H-Stab Anahedral
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@Owain. Your second answer could be used to describe a dihedral as well...correct? In other words, dihedrals help with roll stability in a slip condition, so, what would be the difference between the dihedral and anahedral other than looks?
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Only references I can find for the reasons for the change to anhedral in the final design
The story from a senior TP was that the aircraft required the change to facilitate the final part of the touchdown sequence whilst coming aboard the boat. Can't recall any detail beyond that.
The story from a senior TP was that the aircraft required the change to facilitate the final part of the touchdown sequence whilst coming aboard the boat. Can't recall any detail beyond that.
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F-4 stab anhedral was for aesthetic reasons
The F-4 design was preceded by the F3H-G mock up. Photo below. The wing had no dihederal, nor the tailplane, as was also the case with the preceding in service F3H "Demon". The G never went beyond mock up stage. Within McDonnell the G went by the designation Model 98B.
After much wind-tunnel testing, it was found that the fighter would encounter severe stability problems at high speeds and would as a result probably be limited to speeds below Mach 2. At issue was the rolling effect of the outer wing panels in yawed flight. In order to correct the problem the tailplane was given 23° of anhedral, an increase from the original 15°. This gave the necessary degree of stability, but still left the tailplane free of the jet exhaust and turbulent wing wake. So became the first flying Phantom, the YF4H-1 (Model 98S).
Last edited by Brian Abraham; 30th Oct 2013 at 10:44.
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aviationluver
Apologies in advance if this seems too complicated or 'nerdy', but there are some fairly complicated aerodynamics involved and simple explanations of complex problems often lead to wrong solutions!
"Roll stability" is too indefinite a term for me as there are three lateral aircraft motion modes and they all involve roll. Increasing dihedral of either the wing or the HTP will stabilise the spiral stability and destabilise the dutch roll - anhedral vice versa. From what I have read, in most cases one wants to improve the dutch roll, for which tail anhedral would be a possibility, but putting the special case of the F4 aside, it would probably be easier just to fit a yaw damper, so I think introducing lateral stability into this discussion is a bit of a red herring.
In the longitudinal case one needs to understand that the downwash behind the wing is not constant. It is a maximum near the wing wake, but above and below that the downwash falls the further away one gets.
So if the wake lies below the HTP (cruise AoA range) putting anhedral on the HTP will bring the outer part of the HTP into a larger downwash. At high AoA the wake will move upwards relative to the HTP and if it lies above it then anhedral will reduce the tail downwash.
So "Yer pays yer money and yer takes yer choice". GUESSING from what Croucher wrote, they might have been attacking a high AoA problem, possibly some sort of pitch-up near the stall, but that is a pure guess.
Your second answer could be used to describe a dihedral as well...correct? In other words, dihedrals help with roll stability in a slip condition, so, what would be the difference between the dihedral and anahedral other than looks?
"Roll stability" is too indefinite a term for me as there are three lateral aircraft motion modes and they all involve roll. Increasing dihedral of either the wing or the HTP will stabilise the spiral stability and destabilise the dutch roll - anhedral vice versa. From what I have read, in most cases one wants to improve the dutch roll, for which tail anhedral would be a possibility, but putting the special case of the F4 aside, it would probably be easier just to fit a yaw damper, so I think introducing lateral stability into this discussion is a bit of a red herring.
In the longitudinal case one needs to understand that the downwash behind the wing is not constant. It is a maximum near the wing wake, but above and below that the downwash falls the further away one gets.
So if the wake lies below the HTP (cruise AoA range) putting anhedral on the HTP will bring the outer part of the HTP into a larger downwash. At high AoA the wake will move upwards relative to the HTP and if it lies above it then anhedral will reduce the tail downwash.
So "Yer pays yer money and yer takes yer choice". GUESSING from what Croucher wrote, they might have been attacking a high AoA problem, possibly some sort of pitch-up near the stall, but that is a pure guess.
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I'm sure Brian has the correct answer - TP anhedral will reduce the rolling moment with yaw.
The story of the F4 I heard many moons ago was it started off looking like a real aeroplane and then they bent all sorts of bits up and down to make it look as it does now.
That's the problem with letting the Navy play with our aircraft....................
The story of the F4 I heard many moons ago was it started off looking like a real aeroplane and then they bent all sorts of bits up and down to make it look as it does now.
That's the problem with letting the Navy play with our aircraft....................
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Tut, tut BOAC. 
The F-4 was designed to meet a Navy requirement. Have to give the Air Force credit for recognising a good machine when they clapped eyes on it though. Bit like the Skyraider, A-7 and Sidewinder really.
The F-4 was designed to meet a Navy requirement. Have to give the Air Force credit for recognising a good machine when they clapped eyes on it though. Bit like the Skyraider, A-7 and Sidewinder really.
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Yep, the design grew out of the Demon and Banshee and at one stage ( as the F3H-G/H, which AFAIK only existed as a mock up) it did indeed have a straight T- tail.
Can you shed some light wiggy?
The WWII Beaufighter sported both a straight tailplane, and one with dihedral, depending on mod status. As the pilot notes state, "There are a number of aircraft which have not yet been fitted with a dihedral tailplane. These aircraft will be found to be considerably unstable fore and aft under all conditions of flight and particular care must be taken at the lower speeds and at heavy loads."
Interestingly the pilot notes carry the caution "cloud or night flying is not advisable as handling becomes difficult below 180 m.p.h." Yet the aircraft was used as a night fighter. Have to admire the crews, not only did they have to contend with the enemy, but also their own aircraft.
Brian
You're right, my mistake. I've just looked again at the source and I see I missconstrued what was written...
(never post in a hurry after a long night............)
I'm not aware of any of the design iterations having a T tail,
(never post in a hurry after a long night............)
