Tail Damage - Opening can of worms...

[hitech]

Read my description and think about the consequences of the stab fighting the opposite  elevator vs only changing the AOA with 1 stab. The increased roll with 1 elevator 2 stabs makes since and a lot less with 1 stab.

HiTech

[mtnman]

Read my description and think about the consequences of the stab fighting the opposite  elevator vs only changing the AOA with 1 stab. The increased roll with 1 elevator 2 stabs makes since and a lot less with 1 stab.

HiTech

I think I see what you're talking about, and understand your explanation.  What I'm having trouble with though, is wrapping my mind around the "why's".  

In the case of the film I posted, would the argument be that the net result with only the left elevator missing, and the right elevator deflected downward, is a left roll, but only because of force generated by the left stabilizer?  And that removing the left stabilizer removes that force, so that an application of right elevator produces "pure" up/down deflection, without the roll tendency?

I think I'm close to understanding the argument, and I think I see my "stumbling block".  What's been confusing me (I think)is that I'm seeing the downward-deflected right elevator as producing an upward force, which isn't mirrored on the left side.  To my mind, this would generate a left roll (and it appears to, which makes it an easy trap for me to fall into).  In this explanation, the lifting force is what generates the roll, and it isn't dependent at all on the left stabilizer.  It depends only on the right elevator.  This is also linked with the idea that it's the elevator that directs the pitch of the wing.  I think this view is also due in part to what we see when one flap is stuck down, when the other side isn't.  We attribute that to the extra lift on one side, rather than the reduced lift on the other, when in fact, both must play a part.

But...  That ignores the stabilizer, doesn't it?  What's the stabilizer for?  What's it doing, and how?

To my way of thinking, the stabilizer may or may not generate a "net" positive or "net" negative lift at different points in flight.  It's "net" force effect in level flight has to equate to zero lift though, or the tail would constantly be trying to move upwards (if it generated more than a net of zero).  And if it didn't generate a net lifting force of at least "zero", it'd constantly be trying to "fall".  That doesn't mean it generates no lift, it just means that in level flight it generates "exactly enough" to balance out the weight it carries (the lifting force "nets out" to zero)...

With that idea, the "weather-vane" effect of the tail directing the main wing gets tied in.  

With the weather-vane effect, if the tail moved upwards the airflow would try to blow it back "in line".  In the case of the film I posted, the airflow would be trying to blow both stabilizers back "in line".  However, the right elevator is resisting this force, and the left side (without an elevator) is not resisting this force...  In fact, the left side stabilizer will be working to get "back in line", pushing down on the left side, which is an effect not mirrored by the right stabilizer/elevator combo.  In this explanation, the net effect is still a left roll, but it's due to the downward force applied to the left stabilizer, not due to the lifting effect of the right elevator (necessarily).

Reversing the elevator (right elevator up) would reverse this effect, causing a roll to the right, but again, this roll would be caused by the left stabilizer, not by the right elevator.

So, the "final" idea is that the stabilizer directs the up/down pitch of the wing, primarily through a "weather-vane" effect.  The elevators don't produce an up/down effect themselves, so much as directing the stabilizers (by directing the "lift" upwards or downwards, essentially creating a net positive or net negative lifting force).  The stabilizer isn't just something to hold the elevators, the stabilizer is what stabilizes the planes up/down pitch.

Is that pretty close?

I guess I'm not surprised, then, by the idea that without the left stabilizer the plane doesn't exhibit the same rolling effect.  After all, if it was a "strong" effect, we should also see problems with left rudder creating a left roll.  The off-center effect of the left rudder would produce a large right-roll tendency, but it doesn't.  That's not to say those undesirable forces don't exist, just that the effect is overcome by other factors (the wing, primarily).

Back to the stabilizer idea...

It's amazing really, because we all know that a "conventional" airplane flies just fine without the elevators (unless it needs to change direction), but won't fly a bit without a stabilizer.  We can see that with hand-tossed balsa gliders.  They don't use elevators at all, and fly (er, glide) just fine.  Take the stab off, and it drops the tail hard, and piles in...  

Now, it also sets up an easy experiment.  Buy a $1 balsa glider at the hardware store, and cut off the left or right stab; does it still fly ok?  What about if you used tape to create an "elevator" on one side, before you removed the stab from the other side?  I'd expect the plane to "barrel roll" with one elevator and two stabs, for the reasons above.  I'd then expect the glider to "loop" with the one elevator, and with one stab removed.  Anyone have a buck, and live near a store with balsa gliders?  It sure would be easier for most of us than a bunch of math!

[dtango]

mtnman:  your test film with left stab on but missing left elevator is what I would expect how a plane would respond.

The reason of course is because Cm < 0 for a statically stable airplane.    

This explains both the different reactions you're seeing in the stab on, elevator missing as well as the stab & elevator both missing cases.  I don't have time at the moment to diagram and explain.  I'll try to do that later on but maybe someone with that clue can figure out before I get back !

Tango
412th FS Braunco Mustangs
  

[hitech]

Mntman try visualizing it differently where the complete stab is rotating. R ember that the most force is generated before any plane rotation takes place creating AOA, So as example lets say at full deflection and MAX aoa of the wing, the tail needs 100 lbs down force, but when you first pull the elevator back it is generating 1000lb down force.

As the plane begins to gain AOA the force on the tail diminishes from 1k down to 100 lb.

So now lets split that into 2 stabs same forces.

1 elevator is pulled creating (lets take the right one still attached) 500lb down force. But this time 0 force is on the left stab to begin.

AS AOA increases the left stab is starting to generate up force, now as the plane continues to increase AOA the  right is decreasing lift is increasing force in opposite directions. As the equalize with less AOA then both you will have the left pushing up with about 250 lb and the right pushing down with about 250 lbs creating a roll moment.

Now with just 1 stab and elevator you would end up with full AOA (plane turning faster) but again only 100 lb down force holding the AOA needed I.E. the left stab is trying to stabilize at level AOA and fight your desire to pull max AOA.

The thing you need to remember that is not instantly intuitive is that depending on the CG setup, it dosn't take much force to hold an AOA but takes force to rotate to the AOA.

HiTech

[mtnman]

mtnman:  your test film with left stab on but missing left elevator is what I would expect how a plane would respond.

The reason of course is because Cm < 0 for a statically stable airplane.    

This explains both the different reactions you're seeing in the stab on, elevator missing as well as the stab & elevator both missing cases.  I don't have time at the moment to diagram and explain.  I'll try to do that later on but maybe someone with that clue can figure out before I get back !

Tango
412th FS Braunco Mustangs

If/when you have time, I'd love to see your diagram and read your explanation! 

Keep in mind though, I only have a BA in Art, lol.  When I was in school, my math skills weren't so hot.  As a matter of fact, graduating HS and college for me was at least partly due to the requirement of "x years of math or foreign language."  I opted out of math, and haven't looked inside a math textbook in roughly 20 years, apart from trying to help my kids with grade-school math questions.  HS and college French was my "math or foreign language" choice.

So, when you say "The reason of course is because Cm < 0 for a statically stable airplane.  ", I'm gonna guess you mean "Center of mass", and then google it to see if it's possible that that's what you mean, lol.  Then, looking at the choices given (wow, cervical mucus?) I'm gonna sit back and act like I knew what you meant all the time... (but secretly wonder if you mean http://en.wikipedia.org/wiki/Pitching_moment ?).

My interest level is high, so I'm enjoying this, but it isn't easy for me to grasp entirely. 
  

[dtango]

If/when you have time, I'd love to see your diagram and read your explanation! 

Keep in mind though, I only have a BA in Art, lol.  When I was in school, my math skills weren't so hot.  As a matter of fact, graduating HS and college for me was at least partly due to the requirement of "x years of math or foreign language."  I opted out of math, and haven't looked inside a math textbook in roughly 20 years, apart from trying to help my kids with grade-school math questions.  HS and college French was my "math or foreign language" choice.

So, when you say "The reason of course is because Cm < 0 for a statically stable airplane.  ", I'm gonna guess you mean "Center of mass", and then google it to see if it's possible that that's what you mean, lol.  Then, looking at the choices given (wow, cervical mucus?) I'm gonna sit back and act like I knew what you meant all the time... (but secretly wonder if you mean http://en.wikipedia.org/wiki/Pitching_moment ?).

My interest level is high, so I'm enjoying this, but it isn't easy for me to grasp entirely. 

Haha no worries man.  I'm back at home slaving away at the explanation as we speak.  It's just another way of explaining what HT has been explaining.

Cm = Pitching Moment of an airplane for your info!

Tango
412th FS Braunco Mustangs

[mtnman]

Mntman try visualizing it differently where the complete stab is rotating. R ember that the most force is generated before any plane rotation takes place creating AOA, So as example lets say at full deflection and MAX aoa of the wing, the tail needs 100 lbs down force, but when you first pull the elevator back it is generating 1000lb down force.

As the plane begins to gain AOA the force on the tail diminishes from 1k down to 100 lb.

So now lets split that into 2 stabs same forces.

1 elevator is pulled creating (lets take the right one still attached) 500lb down force. But this time 0 force is on the left stab to begin.

AS AOA increases the left stab is starting to generate up force, now as the plane continues to increase AOA the  right is decreasing lift is increasing force in opposite directions. As the equalize with less AOA then both you will have the left pushing up with about 250 lb and the right pushing down with about 250 lbs creating a roll moment.

Now with just 1 stab and elevator you would end up with full AOA (plane turning faster) but again only 100 lb down force holding the AOA needed I.E. the left stab is trying to stabilize at level AOA and fight your desire to pull max AOA.

The thing you need to remember that is not instantly intuitive is that depending on the CG setup, it dosn't take much force to hold an AOA but takes force to rotate to the AOA.

HiTech

I think this is making sense to me know, believe it or not!  I still want to pick up a cheap balsa glider on my way home from work tomorrow, and see if I get these effects.  Back to the art thing, I like to "see" things, to really understand them.

So, it makes sense that the plane would roll more with both stabs and one elevator, and turning would actually be better if the stab was removed from one side.  It would also seem (to my mind anyway) that the one elevator/stab remaining would be able to hold the 100lbs force from your example, so max AoA could still be reached, and maintained (for a while anyway), but probably not as quickly?  It could obviously hold the initial amount, but is there any reason it wouldn't be able to maintain the max AoA?  Maybe as speed dropped, it wouldn't be able to hold it as long (as slow) as two elevators could?  Would the effect be a "slower" transition to max AoA at high speed, and an inability to maintain max AoA in slow flight as well as two elevators could?  But once the movement was initiated at "mid" speeds, it wouldn't turn much worse than "normal"?

And you're right, it isn't intuitive that the force to hold the AoA would be less than that required to initiate the movement, but then again, I can see what you mean now that you mention it.  

I see that effect in boating I think, even though the application is different, and effected by the thrust of the motor or sail.  In sailing, I see the initial movement of the rudder takes more force than it does to hold it deflected, even though (and maybe because?) the initial rate of turn is less (and it's tough to see a prolonged effect because the relative wind direction is changing with the turn).  In a power boat, I think I feel that too, but it's probably not the same thing, since the "rudder" is providing the thrust as well.

FYI- any responses from me will be slow in coming; back to working 12hr shifts, with an hour drive on each end...

[mtnman]

Haha no worries man.  I'm back at home slaving away at the explanation as we speak.  It's just another way of explaining what HT has been explaining.

Cm = Pitching Moment of an airplane for your info!

Tango
412th FS Braunco Mustangs

LOL!  Awesome!

[dtango]

So what the heck is going on in mtman's flight tests?

A key principle we need to look at is what the purpose of the vertical stabilizers are there on the tail for.  Here's an instructive diagram:



An aircraft has various lift forces acting on it in relationship to it's center of gravity.  The wing provides most of the lift force but left to itself that alone causes stability issues in the pitch (longitudinal) axis.  As seen in figure (a) above the lift (& drag, & thrust etc.) of the wing creates a pitching moment that causes the aircraft to pitch up (or down).  A countering acting force is needed to balance out the wing's pitching moment and that's what one of the main purposes of the tail's vertical stabilizers are for.  As represented in figure (b) the tail vert-stabs along with elevator trim provides the counter pitching moment to keep the plane level in level flight.  The pitching moment, usually labeled Cm is the sum of all the pitching moments about an aircraft.

Here's the catch.  Planes that are designed to be statically stable mathematically must have Cm < 0 (pitching moment always negative).  What this means is that a stable airplane will always pitch back toward the trimmed state.  If aoa increases and nose rises, the vert stabs generate force the opposite direction to rotate the nose back down.  When aoa decreases and the nose falls, the vert stabs generate force the opposite direction to rotate the nose back up.  THE KEY HERE IS THAT THE TAIL (VERTICAL STABILIZERS) PROVIDES THIS CORRECTIVE PITCHING FORCE.  This is important for stability including dealing with when plane stalls why they pitch nose down which is to reduce aoa to get you flying again.  Diagramatically it looks like the following (the sloping downward line from left to right is a Cm < 0 slope).



So what does this have to do with rolling when missing one elevator but both stabs are on?  Everything.  It's basically what HT has been saying about the how the vert-stabs are reacting to changes in aoa and each other.  

So in your specific example with left elevator gone- you pulled nose up by applying elevator input.  To pitch up means that the tail is generating a downward force thanks to the camber change with elevator input.  All would be fine if the left elevator is present but it isn't.  On the left vert stab because some nerdy aero designed the tail so that Cm < 0, generates an upward lift force on the tail to counteract the downward pitching moment.  So on the right vert stab we have a downward moment (elevator input).  On the left stab we have an upward moment instead.  Looking at the aircraft nose or tail on these two forces are COMBINING and basically creating a net rolling moment to the right (just like an right alieron roll - left wing lift vector up, right wing lift vector down).  So voila, pull the stick back and the plane rolls right too.

If you didn't notice, this same set of forces are operating in your first film when you dive down to gain airspeed.  When you pitched nose down, the airplane rolled left instead.  Why?  Because of the assymetric lift load on the tail now acting in the opposite direction.

So there you have it.  Hope that helps!  Maybe not as colorful as the way HT describes it but just my stab at it all!

Tango
412th FS Braunco Mustangs

[WMLute]

I still think mo' damage should be mo' difficult to fly...







off to work on changing my aim point to somewhere else on the nme's plane... 
so much for 8 years of easy kill tail shots...

[Chalenge]

Well Tango managed to explain clearly what a tenured professor of aerodynamics couldnt get across in laymans terms. Well done sir! 

[dtango]

Just a correction - I was said vertical stabilizers.  I mean horizontal stabilizers .  Sort of like saying yeah, my "other right" or "other left".

Tango
412th FS Braunco Mustangs

[hitech]

One small caveat to dtango, only the entire plane Cm must be stable, this includes main wing, fuselage and horizontal stab. The primary wing it's self can be unstable, but the planes net Cm could still be stable.

HiTech

[mtnman]

Thanks guys!  I feel like I have a good understanding of what's going on now!

[Simaril]

HiTech, Chalnege, dTango -

Thanks for taking the time to do this kind of stuff! I love reading these threads, and even though I have no background in engineering its somehow satisfying to wrap my head around lift and vectors and counterbalancing forces.

OK, it makes my head hurt too.

Maybe "satisfying" like "doing a triathalon without training"....



But anyway, thanks!