Dihedral in the tail

[Lon Enloe]

OK, so I'm in the Denver International Airport this week, which has a pedestrian bridge over the taxiway between two of the terminals. I'm looking out as a small private jet passes by, and I notice that the horizontal stabilizer has a quite substantial amount of dihedral in it. Now I'm curious, so I start looking at the rest of the airliners I can see for the remainder of my trip, and it seems that all the ones I could see with conventional tails have dihedral in the horizontal stabs, approximately matching that of their wings. My survey was not exhaustive, but it seemed that the T-tail designs had flat horizontal stabs.

So, my first question is, what's the advantage of dihedral in the horizontal stab, for an airliner?

My second question is, would this (presumed) advantage be worth the extra complexity in the linkage if implemented in a model?

[Sparky Paul]

Tail dihedral is used as the last fix for lateral stability problems.
It can permit a smaller (therefore cheaper) tail to be used.
And it's easier to modify the amount of dihedral the horizontal has..
Wings are kinda cast in concrete, and what comes out of wing-mate is what you're stuck with!
As for it's use on a model, the only reason to use it would be cosmetic, unless the rest of the plane is screwed up and the wing or vertical area can't be adjusted properly.
As the horizontal moves up the vertical, dihedral tends to go away, due to the stablizing effect of moving the horizontal.
Note the varying uses of dihedral in the Twin Beech, the Baron.. low-mounted, flat.. Duke & Queen Air, low-mounted , dihedralled..King Air, Tee-mounted, flat.
It all depends on how the parts interact.

[adam_one]

Other reasons are to prevent the stab being affected by the wings' wake (downwash) and/or the propeller’s blast.
That’s why the T-tail designs have flat horizontal stabs as they are located much higher than the wings.

[Grejen]

I've heard (read) that sometimes a T tail is used simply to afford clearance for vehicles when on the ground in the terminal area. Possibly similar non aerodynamic considerations are at work here? If it has even small amount of dihedral it would sure be a lot more visible from the baggage truck.

[Sparky Paul]

If you compare the C-5 and the Ant-224, similar sized airplanes, same mission, the more practical configuration is on the -224, mounted on the fuselage.
The t-tail on the C-5 adds a lot of material needed for mounting the horizontal up there, and to my way of thinking, is more susceptible to the loss of the entire empennage due to combat damage.
The C-17 follows the C-5 shape, so there must some good reasons for it.

[challenger_i]

There is a LOT of turbulence, generated by those fat fuselages (C-5, C-17). Placing the horizontal tail high up improves stability, with a reasonably sized tail plane. If the tail plane were mounted on the fuselage, it would need to be 25% larger, to provide the same stability.

[adam_one]

Yes, it's all about stab efficiency.
The efficiency of a "conventional" stab (monted on the fuselage) is usually between 0.6 and 0.4 or even down to 0.3 if the stab is located very near the wing wake or on a fat fuselage in disturbed flow, whereas the efficiency of a T-tail is usually about 0.9
However, the T-tail is often more difficult to construct in order to achieve the required rigidity and low weight. It also may be more vulnerable in case of warbirds, even thought the Mig15 has almost one...

[Sparky Paul]

Yes, it's all about stab efficiency.
The efficiency of a "conventional" stab (monted on the fuselage) is usually between 0.6 and 0.4 or even down to 0.3 if the stab is located very near the wing wake or on a fat fuselage in disturbed flow, whereas the efficiency of a T-tail is usually about 0.9
However, the T-tail is often more difficult to construct in order to achieve the required rigidity and low weight. It also may be more vulnerable in case of warbirds, even thought the Mig15 has almost one...
.
The MiG-15 and -17 also have an unrecoverable spin. Some of the kills claimed in Korea were given to the first US pilot who would report a MiG-15 spinning down, usually from a high altitude, when the MiG pilot turned too sharply.
The MiG-17s flying here probably keep out of the manuvering area where departure is guaranteed.
But the C-5 et al will never be manuvered to that portion of the envelope. :)

[adam_one]

Sure, there's always the other side of the coin, especially when you have to push the plane into drastic manoeuvres...
But the MIG15 was one of the first swept back wing jet fighters at the time and despite of some drawbacks, I think it was a good challenge to P-80 Shooting Star and F9F Panther, which prompted the USAF to introduce the F86 Sabre...
The F86 Sabre has got stab dihedral, while the F4 Phantom has stab anhedral, certainly for the same reason.
And the F-104 Starfighter has got a T-tail… :)

[N74463]

Hi gang,
It's probably obvious, but why does horizontal stabilizer dihedral (or anhedral) affect lateral stability? Is it just the same sideslip effect that gives lateral stability with wing dihedral? Do horizontal stabs play a big part in lateral stability?
And why would moving the horizontal stab upward increase stability? Is that just increased distance from the roll axis(and therefore a larger moment arm)?

Joe

[adam_one]

Hi gang,
It's probably obvious, but why does horizontal stabilizer dihedral (or anhedral) affect lateral stability? Is it just the same sideslip effect that gives lateral stability with wing dihedral? Do horizontal stabs play a big part in lateral stability?
And why would moving the horizontal stab upward increase stability? Is that just increased distance from the roll axis(and therefore a larger moment arm)?

Joe
The horizontal stab is mainly to provide longitudinal stability (stability in pitch).
The longitudinal stability depends on the location of the centre of gravity, the
stab area, the stab efficiency and how far the stab is placed from the main wing.
By moving the stab upwards (T-tail) you increase the stab's efficiency.
The horizontal stab has little to do with lateral stability, but it might shadow the vertical fin during a spin, making the recover difficult.
Also in certain circumstances, the wings might block the airflow to the tail surfaces, depending on their location.

[AndyOne]

On a model, it may have some effect, Hanno Pretner (remember him) designed anhedral into the tail-plane of his Curare patern-ship design in the 1970s. He claimed it made negative flick rolls more snappy. He didn't stay with it though as it disappeared from more recent designs of his.

Andy.

[adam_one]

With a conventional plane, the main wing has the major effect on the lateral stability, the stab has very little effect (unless you have a tandem or got a bigger stab than usual), that's why Hanno Prettner didn't keep using stab anhedral on his following designs.

[Sparky Paul]

Dihedral in the tail is intended to enhance lateral stability. If it didn't, it wouldn't be used on full-scales.
On a model (and full scales), anhedral relaxes lateral stability. Rearranging the distrubution of areas and moments removed the need for anhedral.
For full scales which aren't as easily (cheaply) modified, and have a much greater flight envelope, what works well at one point might be seriously bad at another, so a compromise is assembled.. the tip dihedral on the F4 wing-and- anhedral on the horizontal are the most extreme example of this.

[adam_one]

Dihedral in the tail is intended to enhance lateral stability. If it didn't, it wouldn't be used on full-scales.

How do you explain the fact that T-tails don't have dihedral?
My answer is: they don't need it, because they're located higher than the wing and thus away from the wing's turbulence.

[Lon Enloe]

This is a little tangential, but Sparky Paul's comment reminded me of a picture that an ex F-4 jockey showed me a couple of years back--there was an incident where the ground crew forgot to lock the folding wing tips on an F-4, and the crew didn't realize it until they were past the point of no return on the runway, when the tips folded up on them. Fortunately, both tips folded up, so the plane didn't go unstable. The crew flew around for a while burning up fuel while the engineers in St. Louis scratched their heads, ran some calculations, and decided that a landing was possible, which the crew managed to accomplish. Now there's an exercise in lateral stability for you!

[adam_one]

Well, the wing was still there but with less effective area… so, the stab's area got closer to the wing's effective area making the stab more relevant to lateral stability than it was before the incident.
I don't know F-4 in detail, but I guess the ailerons were out of function then, so they had only the vertical fin, or is there a possibility to use elevons or maybe flaperons?

[Lon Enloe]

I don't know--that's a good question. I still occasionally run into the person who showed me the picture and the accompanying article; saw him last night, as a matter of fact, at a banquet both of our kids were at. Next time I run into him, I'll try to remember to ask.

[adam_one]

By judging the picture below it seems that there was the possibility to use flaperons to bank the plane and to do adjustments in the roll axis.
The plane was certainly nose-heavy then, increasing the landing speed.

[Sparky Paul]

There's no control surfaces on the outer panels on the F-4.
Spoilers, ailerons and flaps on the inboards.
An F8 Crusader managed the same wing-folded flight feat,
and an Isreali F-15 lost ALL the right wing in a mid, and recovered safely.
A lot depends on the cool-factor in the pilot seat.

[Thomas B]

How do you explain the fact that T-tails don't have dihedral?
My answer is: they don't need it, because they're located higher than the wing and thus away from the wing's turbulence.

Bumping up this old thread:

You rarely if ever see dihedral on the horizontal stab of a T-tail aircaft because of the bonus gained in vertical tail effectiveness from using the high mounted horizontal tail as an end plate to the top of the vertical tail.

The story is that the horizontal stab can be a little smaller up there on top and the vertical can be a little smaller as well, making it a good trad-off, even allowing for the weight of extra structure needed to make the vertical tail stronger.

Dihedral on lower mounted horizontal tails is to make up for a lack of lateral stability in a particular design and has nothing to do with the pitch stability or horizontal tail area of a design.

I think that T-tails in light aircraft as as much a styling statement as a needed aerodynamic feature, with the exception of high performance sailplanes. They make more sense in larger jet transports with swept wings.

[adam_one]

Bumping up this old thread:
Dihedral on lower mounted horizontal tails is to make up for a lack of lateral stability in a particular design...
If dihedral on horizontal tails were to provide lateral stability, then you should have seen the same approach on some T-tails as well...
Is the Boeing 727 less lateraly stable than the Airbus 320?

[Flight Engineer]

L1011 has no di or anhedral on the horizontal stab DC10 has a lot. If the designer cannot mount the stab at his prefered height he can ad dihedral. Also it is added to help roll stability problems that occur late in the design stage. Saab 340/200 has a lot for this reason.

[adam_one]

Notice how the stab's underside of the L1011 has slightly more dihedral than the main wing's upper side. That's to reduce the effect of the main wing's wake on the stabiliser.
The Saab 340's stab needs a lot of dihedral mainly to prevent the stab from being affected by the prop blast.
The stab's lateral pitch moment is too small to give a significant contribution to lateral stability.
:)

[Thomas B]

Not every aircraft needs its lateral stability adjusted using a stab with dihedral or anhedral. Some need it, some don't and it is difficult to generalize. As Sparky mentioned, swept wings are different beasts.

Designers do not do it to get the tail out of the wing turbulence.

The amount shown on that small L-1011 drawing is not enough to make any kind of arguement in any direction..;)

Flight engineer is exactly right on the SAAB 340. You would be amazed at how much effect that amount of stab dihedral has on lateral stability. It is still getting affected by prop blast, even there.

[adam_one]

I don't agree.
Lateral stability is achieved by the main wing, it is there the lift is produced.
Any lift produced by the stab makes the plane point downwards, the stab's function is to achieve longitudinal stability.
Besides, neither you nor Sparky have explained why T-tails never use dihedral.
:)

[Thomas B]

This NASA report goes into some detail as to how the horizontal tail significantly affects lateral stability, contrary to your opinion...;):

http://naca.larc.nasa.gov/digidoc/report/tr/71/NACA-TR-1171.PDF

I said T-tails rarely use dihedral, not never.

Google up a picture of a HP Victor jet bomber and gaze upon a T tail swept wing aircraft that has dihedral in the tail.

Conversely, the Lockheed Martin C-5 T-tail horizontal tail has noticable anhedral....

If an aircraft needs it, they use it. If it does not need it, they don't. It is just that simple.

[adam_one]

Well, I've done a quick read through that document and didn't find anything telling that the stab dihedral would improve lateral stability… also note that they were testing the vertical and horizontal surfaces apart from the rest of the plane…
At lower right corner on page 355 you can read:
"Particular care was taken in conducting these testes to eliminate horizontal tail dihedral."
Further:
"The results of the investigation to determine the effects of horizontal-tail span and vertical location on the aerodynamic characteristics of an unswept tail assembly in sideslip…
… variations in both horizontal tail span and vertical location produced a strong influence on the rolling moment derivative of the horizontal tail…"

So, I don't think that the test denies the fact that the wing's dihedral is the main contributor to lateral stability.
If a disturbance causes one wing to drop, the lower wing will receive more lift and the aircraft will roll back into the horizontal level.
Whereas in case of the stab, when it produces lift, the plane dives…
:)
During high AoA, the main wing can completely put a straight h/stab in "dirty" air, making pitch stability and control almost impossible, so the angled h/stab allows for a part of the stab to always be in good clean airflow necessary for a reasonable control/pitch stability even at high AoA.

[Thomas B]

I don't agree.
Lateral stability is achieved by the main wing, it is there the lift is produced.

Besides, neither you nor Sparky have explained why T-tails never use dihedral.
:)

From that NASA report:

The addition of a horizontal tail to the fuselage vertical tail combination produced the greatest increase in the magnitude of the lateral force derivative when located at the top of the vertical tail.......etc, etc

I think that shows that lateral stability can be tweaked using modifications and relocations to the horizontal tail...:)

No one is arguing that the wing configuration has a huge effect and is the main contributor on lateral stability. The point being argued is whether or not the tail can have effects on lateral atability.

Like Sparky mentioned.: it is a LOT easier to tweak an aircraft this way than it is to redo a complete wing.

It is just one of the tools in the toolbox.

[adam_one]

The title of this thread is: Dihedral in the tail
And my argument still is:
Dihedral in the tail is to minimize the adverse effects of the wing wake and/or the prop blast on the stab.
Obviously, during a roll all vertical and horizontal surfaces are involved and have their own influence.

Like Sparky mentioned.: it is a LOT easier to tweak an aircraft this way than it is to redo a complete wing.
Not necessarily, lateral stability is often an important critical factor and should therefore be taken into account at an early stage of the design.

[Thomas B]

The title of this thread is: Dihedral in the tail
And my argument still is:
Dihedral in the tail is to minimize the adverse effects of the wing wake and/or the prop blast on the stab.
Obviously, during a roll all vertical and horizontal surfaces are involved and have their own influence.
Not necessarily, lateral stability is often an important critical factor and should therefore be taken into account at an early stage of the design.

We are going to have to agree to disagree.

Can you post any documentation to support your belief? I have posted some that proves that changes to horizontal tails affect lateral stability.

I would love to see some that supports your claim.

Consider the F-86 Sabre. The low swept wing is well below the horizontal tail, which has dihedral. And, with the F-86, there are no wing mounted turbulence generators...;) Why does it have dihedral? Answer it needed if for lateral stability reasons, or other reasons than wing wash and engine turbulence.

When North American developed the F-86 into the FJ-4 Fury, one side effect of changing the wing and the aerodynamics of the aircraft was that the horizontal tail no longer needed dihedral...;)


If you think all factors can be taken into account early in the design process of every aircraft, even with the best CFD to date, you are dreaming. Even the most modern aircraft get tweaked as the testing returns results.

I have been involved in a lot of wind tunnel testing work with a major aerospace company and let me assure you, no one here thinks tails have dihedral just to minimize wing wake and prop blast effects....:)

There are other reasons and lateral stability is one of them...

[adam_one]

We are going to have to agree to disagree.

Can you post any documentation to support your belief? I have posted some that proves that changes to horizontal tails affect lateral stability.
There's no reference on the Nasa document you posted telling that the stab dihedral would improve lateral stability.
Besides, it's well known that the main wing may "shadow" the horizontal stab during high AoA, depending on how large the wing chord is and how far the wing is located from the stab.

Following are some links I've found about factors contributing to aircraft's lateral stability.
None of them makes any reference to stab dihedral as a significant factor regarding lateral stability:
http://www.eng.um.edu.mt/~andebo/aero/mod_stab.htm
http://142.26.194.131/aerodynamics1/Stability/Page5.html

Incidentally, it is the vertical fin that may affect lateral stability:
http://web.usna.navy.mil/~dfr/flttst/ea417SimLab02.pdf#search='aircraft%20lateral%20sta bility'

Yes, we have to agree to disagree...
:)

[Thomas B]

There's no reference on the Nasa document you posted telling that the stab dihedral would improve lateral stability.
Besides, it's well known that the main wing may "shadow" the horizontal stab during high AoA, depending on how large the wing chord is and how far the wing is located from the stab.

Following are some links I've found about factors contributing to aircraft's lateral stability.
None of them makes any reference to stab dihedral as a significant factor regarding lateral stability:
http://www.eng.um.edu.mt/~andebo/aero/mod_stab.htm
http://142.26.194.131/aerodynamics1/Stability/Page5.html

Incidentally, it is the vertical fin that may affect lateral stability:
http://web.usna.navy.mil/~dfr/flttst/ea417SimLab02.pdf#search='aircraft%20lateral%20sta bility'

Yes, we have to agree to disagree...

:)

I notice you skipped over the illustrations provided of aircraft with and without stab dihedral.... what is your explanation for the F-86 vs the FJ-4?...;) It fits none of your reasoning.

Also, quoting a couple of basic one page beginner's primers on aircraft stability and one on an electronic simulator has nothing to do with our topic at hand. The lateral stability effects of a horizontal tail modification, be it placement on the aft fuse, dihedral, anhedral or other modification is a subtle design element and is not somethnig that is covered in a one page explanation of stabilty.

This link is much more pertinant:

http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19930088931_1993088931.pdf

Conclusion 3 on pages 8 and 9 confirms that negative dihedral on a swept wing flighter aircraft has a noticable effect on lateral stability. I think you can safely infer that positive dihedral will also have an effect on lateral stability.

I might have to run down to the plant library at lunch and find a S and C book that has the more definitive proof that you seem to require...;)

The vertical fin has a well known effect on directional and lateral stability, which is not the topic being discussed here.

[adam_one]

Those sites I've posted are just to confirm that stab dihedral is not the main factor for lateral stability.
Also note that during our discussion so far I've assumed a positive stab dihedral, this because a negative stab dihedral will increase the vertical fin projected area and thus assisting the vertical fin on it's function (directional and lateral stability).
Further, a negative stab dihedral may also put the stab totally or partially away from the wing's wake during high AoA, just in the same manner as the positive dihedral does in case of F-86.

[Thomas B]

I am not saying and have never said that the horizontal tail is a major part of the lateral stability of an aircraft.

I repeat: the positive (or negative) dihedral of a horizontal tail is often used as a tweak, a way to enhance and tune the lateral stability of an aircraft without large and majorly expensive and sometimes impossible wing mods. It is a subtle, but highly useful, tool in the designers toolbox.

As they say, "the devil is in the details". This applies when building superior handling and efficient aircraft, and this is one of the details.

To quote from Dr. Jan Roskam, author of the multi volume text Aircraft Design Vol III, Chapter 5, pg 261:

"The overall lateral stability is usually dominated by the inherent lateral stability designed into the wing. Wing sweep, dihedral angle and (wing)location on the fuselage dominate the magnitude and the sign of the stability derivative. Since the wing is designed by performance and operational considerations, the empennage is often used to fine tune the lateral stability of the aircraft. Examples include the F-4 and the AV-8B.

Note that he did not say directional stability...;)

Here are three views of the F-86 and the FJ-4. As you can see from the front views, there is no wing wash issue. Note the respective horizontal tail dihedral amounts in the front view:

[Thomas B]

Just for fun and consideration, take a look at the Martin P6M Seamaster 3-view below and note the huge amount of horizontal tail positive dihedral, at the top of a very tall tail fin. The surface would not have been in the wing or jet wash if it had been created without dihedral...;)

also, I added a pic of the HP Victor jet bomber. Note the large amount of positive dihedral in the very highly mounted vertical tail.....

[nemonis]

Quick thought: What is a V-tail? It is nothing more than a horizontal with lots of positive dihedral and no vertical fin. In this case, the dihedral is used entirely for lateral stability. Why would it not work to add dihedral to a horizontal tail to enhance directional stability in concert with a vertical fin?

God bless!
Wayne J.

[Thomas B]

Actually, the dihedral in a v-tail is mostly for projected vertical tail area and directional stability.

However, it does affect lateral stability as well.

[adam_one]

I am not saying and have never said that the horizontal tail is a major part of the lateral stability of an aircraft.That's in contradiction with your following posts:
#25
Flight engineer is exactly right on the SAAB 340. You would be amazed at how much effect that amount of stab dihedral has on lateral stability..#27
This NASA report goes into some detail as to how the horizontal tail significantly affects lateral stability, contrary to your opinion...;):
You have to choose, either you consider it as a subtle factor or as a significant one... not both. :)

And as I previously mentioned a negative stab dihedral will increase the vertical fin projected area and thus assisting the vertical fin on it's function (both directional and lateral stability).
This because directional and lateral stability are interlinked (you can make a plane roll by applying rudder only).

Both Martin P6M Seamaster and HP Victor jet bomber have huge wing root chords, and care was taken to put their stab well away from the wing's wake, which might affect them at high AoA.
Those are rather exceptional cases tho.

Besides, too much lateral stability is not desirable, especially fighters have often negative wing dihedral (anhedral), which is used to increase the ability to roll.
So, there's no reason to tweak lateral stability in such cases.

[Thomas B]

I think there is not a contradiction. I think any effect on lateral stability can be a significant effect...remember, the devil is in the details. One some aircraft, the subtle tweaks have a large effect, one some they have less.

The literature states over and over that there is an effect on lateral stability caused by the horizontal tail. Thus any change to it is going to have an effect on lateral stability. I find it interesting that you dismiss a statement published by a Doctor of Aerospace Engineering out of hand...:)

I do not buy your reasoning at all on the Seamaster and the Victor.

Consider the C-5...based on your reasoning, it should have positive dihedral as well in the horizontal tail, as it has a rather large wing cord and a high mounted wing root..and yet it has negative dihedral. Wonder why??..;)

How about the example of the An-124?: Huge wing root on the upper fuse, with some negative dihedral in the wing. The horizontal tail (that has no dihedral) is also mounted on top of the fuse, right in the wing and jet wash.

No argument on the fighter wing anhedral issue. but, based on your position, there is no need to make the stabs have negative dihedral, since they have no effect on lateral stability. Yes,I know the negative dihedral has some directional effects, but this tweak is done for more than directional stability. If you only need directional stability, you have the option of adding ventral or dorsal fin area or ventral strakes.

I note that you ignore the F-86 vs FJ-4 example, since it does not fit your reasoning....

[adam_one]

I think there is not a contradiction. I think any effect on lateral stability can be a significant effect...That's not the issue, the issue is how it is achieved, and besides "significant" is not synonymous to "subtle".
I find it interesting that you dismiss a statement published by a Doctor of Aerospace Engineering out of hand... :)You are misinterpreting your Doctor of Aerospace as well as my statements:
"The overall lateral stability is usually dominated by the inherent lateral stability designed into the wing. Wing sweep, dihedral angle and (wing)location on the fuselage dominate the magnitude and the sign of the stability derivative. Since the wing is designed by performance and operational considerations, the empennage is often used to fine tune the lateral stability of the aircraft. Examples include the F-4 and the AV-8B."
The above does not dismiss what I've been saying, "empennage" refers to both fin and stab together and makes no reference to whether it is a negative or positive dihedral.
Anyhow, I don't think you are interested in a serious discussion so, if you really believe that a positive stab dihedral significantly increases the lateral stability of your plane, just go for it.
I won't.

[Thomas B]

Well, you are wrong about the serious discussion part. I considered this an enjoyable and serious, if slightly lighthearted at times, discussion.

I have rasied numerous issues in support of my position and quoted a number of texts and articles concerning my position. I have done so in a polite and respectul manner and have enjoyed the debate...:) I am not quoting my feelings on this matter, but have worked to find data and research to support this.

I am still waiting for you to produce a text or a paper that provides evidence that the horizontal tail does not influence lateral stabilty.

I have offered serveral examples of aircraft that do not match your position.

I have spoken with several aero engineers here at work, who have agreed that the dihedral in the horizontal tail, be it postive or negative, can be used to adjust the lateral stability of an aircraft.

I have built a goodly number of low speed wind tunnel models, several dozen over the years. These tend to have MANY interchageable parts. It is not uncommon for them to have multiple, sometimes dozens, of plug-on horizontal tails, with numerous dihedral settings, both positive and negative. I know for a fact that these parts are used for more than checking directional stability....;)

Think about a swept wing airliner with some dihedral in the wing and horizontal tail in a bank. The aircraft is going to not only have dihedral effect in the wing, with more lift from the wing that is more horizontal and less on the more upraised wing, it is going to have a similar effect, of smaller magnitude, from the horizontal tail. The horizontal tail will have more down loading on one side, due to the more horizontal surface and less from the more upraised horizontal tail. this will impart a small, but sometimes qute useful change in the overall lateral stability of an aircraft.

If you need a tweak to lateral stabiliy, you can make small changes, large changes or even reverse the dihedral to somewhat reduce or increase the overall lateral stability of the aircraft, as needed. This is easier, as stated above, than changing the wing.

I note that you still have not discussed the F-86 vs the FJ-4 example...;)

[adam_one]

I note that you still have not discussed the F-86 vs the FJ-4 example...;)F-86 was one of the first jet fighters, sweptback wing with dihedral, so lateral stability is hardly an issue with this plane type.
The positive stab dihedral in this case I assume is to prevent the stab from being affected by the wing's wake at high angles of attack.

FJ-4 naval jet fighter, sweptback wing with dihedral, so lateral stability is hardly an issue with this plane type either.
Stab without dihedral like many other prop fighter planes, was probably considered by the designers adequate for the task even at high AoA.
Dihedral reduces the stab's projected area, so the designer has to trade certain capabilities to get others.

[da'rock]

And there is a minor detail about surfaces that're connected to larger parts of the airframe... Sometimes the interference drag caused by the joint is worth considering in the design. Where the stab connects to the fuselage might dictate dihedral. Stick the stab down on the top of a round cross section of fuse and you may have a really messy joint. Try to solve that problem by moving the stab down so it comes out of the "vertical" part of the fuse, and you're probably moving it down into turbulence. If you look at the F86 above, you'll notice the slight dihedral on that a/c. At that time in the evolution of fighter a/c, the designers were working with power plants that didn't exactly have surplus power. Matter of fact, the engineers were scratching for anything they could do that'd help those fighters go faster. So was that dihedral for the dihedral effect, or was it nothing more than a drag reduction trick? or both?

And there is a minor detail that whatever "side" area is created by the dihedral works as additional vertical stab area. So you're sorta getting more vertical surface by dihedraling your horizontal surfaces.

And there is a real possibility that whomever designed those a/c stuck them thangs on like that 'cause they thought they looked purty that way. And maybe looked more modern too... No lie.