Stab change

[OOZ662]

Keep in mind that you're countering the torque with your undamaged ailerons.

If you use Combat Trim, I don't believe it compensates properly for it. That could lead to perceived instability as well.

[airbull]

Interesting thread.
I have seen many pictures of rear stab damage. Planes making it home with big bits missing. But they have all been bombers. Has any one seen a picture of a fighter getting home with a rear stab missing. The only ones I have seen are of rudder damage.

[Krusty]

I have, but I couldn't find any with a quick google image search when I last replied. Hence the lack of pictures.

[dirtdart]

I checked out the drawings, thanks for the post.  I guess in reality, without a real aeronautical engineer with the right metal properties, rivet count, design specs, et al, this is just speculation.  Someone said earlier, this is what the BBS are about and I agree.  Thanks for all who participated.

[saantana]

Others report being more manuverable than before with 1/2 the stab missing. May be plane dependent.

I don't notice any change in the spit16 when half of the elevator stab is gone. Wait a minute.. this should not be surprising..

 

[CountD90]

I don't notice any change in the spit16 when half of the elevator stab is gone. Wait a minute.. this should not be surprising..

 

That's cause the UFO16s don't even need the H-Stabs, HTC puts them on just to disguise them as real planes...
 

[Mace2004]

This is an F4u-4 not a -1 but I think the construction is similar.

Those are some nice pictures but what you need to understand is that they only show where major components are relative to each other and how they are assembled, not how the structure transmits loads.  Yes, the stabs bolt on to the fuselage but what you cannot see is the structural member within the fuselage that they're bolted to.  I can guarantee you that the other stab bolts to that same structural member making, from a load perspective what is, in essence, a single spar.  What you need to see is the major load carrying members and how and where stress is produced. 

It is not at all unusual for individual stabs to be built separately but, from a load perspective, once they are bolted to the major load carrying component of the aft fuselage it becomes essentially a single spar.  That major load carrying component may not look like a spar but it carries the load produced by the entire surface.  Look at photos of early jets like the F86 and P80.  You'll notice that the stabs are mounted at the top of the fuselage.  This is to simplify the load carrying structure by mounting it above the tail pipe and it looks kinda like a "knuckle" where the stabs and vertical tail all attach.  This "knuckle" is in turn attached to the aircraft's major load bearing longerons forming the monocoque fuselage.  On later jets like the F8 and A7 series the stabs are mounted lower making it impossible to use a single spar design or knuckle because it would have to go through the tailpipe.  There is; however, a load bearing structure in the tail, it's just built as either a full or partial ring so the tailpipe can go through the center of it but it serves the same function of transmitting lateral loads through the entire tailplane.  The same is even true with the F14 where the aft fuselage is actually built up of rings within each engine nacelle that the stabs (and vertical tails) attach to but then those nacelle rings are joined by a structural member through the flat area between the afterburner cans (called the boat tail).  Again, making a continuous load-bearing structure.  In the more modern cases, the aircraft use what are called "differential tails".  In these designs the horizontal tail surfaces are intended to produce roll by deflecting in opposite directions and intentionally producing large torque moments in the aft fuselage but since they are designed specifically for this type of stress the structures are stronger than non-differential tail designs.

In the pictures where aircraft have landed with a stab missing one of three conditions must have occurred.  Either they were unable to generate normal maximum loads with just a single stab (meaning reduced control power and turning capability and resulting torque), the pilot was not in a situation where he needed maximum control power for the maneuvers he needed (which again means less that normal maximum turning capability and low torque), or reduced stability reduced the control power (and torque) required to generate the required pitch.  This third condition (as suggested by HiTech) is that the reduction in longitudinal static stability results in a lower control power requirement for the required pitching moment.  Now you'd have to look at how much longitudinal stability is reduced by the loss of the stab.  Is it still positive, neutral, or negative?  If it's still positive, then the reduced control power may be able to generate the pitching moment needed but the pilot is definently going to know something major is wrong because he can feel the instability.  If neutral, the pilot would really begin to have real problems controlling the plane and if negative, all his attention would be focused on simply keeping the plane flying (and he would probably fail).  In all these cases the pilot would more likely be more concerned with maintaining control rather than making high performance turns.

It is counter-intuitive (and highly unlikely) that the loss of stability combined with reduced control power requirements would balance out to the point that the airplane flies more or less normally.  My contention is that it is unlikely that an aircraft missing one stab will be able to maintain anywhere near it's normal maximum turn rates either because of insufficient control power, failure of the remaining stab and/or fuselage due to stresses it was not designed to handle or flat out instability.  In none of these cases would the plane fly as if it still had both stabs. 

[FLS]

I think Hitech's point that "turn performance would degrade or possibly remain the same" implies that the elevator normally has more authority than it needs. I don't think anyone said it would fly normally. I thought we were talking about flying better than intuitively expected.

If the stab attachment is built into the fuselage in some manner and has the effect of a continuous spar then it's not a continuous spar but I don't think it's worth quibbling over definitions. I assumed that the attachment points pictured were part of a fuselage structure that was designed for more than the anticipated normal range of loads.  I will happily call it a continuous spar for this discussion. I just don't see where it makes a difference since the individually attached stab is designed for the forces applied to it. You haven't shown where the attachment requires balanced forces on both sides in order to maintain structural integrity which I believe is the point in contention.

[Mace2004]

I think Hitech's point that "turn performance would degrade or possibly remain the same" implies that the elevator normally has more authority than it needs. I don't think anyone said it would fly normally. I thought we were talking about flying better than intuitively expected.

There have plenty of comments from folks that the aircraft missing a stab turns as well or better, that's what pretty much started this thread.  That just shouldn't happen because of structural loads or instability.  If it were true, why add all the extra weight and complexity of that "redundant" stab???  Just leave it off and you're good to go with a lighter and less complex airplane!  

If the stab attachment is built into the fuselage in some manner and has the effect of a continuous spar then it's not a continuous spar but I don't think it's worth quibbling over definitions.

I didn't say it was a continuous spar, I said it acted like one and qualified my comments with "in essence" and "essentially".  This is really, really basic mechanics for levers here so I'll try one more time.  With a complete horizontal stab generating approximately equal lift from tip to tip the resulting stress at the fuselage junction is in essentially a straight line normal (i.e., 90deg) to the spar.  With a single stab the type of stress is completely different, it's not a straight line, it's a twist.  Maybe this example will make sense. Glue a block of wood to a wood deck and try to pull the block straight up with all your strength (say 200lbs).  With good wood glue this would be very difficult or even impossible to do.  That is a "normal" load, in other words it's 90deg to the joint and in the direction of the joint's maximum strength and can easily handle that 200lb pull.  That's the type of stress created by a complete stab.  Torque is a completely different type of stress.  Attach one end of a horizontal 2x4 (a hypothetical single "stab") to the block and pull up on the other end of the 2x4 with the same 200lbs you used previously.  The resulting torque will probably easily twist the joint apart.  This demonstrates that the uneven leverage (or torque) is much more capable of separating the joint than a straight pull so it's not just the load that matters but how it's applied.  If this still doesn't make sense then you'll just have to trust me and yes, my degree is in engineering.

I assumed that the attachment points pictured were part of a fuselage structure that was designed for more than the anticipated normal range of loads.  I will happily call it a continuous spar for this discussion. I just don't see where it makes a difference since the individually attached stab is designed for the forces applied to it.

It has nothing to do with the stab itself, it has everything to do with the fact that the stress applied to the fuselage juncture with a single stab is pure torque, not the normal "forces applied to it" that the aircraft designer intended.

You haven't shown where the attachment requires balanced forces on both sides in order to maintain structural integrity which I believe is the point in contention.

I didn't say it required balanced forces or that it would automatically come apart, I'm saying that it's unlikely that sufficient structural margin exists to handle very high torque stresses that were never intended to be applied.  I've personally seen the tips of F14 horizontal stabs bend downward (as the wingtips bend up) while generating maximum pitch rates.  That's a lot of load on those suckers and I wouldn't want to see what happened if I generated the same load with a single stab.  

Aircraft design is really a balancing act.  Make things too weak and they break, make things too strong and the airplane gets too heavy.  All aircraft are "overdesigned" to a degree to provide a safety margin and that margin is specified in contracts and MILSPEC.  For instance an aircraft with a +6G limit will not fall apart at +6.1G but it will begin to have permanent deformation and/or structural failure at some point.  Take the +6.0g wing and push it to -6.0G and it probably will fail as that's not the load it's designed to handle.  As I mentioned previously, the F14 was specifically designed to handle the torque produced by differential tails, that's what makes it roll (well that and spoilers).  Even so, when they added new computers for the stability augmentation system and tested its improved roll capability the test aircraft was severely overstressed and damaged by the torque. Basically what happened is the increased torque produced by the differential tails with the new computers exceeded the safety margin and almost twisted the back end of the airplane off of the front end.

Still, much of this argument may be moot given HiTech's comments about loads being reduced by reduced static stability but it's an interesting discussion anyway.  I still contend that the airplane will not fly anywhere near "normal" as some people here have claimed and you certainly wouldn't be able to continue to fight with half your stab missing.

[FLS]

I agree with just about everything you're saying Mace, just not your conclusion.

Anecdotal evidence from players isn't convincing and hasn't contradicted Hitech's comment.

Maybe the "extra" stab is redundant in case you lose one. 

When I said "has the effect" I'm agreeing with your  "in essence" and "essentially". I got the teeter totter example the first time. I try to avoid arguing with engineers because they cheat and use "math".    I understand your example, but you haven't convinced me that it applies in this case. Might apply? Certainly. I don't pretend to know differently, but I find it easy to believe that the extra stresses could be handled by mid-war aircraft design. As you noted fighter aircraft designers anticipate combat damage and design for it when they can. Isn't that why Grumman was called the "iron works"? I know we're not just talking about Grumman here but as you know it's their bird in the drawings.

Maybe we'll see changes with the expanded damage model. It could be that finding a point where a single stab would fail has the same difficulties as determining when a flap would fail if left down at too high a speed. It could also be that Hitech has modeled the stress and it's not as great as it might seem.

I don't recall ever losing just one stab in AH so I don't know first hand how it flies afterwards. I do appreciate your attempts to educate me. 

[Gr8pape]

I agree with just about everything you're saying Mace, just not your conclusion.

Anecdotal evidence from players isn't convincing and hasn't contradicted Hitech's comment.

Maybe the "extra" stab is redundant in case you lose one. 

When I said "has the effect" I'm agreeing with your  "in essence" and "essentially". I got the teeter totter example the first time. I try to avoid arguing with engineers because they cheat and use "math".    I understand your example, but you haven't convinced me that it applies in this case. Might apply? Certainly. I don't pretend to know differently, but I find it easy to believe that the extra stresses could be handled by mid-war aircraft design. As you noted fighter aircraft designers anticipate combat damage and design for it when they can. Isn't that why Grumman was called the "iron works"? I know we're not just talking about Grumman here but as you know it's their bird in the drawings.

Maybe we'll see changes with the expanded damage model. It could be that finding a point where a single stab would fail has the same difficulties as determining when a flap would fail if left down at too high a speed. It could also be that Hitech has modeled the stress and it's not as great as it might seem.

I don't recall ever losing just one stab in AH so I don't know first hand how it flies afterwards. I do appreciate your attempts to educate me. 

I agree partly with what your saying but, it's not a Grumman in the picture, I could be wrong but I doubt it, didn't Vought make the Corsair's?

Pretty sure they did, but I could be wrong.

[FLS]

You're right it's Chance-Vought, I tend to think of the Corsair and Hellcat as similar and confused the manufacturer even though I know better.   

[dirtdart]

Well, I also am an Engineer, just not an aero guy.  This seems on the unlikely side.  Me fighting a N1K1 with no outer left wing (aileron) and no left stab at all.  Still rolled left and right.  Could the rudder change the AOA on the left wing enough?  Don't know. 




To actually fly, even somewhat level with say 40-60% of your lift on the port side gone...bah.  Then the guy dropped his gear and the nearly landed, crashed into a tree.

[FLS]

You only need one aileron to roll. You can also roll with the rudder. Cruising speed is not too difficult with part of one wing missing but when you slow down you tend to roll towards the broken wing. Flaps help.

[dirtdart]

The use of aileron in this instance would reduce the lift on the right wing, which makes the nose drop.  Curious.  I would love to see a fluid shot of shot up plane like this and see the lift drag and stresses.