Tail Damage - Opening can of worms...

[Mace2004]

La7, P51 and IL-16 to date in combat low down. I could not see any hit on speed/manouverability at all. This could of course be because I am a Piiiiiiiilat of little skill.
Maybe combat trim was compensating
I am not going to get into any contest of minds on the matter.
It just seems strange that the designers of these air craft thought there was a need for stabs and elevators on both sides of the tail when it seems in AH we only need such an assembly on one side. It would be interesting to know what the detrimental effect of losing both stab and elevator from one side is.

The detrimental side is that the aircraft has reduced pitch stability.  It's not CT that's helping because CT is nothing but a look-up table based upon speed, it doesn't "know" half the stab is missing.  Here's what HiTech had to say in another thread about half-stabs:

With a H stab missing the plane is less stable. Less stability creates the ability for a more rapid AOA changes. Turn performance would decrease , or possibly remain the same.

With 1 elevator destroyed your turn performance will decrease, it the corrisponding stab is then destroyed your turn performance will go back up. We thought this was a bug until we brought it up in our force vector displayer and saw that what is happening is with 1 side  elevator gone and the stab remaining, the stab is fighting the other elevator so you can not generate the AOA you need. Once that stab is removed, the AOA you can generate is restored.

Now, when you think about it, it makes a lot of sense.  Maneuvering requires you have to overcome the stability of the plane in one of its axis, in this case pitch.  Fighters, like race cars tend to have less static stability than say, an airliner or Buick.  This is because they need to be able to change direction quickly so reduced static stability helps with maneuvering but they are harder to fly (or drive). 

In this case, when static pitch stability is reduced by loss of part of the Horizontal Stabilizer then less control power is required for the same maneuver.  That means you could have sufficient control power from a single elevator to generate the same pitching moment generated by a complete stab with elevators BUT this comes at a significant cost of reduced pitch stability which means the pilot should have to work harder to control pitch (if it's possible to control at all).

[Tilt]

Thanks Mace......

[Charge]

"With 1 elevator destroyed your turn performance will decrease, it the corrisponding stab is then destroyed your turn performance will go back up. We thought this was a bug until we brought it up in our force vector displayer and saw that what is happening is with 1 side  elevator gone and the stab remaining, the stab is fighting the other elevator so you can not generate the AOA you need. Once that stab is removed, the AOA you can generate is restored."

The basic profile of an elevator is symmetrical so it does not really contribute as lift to any direction and its effect as a limiting force to tail movement sounds strange. It probably is a limiting force but it should be quite small when compared to functioning side of elevator. Of course it depends on the proportions of elevator plane vs. control surface which is different in different planes.

BTW is it possible to lose "other side" of elevator plane to begin with? Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit, but the control surface itself is separated from the other side so you can lose just the other?

-C+

[Chalenge]

There is lift generated by the stabilizer. If symmetrical airfoils contributed nothing to lift they wouldnt and couldnt be used as airfoils.

Your last questions seems to be saying that the only thing holding a stabilizer on to the airframe is the aircraft skin and nothing could be further from the truth.

[Charge]

"There is lift generated by the stabilizer. If symmetrical airfoils contributed nothing to lift they wouldnt and couldnt be used as airfoils."

Symmetrical airfoils do not generate lift at 0 AoA in relation to airflow, only drag, but any change in AoA moves the pressure gradient to lifting side causing lift to be generated. And it is not tailplanes job to create lift, just to direct the lift vector of the main plane.

"Your last questions seems to be saying that the only thing holding a stabilizer on to the airframe is the aircraft skin and nothing could be further from the truth."

"Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit" 

-C+

[mtnman]

[quote author=Charge link=topic=288694.msg3682487#msg3682487 date=1274897860

"Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit" 

-C+
[/quote]

Not necessarily.  The only example I can give is the F4U, where (as I understand it) they're attached separately.  In fact, I'm pretty sure they're identical, so the left from one plane could be used to replace a damaged right on another...

Someone else posted pictures/diagrams of this, for the F4U.  I think I saved them somewhere.  I'll see if I can find them and post them, unless someone beats me to it.

[Stoney]

"There is lift generated by the stabilizer. If symmetrical airfoils contributed nothing to lift they wouldnt and couldnt be used as airfoils."

Symmetrical airfoils do not generate lift at 0 AoA in relation to airflow, only drag, but any change in AoA moves the pressure gradient to lifting side causing lift to be generated. And it is not tailplanes job to create lift, just to direct the lift vector of the main plane.

"Your last questions seems to be saying that the only thing holding a stabilizer on to the airframe is the aircraft skin and nothing could be further from the truth."

"Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit" 

-C+

http://rwebs.net/avhistory/history/p-47.htm

Some good explanations of the P-47 empenage assembly.  Each torque tube for the elevator had its own bell-crank so theoretically if one was lost, the other would still operate.  Might have a wicked shimmy, but it would still be attached.  Obviously, each aircraft is going to have a different design, although I would expect that most of the conventional U.S. fighters would look like this one.

[Charge]

Interesting page Stoney, thx.

It looks like the elevator plane is a solid spar construction and it is attached to fuselage from the center spar but the attachment suggests that losing the other side totally does not affect the other side at all except that I wouldn't try to pull too much G with half configuration..or I don't know, maybe the momentum is just halved? As it is stated the elevator itself is of separate pieces so losing either of them does not affect the other side either in P47, at least as long as the torque tube remains functional.

Ed. If you compare it to that of Zeke it is very much different. http://rwebs.net/avhistory/history/Zeke32.htm.
Comparing these two I'd say that structurally the Zeke suffers less from losing the other side of the elevator completely since the load bearing part is the fuselage but in P47 the through spar bears the full force, so what happens in P47 when you lose the other side of the elevator completely?

After pondering the tailplane matter a bit more it may indeed be true that if the elevator flap is lost, but the elevator plane itself still remains attached it indeed contributes a balancing force against any movement of the remaining elevator, since if the camber of the whole elevator plane does not change due to movement of the elevator flap, the symmetry of the profile always causes a force opposite of the camber of the other side that tries to control the pitch. How much is it depends on the proportions of elevator plane vs. elevator flap and it varies a bit in different planes.

-C+

[Chalenge]

Symmetrical airfoils do not generate lift at 0 AoA in relation to airflow, only drag, but any change in AoA moves the pressure gradient to lifting side causing lift to be generated. And it is not tailplanes job to create lift, just to direct the lift vector of the main plane.

Now you are saying something altogether different. I wont speak in absolutes but it is generally the case that with symmetrical airfoils that are at zero degrees AOA to the relative wind in non-manuevering flight only (a lot of special cases but this satisfies your specifics) that there is no lift (positive or negative) and that the purpose of this condition is optimum cruise. It is also generally the case that this same situation/condition results in the least induced drag BUT this case also usually occurs at one specific altitude and power configuration. So I would say that stipulating that condition is kind of superfluous.

[hitech]

"With 1 elevator destroyed your turn performance will decrease, it the corrisponding stab is then destroyed your turn performance will go back up. We thought this was a bug until we brought it up in our force vector displayer and saw that what is happening is with 1 side  elevator gone and the stab remaining, the stab is fighting the other elevator so you can not generate the AOA you need. Once that stab is removed, the AOA you can generate is restored."

The basic profile of an elevator is symmetrical so it does not really contribute as lift to any direction and its effect as a limiting force to tail movement sounds strange. It probably is a limiting force but it should be quite small when compared to functioning side of elevator. Of course it depends on the proportions of elevator plane vs. control surface which is different in different planes.

BTW is it possible to lose "other side" of elevator plane to begin with? Isn't it usually made so that there is a spar running through the fuselage so that if the other side breaks off the other side is likely to follow suit, but the control surface itself is separated from the other side so you can lose just the other?

-C+

First let us define lift. Lift is any force produced by air or fluid that is 90 degrees to the velocity vector.

Yes the tail produces lift. The elevator changes a symmetric airfoil into an asymmetric airfoil. This change produces lift. This lift rotates the plane and changes AOA on both the tail and the wing.

With only 1 elevator but not stab missing you are only changing the camber on 1/2 the horizontal stab , but you are producing the same AOA on both stabs hence you can not produce enough AOA on the main wing to generate MAX AOA.

On plane where the CG = the main wing CL when turning there is 0 force being generated by the horizontal stab in the turn. I.E. no force up or down so the only thing that is needed from the tail is a change in it's 0 lift angle. This is accomplished by changing the camber of the stab by moving the elevator. The plane then rotates to the horizontal stabs new 0 left angle. This AOA change creates more lift on the primary wing and creates the turn. So how much force the tail generates is pretty much irrelevant, only how much it can change it's 0 AOA.

Now with 50% less elevator as compared to the stab size, it can not produce the same change in zero lift angle. Once the other half of stab is removed it can again generate the same 0 lift AOA change.

I hope that lets you visualize it better.

HiTech

[Charge]

"... it may indeed be true that if the elevator flap is lost, but the elevator plane itself still remains attached it indeed contributes a balancing force against any movement of the remaining elevator, since if the camber of the whole elevator plane does not change due to movement of the elevator flap, the symmetry of the profile always causes a force opposite of the camber of the other side that tries to control the pitch..."

I was a bit unclear with this but what I meant was only my own realization of what HT tried to say in other thread was actually true: If the camber of the functioning elevator changes and that of the damaged one does not (it cannot, no elevator flap), the damaged one with its symmetrical airflow, now with lifting force produced to wrong side, will resist pitch change that the functioning side tries to achieve acting as a balancing force to any elevator control that tries to change the balance (i.e. 0 AoA) of the remaining symmetrical plane.

"So I would say that stipulating that condition is kind of superfluous."

As a main lifting plane that is true because it would not be airborne at 0 AoA to begin with. But as a contol plane it can be in a neutral state where it does abolutely noting but contributes only drag when a CoG happens to be exactly on the middle of CoL in flight. Any change in elevator flap position will change its camber and thus start to produce lift to either direction. If, for some reason, the CoG happens to be rear of CoL the tail plane needs to produce lift too, to balance the flight i.e. the trim is nose heavy to balance the actual tail heavy flight condition.

Sorry bout the confusion that was on my end but I hope I got it right now...

-C+

[hitech]

Charge: You do.

[Tilt]

When looked at from a force vector point of view would there be any increase in loading across the remaining Stab + elevator when "in use". May such increase approach any structural threshold?

[hitech]

When looked at from a force vector point of view would there be any increase in loading across the remaining Stab + elevator when "in use". May such increase approach any structural threshold?

The forces would come from the stab. The remaining stab still can produce the same force as it could when the other was still there. So given the same elevator deflection, it will still be producing the same force. So from a simple how much force view, if you could not break it before, you could not when 1/2 was gone.

HiTech

[Dawger]

It would be pretty rare to fly an airplane with the CG coincident with the CL.

And it would not be very much fun.

Flying an airplane with the CG aft of CL is scary. I've done it and it is a lot of work. Never want to do it again.

The vast majority of the time the horizontal stabilizer is providing down force. Loss of half an elevator would not do much to that down force but would reduce the ability to change the lift produced by the horizontal stabilizer

Loss of half the stabilizer surface would equal loss of half the tail down force required. There would be significant up elevator required all other things being equal.

I've been in an airplane (Cessna 402) with a stalled tail plane. Luckily the aircraft was in a load and configuration requiring little tail down force at the time and got the tail un-stalled before the nose went too far vertical. I was at 500' AGL at the time.

There have been several fatal tail stall accidents.

We practice jammed elevator drills in the simulator and the procedure is to disconnect the two elevators from each other allowing one pilot to operate the unjammed half. There is some roll moment produced in the this condition  but it is not dramatic.

The chances of losing an elevator are remarkably slim. I would think gunfire would cause more jams/severed cables than controls parting from the aircraft in the tail section. That is one area of the damage model I find lacking in authenticity.