Flap deflection on speed and pitch...

[Anatoli-Kagari9]

Please GOTO:

 

http://forums.eagle.ru/showpost.php?p=2541168&postcount=18

 

to see where my reasoning was "poisoned"....

 

What was I doing wrong ? Performing the test at speeds above safe margin for flap deployment...

 

Thx to all who participated, specially Crumpp and Yo-Yo :-)

Edited November 3, 2015 by jcomm

[BSS_Sniper]

What?

[Anatoli-Kagari9]

What?

 

Basically it had been a long time since I tried it, and on a first test flight I apparently saw no nose drop with flap deflection... I turned out to be wrong - it's ok, I guess, the way it is. Even if momentarilly it tends to lift the nose, soon after it pitches down as it should...

[BSS_Sniper]

I said "what" because your first post had nothing that made sense in it. lol

 

When you first put the flaps down, if you are going faster than you should, it's going to life the plane because of the additional lift. Not all planes are going to pitch down with flaps either, just fyi.

[Captain Orso]

huuuuh? While I'm on final approach--usually still turning into the last leg of approach at about 250-200 knots--I start to drop the flaps--sometimes all at once, because I'm a lazy bastard :D--and I always see a very noticeable lift increase with the nose coming up slightly, but never down. In fact, once the flaps are down +5 I trim the nose down about 1-2° before dropping the landing gear. Once I've trimmed the nose down I have to fly with the stick pulled back about half way--I also have about a +25 curve assigned to the pitch--to maintain my glide path.

[Anatoli-Kagari9]

That's actually the problem...

 

From the manual, and by watching some tubes, it's evident the p51d has a pitching down moment resulting from flap deflection, not pitching up... and it's effective as soon as flap deployment starts...

 

When flaps are deployed in the DCS p51d, the nose rises, and only after a while ( if you don't touch throttle or trim) does it start coming down... to catch up with the lost speed ( ? - I thought it had neutral pitch up stability... )

 

As far as the manual says, and I can watch in some cockpit footage of real p51s being flown, the pilot deploys the flaps and immediately applies up pitch trim ( pulls the pitch trim wheel )

Edited November 2, 2015 by jcomm

[Crumpp]

Stability and subsequently the pitching moment is coefficient of lift based. In other words, it will change with speed. What you see at one speed does not mean it will hold true for another.

 

Application of nose up trim is also not designed for the transition period when the flaps are moving to the desired setting. The trim is for a steady approach condition of flight at that flaps setting.

[Anatoli-Kagari9]

Stability and subsequently the pitching moment is coefficient of lift based. In other words, it will change with speed. What you see at one speed does not mean it will hold true for another.

 

Application of nose up trim is also not designed for the transition period when the flaps are moving to the desired setting. The trim is for a steady approach condition of flight at that flaps setting.

 

That's why I tested it over a good range of speeds... from max cruise to 150 kias. At the lower speed ranges also tested with and without the gear lowered, which on it's own does cause a pitching down moment... It always pitches up when flaps are deployed, not down, and the manual says - deploying flaps makes the aircraft tail heavy - hence, my post :-)

 

I tried always with just the wing tanks filled, never with the fuselage tank.

[Crumpp]

Sounds like it is correct to me.

[Anatoli-Kagari9]

Sounds like it is correct to me.

 

How can it be, if the Manual clearly states that, deploying flaps makes the aircraft nose heavy?

 

Under all circumstances I have tried, including various speeds and configurations, the aircraft always starts by pitching up, considerably, and only after a while pitches down.

 

Gear deployment OTOH creates an immediate pitching down moment.

 

IRL the gliders I've flown with flaps, where in the manual it says - glider becomes nose heavy when flaps positively deployed ( they can in some gliders also be set to "negative" settings ...) - truth is the moment I lower flaps even if the glider balloons, the pitching down is immediate.

 

I asked Dudley Henriques, from the A2A forums, which as an experienced P51 pilot was one of the consultors for their P51d military and Civil versions, and he confirms the pitching down tendency.

Edited November 2, 2015 by jcomm

[Crumpp]

When flaps are deployed in the DCS p51d, the nose rises, and only after a while ( if you don't touch throttle or trim) does it start coming down... to catch up with the lost speed ( ? - I thought it had neutral pitch up stability... )

 

With flaps lowered, the airplane becomes nose heavy.

 

 

The stability characteristics during transition from a clean wing to flaps down is going to depend on CG location and Coefficient of Lift.

 

That can change with conditions.

[Anatoli-Kagari9]

So, although it becomes nose heavy as flaps are deployed, you believe it is possible, given the right conditions, to still observe an initial tendency to pitch up, right ?

 

Maybe that's the case, I really can't tell.

 

Would like to hear from Yo-Yo, because he did talk and gather information from p51 pilots during the development and fine tuning phases of the life of this module.

 

Yo-Yo ?

[Crumpp]

So, although it becomes nose heavy as flaps are deployed, you believe it is possible, given the right conditions, to still observe an initial tendency to pitch up, right ?

 

Sure thing. It depends on the conditions, jcomm.

 

Here is the trim curves for the P-51; It requires a lot of nose down trim at high AoA.

 

 

Yo-Yo definitely knows the specifics....

[bongodriver]

Sure thing. It depends on the conditions, jcomm.

 

Under all circumstances I have tried, including various speeds and configurations, the aircraft always starts by pitching up, considerably, and only after a while pitches down.

 

Hopefully Yo-Yo can answer your question jcomm, on the face of it, it does sound at odds with the manual, it would be surprising if the manual gives the behaviour for a unique set of conditions rather than normal.

Maybe the manual is wrong?

[MiloMorai]

Is that graphic for flaps/landing gear up or down?

[Crumpp]

MiloMorai says:

Is that graphic for flaps/landing gear up or down?

 

Just learn to read it.

 

Landing gear retracted = Drag reduction = Coefficient of lift reduction = Coefficient of moment increase = NOSE PITCHES UP

 

Landing gear extended = Drag increase = Coefficient of lift INCREASE = NOSE PITCHES DOWN

 

Flaps extended = Drag Increase = Coefficient of lift INCREASE = NOSE PITCHES DOWN

 

Flaps retracted = Drag reduction = Coefficient of lift reduction = Coefficient of moment increase = NOSE PITCHES UP

[Anatoli-Kagari9]

So,

 

to sum up, even those graphics agree with the fact that extending flaps and / or gear, should cause a pitching down, not up... moment, and hence, we should see the aircraft pitch down, not up as it does in DCS...

 

The graphics Crumpp posted help understanding the stability vs power regarding the known fact that the p51 had neutral positive pitch stability ( power on ), corresponding to the shallower curve on the lower graph.

 

Power off, it was more "conventional", with normal phugoid behaviour if perturbed in pitch with a pull of the stick and return to neutral...

Edited November 3, 2015 by jcomm

[Crumpp]

o sum up, even those graphics agree with the fact that extending flaps and / or gear, should cause a pitching down, not up... moment

 

And it does cause a pitching down moment in DCS.

 

Problem is wanting the transition to be a pitch down. That is not correct.

 

The change in trim is not the moment about the CG when the flaps are in transition from one position to another.

 

It is after the flaps reach their set position.

 

While the flaps are in transition, the moments created by the change in camber may not even be the dominant force acting on the aircraft.

 

In most conventional tail aircraft, the increasing downwash causes an increase in the tail AoA causing a nose up pitch for a moment while the flaps transition.

 

That is momentary and not something you trim the aircraft, trimming is for steady conditions of flight like maintaining Vref on approach.

[Crumpp]

The graphics Crumpp posted help understanding the stability vs power regarding the known fact that the p51 had neutral positive pitch stability ( power on ), corresponding to the shallower curve on the lower graph.

 

Yes and a very good point. Quite a few World War II designs are neutral power on and stable power off.

[bongodriver]

Is that graphic for flaps/landing gear up or down?

 

As far as I can tell it's a representation of a clean wing in a wind tunnel and possibly a computer model at that.

[Anatoli-Kagari9]

And it does cause a pitching down moment in DCS.

 

Problem is wanting the transition to be a pitch down. That is not correct.

 

The change in trim is not the moment about the CG when the flaps are in transition from one position to another.

 

It is after the flaps reach their set position.

 

While the flaps are in transition, the moments created by the change in camber may not even be the dominant force acting on the aircraft.

 

In most conventional tail aircraft, the increasing downwash causes an increase in the tail AoA causing a nose up pitch for a moment while the flaps transition.

 

That is momentary and not something you trim the aircraft, trimming is for steady conditions of flight like maintaining Vref on approach.

 

 

Ok, maybe that's the case, but let's see what Dudley answers at the A2A forums... After all, he was an experienced p51d pilot and instructor. Asked him if he recalls there being an initial tendency to pitch up... But his answer so far was:

 

"Center of pressure moving back on your airplane with flap deployment with the cg a constant creates a nose down pitch moment. Same on the Mustang.

On the Cessna you get a lot of downwash off the high wing onto the horizontal stabilizer that helps with a positive pitch moment.

A lot depends on the exact design of the plane of course. Some aircraft develop some "weird" moment arms with flap deployment due to where the cg is as relates to expendables moved or dropped in the case of military aircraft.

Dudley Henriques"

[Crumpp]

What he says is absolutely right with one exception

 

On the Cessna you get a lot of downwash off the high wing onto the horizontal stabilizer that helps with a positive pitch moment.

 

It is not just high wing aircraft...it is a result of conventional tail and specific stability characteristics of aircraft at the time you lower flaps. (CG location, Cl, etc..)

[bongodriver]

it could be something as simple as the increased angle of downwash actually stalling the tailplane and causing a pitch down.

[Crumpp]

A quick test reveals a marked nose up pitch when I deploy full flaps above the limitation speeds.

 

Lowering flaps at 165mph or lower shows an almost immediate nose down pitch.

[bongodriver]

I get pitch up at all speeds.