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Sep 17, 2019, 07:39 AM
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Originally Posted by xlcrlee
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?
That might have had effect in the design of the DC-10, but probably not for the Handley-Page Victor or for the F-86 Sabre.

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Sep 17, 2019, 07:42 AM
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The T tail on the F104, along with the wingtip tanks, was to counteract the inertial coupling caused by the long fuselage and short wingspan, and incidentally the switch to a T tail meant that the plane had a marked anhedral to compensate for the dihedral effect of the fin+tail complex. On many commercial jets the T tail was adopted to move the horizontal stabilizer away from wing wake and the jet exhaust. Bombers and commercial planes are optimized for cruise, and are usually designed to be overly stable, so it's fairly common to see a lot of dihedral at least on the ones with a low wing.
Sep 17, 2019, 08:12 AM
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BTW, shouldn’t the F-100 Super Sabre’s taiplane be strongly affected by the wing’s wake?



Apparently that was not much of a concern during the design then...

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Last edited by funfly2; Sep 17, 2019 at 08:42 AM.
Sep 17, 2019, 02:07 PM
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Our model pattern airplanes use anhedral to help eliminate a pitch couple when rudder is applied. Would make sense especially on a carrier based aircraft. Imagine trying to line up on the deck with some rudder input and getting a positive pitch couple at the same time. Of course getting the stabilizer at the correct height in the first place would also solve that issue however on a full scale jet putting the stabilizer in line with the jet exhaust would lead to other more serious issues. An aircraft is nothing more then a bunch of flying compromises.
Sep 17, 2019, 03:07 PM
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Quote:
Originally Posted by exf3bguy
Would make sense especially on a carrier based aircraft. Imagine trying to line up on the deck with some rudder input and getting a positive pitch couple at the same time.
Jets don’t use rudder input to line up with the carrier deck. Prop airplane’s sometimes do, but not jets.
Sep 17, 2019, 03:14 PM
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Quote:
Originally Posted by funfly2
BTW, shouldn’t the F-100 Super Sabre’s taiplane be strongly affected by the wing’s wake?
It most certainly is. Another good example of an airplane with very little vertical offset between the wing and stabilator is the T-38 / F-5. The T-38 flight control system mechanically commands the stabilator to deflect trailing edge down when the flaps are extended in order to prevent a significant nose-up pitch.
Sep 17, 2019, 03:15 PM
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What do they do if landing on a paved runway with a cross wind? My example of a carrier landing my have been wrong however the aerodynamics behind my example concerning models is not.
Sep 17, 2019, 03:18 PM
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Many fighters simply land crabbed in a crosswind. At touchdown the gear is strongly directionally stabilizing.

From the crosswind landing section of the F-4J flight manual: “Make a firm touchdown while maintaining the wings-level crab. Touching down in the crab results in a continuation of a straight track down the runway.”
Last edited by ShoeDLG; Sep 17, 2019 at 03:28 PM.
Sep 17, 2019, 03:19 PM
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I guess that the tailplane gets blanked by the wing at certain AoA on most planes .
Especially those with the tailplane placed higher than the wing.

Sep 17, 2019, 06:32 PM
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Superstall only happens when the stabilizer is in the wake of a stalled wing. There is significant downwash behind non stalled a wing, and it's mostly this downwash that can cause issues, as in the T38 example that ShoeDLG mentioned earlier. This also means that T tails are often more susceptible to superstall than just a high or cruciform tail arrangement. I guess that the ventral fins on the late F104 versions and on the Learjets are also there to force the nose down in a superstall situation.
Sep 17, 2019, 07:13 PM
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Disagree. Think pioneering X-15 (lower vert sub-section dropped off before touchdown).
Sep 18, 2019, 04:45 AM
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But the X15 had a single ventral fin, as did the early F104 revisions, with no side projection. And it had no T tail, so no real issues with superstall. It also had an anhedraled horizontal stabilizer, probably to smooth out the interaction between the wing wake and the elevator. And the fin was designed for a ballistic flight regime, all moving with a very wide trailing edge. The ventral fin was for all purposes the fin of a rocket, and it was there to reduce the dihedral effect of having a single tall vertical fin and the risk of the fin being blanked at high AOA. A similar situation is that of the Mig-23 Flogger, with its folding ventral fin. But the two tail strakes on the Learjet, late F104 and other T tailed planes act as a fences on the rear fuselage, and contribute a strong pitch-down effect at increased AOA.

[edit] I am specifically referring to the extra ventral fins on the F104S-ASA:
Last edited by Brandano; Sep 18, 2019 at 04:58 AM.
Sep 18, 2019, 05:57 AM
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Originally Posted by Brandano
But the X15 had a single ventral fin, as did the early F104 revisions, with no side projection. And it had no T tail, so no real issues with superstall. It also had an anhedraled horizontal stabilizer, probably to smooth out the interaction between the wing wake and the elevator. And the fin was designed for a ballistic flight regime, all moving with a very wide trailing edge. The ventral fin was for all purposes the fin of a rocket, and it was there to reduce the dihedral effect of having a single tall vertical fin and the risk of the fin being blanked at high AOA. A similar situation is that of the Mig-23 Flogger, with its folding ventral fin. But the two tail strakes on the Learjet, late F104 and other T tailed planes act as a fences on the rear fuselage, and contribute a strong pitch-down effect at increased AOA.

[edit] I am specifically referring to the extra ventral fins on the F104S-ASA:
It makes sense.

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Sep 18, 2019, 10:14 AM
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the smart way to add essential rear underside vertical area


Sorry I didn't "fill in the blanks":

1. These type planes which get subjected to "wild" maneuvers need SUB-FIN area, If it wasn't necessary, it would not be there!

2, Because unlike the pioneering X-15 these planes must not only rotate to take off, but land nose-high as well

Any good engineer with half a brain can easily figure out that extending two splayed subfins somewhat ahead of the rear of the body is much more practical than dropping off a subfin.
Sep 18, 2019, 01:27 PM
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Quote:
Originally Posted by xlcrlee
Sorry I didn't "fill in the blanks":

...If it wasn't necessary, it would not be there!
Sure, and the X15 had a ventral fin because it was considered necessary too...

I think that the purpose of those ventral and sub-fins is to improve directional stability during certain flight conditions, such as (but not limited to) when the tail gets blanked or partially blanked by the wing during certain high AoA.

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