Flap Type and Efficiency

[Ex-jazz]

How the flaps are changing the wing airfoil effective AoA and plane attitude.


Edit
I made another render with yellow effective AoA angle line and hopefully with more descriptive texts.

[Mace2004]

A tendency to pitch nose-down rather than up would be a benefit to flap-using planes. The nose up pitching tendency is quite awkward to fight against when trying to fly/gain a guns solution, the nose down tendency is not in my experience.

Actually, I find the nose up pitching moment to be beneficial in this scenario.  Say you need just a few more degrees of lead for your shot, dropping another notch of flaps can pitch the nose up nicely.  Also, the nose up pitching moment helps in coming over the top inverted.  If you are a little too slow the nose can kind of "park" just a little nose high, dropping flaps can help you get the nose around nicely.

[thorsim]

Horizontal stab and a skid?
It will mean a different A.o.A. which makes a different lift, so some difference to the elevators.

yes assuming the stab and wing are into the airflow at the same angle when the flaps are not deployed,

and the deployment of the flaps pitches the plane (up or down) in level flight then the stab in most planes being fixed would then not be inline with the airflow (skid)

no?

t

[Anaxogoras]

Stoney:

Center pressure,Cl,Cd,Zero Lift angle, Max AOA are all accounted for with change of flap settings.

We currently do not account for the change in the Cm curve.

HiTech

Oh come on, am I the only one who doesn't know what this means?

[Stoney]

Oh come on, am I the only one who doesn't know what this means?

As the flaps get lowered, the pitching moment of the flapped area increases--in some cases, rather large changes occur.  HTC is saying that they don't account for this change like they do the other aerodynamic forces that change as the flaps are deployed, if I understand him correctly.

[BnZs]

Actually, I find the nose up pitching moment to be beneficial in this scenario.  Say you need just a few more degrees of lead for your shot, dropping another notch of flaps can pitch the nose up nicely.  Also, the nose up pitching moment helps in coming over the top inverted.  If you are a little too slow the nose can kind of "park" just a little nose high, dropping flaps can help you get the nose around nicely.

Yeah, but if you are doing more than pulling a few degrees of lead for a moment, its awkward. Say it's a crossing snapshot instead of a tracking shot. Or say you've worked your way more to the opponents six and they are at least somewhat "flopping"/abusing Neg-Gs like is typical in game. Pushing forward on the stick to fight that all the time is very awkward to me. I always plug in lots of nose-down trim as soon as it becomes apparent that I'm going to be turning with some flap out, but that's an extra step and button, and the trim works rather slowly.

[hitech]

Do you use the Cl curve for the tailplane instead of the Cm curve?

Another interesting thing that happens when extending flaps and increasing the incidence of the flapped section is that the washout increases so the flapped section of wing stalls first. This lets the ailerons remain effective further into the stall.

So if the P-38 pitches down with flap extension it's setting a higher trim speed with a higher coefficient of lift and if left alone it will nose up when it exceeds that trim speed? I can see where that might be a problem in a loop if it pitched down for every flap setting. Aircraft like the F4U that pitch up with larger flap angles would do better.

When I refer to a Cm curve, I am referring to the Cm curve of the airfoil section only. Not the net moments of force of the entire plane.

HiTech

[FLS]

Hitech
I posted in haste I should have rephrased my question. I thought I was asking if you modeled the tail plane forces without the effect of the down wash from the main wing.

Edit: Slightly off topic. I believe you model spiral slipstream on the tail, is it possible when slow at a high AOA to drop the tail out of the slipstream?

[mtnman]

It's just RC of course, but every RC plane I've flown that had flaps actually pitched nose-up when they were deployed, and needed some down elevator trim to compensate.  Especially if the plane was moving a bit fast.  

All of them have been low-wing planes, mostly WWII fighter types, and a Piper Cherokee.

I actually wrecked an F4U due to a stall that occurred because the plane "ballooned", stalled, and spun on me when I dropped the flaps, and wasn't ready for the nose-up pitch.

Of course the airfoils are different, and thicker...

[Stoney]

My Grumman AA-1B would pitch down hard when I put the flaps down, and it has horribly small flaps.  I had to snatch the yoke into my lap hard and  use about 3 full turns of the pitch trim to relieve the yoke pressure.  Of course, the 64415 airfoil has a pretty high pitching moment relative to other airfoils.

[Mace2004]

yes assuming the stab and wing are into the airflow at the same angle when the flaps are not deployed,

and the deployment of the flaps pitches the plane (up or down) in level flight then the stab in most planes being fixed would then not be inline with the airflow (skid)

Not a skid, a skid is when the airplane is in uncoordinated flight in a turn but your point is getting there. 

First, the wing always produces a nose-down pitching moment whenever it produces lift because the center of lift is behind the Center of Gravity.  Remember that an airplane wants to rotate around its CG.  This is nose-down moment balanced by the horizontal stab which provides a nose-up pitching moment, basically, it pushes down to counteract the upward push of the wing (how's that for a non aero description?).  In stable flight, these two forces are balanced. 

So, now lets say you're stabilized (not accelerating or decelerating, not climbing or descending) and trimmed for level flight with flaps up.  You then drop your flaps.  All flaps will always produce additional nose-down pitching moment.  It's the way they work.  I can actually expand that statement.  All lift from the wing creates a nose-down moment regardless of whether is caused by camber or AOA because the center of lift is behind the CG.  The airplane rotates around the CG and the additional lift behind the CG will want to pitch the nose down.  OK, so much for the wing and flaps but what about the rest of the plane, i.e., the tail surfaces?  Remember that the horizontal stab was providing a nose-up pitching moment which balanced the moment caused by the lift from the wing with flaps up but now the wing lift has increased so the nose wants to  pitch down more.  But what happens to the horizontal stab as the nose pitches down?  It pitches up which increases the horizontal stab's AOA and increases it's lift (remember its lift acts downward) and nose up pitching moment.  In other words, you can think of it as automatically trying to oppose the increased pitching moment from the wing.  So, you have a nose-up pitching moment which is opposite of the nose-down moment created by the flaps.  You can see that whether the plane pitches up or down is a direct result of these two relationships.  If the moment from the flaps is greater than the moment from the tail then the nose will pitch down.  If this is reversed, and the tail has a greater moment the airplane will pitch up. 

Now, this is greatly simplified as there are several other things going on though including the effect of downwash on the tail surface.  Also, this is much clearer with a diagram but I still can't upload it. I hope the verbal description hasn't confused anyone.

Mace

[FLS]

Nice description Mace. Sometimes when thinking about the different forces I forget that they're combined and applied to the CG.

I would just add that, as you know, the tail is a much longer lever arm from the CG than the lever arm of the center of lift so it's in a position to be the main force in determining pitch.

[hitech]

mace

First, the wing always produces a nose-down pitching moment whenever it produces lift because the center of lift is behind the Center of Gravity.

This is not a accurate statement when you are only considering the primary wing. Changing camber and hence adding lift can be done ahead of the CG.  And is on most of AH's planes But life is not even close as so simple as this.

You also have a wing above or below the cg and hence any change in drag also creates a pitching moment.

Also with the change in camber the Cm, I.E. the torque created that does not have any tendency to move the airfoil, only rotate it also changes.

The sum of all these will determine the direction of pitch.

HiTech

[Mace2004]

mace

This is not a accurate statement when you are only considering the primary wing. Changing camber and hence adding lift can be done ahead of the CG.  And is on most of AH's planes But life is not even close as so simple as this.
You also have a wing above or below the cg and hence any change in drag also creates a pitching moment.
Also with the change in camber the Cm, I.E. the torque created that does not have any tendency to move the airfoil, only rotate it also changes.
The sum of all these will determine the direction of pitch.
HiTech

Just keeping it simple HT for those that aren't grasping the relationships.  The fact that there are exceptions and additional complexities is why I mentioned that there is much more going on then just this simple explanation.   I know you need to get into precise details like power-on/power-off, thrust line, gear extension, external stores, horizontal stab position (high, mid, low), downwash effect, twist, etc., for accurate computer modeling but it doesn't change the essential concept.

[Anaxogoras]

The CT does not have a noticable effect on this. The pitch-up is the same.

There is a very simple test everyone can do:

Take a P-38, take off, trim for level flight, accelerate to just shy of 250 on WEP, and then dial in flaps all the time. one stage will pop after another as the speed goes down and watch the P-38 do a beautiful looping without the slightest control input. With a bit of throttle management on the way down, you can essentially loop a 38 this way until it rus of of fuel, without control input. If that is no indication for a very distinct nose-up pitch, I dont know what is.  

I just tried this with Il-2 1946.  Took out a P-38J, 50% fuel, trimmed for level flight at 250mph.  First notch of flaps the nose pitches up, added more notches until they were completely out (Il-2 1946 only has 4 flap settings for P-38), and the aircraft began to loop.  The first time around there was not quite enough speed to get over the top, and it nearly entered a spin, but a little aileron input prevented that.  The flaps jammed in the extended position when airspeed was too great, but the aircraft continued to loop with no elevator input.

So, interesting that two sims have the same pitch-up effect for flaps on the P-38, and maybe others, too.