gravity, then increased lift will cause a nose down pitching moment. Conversely, if the center of lift is forward of the center of gravity, then an increase in lift will cause a nose up pitching moment.

Ideally, increased lift should cause a stick fixed nose down moment. Otherwise, if the nose were pushed up by a gust of wind or turbulence, the increased lift due to increased AOA would cause the nose to keep pitching up until the aircraft stalled and the aircraft is statically unstable.

Generally, high-wing aircraft will pitch up when flaps are lowered because the center of pressure on the flaps is above the center of gravity of the aircraft. Conversely, low wing aircraft will generally pitch down when the flaps are lowered because the center of pressure is below the center of gravity of the aircraft. I say generally because this is dependent on many things including the speed of flap deployment, the airspeed of the aircraft, and the design of the flaps. Note that the deployment of flaps moves the center of lift aft.

Except for those aircraft with a "Combat Flap" setting, only the true experts or truly desperate ever deployed any amount of flap in combat. I have the P-47 flight manual and it states that except in extreme emergencies, the flaps should never be partially deployed and then raised. They should always be deployed fully and then raised.

Even if deploying the flaps causes a nose down pitching moment, it does not necessarily decrease the ability to turn. More back pressure on the stick can overcome the pitching moment and then some due to the increased availability of AOA before stalling.

It has been a long time since I took my last aero classes so if anything above is incorrect, forget I said it.

-Outlaw.

AFSG__Mshine
01-12-2004, 09:15 PM
When I pick my nose in the cockpit, and flick
the booger onto the windscreen, it doesn't
stick like it should.

Can this be fixed in the next patch? http://ubbxforums.ubi.com/infopop/emoticons/icon_rolleyes.gif

WB_Outlaw
01-12-2004, 09:48 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by AFSG__Mshine:
When I pick my nose in the cockpit, and flick
the booger onto the windscreen, it doesn't
stick like it should.

Can this be fixed in the next patch? http://ubbxforums.ubi.com/infopop/emoticons/icon_rolleyes.gif<HR></BLOCKQUOTE>

Try rolling it between your fingers a bit first, but not too much. You want it to be firm enough for a quick release off your finger, but no so firm that it won't deform and stick when it impacts the windscreen. Also, try cleaning off the target if possible. Oil and dust at the impact point can really screw up the adhesive properties of nasal mucus.

-Outlaw.

waterinthefuel
01-12-2004, 11:21 PM
Yes flaps increase lift. Yes, flaps will cause the nose to momentarily rise. Yes, you are wrong.

Flaps also cause an increase in drag.

Kahvikuppi
01-13-2004, 12:47 AM
Gentlemen, thank you for intresting discussion conscernig flaps. According to my experience of flyable RC-models (I don´t fly real aeroplanes) and my theorethical knowledge around this matter FB should be free of bugs. And your discussion seems to support this conception. That is good, for I feel like revealed every time any bad suspicion about the simulator disappears. I like this game/sim so much, that it would "hurt" if it would turn out to be programmed wrong way; aeroplanes going up when supposed to go down etc. Thank you and enjoy sim-piloting!

Aeronautico
01-13-2004, 02:32 AM
Very competent post Outlaw.
Correct, and especially well put.

Aero

RayBanJockey
01-13-2004, 03:30 AM
Every game I have every played flaps make the nose rise.

But yea, if the flaps behaved like the elevator they would make your nose go down.

http://www.geocities.com/adlabs6/B/bin/testsig.gif
To anyone who wants to take away my trim on a slider, "From My Cold Dead HANDS (http://www.talonse.com/supergreg.swf)."

waterinthefuel
01-13-2004, 06:47 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by RayBanJockey:
Every game I have every played flaps make the nose rise.

But yea, if the flaps behaved like the elevator they would make your nose go down.

http://www.geocities.com/adlabs6/B/bin/testsig.gif
_To anyone who wants to take away my trim on a slider, "From My Cold Dead </BLOCKQUOTE>" TARGET=_blank>http://www.talonse.com/supergreg.swf."_<HR></BLOCKQUOTE> (http://www.talonse.com/supergreg.swf.)


But they don't. They elevator pushes down on the tail making the nose rise. The flaps simply increase lift, making the nose go up, slightly. I fly model and full scale, and when flaps are deployed the nose pitches up, period. Not much, but a few degrees, yes.

PF_Welshman
01-13-2004, 08:15 AM
actually flaps do make the plane fly in a nose down state its just you have to push the stick forward ( or trim the plane ) for level flight again to achieve it , try attacking a convoy on a road with combat flaps down , you can hit the trucks while flying level but with a nose down state.

WB_Outlaw
01-13-2004, 08:49 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by waterinthefuel:

But they don't. They elevator pushes down on the tail making the nose rise. The flaps simply increase lift, making the nose go up, slightly. I fly model and full scale, and when flaps are deployed the nose pitches up, period. Not much, but a few degrees, yes.<HR></BLOCKQUOTE>

Saying that the flaps simply increase lift is a massive over-simplification. Depending on the amount of deployment, flaps drastically change the aerodynamics of the aircraft. Drag, center of lift, center of gravity, and many other properties are affected by the deployment of flaps.

While I have not done any calcs myself, I would not compare model aircraft and full size aircraft without a Reynolds number check at the very least.

What full scale aircraft are you referring to? Comparing what you have piloted to anything you have not piloted is not valid IMHO.

-Outlaw.

01-13-2004, 09:21 AM
Two points:

1. Some aircraft pitch up, some pitch down. It depends on many factors.

2. Flaps cause two changes to pitching moment that must be considered.
a) the nose down pitching moment caused by the wings and flaps
b) altered tail reaction caused by flap downwash over the tail.

In most warbirds, even those with low-mounted tails, the nose-down moment of the flaps exceeds the increased tail reaction from flap downwash.

Maybe there's a Bf 109 fan or two out there who recalls reading that in that aircraft, you have to roll in nose up trim to compensate for the flaps -- plain flaps.

That's the other thing... split flaps cause less moment that plain flaps. Butterfly and Fowler type flaps, on the other hand, cause even more nose down moment.

----

The sim as it sits right now is Good Enough(tm), though. World from the developers is that these kind of effects will be addressed in great detail in The Next Sim. Until then, I am happy to live with a little nose-up pitching even if it ain't 100% accurate.

WWSensei
01-13-2004, 09:27 AM
Lots of over simplification going on here.

Flaps alone don't determine pitch-up. CG, length of tail, size of stabilizer and speed all play a factor in it. Wingload and placement as well as dihedral and wing sweep also factor in.

In general a high wing aircraft will pitch-up and low wing aircraft will pitch. There are exceptions and there can be different behavior depending on speed. At very low speed an aircraft may pitch up whereas at a higher speed it would pitch down.

My guess is that 1C merely simplified it to one equation since it wouldn't justify the CPU cost in power to bother with this aspect of the sim.

01-13-2004, 09:38 AM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Flaps alone don't determine pitch-up . . .
In general a high wing aircraft will pitch-up and low wing aircraft will pitch (down).<HR></BLOCKQUOTE>

Agree 100% Couldn't have said it better myself.

and considering that the P.11c and I-153 don't even have flaps, the general reaction of the other flap-equipped aircraft oughta be to pitch down.

Anyway, it's a *minor* thing that any competent sim-pilot can compensate for with a little trim, even if we have to trim the opposite way. After all, it's not like it takes 50 pounds of force to wrestle with our electronic joysticks. http://ubbxforums.ubi.com/infopop/emoticons/icon_wink.gif

Aeronautico
01-13-2004, 05:30 PM
Sorry, I post here the image and comment I put on SimHQ forum on the same subject.

The image doesn't consider the effect of the tail surfaces, but should intuitively illustrate the results the air flow tends to accomplish on the wing profile. I.e. "minimizing" the front exposition to the stream, plus (not drawn) moving backwards and rotating the lift vector (increasing it as well). Am I wrong? I think it does make sense...

http://img13.photobucket.com/albums/v38/Aeronautico/Profiles.jpg

ivankuturkokoff
01-13-2004, 06:11 PM
I have a little real world flight experience on roughly 30 different types (including a flapless 190knot down final type). trying tobe general here as has been pointed out CofG Wing configuration etc all affect trim changes. In all the flapped aeroplanes I have flown the following is typicall.

The first stage of flap taken at circuit speeds results in a slight "ballon" this brought about by the increase in lift with little increase in Drag component, to maintain circuit altitude a slight "check" forward on the stick is required. No great trim change.

Any further flap selection typically is ***ociated with a strong nose down trim change.
this being particularly noticeable when making the final flap selection.

Putting this into FB terms I think this is what should happen in 1G Circuit type speeds.

Selection of Combat flap.
Little trim change (i.e. as it is )

Selection of Take Off flap
Slight trim change requiring check forward of stick to maintain altitude, this simulating the ballon effect of most low flap settings.

Selection of Landing Flap
Strong nose down Trim change requiring definte mall backstick input to maintain the desired nose position

Scen
01-14-2004, 03:20 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ivankuturkokoff:
I have a little real world flight experience on roughly 30 different types (including a flapless 190knot down final type). trying tobe general here as has been pointed out CofG Wing configuration etc all affect trim changes. In all the flapped aeroplanes I have flown the following is typicall.

The first stage of flap taken at circuit speeds results in a slight "ballon" this brought about by the increase in lift with little increase in Drag component, to maintain circuit altitude a slight "check" forward on the stick is required. No great trim change.

Any further flap selection typically is ***ociated with a strong nose down trim change.
this being particularly noticeable when making the final flap selection.

Putting this into FB terms I think this is what should happen in 1G Circuit type speeds.

Selection of Combat flap.
Little trim change (i.e. as it is )

Selection of Take Off flap
Slight trim change requiring check forward of stick to maintain altitude, this simulating the ballon effect of most low flap settings.

Selection of Landing Flap
Strong nose down Trim change requiring definte mall backstick input to maintain the desired nose position<HR></BLOCKQUOTE>

That's also been my real world experience with flying serveral different types of aircraft. Intially when coming into the the white arc and deploying flaps you get a ballooning effect and a slight pitch up. Then as the aircraft starts to settle and bleed airspeed the nose starts to pitch down and it requires some trim especially if you add more flaps.

It's really a function of speed. The purpose of flaps is to increase decent angle without increasing speed. Of course there are several other benfits like slowing down etc.

Scens 2 cents

waterinthefuel
01-14-2004, 11:05 PM
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Scen:
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by ivankuturkokoff:
I have a little real world flight experience on roughly 30 different types (including a flapless 190knot down final type). trying tobe general here as has been pointed out CofG Wing configuration etc all affect trim changes. In all the flapped aeroplanes I have flown the following is typicall.

The first stage of flap taken at circuit speeds results in a slight "ballon" this brought about by the increase in lift with little increase in Drag component, to maintain circuit altitude a slight "check" forward on the stick is required. No great trim change.

Any further flap selection typically is ***ociated with a strong nose down trim change.
this being particularly noticeable when making the final flap selection.

Putting this into FB terms I think this is what should happen in 1G Circuit type speeds.

Selection of Combat flap.
Little trim change (i.e. as it is )

Selection of Take Off flap
Slight trim change requiring check forward of stick to maintain altitude, this simulating the ballon effect of most low flap settings.

Selection of Landing Flap
Strong nose down Trim change requiring definte mall backstick input to maintain the desired nose position<HR></BLOCKQUOTE>

That's also been my real world experience with flying serveral different types of aircraft. Intially when coming into the the white arc and deploying flaps you get a ballooning effect and a slight pitch up. Then as the aircraft starts to settle and bleed airspeed the nose starts to pitch down and it requires some trim especially if you add more flaps.

It's really a function of speed. The purpose of flaps is to increase decent angle without increasing speed. Of course there are several other benfits like slowing down etc.

Scens 2 cents<HR></BLOCKQUOTE>

Basically what I said. I don't know all the physics behind why, but I know when flying aircraft and flaps are deployed the plane pitches up. I DON'T KNOW WHY, I JUST KNOW IT HAPPENS! LOL

It must have something to do with increase in lift. Otherwise, when flaps are retracted and lift is cut, she wouldn't drop like a lead balloon.

I could be wrong, hell I'm proven wrong more often than right on these boards! LOL

HomeboyWu
01-15-2004, 06:33 AM
There are actually quite a number of effects with the deploy of flaps, that can affect pitch, in different ways:

1. increase of lift: The obvious one, as you hold attitude/speed and deploy flaps, lift goes up. Will pitch up (down) if center of lift is in front of (behind) center of gravity.

2. increase of drag: Quite obvious, too. So if the main wing is higher (lower) than C.G., pitches up (down).

3. increased nose-down torque/center of lift moving backwards: Not so well known. These are really only one effect. Imagine a delta-wing deploy some flaps at trailing edge, then you'll know it creates some nose down torque on the main wing. Or you can say it moves the center of lift backwards.

There may be other effects not listed.

So? As you can see the flaps' effects are a lot more complicated than you may think. It may pitch you up or down, depending on your main wing type, speed, and amount of flaps deployed. I guess this theory is quite consistent with our real pilot friends' experiences.