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Sep 13, 2019, 02:38 PM
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Discussion

Why has F-4 tailplanes with anhedral but has dihedral on the wing tips?


Why didnít they keep the original design as shown below?

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Sep 13, 2019, 05:01 PM
Registered User
The tail fin has been shortened to save space in aircraft carrier. To maintain directional stability, tailplane got anhedral to gain more vertical surface on the back. Dihedral on wingtips counters the anhedral on the tail.
Sep 13, 2019, 09:07 PM
Red Merle SJ VIII
Curtis Suter's Avatar
Yep, there is a tail down force on the tail, you can see the airfoil is upside down so it's actual dihedral in the proper direction for stability.

Plus it looks better!

Curtis
F4G Wild Weasel Avionics Maintainer
Sep 14, 2019, 04:33 AM
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Quote:
Originally Posted by Curtis Suter
Yep, there is a tail down force on the tail, you can see the airfoil is upside down so it's actual dihedral in the proper direction for stability.

Plus it looks BadAss!

Curtis
F4G Wild Weasel Avionics Maintainer
Fixed your spelling mistake
Sep 14, 2019, 08:49 AM
AndyKunz's Avatar
The original version looks like an oversize A-4.

Andy
Sep 14, 2019, 08:51 AM
Registered User
Hmm not quite. Anhedral is anhedral no matter which way the wing or tail is lifting. If there's sideslip, one surface will have a different angle compared to the other, and in the case of anhedral that means the plane will tend to roll OUT of the sideslip. That's why it has to be compensated by the dihedral on the wing tips. Davidz90 is right, the early wind tunnel models used straight surfaces, but the small fin and small side projection of the rear fuselage meant that the original design wasn't sufficiently directionally stable. Cranking down the elevators and up the wingtips increases the side area projection behind the centre of mass without having to redesign the entire plane. And since the wing was cranked they took the opportunity to add a dog-tooth to improve high AOA behaviour too.
Sep 14, 2019, 02:09 PM
B for Bruce
BMatthews's Avatar
I had not thought about the wing cranking adding yaw stability. But with the strongly swept wing planform that makes sense. Same with the drooping of the tail.

I wonder if having the tips of the stabilators sitting low like that would also improve their exposure to less disturbed airflow during high angles of attack?
Sep 14, 2019, 02:42 PM
Registered User
IIRC there was definitely some interaction between the elevator and the wing wake that the added anhedral also took care of. Not quite superstall, but buffeting at specific AOA's.
The F4 was a slightly flawed design that was turned into an excellent plane via some late fixes. But it was never meant to be a dogfighter anyway. It could hold its own, but it was really meant to engage targets at long range rather than enter into manoeuvred combat. The early design wasn't even meant to carry a cannon, and the cannon was only re-added after the experience in Corea showed that dogfights were often unavoidable when the enemy had fast and agile planes.
Last edited by Brandano; Sep 14, 2019 at 02:49 PM.
Sep 15, 2019, 06:04 PM
AndyKunz's Avatar
Sounds like the reasoning that generated the LWF program giving us the F-16 and early F-18...

Andy
Sep 15, 2019, 09:17 PM
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ShoeDLG's Avatar
One feature of a stabilator with significant “negative” dihedral is that creating a rolling moment using differential stabilator will create a tendency to yaw in the direction of the roll... In other words differential stabilator will create “proverse yaw”, which can be a desirable feature.
Sep 16, 2019, 08:19 AM
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Quote:
Originally Posted by AndyKunz
Sounds like the reasoning that generated the LWF program giving us the F-16 and early F-18...

Andy
I guess that a good deal of that also comes from the experience with the F5 in the Southvietnamese aviation. Though both the F16 the F18 overgrew their initial concept.
Sep 16, 2019, 09:05 AM
An itch?. Scratch build.
eflightray's Avatar
The anhedral is to get the tailplane as low as possible out of the wing wake.

Moving the tail plane higher can put it into the wing wake and can cause a deep stall at high angles of attack.

Ok so I Googled.
Sep 16, 2019, 11:47 AM
AndyKunz's Avatar
Quote:
Originally Posted by Brandano
Though both the F16 the F18 overgrew their initial concept.
I can see it now at the Lockheed Martin conference room:

"What are we going to name this new plane?"

"Well, 16 + 18 = 34. Let's call it the 'F-34!'"

"We can't do that. We have to make it MORE than just those two planes."

"F-35 then!"

And all the heads bobble in unison.



Andy
Sep 16, 2019, 01:53 PM
Registered User
Thread OP
Quote:
Originally Posted by eflightray
The anhedral is to get the tailplane as low as possible out of the wing wake.

Moving the tail plane higher can put it into the wing wake and can cause a deep stall at high angles of attack.

Ok so I Googled.
Maybe, however some have got it the other way around (dihedral) ...





I think that explanation makes sense when the tailplane is placed at the same level as the main wing, like with F-16:



But thatís not the case with the F-4 Phantom:

Last edited by funfly2; Sep 16, 2019 at 02:58 PM.
Sep 17, 2019, 03:21 AM
Registered User
Disregarding the silly crescent-winged bomber (at a time when T-tails were the rage), do you think the exact position of jet efflux has any effect on these design decisions?


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