Gscholz - Stability Fun

[funkedup]

Originally posted by GScholz
1. Loose tail and the nose goes up. R/L nose goes down because the horizontal stabilizer provides a down force that is in fact "holding your nose up".

2. Deploy flaps nose goes up. R/L nose pitches down because the CL of the wing moves back, further away from the CG.

3. Changes in airspeed requires drastic trimming changes. R/L not so, in fact the Fw190 didn't even have movable trim because it was so stable at all speeds, only small fixed trim tabs were needed. One example how dynamically stable aircraft are just that; stable at all speeds, because speed does not change the balance of forces acting on the pitch axis.

OK Now we are getting somewhere.  


#3 is easy.  First of all, the Fw 190 needed longitudinal pitch trim.  In fact, it needed so much trimming that they had to make the entire horizontal stabilizer movable via an electric actuator!  The pilot adjusted the stabilizer trim with a switch in the cockpit.  The more stable an aircraft is, the more trim it needs.

And speed does change the pitching moment.  The author is correct that speed doesn't directly change the pitching moment.  It's an indirect effect.  You can see it in the first movie here:  http://142.26.194.131/aerodynamics1/Stability/Page12.html

#2 - Yes good point that the aerodynamic center of the wing moves aft, and this does cause a nose-down pitching moment.  Also the flap adds drag to the bottom of the wing, adding a nose-down pitching moment.  
However the primary effect of flaps is to increase lift coefficient (CL) at the current angle of attack.  In order to maintain level flight after flap deployment, the pilot will need to lower his angle of attack to maintain the same CL.  The nose down moments from the a.c. movement of the wing will help do this, but he may still need some forward stick movement.

#1 - Aerodynamic center of the wing will not move at all when the tail is lost.  However, the aerodynamic center OF THE AIRPLANE will move forward.  

Whether the plane pitches nose up or nose down depends on a bunch of variables.  To figure out all the pitching moments we need to consider thrust, pitching moment of the wing and fuselage about their own aerodynamic centers, the movement of the aerodynamic center of the airplane, the movement of the cg of the airplane, etc.  If you want I can dig up the equation tonight and post it.  

But in the simplest case it will boil down to this:
Is the wing's aerodynamic center forward or aft of the CG?
If the wing's a.c. is fwd of the CG, the plane will pitch nose up.
If the wing's a.c. is aft of the CG, the plane will pitch nose down.

HT has said before that the planes in AH (and real WW2 fighters) had wing a.c. forward of the center of gravity.  This makes sense because it means the tail contributes to lift and gives the aircraft maximum maneuverability.  
So aircraft of this type are going to pitch nose up if the tail's lift is taken away.

Note that having the wing a.c. forward of the c.g. does NOT always make the aircraft unstable.  Note the second chart here:  http://142.26.194.131/aerodynamics1/Stability/Page9.html
The author describes the case in AH exactly:  

The c of g however, is sometimes behind the ac of the main wing. If this is the case the wing will be unstable. That is indicated in the diagram to the right by the upward slope of the red line.  
The total stability is simply the sum of the tail and main wing stabilities. In the example above the red and green lines are added to get the blue line.
As long as the total aircraft CM vs. CL graph has a negative slope the aircraft is stable.

[GScholz]

Sorry for being late, had to go sand down a window frame that needs painting.

"#3 is easy.  First of all, the Fw 190 needed longitudinal pitch trim.  In fact, it needed so much trimming that they had to make the entire horizontal stabilizer movable via an electric actuator!  The pilot adjusted the stabilizer trim with a switch in the cockpit.  The more stable an aircraft is, the more trim it needs."

>>> That is the same system fitted to the 109, but I have not read anything about the 190 having it (that does not of course mean it didn't )


"And speed does change the pitching moment.  The author is correct that speed doesn't directly change the pitching moment.  It's an indirect effect.  You can see it in the first movie here:  http://142.26.194.131/aerodynamics1/Stability/Page12.html"

>>> Yes change in engine power will upset the trim balance (in a prop plane) because the propwash adds more lift to the wing than to the tail. However in AH a deadstick plane will start to pitch up in a dive. And with no change in power (say full power) an accelerating plane in AH will pitch up as speed starts to pick up even if trimmed for level flight at that power setting.


"#2 - Yes good point that the aerodynamic center of the wing moves aft, and this does cause a nose-down pitching moment.  Also the flap adds drag to the bottom of the wing, adding a nose-down pitching moment.  
However the primary effect of flaps is to increase lift coefficient (CL) at the current angle of attack.  In order to maintain level flight after flap deployment, the pilot will need to lower his angle of attack to maintain the same CL.  The nose down moments from the a.c. movement of the wing will help do this, but he may still need some forward stick movement.

>>> My limited experience in the C152 trainer (27 hours) tells me otherwise. As I crank out the flaps the nose pitches down and I must correct by pulling the stick a little. Friends here at the local flying club says it's the same with the C182, Archer III and Grumman Traveller they've got. It can of course be different with other planes, but I don't know any.


"#1 - Aerodynamic center of the wing will not move at all when the tail is lost.  However, the aerodynamic center OF THE AIRPLANE will move forward."

>>> Only if the tail is providing positive lift. My premise for this whole discussion is that it don't. If this is wrong then I am wrong in my conclusions.


"Whether the plane pitches nose up or nose down depends on a bunch of variables.  To figure out all the pitching moments we need to consider thrust, pitching moment of the wing and fuselage about their own aerodynamic centers, the movement of the aerodynamic center of the airplane, the movement of the cg of the airplane, etc.  If you want I can dig up the equation tonight and post it."  

>>> Not really necessary, but if you want to who am I to say no.


"But in the simplest case it will boil down to this:
Is the wing's aerodynamic center forward or aft of the CG?
If the wing's a.c. is fwd of the CG, the plane will pitch nose up.
If the wing's a.c. is aft of the CG, the plane will pitch nose down."

>>> And therein lies the whole premise of this debate.


"HT has said before that the planes in AH (and real WW2 fighters) had wing a.c. forward of the center of gravity.  This makes sense because it means the tail contributes to lift and gives the aircraft maximum maneuverability.  
So aircraft of this type are going to pitch nose up if the tail's lift is taken away."

>>> If so, HT is right and I am wrong. If the CG is behind CL/CP or AC the nose will pitch up because the tail is producing lift to balance the plane rather than pushing holding the tail down. I wonder if every WWII plane was balanced in this was ... if any?

"Note that having the wing a.c. forward of the c.g. does NOT always make the aircraft unstable.  Note the second chart here:  http://142.26.194.131/aerodynamics1/Stability/Page9.html
The author describes the case in AH exactly:  "

>>> No, but it would make the wing unstable, and the aircraft would be dynamically unstable, i.e. negative dynamic stability.

http://142.26.194.131/aerodynamics1/Stability/Page3.html
http://142.26.194.131/aerodynamics1/Stability/Page8.html


It nice to have a civil debate, don't you think?


(Btw. now I'm going to play some AH. Will continue this later if necessary.)

[hitech]

naca1144

Check this out, shows the force direction on the speed to tail force chart.

HiTech

[funkedup]

Yes change in engine power will upset the trim balance (in a prop plane) because the propwash adds more lift to the wing than to the tail. However in AH a deadstick plane will start to pitch up in a dive. And with no change in power (say full power) an accelerating plane in AH will pitch up as speed starts to pick up even if trimmed for level flight at that power setting.

Actually the example on the website is set up so that the thrust is not a factor.  Propwash and thrust won't affect the result.  If you are trimmed for a given angle of attack and CL in level flight, and you speed up, there will be excess lift, the plane will accelerate vertically, and you will start a climb.  The stability of the airplane will cause the airplane to "weathervane" and maintain angle of attack, which means the nose goes up.

My limited experience in the C152 trainer (27 hours) tells me otherwise. As I crank out the flaps the nose pitches down and I must correct by pulling the stick a little. Friends here at the local flying club says it's the same with the C182, Archer III and Grumman Traveller they've got. It can of course be different with other planes, but I don't know any.

Those are high wing aircraft I think?  That changes things a bit.

No, but it would make the wing unstable, and the aircraft would be dynamically unstable, i.e. negative dynamic stability.

http://142.26.194.131/aerodynamics1/Stability/Page3.html
http://142.26.194.131/aerodynamics1/Stability/Page8.html

No.  As long as the airplane's aerodynamic center (total effect of wing + tail + fuselage) is behind the CG, the airplane is longitudinally stable in all respects.  The Page 8 example is an airplane with a wing only, no tail.  The tail can stabilize an aircraft even if the CG is aft of the wing's aerodynamic center.   That's the whole point of having a tail, and it's the point of the section I referred you to on Page 9.

It nice to have a civil debate, don't you think?

 Debate is maybe too strong a term...  When I was learning this material I went to the professor's office and asked very similar questions to what you are asking now.  

[GScholz]

Thanks HiTech. I've downloaded the PDF and will study this document with great interest.

(Now I'm off to fly)

[Ecliptik]

>>> If so, HT is right and I am wrong. If the CG is behind CL/CP or AC the nose will pitch up because the tail is producing lift to balance the plane rather than pushing holding the tail down. I wonder if every WWII plane was balanced in this was ... if any?

This should be easy to answer, I think.  If the horizontal stabalizers on the tail have a similar cross section as the wing, (greater surface area on the upper surface than the bottom) then the tail should produce lift.  If the opposite is true, it should act like the rear spoiler on a car and produce a downward force.

I realize I oversimplify, because the net force produced also depends on the angle of attack of a wing (for example, there are stunt planes that have symmetrical wings to enable good inverted flight characteristics).  But in general the above should hold.

[GScholz]

Originally posted by funkedup
Actually the example on the website is set up so that the thrust is not a factor.  Propwash and thrust won't affect the result.  If you are trimmed for a given angle of attack and CL in level flight, and you speed up, there will be excess lift, the plane will accelerate vertically, and you will start a climb.  The stability of the airplane will cause the airplane to "weathervane" and maintain angle of attack, which means the nose goes up.

Of course! Doh!

Originally posted by funkedup
Those are high wing aircraft I think?  That changes things a bit.

The Archer and Traveller are low-wing planes, the Cessna's are high-wing.

Originally posted by funkedup
No.  As long as the airplane's aerodynamic center (total effect of wing + tail + fuselage) is behind the CG, the airplane is longitudinally stable in all respects.  The Page 8 example is an airplane with a wing only, no tail.  The tail can stabilize an aircraft even if the CG is aft of the wing's aerodynamic center.   That's the whole point of having a tail, and it's the point of the section I referred you to on Page 9.

Yes I've studied the report HiTech showed me (although most of the math was over my head), and given the fact that these planes indeed had CG aft of AC the nose should go up if the tail is lost (unless the entire tail section goes fly, I imagine the CG would move far forward then).

Originally posted by funkedup
 Debate is maybe too strong a term...  When I was learning this material I went to the professor's office and asked very similar questions to what you are asking now.  

Yeah, I feel like a school kid now all right!

[FUNKED1]

You shouldn't feel like too much of a kid.   Check out the other thread where I made some grand declaration and then somebody pointed out that I made a big mistake.  

[GScholz]

Hehehe