what advantage does anhedral give on a horizontal stabilizer (other than looking cool)?

What advantage would it give with minimal anhedral on a f3a/3d crossover plane like the new matrix from CA?

What about a plane with more extreme anhedral like an F4?

Just a stab in the dark here but I am thinking that it might help in knife edge flight by providing some lift.

I always wondered that too. For the Curare flown by Pretner back in the early 1980's was pretty sucessful. Was a discussion about this feature in the Pattern forum a good two months ago.

I am working up parts and new plans based upon some Russian copy. The Curare had an even larger amount, something like nine degrees each side.


Wm.

http://www.rcuniverse.com/forum/m_36...tm.htm#3619683

I think this might be the post you are talking about. I remembered reading it but it didnt seem like anybody knew anything for sure or not.

If its a gimmick I can see it on a model plane like the matrix (it does look sharp), but there must be a purpose for it if it was incorporated into the F4.

I figured some of you rocket scientist that post in this forum could get their giant screen calculators out and whip out a formula to tell me why its done.

An engineer once told me that the anhedral on the F-4 tail was to help with low-speed yaw stability. The tail of the F-4, like most airplanes, generates negative lift, so anhedral on the horizontal tail would actually work like dihedral for a wing (it's stabilizing). This seems to imply there is a tradeoff with yaw stability while inverted, which of course wouldn't be a problem for the F-4.

As for RC aerobatic airplanes, especially so-called 3D, I doubt it has any benefit. The anhedral adds effective side area to the tail, which is the same as adding more fin area. It does add it down low, however, which might help knife-edge tracking if it is done properly, but that's only theory, and I believe that was the thinking behind the Curare tail. I remember some discussion about it in the '70s (or was it the '80s?). It seemed to work, but then so do many other designs without anhedral. You would probably have to be an aerodynamicist or an avid experimenter to design it so there is any discernible benefit.

If you think it looks cool, a little bit probably will not hurt. Or if you think you need to redistribute the vertical stabilization area lower, it might help.

As for adding maximim side lift capbility, there would be no benefit at all, since adding tail area is stabilizing. Good knife-edge lift has a lot to do with the proper balance between area ahead of the CG and area behind the CG. All else equal, adding effective fin area, without changing anything else, will make it harder for the rudder to yaw the airplane and generate side-lift. Also, the length of each horizontal stabilizer/elevator would have to be multiplied by the secant of the anhedral angle to keep the same pitch effectiveness, which would of course increase weight slightly.

That is correct from what I remember my father (who helped design the aircraft) telling me.

That's way cool. Was he the guy who decided to try the Coke bottle shape to alleviate the standing wave drag on the fuselage?

That was a FAST airplane. Even some modern fighters have trouble keeping up with it flat out in a straight line. I used to love the disticntive howl those engines made when I and they were still at Hickam AFB.

LOL! Nope. He WAS the guy who developed the variable geometry inlets for it though (the modification that, eventually, sold the Air Force on the plane, and was later incorporated into the Eagle)

Yeah...I used to confuse friends in my early days in the military...we'd be out somewhere on some exercise or something, and I'd say "Hey...here come some Phantoms", and sure enough....zip, there went a couple. They'd always ask how in the **** I could have known that before seeing them...I'd tell them "Nothing in the world sounds like an F-4 Phantom." it was true then, and is STILL true.

Ahnedral on horizontal stab helps also on recovering from spins. As mesae said, low speed response of the horizontal stab is also leveraged with this configuration. I'd rather believe that the Curare had this configuration mostly due to spin recovering than other thing. Undoubtly for me, there is no advantage at all while regarding to knife edge flying...

Gringo: la cosa es con la recuperación del tirabuzón y nada más... ¿te compraste un Matrix? OJO CON EL ALA, NOSOTROS ROMPIMOS 2 AL HILO EN VUELO RECTO Y NIVELADO EN EL PRIMER VUELO DEL AVIÓN!!!!!!! Y NO FUIMOS LOS UNICOS!!!!!!!

(Sorry about the spanish message to Gringo flyier)

Regards
Guille

I was considering writing something speculative about spin recovery that disagrees with the above: I would predict that anhedral would tend to delay upright spin recovery by reducing the amount of the rudder bottom exposed the the free airflow (the major anti-spin force). The opposite would be true in recovering from a negative spin. This compares two airplanes that are identical except that one has an anhedral stab, and the other doesn't. If you raised the attach point of the stab to compensate you might be able to mitigate this effect.

Another thing to consider is that few decent propeller-driven aerobatic airplanes have trouble recovering from spins, anhedral stab or not. Furthermore, any supposed advantage that might be gained by anhedral during a positive spin would be turned to a liability by the opposite effect during a negative spin, or vice versa. Since aerobatic airplanes need to recover precisely from both positive and negative spins, it seems unlikely that anhedral has as much to do with spin recovery as it does with appearance, except, as I mentioned in the earlier post, to re-distribute the effective center of horizontal tail area lower.

Again, this is all speculation based on my knowledge of physics and I have no experimentation to back any of this up. I might be missing something (like a brain). Perhaps Guille could illuminate me.

Anhedral wings are more efficient when producing positive lift because they reduce spanwise flow, converting more of the gas's overall momentum change to lift, rather than vortex generation. This would imply that while the anhedral stab is producing negative lift, while the airplane is positively loaded, it would be less efficient than a zero-hedral stab, and the opposite would be true while flying inverted or while the wing is negatively loaded. You can't get something for nothing. Whatever you gain from an anhedral stab in upright or inverted flight, you will lose during the opposite condition.

I think, in general, adding anhedral to a stab will increase yaw stability slightly, which isn't usually a good thing for an aerobatic plane, unless the fin/rudder/aft fuselage area is reduced proportionately to compensate.

I flew the F4C & D in the US and the F4E in southeast Asia, Laos & North Vietnam.

We were told in training that the anhedral was to keep the stab in good air at high angles of attach. If you'll look at the prograssion of McDonnel aircraft before the F-15, all of them had the same basic design, 2 engines on either side of the center fuselage with an upswept rear fuselage. The engines on the FH Phantom and F2H Banshee were small so it isn't very pronounced. Look at the XF-88 Voodoo and the F3H Demon. Both had the upswept rear fuselage and low stabs. Their next plane was the F-101 Voodoo. It had a high tail, trying to get it in good air. At high angles of attack, the stab would be blanked out and the plane would go crazy. They finally put a stick shaker on it to warn the pilot of very high angles. On the F4, they lowered the stab back down and, because of the upswept rear fuselage, they couldn't get it and lower so they put anhedral in it.

The stabs on the C & D models were different from the E model. For take off in the C & D, you held full back stick and went to full AB. When you got control speed, the nose came up, you eased off the back stick and were airborne. This was different to me who had been used to flying planes off the ground. On landing you ended up with the stick against the back stop just prior to touchdown. On the E, and the Navy's J, they added slotted leading edges to the stab and it did lift downward. These models had very good pitch control at low speeds. You could roll the plane on.

Here's a trivia answer, the original designation and name of the USAF F-4 was the F-110A Spectre. In 1962 DOD forced both services to convert to a common aircraft designation system. The UASF had been numbering them in order since the 1920s and the Navy used a letter for the manufacturer. The F-110 became the F-4C and the F-4H-2 became the F-4B, as I recall.

Guille,

No compre todavia. Vuelo en el club en Formosa y hay 4 o 5 de nosotros que quieren comprar.

¿Como se puede resolver este problema con el ala?

We really need a bowing smiley.

PM me some time if you're ever interested in talking Phantoms, or swapping some memorabilia.

First of all, anhedral is ALWAYS DESTABILIZING unless the aircraft is inverted. As I understand it, the anhedral on the horizontal tail was employed due to a flutter problem at high speeds. The anhedral was added to get the tail out of a flow field of a certain flight condition. I'm not sure if this is the actual reason, but believe me the anhedral was not added to make the aircraft more stable laterally. Adding anhedral to the horizontal tail would destabilize the aircraft laterally, which does 2 things: (1) - It makes the aircraft more maneurverable in roll and (2) - it increases dutch roll damping.

My point is that when discussing stability, you must be careful. Adding anhedral increases dutch-roll damping, which can be viewed as increasing yaw stability, but this would come at the cost of decreasing lateral stability. When discussing aircraft stability the two primary modes are longitudinal and lateral. Directional stability (yaw) comes as a distant third. Therefore, I would seriously doubt that someone would try to decrease lateral stability just to get better dutch-roll performance. Yaw dampers work just as well.

It seems you are saying that anhedral on a horizontal tail is always destabilizing. This can't be true because it depends on whether the tail is generating negative, positive, or zero lift. While the tail generates negative lift, as with an F-4 in approach configuration, anhedral is stabilizing for the tail, since aerodynamically it is dihedral.

Also, stability calculations for RC models can generaly disregard stick-free considerations, since the "stick" is always fixed, from a stability standpoint (servos always hold commanded position unless overpowered), and yaw dampers are seldom necessary, since stick-fixed is more stable than stick-free, and dutch roll is therefore less frequently a problem than with full-scale.

I think it's quite obvious (that) on the F4 the stab had to make "way" for the two "burners" in the back ... but then the stab would be way too high, so the anhedral brings the tips back down (logical design solution) also notice the unpaited (natural metal) portion of the stab& tail near the burners on any paint scheme>>> the paint would probably peel off due to the heat factor...[X(]

I love those Phantoms!!!!!! [sm=thumbup.gif]

Phantoms are cool.

I don't follow your argument. If the anhedral had anything to do with the burners, why would they swing them down closer to the exhaust, then make them heat resistant, rather than just leave them up there horizontal at the attach point, unless there was a compelling aerodynamic reason for the anhedral?

Look at the pics again.... the tips of the stab are not "heat treated" > its the center section which is in the line of "fire"... thus was moved up as far as could be... obviously not far enough so it had to be heat resistant... this is the reason the attach point is so high and the tips of the stab were brought down closer to the thrust line... thus the anhedral

I never implied that the tips were heat resistant. But angling the tips down brings the area close to the fuselage closer to the exhaust. And why is bringing the tips of the stabs closer to the thrust line a more compelling reason for the anhedral than low-speed (high AOA) yaw stability due to an aerodynamic dihedral effect (which was told me by an engineer who had first-hand knowledge of the F-4 project, and corroborated by the son of another engineer with similar qualifications)?

The tail of the F-4 definitely lifts negatively at least some of the time (low speed at least, unless the designers wanted to make the tail less efficient): notice the negative camber and the negatively operating leading edge slots. This means the tail is operating with a pronounced aerodynamic dihedral effect, which by definition would provide some positively stabilizing effect. It also may bring the tail into some "cleaner" air, and there may also be an effect involving entrained airflow influenced by the exhaust, etc.

Furthermore, if it is so desirable to have the stabilizers as close to the thrust line as possible, which you seem to be implying, then why are there so many T-Tail jets with otherwise relatively similar configurations?

Don't relate the aerodynamic configurations of full-scales which have to meet vastly different criteria for performance to most model sport flying!
There just isn't a 1:1 correspondence!
The anhedral on the F-4 -AND- the tip dihedral -AND- the motor location -AND- the need to land on a carrier-AND- the Mach 0.2 to Mach 2.0 flight regime dictate that unique shape.
A model of an F-4 doesn't need any of those aerodynamic things to fly within its limited flight regime.

It looks like you were replying to me. Are you countering something I wrote? Cuz I was countering paradigm's post. He's the one who brought up yaw dampers first .

First of all, there is no such thing as aerodynamic dihedral. The direction of lift - up, down, or none - has nothing to do with dihedral effect. Wing, or horizontal tail dihedral adds lateral stability in the form of C_l_beta (or the derivative of rolling moment coefficient with respect to sideslip). When the a wing or h. tail has dihedral and is subjected to a sideslip, the lateral moving air creates a normal force on the wings. There is a component of this normal force upward on the right wing and downward on the left for dihedral and the opposite for for anhedral assuming the sideslip is positive (coming from the right). Therefore, the force couple created by dihedral will create a rolling moment to the left (away from sideslip - stabilizing), while anhedral will create a rolling moment to the right (into sideslip - destabilizing).

When I say that anhedral is always destabilizing, I mean in the lateral mode. As I said before, you could argue that adding anhedral increase the dutch-roll damping which increases directional stability, but that is always secondary to lateral stability.

And in terms of whether a horizontal tail creates a force upward or downward. For a longitudinally stable aircraft (c.g. is ahead of aircraft aerdynamic center) the horizontal tail will ALWAYS carry a downward force in straight and level flight.

In terms of stick fixed vs. stick free, you are correct in terms of stick-fixed being more stable longitudinally and directionally, but that doesn't mean that dutch roll can be ignored. Dutch roll is however much less restrictive in r/c aircraft because there are no passengers on board to get sick.

Another reason for anhedral on horizontal tail might be to increase the projection area of the vertical fin, improving directional stability.

I was referring do dihedral effects in terms of positive roll correction with sideslip. A high-wing configuration with no geometric dihedral (or even slight geometric anhedral) still can show a positive roll stability effect. Semantics, perhaps.

According to Dr. Warren Phillips, horizontal tails do not have to lift negatively at all times to avoid divergent static pitch stability. They usually do over most or all the approved CG envelop for most certificated light aircraft, but this is not absolutely necessary. There is a discussion of this very subject on and about page 351 of Mechanics of Flight. For efficiency, it is desireable for the tail to be generating zero lift at equilibrium. For meeting Part 23 certification requirements, the tail will usually lift negatively to provide the desired dynamic stability. So-called 3D models do not need (or want) positive dynamic pitch stability and can be designed for zero tail lift at equilibrium, and if the tail volume is great enough, a high degree of static stability and control authority are achieved together. You can go to Amazon.com, look up the above book (by Phillips), and search inside to p. 351 to see what he has to say about it.

Also, I did not write that dutch-roll can or should be ignored. I did write that stick-free considerations can be disregarded, since RC models do not operate stick-free.

(added qualification with emphasis)

The longitudinal stability criterion for dynamic stability is actually the same as static stability. And it is that the aerodynamic center of the aircraft must be behind the c.g. of the aircraft. Now, this means that there will be a negative (nose-down) pitching moment simply caused by the force moment of the lift and weight. Now this moment has to be counteracted by something, and that is usually the horizontal tail.

You are correct that a more efficient design is to place the c.g. at, if not behind the aerodynamic center as this reduces the so-called trim drag penalties. However, this can only be done if a stability augmentation system is used.

R/C aircraft can be designed with lower stability than manned aircraft. In fact there have been tests where large, manned aircraft have been controlled with the c.g. behind the aerodynamic center without augmentation. However, this only works if the aircraft has a large pitching moment of inertia (i.e. the time to double amplitude is high, so the pilot has time to respond). With small r/c aircraft I seriously doubt that anyone could control the aircraft with the c.g. behind the aerodynamic center.

YOU SAID:
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If the anhedral had anything to do with the burners, why would they swing them down closer to the exhaust, then make them heat resistant, rather than just leave them up there horizontal at the attach point...
[/quote]

if i understand "why would they swing them down closer to the exhaust, then make them heat resistant" you mean the tips are swinging down BUT they are NOT heat resistant because they are pointed away from the fuse & the hot zone... don't follow what youre saying... yes the middle part is heat resistant & even if the stab would be horizontal, the middle part would still be in the HOT ZONE ...