Aces High Bulletin Board
General Forums => Aircraft and Vehicles => Topic started by: SFRT - Frenchy on February 14, 2003, 08:28:19 PM
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I was teaching ground school to a bunch of private pilots yesterday. While explaining basic aerodynamics, it hit me on the head :
"In Aces High, when you shoot a guy and he looses his tail, the nose is going straight up"
Is it correct? In general aviation aircrafts (made a lil drawing), the CG is ahead of the Center of lift. So the plane has a tendancy to nose over. The vertical stab/ elevator is made to counter this moment by creating a down force.
If you shoot the tail off :
- You don't have this down force anymore, the nose wants to fall to the earth.
- The CG is even more foward, so the nose wants to nose over.
I know that Hitech is putting a lot of pride and work on his FM modelling ... Am I missing something or the FM in Aces High is not yet totaly based on "aerodynamics"? Maybe on Warbirds the CG is behind the Center of Lift, but I doubt it. I think maybe the moder jet fighters have them behind. As far as airliners, their CG is way ahead of their nose. (about 8ft on a 727).
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seems kinda inefficient.
I would think that if the cg was behind the wings and the tail lifting also you could have tail lift and all of your wing lift helping. so less drag for amount of lift = more lift and speed.
where with the cg forward and the tail pushing down you would you would have enough lift on the main wing to lift the weight of the plane plus whatever downward force is created by the tail. not only less lift but more drag for the amount of lift you do get.
why would they make them with the cg forward? visability (get the wing behind you) is the only thing that I can think of, is that it?
I have no idea as to where the cg is on warbirds, as I know little or nothing about aerodynamics other than the 'common sense' basics.
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Frenchy has it right. CG is forward so you can recover from stalls. Or else you would tail slide. Canard designs are nose heavy too, but the front wing is designed to stall before the rear wing, so nose pitches down in stall too.
ra
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Depends on aircraft, definately.
In my years of shooting down Japanese aircraft in CFS2, I have noticed that an A6M2 Zero will nosedive to a quick death in the shark infested water, while the D3A1 Val (which is modeled rather oddly, i think), when the tail is shot off, will aim skyward and fly in endless corkscrews for hours on end until you finally assign a wingman to blow his bellybutton outta the sky, as it "flies" soo erratically it is not in sights long enough to shoot at...
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Well according to this NASA Site (http://www.grc.nasa.gov/WWW/K-12/airplane/trim.html) what Frenchy said is desirable for stability. If this is the case with AH it doesnt make sense why most AC fall nose up, even bombers (which I assume you would want inerently stable).
The thing is we don't really know what happens to the tail when you loose it in AH. Visually it is gone but that is no garuantee that its mass and lift go to zero also.
Btw Frenchy you mean cg is 8ft ahead of center of lift on a 727?
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Normal airplanes with a aft CG will pitch up at slow speeds. When you loose tail in AH the plane pitches up volently and falls to earth nose high. This makes no sense because now the CG should be very very much forward as the loss of tail weight is more pronounced in CG calculation because of the long tail moments.
I think AH has this wrong.
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trim an aircraft in level fight going about 200 mph. you need down trim to keep it level. once the tail is gone it will have to pitch up.
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Check this out. If you shoot the tail off an aircraft, that takes away the lift for the rear section of the fuselauge. Since that section of the aircraft is no longer producing lift, it is like adding extra weight and thus the nose pitches up.
Thats the only explanation I can think of. I think AH has it wrong.
Now check this out- A b17 was flying in formation and falling bombs tore off its horizontal stabilizers. The plane went into a unrecoverable dive and crashed. I have a sequence of pictures of it.
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Originally posted by Furious
trim an aircraft in level fight going about 200 mph. you need down trim to keep it level. once the tail is gone it will have to pitch up.
That doesnt explain why AH planes fall tail first. The engine is by far the heaviest and densest part of the plane - it would pull the nose down if the balancing tail weight was gone.
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Originally posted by GRUNHERZ
Normal airplanes with a aft CG will pitch up at slow speeds.
There are no normal aeroplanes with an aft G of G.
every non-canard aeroplane you'll ever see has a C of G forward of the MAC (Mean Aerodynamic Chord) line.
Check this thread (http://www.hitechcreations.com/forums/showthread.php?threadid=65971&referrerid=710)
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Im saying if the CG is shifted too far aft for some reason.
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This problem is often what I have thought, too.
In many other games, though not all of them can necessarily come within the levels of historic FM realism AH boasts, when the tail/aft section is fatally damaged, it depicts the aircraft going into a rapid, irrecoverable dive.
Come to think of it, in many WWII footages and gun cams I've seen, when a plane has been hit with a long burst, there's this typical smokey explosion with fumes and sometimes fire, and the plane noses down and plunges to the ground. I've never seen the instance like in AH before. (Though admittably I'm not really an adept when it comes to having seen many films or so..)
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In my meeger opinion as a layman in aerophysics :) , I think davidpt40 has got it right.
Due to the characteristic of how damage is delivered in AH, a damage to the aft section seems remove every influence the tail section control surfaces can inflict upon flight, instantly, abruptly, and cleanly. It's like a plane flies by, and then suddenly the aft stab section disappears into nowhere, and with it all the lift and influence it produced..
Frankly, I think it's a bit absurd seeing a plane instantly stop travelling forward(it almost does that..), nose up 90 degrees straight, and decend and crash on it's tail like a helicopter landing..
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Misconception: You can move the cg behind the cp of the main wing and a plane will still be stable. You can not move it behind the total center of lift generated by both the tail and the wing.
Per your diagram both surfaces are producing up forces, but the tail is producing more torque than the main wing, hence it is still stable.
When the HStab is removed in AH all forces from it are removed from the flight model. Hence why they fly nose up do to physics.
HiTech
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Hitech but why does the AH plane keep falling nose high 90 degrees vertical? The engine is by far the greatest concentracion of weight in the plane - seems to me it should fall nose down after any wing/tail aerodynamic forces are negated by the 90 degree attitude of the falling tailess plane.
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Per your diagram both surfaces are producing up forces
I can't see how, it looks like the tail is counteracting the designed nose heaviness by creating negative lift. That is one of the attractions of a canard design, both wings produce lift, unlike a conventional design.
ra
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Originally posted by hitech
Misconception: You can move the cg behind the cp of the main wing and a plane will still be stable. You can not move it behind the total center of lift generated by both the tail and the wing.
Per your diagram both surfaces are producing up forces, but the tail is producing more torque than the main wing, hence it is still stable.
When the HStab is removed in AH all forces from it are removed from the flight model. Hence why they fly nose up do to physics.
HiTech
FWIW this is exactly how the professors taught it to me.
I'm sure some planes have a CG forward of the wing's center of lift. But for best performance you want a CG aft of the wing's center of lift, but forward of the combined center of lift of the wing and the horizontal stabilizer. In Frenchy's picture, this point (the aerodynamic center of aircraft) would be somewhere between the wing and the tail.
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Originally posted by hitech
Per your diagram both surfaces are producing up forces, but the tail is producing more torque than the main wing, hence it is still stable.
When the HStab is removed in AH all forces from it are removed from the flight model. Hence why they fly nose up do to physics.
HiTech
Heuuu ... Hitech ... are you talking about my diagram? When you had your private pilot licence, what book did you used? If by any chance, you used the comon "Private Pilot Manual" by Jeppesen, the light brown one in color this one (http://www.marvgolden.com/books/private_pilot_books.htm) ... you may want to check page 3-29.
I showed your answer to an Aeronautical Engineer this morning, who also happen to own a Cj6 (Chineese Yak52). He said that the second part of your answer didn't really made sense.
Would you mind to clarify it for me please?
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There is an old saying in r/c aircraft: A nose heavy plane flies badly, a tail heavy plane flies once.
The tail actually produces downforce to hold the nose up when flying level. It increases this downforce to pitch the aircraft up, and decreases or even provides lift to force the nose down.
You should try and kick this question over to Mary Shafer at sci.aeronautics, she has a gift for these kind of explanations.
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Originally posted by GRUNHERZ
Hitech but why does the AH plane keep falling nose high 90 degrees vertical? The engine is by far the greatest concentracion of weight in the plane - seems to me it should fall nose down after any wing/tail aerodynamic forces are negated by the 90 degree attitude of the falling tailess plane.
It seems to me that if the prop is still spinning and producing thrust, then once the nose pitches up you basically have a helicopter and this would keep the nose up.
F.
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this whole thread is silly, when your tail is shot off , your going to crash , does it really make a difference if you hit nose first or tail first?
lets get back to something interesting like "how can you put 125% fuel in a airplane" or "why do i get killed with one shot but the enemy won't die after ( insert large number here ) shots"
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Originally posted by john9001
this whole thread is silly, when your tail is shot off , your going to crash , does it really make a difference if you hit nose first or tail first?
If one thing is wrong with damage -> flight model, then it is reasonable to expect that other factors can be wrong as well - with a plane that doesn't go down.
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John, you misunderstand our interest:
No one's suggesting anything's wrong with the FM. As was mentioned in the linked thread, there's enough real life observations (with models) to show AH's FM exhibits the same behaviour as real life.
The point of the question is not to throw doubt on AH's flight modelling, but to ask for some of the gaps in our aerodynamic knowledge to be filled.
I know aircraft have a forward centre of gravity; it was my job to balance all DHL's aircraft traveling through scandanavia for some years; and most of 'em made it.
I know an aircraft that loses it's tail flutters nose up down to the ground, because I've seen enough models do precisely that.
I don't know why that should be, because with my limited understanding of the physics, they should go down nose first, and that's what I'm interested in understanding.
I've a clip of a Blenheim lose it's tail after a collision at an air show: It goes straight up, then sinks nose high into the ground, along with (presumably) with four men screaming their last. Why didn't it plough nose first into the ground? That's what I'm trying to understand.
<edit >
I've just checked the clip again, It's Beauforts, an MPEG of 2,355 Mb if any one wants it, and it looks exactly like AH.
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sorry , i forgot to put a :) in my post
:D
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Originally posted by GRUNHERZ
That doesnt explain why AH planes fall tail first. The engine is by far the heaviest and densest part of the plane - it would pull the nose down if the balancing tail weight was gone.
in a non power off situation yes the nose should fall if tail is gone, but in a power on , the nose is trying to go up and the tail has to counter that or u would climb all the time.
granted it shouldnt fall from 30k tail down the whole flight,
but i have seen RL guncam footage of a LW fighter losing its
tail and go nose up.
whels
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Yep whels, my whole issue was after the plane stopped flying and started falling vertically it doesnt make sense that it would continue descending nose up regardless of prop blast.
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GRUNHERZ what about the sideways stalls of some planes like P51 and Ta152? Or 109 and spit5, easy to induce?
P51 and Ta152 both also have flat stalls, 51 does it inverted at least, spins sometimes, stays that way to the ground.
152 can do a tail centered backwards >=45deg spin to the ground too.
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I'm not sure what you are talking about moot.
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moot now your into a hole new area, btw center of lift is not a constant on an airfoil, the cp moves with changes in AOA and can change drasticly with post stall AOA's. Also the CP/CG relationship is not the only force movinging the nose, CM also can have a large effect at post stall AOA's.
On the orignal debate I assure everone the physics are correct, all your realy disscussing is to move the CG more forward on the plane, and it would then be nose down with the VStab gone.
Think of the p51 in which everyone has heard the story of instablity with the aft gas tank full. Note the plane still flew, therefore it didn't have the cg behind the "Planes" Center of lift but they were very close. Now when you remove the tail the CP of the entire plane will move forward. Hence nose up.
It's much easyer to think of the Center of lift of "Plane" i.e. both wing and tail combined when trying to understand stablility.
And frenchy, most pilots realy don't know much about the physics of flight. I have had length disccussions with multiple CFI's and most just have a very basic understanding and would draw the same conclusion from your diagram.
HiTech
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Frenchy I think this is what Hitech meant when he made the reference to your diagram.
a.c. = aerodynamic center
As long as the c.g. is forward of the airplane's a.c., the airplane is longitudinally stable.
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You could put the c.g. ahead of the wing's a.c. and the plane would be plenty stable, but the wing would be fighting the tail. The wing would have to create extra lift to make up for the downforce from the tail. This would in create more drag and lower the maximum g that the airplane could achieve. So for high performance aircraft you will always find the c.g. behind the wing's a.c. but ahead of the airplane's a.c.
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I knew squat about planes before AH.
Thanks for the explanation HT.
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But hitech why do AH planes keep falling nose up?
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because there's no lift to keep it flat once horstab is off?
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i thought i had a pretty good understanding of this stuff
but now i'm more confused then ever
and frenchy - i have that book :)
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Is there a point on using CPU power to compute the FM of a wreck ?
Is AH still doing any computation on such a "part-less" plane or does the AH's engine just discard it ?
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Originally posted by GRUNHERZ
But hitech why do AH planes keep falling nose up?
because as the nose comes down the wing begins to generate lift? so depending on the new cg the AC either cartwheels with an upward nose moment or "nods" down with the nose mostly up? (assuming wings are in perfect working order)
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Originally posted by Seeker
I've a clip of a Blenheim lose it's tail after a collision at an air show: It goes straight up, then sinks nose high into the ground, along with (presumably) with four men screaming their last. Why didn't it plough nose first into the ground? That's what I'm trying to understand.
<edit >
I've just checked the clip again, It's Beauforts, an MPEG of 2,355 Mb if any one wants it, and it looks exactly like AH.
Hi Seeker,
Can you email me a copy of that MPEG please?
Also, the behaviour you are asking about, appears to be perfectly correct to me, and is exactly what I would expect from the excellent flight model in AcesHigh. I think the confusion in this discussion has been caused by some misunderstanding of the system of forces shown in the stability diagram, and in trying to apply that to explain what happens when the tail has gone away.
Just bear with me while I build an image of what happens… There you are flying along, minding your own business, in level flight at constant speed. At this point the stability diagram shown in this thread is doing us proud! All the forces on your aircraft are balanced. The prop’ thrust is balanced with drag, the lift with weight and so on. All the moments are balanced too, the pitching yawing and rolling moments are all in equilibrium. The nose down pitching moment caused by the weight and lift couple is balanced at the tail, and so on.
Now, since the question only concerns what direction the nose points when the tail goes away, let’s just think about those pitching moments. Most folk think like this… During flight the weight and lift couple was trying to rotate the nose downwards, and the tail was preventing this from happening. So, if we remove the tail, there will be nothing to prevent that rotation, and the nose will drop… Not so fast! That’s not what happens, you might not see this right away, so bear with me, I’m going to go slowly… But first, the flaw in that reasoning is that it overlooks the fact that the nose down pitching moment that existed during controlled flight, also goes away with the tail. You see, the tail wasn’t only responsible for the balancing moment, it was also indirectly responsible for the lift that was produced the nose down pitching moment in the first place. Once the tail has gone, that stability diagram no longer applies… So what does happen?
Firstly, the wings are only producing lift when they are forced to do so by the control surfaces at the tail. Those surfaces (using a small force but long lever arm) rotate the wings against the airflow, forcing the wings to fly at an angle to the free air stream, thereby causing downwash, and thus lift. When the elevators go away with the tail, the wings will begin to move upwards, due to the lift already there, but they won’t go far because as they move the lift decays rapidly until the wings weather vane, and no longer produce any lift. That all happens in just a few degrees, so when the tail goes away, the nose might move down slightly, but only momentarily, because now that only leaves an engine, with the wings and forward/mid fuselage acting as little more than dead weight that simply wants to fall downwards, with a propeller attached to it that is still producing thrust.
Now, all you really need to consider at this point is how a heavy lump of metal with a propeller attached to it would fall. I think most people can see intuitively, that the heavy lump would fall first, dragging the propeller behind it. An admittedly weak analogy would be the stable condition that arises with a man hanging beneath a parachute. The aircraft falls, dragging the prop behind it, and falls more slowly because the prop is producing thrust and slowing it down.
That's exactly what happens in AcesHigh... Kudos HT!
Hope that helps.
Badboy
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There is a really easy way to see what happens when the horizontal stabilizor is removed from an airplane in flight - purchase a balsa wood glider and become a flight test engineer :D
MiG
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funked's diagram is exactly the way ah is set up for most planes.
HiTech
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Originally posted by funkedup
Frenchy I think this is what Hitech meant when he made the reference to your diagram.
a.c. = aerodynamic center
As long as the c.g. is forward of the airplane's a.c., the airplane is longitudinally stable.
Stretching back to my aerodynamics courses in college - Lifting bodies use the of concept of aerodynamic center in lieu of a conventional aircraft's center of pressure. It also works for aircraft that are longitudinally unstable, like the F-16. None of the aircraft in AH should be modeled with the horizontal stabilizer providing lift in stabilized straight and level flight. The aircraft's total CP has to be in front of the CG in an aircraft with a conventional tail to be capable of longitudinally stable flight.
Anyone that doubts that the horizontal tail's "lifting" force acts downward needs to go out and fly a C-182. (Remember, the elevator goes up to force the tail down.) You'll get a real appreciation for just how much downward force is required when you flare for landing. The yoke is at its aft limit of travel at touchdown.
MiG
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* Layman's questions! :) *
That's a very good explanation Badboy! I think I can at least vaguely get a picture of what you are talking about..
But I'm still a bit confused.
First, umm.. can it not be considered that the case you explained is sort of like a 'purely theoretical' one? I mean, if you say it happens that way, no doubt you're right then. :)
What I mean is, in the case you describe, wouldn't it be a case when a plane is flying around, and then suddenly the H-stabs just disappears into thin air? How would it compare to real life, when an attacking plane shoots from behind, and gradually the H-stab is tattered, pieces disappearing, falling off, and then finally breaking off? Would it be the same in behavior, as like in AH the H-stabs and all its forces gets removed in an instant?
.. and ummm.. as I've said, I've not seen too many clips and footages, but I've still seen quote some cases where the tail end gets very badly damaged on the target plane. Seeker says his footage is like in AH, but I've never seen such thing happen.
Would this "nose up, tail down, fall vertical like a helicopter" sort of behavior be a rare case in real life? :confused: Or was it the general case of planes with their aft sections badly damaged? Also, what would be the difference in behavior between a 'badly damaged' aft section and the aft section/tail/stabilizers being totally removed?
.. and third question.. as in some cases I've seen, when the stabilizers and tails get battered into rags, and the plane begins to dive and auger, what kind of damage would cause this behavior? Or, is that kind of behavior due to the pilot bailing out and giving up control of the plane, not relevant with the forces working on the plane?? :confused:
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Frenchy, there CG is supposed to be ahead of the Aerodynamic Center of the aircraft(the point around which the moments caused by lift stay constant even when the AOA is changing) for the machine to be stable.
The aerodynamic center is calculated taking into account the effects of the tail as well. Also, the aerodynamic center is not the same point as the center of lift of the wing. Depending on loading and flying conditions, the CG and the center of lift move around (for instance if flaps are lowered or not), the tail is used to create a moment either downward or upward.
Therefore, I think it is possible for planes to go up when they lose their tail. Not all of them and not always, but SOME would certainly exhibit this tendency.
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hmm...I guess funkedup's picture explains it better :)
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Perfect, Badboy,
I think I'm getting it now :-)
The film clip's on it's way to your blue yonder address.
I *think* it's from a post war airshow in New zealand; and it shows a sight familiar to any one who flies AH:
Two planes touch, and one loses it's tail. The tailless plane immeadiatly rears nose up until it's speed decays, then flutters about, ultimately smacking the water nose first; followed very quickly by the second Beau.
Very tragic.
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Just get a balsa glider and test the theory by flying it normally, then remove the tail and fly it.
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AFAIK, the profile of the horizontal stab is a lift generator one. If you remove this, the lack of lift, if everything remains the same, the tail will fall.
Just an uneducated €0.02
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The biggest problem I see in this discussion, is the question "what means shooting of the tail?".
If it means, shooting a lot of holes into the elevator disabling the lift, then the nose will go up, as it is modelled.
If it means, cutting away half the fuselage , the center of gracity will move forward drastically (wherever the lift may be at the moment)and the nose will go down.
It simply depends on how much mass is lost on the tail.
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Bump for Fester.
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Bump For Pitch Up question
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Look at it this way lets take a full loaded Me-109G and balance it on the head of a pin so its level this is the center of gravity. This could even be the aft CG limit. Then put the landing gear down and the CG moves forward and the nose points down. Now cut the enpennage off about about 4 ft. ahead of the vertical stabilizer now what happens the plane noses way over and way out of balance. Now how does this happen?
I like the helicopter bit were the engines pointing straight up and hovering there. LOL
So how did the main wing pitch up anyhow? By the horizontal stabilizer creating tail down force pitching the nose up in a high angle of attack and so you shoot the tail off completly, about a foot behind the canopy and now what happens to the nose, it will pitch over in a hard way nose down.
The horizontal stabilizer is an up side down airfoil creating a down ward force or lift, it also puts the main wing into different angles of attack. To change the angle of attack then change the elevator position up or down which decrease or increases lift on the horizontal stabilizer.
The prop wash also impacks on the leading edge of the main wing and also pitches the nose down.
Read for real pilots in A & V theres good sights and good reading.
Frenchy dont back down your right.
And the same for you to Grunherz, why does the nose point up, has it turned into an AH Helicopter, but were is the tail rotor? It wouldnt supprise me.
Straiga
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Was browsing and found this:
(http://www.badz.pwp.blueyonder.co.uk/images/Stability2.jpg)
This extract shows an example of stability (achieved with longitudinal dihedral) with CG aft of lift, and in this case there can be no doubt that the nose would pitch up if the tail went away.
Badboy
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Thanks for the find badboy
HiTech
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Originally posted by NUKE
Just get a balsa glider and test the theory by flying it normally, then remove the tail and fly it.
damn Nuke!
I was scrolling down to suggest that very thing!:aok
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Just looking it over and learning myself. Straigia does seem correct on this one.
Look at the force of lift down the tail is providing.
The fuselage connecting them acts as a lever and the CG a fulcrum. The tail pushes down keeping the wing from pitching forward and reducing the angle of attack.
Remove that downward force created by the tail and the wing will pitch up with the nose coming forward.
What does the value "M" represent in this diagram?
Also the CG is in a different location for level flight, Correct? So won't it shift when the downward lift of the tail is radically removed along with the weight of the tail? I would think it would have to shift forward to compensate and violently pitch the nose forward.
Crumpp
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Originally posted by Crumpp
What does the value "M" represent in this diagram?
The M = Moment, which is similar to a torque, in as much as it describes a turning effect, or a force that causes rotation only. It provides a quantitative description of the tendency of the wing to want to rotate and the units are the same as torque either newton.metres or pounds.foot.
Originally posted by Crumpp
Also the CG is in a different location for level flight, Correct? So won't it shift when the downward lift of the tail is radically removed along with the weight of the tail? I would think it would have to shift forward to compensate and violently pitch the nose forward.
Yes, if the tail comes off the CG would move forward, however, the main argument for the aircraft pitching nose down has been that the centre of gravity was in front of the aerodynamic centre producing a nose down couple, in this diagram there is stable and balanced flight with a nose up couple, so in this example, if the tail is removed the nose must initially pitch up. Once the nose pitches up, what’s left of the aircraft isn’t flying anymore, and stability considerations no longer apply. Now, all you really need to consider at this point is how a heavy lump of metal with a propeller attached to it would fall. I think most people can see intuitively, that the heavy lump would fall first, dragging the propeller behind it.
Hope that helps…
Badboy
PS
Seeker had some video footage of a bomber losing its tail due to a collision at an air show, it showed the aircraft lose its tail and go nose up initially, eventually going into a dive. I’ve tried to find it without any luck :(
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Noob's point of view: when you get your tail shot off your plane briefly turnes into a helicopter :lol
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Yes, if the tail comes off the CG would move forward,
Thanks Badboy for the explaination. Then that opens some more questions. If the CG moves forward in proportion to the speed at which the force shifts from the tail, won't it it end up ahead of the Center of lift pitching the nose down as the bomber does in the crash video?
Certainly an engine and prop in freefall will fall with the prop up. Will an engine with half a fuselage and a wing attached do the same?
I thought M stood for momentum. That also adds to the forces pushing the nose down correct?
Crumpp
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Crump CG isnt effected by any force. It is just the center of gravity i.e. where the plane always balances at. In some ways you can think of it as a fulcrum.
HiTech
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Originally posted by Crumpp
Thanks Badboy for the explaination. Then that opens some more questions. If the CG moves forward in proportion to the speed at which the force shifts from the tail, won't it it end up ahead of the Center of lift pitching the nose down as the bomber does in the crash video?
Yes that bomber did end up nose down eventually, and it didn’t pitch up all the way either, but it was also a very heavy bomber with a large moment of inertia, (which is a measure of an objects resistance to rotation) which means it wouldn’t have rotated quickly longitudinally anyway.
Also, in the situation shown in that diagram, I’m speculating that if the tail comes off of a fighter with that configurartion, there would be an initial pitch up, possibly to a large degree, after that, the CG would move forward (because the mass and distribution of mass has now changed and that changes the position of the centre of gravity, forward in this case) but it would be too late already, by that time the aircraft would be nothing more than a lump of falling debris with a propeller attached to it.
Originally posted by Crumpp
Certainly an engine and prop in freefall will fall with the prop up. Will an engine with half a fuselage and a wing attached do the same?
Why not? They are both just ballistic at that point, a wing is just a lump of metal without the elevator to hold it at an angle to the airflow in order to create lift.
Originally posted by Crumpp
I thought M stood for momentum. That also adds to the forces pushing the nose down correct?
Not in this case, the M does stand for a Moment. Momentum is different, it involves the product of mass and speed and if the nose were already rotating up or down at the time the tail came off, momentum would tend to make it continue in whichever direction it was going at the time, but I don’t think that would be enough to change anything, other than perhaps just the rate at which it happened. So, some rotational momentum might speed up, or slow down what ever nose pitching tendency was going to happen, but I don’t think it would be strong enough to change it.
Hope that helps…
Badboy
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Also, in the situation shown in that diagram, I’m speculating that if the tail comes off of a fighter with that configurartion, there would be an initial pitch up, possibly to a large degree, after that, the CG would move forward (because the mass and distribution of mass has now changed and that changes the position of the centre of gravity, forward in this case) but it would be too late already, by that time the aircraft would be nothing more than a lump of falling debris with a propeller attached to it.
Thanks again for the reply, Badboy. Very informative.
Why in a fighter would it be slower?
Seems to me it would be faster. Less mass means less resistance to change. It is already moving that way because the CG is shifting correct?
Thanks Hitech for your reply as well. If the CG can be thought of as a fulcrum will it not move instantly to balance? Since it can no longer balance without the tail won't it shift forward?
Why not? They are both just ballistic at that point, a wing is just a lump of metal without the elevator to hold it at an angle to the airflow in order to create lift.
Yes but the wing is still a large hunk of drag rather like a sail or a streamer, right?
When the CG shifts foward the largest amount of drag would fall last. The prop is still trying to pull the engine, correct? Thrust out front and drag in the rear?
A good illustration is your legs in freefall. If you invert and fall feet first you must minimize the drag of your legs by putting them together and pointing your toes to the earth. Have them apart or your toes not pointed to the earth and you will flip. Falling head first to the earth. It is tough to master but necessary if you want to skysurf.
Does that make sense?
Crumpp
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Originally posted by Crumpp
Thanks again for the reply, Badboy. Very informative.
Why in a fighter would it be slower?
It wouldn’t, and I don’t think I said it would?
Originally posted by Crumpp
Seems to me it would be faster. Less mass means less resistance to change. It is already moving that way because the CG is shifting correct?
I agree with the first part of what you say here, but not sure about the part where the CG is shifting, because it doesn’t really shift, as such. When you hear that expression it normally implies something else is doing the shifting, like the cargo, the passengers, or the fuel. The CG itself is just an imaginary point, as HiTech said, the point the plane balances at. So when the tail is there the CG is in one place, The moment it goes away it is somewhere else, but its original location is what causes the original motion, after that, as I said it is just a lump of metal falling.
Originally posted by Crumpp
Yes but the wing is still a large hunk of drag rather like a sail or a streamer, right?
When the CG shifts foward the largest amount of drag would fall last. The prop is still trying to pull the engine, correct? Thrust out front and drag in the rear?
I think a lump of metal with a propeller attached, will fall in such a way that it drags the propeller behind it, just like a parachute. Same thing happens with small seeds that spin to the ground like helicopters with the heavy lump underneath, like the one below. Remember, the propeller is still producing thrust, and that's creating a lot more resistance to motion than the drag of the wings and fuselage, so it should fall last.
(http://www.badz.pwp.blueyonder.co.uk/images/Seed.jpg)
Just seems natural to me.
Hope that helps…
Badboy
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http://www.onlineaviation.org/My%20Documents/weight.htm
Crump CG isnt effected by any force. It is just the center of gravity i.e. where the plane always balances at. In some ways you can think of it as a fulcrum.
IM LMAO, Well some force just shot the tail away, that CG location is now going the move. The new lever is now the weight of the engine and the fulcrum that is back by the CG has no leverage from the opposit side, to balance. The CG now moves forward. So quess what the nose pitches forward. Due to good ole plain gravity. The prop is now going to take the rest of the airframe straight down to the seen of the accident. Read the ME-109 balancing on a pin in my post.
The prop cant hold the weight of the airplane in the air the main wing only can in level flight. The main wing has to produce more lift then total weight of the airplane.
Please dont explain MACH TUCK I wouldnt want you to make my point for me. Same goes for Tail Stalls due to Icing.
Im still laughing.
Its ok to admit that your wrong some may laugh but its still ok.
Crumpp you have a good understanding of the concept.
Straiga
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I think a lump of metal with a propeller attached, will fall in such a way that it drags the propeller behind it, just like a parachute.
Wholeheartily agree, Badboy.
But we are not talking just the engine/prop in freefall.
I think though with the rest of the aircraft minus tail attached, Straiga is correct.
I don't want to get in the middle of a "engineer" fight because I have the "weakest" resume at the table between Badboy, Straiga, and Hitech. I know Hitech threw out the insult about your teaching ability Straiga which was surprising. I have found him to be more than willing to help or answer questions. He is a nice guy and I would suggest giving them a call.
That said looking at the science and Badboy's bomber crash, it makes perfect sense what Straiga is saying.
Crumpp
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That is suprising. Thanks Hitech
But did I ever deserve the wise remark to begin with.
Im just trying to express a point of view with alot of practical experience to back it up. I cant even tell you how many different airplanes I have flown in over 30 years of flying. From a 1000 lbs to 550,000 lbs. of aluminum. Is there anybody else with the same experince lets talk.
But if you bark at me I will bark back its kinda fun.
Some people probably think Im not telling the truth about my flying experience or think I dont have a single license in my pocket. Talk to JB 11 or I think he goes under Pokesz now, he has see them. I dont think his really believed how my big my house was either until he saw it, I told him it was a 3 story 6400 sq ft. Im trying to help him get his rotorcraft rating.
Plus I dont have to prove anything to anybody, I could care less what anybody thinks frankly. Why do I have to blow sunshine up anybodies you know what, to be cool or something. Give me a brake I have better things to do. But if some times you want to read what I have to say you might learn something. If it sounds like Im mad about this, Im not at all, far from it.
Its takes me about a second in a day to pay for flying in AHII for a month. I enjoy flying in AHII I also enjoy the friend ships and some team work. Its like playing chess, what is the other guy going to do. Some guys really hate me in a tank fight, which is good they keep dying from frustration.
You know you can read a book riding a bike, but its different when you ride one. Also what you read out of a book is not always the way it is either.
Straiga
Straiga
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Okeedokee. You have a fat kid and a skinny kid on a teeter totter. The fulcrum is a literal CG and a metaphorical CL (supporting the sytem) which are co-located. This of course has to be closer to the fat kid for the system to balance. The fat kid is the engine and prop and the skinny kid is the empennage. The skinny kid falls off the teeter totter (or part the the skinny kid...the horizontal stab). According to what I think I'm hearing is when the skinny kid falls off the fat kid goes up. The prop doesn't have enough area to act as a parachute and if it producing thrust is insensible of which way is up unless informed by the CG relative to the CL of the system. When you are considering the plummeting carcass of an erstwhile single engine aircraft what you really have is a light aluminum structure with most the heavy stuff at one end.
Also although the CG shift is undoubtedly a factor I don't consider it to be the primary factor in the pitch down phenomena. The primary factors are the negative pitching moment of the foil and the loss of the downforce on the tail which is the condition that most airplanes operate in most of the time.
That was an interesting article that bad boy posted but I'm just unaware of any aircraft that carry the tail around with certain exceptions. F-16s for instance run the CG way back and that oddball NASA plane with the swept forward wings (X29 or something?). These planes are flown by computers however.
The article has the CG behind the aerodynamic center but by the look of the tail it is still placed ahead of the Cl (the tail looks configured to lift down).
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Casca you crack me up. Sometimes you have to put it simple terms so they can understand it. WTG
Straiga
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Im just trying to express a point of view with alot of practical experience to back it up. I cant even tell you how many different airplanes I have flown in over 30 years of flying. From a 1000 lbs to 550,000 lbs. of aluminum. Is there anybody else with the same experince lets talk
Straiga: What erks me is your assumption that flying experince equates to knowledge of physics.
I constantly see you make inncorect statements like the torque on moving engines off center line, and then claming your pilot experience makes you an expert. Your expertise is flying and managing an airplane. That dosn't equate to detailed knowledge of what makes a plane fly.
IM LMAO, Well some force just shot the tail away, that CG location is now going the move.
The CG would shift do to a change in total mass locations nothing to do with forces.
That was an interesting article that bad boy posted but I'm just unaware of any aircraft that carry the tail around with certain exceptions. F-16s for instance run the CG way back and that oddball NASA plane with the swept forward wings (X29 or something?). These planes are flown by computers however.
Cacsa: The confusion I belive normaly come from a definition of CL on an airplane. Is that CL the wing CL, or the CL of the horizontal stab and all other components of the plane combined.
I belive it also is normaly described that way because it is much easyier to describe in basic flight training. Hence why most people think most tail planes are normaly creating a downward force.
For a plane to be stable the combined CL must be behind the CG, but that is an entirely different statment than the Wings CL must be behind the CG. Of the planes we have found with documentation on tail forces in flight. They have produce up forces when in level flight. The resone is that producing down force requires the primary wing to produce more up force to maintian level flight, hence more drag. When you are designing a fast fighter would that be the normal condition you would wish to set it up in?
HiTech
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Straiga you are not reasoning with a dynamic model.
The shift of cg can be not enought to pull the nose down, it depend off the sum of all the forces involved.
I won't post more as I've trouble translating from French :)
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Hmmmm....
As anyone who's ever lost he tail on an R/C plane will attest....
The sucker comes down nose high:( :mad: :confused:
Now thats as close to real life as I want to get...
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Originally posted by hitech
Casca: The confusion I belive normaly come from a definition of CL on an airplane. Is that CL the wing CL, or the CL of the horizontal stab and all other components of the plane combined.
I belive it also is normaly described that way because it is much easyier to describe in basic flight training. Hence why most people think most tail planes are normaly creating a downward force.
For a plane to be stable the combined CL must be behind the CG, but that is an entirely different statment than the Wings CL must be behind the CG. Of the planes we have found with documentation on tail forces in flight. They have produce up forces when in level flight. The resone is that producing down force requires the primary wing to produce more up force to maintian level flight, hence more drag. When you are designing a fast fighter would that be the normal condition you would wish to set it up in?
HiTech
Apologies in advance for what I anticipate will be a lengthly post. I think I see what you are driving at and if you have information that edifies me I am, of course, eager to learn. I would certainly be interested in seeing a case or two of aircraft that are set up intentionally tail heavy.
To answer the question of the normal condition I would wish to set a fighter up in I would answer, in general terms, that I would want the CG to coincide with the CL at the rearmost allowable loading of the load schedule. This would provide the highest speed, the lowest stall speed, the best fuel economy, probably something approaching neutral static stability (but thats a big enough subject on its own, lets don't go there) and would have a salutory effect on controlability and manuverability (two other big subjects, so let's ignore them for now).
As for considering the Cl of the system instead of the Cl of the foil in isolation you make a valid point. That being said the CG of every weight and balance schedule since Orville and Wilber are referenced with respect to LEMAC (leading edge of the mean aerodynamic chord) of the airfoil. 20% to 30% are average ballpark figures for most plain vanialla airfoils. Now there is every possibility that the aerodynamisicts are dumbing it down so that the mechanic reading the scales can grasp it. What you say may be entirely true, it's just that I have never seen it.
I looked for some weight and balance documents online but they were pretty thin. I was able to come up with a Technical Order W&B schedule for a P-47 which sets the CG limit at 25% to 32% MAC. This does not appear to be a tail heavy aircraft at first glance.
Among the documents I have laying around the office is the Fifth Edition Textbook of the Transportation Safety Institute run by the Federal Aviation Administration. This is the premier aviation safety and accident investigation training organization in the world (apologies to Cranfield in Jolly old England, that's just the way it is). All NTSB investigators go through this school and the majority of investigators for almost any major aviation regulatory agency on the globe find themselves there at one time or another. Many academic luminaries have been and are currently associated with this organization. Plug: My university, CMSU, is offering our Master of Science in Aviation Safety in collaboration with them via distance learning so if anyone has a yen to kick tin, this is your chance. :)
I'm including a paragraph or two and a couple of illustrations. If I turn out to be wrong about all of this I will cheerfully admit it and notify TSI to change their text.
To give the quote some context, what is being taught in this portion of the text is to use wreckage distribution and failure modes to determine accident causes.
"To be able to analyze an inflight breakup, it first must be clearly understood how an aircraft is aerodynamically loaded in its normal configuration. Reference is made to Fig. CIII-29. It must be remembered that an aircraft rotates around the center of gravity, and the center of gravity is located in close proximity to the quarter chord point of the mean aerodynamic chord (that would be 25% LEMAC mentioned above). The center of gravity in this illustration may be assumed to be in the center of the aircraft and in the area of the spar outline. The arrow under the engine represents the weight of all items ahead of the CG and the arrow under the aft fuselage represents the weight of all items aft of the CG. The sum of the moments of these two forces around the CG, in addition to the wing pitching moment (my italics, it seems to keep getting lost in the shuffle), if it is a cambered airfoil, will result in a nosedown pitch. This resultant nosedown pitch is prevented by a download on the tail. This download then brings the pitching moment to zero, and my be considered as that balancing force which places the aircraft in a state of equilibrium as far as pitching moments are concerned."
The following illustration is on page 181 of the text and shows the condition that we have been discussing.
No more breakfast forever Part 1
(http://www.pcspray.com/pics/mooney1.jpg)
This illustration is on page 182 of the text.
Headed for Page Two on Paul Harvey
(http://www.pcspray.com/pics/mooney2.jpg)
This is of course the typical pitch down and breakup sequence (wings failing down if we are going fast enough).
Wish they had used a Piper instead of a Mooney. I used to have a Mooney, great ship. Someone in the game said you had an RV-8. I'm planning on building one at some point (any opinions on the Eggenfellner setup? I'm thinkin we are gonna lose 100LL at some point so the angle valve IO360 might become problematic). Probably belongs in a different thread.
Here at the school we have a V Tail Bonanza in pieces at the airport. We lay it out twice a year for the final in the Accident Investigation class. It belonged to a mortician who was picking up a body with two surviving family members. On the way back he lost it in IMC and wound up in a spiral dive. Upon popping out the the clouds and seeing the ground he hauled back on the yoke hard enough to cause one of the stabilizers and ruddervators to come off of the airplane. With the remaining ruddervator he managed to stay up about ten minutes after which it also left the airplane. It, no surprise here, pitched down and the wings failed in the typical downward direction. This aircraft was right at or out of rear CG limits (the body was in a coffin projecting into the tail cone).
I think the above fairly summarizes how I've arrived at my conclusions although they are not the only sources. If you can show me where I'm off track I'm all ears.
Casca
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Anybody want to host the Beaufort crash clip for linking here?
2.3 Mb.
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Deleted for flame.
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Casca:
http://naca.larc.nasa.gov/reports/1947/naca-tn-1483/naca-tn-1483.pdf
Check out page 19 shows tail loads at various conditions and CG's
Note: Up = a positive load.
HiTech
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Interesting picture Casca posts of a Mooney cracking up - the irony though Mooney's have the best in flight breakup record of any plane in history. Atleast they could've used a Bonanza.
Wolfala
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Thanks for the link appreciate the reply. I think we are going to have to agree to disagree on this one. What the chart on page 19 shows me (and feel free to correct me if I have mis-interpeted this) is that it's possible to get the horizontal to see a positive (up) load if we are mushing around lower power settings and/or higher AoA. No argument from me there. What the graph on page 47 (n = load factor) makes abundantly clear to me is that at load factor of 1 and 29.7% MAC CG (which is admittedly not the most rearward CG condition) the tail is operating with a negative (down) load at any speed in excess of about 75 mph power on and about 82 mph power off.
That is a very interesting document by the way.
Thanks
Casca
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Actualy it sounds like we now agree Casca. Just understand most planes in AH are setup with a fairly aft CG and that when the tail is removed no change in mass takes place.
Hence why they go nose up, and nothing is wrong with the physics.
My basic agument is always about how a plane can still be stable with CG behind the main wings CL.
One question how do pick off the speed from that document? I saw the CL and density, didn't convert it to speed.
One other thing that suprised me is the positive tail load in the 3g loads.
HiTech
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I wasn't going by the speed but by the engine hp power settings listed on the left. Yeah, I was suprised by the positive loadings at the higher Gs. I think at 8 G it went to 400 mph or something before neutral load (that chart was a few pages later I think).
Seriously on the RV-8 thing.
1. Do you have one?
2. Are you familiar with the Eggenfellner Subaru conversion?
3. What are you running on yours? (if in fact you have one).
If you don't want to respond here why don't we start another thread?
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Hitech - even a better question is perhaps more of a statement.
Most civilian aircraft are built with a positive stability design (the airframes are designed to bring the aircraft into level/horizantal flight if deflection is removed on the controls) vs WWII fighters are stability neutral (aircraft are designed to maintain position from last movement on the controls).
Frenchy: that being said, it makes perfect sense for a fighter aircraft, with a neutral stability design, to go belly up if your tail is shot off because it's primarily the tail that gives our birds that ability.
Couple of points:
1. I've had my tail knocked off a few times. Depending on my speed, AoA, and likewise, I never knew it was off until my elevations controls were touched, especially if my throttle was idle. I would eventually slide left or right and when speed was low, I would THEN go belly up as the tail was no longer providing resistance to air.
2. Shooting off an aircrafts tail has a final result. Engine torque - will cause the aircraft to yaw-left or right sharply. Fuselage by design is an airfoil - upwards pressure. Wing lift CoG will also cause the tail to drop.
3. Horizontal stability is removed from the tail - and seeing how the tails are all tubular in design, they have little or no resistance to gravity and air pressure where as the front engine and wings are causing your aircraft nose to rise. That will cause the nose of the aircraft to sharply nose up.
A lot of other factors could impact a 'nose up - missing tail'. Like I've said before, I've had a tail shot off many a times and not know it until 10 seconds later when I saw 'sky-ground-sky-ground-sky-ground'. :D
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In this test you can see the determination of the pitching-moment coefficient for the airplane with less tail, shows CM to be -5.2 power on and -4.72 power off (per pounds per sq.ft.). This is the pitch down moment of the airplane with out a tail. At -5.2 the nose pitches down with power on and less at -4.72 with power off. This means that the main wing produces a down force with out the tail.
Cascas, drawing also shows in a convential airplane when the tail feathers leave the airframe the tail will rise because of no more down force. The main wing is producing a downward force also pitching down. The enpennage is the leverage, oposite the engine and the main wing. It also changes the angle of attack either positive or negative on the main wing.
If you cut the airplane down the middle how much will the engine, prop , forward fuselage and the front part of the wing with the fuel inside wiegh?
The main wing produces a downward force also.
Now how much will the airlerons,flaps, rear fuselage with enpennage weigh? Less than the nose section?
The enpennage also produces a downward force.
Try any wieght and balance form and from the forward datum to the rearward datum add up the wieght difference between the two halfs.
Wind tunnel test shows -0.038 Power off and -0.501 power on.
QUOTE: Slipping to the right or left is found to cause an increase in tail down load.
QUOTE: At the start of the manuever the surface results in a down load on the elevator(up elevator), which induces a slight initial download on the stabilizer. The pitching up of the airplane resulting from this down load cause an increase in angle of attack of the tail (inverted airfoil) which in turns reduces the initial downward load caused by the elevator. The inverted tail plane can be seen in Badboys post at the top of the page.
This test is basicly a test to prove how the force of either pushing forward or pulling back on the elevator, and to see how much force the horinzontal tail will produce from a nuetral position. Using varing airspeeds, power on or off, CG forward and aft. Side slips in pitch up and pitch down in turns and ect. ect. ect.... But it does not show that the horizontal stabilizer produces any positive lift by itself without elevator input. If any thing it shows the HS produces a downward force normally.
The largest up tail load (full forward stick) will occure with the CG rearward at a large value of the airplane load factor, moderate airspeed, and low altitude with power off.
The largest downtail load however, will occure in the high-speed power range and at high altitudes with a large negative load factor and with a rearward center of gravity.
Straiga
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Badboy,
Wouldnt you say the CG is closer to the main wings center of pressure, than the to enpennage. Then the fulcrum might be between the CP and GC or there abouts. Now shoot the tail away and the CG and CP are now real close to the fulcrum with little or no leverage at the fulcrum. The new leverage is the engine and it is farther from the fulcrum and CG , CP location. The engine and prop combo wieghs more than the remaining airframe I would bet. So the nose will pitch down with the main wings downward pitching force along with it.
When you look at the picture you provide at the top of the page, showing the Center of pressure ahead of the CG you assume that the COP will pitch up the airframe. But when you look further forward it is the engine with a lot more leverage than the COP from the fulcrum. Which will pitch the nose down, when your tail gets shot off.
Straiga
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This is an interesting thread. It's refreshing to see arguments backed up with numbers.
May I make a layman's observation?
All of the equations and scenarios being presented here are descriptions of either "tail-on" or "tail-off". What happened to "as the tail wrenches from the fuselage"?
A tail coming off of an aircraft in flight has got to be a terribly violent event. I doubt that any tail has ever come off an aircraft cleanly, as to impart no reaction to the fuselage, as your equations describe. Also, in however many times this may have happened in real-life, I doubt that any two occurences were alike, so historical data (if any) does not help much.
Also, figure in the fact that the aircraft is probably engaged in a dogfight, ie., maneuvering violently. I would think that any residual inertia from the unitary fuselage would be somewhat imparted to the remaining pieces... for example, the pitching moment for a 28'-long fuselage is suddenly applied to a 14'-long nose. It would be like a skater bringing her arms inward for a faster spin.
Anyway, I don't know anything for sure, but I am pretty certain that the "as the tail rips off" part of the equation is a bigger factor in all of this than you realize. It's the ultimate wildcard.
It doesn't seem to me that a pitch-up is inherently wrong... or right. As far as the game is concerned... pitch-up or pitch-down, you're in for a short, nasty ride. :)
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Way to revieve a 2 year old question :aok
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Originally posted by hitech
Just understand most planes in AH are setup with a fairly aft CG and that when the tail is removed no change in mass takes place.
The light bulb just lit for me. No change in mass; no change in CG position aft of CL, yet all HS forces and effects disappear.
The pitch up exhibited in AH only occurs (as it should) when the affected aircraft (now a falling body of the same mass and CG with a variety of aerodynamic effects) was in an attitude that would exhibit the behavior when the tail was 'removed.'
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How about when the mass of the tail is removed. Does this not change the effect on mass?
Straiga
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How about when the mass of the tail is removed. Does this not change the effect on mass?
I would certainly think so. If the CG acts as a fulcrum and tail applies downward lift to hold the wing up at high angles of attack then you can think of it like two kids on the teeter-toter. When the low one gets off suddenly, the high kid gets a sore butt.
Crumpp
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Yes removeing tail would effect the CG, but we do not model that change.
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A nice thread.
After considering the foregoing posts, in my non-specialist eye I can see occasions where a tail-less bird noses up and other occasions where it would nose down.
First, aircraft CG does shift forward slightly due to weight lost from the tail. But how much does a duralumin and sometimes fabric-covered structure weigh? If only the horizontal stabs and elevators go missing, but the rest of the aft fuselage in tact, then weight loss is likely much less than…? 300 lbs? and that from planes ranging from three to seven tons. Even if you include the aft fuselage, there certainly isn't a major loss to the total mass of the aircraft. So the resulting CG shift is not great. It would not seem to be enough to instantly drive the nose down.
Other points worth mentioning related to this question: Thrust line and wing location on the airframe, i.e. high-wing, mid-wing (Wildcat), or low-wing like most of WWII fighters. The moment produced by wing-drag and thrust line on a high-wing plane at high power is a moment to lift the nose. A low-wing plane at high power settings produces a moment that drops the nose. For a mid-wing, like the Wildcat, where thrust line coincides with wing placement, there is no moment induced by high power settings. I believe the thrust induced moment must be considered in relation to all the other aerodynamic factors when the tail is lost. For a low-wing airframe this moment contributes to nosing down.
Nevertheless, it seems reasonable that a tail-less bird would "nose up" in the following conditions: High power setting, lower speed range, nose-up attitude (high AoA) as in a climb or turn, for air combat maneuvering.
In this case, the stick is pulled back to cause the tail surface to produce negative lift (relative to the total lift vector on the plane), forcing an increase in AoA, increasing the lift of the main wing. If at this point the tail surface is lost, the negative force aft is lost, resulting in a momentary drop of the nose and AoA. However, with this immediate decrease in AoA, much lift is also lost from the main wing. At lower speeds the plane ceases controlled "flying" and shortly resumes nosing up, due to high engine/propeller thrust downward, pulling the plane upward, with the weight of the aircraft being pulled down under the propeller by gravity. This was pointed out by Badboy and we see it modeled in AH II. Bravo. And if you immediately shut off the engine you'd presume the plane would eventually assume a nose-down attitude.
Nevertheless, I don't see any reason why a plane in high-speed level flight, or especially in a dive (how about straight down), that loses its tail, would pitch up and start a descending hover.
Comments have been made that the tail-plane provides lift, in conjunction with the main wing, and that its removal causes the tail to drop. Others are saying the tail provides "negative lift" to stabilize the plane, and keep the nose, at the other end of the "teeter totter," level. The diagram posted by Badboy supports the idea of some amount of positive lift of tail surfaces for certain aircraft. However the text indicates that this effect occurs in conditions of "intermediate" AoA. So when do WWII fighter aircraft fly at "intermediate" AoA? Economy cruise? I doubt if it is when you are yanking the stick around in ACM. Moreover that diagram shows a negative lift vector for the tail, and the camber of the elevator is inverted from that of the main wing.
The F-4 Phantoms I used to work on definitely had a tail surface designed for highest aerodynamic efficiency producing negative lift. But that plane also traversed the regions of mach 2, so you can't make direct comparisons between planes.
My personal opinion (purely that) is that tail surfaces of WWII fighters were designed for something close to neutral lift in level flight for most loads and speeds. Here's why. The vertical stabilizer and rudder do not contribute a force vector until you mash the rudder pedal. Until then, the two opposite and equal vectors on the vert stab and rudder result in a net-zero force vector. Nevertheless, the vert stab and rudder provide an aerodynamically stabilizing force that prevents the tail from wallowing around. So with the horizontal stabilizers. They primarily provide a stabilizing force that keeps the AoA where you want it. While the horizontal tail could contribute to total lift at intermediate AoA in steady flight, it seems obvious to me that during combat maneuvers the tail surface provides either excess lift (driving the nose down) or negative lift (driving the nose up).
So is there any further evidence from which to draw conclusions?
I conducted an interesting experiment to discover AH II 's model of the force generated by the tail, relative to the main wing, during high-speed level flight. I wanted to know whether the tail produces:
• Positive lift, like the main wing, lifting the weight of the tail
• Or negative lift, opposite that of the main wing, pushing down on the tail and forcing the nose up at the other end of the "teeter totter"
I used:
• P-38, 40, 47, 51D, the F4U, F6F, FM2, Tempest, Spitfire Mk IX and FW 190D
• 100 percent fuel load
• no drop tanks
• initial trim set for level flight at low power
I found that as speed increases beyond 200 IAS, each of these planes noses up . So the net-effect of all aerodynamic forces is a net negative lift tail-plane, pushing the tail down, lifting the nose, and this force increases with speed. This also seems to indicate the tail-plane is not bearing any (or much) of the weight as speed increases. The pilot must change the tail's trim to counter this effect as speed increases, to raise the tail and lower the nose. (Probably most people in AH use auto trim and never noticed.)
In RL this effect would doubtless be due to longitudinal dihedral designed into the airframe (also mentioned in Badboy's diagram) to contribute to aerodynamic stability. And this nosing up occurs despite the center of lift creeping aft as speed increases.
So, think about it. Based on the FM in AH, as speed increases in level flight, the tail generates increasing negative lift in relation to the main wing, raising the nose. So much so that you have to seriously trim against it. What will happen if in high-speed level flight you suddenly remove that aerodynamic force pushing down on the tail? Seems to me that the aft of the plane will rise and the nose will drop. Several posts above we see diagrams of the disintegrating Mooney. They didn't come out of nowhere. Add to that the nose-down moment produced because the thrust-line is above the main wing drag vector in low-wing airframes and it seems nose down is inevitable. Moreover, in level flight or in a dive, the line of thrust is not far enough above the center of mass to let gravity pull the plane below the propeller into a state of descending hover.
So it seems to me that at higher speeds in level flight, or a dive, the plane is prone to nose down if the tail-plane suddenly ceases to exist. It seems unlikely that it will quickly pitch up and begin hovering.
That said, AH is tops! Thanks Hitech and all your crew for the fun.
Regards,
Cement
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Yes removeing tail would effect the CG, but we do not model that change.
Thanks Hitech!
AH is the most realistic FM for the MMOL flight sims. Customer service is above reproach.
What a great thread. I certainly learned alot. Thanks for the help!
Crumpp
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Almost all conventional (not flying-wing) airfoils have significant negative moments. Meaning most WWII aircraft wings want to pitch nose down, hard. If the flaps are out, even more so. If the aircraft was in level flight and the tail and prop went away simultaneously the aircraft would pitch nose down 'till it found a stable configuration, most likely an inverted flat spin.
The questions that are hard to answer is what if the aircraft is manuevering violently when it loses the tail and what effect does the prop have? I don't know the answer to those.
I have a question regarding HT's comment:
"Just understand most planes in AH are setup with a fairly aft CG"
By this does he mean the CG is on/forward of the center of lift or actually behind it?
So it seems in a straight and level situation the plane should nose down immediately perhaps followed by a hang from the prop if it's still running. I can only envision a pitch up during aggressive manuevering.
g00b
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g00B,
Because AH doesnt model the CG change when the tail mass is removed. The planes will not show a pitch down moment. I think that if the CG was modeled in all aspects of flight the nose down moment would happen more offen than a nose up moment.
Straiga
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so the physics are correct but incomplete in that the plane with no tail still has the total weight and weight distribution (the weight of the missing tail remains and CG is unchanged)
I am curious if the change in CG and the lost weight of the tail would indeed push the CG forward on the aircraft whos tail was removed.
surely such a large amount of fuselage missing would make a large impact on the CG of the remains of the plane.
can you clarify HT?
that CG change and weight loss of the missing tail is not calculated.
but the missing lift of the removed tail is calculated?
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That is correct Citabria.
Removeing the tail on a real plane would obviosly move the CG forward, how much it would be moved could be calculated.
But in AH it realy is not worth the effort to do that caculation.
HiTech
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Yeah, I guess if your tail is shot off, your kaput. How you fall is completely irrevelant - nose up or down. :D
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the f4f will fly without a tail.:aok
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IMHO, the wing's natural tendency to pitch down would overcome ANY rearward CG situation and "most" manuevering situations. I just ran some polars on the P-51 airfoils and it has a huge negative moment. I'll presume most WWII aircraft did as well.
g00b
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I took a more extended look at the very interesting NACA Technical Note No. 1483 and thought I'd add a comment or two. First, I wouldn't know if it is worth the time to code into AH II the precise reaction of an aircraft losing its horizontal tail. But on the supposition that some day it might be, according to NACA there is a lot more at play here than just c.g. and center of lift.
Three load components for the tail
On page 9 at the top of the page is the formula used to calculate total load on the tail at any given moment in flight. There are three main components to the solution, L1, L2, L3, each of which relate to specific aerodynamic issues. According to the explanation of the first paragraph on page 9, only L1 and L2 are involved if there is no pitching angular acceleration.
Steady-level-flight pitch down moment
Straiga points out in his post above that on page 10, below formula (4), we read that the steady-level-flight data shown by figure 20 indicates a force on the tail of -5.2 lbs/sq ft power-on, and -4.72 lbs/sq ft. power-off. From this the Cm or pitching moment coefficient of the airplane less tail is calculated and is -0.0552 power-on and -0.0501 power-off. That means in steady-level-flight, a tail less plane is prone to nose down whether power-on or power-off.
In fact, the L1 component of the total tail load (measured during condition of zero main-wing lift) is shown in the graph of figure 21 on page 48. An aircraft in a zero-lift condition (dive) shows increasing negative load on the horizontal tail as speed increases. So for an aircraft at altitude in a no-lift dive that loses its tail, the nose will want to continue to drop, not rise.
The L2 component of total tail load is shown in the graph of figure 22 on page 49. The graph shows the effect of g load forces on the tail load for varying c.g.s. Presumably the center line of the graph, 30 percent MAC, is something near average for fighters. So, in normal steady flight of 1 g at 30 percent MAC the L2 component shows to be 400 lbs up load. But don't forget, this tail load factor must be added to L1.
Loads L1 and L2 together
The combination of the L1 and L2 loads in steady level flight power-off are shown in the graph of figure 23 on page 50. Print the page and draw in a curve precisely between the solid black curves labeled 0 and 2 to show the tail load for steady flight at 1 g. At about 200 mph the tail load is zero, at 300 we are getting close to -1000 lbs on the tail. And this is even with the c.g pushed far back at 34 percent MAC. Moreover, as mentioned above, power-on would add further nose-down moment, requiring further negative load on the tail to balance the flight.
Flight data shows down load on tail at higher speeds.
Table 1 on page 19 records data during level flight, power-on and power-off.
No speed indication is given, but engine power is in the fourth column "bhp."
The load at the c.g. is given in the sixth column "n c.g. (g)"
The coefficient of lift is given in the seventh column "CL"
Dynamic pressure is given in the eleventh column "q (lb/sq ft)"
The load on the tail is shown in the last three columns.
In all the runs of flights 4, 5, and 13 the load n at c.g. stayed close to 1 g. In every case in those runs where the bhp went above about 500 (indicating a higher speed) the tail load was negative. In those cases, the coefficient of lift "CL" is low, and the dynamic pressure "q" is high, so presumably, even with power off, if the coefficient of lift "CL" is low, and the dynamic pressure "q" is high, the aircraft would seem to be moving at a higher speed. On all those power-off runs we again see a negative load for the tail.
NACA says on page 15 in the third paragraph regarding steady-flight:
"The largest up tail load will occur the center of gravity rearward, a large value of airplane load factor, moderate airspeed, and low altitude with power off."
"The largest down tail load, however, will occur in high-speed, power-on flight at high altitude, with a large negative load factor and with rearward center-of-gravity location."
It seems then, that the current "nose up" attitude assumed by tail less planes is okay for slower speeds and g maneuvers. But for high speed level flight, and dives, the nose apparently should drop on a plane that loses its tail.
Nevertheless, thanks again for a fine product.
Cement1
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im amused by the fact it took two years to get the most simple of answers out of this discussion...
i always assumed that ah did not model total loss of the rear of the aircraft... like ht said, it wouldnt really be worth it. you're dead anyway!
just curious though - does this mean that the damage model is completely seperate from the weight modelling of the planes? not that it would matter i guess because the only drastic change in weight would be from a catastrophic event such as a missing wing but in theory if you lost a lot of parts it could change the center of gravity, could it not?
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Well, moose, after reading the NACA tech note 1483 several times, I wouldn't say the answers to the question of a tail less plane are simple really. NACA says that there are variables involved that even they didn't have the data for when they published the report. But the empirical data they measured during flight made up for that lack to some degree.
But I too would be interested to know if the damage model is completely separate from the normal flight model.
Best regards.
Cement1
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I like paste it tastes funny!
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Originally posted by hitech
And frenchy, most pilots realy don't know much about the physics of flight. I have had length disccussions with multiple CFI's and most just have a very basic understanding and would draw the same conclusion from your diagram.
HiTech
Sorry HT, goota call you up on this statement. It should read "most pilots realy don't know ANYTHING about the physics of flight."
I've been working on my PPL over the last few years, after doing an honours Aero Eng degree almost 10 years ago (ugggh scary thought). I have found many errors in the textbooks used to decribe flight to students. I even had to correct the exams they were giving me for weight and balance and performance. Scary to think of the number of students that went through before me who are probably flying airliners by now ...
Most pilots know that when they get in an airplane they can turn the key or hit the button and it will make noise, roll down the runway and take them (hopefully) where they want to go. That's about it for most of them. The ones that actually understand correctly how it all works are few and far between...