Some question about ventral fin ?

[garrya]

If any one not know yet , ventral fin is the little fin below fuselage. What are their purpose ? why some aircraft like Su-27 , YF-12 , F-16 , Mig-21 , Mig-31 have them




while some others like F-35 , F-15 , F-18 , Mig-29 , Mirage , SR-71 doesnt have them?

[Dragon029]

Yaw stability, particularly while at high angles of attack; why some jets need them and others don't is a tad complex, but it ultimately comes down to them being extra-unstable (nor not) while at high angles of attack, sometimes because they have a large side profile up the front and not enough rudder, sometimes because their rudder doesn't get clean air coming over the wing or elevators, etc.

Not really sure how this is relevant to the F-35 though.

[garrya]

Yaw stability, particularly while at high angles of attack

Then why would something like YF-12 has ventral fin while something like F-18E , F-35 doesnt ?.

why some jets need them and others don't is a tad complex, but it ultimately comes down to them being extra-unstable (nor not) while at high angles of attack, sometimes because they have a large side profile up the front and not enough rudder, sometimes because their rudder doesn't get clean air coming over the wing or elevators, etc

.
I find it very strange :
At first i thought that only aircraft flying faster than Mach 1.8 need ventral fin since all aicraft that slower than Mach 1.8 such as Rafale , Typhoon , F-35 , F-18 doesn't have them. But then i looked at Mig-29 and F-15 both can fly at speed up to Mach 2.5 and neither got ventral fin

Then i speculate that only aircraft with single vertical stabilizer would need ventral fin ( Ex: F-16 , Mig-21 , J-17) then i looked at Su-27 , Mig-31 both have twin stabilizer and both get ventral fin

Not really sure how this is relevant to the F-35 though.

Just caused it does not have it so i wonder what sort of aerodynamic quality we could expect from the lack of ventral fin

[KamenRiderBlade]

Fly By Wire takes care of lots of minor things.

All the aircraft that you listed, which are Fly By Wire, which are more traditional mechanical linkages?

[garrya]

Fly By Wire takes care of lots of minor things.

All the aircraft that you listed, which are Fly By Wire, which are more traditional mechanical linkages?

Still not adding up
F-16 , Su-27 , F-18 , Mig-35 are FBW : 2 of them has ventral fin , 2 of them dont
F-15 , Mig-21 doesn't have FBW : 1 one them has ventral fin , the other doesn't

[KamenRiderBlade]

Fly By Wire takes care of lots of minor things.

All the aircraft that you listed, which are Fly By Wire, which are more traditional mechanical linkages?

Still not adding up
F-16 , Su-27 , F-18 , Mig-35 are FBW : 2 of them has ventral fin , 2 of them dont
F-15 , Mig-21 doesn't have FBW : 1 one them has ventral fin , the other doesn't

Then I have no idea, where's our resident Aerodynamicists?

[Dragon029]

Because aircraft aren't built equally; the Su-27 doesn't have the same CG / CP relationship on the vertical axis as the F-16, or the F/A-18 or MiG-35, etc. Put an F-16 and an F/A-18 at 70 degrees angle of attack, at a low altitude and only one of them will survive it. Fly-by-wire performance and capabilities also differ between aircraft, some have faster update rates, some use more advanced modelling to use control surfaces in different ways, etc. There's also the matter of how fast a control surface can move, as well as how large they are, etc.

Ultimately, jets have ventral fins because they need them for yaw stability.

[sprstdlyscottsmn]

Ultimately, jets have ventral fins because they need them for yaw stability.

Dragon has half the answer. They have them, if they have them, for directional stability. The part he is missing is that they need the added stability in different regimes. Let me roll through a progressive look at the listed aircraft.

F-16/MiG-21 - Single fin blanked out from good airflow during maneuvers (already discussed) needs additional stability for higher AoA.
F/A-18/MiG-29/F-35/F-22 - outwardly canted twin fins biting into airflow vortex from LERX/Chines does not need them.
Su-27 - twin fins are not canted outward, still needs them but they are reduced in relative size.
F-15 - twin fins are not canted outward and has no LERX, loses Yaw control at high AOA, could use them but dash speed was the priority in the design
MiG-31 - Mach 3 speeds required small vertical tail, tall fore body required additional stability at Mach 3
SR-71 - Went with all moving tail, low AoA aircraft, very slim fore body and full strakes, no ventrals needed
YF-12 - taller fore body and shorter strakes than the SR-71 mean more sideload can be applied to the nose, the short verticals need some help.

In short, everything is a compromise.

[sferrin]

SR-71 - Went with all moving tail, low AoA aircraft, very slim fore body and full strakes, no ventrals needed
YF-12 - taller fore body and shorter strakes than the SR-71 mean more sideload can be applied to the nose, the short verticals need some help.

I've heard this many times. How do the chines have any effect whatsoever in the yaw plane? The YF-12A had the 3- ventrals because of the increased profile of the radome up front. The trainer Blackbird had two ventrals due to the stepped canopy up front.

What I find interesting about the ventral fin thing in general is how in the 60s it was common to keep a single vertical and add ventrals, even if they needed to be folding, whereas in the 70s and beyond they just went with two tails.

[steve2267]

SR-71 - Went with all moving tail, low AoA aircraft, very slim fore body and full strakes, no ventrals needed
YF-12 - taller fore body and shorter strakes than the SR-71 mean more sideload can be applied to the nose, the short verticals need some help.

I've heard this many times. How do the chines have any effect whatsoever in the yaw plane? The YF-12A had the 3- ventrals because of the increased profile of the radome up front. The trainer Blackbird had two ventrals due to the stepped canopy up front.

You can think of flight at an angle of sideslipe as having two components: flow along the longitudinal axis and flow along the yaw axis (or in the yaw plane). Considering only the sideslip case, i.e. ignoring angle of attack (i.e. either α=0 or very low α), the cross flow for the YF-12A nose / radome will be close to flow around a circular cross section. Cross flow for the SR-71 nose will be a circular cross section but with the addition of the chines, which are substantial. That is, the ratio of the chine height to the circular diameter is pretty sufficient. Without going and finding my copy of Hoerner, I feel reasonably confident in stating that the drag coefficient for a circular cross section is much higher than for a circular cross section with the chins (along a symmetrical plane with the flow). Since at the nose of the aircraft, the moment arm to the CG of the aircraft will be (most probably) the largest possible, a small change in cross-flow drag coefficient could result in an appreciable change in the vehicle yaw coefficient. So you need the ventrals at that point.

[sferrin]

You can think of flight at an angle of sideslipe as having two components: flow along the longitudinal axis and flow along the yaw axis (or in the yaw plane). Considering only the sideslip case, i.e. ignoring angle of attack (i.e. either α=0 or very low α), the cross flow for the YF-12A nose / radome will be close to flow around a circular cross section. Cross flow for the SR-71 nose will be a circular cross section but with the addition of the chines, which are substantial. That is, the ratio of the chine height to the circular diameter is pretty sufficient. Without going and finding my copy of Hoerner, I feel reasonably confident in stating that the drag coefficient for a circular cross section is much higher than for a circular cross section with the chins (along a symmetrical plane with the flow). Since at the nose of the aircraft, the moment arm to the CG of the aircraft will be (most probably) the largest possible, a small change in cross-flow drag coefficient could result in an appreciable change in the vehicle yaw coefficient. So you need the ventrals at that point.

Funny story on the YF-12A that went to NASA. At one point the folding ventral departed the aircraft. They recovered it but upon further testing discovered it wasn't necessary so they left it off.

[garrya]

Ultimately, jets have ventral fins because they need them for yaw stability.

Dragon has half the answer. They have them, if they have them, for directional stability. The part he is missing is that they need the added stability in different regimes. Let me roll through a progressive look at the listed aircraft.

F-16/MiG-21 - Single fin blanked out from good airflow during maneuvers (already discussed) needs additional stability for higher AoA.
F/A-18/MiG-29/F-35/F-22 - outwardly canted twin fins biting into airflow vortex from LERX/Chines does not need them.
Su-27 - twin fins are not canted outward, still needs them but they are reduced in relative size.
F-15 - twin fins are not canted outward and has no LERX, loses Yaw control at high AOA, could use them but dash speed was the priority in the design
MiG-31 - Mach 3 speeds required small vertical tail, tall fore body required additional stability at Mach 3
SR-71 - Went with all moving tail, low AoA aircraft, very slim fore body and full strakes, no ventrals needed
YF-12 - taller fore body and shorter strakes than the SR-71 mean more sideload can be applied to the nose, the short verticals need some help.

In short, everything is a compromise.

Thank you for the very detail answer. Btw , why all aircraft with delta, semi delta wing like LCA , Mirage , Rafale , Typhoon , F-102 , F-106 , Concorde doesnot have ventral fin ? is that because they also desired dash capability ?

[garrya]

Adding answer from Johnwill

[sprstdlyscottsmn]

Johnwill to the rescue!

[garrya]

Johnwill to the rescue!

I would like to hear answer from both you please