Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: Chalenge on May 08, 2010, 09:45:33 PM
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When the tail of your plane is damaged and the stab and elevator on one side appears to be missing there should be a rolling component to any input of elevator... but there isnt.
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I'm not sure what should happen, but I don't think there is enough of a decline in flight efficiency after you lose the one side.
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Hm...never really thought about it, but i think you are right..the increased drag on one side of the plae should create a rolling effect, just like alierons do...good catch..
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with both elevators gone should airplane actually go down, instead of pushing up like it does now. I have lost both of them and my spit8 kept climbing was able to keep control if i kept it on a slow turn. I believe i gained about 10k in altitude till i got bored and augured.
semp
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with both elevators gone should airplane actually go down, instead of pushing up like it does now. I have lost both of them and my spit8 kept climbing was able to keep control if i kept it on a slow turn. I believe i gained about 10k in altitude till i got bored and augured.
semp
Actually it depends on speed. Your wings produce lift and will cause the plane to nose up with speed. Elevators only control pitch when u want it to. If you notice on most if not all planes, when in level flight at speed, your trim is actually in the - (elevator down) some to keep the plane level and to counterack the lift of the main wings.
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with both elevators gone should airplane actually go down, instead of pushing up like it does now. I have lost both of them and my spit8 kept climbing was able to keep control if i kept it on a slow turn. I believe i gained about 10k in altitude till i got bored and augured.
If there were negative lift generated on the horizontal stabilizer (holding the nose 'up') then there would be a double induced drag penalty which designers of the era were certainly aware of and I would say this was avoided in all of the cases of aircraft in AH. It is not a requirement of course and it is possible that some aircraft were designed with negative lift but I find it highly doubtful that any designer of a successful war plane would design anything that inefficient. That said most airplanes carry some negative lift over a small portion of their flight envelope. If you should lose a tail in one of those portions of flight your nose will drop very rapidly.
If you accept this (and I think you will) then you will realize that mostly the tails generate positive lift and so when a piece of a tail comes off then the nose becomes more likely to rise (tail heavy) and when the entire tail is gone the nose balloons (which is how the game represents things). If you fly an airplane with an elevator partially missing (one side gone) you should see more drag generated on that side and the airplane will tend to fly tail heavy with a slight yaw (less positive lift on the tail and unequal drag). AH represents that as expected.
Where the representation does not agree with what I expect is when you apply elevator in either direction with the stab and elevator surface missing on one side. I would expect that the aircraft would roll with a slight yaw tendency as well. Of course I could be mistaken but I still thought it best to bring it up so that if I am right and the representation is off this little bit then it can be corrected and AH will be even more accurate which is something I think we all want.
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If there were negative lift generated on the horizontal stabilizer (holding the nose 'up') then there would be a double induced drag penalty which designers of the era were certainly aware of and I would say this was avoided in all of the cases of aircraft in AH. It is not a requirement of course and it is possible that some aircraft were designed with negative lift but I find it highly doubtful that any designer of a successful war plane would design anything that inefficient. That said most airplanes carry some negative lift over a small portion of their flight envelope. If you should lose a tail in one of those portions of flight your nose will drop very rapidly.
If you accept this (and I think you will) then you will realize that mostly the tails generate positive lift and so when a piece of a tail comes off then the nose becomes more likely to rise (tail heavy) and when the entire tail is gone the nose balloons (which is how the game represents things). If you fly an airplane with an elevator partially missing (one side gone) you should see more drag generated on that side and the airplane will tend to fly tail heavy with a slight yaw (less positive lift on the tail and unequal drag). AH represents that as expected.
Where the representation does not agree with what I expect is when you apply elevator in either direction with the stab and elevator surface missing on one side. I would expect that the aircraft would roll with a slight yaw tendency as well. Of course I could be mistaken but I still thought it best to bring it up so that if I am right and the representation is off this little bit then it can be corrected and AH will be even more accurate which is something I think we all want.
just saying with elevators gone plane should go down. a big commercial airplane crashed a couple of years ago, its elevators where gone and it flipped and nosed down streight into the water. now i am no expert, but i dont think it is any different with ww2 fiters. elevators are used to keep airplane going up or down, if they are gone then the airplane will go down trying to catch them :D.
but in honesty plane still flying with both elevators gone is one of those thing that only happen in ah, kinnda like a plane flying at full speed with 1/2 a wing gone.
semp
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I think that after the forces all stabilize that aircraft in AH act exactly like the airliner you mentioned and they will go straight down.
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There is actually unfortunate footage of an airshow crash showing similar nose-up tendency when stabilizers break off. It was a Blenheim or something.
Also, you ever try building paper planes and such as a kid? I remember trying flying wings and stuff I built. They don't just glide, they spiral all over (often leading edge flipping up).
I buy that the nose would go up after a slight pause. I think that's right.
I also agree that you should roll with 1 stab gone. It's like a rudder. Also, using rudder and that elevator so that both pointed towards each other (like in a coordinated turn) would horribly push your tail at a 45-degree down angle toward the missing stab.
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The question Krusty, is how conspicuous will the resulting rolling moment be, in conjunction with the rest of the forces on the plane. I'd contend that it would be very small--perhaps even imperceptible in-game. Doesn't mean its not there--just too light to matter.
Guncrasher, there are some excellent primers about aerodynamics out there. I suggest you find one that will explain the forces involved in flight, and use your question/notion about the pitching tendencies of aircraft after losing elevator authority to guide your study.
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I have had this happen with a scale model and I would not express the effect as 'imperceptible' at all. The real question I have is whether the representation of the stabilizer and elevator as missing indicates they are actually gone (removed completely) or if they have been reduced to swiss cheese surfaces. I really think HT isnt through making the changes to tail damages and we have more to see yet.
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The question Krusty, is how conspicuous will the resulting rolling moment be, in conjunction with the rest of the forces on the plane. I'd contend that it would be very small--perhaps even imperceptible in-game. Doesn't mean its not there--just too light to matter.
Guncrasher, there are some excellent primers about aerodynamics out there. I suggest you find one that will explain the forces involved in flight, and use your question/notion about the pitching tendencies of aircraft after losing elevator authority to guide your study.
I disagree entirely. Is the kick from a rudder insignificant? Most times folks are jamming that elevator, and it would roll you just as hard as a rudder, if not more (usually more surface on elevators).
It's a rudder, just mounted at a different angle. Why would the forces be any different?
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I disagree entirely. Is the kick from a rudder insignificant? Most times folks are jamming that elevator, and it would roll you just as hard as a rudder, if not more (usually more surface on elevators).
It's a rudder, just mounted at a different angle. Why would the forces be any different?
The fact that the rudder is perpendicular to the wing seems like a significant difference.
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Nope. It's a control surface. There are such things as V-tail setups as well. They work in conjunction with each other, and when both are deflected into each other, the force combines.
So, similar to an off-set-vtail, the single h-stab should work just like a rudder because it has nothing on the other side balancing the forces out. Same as a rudder. Doesn't matter what way the rudder is angled.
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Interesting, are we talking WWI or WW2 aircraft? IIRC, damage effects in WWI were given an update that was said to be carried over into WW2 craft in possible future updates.
Good food for thought, opening a can of chili :neener:
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When rudder input is given, it doesn't cause a roll. It causes a yaw. The yaw is what leads to the roll- in many planes, anyway.
That's actually pretty easy to see in RC planes that have no dihedral. Or, at least that's probably the easiest way for many of us to see that effect. You see that rudder-input "wags the tail", but doesn't directly lead to roll, and especially on aerobatic planes will lead to very little roll at all. Actually, in aerobatic planes the effect of rudder leading to roll, or aileron input leading to yaw, or elevator input leading to either roll or yaw (or both) is undesirable, and pains are taken to minimize those effects (through design).
When the yaw is induced, more lift is generated by the wing that "moves forward" faster, while at the same time the other wing generates less lift. This alone will cause some roll. On planes with dihedral or polyhedral, even more roll will be generated because the wing that moves forward will have its bottom presented to the slipstream, which will deflect it upward. It leads to a roll, but it's a "dirty" roll. It is, however enough of a roll to control 2-channel RC planes via rudder and aileron as long as enough dihedral or polyhedral is present. An airplane with too little dihedral or polyhedral doesn't generate enough roll through rudder input to be controllable. And if "extra" rudder is given in an effort to achieve that roll, things get ugly.
Keep in mind that while the rudder swings the nose to one side, it doesn't alter the flight direction to the same extreme. This results in a a slip or "crab".
So in effect, it isn't the rudder that causes the roll, it's the design and orientation of the wing... And due to those, left rudder leads to a left roll...
Now, to make things confusing, think about what you guys are arguing with the "rudder-induced roll" idea, and how you think that should translate to an "elevator-induced roll"...
If what you're claiming were true, then an input of left rudder would cause a roll to the right. That's not what happens though, is it?
Kind of intriguing, huh?
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The same way it works on F-14s or any other plane with elevons. Sorry but the way it works right now is just wrong.
EDIT: UNLESS like I said the representation of the surface being missing is one of simplifying graphics and yes Im hoping the new damage system will take care of that.
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The same way it works on F-14s or any other plane with elevons. Sorry but the way it works right now is just wrong.
EDIT: UNLESS like I said the representation of the surface being missing is one of simplifying graphics and yes Im hoping the new damage system will take care of that.
Not really "apples to apples"... I'm not arguing that the tail damage here is correct either, I can't prove it one way or the other. And really, I like these threads, because I can learn something from them.
However, it appears that people are jumping to gut conclusions without thinking things through entirely.
In the F14 example, you have a specially designed airplane specifically designed to roll due to elevon control (aided by wing-mounted spoilers and rudders). The entire wing design is also completely different. To think that a WWII fighter would "accidentally" roll "the same way" with only 1/2 of its elevator, as an F14 with two fully functioning elevons, and spoilers, and two rudders seems kind of far-fetched. If a damaged WWII plane could do it (without those benefits), and 1/2 of it's it looks like the F14 wasn't designed very efficiently.
In the end, we're looking at a damaged or missing elevator, not a set of elevons. I'm not "buying" the theory... yet...
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No. Sorry. Spoilers on the F14 do not help it roll. It doesnt take a full flying elevon to make an airplane roll either. If a single stab+elevator combination could not cause an airplane to roll then two of them could not cause an airplane to loop.
Its that simple.
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No. Sorry. Spoilers on the F14 do not help it roll. It doesnt take a full flying elevon to make an airplane roll either. If a single stab+elevator combination could not cause an airplane to roll then two of them could not cause an airplane to loop.
Its that simple.
No need to apologize... I think you're the sailplane pilot, right? Don't confuse those spoilers with the ones the F14 uses. It isn't that simple. I'm an RC sailplane buff, and understand the use of spoilers on those type of planes. Different application.
In here-
http://www.globalsecurity.org/military/systems/aircraft/f-14-design.htm
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"The design of the F-14B allows for incredible pitch authority as well as good roll control to produce an extremely agile fighter. Rolling maneuvers are accomplished through the use of differential horizontal stabilator and 8 spoilers located on top of the wings. Its general arrangement consists of a long nacelle containing the large nose radar and 2 crew positions extending well forward and above the widely spaced engines. The engines are parallel to a central structure that flattens towards the tail; butterfly-shaped airbrakes are located between the fins on the upper and lower surfaces. Altogether, the fuselage forms more than half of the total aerodynamic lifting surface. With the wings in the 20 degree position most of the lifting force comes from the wings, in the 68 degree position over 60% of the lift is generated from the fuselage itself".
"Lateral control is achieved by long-span spoilers, ahead of flaps, and tailerons. Automatic leading-edge slats assist maneuvering, and strakes emerge from the wing glove leading-edge at high airspeeds. The automatic wing sweep has manual override, with automatic scheduling of control with airspeed and autostabilisation and angle of attack protection provided by the autopilot and automatic carrier landing system (ALCS). Airbrake panels are located above and below tail, between the twin fins and rudders. For roll control below 57 deg (this is referring to wing-sweep, not degree of bank), the F-14 uses spoilers located along the upper wing near the trailing edge in conjunction with its all-moving, swept tailplanes, which are operated differentially; above 57-deg sweep, the tailplanes operate alone. For unswept, low-speed combat maneuvering, the outer 2 sections of trailing edge flaps can be deployed at 10 deg and the nearly full-span leading-edge slats are drooped to 8.5 deg. At speeds above Mach 1.0, the glove vanes in the leading edge of the fixed portion of the wing extend to move the aerodynamic center forward and reduce loads on the tailplane".
"The tail control surfaces on F-14s are known as "rolling tails", in that the aircraft does not have ailerons on the wings to control roll. Roll control is instead provided at low speeds by wing-mounted spoilers and at high speeds by differential horizontal stabilizer deflection. This configuration also produces side force, or yaw, which contributed to the inadvertent spin entries. This large tail configuration is to aid in takeoff from aircraft carriers, by providing more pitch moment".
Also, for more detail look here (if nothing else, read the first paragraph in the Introduction)-
http://www.seas.ucla.edu/~mcloskey/PDF/jgcd98.pdf
So, I'm still of the opinion that 1/2 of an elevator doesn't equate to an elevon, stabilitator, or "rolling tail" system.
I'd love to see an aero-engineer step in and give us some clarity, but I think a huge part of the equation here is that so much of the F14's lift is generated by its fuselage, where that isn't the case with conventional WWII planes. Having the lift located so close to the center of the roll axis is what makes the flying tail possible. Even at that, it isn't "really" adequate until the wings are swept, which centralizes the lift even more-so. In effect, the F14 is rolling like an arrow spins with helical fletching. "Normal" planes don't do that, and in its more conventional (un-swept) configuration, the F14 doesn't even do that without help from its wing-mounted control surfaces...
In reality, I think we might see some roll effect from a WWII plane with only one elevator. But which way would it roll? If the right elevator were removed, we'd probably argue that up-deflection would give a left roll, and down-deflection would give a right roll... But would that really be the case?
Deflecting the elevator would cause asymmetrical drag on the left side of the plane, and it would occur for either up or down elevator. Ever stick you hand out of the cockpit of an airplane in flight? The few times I've done it, the plane yawed to that side, due to the asymmetrical drag. Enough so that the pilot (I was in back) immediately looked back to see what was going on. And I only stuck my hand out part-way, and just for a second...
So, if that were to have any effect on the WWII plane, deflecting the left elevator would cause a left yaw, resulting in a left roll (regardless of whether the deflection were up or down). Now, would that be the end result? Or is it an effect that would lead to another, or be cancelled out by another (like adverse yaw from aileron deflection, which is overcome by rudder?).
It's not simple.
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And again, I'm not arguing that the tail damage model is correct, since I don't know for a fact one way of the other...
I'm just playing devil's advocate here, and pointing out potential flaws I see in the argument. The F14 info took me all of 30 seconds to locate, which is more time than I've accumulated researching them in my entire life. I have less interest in jets than I do in my septic tank.
As a result, I could easily be wrong...
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And again, I'm not arguing that the tail damage model is correct, since I don't know for a fact one way of the other...
I'm just playing devil's advocate here, and pointing out potential flaws I see in the argument. The F14 info took me all of 30 seconds to locate, which is more time than I've accumulated researching them in my entire life. I have less interest in jets than I do in my septic tank.
As a result, I could easily be wrong...
You might be wrong, but the 30 seconds of effort you used to look this information up is a stab at providing some evidence to support your argument, versus what some folks in this forum do by firing an opinion downrange with nothing more than their occluded intuition to guide them.
Comparing the flight dynamics and control system of the F-14 to a Spitfire, for example, is most definitely apples to oranges.
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Sitting behind a desk you will never experience the chance that can come upon you when things like midair collisions occur to your radio control aircraft. If you accept the nature of a video game over reality then all you are is a funnel for bad information.
Stoney have you sat down and played with the formulas?
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Sitting behind a desk you will never experience the chance that can come upon you when things like midair collisions occur to your radio control aircraft. If you accept the nature of a video game over reality then all you are is a funnel for bad information.
Stoney have you sat down and played with the formulas?
I'm actually having trouble figuring out which ones to use for this. Most of the stability equations I have assume uniform pitch inputs from the elevator, so they won't really have any merit here.
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Sitting behind a desk you will never experience the chance that can come upon you when things like midair collisions occur to your radio control aircraft. If you accept the nature of a video game over reality then all you are is a funnel for bad information.
Stoney have you sat down and played with the formulas?
I'm confused as to what your point is, and/or if it's directed towards me?
I've been flying RC planes for 25 years or more. I've built (mostly scratch-building, with my own self-drawn plans, but also a bunch of kit-planes) and flown sailplanes (thermal and slope), high and low-wing "sport" planes, semi-scale (civilian as well as WWI and WWII fighters), float planes, combat-streamer planes, and aerobatic biplanes (Ultimates and Pitts). Throw in some RC "bird-type" gliders as well, and even some of the modern "simplistic" helicopters.
I've experienced all sorts of mid-air mishaps (even a lost elevator, believe it or not, with a safe, uneventful-but-stressful landing). Flutter (aileron, which led to a disintegrated wing at high speed and low altitude, an engine-mount failure (again, a safe landing, with the engine hanging by the throttle-servo push-rod) flap failure on one wing (flap stuck down on one side), a $10,000 mishap with a hospital window, and others...
If by chance you're referring to me with "sitting behind a desk" and the "funneling bad info", feel free to post some evidence of your own.
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I wasnt talking of or to anyone in particular but since you say you have so much radio control experience you should be aware that the trimming procedure for elevators involves looping the airplane and 'trimming out' any rolling tendency. This is why the majority of precision aerobatic planes like the Sukhoi series have dual elevator horns and the difference of 1/2 clevis turn can make a very large difference which is why the even larger scale aircraft tend to have finer adjustments.
Its just as important in full size aircraft.
By the way my model aircraft flying extends back to the 1960s and controllers that just barely qualified as radios. These days I fly larger and slower planes to compensate for age reflexes and eyesight.
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I wasnt talking of or to anyone in particular but since you say you have so much radio control experience you should be aware that the trimming procedure for elevators involves looping the airplane and 'trimming out' any rolling tendency. This is why the majority of precision aerobatic planes like the Sukhoi series have dual elevator horns and the difference of 1/2 clevis turn can make a very large difference which is why the even larger scale aircraft tend to have finer adjustments.
Its just as important in full size aircraft.
By the way my model aircraft flying extends back to the 1960s and controllers that just barely qualified as radios. These days I fly larger and slower planes to compensate for age reflexes and eyesight.
Hehe, I'm amazed at how fast my planes seem to be flying these days too! I'm finding that I'm having difficulty keeping up with them, and am transitioning back into slower, more docile planes myself. I'm flying an el-cheapo ASH-26 right now, and am actually tempted to shelve it for a floater.
I've never flown single-wing aerobatic planes, and have also never noticed a very pronounced rolling tendency with elevator trim. Some, maybe, but nothing extreme, and I sure couldn't peg it as being elevator-related anyway (it could have been any of a multitude of other factors). The plane that lost an elevator was one of my scratch-built Ultimate Bipes, and I don't recall any roll issues when I brought it in. I wasn't really watching for it though, and it's been years since it happened. It's the only plane I've flown that had separate elevator servos.
Having a flap jam down (on an F4U) was a much more traumatic experience. I was able to get that plane down by dropping my other flaps, but it was scary. There was nothing like that much of a roll effect when I lost the elevator; I'm sure I'd have remembered it. Those Ultimates rolled like mad anyway though, so maybe I just didn't associate it with the elevator? That plane landed pretty "normally". It felt like I had "bragging rights", but in truth it wasn't that hard.
If it was a "huge" issue, you'd think we'd need a vertical stab and rudder under the fuselage too. Otherwise we'd have "huge" issues with roll being associated with rudder input, and it would be opposite of rudder deflection... Right rudder would create left roll. The only reason it doesn't is because of the wing...
Flying aerobatic planes, I'm sure you're familiar with the fact that planes without dihedral sure don't roll well/quickly/immediately with rudder input, do they? Which again makes me wonder... If right rudder creates right roll due to the dihedral of the wing, why does removing the dihedral not result in a right rudder/left roll effect?
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It depends on the design of the plane. Rudder will in some cases cause a stall of a portion of the wing due to the fuselage nose disturbing the smooth flow of air over the wing center and with the unequal lift comes roll. I suspect you have just forgotten how the loss of one elevator effected flight or your experience took over and you didnt have difficulty in making changes to land the plane or since the ailerons are so over-whelming on the Ultimate it was easy to retrim. I have flown planes with the elevator control reversed without difficulty but I brought it down immediately and reversed the throw.
Im not sure what has you thinking right rudder would cause left roll? I have never experienced that. I have flown a straight wing (zero dihedral) Ugly Stick and yes it rolled on rudder. It was more of a barrel roll but it rolled.
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I have less interest in jets than I do in my septic tank.
HERESY!!!!
I shall have you burned and darn you to heck! :devil
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It depends on the design of the plane. Rudder will in some cases cause a stall of a portion of the wing due to the fuselage nose disturbing the smooth flow of air over the wing center and with the unequal lift comes roll. I suspect you have just forgotten how the loss of one elevator effected flight or your experience took over and you didnt have difficulty in making changes to land the plane or since the ailerons are so over-whelming on the Ultimate it was easy to retrim. I have flown planes with the elevator control reversed without difficulty but I brought it down immediately and reversed the throw.
Im not sure what has you thinking right rudder would cause left roll? I have never experienced that. I have flown a straight wing (zero dihedral) Ugly Stick and yes it rolled on rudder. It was more of a barrel roll but it rolled.
Without wings (just a fuselage and tail), right rudder would cause the fuselage to twist/rotate to the left. Just like helical twist fletch's would do to an arrow shaft. The fuselage would "spin" opposite of the rudder deflection (if we ignore torgue, etc). It's an easy experiment actually, if you're having trouble visualizing it. Take a soda straw, cut 1" slits in one end and insert a paper elevator and rudder. Put a paper clip on the other end. Bend in some right rudder. Toss it, it rolls left.
You can't/don't see that effect with wings, because wings reverse that effect. Even with no dihedral, one wing moves forward faster than the other, generates enough excess lift to raise a bit, and exposes its underside to the slipstream (the plane will be in a bit of a skid- uncoordinated), and initiates a roll. With wings, left rudder gives left roll. Without wings, left rudder gives right roll.
Back to the (eeeew) F14... Ever have or seen a spoiler on a sailplane malfunction? (If not, buy a CMPro glider, mine are now packing-taped "closed", and the servo removed). The effect is that one wing loses lift (the one with the good spoiler), and one heck of a roll ensues. It isn't really a roll though, just one wing dropping fast. I imagine that in slome variety is how they get the F14 to roll with spoilers when the wings are "out". The wing without the spoiler must still be generating lift (and it's going faster) so a spiral dive eventually results with a glider. I wonder if that lift is enough to get the full roll out of the F14? Otherwise it seems like it should get "stuck" when it reaches a 90 degree bank?
FWIW, I just lost my (F4U) right elevator in a fight. With some difficulty, I was able to kill the guy. The horizontal stab remained intact. Left elevator remained.
Down elevator results in the nose dropping, and right roll... Up elevator results in the nose lifting, and left roll. That was without auto-trim. Maybe auto-trim dampens the effect?
Think about that for a minute... If down (left)elevator gives me a right roll, what happens when I reach 90 degrees right bank, and the elevator is vertical? It's now "kind of" like a rudder... And it's effectively deflected left, but I'm rolling right...
Ailerons too... They create a roll/movement opposite of the deflection (except for the adverse yaw effect). Why would the rudder create movement toward the deflection?
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Auto-trim (eww)!
If it works as you say maybe its new but I didnt read that in the notes. Of course I wasnt trying to fight anyone after I had mine part company with the plane.
Rudders are usually mounted too close to the fuselage centerline to work like your thinking. Also notice that fletchings on arrows are attached in a spiral pattern (at least sort of) in order to get the stabilizing spin. Wings are not magical reversers. :D Notice that the elevator direction matches aileron direction to get similar rolls? Nothing magical.
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Rudders are usually mounted too close to the fuselage centerline to work like your thinking. Also notice that fletchings on arrows are attached in a spiral pattern (at least sort of) in order to get the stabilizing spin. Wings are not magical reversers. :D Notice that the elevator direction matches aileron direction to get similar rolls? Nothing magical.
Elevators are mounted just as close to the fuselage center-line, and cause the effect I'm talking about (and that you've been arguing for...) when one is missing. You argued that the elevator should cause rotation away from its deflection, but are now saying the rudder should cause rotation toward the deflection? Weird, or you're just not thinking it through yet.
In addition, the F4U rudder is just plain huge, and the vertical stabilizer is small. In a plane like this the effect should be pretty obvious, unless some other force is taking over (like the wing...).
So, I guess I can go find some pictures to clarify. But while I do that, lemme ask this. In a case where the right elevator is missing, and down deflection is given to the left elevator, rotation direction would be "lifting" on the left (or clockwise rotation, viewed from the rear), right? Now, ignoring the wing, we add some left rudder. Should this stop rotation, or increase/aid it? Maybe even reverse it? Viewed from the rear, apply up elevator (to the left side), and right rudder. Which way does should it rotate? Or shouldn't it?
It really doesn't take much deflection at all to get and arrow to rotate, and as a matter of fact, an arrow with straight fletch's (no helical) still rotates, especially if feathers are used.
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No your missing the point. The rudder is typically above AND below the centerline. Usually a little more above but also shaped to be a broader surface near the bottom. Not in every case obviously but it is in nearly every case for fighters. Elevators are nothing like that.
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No your missing the point. The rudder is typically above AND below the centerline. Usually a little more above but also shaped to be a broader surface near the bottom. Not in every case obviously but it is in nearly every case for fighters. Elevators are nothing like that.
Depends on the aircraft. In AH2, most aircraft have the rudder mounted above the horizontal stab. The F4U series is a good example, as the entire rudder control surface is above the fuselage. That's why I keep saying that this is a very complicated issue. For those that claim the rolling moment is not what it should be, I'd like to see you diagram out all the forces that are involved here and post them up for us to see.
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Well I see how mtnman is looking at it though. He sees the elevator to left and right of the center.
I think the difference in stability can be portrayed as four legs of a chair in the case of wing and stab. In the case of the rudder its a three legged chair. Maybe that makes it a little more apparent what is happening.
Actually I would think of it as a three legged chair with a midget tugging at one of the legs. :D
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No your missing the point. The rudder is typically above AND below the centerline. Usually a little more above but also shaped to be a broader surface near the bottom. Not in every case obviously but it is in nearly every case for fighters. Elevators are nothing like that.
No, I see your point, but I'd say it typically isn't below the center-line. Some planes have it a bit below, but not most. Aerobatic planes in particular have extremely broadened lower portions of the rudder, coupled with generally "short" or "stubby" vertical rudder surfaces in order to minimize the rotational force. Aerobatic planes are designed to give as "pure" of an input as possible in all three axis. Rudder throw, for example, is desired to give yaw force, but not pitch or roll force. Most planes show at least some undesirable traits as a result of control surface deflection.
And even with the broader surface equal to or very slightly below the center-line, there's still going to be at least some "opposite" rotational force exerted, for two reasons.
One, the further from center, the greater the leverage applied. So the top of the rudder is applying more rotational force than the bottom of it. And two, even with the broader surface equal to or slightly below the center-line, there's little or no opposite rotational force applied, for the same reasons. In order to cancel out a clockwise rotation, the bottom would need to apply an equal counter-clockwise force.
Back to the F14... How much of the rudder is below the center-line? None. And there are two of them... So why (apart from the wings) wouldn't left rudder result in a right roll? If one of the stabilators were removed, would the other one still be able to impart a rotational force to the plane? Why would removing it, and re-attaching it 90ish degrees around the circumference of the tail cancel that effect out? Or better yet, reverse the effect? If down left elevator pushes the tail up and rotates right, left rudder should be expected to have the same effect.
It's really the same argument as the initial post... Without the opposite elevator to cancel the rotational force, losing an elevator could be expected to result in at least some rotational force, rather than a "pure" up/down force.
(http://i107.photobucket.com/albums/m309/Mtnman_03/rotational-axis.jpg)
Which brings me back to the original post again... Even though I think that having only one elevator should result in some unwanted rotational force (which, in the case of the F4U, it does), I don't think it'd be all that extreme.
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Mtnman... for the moment consider an aircraft in a slip. I know you had to have slipped an airplane or two. In a right hand slip (right wing low) the nose is left of the velocity vector and you are holding left rudder. What happens in this case is lift is generated by the vertical stabilizer and it has a corrective measure upon the yaw. So if you dont hold the left rudder then the plane will yaw to the right and restore the moment (so to speak) which will cause the airplane nose to point into the relative wind and return the sideslip angle to zero. Notice the plane does not roll to the right except the roll you add with aileron in order to hold the slip.
Im not sure but I dont think the rudder on the F-14 is very effective either (at yawing with coupled roll).
Anyway take the example of the slip I used and apply it to an aircraft with one stabilizer missing and you will see there is a pretty big difference.
EDIT: The F-14 actually has a counterpart in WWII that you can consider to be very similar in trait. The P-38 has the vertical stabilizers placed directly behind the propellers for increased stability. In a similar manner the F-14 has the twin stabilizers mounted where they can make use of wingtip vortices and flow entrainment caused by the exhaust of the two engines. Both of these airplanes are making use of the sidewash gradient that should become obvious in the slipping example I gave.
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Mtnman... for the moment consider an aircraft in a slip. I know you had to have slipped an airplane or two. In a right hand slip (right wing low) the nose is left of the velocity vector and you are holding left rudder. What happens in this case is lift is generated by the vertical stabilizer and it has a corrective measure upon the yaw. So if you dont hold the left rudder then the plane will yaw to the right and restore the moment (so to speak) which will cause the airplane nose to point into the relative wind and return the sideslip angle to zero. Notice the plane does not roll to the right except the roll you add with aileron in order to hold the slip.
Im not sure but I dont think the rudder on the F-14 is very effective either (at yawing with coupled roll).
Anyway take the example of the slip I used and apply it to an aircraft with one stabilizer missing and you will see there is a pretty big difference.
EDIT: The F-14 actually has a counterpart in WWII that you can consider to be very similar in trait. The P-38 has the vertical stabilizers placed directly behind the propellers for increased stability. In a similar manner the F-14 has the twin stabilizers mounted where they can make use of wingtip vortices and flow entrainment caused by the exhaust of the two engines. Both of these airplanes are making use of the sidewash gradient that should become obvious in the slipping example I gave.
Sorry about the long time between replies, I'm working this weekend, and with the drive it's a 15hr day... Yes, I've flown slips. RC as well as "full-size" planes, and of course in the game.
The slip argument is a great example, but actually illustrates my point better than yours....
Again, you're not going to see the left rudder/right roll effect with a "normal" plane, because it has wings, and those are overcoming that effect. get rid of the wings (back to the plastic straw, where it's really pretty obvious), and you would. Just because the effect is canceled by the wings though, doesn't mean it isn't there. It's the same rolling effect seen with one elevator without a countering surface (opposite elevator) to overcome it...
The slip proves the point actually. In a right-wing low slip, with the nose pointed to the left, the very reason the nose is left is due to the right-push of the left rudder. While the nose is to the left, it's only because the left rudder forces the tail right, out into the slipstream, and with the plane pivoting around it's yaw axis, the nose goes left...
"Lift" isn't generated by the vertical stab/rudder. That's not what forces it out into the slipstream, and that's not what brings it back in line. "Lift" would "draw" it left or right, which doesn't happen. It's "pushed" out there by the deflected rudder, and "pushed" back in line when the rudder is released. A much better analogy is a weather-vane. Lift is also the force that pulls opposite of gravity, but what matters here is that the tail isn't "drawn" anywhere, it's pushed/forced.
That very "push" is what leads to a left-rudder/right roll force. In order for there to be no roll force, the rudder would need to exert equal force above and below the aerodynamic center-line. If it exerts more force either above or below that center-line, a roll force will be present. It's exactly the same argument as the one this thread began with... The reason you don't see the roll effect from the elevators is because they "normally" overcome each other. The reason you don't see it from the rudder is because the wings overcome it...
Back to the straw again (you really should try it, it illustrates the point quite nicely). If you only put one "stabilizer" on it, bent to simulate control surface deflection, and give it a toss, you'll see two things (and it doesn't matter if you toss it with the stabilizer horizontal and call it an elevator, or toss it with the stabilizer vertical and call it a rudder, the effect is the same). For one, the deflected surface will push the "tail" away from the deflected surface. Always. Second, it will exert a roll force, and it will also be away from the deflected surface. Always.
The final result is a spinning straw that rolls away from the deflected surface (exactly like the opening post argues) as well as "rotates" (for lack of a better word, I'm tired). Imagine the tip of the straw as the tip of a funnel, with the "tail" of the straw following a path around the top of the funnel. The speed of the roll, and the relation it has with the "rotation" is variable, and depends on the size of the surface, it's height/distance that it protrudes from "center", the speed of the "toss", the balance point/CoG of the straw, and the time "in flight". I imagine if it had a long enough "flight", it'd try to rotate around the CoG, almost like two funnels taped together, large open ends away from each other. With the smaller funner being the front of the "plane" and the larger being the rear.
That rotation around the CoG is exactly what's happening in a slip, where the nose is force left with left rudder, all because the tail is being forced right (with left rudder).
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You have been talking about moving the tail right and I have been talking about moving the nose. :neener:
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You have been talking about moving the tail right and I have been talking about moving the nose. :neener:
Agreed... So, what moves the nose... And what direction of force is the tail applying to do that? :D
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What puzzles me is that (in AH) if I lose both the elevator and horizontal stab from one side there seems to be hardly any detrimental effect. (as long as I have a stab and elevator on the other side) or is this because I have combat trim on at the time?
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What puzzles me is that (in AH) if I lose both the elevator and horizontal stab from one side there seems to be hardly any detrimental effect. (as long as I have a stab and elevator on the other side) or is this because I have combat trim on at the time?
In which aircraft, and in what flight condition?
Again, I'd ask anyone that thinks it should be modeled differently to sketch out the control surfaces and plot all of the vectors that are acting on the aircraft in a specific flight condition, and then explain, using that graphic and available stability equations, why it should be different than it "feels" in-game.
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I have to wait until I lose another stab... might take a few months though. :D
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In which aircraft, and in what flight condition?
La7, P51 and IL-16 to date in combat low down. I could not see any hit on speed/manouverability at all. This could of course be because I am a Piiiiiiiilat of little skill.
Maybe combat trim was compensating
I am not going to get into any contest of minds on the matter.
It just seems strange that the designers of these air craft thought there was a need for stabs and elevators on both sides of the tail when it seems in AH we only need such an assembly on one side. It would be interesting to know what the detrimental effect of losing both stab and elevator from one side is.
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La7, P51 and IL-16 to date in combat low down. I could not see any hit on speed/manouverability at all. This could of course be because I am a Piiiiiiiilat of little skill.
Maybe combat trim was compensating
I am not going to get into any contest of minds on the matter.
It just seems strange that the designers of these air craft thought there was a need for stabs and elevators on both sides of the tail when it seems in AH we only need such an assembly on one side. It would be interesting to know what the detrimental effect of losing both stab and elevator from one side is.
Well, next time you lose the entire side of a stab/elevator, roll her over on one side and try to pull max G.
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La7, P51 and IL-16 to date in combat low down. I could not see any hit on speed/manouverability at all. This could of course be because I am a Piiiiiiiilat of little skill.
Maybe combat trim was compensating
I am not going to get into any contest of minds on the matter.
It just seems strange that the designers of these air craft thought there was a need for stabs and elevators on both sides of the tail when it seems in AH we only need such an assembly on one side. It would be interesting to know what the detrimental effect of losing both stab and elevator from one side is.
The detrimental side is that the aircraft has reduced pitch stability. It's not CT that's helping because CT is nothing but a look-up table based upon speed, it doesn't "know" half the stab is missing. Here's what HiTech had to say in another thread about half-stabs:
With a H stab missing the plane is less stable. Less stability creates the ability for a more rapid AOA changes. Turn performance would decrease , or possibly remain the same.
With 1 elevator destroyed your turn performance will decrease, it the corrisponding stab is then destroyed your turn performance will go back up. We thought this was a bug until we brought it up in our force vector displayer and saw that what is happening is with 1 side elevator gone and the stab remaining, the stab is fighting the other elevator so you can not generate the AOA you need. Once that stab is removed, the AOA you can generate is restored.
Now, when you think about it, it makes a lot of sense. Maneuvering requires you have to overcome the stability of the plane in one of its axis, in this case pitch. Fighters, like race cars tend to have less static stability than say, an airliner or Buick. This is because they need to be able to change direction quickly so reduced static stability helps with maneuvering but they are harder to fly (or drive).
In this case, when static pitch stability is reduced by loss of part of the Horizontal Stabilizer then less control power is required for the same maneuver. That means you could have sufficient control power from a single elevator to generate the same pitching moment generated by a complete stab with elevators BUT this comes at a significant cost of reduced pitch stability which means the pilot should have to work harder to control pitch (if it's possible to control at all).
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Thanks Mace......
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"With 1 elevator destroyed your turn performance will decrease, it the corrisponding stab is then destroyed your turn performance will go back up. We thought this was a bug until we brought it up in our force vector displayer and saw that what is happening is with 1 side elevator gone and the stab remaining, the stab is fighting the other elevator so you can not generate the AOA you need. Once that stab is removed, the AOA you can generate is restored."
The basic profile of an elevator is symmetrical so it does not really contribute as lift to any direction and its effect as a limiting force to tail movement sounds strange. It probably is a limiting force but it should be quite small when compared to functioning side of elevator. Of course it depends on the proportions of elevator plane vs. control surface which is different in different planes.
BTW is it possible to lose "other side" of elevator plane to begin with? Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit, but the control surface itself is separated from the other side so you can lose just the other?
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There is lift generated by the stabilizer. If symmetrical airfoils contributed nothing to lift they wouldnt and couldnt be used as airfoils.
Your last questions seems to be saying that the only thing holding a stabilizer on to the airframe is the aircraft skin and nothing could be further from the truth.
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"There is lift generated by the stabilizer. If symmetrical airfoils contributed nothing to lift they wouldnt and couldnt be used as airfoils."
Symmetrical airfoils do not generate lift at 0 AoA in relation to airflow, only drag, but any change in AoA moves the pressure gradient to lifting side causing lift to be generated. And it is not tailplanes job to create lift, just to direct the lift vector of the main plane.
"Your last questions seems to be saying that the only thing holding a stabilizer on to the airframe is the aircraft skin and nothing could be further from the truth."
"Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit" :huh
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[quote author=Charge link=topic=288694.msg3682487#msg3682487 date=1274897860
"Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit" :huh
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[/quote]
Not necessarily. The only example I can give is the F4U, where (as I understand it) they're attached separately. In fact, I'm pretty sure they're identical, so the left from one plane could be used to replace a damaged right on another...
Someone else posted pictures/diagrams of this, for the F4U. I think I saved them somewhere. I'll see if I can find them and post them, unless someone beats me to it.
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"There is lift generated by the stabilizer. If symmetrical airfoils contributed nothing to lift they wouldnt and couldnt be used as airfoils."
Symmetrical airfoils do not generate lift at 0 AoA in relation to airflow, only drag, but any change in AoA moves the pressure gradient to lifting side causing lift to be generated. And it is not tailplanes job to create lift, just to direct the lift vector of the main plane.
"Your last questions seems to be saying that the only thing holding a stabilizer on to the airframe is the aircraft skin and nothing could be further from the truth."
"Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit" :huh
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http://rwebs.net/avhistory/history/p-47.htm
Some good explanations of the P-47 empenage assembly. Each torque tube for the elevator had its own bell-crank so theoretically if one was lost, the other would still operate. Might have a wicked shimmy, but it would still be attached. Obviously, each aircraft is going to have a different design, although I would expect that most of the conventional U.S. fighters would look like this one.
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Interesting page Stoney, thx.
It looks like the elevator plane is a solid spar construction and it is attached to fuselage from the center spar but the attachment suggests that losing the other side totally does not affect the other side at all except that I wouldn't try to pull too much G with half configuration..or I don't know, maybe the momentum is just halved? As it is stated the elevator itself is of separate pieces so losing either of them does not affect the other side either in P47, at least as long as the torque tube remains functional.
Ed. If you compare it to that of Zeke it is very much different. http://rwebs.net/avhistory/history/Zeke32.htm.
Comparing these two I'd say that structurally the Zeke suffers less from losing the other side of the elevator completely since the load bearing part is the fuselage but in P47 the through spar bears the full force, so what happens in P47 when you lose the other side of the elevator completely?
After pondering the tailplane matter a bit more it may indeed be true that if the elevator flap is lost, but the elevator plane itself still remains attached it indeed contributes a balancing force against any movement of the remaining elevator, since if the camber of the whole elevator plane does not change due to movement of the elevator flap, the symmetry of the profile always causes a force opposite of the camber of the other side that tries to control the pitch. How much is it depends on the proportions of elevator plane vs. elevator flap and it varies a bit in different planes.
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Symmetrical airfoils do not generate lift at 0 AoA in relation to airflow, only drag, but any change in AoA moves the pressure gradient to lifting side causing lift to be generated. And it is not tailplanes job to create lift, just to direct the lift vector of the main plane.
Now you are saying something altogether different. I wont speak in absolutes but it is generally the case that with symmetrical airfoils that are at zero degrees AOA to the relative wind in non-manuevering flight only (a lot of special cases but this satisfies your specifics) that there is no lift (positive or negative) and that the purpose of this condition is optimum cruise. It is also generally the case that this same situation/condition results in the least induced drag BUT this case also usually occurs at one specific altitude and power configuration. So I would say that stipulating that condition is kind of superfluous.
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"With 1 elevator destroyed your turn performance will decrease, it the corrisponding stab is then destroyed your turn performance will go back up. We thought this was a bug until we brought it up in our force vector displayer and saw that what is happening is with 1 side elevator gone and the stab remaining, the stab is fighting the other elevator so you can not generate the AOA you need. Once that stab is removed, the AOA you can generate is restored."
The basic profile of an elevator is symmetrical so it does not really contribute as lift to any direction and its effect as a limiting force to tail movement sounds strange. It probably is a limiting force but it should be quite small when compared to functioning side of elevator. Of course it depends on the proportions of elevator plane vs. control surface which is different in different planes.
BTW is it possible to lose "other side" of elevator plane to begin with? Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit, but the control surface itself is separated from the other side so you can lose just the other?
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First let us define lift. Lift is any force produced by air or fluid that is 90 degrees to the velocity vector.
Yes the tail produces lift. The elevator changes a symmetric airfoil into an asymmetric airfoil. This change produces lift. This lift rotates the plane and changes AOA on both the tail and the wing.
With only 1 elevator but not stab missing you are only changing the camber on 1/2 the horizontal stab , but you are producing the same AOA on both stabs hence you can not produce enough AOA on the main wing to generate MAX AOA.
On plane where the CG = the main wing CL when turning there is 0 force being generated by the horizontal stab in the turn. I.E. no force up or down so the only thing that is needed from the tail is a change in it's 0 lift angle. This is accomplished by changing the camber of the stab by moving the elevator. The plane then rotates to the horizontal stabs new 0 left angle. This AOA change creates more lift on the primary wing and creates the turn. So how much force the tail generates is pretty much irrelevant, only how much it can change it's 0 AOA.
Now with 50% less elevator as compared to the stab size, it can not produce the same change in zero lift angle. Once the other half of stab is removed it can again generate the same 0 lift AOA change.
I hope that lets you visualize it better.
HiTech
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"... it may indeed be true that if the elevator flap is lost, but the elevator plane itself still remains attached it indeed contributes a balancing force against any movement of the remaining elevator, since if the camber of the whole elevator plane does not change due to movement of the elevator flap, the symmetry of the profile always causes a force opposite of the camber of the other side that tries to control the pitch..."
I was a bit unclear with this but what I meant was only my own realization of what HT tried to say in other thread was actually true: If the camber of the functioning elevator changes and that of the damaged one does not (it cannot, no elevator flap), the damaged one with its symmetrical airflow, now with lifting force produced to wrong side, will resist pitch change that the functioning side tries to achieve acting as a balancing force to any elevator control that tries to change the balance (i.e. 0 AoA) of the remaining symmetrical plane.
"So I would say that stipulating that condition is kind of superfluous."
As a main lifting plane that is true because it would not be airborne at 0 AoA to begin with. But as a contol plane it can be in a neutral state where it does abolutely noting but contributes only drag when a CoG happens to be exactly on the middle of CoL in flight. Any change in elevator flap position will change its camber and thus start to produce lift to either direction. If, for some reason, the CoG happens to be rear of CoL the tail plane needs to produce lift too, to balance the flight i.e. the trim is nose heavy to balance the actual tail heavy flight condition.
Sorry bout the confusion that was on my end but I hope I got it right now...
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Charge: You do.
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When looked at from a force vector point of view would there be any increase in loading across the remaining Stab + elevator when "in use". May such increase approach any structural threshold?
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When looked at from a force vector point of view would there be any increase in loading across the remaining Stab + elevator when "in use". May such increase approach any structural threshold?
The forces would come from the stab. The remaining stab still can produce the same force as it could when the other was still there. So given the same elevator deflection, it will still be producing the same force. So from a simple how much force view, if you could not break it before, you could not when 1/2 was gone.
HiTech
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It would be pretty rare to fly an airplane with the CG coincident with the CL.
And it would not be very much fun.
Flying an airplane with the CG aft of CL is scary. I've done it and it is a lot of work. Never want to do it again.
The vast majority of the time the horizontal stabilizer is providing down force. Loss of half an elevator would not do much to that down force but would reduce the ability to change the lift produced by the horizontal stabilizer
Loss of half the stabilizer surface would equal loss of half the tail down force required. There would be significant up elevator required all other things being equal.
I've been in an airplane (Cessna 402) with a stalled tail plane. Luckily the aircraft was in a load and configuration requiring little tail down force at the time and got the tail un-stalled before the nose went too far vertical. I was at 500' AGL at the time.
There have been several fatal tail stall accidents.
We practice jammed elevator drills in the simulator and the procedure is to disconnect the two elevators from each other allowing one pilot to operate the unjammed half. There is some roll moment produced in the this condition but it is not dramatic.
The chances of losing an elevator are remarkably slim. I would think gunfire would cause more jams/severed cables than controls parting from the aircraft in the tail section. That is one area of the damage model I find lacking in authenticity.
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The ONLY thing that confuses me is if I loose an elevator it is very noticeable. The plane flies drastically different and is greatly impaired in its maneuverability.
But if I loose and elevator AND a v-stab it is only slightly impaired in it's maneuverability.
Someone please explain that to me.
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A whole stab means there is a whole stability and with only half the elevator area it only has half the influence. With only half the stab and half of the elevator the stabilizing effect is decreased but the elevator can still influence the same change to that half.
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Lets see if I understand what you are saying.
With only 1/2 the surfaces I can get close to normal performance.
With only 1/2 and Elev. but a FULL V-Stab those two forces are fighting one another and the plane performance suffers.
So with this logic if I lost 1/2 a V-Stab and still had a full Elev. I would also suffer greatly in performance.
Just seems odd to me that more damage = better performance.
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Even if you had a full elevator and half the stab area (meaning the left or right is missing) only the half elevator that has a stab is actually producing beneficial lift (positive or negative). The half without the stab portion would primarily be producing drag but more likely it would be adding a non-beneficial load on the other elevator.
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I don't think it works that way. You still have only half as much surface deflected into the wind to pitch the tail up or down. Also the CoG would be altered, the plane would fly all squirrelly, and any use of the remaining stab+elevator would include a rolling motion (just like a rudder).
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I don't think it works that way. You still have only half as much surface deflected into the wind to pitch the tail up or down. Also the CoG would be altered, the plane would fly all squirrelly, and any use of the remaining stab+elevator would include a rolling motion (just like a rudder).
Which is what I kinda figured would happen.
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Then what are we in disagreement about? :)
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I don't think it works that way. You still have only half as much surface deflected into the wind to pitch the tail up or down. Also the CoG would be altered, the plane would fly all squirrelly, and any use of the remaining stab+elevator would include a rolling motion (just like a rudder).
The CG wouldn't change that much. Elevators don't weigh much at all, relative to the other parts of the aircraft. Exactly how much "rolling motion" would be created? You say that like you've done the math, but really you're just stating something you think will happen.
What would change is the ability of the elevator/horizontal stab to create the required trim moment about the pitch axis because with half of the horizontal stab and elevator missing, the tail would only be able to generate 1/2 the lift as it would in its original state.
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Also the CoG would be altered, the plane would fly all squirrelly, and any use of the remaining stab+elevator would include a rolling motion (just like a rudder).
The CG would shift slightly forward, though, not back. Planes that are designed to be more maneuverable/less stable generally have their CG further back than planes that are designed to be more stable/less maneuverable. There's a trade-off involved; less stable is more maneuverable. More stable is less maneuverable. So, a slight shift forward could actually make the plane less "squirley".
The required CoG of a plane isn't a precise point, it's more of a "range". Flight testing will reveal the "best" point within that range, but there's some allowable shift forward and backward from that point. Things like cargo, fuel, or passengers, will shift the actual CoG forward or backwards. If the fuel or cargo (ordinance) are moved, or burned, or "dropped", the CoG will change (along with the current wing-loading), if that weight was initially placed anywhere forward or rearward of the CoG. Shifting it forward (as long as the forward limit isn't surpassed) will make the plane more difficult/slower to re-direct (which is also what we generally consider to be more "stable"), shifting the CoG rearward makes the plane easier/quicker to re-direct, until it reaches a point where it's too "squirly" to control (less stable).
That rolling movement you mention is present in AH from what I've seen.
I'm not arguing that a damaged plane should be more stable, just that a slight forward shift of the CoG shouldn't be a deal-breaker.
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The CG would shift slightly forward, though, not back. Planes that are designed to be more maneuverable/less stable generally have their CG further back than planes that are designed to be more stable/less maneuverable. There's a trade-off involved; less stable is more maneuverable. More stable is less maneuverable. So, a slight shift forward could actually make the plane less "squirley".
The required CoG of a plane isn't a precise point, it's more of a "range". Flight testing will reveal the "best" point within that range, but there's some allowable shift forward and backward from that point. Things like cargo, fuel, or passengers, will shift the actual CoG forward or backwards. If the fuel or cargo (ordinance) are moved, or burned, or "dropped", the CoG will change (along with the current wing-loading), if that weight was initially placed anywhere forward or rearward of the CoG. Shifting it forward (as long as the forward limit isn't surpassed) will make the plane more difficult/slower to re-direct (which is also what we generally consider to be more "stable"), shifting the CoG rearward makes the plane easier/quicker to re-direct, until it reaches a point where it's too "squirly" to control (less stable).
That rolling movement you mention is present in AH from what I've seen.
I'm not arguing that a damaged plane should be more stable, just that a slight forward shift of the CoG shouldn't be a deal-breaker.
Just remember that fore and aft CG changes affect pitch stability only. There are two other axes involved. Given that the CG movement forward would be very small (again because empenage control surfaces are typically very light), I'd say it would be difficult to argue that the CG shift would be a stabilizing influence on the plane. The loss of half of the pitch dampening force of the aircraft would decrease stability more.
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Just remember that fore and aft CG changes affect pitch stability only. There are two other axes involved. Given that the CG movement forward would be very small (again because empenage control surfaces are typically very light), I'd say it would be difficult to argue that the CG shift would be a stabilizing influence on the plane. The loss of half of the pitch dampening force of the aircraft would decrease stability more.
Agreed. That's the reason for my last sentence.
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I would like to see film of the rolling motion when a half-stab is missing. I dont like to get shot so it may be a while I get to experience it. :D
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I would like to see film of the rolling motion when a half-stab is missing. I dont like to get shot so it may be a while I get to experience it. :D
Mtnman will have it up soon. :P
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I would like to see film of the rolling motion when a half-stab is missing. I dont like to get shot so it may be a while I get to experience it. :D
LOL, Thanks Spikes! Spikes was nice to enough to head into the DA with me and shoot parts off for me. Nice shootin' too. Popped the elevator off first try, and let me film a bit, and then popped the stab off, and let me film again...
With just the elevator gone (left) the roll tendency is pretty obvious. What I did was level out, and manually trim for level flight, and then use a finger to apply elevator doing my best to avoid any input of aileron. My feet were off of the pedals as well, to eliminate rudder input. This is the effect I'd expect, and have seen several times...
http://www.4shared.com/file/jJD0iSR9/Test_film-_left_elev_shot_off.html
Then, with the left stabilizer removed, things weren't as I expected. First, using only up/down elevator trim, the roll effect wasn't present. Then, using the elevator "normally", there was no rolling effect either. The only big difference was that in my initial application of up elevaotr, i snapped into a spin very quickly/easily. I didn't pull back hard at all, and was surprised to get that result. When I tried again, with a smoother, more subtle application, there were no nasty results, and also no rolling effect. I'm not sure which order these next three films are in, I named them too similarly...
http://www.4shared.com/file/ztu509Sy/Test_film-_L_stab_and_elevator.html
http://www.4shared.com/file/RfgDr61F/Test_film_L_stab_and_elev_shot.html
http://www.4shared.com/file/NMxRPE5L/Test_film-_L_stab_and_elevator.html
Thanks again Spikes!
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Not what I would expect either.
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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
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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!
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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
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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
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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. :)
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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
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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...
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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!
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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:
(http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability/TH26G3.jpg)
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).
(http://www.centennialofflight.gov/essay/Theories_of_Flight/Stability/TH26G4.jpg)
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
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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...
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Well Tango managed to explain clearly what a tenured professor of aerodynamics couldnt get across in laymans terms. Well done sir! :aok
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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
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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
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Thanks guys! I feel like I have a good understanding of what's going on now!
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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!